Publications

* implies equal contribution

Sorted by date or topic

2017

(304)
"Benzoin Radicals as Reducing Agent for Synthesizing Ultrathin Copper Nanowires",

Fan Cui, Letian Dou, Qin Yang, Yi Yu, Zhiqiang Niu, Yuchun Sun, Hao Liu, Ahmad Dehestani, Kerstin Schierle-Arndt, and Peidong Yang. J. Am. Chem. Soc., in press.

(303)
“Structure-sensitive CO2 electroreduction to hydrocarbons on ultrathin five-fold twinned copper nanowires”,

Yifan Li, Fan Cui, Michael B. Ross, Dohyung Kim, Yuchun Sun, and Peidong Yang. Nano Letters, In press.

(302)
"Revealing the Size-Dependent D-D Excitations of Cobalt Nanoparticles Using Soft X-ray Spectroscopy",

Zhangzhang Cui, Chenlu Xie, Xuefei Feng, Nigel Becknell, Peidong Yang, Yalin Lu, Xiaofang Zhai, Xiaosong Liu, Wanli Yang, Yi-De Chuang, Jinghua Guo, J. Phys. Chem. Lett, In press.

(301)
"Structural, optical, and electrical study of phase controlled cesium lead iodide nanowires",

Minliang Lai, Qiao Kong, Connor G. Bischak, Yi Yu, Letian Dou, Sam Eaton, Naomi S. Ginsberg, Peidong Yang, Nano Research, In press.

(300)
"Cyborgian Material Design for Solar Fuel Production: The Emerging Photosynthetic Biohybrid Systems",

Kelsey Sakimoto, Nikolay Kornienko, Peidong Yang, Acct. Chem. Res, In press.

(299)
"Polymer Encapsulation of Perovskite Nanocrystals: Enhanced Water and Light Stability and Polarization",

Shilpa Raja, Yehonadav Bekenstein, Matthew Koc, Stefan Fischer, Dandan Zhang, Liwei Lin, Robert Ritchie, Peidong Yang, A. Paul Alivisatos, ACS Applied Materials & Interfaces, In press.

(298)
"Investigation of Phonon Coherence and Backscattering using Silicon Nanomeshes",

Jaeho Lee, Woochul Lee, Geoff Wehmeyer, Scott Dhuey, Deirdre L Olynick, Stefano Cabrini, Chris Dames, Jeffrey J Urban, and Peidong Yang. Nature Communications 8, 14054 (2017).

Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction.

[pdf] [SI]

(297)
“Plasmon-Enhanced Photocatalytic CO2 Conversion within Metal-Organic Frameworks Under Visible Light”,

Kyung Min Choi*, Dohyung Kim*, Bunyarat Rungtaweevoranit, Christopher A Trickett, Jesika Trese Deniz Barmanbek, Ahmad S Alshammari, Peidong Yang, and Omar M Yaghi. J. Am. Chem. Soc. 139, 356–362 (2017).

DOI: 10.1021/jacs.6b11027"

Materials development for artificial photosynthesis, in particular, CO2 reduction, has been under extensive efforts, ranging from inorganic semiconductors to molecular complexes. In this report, we demonstrate a metal–organic framework (MOF)-coated nanoparticle photocatalyst with enhanced CO2 reduction activity and stability, which stems from having two different functional units for activity enhancement and catalytic stability combined together as a single construct. Covalently attaching a CO2-to-CO conversion photocatalyst ReI(CO)3(BPYDC)Cl, BPYDC = 2,2′-bipyridine-5,5′-dicarboxylate, to a zirconium MOF, UiO-67 (Ren-MOF), prevents dimerization leading to deactivation. By systematically controlling its density in the framework (n = 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell), the highest photocatalytic activity was found for Re3-MOF. Structural analysis of Ren-MOFs suggests that a fine balance of proximity between photoactive centers is needed for cooperatively enhanced photocatalytic activity, where an optimum number of Re complexes per unit cell should reach the highest activity. Based on the structure–activity correlation of Ren-MOFs, Re3-MOF was coated onto Ag nanocubes (Ag⊂Re3-MOF), which spatially confined photoactive Re centers to the intensified near-surface electric fields at the surface of Ag nanocubes, resulting in a 7-fold enhancement of CO2-to-CO conversion under visible light with long-term stability maintained up to 48 h.

[pdf] [SI]

2016

(296)
“Atomic Resolution Imaging of Halide Perovskites”,

Yi Yu*, Dandan Zhang*, Christian Kisielowski, Letian Dou, Nikolay Kornienko, Yehonadav Bekenstein, Andrew B Wong, A Paul Alivisatos, and Peidong Yang. Nano Letters 16, 7530–7535 (2016).

DOI: 10.1073/pnas.1610554113

The radiation-sensitive nature of halide perovskites has hindered structural studies at the atomic scale. We overcome this obstacle by applying low dose-rate in-line holography, which combines aberration-corrected high-resolution transmission electron microscopy with exit-wave reconstruction. This technique successfully yields the genuine atomic structure of ultrathin two-dimensional CsPbBr3 halide perovskites, and a quantitative structure determination was achieved atom column by atom column using the phase information of the reconstructed exit-wave function without causing electron beam-induced sample alterations. An extraordinarily high image quality enables an unambiguous structural analysis of coexisting high-temperature and low-temperature phases of CsPbBr3 in single particles. On a broader level, our approach offers unprecedented opportunities to better understand halide perovskites at the atomic level as well as other radiation-sensitive materials.

[pdf] [SI]

(295)
“Shaping electrocatalysis through tailored nanomaterials”,

Yijin Kang, Peidong Yang, Nenad M Markovic, and Vojislav R Stamenkovic. Nano Today 11, 587–600 (2016).

DOI: 10.1016/j.nantod.2016.08.008

Electrocatalysis is a subclass of heterogeneous catalysis that is aimed towards increase of the electrochemical reaction rates that are taking place at the surface of electrodes. Real-world electrocatalysts are usually based on precious metals in the form of nanoparticles due to their high surface-to-volume ratio, which enables better utilization of employed materials. Ability to tailor nanostructure of an electrocatalyst is critical in order to tune their electrocatalytic properties. Over the last decade, that has mainly been achieved through implementation of fundamental studies performed on well-defined extended surfaces with distinct single crystalline and polycrystalline structures. Based on these studies, it has been demonstrated that performance of an electrocatalyst could be significantly changed through the control of size, composition, morphology and architecture of employed nanomaterials. This review outlines the following steps in the process of rational development of an efficient electrocatalyst: 1) electrochemical properties of well-defined surfaces, 2) synthesis and characterization of different classes of electrocatalysts, and 3) correlation between physical properties (size, shape, composition and morphology) and electrochemical behavior (adsorption, electrocatalytic activity and durability) of electrocatalyst. In addition, this is a brief summary of the novel research platforms in the development of functional nanomaterials for energy conversion and storage applications such as fuel cells electrolyzers and batteries.

[pdf]

(294)
“Ultrathin Colloidal Cesium Lead Halide Perovskite Nanowires”,

Dandan Zhang*, Yi Yu*, Yehonadav Bekenstein*, Andrew B Wong, A Paul Alivisatos, and Peidong Yang. J. Am. Chem. Soc. 138, 13155–13158 (2016).

DOI: 10.1021/jacs.6b08373

Highly uniform single crystal ultrathin CsPbBr3 nanowires (NWs) with diameter of 2.2 ± 0.2 nm and length up to several microns were successfully synthesized and purified using a catalyst-free colloidal synthesis method followed by a stepwise purification strategy. The NWs have bright photoluminescence (PL) with a photoluminescence quantum yield (PLQY) of about 30% after surface treatment. Large blue-shifted UV− vis absorption and PL spectra have been observed due to strong two-dimensional quantum confinement effects. A small angle X-ray scattering (SAXS) pattern shows the periodic packing of the ultrathin NWs along the radial direction, demonstrates the narrow radial distribution of the wires, and emphasizes the deep intercalation of the surfactants. Despite the extreme aspect ratios of the ultrathin NWs, their composition and the resulting optical properties can be readily tuned by an anion-exchange reaction with good morphology preservation. These bright ultrathin NWs may be used as a model system to study strong quantum confinement effects in a one-dimensional halide perovskite system.

[pdf] [SI]

(293)
“Spectroscopic elucidation of energy transfer in hybrid inorganic-biological organisms for solar-to-chemical production”,

Nikolay Kornienko, Kelsey K. Sakimoto, David M Herlihy, Son C Nguyen, A. Paul Alivisatos, Charles B. Harris, Adam Schwartzberg, and Peidong Yang. Proc. Natl. Acad. Sci. 113, 11750–11755 (2016).

DOI: 10.1021/jacs.6b08373

The rise of inorganic–biological hybrid organisms for solar-to-chemical production has spurred mechanistic investigations into the dynamics of the biotic–abiotic interface to drive the development of next-generation systems. The model system, Moorella thermoacetica–cadmium sulfide (CdS), combines an inorganic semiconductor nanoparticle light harvester with an acetogenic bacterium to drive the photosynthetic reduction of CO2 to acetic acid with high efficiency. In this work, we report insights into this unique electrotrophic behavior and propose a charge-transfer mechanism from CdS to M. thermoacetica. Transient absorption (TA) spectroscopy revealed that photoexcited electron transfer rates increase with increasing hydrogenase (H2ase) enzyme activity. On the same time scale as the TA spectroscopy, time-resolved infrared (TRIR) spectroscopy showed spectral changes in the 1,700–1,900-cm−1 spectral region. The quantum efficiency of this system for photosynthetic acetic acid generation also increased with increasing H2ase activity and shorter carrier lifetimes when averaged over the first 24 h of photosynthesis. However, within the initial 3 h of photosynthesis, the rate followed an opposite trend: The bacteria with the lowest H2ase activity photosynthesized acetic acid the fastest. These results suggest a two-pathway mechanism: a high quantum efficiency charge-transfer pathway to H2ase generating H2 as a molecular intermediate that dominates at long time scales (24 h), and a direct energy-transducing enzymatic pathway responsible for acetic acid production at short time scales (3 h). This work represents a promising platform to utilize conventional spectroscopic methodology to extract insights from more complex biotic–abiotic hybrid systems.

[pdf]

(292)
“Insights into the Mechanism of Tandem Alkene Hydroformylation over Nanocrystal Catalyst with Multiple Interfaces”,

Ji Su, Chenlu Xie, Chen Chen, Yi Yu, Griffin Kennedy, Gabor A. Somorjai, and Peidong Yang. J. Am. Chem. Soc. 138, 11568–11574 (2016).

DOI: 10.1021/jacs.6b03915

The concept of tandem catalysis, where sequential reactions catalyzed by different interfaces in single nanostructure give desirable product selectively, has previously been applied effectively in the production of propanal from methanol (via carbon monoxide and hydrogen) and ethylene via tandem hydroformylation. However, the underlying mechanism leading to enhanced product selectivity has remained elusive due to the lack of stable, well-defined catalyst suitable for in-depth comprehensive study. Accordingly, we present the design and synthesis of a three-dimensional (3D) catalyst CeO2–Pt@mSiO2 with well-defined metal–oxide interfaces and stable architecture and investigate the selective conversion of ethylene to propanal via tandem hydroformylation. The effective production of aldehyde through the tandem hydroformylation was also observed on propylene and 1-butene. A thorough study of the CeO2–Pt@mSiO2 under different reaction and control conditions reveals that the ethylene present for the hydroformylation step slows down initial methanol decomposition, preventing the accumulation of hydrogen (H2) and favoring propanal formation to achieve up to 80% selectivity. The selectivity is also promoted by the fact that the reaction intermediates produced from methanol decomposition are poised to directly undergo hydroformylation upon migration from one catalytic interface to another. This synergistic effect between the two sequential reactions and the corresponding altered reaction pathway, compared to the single-step reaction, constitute the key advantages of this tandem catalysis. Ultimately, this in-depth study unravels the principles of tandem catalysis related to hydroformylation and represents a key step toward the rational design of new heterogeneous catalysts.

[pdf] [SI]

(291)
"Cysteine−Cystine Photoregeneration for Oxygenic Photosynthesis of Acetic Acid from CO2 by a Tandem Inorganic−Biological Hybrid System",

Kelsey K Sakimoto, Stephanie J Zhang, and Peidong Yang. Nano Letters 16, 5883–5887 (2016).

DOI: 10.1021/acs.nanolett.6b02740"

Tandem “Z-scheme” approaches to solar-to-chemical production afford the ability to independently develop and optimize reductive photocatalysts for CO2 reduction to multicarbon compounds and oxidative photocatalysts for O2 evolution. To connect the two redox processes, molecular redox shuttles, reminiscent of biological electron transfer, offer an additional level of facile chemical tunability that eliminates the need for solid-state semiconductor junction engineering. In this work, we report a tandem inorganic–biological hybrid system capable of oxygenic photosynthesis of acetic acid from CO2. The photoreductive catalyst consists of the bacterium Moorella thermoacetica self-photosensitized with CdS nanoparticles at the expense of the thiol amino acid cysteine (Cys) oxidation to the disulfide form cystine (CySS). To regenerate the CySS/Cys redox shuttle, the photooxidative catalyst, TiO2 loaded with cocatalyst Mn(II) phthalocyanine (MnPc), couples water oxidation to CySS reduction. The combined system M. thermoacetica–CdS + TiO2–MnPc demonstrates a potential biomimetic approach to complete oxygenic solar-to-chemical production.

[pdf] [SI]

(290)
“Directed assembly of nanoparticle catalysts on nanowire photoelectrodes for photoelectrochemical CO2 reduction”,

Qiao Kong*, Dohyung Kim*, Chong Liu, Yi Yu, Yude Su, Yifan Li, and Peidong Yang. Nano Letters 16, 5675–5680 (2016).

DOI: 10.1021/acs.nanolett.6b02321

Reducing carbon dioxide with a multicomponent artificial photosynthetic system, closely mimicking nature, represents a promising approach for energy storage. Previous works have focused on exploiting light-harvesting semiconductor nanowires (NW) for photoelectrochemical water splitting. With the newly developed CO2 reduction nanoparticle (NP) catalysts, direct interfacing of these nanocatalysts with NW light absorbers for photoelectrochemical reduction of CO2 becomes feasible. Here, we demonstrate a directed assembly of NP catalysts on vertical NW substrates for CO2-to-CO conversion under illumination. Guided by the one-dimensional geometry, well-dispersed assembly of Au3Cu NPs on the surface of Si NW arrays was achieved with facile coverage tunability. Such Au3Cu NP decorated Si NW arrays can readily serve as effective CO2 reduction photoelectrodes, exhibiting high CO2-to-CO selectivity close to 80% at −0.20 V vs RHE with
suppressed hydrogen evolution. A reduction of 120 mV overpotential compared to the planar (PL) counterpart was observed resulting from the optimized spatial arrangement of NP catalysts on the high surface area NW arrays. In addition, this system showed consistent photoelectrochemical CO2 reduction capability up to 18 h. This simple photoelectrode assembly process will lead to further progress in artificial photosynthesis, by allowing the combination of developments in each subfield to create an efficient light-driven system generating carbon-based fuels.

[pdf] [SI]

(289)
“Anisotropic phase segregation and migration of Pt in nanocrystals en route to nanoframe catalysts”,

Zhiqiang Niu*, Nigel Becknell*, Yi Yu, Dohyung Kim, Chen Chen, Nikolay Kornienko, Gabor A Somorjai, and Peidong Yang. Nat. Mater. 15, 1188–1194 (2016).

DOI: 10.1038/nmat4724

Compositional heterogeneity in shaped, bimetallic nanocrystals offers additional variables to manoeuvre the functionality of the nanocrystal. However, understanding how to manipulate anisotropic elemental distributions in a nanocrystal is a great challenge in reaching higher tiers of nanocatalyst design. Here, we present the evolutionary trajectory of phase segregation in Pt–Ni rhombic dodecahedra. The anisotropic growth of a Pt-rich phase along the ⟨111⟩ and ⟨200⟩ directions at the initial growth stage results in Pt segregation to the 14 axes of a rhombic dodecahedron, forming a highly branched, Pt-rich tetradecapod structure embedded in a Ni-rich shell. With longer growth time, the Pt-rich phase selectively migrates outwards through the 14 axes to the 24 edges such that the rhombic dodecahedron becomes a Pt-rich frame enclosing a Ni- rich interior phase. The revealed anisotropic phase segregation and migration mechanism o ers a radically different approach to fabrication of nanocatalysts with desired compositional distributions and performance.

[pdf] [SI]

(288)
“A Molecular Surface Functionalization Approach to Tuning Nanoparticle Electrocatalysts for Carbon Dioxide Reduction”,

Zhi Cao, Dohyung Kim, Dachao Hong, Yi Yu, Jun Xu, Song Lin, Xiaodong Wen, Eva M Nichols, Keunhong Jeong, Jeffrey A Reimer, Peidong Yang, and Christopher J Chang. J. Am. Chem. Soc. 138, 8120–8125 (2016).

DOI: 10.1021/jacs.6b02878

Conversion of the greenhouse gas carbon dioxide (CO2) to value-added products is an important challenge for sustainable energy research, and nanomaterials offer a broad class of heterogeneous catalysts for such transformations. Here we report a molecular surface functionalization approach to tuning gold nanoparticle (Au NP) electrocatalysts for reduction of CO2 to CO. The N-heterocyclic (NHC) carbene-functionalized Au NP catalyst exhibits improved faradaic efficiency (FE = 83%) for reduction of CO2 to CO in water at neutral pH at an overpotential of 0.46 V with a 7.6-fold increase in current density compared to that of the parent Au NP (FE = 53%). Tafel plots of the NHC carbene-functionalized Au NP (72 mV/decade) vs parent Au NP (138 mV/decade) systems further show that the molecular ligand influences mechanistic pathways for CO2 reduction. The results establish molecular surface functionalization as a complementary approach to size, shape, composition, and defect control for nanoparticle catalyst design.

[pdf] [SI]

(287)
“Synthesis of Composition Tunable Cesium Lead Halide Nanowires through Anion-Exchange Reactions”,

Dandan Zhang, Yiming Yang, Yehonadav Bekenstein, Yi Yu, Natalie A Gibson, Andrew B Wong, Samuel W Eaton, Nikolay Kornienko, Qiao Kong, Minliang Lai, A Paul Alivisatos, Stephen R Leone, and Peidong Yang, J. Am. Chem. Soc. 138, 7236–7239 (2016).

DOI: 10.1021/jacs.6b03134

Here, we demonstrate the successful synthesis of brightly emitting colloidal cesium lead halide (CsPbX3, X = Cl, Br, I) nanowires (NWs) with uniform diameters and tunable compositions. By using highly monodisperse CsPbBr3 NWs as templates, the NW composition can be independently controlled through anion-exchange reactions. CsPbX3 alloy NWs with a wide range of alloy compositions can be achieved with well-preserved morphology and crystal structure. The NWs are highly luminescent with photoluminescence quantum yields (PLQY) ranging from 20% to 80%. The bright photoluminescence can be tuned over nearly the entire visible spectrum. The high PLQYs together with charge transport measurements exemplify the efficient alloying of the anionic sublattice in a one-dimensional CsPbX3 system. The wires increased functionality in the form of fast photoresponse rates and the low defect density suggest CsPbX3 NWs as prospective materials for optoelectronic applications.

[pdf] [SI]

(286)
“Thermal Transport in Silicon Nanowires at High Temperature up to 700 K”,

Jaeho Lee, Woochul Lee, Jongwoo Lim, Yi Yu, Qiao Kong, Jeffrey J Urban, and Peidong Yang, Nano Letters 16, 4133–4140 (2016).

DOI: 10.1021/acs.nanolett.6b00956

Thermal transport in silicon nanowires has captured the attention of scientists for understanding phonon transport at the nanoscale, and the thermoelectric figure-of-merit (ZT) reported in rough nanowires has inspired engineers to develop cost-effective waste heat recovery systems. Thermoelectric generators composed of silicon target high-temperature applications due to improved efficiency beyond 550 K. However, there have been no studies of thermal transport in silicon nanowires beyond room temperature. High-temperature measurements also enable studies of unanswered questions regarding the impact of surface boundaries and varying mode contributions as the highest vibrational modes are activated (Debye temperature of silicon is 645 K). Here, we develop a technique to investigate thermal transport in nanowires up to 700 K. Our thermal conductivity measurements on smooth silicon nanowires show the classical diameter dependence from 40 to 120 nm. In conjunction with Boltzmann transport equation, we also probe an increasing contribution of high-frequency phonons (optical phonons) in smooth silicon nanowires as the diameter decreases and the temperature increases. Thermal conductivity of rough silicon nanowires is significantly reduced throughout the temperature range, demonstrating a potential for efficient thermoelectric generation (e.g., ZT = 1 at 700 K).

[pdf] [SI]

(285)
“Semiconductor Nanowire Lasers”,

Samuel W. Eaton, Anthony Fu, Andrew B. Wong, C.Z. Ning, Peidong Yang, Nature Mater. Rev, 1, 16028, 2016.

The discovery and continued development of the laser has revolutionized both science and industry. The advent of miniaturized, semiconductor lasers has made this technology an integral part of everyday life. Exciting research continues with a new focus on nanowire lasers because of their great potential in the field of optoelectronics. In this Review, we explore the latest advancements in the development of nanowire lasers and offer our perspective on future improvements and trends. We discuss fundamental material considerations and the latest, most effective materials for nanowire lasers. A discussion of novel cavity designs and amplification methods is followed by some of the latest work on surface plasmon polariton nanowire lasers. Finally, exciting new reports of electrically pumped nanowire lasers with the potential for integrated optoelectronic applications are described.

[pdf] [Cover Highlight]

(284)
“Atomic Structure of Ultrathin Gold Nanowires”,

Yi Yu, Fan Cui, Jianwei Sun, Peidong Yang, Nano. Lett, 16, 3078, 2016.

Understanding of the atomic structure and
stability of nanowires (NWs) is critical for their applications in
nanotechnology, especially when the diameter of NWs reduces
to ultrathin scale (1−2 nm). Here, using aberration-corrected
high-resolution transmission electron microscopy (ACHRTEM),
we report a detailed atomic structure study of the
ultrathin Au NWs, which are synthesized using a silanemediated
approach. The NWs contain large amounts of
generalized stacking fault defects. These defects evolve upon
sustained electron exposure, and simultaneously the NWs
undergo necking and breaking. Quantitative strain analysis
reveals the key role of strain in the breakdown process.
Besides, ligand-like morphology is observed at the surface of the NWs, indicating the possibility of using AC-HRTEM for surface
ligand imaging. Moreover, the coalescence dynamic of ultrathin Au NWs is demonstrated by in situ observations. This work
provides a comprehensive understanding of the structure of ultrathin metal NWs at atomic-scale and could have important
implications for their applications.

[pdf]

(283)
“Growth and Photoelectrochemical Energy Conversion of Wurtzite Indium Phosphide Nanowire Arrays”,

Nikolay Kornienko, Natalie Gibson, Hao Zhang, Samuel W. Eaton, Yi Yu, Shaul Aloni, Stephen R. Leone, Peidong Yang, ACS Nano, 10, 5525, 2016.

Photoelectrochemical (PEC) water splitting into hydrogen and oxygen is a promising strategy to absorb solar energy and directly convert it into a dense storage medium in the form of chemical bonds. The continual development and improvement of individual components of PEC systems is critical toward increasing the solar to fuel efficiency of prototype devices. Within this context, we describe a study on the growth of wurtzite indium phosphide (InP) nanowire (NW) arrays on silicon substrates and their subsequent implementation as light-absorbing photocathodes in PEC cells. The high onset potential (0.6 V vs the reversible hydrogen electrode) and photocurrent (18 mA/cm2) of the InP photocathodes render them as promising building blocks for high performance PEC cells. As a proof of concept for overall system integration, InP photocathodes were combined with a nanoporous bismuth vanadate (BiVO4) photoanode to generate an unassisted solar water splitting efficiency of 0.5%.

[pdf]

(282)
“Single nanowire photoelectrochemistry”,

Y. Su*, C. Liu*, S. Brittman, J. Tang, A. Fu, P. Yang, Nature Nano., 11, 609, 2016

DOI: 10.1038/nnano.2016.30

(281)
“Lasing in Robust Cesium Lead Halide Perovskite Nanowires”,

S. W. Eaton, M. Lai, N. Gibson, A. B. Wong, L. Dou, J. Ma, L. Wang, S. R. Leone, P. Yang, PNAS, 113, 1993, 2016.

The rapidly growing field of nanoscale lasers can be advanced through the discovery of new, tunable light sources. The emission wavelength tunability demonstrated in perovskite materials is an attractive property for nanoscale lasers. Whereas organic–inorganic lead halide perovskite materials are known for their instability, cesium lead halides offer a robust alternative without sacrificing emission tunability or ease of synthesis. Here, we report the low-temperature, solution-phase growth of cesium lead halide nanowires exhibiting low-threshold lasing and high stability. The as-grown nanowires are single crystalline with well-formed facets, and act as high-quality laser cavities. The nanowires display excellent stability while stored and handled under ambient conditions over the course of weeks. Upon optical excitation, Fabry–Pérot lasing occurs in CsPbBr3 nanowires with an onset of 5 μJ cm−2 with the nanowire cavity displaying a maximum quality factor of 1,009 ± 5. Lasing under constant, pulsed excitation can be maintained for over 1 h, the equivalent of 109 excitation cycles, and lasing persists upon exposure to ambient atmosphere. Wavelength tunability in the green and blue regions of the spectrum in conjunction with excellent stability makes these nanowire lasers attractive for device fabrication.

[pdf]

(280)
"TiO2/BiVO4 Nanowire Heterostructure Photoanodes based on Type II Band Alignment",

J. Resasco; H. Zhang; N. Kornienko; N. Becknell; H. Lee; J. Guo; A. Briseno; P. Yang, ACS Central Science, 2, 80, 2016.

Metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells. However, their performance is often limited by poor charge carrier transport. We show that this problem can be addressed by using separate materials for light absorption and carrier transport. Here, we report a Ta:TiO2|BiVO4 nanowire photoanode, in which BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. Electrochemical and spectroscopic measurements provide experimental evidence for the type II band alignment necessary for favorable electron transfer from BiVO4 to TiO2. The host–guest nanowire architecture presented here allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent density of 2.1 mA/cm2 at 1.23 V vs RHE.

[pdf]

(279)
"Solution Processed Copper reduced-Graphene-Oxide Core-shell Nanowire Transparent Conductors",

L. Dou; F. Cui; Y. Yu; G. Khanarian; S. Eaton; Q. Yang; J. Resasco; C. Schildknecht; K. Schierle; P. Yang, ACS Nano, 10, 2600, 2016.

Copper nanowire (Cu NW) based transparent conductors are promising candidates to replace ITO (indium–tin-oxide) owing to the high electrical conductivity and low-cost of copper. However, the relatively low performance and poor stability of Cu NWs under ambient conditions limit the practical application of these devices. Here, we report a solution-based approach to wrap graphene oxide (GO) nanosheets on the surface of ultrathin copper nanowires. By mild thermal annealing, GO can be reduced and high quality Cu r-GO core–shell NWs can be obtained. High performance transparent conducting films were fabricated with these ultrathin core–shell nanowires and excellent optical and electric performance was achieved. The core–shell NW structure enables the production of highly stable conducting films (over 200 days stored in air), which have comparable performance to ITO and silver NW thin films (sheet resistance ∼28 Ω/sq, haze ∼2% at transmittance of ∼90%).

[pdf]

(278)
“Simultaneous Thermoelectric Property Measurement and Incoherent Phonon Transport in Holey Silicon”,

J. Lim, H. Wang, J. Tang, S. C. Andrews, J. Lee, D. Lee, T. P. Russell, P. Yang, ACS Nano, 10(1), 124, 2016

Block copolymer patterned holey silicon (HS) was successfully integrated into a microdevice for simultaneous measurements of Seebeck coefficient, electrical conductivity, and thermal conductivity of the same HS microribbon. These fully integrated HS microdevices provided excellent platforms for the systematic investigation of thermoelectric transport properties tailored by the dimensions of the periodic hole array, that is, neck and pitch size, and the doping concentrations. Specifically, thermoelectric transport properties of HS with a neck size in the range of 16–34 nm and a fixed pitch size of 60 nm were characterized, and a clear neck size dependency was shown in the doping range of 3.1 × 1018 to 6.5 × 1019 cm–3. At 300 K, thermal conductivity as low as 1.8 ± 0.2 W/mK was found in HS with a neck size of 16 nm, while optimized zT values were shown in HS with a neck size of 24 nm. The controllable effects of holey array dimensions and doping concentrations on HS thermoelectric performance could aid in improving the understanding of the phonon scattering process in a holey structure and also in facilitating the development of silicon-based thermoelectric devices.

[pdf] [SI]

(277)
“Low-Temperature Solution-Phase Growth of Silicon and New Silicon-Containing Alloy Nanowires”,

J. Sun*, F. Cui*, C. Kisielowski, Y. Yu, N. Kornienko, P. Yang, J. Phys. Chem. C, In Press, 2016

Low-temperature synthesis of crystalline silicon and silicon-containing nanowires remains a challenge in synthetic chemistry due to the lack of sufficiently reactive Si precursors. We report that colloidal Si nanowires can be grown using tris(trimethylsilyl)silane or trisilane as the Si precursor by a Ga-mediated solution–liquid–solid (SLS) approach at temperatures of about 200 °C, which is more than 200 °C lower than that reported in the previous literature. We further demonstrate that the new Si chemistry can be adopted to incorporate Si atoms into III–V semiconductor lattices, which holds promise to produce a new Si-containing alloy semiconductor nanowire. This development represents an important step toward low-temperature fabrication of Si nanowire-based devices for broad applications.

[pdf] [SI]

(276)
"Self-photosensitization of Nonphotosynthetic Bacteria for Solar-to-chemical Production",

K. K. Sakimoto, A. B. Wong, P. Yang, Science, 351(6268), 74, 2016

Improving natural photosynthesis can enable the sustainable production of chemicals. However, neither purely artificial nor purely biological approaches seem poised to realize the potential of solar-to-chemical synthesis. We developed a hybrid approach, whereby we combined the highly efficient light harvesting of inorganic semiconductors with the high specificity, low cost, and self-replication and -repair of biocatalysts. We induced the self-photosensitization of a nonphotosynthetic bacterium, Moorella thermoacetica, with cadmium sulfide nanoparticles, enabling the photosynthesis of acetic acid from carbon dioxide. Biologically precipitated cadmium sulfide nanoparticles served as the light harvester to sustain cellular metabolism. This self-augmented biological system selectively produced acetic acid continuously over several days of light-dark cycles at relatively high quantum yields, demonstrating a self-replicating route toward solar-to-chemical carbon dioxide reduction.

[pdf] [SI] [Science Magazine Perspective]

2015

(275)
"Highly Luminescent Colloidal Nanoplates of Perovskite Cesium Lead Halide and Their Oriented Assemblies",

Y. Bekenstein, B. A. Koscher, S. W. Eaton, P. Yang, A. P. Alivisatos, JACS, 137(51), 16008, 2015

Anisotropic colloidal quasi-two-dimensional nanoplates (NPLs) hold great promise as functional materials due to their combination of low dimensional optoelectronic properties and versatility through colloidal synthesis. Recently, lead-halide perovskites have emerged as important optoelectronic materials with excellent efficiencies in photovoltaic and light-emitting applications. Here we report the synthesis of quantum confined all inorganic cesium lead halide nanoplates in the perovskite crystal structure that are also highly luminescent (PLQY 84%). The controllable self-assembly of nanoplates either into stacked columnar phases or crystallographic-oriented thin-sheet structures is demonstrated. The broad accessible emission range, high native quantum yields, and ease of self-assembly make perovskite NPLs an ideal platform for fundamental optoelectronic studies and the investigation of future devices.

[pdf] [SI]

(274)
“Atomic Structure of Pt3Ni Nanoframe Electrocatalysts by In situ X-ray Absorption Spectroscopy”,

N. Becknell, Y. Kang, C. Chen, J. Resasco, N. Kornienko, J. Guo, N. M. Markovic, G. A. Somorjai, V. R. Stamenkovic, P. Yang, JACS, 137(50), 15817, 2015

Understanding the atomic structure of a catalyst is crucial to exposing the source of its performance characteristics. It is highly unlikely that a catalyst remains the same under reaction conditions when compared to as-synthesized. Hence, the ideal experiment to study the catalyst structure should be performed in situ. Here, we use X-ray absorption spectroscopy (XAS) as an in situ technique to study Pt3Ni nanoframe particles which have been proven to be an excellent electrocatalyst for the oxygen reduction reaction (ORR). The surface characteristics of the nanoframes were probed through electrochemical hydrogen underpotential deposition and carbon monoxide electrooxidation, which showed that nanoframe surfaces with different structure exhibit varying levels of binding strength to adsorbate molecules. It is well-known that Pt-skin formation on Pt–Ni catalysts will enhance ORR activity by weakening the binding energy between the surface and adsorbates. Ex situ and in situ XAS results reveal that nanoframes which bind adsorbates more strongly have a rougher Pt surface caused by insufficient segregation of Pt to the surface and consequent Ni dissolution. In contrast, nanoframes which exhibit extremely high ORR activity simultaneously demonstrate more significant segregation of Pt over Ni-rich subsurface layers, allowing better formation of the critical Pt-skin. This work demonstrates that the high ORR activity of the Pt3Ni hollow nanoframes depends on successful formation of the Pt-skin surface structure.

[pdf] [SI]

(273)
"A Unified Initiative to Harness Earth's Microbiomes",

Unified Microbiome Initiative Consortium, Science, 350(6265), 507, 2015

Despite their centrality to life on Earth, we know little about how microbes (1) interact with each other, their hosts, or their environment. Although DNA sequencing technologies have enabled a new view of the ubiquity and diversity of microorganisms, this has mainly yielded snapshots that shed limited light on microbial functions or community dynamics. Given that nearly every habitat and organism hosts a diverse constellation of microorganisms—its “microbiome”—such knowledge could transform our understanding of the world and launch innovations in agriculture, energy, health, the environment, and more (see the photo). We propose an interdisciplinary Unified Microbiome Initiative (UMI) to discover and advance tools to understand and harness the capabilities of Earth’s microbial ecosystems. The impacts of oceans and soil microbes on atmospheric CO2 are critical for understanding climate change (2). By manipulating interactions at the root-soil-microbe interface, we may reduce agricultural pesticide, fertilizer, and water use enrich marginal land and rehabilitate degraded soils. Microbes can degrade plant cell walls (for biofuels), and synthesize myriad small molecules for new bioproducts, including antibiotics (3). Restoring normal human microbial ecosystems can save lives [e.g., fecal microbiome transplantation for Clostridium difficile infections (4)]. Rational management of microbial communities in and around us has implications for asthma, diabetes, obesity, infectious diseases, psychiatric illnesses, and other afflictions (5, 6). The human microbiome is a target and a source for new drugs (7) and an essential tool for precision medicine (8).

[pdf] [SI]

(272)
"Synthesis of PtCo3 Polyhedral Nanoparticles and Evolution to Pt3Co Nanoframes",

N. Becknell, C. Zheng, C. Chen, Y. Yu, P. Yang, Surf. Sci., 648, 328-332, 2015.

Bimetallic nanoframes have great potential for achieving new levels of catalytic activity in various heterogeneous reactions due to their high surface area dispersion of expensive noble metals on the exterior and interior surfaces of the structure. PtCo3 nanoparticles with polyhedral shapes were synthesized by a hot-injection method. Scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy (EDS) showed that these nanoparticles demonstrated elemental segregation of platinum to the edges of the polyhedron, forming the basis for a framework nanostructure. The process of preferential oxidative leaching which removed cobalt from the interior of the framework was tracked by EDS and inductively coupled plasma optical emission spectroscopy. This evolution procedure left the platinum-rich edges intact to form a Pt3Co nanoframe. This is the first reported synthesis of a platinum–cobalt nanoframe and could have potential applications in catalytic reactions such as oxygen reduction.

[pdf]

(271)
“Metal-Organic Frameworks for Electrocatalytic Reduction of Carbon Dioxide”,

N. Kornienko*, Y. Zhao*, C. S. Kley, C. Zhu, D. Kim, S. Lin, C. J. Chang, O. M. Yaghi, P. Yang, JACS, 137(44), 14129, 2015.

A key challenge in the field of electrochemical carbon dioxide reduction is the design of catalytic materials featuring high product selectivity, stability, and a composition of earth-abundant elements. In this work, we introduce thin films of nanosized metal–organic frameworks (MOFs) as atomically defined and nanoscopic materials that function as catalysts for the efficient and selective reduction of carbon dioxide to carbon monoxide in aqueous electrolytes. Detailed examination of a cobalt–porphyrin MOF, Al2(OH)2TCPP-Co (TCPP-H2 = 4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrabenzoate) revealed a selectivity for CO production in excess of 76% and stability over 7 h with a per-site turnover number (TON) of 1400. In situ spectroelectrochemical measurements provided insights into the cobalt oxidation state during the course of reaction and showed that the majority of catalytic centers in this MOF are redox-accessible where Co(II) is reduced to Co(I) during catalysis.

[pdf] [SI]

(270)
“Widely Tunable Distributed Bragg Reflectors Integrated into Nanowire Waveguides”,

A. Fu, H. Gao, P. Petrov, P. Yang, Nano Lett., 15(10), 6909, 2015.

Periodic structures with dimensions on the order of the wavelength of light can tailor and improve the performance of optical components, and they can enable the creation of devices with new functionalities. For example, distributed Bragg reflectors (DBRs), which are created by periodic modulations in a structure’s dielectric medium, are essential in dielectric mirrors, vertical cavity surface emitting lasers, fiber Bragg gratings, and single-frequency laser diodes. This work introduces nanoscale DBRs integrated directly into gallium nitride (GaN) nanowire waveguides. Photonic band gaps that are tunable across the visible spectrum are demonstrated by precisely controlling the grating’s parameters. Numerical simulations indicate that in-wire DBRs have significantly larger reflection coefficients in comparison with the nanowire’s end facet. By comparing the measured spectra with the simulated spectra, the index of refraction of the GaN nanowire waveguides was extracted to facilitate the design of photonic coupling structures that are sensitive to phase-matching conditions. This work indicates the potential to design nanowire-based devices with improved performance for optical resonators and optical routing.

[pdf] [SI]

(269)
"Synthesis of Ultrathin Copper Nanowires Using Tris(trimethylsilyl)silane for High-Performance and Low-Haze Transparent Conductors",

F. Cui, Y. Yu, L. Dou, J. Sun, Q. Yang, C. Schildknecht, K. Schierle-Arndt, P. Yang, Nano Lett., 15(11), 7610, 2015.

Colloidal metal nanowire based transparent conductors are excellent candidates to replace indium–tin–oxide (ITO) owing to their outstanding balance between transparency and conductivity, flexibility, and solution-processability. Copper stands out as a promising material candidate due to its high intrinsic conductivity and earth abundance. Here, we report a new synthetic approach, using tris(trimethylsilyl)silane as a mild reducing reagent, for synthesizing high-quality, ultrathin, and monodispersed copper nanowires, with an average diameter of 17.5 nm and a mean length of 17 μm. A study of the growth mechanism using high-resolution transmission electron microscopy reveals that the copper nanowires adopt a five-fold twinned structure and evolve from decahedral nanoseeds. Fabricated transparent conducting films exhibit excellent transparency and conductivity. An additional advantage of our nanowire transparent conductors is highlighted through reduced optical haze factors (forward light scattering) due to the small nanowire diameter.

[pdf] [SI]

(268)
"Atomically Thin Two-Dimensional Organic-Inorganic Hybrid Perovskites",

L. Dou*, A. B. Wong*, Y. Yu*, M. Lai, N. Kornienko, S. W. Eaton, A. Fu, C. G. Bischak, J. Ma, T. Ding, N. S. Ginsberg, L. Wang, A. P. Alivisatos, P. Yang, Science, 349(6255), 1518, 2015.

Organic-inorganic hybrid perovskites, which have proved to be promising semiconductor materials for photovoltaic applications, have been made into atomically thin two-dimensional (2D) sheets. We report the solution-phase growth of single- and few-unit-cell-thick single-crystalline 2D hybrid perovskites of (C4H9NH3)2PbBr4 with well-defined square shape and large size. In contrast to other 2D materials, the hybrid perovskite sheets exhibit an unusual structural relaxation, and this structural change leads to a band gap shift as compared to the bulk crystal. The high-quality 2D crystals exhibit efficient photoluminescence, and color tuning could be achieved by changing sheet thickness as well as composition via the synthesis of related materials.

[pdf] [SI]

(267)
"Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water",

S. Lin*, C. Diercks*, Y. Zhang*, N. Kornienko, E. M. Nichols, Y. Zhao, A. R. Paris, D. Kim, P. Yang, O. M. Yaghi, C. J. Chang, Science, 349(6253), 1208, 2015.

Conversion of carbon dioxide (CO2) to carbon monoxide (CO) and other value-added carbon products is an important challenge for clean energy research. Here we report modular optimization of covalent organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000, with initial turnover frequency of 9400 hour−1) at pH 7 with an overpotential of –0.55 volts, equivalent to a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.

[pdf] [SI]

(266)
"Hybrid Bioinorganic Approach to Solar-to-Chemical Conversion",

E.M. Nichols, J.J. Gallagher, C. Liu, Y. Su, J. Resasco, Y. Yu, Y. Sun, P. Yang, M.C.Y. Chang, C.J. Chang, PNAS, 112(37), 11461, 2015

Natural photosynthesis harnesses solar energy to convert CO2 and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO2 to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts and Methanosarcina barkeri as a biocatalyst for CO2 fixation, we demonstrate robust and efficient electrochemical CO2 to CH4 conversion at up to 86% overall Faradaic efficiency for ≥7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO2, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.

[pdf] [SI]

(265)
“Solution-phase Synthesis of Cesium Lead Halide Perovskite Nanowires”,

D. Zhang, S.W. Eaton, Y. Yu, L. Dou, P. Yang, JACS, 137(29), 9230–9233, 2015.

Halide perovskites have attracted much attention over the past 5 years as a promising class of materials for optoelectronic applications. However, compared to hybrid organic–inorganic perovskites, the study of their pure inorganic counterparts, like cesium lead halides (CsPbX3), lags far behind. Here, a catalyst-free, solution-phase synthesis of CsPbX3 nanowires (NWs) is reported. These NWs are single-crystalline, with uniform growth direction, and crystallize in the orthorhombic phase. Both CsPbBr3 and CsPbI3 are photoluminescence active, with composition-dependent temperature and self-trapping behavior. These NWs with a well-defined morphology could serve as an ideal platform for the investigation of fundamental properties and the development of future applications in nanoscale optoelectronic devices based on all-inorganic perovskites.

[pdf] [SI]

(264)
“Growth and Anion Exchange Conversion of CH3NH3PbX3 Nanorod Arrays for Light-Emitting Diodes”,

A.B. Wong, M. Lai, S.W. Eaton, Y. Yu, E. Lin, L. Dou, A. Fu, P. Yang, Nano. Lett., 15(8), 5519–5524, 2015.

The nanowire and nanorod morphology offers great advantages for application in a range of optoelectronic devices, but these high-quality nanorod arrays are typically based on high temperature growth techniques. Here, we demonstrate the successful room temperature growth of a hybrid perovskite (CH3NH3PbBr3) nanorod array, and we also introduce a new low temperature anion exchange technique to convert the CH3NH3PbBr3 nanorod array into a CH3NH3PbI3 nanorod array while preserving morphology. We demonstrate the application of both these hybrid perovskite nanorod arrays for LEDs. This work highlights the potential utility of postsynthetic interconversion of hybrid perovskites for nanostructured optoelectronic devices such as LEDs, which enables new strategies for the application of hybrid perovskites.

[pdf] [SI]

(263)
“Operando Spectroscopic Analysis of an Amorphous Cobalt Sulfide Hydrogen Evolution Electrocatalysts”,

N. Kornienko, J. Resasco, N. Becknell, C. Jiang, J. Guo, S. Leone, P. Yang, JACS, 137(23), 7448-7455, 2015.

The generation of chemical fuel in the form of molecular H2 via the electrolysis of water is regarded to be a promising approach to convert incident solar power into an energy storage medium. Highly efficient and cost-effective catalysts are required to make such an approach practical on a large scale. Recently, a number of amorphous hydrogen evolution reaction (HER) catalysts have emerged that show promise in terms of scalability and reactivity, yet remain poorly understood. In this work, we utilize Raman spectroscopy and X-ray absorption spectroscopy (XAS) as a tool to elucidate the structure and function of an amorphous cobalt sulfide (CoSx) catalyst. Ex situ measurements reveal that the as-deposited CoSx catalyst is composed of small clusters in which the cobalt is surrounded by both sulfur and oxygen. Operando experiments, performed while the CoSx is catalyzing the HER, yield a molecular model in which cobalt is in an octahedral CoS2-like state where the cobalt center is predominantly surrounded by a first shell of sulfur atoms, which, in turn, are preferentially exposed to electrolyte relative to bulk CoS2. We surmise that these CoS2-like clusters form under cathodic polarization and expose a high density of catalytically active sulfur sites for the HER.

[pdf] [SI]

(262)
“Core-shell CdS-Cu2S Nanorod Array Solar Cells”,

A.B. Wong, S. Brittman, Y. Yu, N. Dasgupta, P. Yang, Nano Lett., 15(6), 4096-4101, 2015.

As an earth-abundant p-type semiconductor, copper sulfide (Cu2S) is an attractive material for application in photovoltaic devices. However, it suffers from a minority carrier diffusion length that is less than the length required for complete light absorption. Core–shell nanowires and nanorods have the potential to alleviate this difficulty because they decouple the length scales of light absorption and charge collection. To achieve this geometry using Cu2S, cation exchange was applied to an array of CdS nanorods to produce well-defined CdS–Cu2S core–shell nanorods. Previous work has demonstrated single-nanowire photovoltaic devices from this material system, but in this work, the cation exchange chemistry has been applied to nanorod arrays to produce ensemble-level devices with microscale sizes. The core–shell nanorod array devices show power conversion efficiencies of up to 3.8%. In addition, these devices are stable when measured in air after nearly one month of storage in a desiccator. These results are a first step in the development of large-area nanostructured Cu2S-based photovoltaics that can be processed from solution.

[pdf] [SI]

(261)
"A Lower Threshold for Nanowire Lasers",

A. Fu, P. Yang, Nature Mater. 14, 557-558, 2015.

[pdf]

(260)
“Stabilization of 4H Hexagonal Phase of Gold in Nanoribbon Form”,

Z. Fan, M. Bosman, X. Huang, D. Huang, Y. Yu, Y.A. Akimov, L. Wu, Y. Li, J. Wu, Q. Liu, C.E. Png, C.L. Gan, P. Yang, H. Zhang, Nature Comm., 6, 7684, 2015

Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

[pdf] [SI]

(259)
“Solution Phase Synthesis of Indium Gallium Phosphide Alloy Nanowires”,

N. Kornienko, D. D. Whitmore, Y. Yu, S. Leone, P. Yang, ,ACS Nano, 9(4), 3951-3960, 2015.

The tunable physical and electronic structure of III–V semiconductor alloys renders them uniquely useful for a variety of applications, including biological imaging, transistors, and solar energy conversion. However, their fabrication typically requires complex gas phase instrumentation or growth from high-temperature melts, which consequently limits their prospects for widespread implementation. Furthermore, the need for lattice matched growth substrates in many cases confines the composition of the materials to a narrow range that can be epitaxially grown. In this work, we present a solution phase synthesis for indium gallium phosphide (InxGa1–xP) alloy nanowires, whose indium/gallium ratio, and consequently, physical and electronic structure, can be tuned across the entire x = 0 to x = 1 composition range. We demonstrate the evolution of structural and optical properties of the nanowires, notably the direct to indirect band gap transition, as the composition is varied from InP to GaP. Our scalable, low-temperature synthesis affords compositional, structural, and electronic tunability and can provide a route for realization of broader InxGa1–xP applications.

[pdf] [SI]

(258)
“Nanowire-Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals”,

C. Liu, J. J. Gallagher, K. K. Sakimoto, E. M. Nichols, C. J. Chang, M. C. Y. Chang, P. Yang, Nano Lett., 15(5), 3634-3639, 2015.

Direct solar-powered production of value-added chemicals from CO2 and H2O, a process that mimics natural photosynthesis, is of fundamental and practical interest. In natural photosynthesis, CO2 is first reduced to common biochemical building blocks using solar energy, which are subsequently used for the synthesis of the complex mixture of molecular products that form biomass. Here we report an artificial photosynthetic scheme that functions via a similar two-step process by developing a biocompatible light-capturing nanowire array that enables a direct interface with microbial systems. As a proof of principle, we demonstrate that a hybrid semiconductor nanowire–bacteria system can reduce CO2 at neutral pH to a wide array of chemical targets, such as fuels, polymers, and complex pharmaceutical precursors, using only solar energy input. The high-surface-area silicon nanowire array harvests light energy to provide reducing equivalents to the anaerobic bacterium, Sporomusa ovata, for the photoelectrochemical production of acetic acid under aerobic conditions (21% O2) with low overpotential (η < 200 mV), high Faradaic efficiency (up to 90%), and long-term stability (up to 200 h). The resulting acetate (∼6 g/L) can be activated to acetyl coenzyme A (acetyl-CoA) by genetically engineered Escherichia coli and used as a building block for a variety of value-added chemicals, such as n-butanol, polyhydroxybutyrate (PHB) polymer, and three different isoprenoid natural products. As such, interfacing biocompatible solid-state nanodevices with living systems provides a starting point for developing a programmable system of chemical synthesis entirely powered by sunlight.

[pdf] [SI]

(257)
“Ballistic phonon transport in holey silicon nanostructures”,

J. Lee, J. Lim, P. Yang, Nano Lett., 15(5), 3273-3279, 2015.

When the size of semiconductors is smaller than the phonon mean free path, phonons can carry heat with no internal scattering. Ballistic phonon transport has received attention for both theoretical and practical aspects because Fourier’s law of heat conduction breaks down and the heat dissipation in nanoscale transistors becomes unpredictable in the ballistic regime. While recent experiments demonstrate room-temperature evidence of ballistic phonon transport in various nanomaterials, the thermal conductivity data for silicon in the length scale of 10–100 nm is still not available due to experimental challenges. Here we show ballistic phonon transport prevails in the cross-plane direction of holey silicon from 35 to 200 nm. The thermal conductivity scales linearly with the length (thickness) even though the lateral dimension (neck) is as narrow as 20 nm. We assess the impact of long-wavelength phonons and predict a transition from ballistic to diffusive regime using scaling models. Our results support strong persistence of long-wavelength phonons in nanostructures and are useful for controlling phonon transport for thermoelectrics and potential phononic applications.

[pdf] [SI]

(256)
“Mesoscopic Constructs of Ordered and Oriented Metal-Organic Frameworks on Plasmonic Silver Nanocrystals”,

Y. Zhao*, N. Kornienko*, Z. Liu, C. Zhu, S. Asahina, T. Kuo, W. Bao, C. Xie, A. Hexemer, O. Terasaki, P. Yang, O. M. Yaghi, JACS, 137(6), 2199-2202, 2015.

We enclose octahedral silver nanocrystals (Ag NCs) in metal−organic frameworks (MOFs) to make mesoscopic constructs Oh-nano-Ag⊂MOF in which the interface between the Ag and the MOF is pristine and the MOF is ordered (crystalline) and oriented on the Ag NCs. This is achieved by atomic layer deposition of aluminum oxide on Ag NCs and addition of a tetra-topic porphyrin based linker, 4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)-tetrabenzoic acid (H4TCPP), to react with alumina and make MOF [Al2(OH)2TCPP] enclosures around Ag NCs. Alumina thickness is precisely controlled from 0.1 to 3 nm, thus allowing control of the MOF thickness from 10 to 50 nm. Electron microscopy and grazing angle X-ray diffraction confirm the order and orientation of the MOF by virtue of the porphyrin units being perpendicular to the planes of the Ag. We use surface-enhanced Raman spectroscopy to directly track the metalation process on the porphyrin and map the distribution of the metalated and unmetalated linkers on a single-nanoparticle level.

[pdf]

(255)
"Artificial Photosynthesis for Sustainable Fuel and Chemical Production",

D. Kim, K. K. Sakimoto, D. Hong, P. Yang, Angew Chem. Int. Ed. ,54, 2-10, 2015.

The apparent incongruity between the increasing consumption of fuels and chemicals and the finite amount of resources has led us to seek means to maintain the sustainability of our society. Artificial photosynthesis, which utilizes sunlight to create high-value chemicals from abundant resources, is considered as the most promising and viable method. This Minireview describes the progress and challenges in the field of artificial photosynthesis in terms of its key components: developments in photoelectrochemical water splitting and recent progress in electrochemical CO2 reduction. Advances in catalysis, concerning the use of renewable hydrogen as a feedstock for major chemical production, are outlined to shed light on the ultimate role of artificial photosynthesis in achieving sustainable chemistry.

[pdf]

2014

(254)
“Introductory Lecture: Systems Materials Engineering Approach for Solar-to-Chemical Conversion”,

C. Liu, P. Yang, Faraday Discussions, 176, 9-16, 2014.

Solar-to-chemical (STC) production using a fully integrated system is an attractive goal, but to-date there has yet to be a system that can demonstrate the required efficiency or durability, or could be manufactured at a reasonable cost. One can learn a great deal from the natural photosynthesis where the conversion of carbon dioxide and water to carbohydrates is routinely carried out at a highly coordinated system level. There are several key features worth mentioning in these systems: spatial and directional arrangement of the light-harvesting components; charge separation and transport; as well as the desired chemical conversion at catalytic sites in compartmentalized spaces. In order to design an efficient artificial photosynthetic materials system, at the level of the individual components better catalysts need to be developed, new light-absorbing semiconductor materials will need to be discovered, architectures will need to be designed for effective capture and conversion of sunlight, and more importantly, processes need to be developed for the efficient coupling and integration of the components into a complete artificial photosynthetic system.

[pdf]

(253)
"MoS2-Wrapped Silicon Nanowires for Photoelectrochemical Water Reduction",

L. Zhang*, C. Liu*, A. B. Wong, J. Resasco, P. Yang, Nano Research DOI:10.1007/s12274-014-0673-y, 2015.

Integration of molybdenum disulfide (MoS2) onto high surface area photocathodes is highly desired to minimize the overpotential for the solar-powered hydrogen evolution reaction (HER). Semiconductor nanowires (NWs) are beneficial for use in photoelectrochemistry because of their large electrochemically available surface area and inherent ability to decouple light absorption and the transport of minority carriers. Here, silicon (Si) NW arrays were employed as a model photocathode system for MoS2 wrapping, and their solar-driven HER activity was evaluated. The photocathode is made up of a well-defined MoS2/TiO2/Si coaxial NW heterostructure, which yielded photocurrent density up to 15 mA/cm2 (at 0 V vs. the reversible hydrogen electrode (RHE)) with good stability under the operating conditions employed. This work reveals that earth-abundant electrocatalysts coupled with high surface area NW electrodes can provide performance comparable to noble metal catalysts for photocathodic hydrogen evolution.

[pdf] [SI]

(252)
"Phase-selective Cation-exchange Chemistry in Sulfide Nanowire System",

D. Zhang, A. Wong, Y. Yu, S. Brittman, J. Sun, B. Brandon, P. Alivisatos, P. Yang, JACS,136(50), 17430–17433 , 2014.

As a cation-deficient, p-type semiconductor, copper sulfide (Cu2–xS) shows promise for applications such as photovoltaics, memristors, and plasmonics. However, these applications demand precise tuning of the crystal phase as well as the stoichiometry of Cu2–xS, an ongoing challenge in the synthesis of Cu2–xS materials for a specific application. Here, a detailed transformation diagram of cation-exchange (CE) chemistry from cadmium sulfide (CdS) into Cu2–xS nanowires (NWs) is reported. By varying the reaction time and the reactants’ concentration ratio, the progression of the CE process was captured, and tunable crystal phases of the Cu2–xS were achieved. It is proposed that the evolution of Cu2–xS phases in a NW system is dependent on both kinetic and thermodynamic factors. The reported data demonstrate that CE can be used to precisely control the structure, composition, and crystal phases of NWs, and such control may be generalized to other material systems for a variety of practical applications.

[pdf] [SI]

(251)
"All Inorganic Semiconductor Nanowire Mesh for Direct Solar Water Splitting",

B.  Liu,  C. H. Wu, J. W.  Miao, P.  Yang, ACS Nano. 8,11739, 2014.

The generation of chemical fuels via direct solar-to-fuel conversion from a fully integrated artificial photosynthetic system is an attractive approach for clean and sustainable energy, but so far there has yet to be a system that would have the acceptable efficiency, durability and can be manufactured at a reasonable cost. Here, we show that a semiconductor mesh made from all inorganic nanowires can achieve unassisted solar-driven, overall water-splitting without using any electron mediators. Free-standing nanowire mesh networks could be made in large scales using solution synthesis and vacuum filtration, making this approach attractive for low cost implementation.

[pdf] [SI]

(250)
"Three-Dimensional Spirals of Atomic Layered MoS2",

L. Zhang, K. Liu, A. B. Wong, J. Kim, X. Hong, C.  Liu, T.  Cao, S. G. Louie, F. Wang, P.  Yang, Nano Lett., 2014, 14 (11), pp 6418–6423.

Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena  distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks gradually to the summit when spiraling up. All the layers in the spiral assume an AA lattice stacking, which is in contrast to the centrosymmetric AB stacking in natural MoS2 crystals. We show that the noncentrosymmetric MoS2 spiral leads to a strong bulk second-order optical nonlinearity. In addition, we found that the growth of spirals involves a dislocation mechanism, which can be generally applicable to other 2D TMD materials.

[pdf] [SI]

(249)
"Evolution of Interlayer Coupling in Twisted Molybdenum Disulfide Bilayers",

K. Liu*, L.  Zhang*, T.  Cao, C*.  Jin, D.  Qiu, Q. Zhou, A.  Zettl, P.  Yang, S. Louie, F. Wang, Nature Comm.,  5, 4966, 2014.

Van der Waals coupling is emerging as a powerful method to engineer physical properties of atomically thin two-dimensional materials. In coupled graphene–graphene and graphene–boron nitride layers, interesting physical phenomena ranging from Fermi velocity renormalization to Hofstadter’s butterfly pattern have been demonstrated. Atomically thin transition metal dichalcogenides, another family of two-dimensional-layered semiconductors, can show distinct coupling phenomena. Here we demonstrate the evolution of interlayer coupling with twist angles in as-grown molybdenum disulfide bilayers. We find that the indirect bandgap size varies appreciably with the stacking configuration: it shows the largest redshift for AA- and AB-stacked bilayers, and a significantly smaller but constant redshift for all other twist angles. Our observations, together with ab initio calculations, reveal that this evolution of interlayer coupling originates from the repulsive steric effects that leads to different interlayer separations between the two molybdenum disulfide layers in different stacking configurations.

[pdf]

(248)
"Salt-Induced Self-Assembly of Bacteria on Nanowire Arrays",

K. K. Sakimoto, C.  Liu, J. Lim, P. Yang, Nano. Lett, 14(9), 5471, 2014.

Studying bacteria−nanostructure interactions is crucial to gaining controllable interfacing of biotic and abiotic components in advanced biotechnologies. For bioelectrochemical systems, tunable cell−electrode architectures offer a path
toward improving performance and discovering emergent properties. As such, Sporomusa ovata cells cultured on vertical silicon nanowire arrays formed filamentous cells and aligned parallel to the nanowires when grown in increasing ionic concentrations. Here, we propose a model describing the kinetic and the thermodynamic driving forces of bacteria−nanowire interactions.

[pdf] [SI]

(247)
"Uniform Doping of Metal Oxide Nanowires Using Solid State Diffusion",

J. Resasco, N. P. Dasgupta, J. R. Rosell, J. Guo, P. Yang, JACS, 136, 10521, 2014

 

The synthesis of one-dimensional nanostructures with specific properties is often hindered by difficulty in tuning the material composition without sacrificing morphology and material quality. Here, we present a simple solid state diffusion method utilizing atomic layer deposition to controllably alter the composition of metal oxide nanowires. This compositional control allows for modification of the optical, electronic, and electrochemical properties of the semiconductor nanowires. Using this method and a novel process for manganese oxide atomic layer deposition, we produced manganese-doped rutile TiO2 nanowires and investigated their structural and  photoelectrochemical properties. A homogeneous incorporation of the Mn dopant into the rutile lattice was observed, and the local chemical environment of the Mn was determined using X-ray absorption spectroscopy. The doping process resulted in a tunable enhancement in the electrocatalytic activity for water oxidation, demonstrating that this simple and general method can be used to control the properties of one-dimensional nanostructures for use in a variety of applications including solar-to-fuel
generation.

[pdf] [SI]

(246)
"Synergistic Geometric and Electronic Effects for Electrochemical Reduction of Carbon Dioxide Using Gold–copper Bimetallic Nanoparticles",

D. Kim, J. Resasco, Y.  Yu, A.M. Asiri, P. Yang, Nature Comm., 5, 4948, 2014

Highly efficient and selective electrochemical reduction of carbon dioxide represents one of the biggest scientific challenges in artificial photosynthesis, where carbon dioxide and water are converted into chemical fuels from solar energy. However, our fundamental understanding of the reaction is still limited and we do not have the capability to design an outstanding catalyst with great activity and selectivity a priori. Here we assemble uniform gold–copper bimetallic nanoparticles with different compositions into ordered monolayers, which serve as a well-defined platform to understand their fundamental catalytic activity in carbon dioxide reduction. We find that two important factors related to intermediate binding, the electronic effect and the geometric effect, dictate the activity of gold–copper bimetallic nanoparticles. These nanoparticle monolayers also show great mass activities, outperforming conventional carbon dioxide reduction catalysts. The insights gained through this study may serve as a foundation for designing better carbon dioxide electrochemical reduction catalysts.

[pdf]

(245)
“Epitaxially Aligned Cuprous Oxide Nanowires for All-Oxide, Single-Wire Solar Cells”,

S. Brittman*, Y. Yoo*, N. P. Dasgupta, S. Kim, B. Kim, P. Yang, Nano. Lett., 14, 4665, 2014.

As a p-type semiconducting oxide that can absorb visible light, cuprous oxide (Cu2O) is an attractive material for solar energy conversion. This work introduces a high-temperature, vapor-phase synthesis that produces faceted Cu2O nanowires that grow epitaxially along the surface of a lattice-matched, single-crystal MgO substrate. Individual wires were then fabricated into singlewire, all-oxide diodes and solar cells using low-temperature atomic
layer deposition (ALD) of TiO2 and ZnO films to form the heterojunction. The performance of devices made from pristine Cu2O wires and chlorine-exposed Cu2O wires was investigated under one-sun and laser illumination. These faceted wires allow the fabrication of well-controlled heterojunctions that can be used to investigate the interfacial properties of all-oxide solar cells.

[pdf] [SI]

(244)
“Simultaneously Efficient Light Absorption and Charge Separation in WO3/BiVO4 Core/Shell Nanowire Photoanode for Photoelectrochemical Water Oxidation”,

P. M. Rao, L. Cai, C. Liu, I. Cho, C. Lee, J. M. Weisse, P. Yang, X. Zheng, Nano. Lett. 14, 1099, 2014.

We report a scalably synthesized WO3/BiVO4 core/shell nanowire photoanode in which BiVO4 is the primary light-absorber and WO3 acts as an electron conductor. These core/shell nanowires achieve the highest product of light absorption and charge separation efficiencies among BiVO4-based photoanodes to date and, even without an added catalyst, produce a photocurrent of 3.1 mA/cm2 under simulated sunlight and an incident photon-to-current conversion efficiency of 60% at 300–450 nm, both at a potential of 1.23 V versus RHE.

[pdf] [SI]

(243)
“Semiconductor Nanowires: Synthesis, Characterization, and Applications”,

N. P. Dasgupta, J. Sun, C. Liu, S. Brittman, S. C. Andrews, J. Lim, H. Gao, R. Yan, P. Yang, Adv. Mater. 26, 2137, 2014.

Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of “bottom-up” growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

[pdf]

(242)
“Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces”,

C. Chen*, Y. Kang*, Z. Huo, Z. Zhu, W. Huang, H. Xin, J. D. Snyder, D. Li, J. A. Herron, M. Mavrikakis, M. Chi, K. L. More, Y. Li, N. M. Markovic, G. A. Somorjai, P. Yang, V. R. Stamenkovic, Science, 343,1319, 2014.

Control of structure at the atomic level can precisely and effectively tune catalytic properties of materials, enabling enhancement in both activity and durability. We synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of platinum-nickel (Pt-Ni) bimetallic nanocrystals. The starting material, crystalline PtNi3 polyhedra, transforms in solution by interior erosion into Pt3Ni nanoframes with surfaces that offer three-dimensional molecular accessibility. The edges of the Pt-rich PtNi3 polyhedra are maintained in the final Pt3Ni nanoframes. Both the interior and exterior catalytic surfaces of this open-framework structure are composed of the nanosegregated Pt-skin structure, which exhibits enhanced oxygen reduction reaction (ORR) activity. The Pt3Ni nanoframe catalysts achieved a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity, respectively, for this reaction (relative to state-of-the-art platinum-carbon catalysts) during prolonged exposure to reaction conditions.

[pdf] [SI]

(241)
“Semiconductor nanowires for artificial photosynthesis”,

C. Liu, N. Dasgupta, P. Yang, Chem. Mater., 1, 26, 2014.

In this Perspective, we discuss current challenges in artificial photosynthesis research, with a focus on the benefits of a nanowire morphology. Matching the flux between electrocatalysts and light-absorbers, and between individual semiconducting light-absorbers, are two major issues to design economically viable devices for artificial photosynthesis. With the knowledge that natural photosynthesis is an integrated nanosystem, individual building blocks of biomimetic artificial photosynthesis are discussed. Possible research directions are presented under an integrated device design scheme, with examples of our current progress in these areas. Coupling all of the components together, including electrocatalysts, light-absorbers, and charge transport units, is crucial due to both fundamental and practical considerations. Given the advantages of one-dimensional nanostructures, it is evident that semiconductor nanowires can function as essential building blocks and help to solve many of the issues in artificial photosynthesis.

[pdf]

(240)
"Alumina Coated Ag Nanocrystal Monolayer as Surface-Enhanced Raman Spectroscopy Platforms for Direct Spectroscopic Detection of Water Splitting Reaction Intermediates",

X. Ling*, R. Yan*, S. Lo, D. T. Hoang, C. Liu, M. A. Fardy, S. B. Khan, A. M. Asiri, S. M. Bawaked, P. Yang, Nano Research, 7, 132, 2014.

A novel Ag-alumina hybrid surface-enhanced Raman spectroscopy (SERS) platform has been designed for the spectroscopic detection of surface reactions in the steady state. Single crystalline and faceted silver (Ag) nanoparticles with strong light scattering were prepared in large quantity, which enables their reproducible self-assembly into large scale monolayers of Raman sensor arrays by the Langmuir-Blodgett technique. The close packed sensor film contains high density of sub-nm gaps between sharp edges of Ag nanoparticles, which created large local electromagnetic fields that serve as “hot spots” for SERS enhancement. The SERS substrate was then coated with a thin layer of alumina by atomic layer deposition to prevent charge transfer between Ag and the reaction system. The photocatalytic water splitting reaction on a monolayer of anatase TiO2 nanoplates decorated with Pt co-catalyst nanoparticles was employed as a model reaction system. Reaction intermediates of water photo-oxidation were observed at the TiO2/solution interface under UV irradiation. The surface-enhanced Raman vibrations corresponding to peroxo, hydroperoxo and hydroxo surface intermediate species were observed on the TiO2 surface, suggesting that the photo-oxidation of water on these anatase TiO2 nanosheets may be initiated by a nucleophilic attack mechanism.

[pdf]

2013

(239)
“Electrodeposited Cobalt Sulfide Catalyst for Electrochemical and Photoelectrochemical Hydrogen Generation from Water”,

Y.  Sun*, C. Liu*, D.  C. Grauer,  J.  R. Long, P.  Yang,   C. J. Chang, JACS. 135, 17699, 2013.

A cobalt-sulfide (Co–S) film prepared via electrochemical deposition on conductive substrates is shown to behave as an efficient and robust catalyst for electrochemical and photoelectrochemical hydrogen generation from neutral pH water. Electrochemical experiments demonstrate that the film exhibits a low catalytic onset overpotential (η) of 43 mV, a Tafel slope of 93 mV/dec, and near 100% Faradaic efficiency in pH 7 phosphate buffer. Catalytic current densities can approach 50 mA/cm2 and activity is maintained for at least 40 h. The catalyst can also be electrochemically coated on silicon, rendering a water-compatible photoelectrochemical system for hydrogen production under simulated 1 sun illumination. The facile preparation of this Co–S film, along with its low overpotential, high activity, and long-term aqueous stability, offer promising features for potential use in solar energy applications.

[pdf] [SI]

(238)
“Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel N-Heterocyclic Carbene-Isoquinoline Complex”,

V. Thoi*, N. Kornienko*, C. Margarit, P. Yang, C. Chang, JACS, 135, 14413, 2013.

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene–amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni(Prbimiq1)]2+ (1c, wherePrbimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (Ecat = −1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s–1, respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion.

[pdf] [SI]

(237)
"Femtosecond M2,3-edge spectroscopy of transition metal oxides: photoinduced oxidation state change in α-Fe2O3",

J. Vura-Weis, C. Jiang, C. Liu, H. Gao, J. M. Lucas, F. M.F. de Groot, P. Yang, A. P. Alivisatos, S. R. Leone, JPC Lett. 4, 3667, 2013.

Oxidation-state-specific dynamics at the Fe M2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe2O3 thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at 57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe3+ and photoinduced Fe2+ state confirm this assignment and exclude the alternative d–d excitation. The Fe2+ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.

[pdf] [SI]

(236)
“Atomic Layer Deposition of Platinum Catalysts on Nanowire Surfaces for Photoelectrochemical Water Reduction”,

N. P. Dasgupta*, C. Liu*, S. Andrews, F. B. Prinz, P.  Yang, JACS, 135, 12932, 2013.

The photocathodic hydrogen evolution reaction (HER) from p-type Si nanowire (NW) arrays was evaluated using platinum deposited by atomic layer deposition (ALD) as a HER cocatalyst. ALD of Pt on the NW surface led to a highly conformal coating of nanoparticles with sizes ranging from 0.5 to 3 nm, allowing for precise control of the Pt loading in deep submonolayer quantities. The catalytic performance was measured using as little as 1 cycle of Pt ALD, which corresponded to a surface mass loading of 10 ng/cm2. The quantitative exploration of the lower limits of Pt cocatalyst loading reported here, and its application to high-surface-area NW photoelectrodes, establish a general approach for minimizing the cost of precious-metal cocatalysts for efficient and affordable solar-to-fuel applications.

[pdf] [SI]

(235)
"Ta3N5 nanowire bundles as visible-light-responsive photoanodes",

C. Wu, C. Hahn, S.  B. Khan, A.  M. Asiri, S.  M. Bawaked, P.  Yang, Chem. Asia J.  8, 2354, 2013.

Ta3N5 nanowire bundles (NWBs) were synthesized using a molten salt reaction followed by nitridation. The bundles consisted of single crystalline Ta3N5 nanowires which aligned along the same crystallographic growth direction. O2 evolution and photoelectrochemical measurements demonstrate the potential of Ta3N5 NWBs as a visible-light-responsive photoanode.

[pdf] [SI]

(234)
“Zigzag Inversion Domain Boundaries in Indium Zinc Oxide-Based Nanowires: Structure and Formation”,

A. P. Goldstein, S.  C. Andrews, R. F. Berger, V. Radmilovic, J. B. Neaton, P.  Yang, ACS Nano  In Press, 2013.

Existing models for the crystal structure of indium zinc oxide (IZO) and indium iron zinc oxide (IFZO) conflict with electron microscopy data. We propose a model based on imaging and spectroscopy of IZO and IFZO nanowires and verify it using density functional theory. The model features a {12̅1} “zigzag” layer, which is an inversion domain boundary containing 5-coordinate indium and/or iron atoms. Higher  values are observed for greater proportion of iron. We suggest a mechanism of formation in which the basal inclusion and the zigzag diffuse inward together from the surface of the nanowire.

(233)
"Large Scale Synthesis of Transition-metal Doped TiO2 Nanowires with Low Overpotential",

B Liu*, H. M. Chen*, C. Liu, S. C. Andrews, C. Hahn, P. Yang, JACS. 135, 9995, 2013.

Practical implementation of one-dimensional semiconductors into devices capable of exploiting their novel properties is often hindered by low product yields, poor material quality, high production cost, or overall lack of synthetic control. Here, we show that a molten-salt flux scheme can be used to synthesize large quantities of high-quality, single-crystalline TiO2 nanowires with controllable dimensions. Furthermore, in situ dopant incorporation of various transition metals allows for the tuning of optical, electrical, and catalytic properties. With this combination of control, robustness, and scalability, the molten-salt flux scheme can provide high-quality TiO2 nanowires to satisfy a broad range of application needs from photovoltaics to photocatalysis.

[pdf] [SI]

(232)
“A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting”,

C. Liu*, J. Tang*, H. Chen, B. Liu, P. Yang, Nano Lett, 13, 2989, 2013.

Artificial photosynthesis, the biomimetic approach to converting sunlight’s energy directly into chemical fuels, aims to imitate nature by using an integrated system of nanostructures, each of which plays a specific role in the sunlight-to-fuel conversion process. Here we describe a fully integrated system of nanoscale photoelectrodes assembled from inorganic nanowires for direct solar water splitting. Similar to the photosynthetic system in a chloroplast, the artificial photosynthetic system comprises two semiconductor light absorbers with large surface area, an interfacial layer for charge transport, and spatially separated cocatalysts to facilitate the water reduction and oxidation. Under simulated sunlight, a 0.12% solar-to-fuel conversion efficiency is achieved, which is comparable to that of natural photosynthesis. The result demonstrates the possibility of integrating material components into a functional system that mimics the nanoscopic integration in chloroplasts. It also provides a conceptual blueprint of modular design that allows incorporation of newly discovered components for improved performance.

[pdf] [SI]

(231)
"Bacterial Recognition of Silicon Nanowire Arrays",

H. Jeong, I. Kim, P. Karam, H. Choi, P. Yang, Nano Lett, 13, 2864, 2013.

Understanding how living cells interact with nanostructures is integral to a better understanding of the fundamental principles of biology and the development of next-generation biomedical/bioenergy devices. Recent studies have demonstrated that mammalian cells can recognize nanoscale topographies and respond to these structures. From this perspective, there is a growing recognition that nanostructures, along with their specific physicochemical properties, can also be used to regulate the responses and motions of bacterial cells. Here, by utilizing a well-defined silicon nanowire array platform and single-cell imaging, we present direct evidence that Shewanella oneidensis MR-1 can recognize nanoscale structures and that their swimming patterns and initial attachment locations are strongly influenced by the presence of nanowires on a surface. Analyses of bacterial trajectories revealed that MR-1 cells exhibited a confined diffusion mode in the presence of nanowires and showed preferential attachment to the nanowires, whereas a superdiffusion mode was observed in the absence of nanowires. These results demonstrate that nanoscale topography can affect bacterial movement and attachment and play an important role during the early stages of biofilm formation.

[pdf] [SI]

(230)
“Semiconductor Nanowires for Photovoltaic and Photoelectrochemical Energy Conversion”,

N. P. Dasgupta, P.  Yang, Frontiers of Physics, 9, 289, 2014.

Semiconductor nanowires (NW) possess several beneficial properties for efficient conversion of solar energy into electricity and chemical energy. Due to their efficient absorption of light, short distances for minority carriers to travel, high surface-to-volume ratios, and the availability of scalable synthesis methods, they provide a pathway to address the low cost-to-power requirements for wide-scale adaptation of solar energy conversion technologies. Here we highlight recent progress in our group towards implementation of NW components as photovoltaic and photoelectrochemical energy conversion devices. An emphasis is placed on the unique properties of these one-dimensional (1D) structures, which enable the use of abundant, low-cost materials and improved energy conversion efficiency compared to bulk devices.

[pdf]

(229)
“Energy and environment policy case for a global project on artificial photosynthesis”,

T. A. Faunce et al. Energy & Environ. Sci., 6, 695, 2013.

A policy case is made for a global project on artificial photosynthesis including its scientific justification, potential governance structure and funding mechanisms.

[pdf]

(228)
"Effect of Thermal Annealing in Ammonia on the Properties of InGaN Nanowires with Different Indium Concentrations",

C. Hahn, A. A. Cordones, S. C. Andrews, H. Gao, A. Fu, S. R. Leone, P. Yang, J. Phys. Chem. C. , 117, 3627, 2013

The utility of an annealing procedure in ammonia ambient is investigated for improving the optical characteristics of InxGa1–xN nanowires (0.07 ≤ x ≤ 0.42) grown on c-Al2O3 using a halide chemical vapor deposition method. Morphological studies using scanning electron microscopy confirm that the nanowire morphology is retained after annealing in ammonia at temperatures up to 800 °C. However, significant indium etching and composition inhomogeneities are observed for higher indium composition nanowires (x = 0.28, 0.42), as measured by energy-dispersive X-ray spectroscopy and Z-contrast scanning transmission electron microscopy. Structural analyses, using X-ray diffraction and high-resolution transmission electron microscopy, indicate that this is a result of the greater thermal instability of higher indium composition nanowires. The effect of these structural changes on the optical quality of InGaN nanowires is examined using steady-state and time-resolved photoluminescence measurements. Annealing in ammonia enhances the integrated photoluminescence intensity of InxGa1–xN nanowires by up to a factor of 4.11 ± 0.03 (for x = 0.42) by increasing the rate of radiative recombination. Fitting of photoluminescence decay curves to a Kohlrausch stretched exponential indicates that this increase is directly related to a larger distribution of recombination rates from composition inhomogeneities caused by annealing. The results demonstrate the role of thermal instability on the improved optical properties of InGaN nanowires annealed in ammonia.

[pdf] [SI]

(227)
“Oriented Assembly of Polyhedral Plasmonic Nanoparticle Clusters”,

J. Henzie, S. Andrews, X. Ling, Z. Li, P. Yang, Proc. Natl. Acad. Sci. USA, Early Edition, 2013.

Shaped colloids can be used as nanoscale building blocks for the construction of composite, functional materials that are completely assembled from the bottom up. Assemblies of noble metal nanostructures have unique optical properties that depend on key structural features requiring precise control of both position and connectivity spanning nanometer to micrometer length scales. Identifying and optimizing structures that strongly couple to light is important for understanding the behavior of surface plasmons in small nanoparticle clusters, and can result in highly sensitive chemical and biochemical sensors using surface-enhanced Raman spectroscopy (SERS). We use experiment and simulation to examine the local surface plasmon resonances of different arrangements of Ag polyhedral clusters. High-resolution transmission electron microscopy shows that monodisperse, atomically smooth Ag polyhedra can self-assemble into uniform interparticle gaps that result in reproducible SERS enhancement factors from assembly to assembly. We introduce a large-scale, gravity-driven assembly method that can generate arbitrary nanoparticle clusters based on the size and shape of a patterned template. These templates enable the systematic examination of different cluster arrangements and provide a means of constructing scalable and reliable SERS sensors.

[pdf] [SI]

(226)
"Cleaved-Coupled Nanowire Lasers",

H. Gao*, A. Fu*, S. C. Andrews, P. Yang, Proc. Natl. Acad. Sci. USA, 110, 865, 2013.

The miniaturization of optoelectronic devices is essential for the continued success of photonic technologies. Nanowires have been identified as potential building blocks that mimic conventional photonic components such as interconnects, waveguides, and optical cavities at the nanoscale. Semiconductor nanowires with high optical gain offer promising solutions for lasers with small footprints and low power consumption. Although much effort has been directed toward controlling their size, shape, and composition, most nanowire lasers currently suffer from emitting at multiple frequencies simultaneously, arising from the longitudinal modes native to simple Fabry–Pérot cavities. Cleaved-coupled cavities, two Fabry–Pérot cavities that are axially coupled through an air gap, are a promising architecture to produce single-frequency emission. The miniaturization of this concept, however, imposes a restriction on the dimensions of the intercavity gaps because severe optical losses are incurred when the cross-sectional dimensions of cavities become comparable to the lasing wavelength. Here we theoretically investigate and experimentally demonstrate spectral manipulation of lasing modes by creating cleaved-coupled cavities in gallium nitride (GaN) nanowires. Lasing operation at a single UV wavelength at room temperature was achieved using nanoscale gaps to create the smallest cleaved-coupled cavities to date. Besides the reduced number of lasing modes, the cleaved-coupled nanowires also operate with a lower threshold gain than that of the individual component nanowires. Good agreement was found between the measured lasing spectra and the predicted spectral modes obtained by simulating optical coupling properties. This agreement between theory and experiment presents design principles to rationally control the lasing modes in cleaved-coupled nanowire lasers.

[pdf] [SI]

(225)
"Mesoporous Co3O4 as an electrocatalyst for water oxidation",

H. Tüysüz, Y. Hwang, S. Khan, A. Asiri, P. Yang, Nano Research, 6, 47, 2013.

Mesoporous Co3O4 has been prepared using porous silica as a hard template via a nanocasting route and its electrocatalytic properties were investigated as an oxygen evolution catalyst for the electrolysis of water. The ordered mesostructured Co3O4 shows dramatically increased catalytic activity compared to that of bulk Co3O4. Enhanced catalytic activity was achieved with high porosity and surface area, and the water oxidation overpotential (η) of the ordered mesoporous Co3O4decreases significantly as the surface area increases. The mesoporous Co3O4 also shows excellent structural stability in alkaline media. After 100 min under 0.8 V (versus Ag/AgCl) applied bias, the sample maintains the ordered mesoporous structure with little deactivation of the catalytic properties.

[pdf]

(224)
"Photocatalytic generation of hydrogen from water using a cobalt pentapyridine complex in combination with molecular and semiconductor nanowire photosensitizers",

Y. Sun, J. Sun, J.R. Long, P. Yang, C. J. Chang, Chem. Sci. 4, 118, 2013.

Recently, a family of cobalt pentapyridine complexes of the type [(R-PY5Me2)Co(H2O)])(CF3SO3)2, (R = CF3, H, or NMe2; PY5Me2 = 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine) were shown to catalyze the electrochemical generation of hydrogen from neutral aqueous solutions using a mercury electrode. We now report that the CF3 derivative of this series, [(CF3PY5Me2)Co(H2O)](CF3SO3)2(1), can also operate in neutral water as an electrocatalyst for hydrogen generation under soluble, diffusion-limited conditions on a glassy carbon electrode, as well as a photocatalyst for hydrogen production using either molecular or semiconductor nanowire photosensitizers. Owing to its relatively low overpotential compared to other members of the PY5 family, complex1 exhibits multiple redox features on glassy carbon, including a one-proton, one-electron coupled oxidative wave. Further, rotating disk electrode voltammetry measurements reveal the efficacy of 1 as a competent hydrogen evolution catalyst under soluble, diffusion-limited conditions. In addition, we establish that 1 can also generate hydrogen from neutral water under photocatalytic conditions with visible light irradiation (λirr ≥ 455 nm), using [Ru(bpy)3]2+ as a molecular inorganic chromophore and ascorbic acid as a sacrificial donor. Dynamic light scattering measurements show no evidence for nanoparticle formation for the duration of the photolytic hydrogen evolution experiments. Finally, we demonstrate that 1 is also able to enhance the hydrogen photolysis yield of GaP nanowires in water, showing that this catalyst is compatible with solid-state photosensitizers. Taken together, these data establish that the well-defined cobalt pentapyridine complex [(CF3PY5Me2)Co(H2O)]2+ is a versatile catalyst for hydrogen production from pure aqueous solutions using either solar or electrical input, providing a starting point for integrating molecular systems into sustainable energy generation devices.

[pdf] [SI]

2012

(223)
“Synthesis and Photocatalytic Properties of Single Crystalline (Ga1-xZnx)(N1-xOx) Nanotubes”,

C. Hahn*, M. A. Fardy*, C. Nguyen, M. Natera-Comte, S. C. Andrews, P. Yang, Israel J. Chem. 52, 1111, 2012.

Recently, (Ga1-xZnx)(N1-xOx) has gained widespread attention as a comparatively high efficiency photocatalyst for visible-light-driven overall water splitting. Despite significant gains in efficiency over the past several years, a majority of the photogenerated carriers recombine within bulk powders. To improve the photocatalytic activity, we used an epitaxial casting method to synthesize single-crystalline, high surface area (Ga1-xZnx)(N1-xOx) nanotubes with ZnO compositions up to x=0.10. Individual nanotubes showed improved homogeneity over powder samples due to a well defined epitaxial interface for ZnO diffusion into GaN. Absorption measurements showed that the ZnO incorporation shifts the absorption into the visible region with a tail out to 500 nm. Gas chromatography (GC) was used to compare the solar water splitting activity of (Ga1-xZnx)(N1-xOx) nanotubes (x=0.05–0.10) with similar composition powders. Cocatalyst decorated samples were dispersed in aqueous solutions of CH3OH and AgO2CCH3 to monitor the H+ reduction and H2O oxidation half reactions, respectively. The nanotubes were found to have approximately 1.5–2 times higher photocatalytic activity than similar composition powders for the rate limiting H+ reduction half reaction. These results demonstrate that improvements in homogeneity and surface area using the nanotube geometry can enhance the photocatalytic activity of GaN:ZnO for solar water splitting.

[pdf] [SI]

(222)
“Membrane-Protein Binding Measured with Solution-Phase Plasmonic Nanocube Sensors”,

H. Wu, J. Henzie, W. Lin, C. Rhodes, Z. Li, E. Sartorel, J. Thorner, P. Yang, J. T. Groves, Nature Methods, 9, 1189, 2012.

We describe a solution-phase sensor of lipid-protein binding based on localized surface plasmon resonance (LSPR) of silver nanocubes. When silica-coated nanocubes are mixed in a suspension of lipid vesicles, supported membranes spontaneously assemble on their surfaces. Using a standard laboratory spectrophotometer, we calibrated the LSPR peak shift due to protein binding to the membrane surface and then characterized the lipid-binding specificity of a pleckstrin homology domain protein.

[pdf] [SI]

(221)
“Zn-doped p-type Gallium Phosphide Nanowire Photocathodes from a Surfactant-free Solution Synthesis”,

C. Liu, J. Sun, J. Tang, P. Yang, Nano. Lett,  12, 5407, 2012.

Gallium phosphide (GaP) nanowire photocathodes synthesized using a surfactant-free solution–liquid–solid (SLS) method were investigated for their photoelectrochemical evolution of hydrogen. Zinc as a p-type dopant was introduced into the nanowires during synthesis to optimize the photocathode’s response. Investigation of the electrical properties of Zn-doped GaP nanowires confirmed their p-type conductivity. After optimization of the nanowire diameter and Zn doping concentration, higher absorbed photon-to-current efficiency (APCE) over the spectrum was achieved. The versatility of the SLS synthesis and the capability to control the electrical properties suggest that our approach could be generalized to other III–V and II–VI semiconductors.

[pdf] [SI]

(220)
“Towards Systems Materials Engineering”,

P. Yang, J. Tarascon, Nature Mater., 11, 560, 2012.

System-level planning of theoretical and experimental efforts is increasingly important for the development of modern materials science.

[pdf]

(219)
“Nanowire building blocks: from flux line pinning to artificial photosynthesis (MRS Medal Address)”,

P. Yang, MRS Bulletin, 37, 806, 2012.

Semiconductor nanowires, by definition, typically have nanoscale cross-sectional dimensions, with lengths spanning from hundreds of nanometers to millimeters. These subwavelength structures represent a new class of semiconductor materials for investigating light generation, propagation, detection, amplification, and modulation. After more than a decade of research, nanowires can now be synthesized and assembled with specific compositions, heterojunctions, and architectures. This has led to a host of nanowire photonic and electronic devices, including photodetectors, chemical and gas sensors, waveguides, LEDs, microcavity lasers, and nonlinear optical converters. Nanowires also represent an important class of nanostructure building blocks for photovoltaics as well as direct solar-to-fuel conversion because of their high surface area, tunable bandgap, and efficient charge transport and collection. This article gives a brief history of nanowire research for the past two decades and highlights several recent examples in our lab using semiconductor nanowires and their heterostructures for the purpose of solar energy harvesting and waste heat recovery.

[pdf]

(218)
“Photoelectrochemical Properties of TiO2 Nanowire Arrays: A Study on the Dependence of Length and Atomic Layer Deposition Coating”,

Y. J. Hwang, C. Hahn, B. Liu, P. Yang, ACS Nano, 6, 5060, 2012.

We report that the length and surface properties of TiO2 nanowires can have a dramatic effect on their photoelectrochemical properties. To study the length dependence, rutile TiO2 nanowires (0.28–1.8 μm) were grown on FTO substrates with different reaction times (50–180 min) using a hydrothermal method. Nanowires show an increase in photocurrent with length, and a maximum photocurrent of 0.73 mA/cm2 was measured (1.5 V vs RHE) for 1.8 μm long nanowires under AM 1.5G simulated sunlight illumination. While the incident photon to current conversion efficiency (IPCE) increases linearly with photon absorptance (1–10–α×length) with near band gap illumination (λ = 410 nm), it decreases severely at shorter wavelengths of light for longer nanowires due to poor electron mobility. Atomic layer deposition (ALD) was used to deposit an epitaxial rutile TiO2 shell on nanowire electrodes which enhanced the photocatalytic activity by 1.5 times (1.5 V vs RHE) with 1.8 μm long nanowires, reaching a current density of 1.1 mA/cm2 (61% of the maximum photocurrent for rutile TiO2). Additionally, by fixing the epitaxial rutile shell thickness and studying photoelectrochemical (PEC) properties of different nanowire lengths (0.28–1.8 μm), we found that the enhancement of current increases with length. These results demonstrate that ALD coating improves the charge collection efficiency from TiO2 nanowires due to the passivation of surface states and an increase in surface area. Therefore, we propose that epitaxial coating on materials is a viable approach to improving their energy conversion efficiency.

[pdf] [SI]

(217)
“Quantifying Surface Roughness Effects on Phonon Transport in Silicon Nanowires”,

J. Lim*, K. Hippalgaonkar*, S. Andrews, A. Majumdar, P. Yang, Nano Lett., 12, 2475, 2012.

Although it has been qualitatively demonstrated that surface roughness can reduce the thermal conductivity of crystalline Si nanowires (SiNWs), the underlying reasons remain unknown and warrant quantitative studies and analysis. In this work, vapor–liquid–solid (VLS) grown SiNWs were controllably roughened and then thoroughly characterized with transmission electron microscopy to obtain detailed surface profiles. Once the roughness information (root-mean-square, σ, correlation length, L, and power spectra) was extracted from the surface profile of a specific SiNW, the thermal conductivity of the same SiNW was measured. The thermal conductivity correlated well with the power spectra of surface roughness, which varies as a power law in the 1–100 nm length scale range. These results suggest a new realm of phonon scattering from rough interfaces, which restricts phonon transport below the Casimir limit. Insights gained from this study can help develop a more concrete theoretical understanding of phonon–surface roughness interactions as well as aid the design of next generation thermoelectric devices.

[pdf] [SI]

(216)
“Si/InGaN Core/Shell Hierarchical Nanowire Arrays and their Photoelectrochemical Properties”,

Y. J. Hwang, C. H. Wu, C. Hahn, H. E. Jeong, P. Yang, Nano Lett., 12, 1678, 2012.

Three-dimensional hierarchical nanostructures were synthesized by the halide chemical vapor deposition of InGaN nanowires on Si wire arrays. Single phase InGaN nanowires grew vertically on the sidewalls of Si wires and acted as a high surface area photoanode for solar water splitting. Electrochemical measurements showed that the photocurrent density with hierarchical Si/InGaN nanowire arrays increased by 5 times compared to the photocurrent density with InGaN nanowire arrays grown on planar Si (1.23 V vs RHE). High-resolution transmission electron microscopy showed that InGaN nanowires are stable after 15 h of illumination. These measurements show that Si/InGaN hierarchical nanostructures are a viable high surface area electrode geometry for solar water splitting.

[pdf] [SI]

(215)
“Crystal cuts on the nanoscale”,

P. Yang, Nature, 482, 41, 2012.

A simple method has been developed to control the shape of nanoscale cuprous oxide crystals. Some shapes turn out to be much better than others as catalysts for a light-activated reaction.

[pdf]

(214)
“Self-assembly of uniform polyhedral silver nanocrystals into densely packed supercrystals”,

J. Henzie, M. Grünwald, A. Widmer-Cooper, P. L. Geissler, P. Yang, Nature Mater., 11, 31, 2012.

Understanding how polyhedra pack into extended arrangements is integral to the design and discovery of crystalline materials at all length scales. Much progress has been made in enumerating and characterizing the packing of polyhedral shapes, and the self-assembly of polyhedral nanocrystals into ordered superstructures. However, directing the selfassembly of polyhedral nanocrystals into densest packings requires precise control of particle shape, polydispersity, interactions and driving forces. Here we show with experiment and computer simulation that a range of nanoscale Ag polyhedra can self-assemble into their conjectured densest packings. When passivated with adsorbing polymer, the polyhedra behave as quasi-hard particles and assemble into millimetre-sized three-dimensional supercrystals by sedimentation. We also show, by inducing depletion attraction through excess polymer in solution, that octahedra form an exotic superstructure with complex helical motifs rather than the densest Minkowski lattice. Such large-scale Ag supercrystals may facilitate the design of scalable three-dimensional plasmonic metamaterials for sensing, nanophotonics and photocatalysis.

[pdf] [SI] [Cover Highlight]

(213)
“Nanowire-based Single Cell Endoscopy”,

R. Yan*, J. Park*, Y. Choi, C. Heo, S. Yang, L. P. Lee, P. Yang, Nature Nanotech., 7, 191, 2012.

One-dimensional smart probes based on nanowires and nanotubes that can safely penetrate the plasma membrane and enter biological cells are potentially useful in high-resolution123456 and high-throughput78 gene and drug delivery, biosensing69 and single-cell electrophysiology610. However, using such probes for optical communication across the cellular membrane at the subwavelength level remains limited. Here, we show that a nanowire waveguide attached to the tapered tip of an optical fibre can guide visible light into intracellular compartments of a living mammalian cell, and can also detect optical signals from subcellular regions with high spatial resolution. Furthermore, we show that through light-activated mechanisms the endoscope can deliver payloads into cells with spatial and temporal specificity. Moreover, insertion of the endoscope into cells and illumination of the guided laser did not induce any significant toxicity in the cells.

[pdf] [SI]

(212)
“Plasmon Enhanced-Photocatalytic Activity of Iron Oxide on Gold Nanopillars”,

H. Gao*, C. Liu*, H. E. Jeong, P. Yang, ACS Nano., 6, 234, 2012.

Photocatalytic water splitting represents a promising way to produce renewable hydrogen fuel from solar energy. Ultrathin semiconductor electrodes for water splitting are of particular interest because the optical absorption occurs in the region where photogenerated charge carriers can effectively contribute to the chemical reactions on the surface. It is therefore important to manipulate and concentrate the incident light so that more photons can be absorbed within the thin film. Here we show an enhanced photocurrent in a thin-film iron oxide photoanode coated on arrays of Au nanopillars. The enhancement can be attributed primarily to the increased optical absorption originating from both surface plasmon resonances and photonic-mode light trapping in the nanostructured topography. The resonances can be tuned to a desirable wavelength by varying the thickness of the iron oxide layer. A net enhancement as high as 50% was observed over the solar spectrum.

[pdf] [SI]

2011

(211)
“Surfactant-Free, Large-Scale, Solution-Liquid-Solid (SLS) Growth of Gallium Phosphide Nanowires and Their Use for Visible-Light-Driven Hydrogen Production from Water Reduction”,

J. Sun, C. Liu, P. Yang, J. Am. Chem. Soc., 133, 19306, 2011.

Colloidal GaP nanowires (NWs) were synthesized on a large scale by a surfactant-free, self-seeded solution–liquid–solid (SLS) method using triethylgallium and tris(trimethylsilyl)phosphine as precursors and a noncoordinating squalane solvent. Ga nanoscale droplets were generated in situ by thermal decomposition of the Ga precursor and subsequently promoted the NW growth. The GaP NWs were not intentionally doped and showed a positive open-circuit photovoltage based on photoelectrochemical measurements. Purified GaP NWs were used for visible-light-driven water splitting. Upon photodeposition of Pt nanoparticles on the wire surfaces, significantly enhanced hydrogen production was observed. The results indicate that colloidal surfactant-free GaP NWs combined with potent surface electrocatalysts could serve as promising photocathodes for artificial photosynthesis.

[pdf] [SI]

(210)
“Absorption of Light in a Single-nanowire Silicon Solar Cell Decorated with an Octahedral Silver Nanocrystal”,

S. Brittman*, H. Gao*, E. C. Garnett, P. Yang, Nano Lett., 11, 5189, 2011.

In recent photovoltaic research, nanomaterials have offered two new approaches for trapping light within solar cells to increase their absorption: nanostructuring the absorbing semiconductor and using metallic nanostructures to couple light into the absorbing layer. This work combines these two approaches by decorating a single-nanowire silicon solar cell with an octahedral silver nanocrystal. Wavelength-dependent photocurrent measurements and finite-difference time domain simulations show that increases in photocurrent arise at wavelengths corresponding to the nanocrystal’s surface plasmon resonances, while decreases occur at wavelengths corresponding to optical resonances of the nanowire. Scanning photocurrent mapping with submicrometer spatial resolution experimentally confirms that changes in the device’s photocurrent come from the silver nanocrystal. These results demonstrate that understanding the interactions between nanoscale absorbers and plasmonic nanostructures is essential to optimizing the efficiency of nanostructured solar cells.

[pdf] [SI]

(209)
“Multinozzle Emitter Arrays for Nanoelectrospray Mass Spectrometry”,

P. Mao, H.-T. Wang, P. Yang, D. Wang, Anal. Chem., 83, 6082, 2011.

Mass spectrometry (MS) is the enabling technology for proteomics and metabolomics. However, dramatic improvements in both sensitivity and throughput are still required to achieve routine MS-based single cell proteomics and metabolomics. Here, we report the silicon-based monolithic multinozzle emitter array (MEA) and demonstrate its proof-of-principle applications in high-sensitivity and high-throughput nanoelectrospray mass spectrometry. Our MEA consists of 96 identical 10-nozzle emitters in a circular array on a 3 in. silicon chip. The geometry and configuration of the emitters, the dimension and number of the nozzles, and the micropillar arrays embedded in the main channel can be systematically and precisely controlled during the microfabrication process. Combining electrostatic simulation and experimental testing, we demonstrated that sharpened-end geometry at the stem of the individual multinozzle emitter significantly enhanced the electric fields at its protruding nozzle tips, enabling sequential nanoelectrospray for the high-density emitter array. We showed that electrospray current of the multinozzle emitter at a given total flow rate was approximately proportional to the square root of the number of its spraying-nozzles, suggesting the capability of high MS sensitivity for multinozzle emitters. Using a conventional Z-spray mass spectrometer, we demonstrated reproducible MS detection of peptides and proteins for serial MEA emitters, achieving sensitivity and stability comparable to the commercial capillary emitters. Our robust silicon-based MEA chip opens up the possibility of a fully integrated microfluidic system for ultrahigh-sensitivity and ultrahigh-throughput proteomics and metabolomics.

[pdf] [SI]

(208)
“Templated Synthesis of Shape Controlled Ordered TiO2 Cage Structures”,

Y. Deng, H. Tüysüz, J. Henzie, P. Yang, Small, 7, 2037, 2011.

Based on a combination of colloidal self-assembly and atomic layer deposition, a facile approach is developed to create novel, high-quality, ordered cage structures of anatase TiO2 with shape and morphology control using Ag nanocrystals of different shapes as templates.

[pdf]

(207)
“High Quantum Efficiency of Band-Edge Emission from ZnO Nanowires”,

D. Gargas*, H. Gao*, P. Yang, Nano Lett. 11, 3792, 2011.

External quantum efficiency (EQE) of photoluminescence as high as 20% from isolated ZnO nanowires were measured at room temperature. The EQE was found to be highly dependent on photoexcitation density, which underscores the importance of uniform optical excitation during the EQE measurement. An integrating sphere coupled to a microscopic imaging system was used in this work, which enabled the EQE measurement on isolated ZnO nanowires. The EQE values obtained here are significantly higher than those reported for ZnO materials in forms of bulk, thin films or powders. Additional insight on the radiative extraction factor of one-dimensional nanostructures was gained by measuring the internal quantum efficiency of individual nanowires. Such quantitative EQE measurements provide a sensitive, noninvasive method to characterize the optical properties of low-dimensional nanostructures and allow tuning of synthesis parameters for optimization of nanoscale materials.

[pdf] [SI]

(206)
“Light Induced Charge Transport within a Single Asymmetric Nanowire”,

C. Liu*, Y. J. Hwang*, H. E. Jeong, P. Yang, Nano Lett., 11, 3755 , 2011.

Artificial photosynthetic systems using semiconductor materials have been explored for more than three decades in order to store solar energy in chemical fuels such as hydrogen. By mimicking biological photosynthesis with two light-absorbing centers that relay excited electrons in a nanoscopic space, a dual-band gap photoelectrochemical (PEC) system is expected to have higher theoretical energy conversion efficiency than a single band gap system. This work demonstrates the vectorial charge transport of photogenerated electrons and holes within a single asymmetric Si/TiO2 nanowire using Kelvin probe force microscopy. Under UV illumination, higher surface potential was observed on the n-TiO2 side, relative to the potential of the p-Si side, as a result of majority carriers’ recombination at the Si/TiO2interface. These results demonstrate a new approach to investigate charge separation and transport in a PEC system. This asymmetric nanowire heterostructure with a dual band gap configuration and simultaneously exposed anode and cathode surfaces represents an ideal platform for the development of technologies for the generation of solar fuels, although better photoanode materials remain to be discovered.

[pdf] [SI]

(205)
“Solution processed core-shell nanowires for efficient photovoltaic cells”,

J. Tang*, Z. Huo*, S. Brittman, H. Gao, P. Yang, Nature Nanotech., 6, 568, 2011.

Semiconductor nanowires are promising for photovoltaic applications1234567891011, but, so far, nanowire-based solar cells have had lower efficiencies than planar cells made from the same materials6789101213, even allowing for the generally lower light absorption of nanowires. It is not clear, therefore, if the benefits of the nanowire structure, including better charge collection and transport14 and the possibility of enhanced absorption through light trapping415, can outweigh the reductions in performance caused by recombination at the surface of the nanowires and at p–n junctions. Here, we fabricate core–shell nanowire solar cells with open-circuit voltage and fill factor values superior to those reported for equivalent planar cells, and an energy conversion efficiency of ~5.4%, which is comparable to that of equivalent planar cells despite low light absorption levels16. The device is made using a low-temperature solution-based cation exchange reaction17181920,21 that creates a heteroepitaxial junction between a single-crystalline CdS core and single-crystalline Cu2S shell. We integrate multiple cells on single nanowires in both series and parallel configurations for high output voltages and currents, respectively. The ability to produce efficient nanowire-based solar cells with a solution-based process and Earth-abundant elements222324could significantly reduce fabrication costs relative to existing high-temperature bulk material approaches.

[pdf] [SI]

(204)
“Epitaxial growth of InGaN nanowire arrays for light emitting diodes”,

C. Hahn, Z. Zhang, A. Fu, C. H. Wu, Y. J. Hwang, D. J. Gargas, P. Yang, ACS Nano, 5, 3970, 2011.

Significant synthetic challenges remain for the epitaxial growth of high-quality InGaN across the entire compositional range. One strategy to address these challenges has been to use the nanowire geometry because of its strain relieving properties. Here, we demonstrate the heteroepitaxial growth of InxGa1–xN nanowire arrays (0.06 ≤ x ≤ 0.43) on c-plane sapphire (Al2O3(001)) using a halide chemical vapor deposition (HCVD) technique. Scanning electron microscopy and X-ray diffraction characterization confirmed the long-range order and epitaxy of vertically oriented nanowires. Structural characterization by transmission electron microscopy showed that single crystalline nanowires were grown in the 002 direction. Optical properties of InGaN nanowire arrays were investigated by absorption and photoluminescence measurements. These measurements show the tunable direct band gap properties of InGaN nanowires into the yellow-orange region of the visible spectrum. To demonstrate the utility of our HCVD method for implementation into devices, LEDs were fabricated from InxGa1–xN nanowires epitaxially grown on p-GaN(001). Devices showed blue (x= 0.06), green (x = 0.28), and orange (x = 0.43) electroluminescence, demonstrating electrically driven color tunable emission from this p–n junction.

[pdf] [SI]

(203)
“Nanocrystal Bilayer for Tandem Catalysis”,

Y. Yamada, C. Tsung, W. Huang, Z. Huo, S. E. Habas, T. Soejima, C. E. Aliaga, G. A. Somorjai, P. Yang, Nature Chem., 3, 372, 2011.

Supported catalysts are widely used in industry and can be optimized by tuning the composition and interface of the metal nanoparticles and oxide supports. Rational design of metal–metal oxide interfaces in nanostructured catalysts is critical to achieve better reaction activities and selectivities. We introduce here a new class of nanocrystal tandem catalysts that have multiple metal–metal oxide interfaces for the catalysis of sequential reactions. We utilized a nanocrystal bilayer structure formed by assembling platinum and cerium oxide nanocube monolayers of less than 10 nm on asilica substrate. The two distinct metal–metal oxide interfaces, CeO2–Pt and Pt–SiO2, can be used to catalyse two distinct sequential reactions. The CeO2–Pt interface catalysed methanoldecomposition to produce CO and H2, which were subsequently used for ethylene hydroformylation catalysed by the nearby Pt–SiO2 interface. Consequently, propanal was produced selectively frommethanol and ethylene on the nanocrystal bilayer tandem catalyst. This new concept of nanocrystal tandem catalysis represents a powerful approach towards designing high-performance, multifunctional nanostructured catalysts.

[pdf] [SI]

(202)
“Efficiency Enhancement of Copper Contaminated Radial p-n Junction Solar Cells”,

A. Boukai , P. Haney, A. Katzenmeyer, G. M. Gallatin, A. A. Talin, P. Yang, Chem. Phys. Lett., 501, 153, 2011.

Radial p–n junction solar cells have been predicted theoretically to have better efficiencies than their planar counterparts due to a decrease in the distance required to collect minority carriers relative to carrier diffusion length. This advantage is also significantly enhanced when the diffusion length is much smaller than the absorption length. The radial p–n junctions studied here consist of micron-scale to nano-scale diameter holes etched into a copper contaminated silicon wafer. Radial p–n junctions contaminated with copper impurities show roughly a twofold increase in efficiency than similarly contaminated planar p–n junction solar cells; however the enhancement is a strong function of the radial junction pitch, with maximum enhancement occurring for a pitch that is twice the carrier diffusion length.

[pdf]

(201)
“Metal Coated Zinc Oxide Nanocavities”,

C. Ni, S. Chang, D. J. Gargas, M. C. Moore, P. Yang, S. L. Chuang, IEEE J. Of Quantum Electronics, 47, 245, 2011.

We theoretically demonstrate that metals can be useful for increasing the quality factor and confinement factor of a zinc oxide (ZnO) nanocavity. For small cavities, the advantages of low radiation loss and significant mode confinement due to metal coating outweigh the disadvantage of absorption loss from metal and efficiently lower the threshold material gain. The performances of ZnO cavities without metal coating, with aluminum (Al) coating, and with silver (Ag) coating are investigated. The results indicate that while surface-wave-like plasmonic modes are lossy due to metal loss, the performances of well-confined dielectric modes are indeed improved significantly as a result of metal. Both Al and Ag can significantly reduce the threshold material gain of the uncoated ZnO cavity from 16 613 cm-1 to less than 5000 cm-1. In particular, the threshold material gain of Ag-coated cavity is reduced to only 3206 cm-1.

[pdf]

(200)
“Catalytic properties of Pt cluster decorated CeO2 nanostructures”,

L. Feng, D. T. Hoang, C. Tsung, W. Huang, S. Lo, J. B. Wood, H. Wang, J. Tang, P. Yang, Nano Research, 4, 61, 2011.

Uniform clusters of Pt have been deposited on the surface of capping-agent-free CeO2 nanooctahedra and nanorods using electron beam (e-beam) evaporation. The coverage of the Pt nanocluster layer can be controlled by adjusting the e-beam evaporation time. The resulting e-beam evaporated Pt nanocluster layers on the CeO2 surfaces have a clean surface and clean interface between Pt and CeO2. Different growth behaviors of Pt on the two types of CeO2 nanocrystals were observed, with epitaxial growth of Pt on CeO2 nanooctahedra and random growth of Pt on CeO2 nanorods. The structures of the Pt clusters on the two different types of CeO2 nanocrystals have been studied and compared by using them as catalysts for model reactions. The results of hydrogenation reactions clearly showed the clean and similar chemical surface of the Pt clusters in both catalysts. The support-dependent activity of these catalysts was demonstrated by CO oxidation. The Pt/CeO2nanorods showed much higher activity compared with Pt/CeO2 nanooctahedra because of the higher concentration of oxygen vacancies in the CeO2 nanorods. The structure-dependent selectivity of dehydrogenation reactions indicates that the structures of the Pt on CeO2 nanorods and nanooctahedra are different. Thes differences arise because the metal deposition behaviors are modulated by the strong metal-metal oxide interactions.

[pdf]

(199)
“Atomic-Level Control of the Thermoelectric Properties in Polytypoid Nanowires”,

S. C. Andrews*, M. A. Fardy*, M. C. Moore*, S. Aloni, M. Zhang, V. Radmilovic, P. Yang, Chem. Sci., 2, 706, 2011.

Thermoelectric materials have generated interest as a means of increasing the efficiency of power generation through the scavenging of waste heat. Materials containing nanometer-sized structural and compositional features can exhibit enhanced thermoelectric performance due to the decoupling of certain electrical and thermal properties, but the extent to which these features can be controlled is often limited. Here we report a simple synthesis of M2O3(ZnO)n (M= In, Ga, Fe) nanowires with controllable polytypoid structures, where the nanostructured features are tuned by adjusting the amount of metal precursor. After the introduction of nanometer-scale features (individual atomic layers and alloying), thermal and electrical measurements on single In2-xGaxO3(ZnO)n nanowires reveal a simultaneous improvement in all contributing factors to the thermoelectric figure of merit, indicating successful modification of the nanowire transport properties.

[pdf] [SI]

2010

(198)
"Holey Silicon as an Efficient Thermoelectric Material",

J. Tang*, H. Wang*, D. H. Lee, M. Fardy, Z. Huo, T. P. Russell, P. Yang, Nano Lett., 10, 4279, 2010.

This work investigated the thermoelectric properties of thin silicon membranes that have been decorated with high density of nanoscopic holes. These “holey silicon” (HS) structures were fabricated by either nanosphere or block-copolymer lithography, both of which are scalable for practical device application. By reducing the pitch of the hexagonal holey pattern down to 55 nm with 35% porosity, the thermal conductivity of HS is consistently reduced by 2 orders of magnitude and approaches the amorphous limit. With a ZT value of 0.4 at room temperature, the thermoelectric performance of HS is comparable with the best value recorded in silicon nanowire system.

[pdf] [SI]

(197)
“Synthesis of metal sulfide nanomaterials via thermal decomposition of single-source precursors”,

I. J. Plante, T. W. Zeid, P. Yang, T. Mokari, J. Mater. Chem., 20,6612, 2010.

In this report, we present a synthetic method for the formation of cuprous sulfide (Cu2S) and lead sulfide (PbS) nanomaterials directly on substrates from the thermolysis of single-source precursors. We find that the final morphology and arrangement of the nanomaterials may be controlled through the concentration of the dissolved precursors and choice of solvent. One-dimensional (1-D) morphologies may also be grown onto substrates with the addition of a metal catalyst layer through solution-liquid-solid (SLS) growth. These synthetic techniques may be expanded to other metal sulfide materials.

[pdf]

(196)
“Whispering Gallery Mode Lasing from Zinc Oxide Hexagonal Nanodisks”,

D. J. Gargas, M. C. Moore, A. Ni, S.-W. Chang, Z. Zhang, S.-L. Chuang, P. Yang, ACS Nano, 4, 3270, 2010.

Disk-shaped semiconductor nanostructures provide enhanced architectures for low-threshold whispering gallery mode (WGM) lasing with the potential for on-chip nanophotonic integration. Unlike cavities that lase via Fabry−Perot modes, WGM structures utilize low-loss, total internal reflection of the optical mode along the circumference of the structure, which effectively reduces the volume of gain material required for lasing. As a result, circularly resonant cavities provide much higher quality (Q) factors than lower reflection linear cavities, which makes nanodisks an ideal platform to investigate lasing nanostructures smaller than the free-space wavelength of light (i.e., subwavelength laser). Here we report the bottom-up synthesis and single-mode lasing properties of individual ZnO disks with diameters from 280 to 900 nm and show finite difference time domain (FDTD) simulations of the whispering gallery mode inside subwavelength diameter disks. These results demonstrate ultraviolet WGM lasing in chemically synthesized, isolated nanostructures with subwavelength diameters.

[pdf] [SI]

(195)
“Semiconductor Nanowires: What's Next?”,

P. Yang, M. Fardy, R. Yan, Nano Lett. 10, 1529, 2010.

In this perspective, we take a critical look at the research progress within the nanowire community for the past decade. We discuss issues on the discovery of fundamentally new phenomena versus performance benchmarking for many of the nanowire applications. We also notice that both the bottom-up and top-down approaches have played important roles in advancing our fundamental understanding of this new class of nanostructures. Finally we attempt to look into the future and offer our personal opinions on what the future trends will be in nanowire research.

[pdf]

(194)
“Oligo- and Poly-thiophene/ZnO Hybrid Nanowire Solar Cells”,

A. L. Briseno, T. W. Holcombe, A. I. Boukai, E. C. Garnett, S. W. Shelton, J. M. J. Fréchet, P. Yang, Nano Lett. 10, 334, 2010.

We demonstrate the basic operation of an organic/inorganic hybrid single nanowire solar cell. End-functionalized oligo- and polythiophenes were grafted onto ZnO nanowires to produce p−n heterojunction nanowires. The hybrid nanostructures were characterized via absorption and electron microscopy to determine the optoelectronic properties and to probe the morphology at the organic/inorganic interface. Individual nanowire solar cell devices exhibited well-resolved characteristics with efficiencies as high as 0.036%, Jsc = 0.32 mA/cm2,Voc = 0.4 V, and a FF = 0.28 under AM 1.5 illumination with 100 mW/cm2 light intensity. These individual test structures will enable detailed analysis to be carried out in areas that have been difficult to study in bulk heterojunction devices.

[pdf] [SI]

(193)
“Room-temperature Formation of Hollow Cu2O Nanoparticles”,

L. Hung, C. Tsung, W. Huang, P. Yang, Adv. Mater. 22, 1910, 2010.

Monodisperse Cu and Cu2O nanoparticles (NPs) are synthesized using tetradecylphosphonic acid as a capping agent. Dispersing the NPs in chloroform and hexane at room temperature results in the formation of hollow Cu2O NPs and Cu@Cu2O core/shell NPs, respectively. The monodisperse Cu2O NPs are used to fabricate hybrid solar cells with efficiency of 0.14% under AM 1.5 and 1 Sun illumination.

[pdf]

(192)
“Light Trapping in Silicon Nanowire Solar Cells”,

E. Garnett, P. Yang, Nano Lett., 10, 1082, 2010.

Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n2  25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between improved absorption and increased surface recombination; for nanowire arrays fabricated from 8 μm thick silicon films, the enhanced absorption can dominate over surface recombination, even without any surface passivation. These nanowire devices give efficiencies above 5%, with short-circuit photocurrents higher than planar control samples.

[pdf] [SI]

(191)
“Anisotropic Etching of Silver Nanoparticles for Plasmonic Structures Capable of Single-Particle SERS”,

M. J. Mulvihill, X. Y. Ling, J. Henzie, P. Yang, J. Am. Chem. Soc.  132, 268, 2010.

The understanding of the localized surface plasmons (LSPs) that occur at the geometrically bounded surface of metal nanoparticles continues to advance as new and more complex nanostructures are found. It has been shown that the oscillation of electrons at the metal dielectric interface is strongly dependent on the size, symmetry, and proximity of nanoparticles. Here, we present a new method to chemically control the shape of silver nanocrystals by using a highly anisotropic etching process. Tuning of the etchant strength and reaction conditions allows the preparation of new nanoparticle shapes in high yield and purity, which cannot be synthesized with conventional nanocrystal growth methods. The etching process produces intraparticle gaps, which introduce modified plasmonic characteristics and significant scattering intensity in the near-infrared. These new silver particles serve as excellent substrates for wavelength-tunable, single-particle surface enhanced Raman spectroscopy (spSERS).

[pdf] [SI]

(190)
"Semiconductor Nanowires for Energy Conversion",

A. Hochbaum, P. Yang, Chem. Rev(Invited Review), 110, 527, 2010.

[pdf]

2009

(189)
“Direct Photonic-Plasmonic Coupling and Routing in Single Nanowires”,

R. Yan, P. Pausauskie, J. Huang, P. Yang, Proc. Natl. Acad. Sci. USA, 106, 21045, 2009.

Metallic nanoscale structures are capable of supporting surface plasmon polaritons (SPPs), propagating collective electron oscillations with tight spatial confinement at the metal surface. SPPs represent one of the most promising structures to beat the diffraction limit imposed by conventional dielectric optics. Ag nano wires have drawn increasing research attention due to 2D sub-100 nm mode confinement and lower losses as compared with fabricated metal structures. However, rational and versatile integration of Ag nanowires with other active and passive optical components, as well as Ag nanowire based optical routing networks, has yet to be achieved. Here, we demonstrate that SPPs can be excited simply by contacting a silver nanowire with a SnO2 nanoribbon that serves both as an unpolarized light source and a dielectric waveguide. The efficient coupling makes it possible to measure the propagation-distance-dependent waveguide spectra and frequency-dependent propagation length on a single Ag nanowire. Furthermore, we have demonstrated prototypical photonic-plasmonic routing devices, which are essential for incorporating low-loss Ag nanowire waveguides as practical components into high-capacity photonic circuits.

[pdf] [SI]

(188)
“Single crystalline mesoporous silicon nanowires”,

A. Hochbaum, D. Gargas, P. Yang, Nano Lett., 9, 3550, 2009.

Herein we demonstrate a novel electroless etching synthesis of monolithic, single-crystalline, mesoporous silicon nanowire arrays with a high surface area and luminescent properties consistent with conventional porous silicon materials. These porous nanowires also retain the crystallographic orientation of the wafer from which they are etched. Electron microscopy and diffraction confirm their single-crystallinity and reveal the silicon surrounding the pores is as thin as several nanometers. Confocal fluorescence microscopy showed that the photoluminescence (PL) of these arrays emanate from the nanowires themselves, and their PL spectrum suggests that these arrays may be useful as photocatalytic substrates or active components of nanoscale optoelectronic devices.

[pdf] [SI]

(187)
“Nanofluidic diodes based on heterojunction nanotubes”,

R. Yan, W. Liang, R. Fan, P. Yang, Nano Lett., 9, 3820, 2009.

The mechanism of tuning charge transport in electronic devices has recently been implemented into the nanofluidic field for the active control of ion transport in nanoscale channels/pores. Here we report the first synthesis of longitudinal heterostructured SiO2/Al2O3 nanotubes. The ionic transport through these nanotube heterojunctions exhibits clear current rectification, a signature of ionic diode behavior. Such nanofluidic diodes could find applications in ion separation and energy conversion.

[pdf] [SI]

(186)
“Nanowire Photonics”,

R. Yan, D. Gargas, P. Yang, Nature Photonics (Invited Review), 3, 569, 2009.

Semiconductor nanowires, by definition, typically have cross-sectional dimensions that can be tuned from 2–200 nm, with lengths spanning from hundreds of nanometres to millimetres. These subwavelength structures represent a new class of semiconductor materials for investigating light generation, propagation, detection, amplification and modulation. After more than a decade of research, nanowires can now be synthesized and assembled with specific compositions, heterojunctions and architectures. This has led to a host of nanowire photonic devices including photodetectors, chemical and gas sensors, waveguides, LEDs, microcavity lasers, solar cells and nonlinear optical converters. A fully integrated photonic platform using nanowire building blocks promises advanced functionalities at dimensions compatible with on-chip technologies.

[pdf]

(185)
“Thermoelectrical properties of p-type PbSe nanowires”,

W. Liang, O. Rabin, A. Hochbaum, M. Fardy, M. Zhang, P. Yang, Nano Res., 2, 394, 2009.

The thermoelectric properties of individual solution-phase synthesized p-type PbSe nanowires have been examined. The nanowires showed near degenerately doped charge carrier concentrations. Compared to the bulk, the PbSe nanowires exhibited a similar Seebeck coefficient and a significant reduction in thermal conductivity in the temperature range 20 K to 300 K. Thermal annealing of the PbSe nanowires allowed their thermoelectric properties to be controllably tuned by increasing their carrier concentration or hole mobility. After optimal annealing, single PbSe nanowires exhibited a thermoelectric figure of merit (ZT) of 0.12 at room temperature.

[pdf]

(184)
“Field-Effect Modulation of Seebeck Coefficient in Single PbSe Nanowires”,

W. Liang, A. Hochbaum, M. Fardy, M. Zhang, P. Yang, Nano Lett., 9, 1689, 2009.

In this Letter, we present a novel strategy to control the thermoelectric properties of individual PbSe nanowires. Using a field-effect gated device, we were able to tune the Seebeck coefficient of single PbSe nanowires from 64 to 193 μV·K−1. This direct electrical field control of σ and S suggests a powerful strategy for optimizing ZT in thermoelectric devices. These results represent the first demonstration of field-effect modulation of the thermoelectric figure of merit in a single semiconductor nanowire. This novel strategy for thermoelectric property modulation could prove especially important in optimizing the thermoelectric properties of semiconductors where reproducible doping is difficult to achieve.

[pdf] [SI]

(183)
“Sub-10 nm Platinum Nanocrystals with Size and Shape Control: Catalytic Study for Ethylene and Pyrrole Hydrogenation”,

C. Tsung, J. N. Kuhn, W. Huang, C. Aliaga, G. A. Somorjai, P. Yang, J. Am. Chem. Soc.,131, 5816, 2009.

Platinum nanocubes and nanopolyhedra with tunable size from 5 to 9 nm were synthesized by controlling the reducing rate of metal precursor ions in a one-pot polyol synthesis. A two-stage process is proposed for the simultaneous control of size and shape. In the first stage, the oxidation state of the metal ion precursors determined the nucleation rate and consequently the number of nuclei. The reaction temperature controlled the shape in the second stage by regulation of the growth kinetics. These well-defined nanocrystals were loaded into MCF-17 mesoporous silica for examination of catalytic properties. Pt loadings and dispersions of the supported catalysts were determined by elemental analysis (ICP-MS) and H2 chemisorption isotherms, respectively. Ethylene hydrogenation rates over the Pt nanocrystals were independent of both size and shape and comparable to Pt single crystals. For pyrrole hydrogenation, the nanocubes enhanced ring-opening ability and thus showed a higher selectivity to n-butylamine as compared to nanopolyhedra.

[pdf]

(182)
“Dopant profiling and surface analysis of silicon nanowires using capacitance–voltage measurements”,

E. Garnett, Y. Tseng, D. Khanal, J. Wu, J. Bokor, P. Yang, Nature Nanotech., 4, 311, 2009.

Silicon nanowires are expected to have applications in transistors, sensors, resonators, solar cells and thermoelectric systems1, 2, 3, 4, 5. Understanding the surface properties and dopant distribution will be critical for the fabrication of high-performance devices based on nanowires6. At present, determination of the dopant concentration depends on a combination of experimental measurements of the mobility and threshold voltage7, 8 in a nanowire field-effect transistor, a calculated value for the capacitance, and two assumptions—that the dopant distribution is uniform and that the surface (interface) charge density is known. These assumptions can be tested in planar devices with the capacitance–voltage technique9. This technique has also been used to determine the mobility of nanowires10, 11, 12, 13, but it has not been used to measure surface properties and dopant distributions, despite their influence on the electronic properties of nanowires14, 15. Here, we measure the surface (interface) state density and the radial dopant profile of individual silicon nanowire field-effect transistors with the capacitance–voltage technique.

[pdf] [SI]

(181)
“Nanowire-based all-oxide solar cell”,

B. Yuhas, P. Yang, J. Am. Chem. Soc., 131, 3756, 2009.

We present an all-oxide solar cell fabricated from vertically oriented zinc oxide nanowires and cuprous oxide nanoparticles. Our solar cell consists of vertically oriented n-type zinc oxide nanowires, surrounded by a film constructed from p-type cuprous oxide nanoparticles. Our solution-based synthesis of inexpensive and environmentally benign oxide materials in a solar cell would allow for the facile production of large-scale photovoltaic devices. We found that the solar cell performance is enhanced with the addition of an intermediate oxide insulating layer between the nanowires and the nanoparticles. This observation of the important dependence of the shunt resistance on the photovoltaic performance is widely applicable to any nanowire solar cell constructed with the nanowire array in direct contact with one electrode.

[pdf]

(180)
“Sum Frequency Generation and Catalytic Reaction Studies of the removal of the organic capping agents from Pt nanoparticles by UV ozone treatment”,

C. Aliaga, J. Park, Y. Yamada, H. Lee, C. Tsung, P. Yang, G. Somorjai, J. Phys. Chem. C, 113, 6150, 2009.

We report the structure of the organic capping layers of platinum colloid nanoparticles and their removal by UV−ozone exposure. Sum frequency generation vibrational spectroscopy (SFGVS) studies identify the carbon−hydrogen stretching modes on poly(vinylpyrrolidone) (PVP) and tetradecyl tributylammonium bromide (TTAB)-capped platinum nanoparticles. We found that the UV−ozone treatment technique effectively removes the capping layer on the basis of several analytical measurements including SFGVS, X-ray photoelectron spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The overall shape of the nanoparticles was preserved after the removal of capping layers, as confirmed by transmission electron microscopy (TEM). SFGVS of ethylene hydrogenation on the clean platinum nanoparticles demonstrates the existence of ethylidyne and di-σ-bonded species, indicating the similarity between single-crystal and nanoparticle systems.

[pdf]

(179)
“Self-organized ultrathin oxide nanocrystals”,

Z. Huo, F. Tsung, W. Huang, M. Fardy, R. Yan, Y. Li, X. Zhang, P. Yang, Nano Lett., 9, 1260, 2009.

Sub-2-nm (down to one-unit cell) uniform oxide nanocrystals and highly ordered superstructures were obtained in one step using oleylamine and oleic acid as capping and structure directing agents. The cooperative nature of the nanocrystal growth and assembly resulted in mesoscopic one-dimensional ribbon-like superstructures made of these ultrathin nanocrystals. The process reported here is general and can be readily extended to the production of many other transition metal (TiO2, ZnO, Nb2O5) and rare earth oxide (Eu2O3, Sm2O3, Er2O3, Y2O3, Tb2O3, and Yb2O3) systems.

[pdf] [SI]

(178)
“Magnetotransport in Co doped ZnO nanowires”,

W. Liang, B. Yuhas, P. Yang, Nano Lett., 9, 892, 2009.

Electrical and magnetotransport measurements were performed on individual Co-doped ZnO dilute magnetic semiconductor nanowires. The electron transport studies show that the electron mobility could be as high as 75 cm2/(V·s), and we observed positive magnetoresistivity (MR) at low magnetic field and negative MR at higher magnetic field. s−d exchange-induced spin splitting of the conduction band could account for positive MR while suppression of weak localization of impurity centers could account for the negative MR. Lowering the carrier concentration in these nanowires through the application of a gate voltage tends to induce a larger magnitude MR as well as additional fine structure in the MR curves.

[pdf]

(177)
“Imaging Single ZnO Vertical Nanowire Laser Cavities Using UV-laser Scanning Confocal Microscopy”,

D. Gargas, M.E. Toimil-Molares, P. Yang, J. Am. Chem. Soc., 131, 2125, 2009.

We report the fabrication and optical characterization of individual ZnO vertical nanowire laser cavities. Dilute nanowire arrays with interwire spacing >10 μm were produced by a modified chemical vapor transport (CVT) method yielding an ideal platform for single nanowire imaging and spectroscopy. Lasing characteristics of a single vertical nanowire are presented, as well as high-resolution photoluminescence imaging by UV-laser scanning confocal microscopy. In addition, three-dimensional (3D) mapping of the photoluminescence emission performed in both planar and vertical dimensions demonstrates height-selective imaging useful for vertical nanowires and heteronanostructures emerging in the field of optoelectronics and nanophotonics.

[pdf] [SI]

(176)
“High density n-Si/n-TiO2 core/shell nanowire arrays with enhanced photoactivity”,

Y. J. Hwang, A. Bukai, P. Yang, Nano Lett., 9, 410, 2009.

There are currently great needs to develop low-cost inorganic materials that can efficiently perform solar water splitting as photoelectrolysis of water into hydrogen and oxygen has significant potential to provide clean energy. We investigate the Si/TiO2 nanowire heterostructures to determine their potential for the photooxidation of water. We observed that highly dense Si/TiO2 core/shell nanowire arrays enhanced the photocurrent by 2.5 times compared to planar Si/TiO2 structure due to their low reflectance and high surface area. We also showed that n-Si/n-TiO2 nanowire arrays exhibited a larger photocurrent and open circuit voltage than p-Si/n-TiO2 nanowires due to a barrier at the heterojunction.

[pdf] [SI]

(175)
“Optoelectronics: Combining chemical worlds”,

A. L. Briseno, P. Yang, Nature Mater., 8, 7, 2009.

Using self-assembly and electrodeposition, complementary organic and inorganic building blocks are combined to form a lamellar hybrid that is an efficient photoconductor.

[pdf]

(174)
“Thermally Stable Nanocatalyst for High Temperature Reactions: Pt-Mesoporous Silica Core-Shell Nanoparticles”,

S. Joo, J. Park, C. Tsung, Y. Yamada, P. Yang, G. A. Somorjai, Nature Mater., 8, 126, 2009.

Recent advances in colloidal synthesis enabled the precise control of the size, shape and composition of catalytic metal nanoparticles, enabling their use as model catalysts for systematic investigations of the atomic-scale properties affecting catalytic activity and selectivity. The organic capping agents stabilizing colloidal nanoparticles, however, often limit their application in high-temperature catalytic reactions. Here, we report the design of a high-temperature-stable model catalytic system that consists of a Pt metal core coated with a mesoporous silica shell (Pt@mSiO2). Inorganic silica shells encaged the Pt cores up to 750 °C in air and the mesopores providing direct access to the Pt core made the Pt@mSiO2 nanoparticles as catalytically active as bare Pt metal for ethylene hydrogenation and CO oxidation. The high thermal stability of Pt@mSiO2 nanoparticles enabled high-temperature CO oxidation studies, including ignition behaviour, which was not possible for bare Pt nanoparticles because of their deformation or aggregation. The results suggest that the Pt@mSiO2 nanoparticles are excellent nanocatalytic systems for high-temperature catalytic reactions or surface chemical processes, and the design concept used in the Pt@mSiO2 core–shell catalyst can be extended to other metal/metal oxide compositions.

[pdf] [SI] [Cover Highlight]

2008

(173)
“Self-Organized Silver Nanoparticles for Three-Dimensional Plasmonic Crystals”,

A. Tao, D. Ceperley, P. Sinsermsuksakul, A. R. Neureuther, P. Yang, Nano Lett.8, 4033, 2008.

Metal nanostructures that support surface plasmons are compelling as plasmonic circuit elements and as the building blocks for metamaterials. We demonstrate here the spontaneous self-assembly of shaped silver nanoparticles into three-dimensional plasmonic crystals that display a frequency-selective response in the visible wavelengths. Extensive long-range order mediated by exceptional colloid monodispersity gives rise to optical passbands that can be tuned by particle volume fraction. These metallic supercrystals present a new paradigm for the fabrication of plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography.

[pdf] [SI]

(172)
“Near-Monodisperse Ni-Cu Bimetallic Nanocrystals of Variable Composition: Controlled Synthesis and Catalytic Activity for H2 Generation”,

Zhang, Y.; Huang, W.; Habas, S. E.; Kuhn, J. N.; Grass, M. E.; Yamada, Y.; Yang, P.; Somorjai, G. A. J. Phys. Chem. C, 112(32), 12092-12095, 2008.

Near-monodisperse Ni1−xCux (x = 0.2−0.8) bimetallic nanocrystals were synthesized by a one-pot thermolysis approach in oleylamine/1-octadecene, using metal acetylacetonates as precursors. The nanocrystals form large-area 2D superlattices, and display a catalytic synergistic effect in the hydrolysis of NaBH4 to generate H2 at x = 0.5 in a strongly basic medium. The Ni0.5Cu0.5 nanocrystals show the lowest activation energy, and also exhibit the highest H2 generation rate at 298 K.

[pdf] [SI]

(171)
“Silicon Nanowire Radial p-n Junction Solar Cells”,

E. C. Garnett, P. Yang, J. Am. Chem. Soc., 130, 9224, 2008.

We have demonstrated a low-temperature wafer-scale etching and thin film deposition method for fabricating silicon n−p core−shell nanowire solar cells. Our devices showed efficiencies up to nearly 0.5%, limited primarily by interfacial recombination and high series resistance. Surface passivation and contact optimization will be critical to improve device performance in the future.

[pdf] [SI]

(170)
“Vertical Nanowire Array Based Light Emitting Diodes”,

E. Lai, W. Kim, P. Yang, Nano Res., 1, 123, 2008.

Electroluminescence from a nanowire array-based light emitting diode is reported. The junction consists of a p-type GaN thin film grown by metal organic chemical vapor deposition (MOCVD) and a vertical n-type ZnO nanowire array grown epitaxially from the thin film through a simple low temperature solution method. The fabricated devices exhibit diode like current voltage behavior. Electroluminescence is visible to the human eye at a forward bias of 10 V and spectroscopy reveals that emission is dominated by acceptor to band transitions in the p-GaN thin film. It is suggested that the vertical nanowire architecture of the device leads to waveguided emission from the thin film through the nanowire array.

[pdf]

(169)
“Thermal Conductance of Thin Silicon Nanowires”,

R. Chen, A. I. Hochbaum, P. Murphy, J. Moore, P. Yang, A. Majumdar, Phys. Rev. Lett., 101, 105501, 2008.

The thermal conductance of individual single crystalline silicon nanowires with diameters less than 30 nm has been measured from 20 to 100 K. The observed thermal conductance shows unusual linear temperature dependence at low temperatures, as opposed to the T3 dependence predicted by the conventional phonon transport model. In contrast to previous models, the present study suggests that phonon-boundary scattering is highly frequency dependent, and ranges from nearly ballistic to completely diffusive, which can explain the unexpected linear temperature dependence.

[pdf]

(168)
“Sub-Two Nanometer Single Crystal Au Nanowires”,

Z. Huo, F. Tsung, W. Huang, X. Zhang, P. Yang, Nano Lett., 8, 2041, 2008.

Ultrathin single crystal Au nanowires with diameter of 1.6 nm and length of few micrometers were synthesized with high yield by simply mixing HAuCl4 and oleylamine at room temperature. High resolution transmission electron microscopy studies revealed that all of these nanowires are single crystalline and grew along the [111] direction. The valency evolution of the gold species during the synthesis was studied by X-ray photoelectron spectroscopy, which showed a clear Au3+ → Au+ → Au stepwise reduction at different reaction stages. Small angle X-ray scattering and small-angle X-ray diffraction suggest mesostructure formation upon HAuCl4 and oleylamine mixing. The slow in situ reduction of this mesostructure leads to the formation of ultrathin nanowires in solution. This novel nanowire growth mechanism relies on cooperative interaction, organization, and reaction between inorganic precursor salts and oleylamine.

[pdf]

(167)
“Surface-Enhanced Raman Spectroscopy for Trace Arsenic Detection in Groundwater”,

M. Mulvihill, A. Tao, K. Benjauthrit, J. Arnold, P. Yang, Angew. Chem. Int. Ed., 47,6456, 2008.

Getting to the bottom of groundwater: The development of a reliable, portable, and simple-to-use device for detecting arsenic in groundwater is urgently needed in developing nations such as Bangladesh, where contaminated groundwater is at the root of a public health crisis. Toward this end, a highly sensitive platform utilizing surface-enhanced Raman spectroscopy (SERS, see picture) is used to quantitatively detect arsenate in water down to 1 ppb.

[pdf]

(166)
“Langmuir-Blodgettry of Nanocrystals and Nanowires”,

A. R. Tao, J. Huang, P. Yang, Acct. Chem. Res. , 41,1662, 2008.

Although nanocrystals and nanowires have proliferated new scientific avenues in the study of their physics and chemistries, the bottom-up assembly of these small-scale building blocks remains a formidable challenge for device fabrication and processing. An attractive nanoscale assembly strategy should be cheap, fast, defect tolerant, compatible with a variety of materials, and parallel in nature, ideally utilizing the self-assembly to generate the core of a device, such as a memory chip or optical display. Langmuir−Blodgett (LB) assembly is a good candidate for arranging vast numbers of nanostructures on solid surfaces. In the LB technique, uniaxial compression of a nanocrystal or nanowire monolayer floating on an aqueous subphase causes the nanostructures to assemble and pack over a large area. The ordered monolayer can then be transferred to a solid surface en masse and with fidelity.

In this Account, we present the Langmuir−Blodgett technique as a low-cost method for the massively parallel, controlled organization of nanostructures. The isothermal compression of fluid-supported nanoparticles or nanowires is unique in its ability to achieve control over nanoscale assembly by tuning a macroscopic property such as surface pressure. Under optimized conditions (e.g., surface pressure, substrate hydrophobicity, and pulling speed), it allows continuous variation of particle density, spacing, and even arrangement. For practical application and device fabrication, LB compression is ideal for forming highly dense assemblies of nanowires and nanocrystals over unprecedented surface areas. In addition, the dewetting properties of LB monolayers can be used to further achieve patterning within the range of micrometers to tens of nanometers without a predefined template. The LB method should allow for easy integration of nanomaterials into current manufacturing schemes, in addition to fast device prototyping and multiplexing capability.

[pdf]

(165)
“Chemistry and physics of silicon nanowires”,

P. Yang, Dalton Trans., 33, 4387, 2008.

This article provides a short overview of the current status of the silicon nanowire research including its synthetic chemistry and physical property characterization, with examples drawn mainly from the author’s lab.

[pdf]

(164)
"Self-Transducing Silicon Nanowire Electromechanical Systems at Room Temperature",

R. He*, X. L. Feng*, M. L. Roukes, P. Yang, Nano Lett., 8, 1756, 2008.

Electronic readout of the motions of genuinely nanoscale mechanical devices at room temperature imposes an important challenge for the integration and application of nanoelectromechanical systems (NEMS). Here, we report the first experiments on piezoresistively transduced very high frequency Si nanowire (SiNW) resonators with on-chip electronic actuation at room temperature. We have demonstrated that, for very thin (90 nm down to 30 nm) SiNWs, their time-varying strain can be exploited for self-transducing the devices’ resonant motions at frequencies as high as 100 MHz. The strain of wire elongation, which is only second-order in doubly clamped structures, enables efficient displacement transducer because of the enhanced piezoresistance effect in these SiNWs. This intrinsically integrated transducer is uniquely suited for a class of very thin wires and beams where metallization and multilayer complex patterning on devices become impractical. The 30 nm thin SiNW NEMS offer exceptional mass sensitivities in the subzeptogram range. This demonstration makes it promising to advance toward NEMS sensors based on ultrathin and even molecular-scale SiNWs, and their monolithic integration with microelectronics on the same chip.

[pdf]

(163)
"Synthesis of Lead Chalcogenide Alloy and Core–Shell Nanowires",

T. Mokari, S. Habas, M. Zhang, P. Yang, Angew. Chem. Int. Ed., 47, 5605, 2008.

Getting the grip on wires: Lead chalcogenide heterostructures, including alloy and core–shell nanowires were achieved by solution phase synthesis (see HRTEM image of a PbSe–PbTe core–shell nanowire). Structural control was gained by changing the growth parameters. The new heterostructures have a potential to provide better thermoelectric materials compared to the pure PbSe nanowires.

[pdf]

(162)
“Highly Selective Synthesis of Catalytically Active Monodisperse Rhodium Nanocubes”,

Y. Zhang, M. E. Grass, J. N. Kuhn, F. Tao, S. E. Habas, W. Huang, P. Yang, G. A. Somorjai, J. Am. Chem. Soc., 130, 5868, 2008.

Monodisperse sub-10 nm Rh nanocubes were synthesized with high selectivity (>85%) by a seedless polyol method. The {100} faces of the Rh NCs were effectively stabilized by chemically adsorbed Br ions from trimethyl(tetradecyl)ammonium bromide (TTAB). This simple one-step polyol route can be readily applied to the preparation of Pt and Pd nanocubes. Moreover, the organic molecules of PVP and TTAB that encapsulated the Rh nanocubes did not prevent catalytic activity for pyrrole hydrogenation and CO oxidation.

[pdf] [SI]

(161)
"Localized Pd Overgrowth on Cubic Pt Nanocrystals for Enhanced Electrocatalytic Oxidation of Formic Acid",

H. Lee, S. E. Habas, G. A. Somorjai, P. Yang, J. Am. Chem. Soc. , 130, 5406, 2008.

Binary Pt/Pd nanoparticles were synthesized by localized overgrowth of Pd on cubic Pt seeds for the investigation of electrocatalytic formic acid oxidation. The binary particles exhibited much less self-poisoning and a lower activation energy relative to Pt nanocubes, consistent with the single crystal study.

[pdf] [SI]

(160)
“Selective Growth of Metal and Binary Metal Tips on CdS Nanorods”,

S. E. Habas, P. Yang, T. Mokari, J. Am. Chem. Soc., 130, 3294, 2008.

Here, we demonstrate an approach for the selective growth of Pt, PtNi, and PtCo on CdS nanorods. The hybrid nanostructures prepared via an organometallic synthesis have promise for photocatalytic and magnetic applications.

[pdf] [SI]

(159)
“Shape Control of Colloidal Metal Nanocrystals”,

A. Tao, S. Habas, P. Yang, Small (Invited Review), 4, 310, 2008.

Colloidal metal nanoparticles are emerging as key materials for catalysis, plasmonics, sensing, and spectroscopy. Within these applications, control of nanoparticle shape lends increasing functionality and selectivity. Shape-controlled nanocrystals possess well-defined surfaces and morphologies because their nucleation and growth are controlled at the atomic level. An overall picture of shaped metal particles is presented, with a particular focus on solution-based syntheses for the noble metals. General strategies for synthetic control are discussed, emphasizing key factors that result in anisotropic, nonspherical growth such as crystallographically selective adsorbates and seeding processes.

[pdf]

(158)
“Gated proton transport in aligned mesoporous silica films”,

R. Fan, S. Huh, R. Yan, J. Arnold, P. Yang, Nature Materials7, 303, 2008.

Modulated proton transport plays significant roles in biological processes1 such as ATP synthesis2, 3 as well as in technologically important applications including, for example, hydrogen fuel cells4, 5. The state-of-the-art proton-exchange membrane is the sulphonated tetrafluoroethylene copolymer Nafion developed by DuPont in the late 1960s, with a high proton conductivity6. However, actively switchable proton conduction, a functional mimic of the ion transport within a cell membrane, has yet to be realized. Herein, we report the electrostatic gating of proton transport within aligned mesoporous silica thin film. It is observed that surface-charge-mediated transport is dominant at low proton concentrations. We have further demonstrated that the proton conduction can be actively modulated by two–fourfold with a gate voltage as low as 1 V. Such artificial gatable ion transport media could have potential applications in nanofluidic chemical processors, biomolecular separation and electrochemical energy conversion.

[pdf] [SI]

(157)
“Lilliputian light sticks”,

M. Fardy, P. Yang, Nature, 451, 408,2008.

Building two different fluorescing dyes into a composite organic nanocrystal makes a tunable light generator. At just the right dye proportions, a low-cost, highly efficient source of white light is the result.

[pdf]

(156)
“Dynamic manipulation and separation of individual semiconducting and metallic nanowires using optoelectronic tweezers”,

A. Jamshidi*, P. J. Pauzauskie*, P. J. Schuck, A. T. Ohta, P. Chiou, J. Chou, P. Yang, M. C. Wu, Nature Photonics, 2, 85, 2008.

The synthesis of nanowires has advanced in the past decade to the point where a vast range of insulating, semiconducting and metallic materials are available for use in integrated, heterogeneous optoelectronic devices at nanometre scales. However, a persistent challenge has been the development of a general strategy for the manipulation of individual nanowires with arbitrary composition. Here we report that individual semiconducting and metallic nanowires with diameters below 20 nm are addressable with forces generated by optoelectronic tweezers. Using 100,000 times less optical power density than optical tweezers, optoelectronic tweezers are capable of transporting individual nanowires with speeds four times greater than the maximum speeds achieved by optical tweezers. A real-time array of silver nanowires is formed using photopatterned virtual electrodes, demonstrating the potential for massively parallel assemblies. Furthermore, optoelectronic tweezers enable the separation of semiconducting and metallic nanowires, suggesting a broad range of applications for the separation and heterogeneous integration of one-dimensional nanoscale materials.

[pdf] [SI] [vid] [Cover Highlight]

(155)
“Enhanced thermoelectric performance of rough silicon nanowires”,

A. I. Hochbaum*, R. Chen*, R. D. Delgado, W. Liang, E.  C. Garnett, M. Najarian, A. Majumdar, P. Yang, Nature, 451, 163, 2008.

Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent1. While nanostructured thermoelectric materials can increase ZT > 1 (refs 2–4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

[pdf] [SI]

(154)
"Adsorption and Co-adsorption of Ethylene and Carbon Monoxide on Silica-Supported Monodisperse Pt Nanoparticles: Volumetric Adsorption and Infrared Spectroscopy Studies",

R. M. Rioux, J. D. Hoefelmeyer, M. Grass, H. Song, K. Niesz, P. Yang, G. A. Somorjai, Langmuir, 24, 198, 2008.

The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 1.7 to 7.1 nm by diffuse-reflectance infrared spectroscopy and chemisorption. Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure. Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C−O stretch:  ν(CO) = 2075 cm-1 (1.9 nm); 2079 cm-1 (2.9 nm); 2082 cm-1 (3.6 nm); and 2090 cm-1 (7.1 nm). The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providing semiquantitative surface coverage information. Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopic features at 1340, 1420, and 1500 cm-1 assigned to ethylidyne, di-σ-bonded ethylene, and π-bonded ethylene, respectively. The ratio of these surface species is highly dependent on the Pt particle size. At room temperature, Pt particles stabilize ethylidyne as well as di-σ- and π-bonded ethylene; however, ethylidyne predominated on the surfaces of larger particles. Ethylidyne was the only identifiable species at 403 K, with its formation being more facile on larger particles. Co-adsorption experiments reveal that the composition of the surface layer is dependent on the order of exposure to gases. Exposure of a C2H4-covered Pt surface to CO resulted in an 50% decrease in chemisorbed CO compared to a fresh Pt surface. The ν(CO) appeared at 2050 cm-1 on Pt/SBA-15 pretreated with C2H4 at room temperature. The di-σ-bonded and π-bonded species are the most susceptible to displacement from the surface by CO. The formation of ethylidyne appeared to be less sensitive to the presence of adsorbed carbon monoxide, especially on larger particles. Upon exposure of C2H4 to a CO-covered Pt surface, little irreversible uptake occurred due to nearly 100% site blocking. These results demonstrate that carbon monoxide competes directly with ethylene for surface sites, which will have direct implications on the poisoning of the heterogeneously catalyzed conversion of hydrocarbons.

[pdf]

2007

(153)
“ZnO-TiO2 core-sheath nanorod/P3HT solar cell”,

L. Greene, M. Law, B. Yuhas, P. Yang, J. Phys. Chem. C, 111, 18451, 2007.

We evaluate an ordered organic−inorganic solar cell architecture based on ZnO−TiO2 core−shell nanorod arrays encased in the hole-conducting polymer P3HT. Thin shells of TiO2 grown on the ZnO nanorods by atomic layer deposition significantly increase the voltage and fill factor relative to devices without shells. We find that the core−shell cells must be exposed to air to reproducibly attain efficiencies higher than 0.05%. Cells stored in air for 1 month are 0.29% efficient.

[pdf]

(152)
“Complete Composition Tunability of InGaN Nanowires using a Combinatorial Approach”,

T.  Kuykendall, P. Ulrich, S. Aloni, P. Yang, Nature Mater. 6, 951, 2007.

The III nitrides have been intensely studied in recent years because of their huge potential for everything from high-efficiency solid-state lighting and photovoltaics to high-power and temperature electronics1, 2, 3. In particular, the InGaN ternary alloy is of interest for solid-state lighting and photovoltaics because of the ability to tune the direct bandgap of this material from the near-ultraviolet to the near-infrared region. In an effort to synthesize InGaN nitride, researchers have tried many growth techniques4, 5, 6, 7, 8, 9, 10,11, 12, 13. Nonetheless, there remains considerable difficulty in making high-quality InGaN films and/or freestanding nanowires with tunability across the entire range of compositions. Here we report for the first time the growth of single-crystalline InxGa1-xN nanowires across the entire compositional range from x=0 to 1; the nanowires were synthesized by low-temperature halide chemical vapour deposition9and were shown to have tunable emission from the near-ultraviolet to the near-infrared region. We propose that the exceptional composition tunability is due to the low process temperature and the ability of the nanowire morphology to accommodate strain-relaxed growth14, which suppresses the tendency toward phase separation that plagues the thin-film community.

[pdf] [SI]

(151)
“Platinum Nanoparticle Shape Effects on Benzene Hydrogenation Selectivity”,

K. M. Bratlie, H. Lee, K. Komvopoulos, P. Yang, G. A. Somorjai, Nano Lett. 7, 3097, 2007.

Benzene hydrogenation was investigated in the presence of a surface monolayer consisting of Pt nanoparticles of different shapes (cubic and cuboctahedral) and tetradecyltrimethylammonium bromide (TTAB). Infrared spectroscopy indicated that TTAB binds to the Pt surface through a weak C−H···Pt bond of the alkyl chain. The catalytic selectivity was found to be strongly affected by the nanoparticle shape. Both cyclohexane and cyclohexene product molecules were formed on cuboctahedral nanoparticles, whereas only cyclohexane was produced on cubic nanoparticles. These results are the same as the product selectivities obtained on Pt(111) and Pt(100) single crystals in earlier studies. The apparent activation energy for cyclohexane production on cubic nanoparticles is 10.9 ± 0.4 kcal/mol, while for cuboctahedral nanoparticles, the apparent activation energies for cyclohexane and cyclohexene production are 8.3 ± 0.2 and 12.2 ± 0.4 kcal/mol, respectively. These activation energies are lower, and corresponding turnover rates are three times higher than those obtained with single-crystal Pt surfaces.

[pdf]

(150)
“One-step polyol synthesis and langmuir-blodgett monolayer formation of size-tunable monodisperse rhodium nanocrystals with catalytically active (111) surface structures”,

Y. Zhang, M. Grass, S. Habas, F. Tao, T. Zhang, P. Yang, G. Somorjai, J. Phys.Chem. C, 111, 12243, 2007.

Size-tunable monodisperse Rh nanocrystals can offer unique properties for many heterogeneous catalytic reactions (such as hydrogenation, hydroformylation, and hydrocarbonylation) of both scientific and technological interest. In this article, we report the synthesis of monodisperse, well-shaped Rh nanocrystals in a range of 5−15 nm by a one-step polyol reduction at temperatures of 170−230 °C under Ar, using rhodium(III) acetylacetonate [Rh(acac)3] as the source of metal ions, 1,4-butanediol as the reducing solvent, and poly(vinylpyrrolidone) as the capping agent. Two-dimensional projects of the nanocrystals are polygons, dominated by hexagons, pentagons, and triangles with catalytically active (111) surfaces (>65% yield). Over 45% of the polygons are multiple (111) twinned particles (hexagons and pentagons), favored by thermodynamics. To achieve size uniformity, adjustment of the reduction kinetics of Rh(acac)3 in the nucleation and crystal growth stages has been shown to depend upon several synthetic parameters including an Ar or air atmosphere, reaction temperature and time, and Rh(acac)3 concentration. Due to the present well-controlled polyol reduction kinetics, the size of the Rh nanocrystals can be tuned by changing the Rh(acac)3 concentration in a proper range. Monolayer films of the Rh polygons have been formed on silicon wafers by the Langmuir−Blodgett method and have been used as model heterogeneous catalysts for the study of ethylene hydrogenation.

[pdf]

(149)
“Shape, size and assembly control of PbTe nanocrystals”,

T. Moraki, M. Zhang, P. Yang, J. Am. Chem. Soc. 129, 9864, 2007.

In this communication, we demonstrate an approach for shape control of PbTe nanocrystals. We succeeded in synthesizing three different shapes of PbTe nanoparticles, including cubes, cuboctahedra, and octahedra. These morphologies were prepared by changing the molar ratio between the Pb and Te precursors and also by changing the surfactant. Langmuir−Blodgett films were prepared using these PbTe nanocrystals.

[pdf]

(148)
“Shaping binary metal nanocrystals through epitaxial seeded growth”,

S. Habas, H. Lee, V. Radmilovic,  G. Somorjai, P. Yang, Nature Mater. 6, 692, 2007.

Morphological control of nanocrystals has become increasingly important, as many of their physical and chemical properties are highly shape dependent. Nanocrystal shape control for both single- and multiple-material systems, however, remains empirical and challenging. New methods need to be explored for the rational synthetic design of heterostructures with controlled morphology. Overgrowth of a different material on well-faceted seeds, for example, allows for the use of the defined seed morphology to control nucleation and growth of the secondary structure. Here, we have used highly faceted cubic Pt seeds to direct the epitaxial overgrowth of a secondary metal. We demonstrate this concept with lattice-matched Pd to produce conformal shape-controlled core–shell particles, and then extend it to lattice-mismatched Au to give anisotropic growth. Seeding with faceted nanocrystals may have significant potential towards the development of shape-controlled heterostructures with defined interfaces.

[pdf] [SI]

(147)
“Very high frequency silicon nanowire electromechanical resonators”,

X. Feng, R. He, P. Yang, M. Roukes, Nano Lett. 7, 1953, 2007.

We demonstrate very high frequency (VHF) nanomechanical resonators based upon single-crystal silicon nanowires (SiNWs), which are prepared by the bottom-up chemical synthesis. Metallized SiNW resonators operating near 200 MHz are realized with quality factor Q ≈ 2000−2500. Pristine SiNWs, with fundamental resonances as high as 215 MHz, are measured using a VHF readout technique that is optimized for these high resistance devices. The pristine resonators provide the highest Q‘s, as high as Q ≈ 13 100 for an 80 MHz device. SiNWs excel at mass sensing; characterization of their mass responsivity and frequency stability demonstrates sensitivities approaching 10 zeptograms. These SiNW resonators offer significant potential for applications in resonant sensing, quantum electromechanical systems, and high frequency signal processing.

[pdf]

(146)
“Tunable plasmonic lattices of silver nanocrystals”,

A. R. Tao, P. Sinsermsuksakul, P. Yang, Nature Nanotech., 2, 435, 2007.

Silver nanocrystals are ideal building blocks for plasmonic materials that exhibit a wide range of unique and potentially useful optical phenomena. Individual nanocrystals display distinct optical scattering spectra and can be assembled into hierarchical structures that couple strongly to external electromagnetic fields. This coupling, which is mediated by surface plasmons, depends on the shape and arrangement of the nanocrystals. Here we demonstrate the bottom-up assembly of polyhedral silver nanocrystals into macroscopic two-dimensional superlattices using the Langmuir–Blodgett technique. Our ability to control interparticle spacing, density and packing symmetry allows for tunability of the optical response over the entire visible range. This assembly strategy offers a new, practical approach to making novel plasmonic materials for application in spectroscopic sensors, subwavelength optics and integrated devices that utilize field-enhancement effects.

[pdf] [SI] [Cover Highlight]

(145)
“Tunable nanowire nonlinear optical probe”,

Y. Nakayama*, P.  J. Pauzauskie*, A. Radenovic*, R. M. Onorato*, R. J. Saykally, J. Liphardt,  P. Yang, Nature, 447, 1098, 2007.

One crucial challenge for subwavelength optics has been the development of a tunable source of coherent laser radiation for use in the physical, information and biological sciences that is stable at room temperature and physiological conditions. Current advanced near-field imaging techniques using fibre-optic scattering probes1, 2 have already achieved spatial resolution down to the 20-nm range. Recently reported far-field approaches for optical microscopy, including stimulated emission depletion3, structured illumination4, and photoactivated localization microscopy5, have enabled impressive, theoretically unlimited spatial resolution of fluorescent biomolecular complexes. Previous work with laser tweezers6, 7, 8 has suggested that optical traps could be used to create novel spatial probes and sensors. Inorganic nanowires have diameters substantially below the wavelength of visible light and have electronic and optical properties9, 10 that make them ideal for subwavelength laser and imaging technology. Here we report the development of an electrode-free, continuously tunable coherent visible light source compatible with physiological environments, from individual potassium niobate (KNbO3) nanowires. These wires exhibit efficient second harmonic generation, and act as frequency converters, allowing the local synthesis of a wide range of colours via sum and difference frequency generation. We use this tunable nanometric light source to implement a novel form of subwavelength microscopy, in which an infrared laser is used to optically trap and scan a nanowire over a sample, suggesting a wide range of potential applications in physics, chemistry, materials science and biology.

[pdf] [SI] [Cover Highlight]

(144)
“Interfacing silicon nanowires with mammalian cells”,

W. Kim, J.  K. Ng, M.  E. Kunitake, B.  R. Conklin, P. YangJ. Am. Chem. Soc129, 7228, 2007.

We present the first demonstration of a direct interface of silicon nanowires with mammalian cells such as mouse embryonic stem (mES) cells and human embryonic kidney (HEK 293T) cells without any external force. The cells were cultured on a silicon (Si) substrate with a vertically aligned SiNW array on it. The penetration of the SiNW array into individual cells naturally occurred during the incubation. The cells survived up to several days on the nanowire substrates. The longevity of the cells was highly dependent on the diameter of SiNWs. Furthermore, successful maintenance of cardiac myocytes derived from mES cells on the wire array substrates was observed, and gene delivery using the SiNW array was demonstrated. Our results suggest that the nanowires can be potentially utilized as a powerful tool for studying intra- and intercellular biological processes.

[pdf]

(143)
"Synthesis and Thermoelectrical Characterization of Lead Chalcogenide Nanowires",

M. Fardy*, A. I. Hochbaum*, J. Goldberger, P. Yang,  Adv. Mater. 19, 3047, 2007.

Single-crystalline arrays of PbS, PbSe, and PbTe nanowires (Figure: PbS) with diameters ranging from 40-200 nm and lengths up to 100 μm have been synthesized by a chemical vapor transport approach. Electrical and thermal characterization was performed to investigate their potential as thermoelectric materials. Compared to bulk, the nanowires exhibit reduced thermal conductivity below 100 K by up to 3 orders of magnitude, suggesting that they may be promising thermoelectric materials.

[pdf]

(142)
“Microfabricated Monolithic Multinozzle Emitters for Nanoelectrospray Mass Spectrometry”,

W. Kim, M. Guo, P. Yang, D. Wang, Anal. Chem. 79, 3703, 2007.

Mass spectrometry is the enabling technology for proteomics. To fully realize the enormous potential of labon-a-chip in proteomics, a major advance in interfacing microfluidics with mass spectrometry is needed. Here, we report the first demonstration of monolithic integration of multinozzle electrospray emitters with a microfluidic channel via a novel silicon microfabrication process. These microfabricated monolithic multinozzle emitters (M3 emitters) can be readily mass-produced from silicon wafers. Each emitter consists of a parallel silica nozzle array protruding out from a hollow silicon sliver with a conduit size of 100  10 ím. The dimension and number of freestanding nozzles can be systematically and precisely controlled during the fabrication process. Once integrated with a mass spectrometer, M3 emitters achieved sensitivity and stability in peptide and protein detection comparable to those of commercial silica-based capillary nanoelectrospray tips. These M3 emitters may play a role as a critical component in a fully integrated silicon/silica-based micro total analysis system for proteomics.

[pdf]

(141)
“Growth and Electrical Characteristics of Platinum Nanoparticle Catalyzed Silicon Nanowires”,

E. Garnett, W. Liang, P. Yang, Adv. Mater. 19, 2946, 2007.

Pt nanoparticle catalysts are used to synthesize Si nanowires. The standard deviations of the starting colloid and the resulting wire diameters are essentially the same, whereas the wires were 22 % larger than the particles. The figure shows current–voltage plots of a silicon nanowire different gate voltages.

[pdf]

(140)
"Suspended Mechanical Structures Based on Elastic Silicon Nanowire Arrays",

A. San Paulo, N. Arellano, R. He, C. Carraro, R. Maboudian, R. Howe, J. Bokor, P. Yang, Nano Lett. 7, 1100, 2007.

We demonstrate a bottom-up/top-down combined method for the fabrication of horizontally suspended, well-oriented and size-controlled Si nanowire arrays. Mechanical beamlike structures composed of multiple ordered arrays consecutively linked by transversal microspacers are obtained by this method. Such structures are used to investigate the mechanical elasticity of the nanowire arrays by atomic force microscopy. Our results point out important differences in the morphology and mechanical behavior of the fabricated nanowire-based structures with respect to equivalent bulk material structures.

[pdf]

(139)
“Probing the local coordination environment for transition metal dopants in zinc oxide”,

B. D. Yuhas, S. Fakra, M. A. Marcus, P. Yang, Nano Lett. 7, 905, 2007.

It is hypothesized that a highly ordered, relatively defect-free dilute magnetic semiconductor system should act as a weak ferromagnet. Transition-metal-doped ZnO nanowires, being single crystalline, single domain, and single phase, are used here as a model system for probing the local dopant coordination environments using X-ray absorption spectroscopy and diffraction. Our X-ray spectroscopic data clearly show that the dopant resides in a uniform environment, and that the doping does not induce a large degree of disorder in the nanowires. This homogeneous nature of the doping inside the oxide matrix correlates well with observed weakly ferromagnetic behavior of the nanowires.

[pdf]

(138)
"High-brightness gallium nitride nanowire UV-blue light emitting diodes",

S. Lee, T.Kim , K. Choi, S. Lee, P. Yang, Phil. Mag. 87, 2105, 2007.

We report on high-brightness GaN nanowire UV-blue light emitting diodes (LEDs), which are fabricated by coupling of n-GaN nanowires and p-GaN substrates using two assembly methods, random dispersion (RD) and dielectrophoresis assisted assembly deposition (DAAD). These GaN nanowire LEDs have bright UV-blue emission (411-437 nm) from the n-GaN nanowire/p-GaN substrate junction and the light emission is strong enough to be observed with the naked eye even for a single GaN nanowire LED. The results reported here should have significant implications for the fabrication of highly efficient, low-cost UV-blue LEDs with low power consumption, as compared to conventional thin-film based GaN LEDs.

[pdf]

(137)
“One step patterning of aligned nanowire arrays by programmed dip coating”,

J. Huang, R. Fan, S. Connor, P. Yang, Angew Chem Int Ed. 46, 2414, 2007.

Dip, stick, slip: Dip coating, a widely used industrial process for making thin films, is used to align and position nanowires by means of the stick–slip motion of the solvent meniscus. Nanowire arrays with predefined spacing can be readily “printed” on a large substrate with tunable wire density (see picture), thus providing a facile method for producing nanowire-based devices.

[pdf]

(136)
“Multi-functional Nanowire Evanescent Wave Optical Sensors”,

D. J. Sirbuly, A. R. Tao, M. Law, R. Fan, P. Yang, Adv. Mater. 19, 66, 2007.

A photonic sensing platform that utilizes the evanescent field of a subwavelength nanowire waveguide to perform optical spectroscopy on femtoliter probe volumes is demonstrated. Each evanescent sensor is capable of carrying out absorbance, fluorescence, and surface-enhanced Raman spectroscopy measurements on the same analyte while operating within a microfluidic flow cell (see figure).

[pdf]

2006

(135)
“Propagation of guided modes in curved nanoribbon waveguides",

Z. Ye, X. Hu, M. Li, K. M. Ho, P. Yang, Appl. Phys. Lett., 89, 241108, 2006.

The authors develop a plane-wave-based transfer matrix method in curvilinear coordinates to study the guided modes in curved nanoribbon waveguides. The problem of a curved structure is transformed into an equivalent one of a straight structure with spatially dependent tensors of dielectric constant and magnetic permeability. The authors investigate the coupling between the eigenmodes of the straight part and those of the curved part when the waveguide is bent. The authors show that curved sections can result in strong oscillations in the transmission spectrum similar to the recent experimental results of Lawet al. [Science 305, 1269 (2004)] .

[pdf]

(134)
“Probing the interaction of poly(vinylpyrrolidone) with platinum nanocrystals by UV-Raman and FTIR”,

Borodko Y, Habas SE, Koebel M, P. Yang, F. Heinz, G. Somorjai, J. Phys. Chem. B, 110 (46): 23052-23059, 2006.

The vibrational spectra of platinum nanoparticles (2.4−9 nm) capped with poly(N-vinylpyrrolidone) (PVP) were investigated by deep UV−Raman and FTIR spectroscopy and compared with those of pure PVP. Raman spectra of PVP/Pt show selective enhancement of C=O, C−N, and CH2 vibrational modes attributed to the pyrrolidone ring. Selective enhancement of ring vibrations is attributed both to the resonance Raman effect and SERS chemical enhancement. A red shift of the PVP carbonyl frequency on the order of 60 cm-1indicates the formation of strong >C=O−Pt bonds. It is concluded that PVP adheres to the nanoparticles through a charge-transfer interaction between the pyrrolidone rings and surface Pt atoms. Heating the Pt nanoparticles under reducing conditions initiates the decomposition of the capping agent, PVP, at a temperature 100 °C below that of pure PVP. Under oxidizing conditions, both PVP/Pt and PVP degrade to form amorphous carbon=

[pdf]

(133)
“Morphology control of catalytically active Platinum nanocrystals”,

H. Lee, S. Habas, S. Kweskin, D. Butcher, G. Somorjai, P. Yang, Angew. Chem. Int. Ed. 45, 7824, 2006.

Activity revived: Platinum nanoparticles (cuboctahedra, cubes, and porous particles, see picture, from left to right) capped with alkylammonium ions are synthesized by manipulating the reduction kinetics. The catalytic activity of these nanoparticles is superior to that of nanoparticles whose shape was controlled with a polymeric capping agent and foreign metal ions.

[pdf]

(132)
“Giant piezoresistance effect in silicon nanowires”,

R. He, P. YangNature Nanotech., 1, 42, 2006.

The piezoresistance effect of silicon1 has been widely used in mechanical sensors2, 3, 4, and is now being actively explored in order to improve the performance of silicon transistors5, 6. In fact, strain engineering is now considered to be one of the most promising strategies for developing high-performance sub-10-nm silicon devices7. Interesting electromechanical properties have been observed in carbon nanotubes8, 9. In this paper we report that Si nanowires possess an unusually large piezoresistance effect compared with bulk. For example, the longitudinal piezoresistance coefficient along the <111> direction increases with decreasing diameter for p-type Si nanowires, reaching as high as −3,550 × 10−11 Pa–1, in comparison with a bulk value of −94 × 10−11 Pa−1. Strain-induced carrier mobility change and surface modifications have been shown to have clear influence on piezoresistance coefficients. This giant piezoresistance effect in Si nanowires may have significant implications in nanowire-based flexible electronics, as well as in nanoelectromechanical systems.

[pdf]

(131)
"ZnO-Al2O3 and ZnO-TiO2 core-shell nanowire dye-sensitized solar cells",

M. Law, L. E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt, P. Yang, J. Phys. Chem. B, 110, 22652, 2006.

We describe the construction and performance of dye-sensitized solar cells (DSCs) based on arrays of ZnO nanowires coated with thin shells of amorphous Al(2)O(3) or anatase TiO(2) by atomic layer deposition. We find that alumina shells of all thicknesses act as insulating barriers that improve cell open-circuit voltage (V(OC)) only at the expense of a larger decrease in short-circuit current density (J(SC)). However, titania shells 10-25 nm in thickness cause a dramatic increase in V(OC) and fill factor with little current falloff, resulting in a substantial improvement in overall conversion efficiency, up to 2.25% under 100 mW cm(-2) AM 1.5 simulated sunlight. The superior performance of the ZnO-TiO(2) core-shell nanowire cells is a result of a radial surface field within each nanowire that decreases the rate of recombination in these devices. In a related set of experiments, we have found that TiO(2) blocking layers deposited underneath the nanowire films yield cells with reduced efficiency, in contrast to the beneficial use of blocking layers in some TiO(2) nanoparticle cells. Raising the efficiency of our nanowire DSCs above 2.5% depends on achieving higher dye loadings through an increase in nanowire array surface area.

[pdf]

(130)
“Nanowire Photonics”,

P. Pauzauskie, P. Yang, Materials Today, 9, 36, 2006.

The development of integrated electronic circuitry ranks among the most disruptive and transformative technologies of the 20th century. Even though integrated circuits are ubiquitous in modern life, both fundamental and technical constraints will eventually test the limits of Moore’s law. Nanowire photonic circuitry constructed from myriad onedimensional building blocks offers numerous opportunities for the development of next-generation optical information processors and spectroscopy. However, several challenges remain before the potential of nanowire building blocks is fully realized. We cover recent advances in nanowire synthesis, characterization, lasing, integration, and the eventual application to relevant technical and scientific questions.

[pdf]

(129)
“Monodisperse platinum nanoparticles of well-defined shape: Synthesis, characterization, catalytic properties and future prospects”,

R. M. Rioux, H. Song, M. Grass, S. Habas, K. Niesz, J. D. Hoefelmeyer, P. Yang, G. A. Somorjai, Topic Cata. 39 (3-4), 167 2006.

Monodisperse platinum nanoparticles with well-defined faceting have been synthesized by a modified polyol process with the addition of silver ions. Pt nanoparticles are encapsulated in mesoporous silica during in situ hydrothermal growth of the high surface area support. Removal of the surface regulating polymer, poly(vinylpyrrolidone), was achieved using thermal oxidation-reduction treatments. Catalysts were active for ethylene hydrogenation after polymer removal. Rates for ethylene hydrogenation decreased in accordance with the amount of Ag retained in the Pt nanoparticles after purification. Ag is most likely present on the Pt particle surface as small clusters. Future prospects for these catalysts for use in low temperature selective hydrogenation reactions are discussed.

[pdf]

(128)
“Solution Grown Zinc Oxide Nanowires”,

L. Greene, B. Yuhas, M. Law, P. Yang, Inorg. Chem. 45, 7535, 2006.

We review two strategies for growing ZnO nanowires from zinc salts in aqueous and organic solvents. Wire arrays with diameters in the nanoscale regime can be grown in an aqueous solution of zinc nitrate and hexamethylenetetramine. With the addition of poly(ethylenimine), the lengths of the wires have been increased to 25 μm with aspect ratios over 125. Additionally, these arrays were made vertical by nucleating the wires from oriented ZnO nanocrystals. ZnO nanowire bundles have been produced by decomposing zinc acetate in trioctylamine. By the addition of a metal salt to the solution, the ZnO wires can be doped with a range of transition metals. Specifically, ZnO nanowires were homogeneously doped with cobalt and showed a marked deviation from paramagnetic behavior. We conclude by highlighting the use of these solution-grown nanowire arrays in dye-sensitized solar cells. The nanowire cells showed an improvement in the charge collection efficiency over traditional nanoparticle cells.

[pdf]

(127)
“Electrical Characteristics and Chemical Stability of Non-Oxidized, Methyl-Terminated Silicon Nanowires”,

H. Haick, P. T. Hurley, A. Hochbaum, P. Yang, N. S. Lewis, J. Am. Chem. Soc. 128, 8990, 2006.

Silicon nanowires (Si NWs) modified by covalent Si−CH3 functionality, with no intervening oxide, show atmospheric stability, high conductance values, low surface defect levels, and allow for the formation of air-stable Si NW Field-Effect Transistors (FETs) having on−off ratios in excess of 105 over a relatively small gate voltage swing (±2 V).

[pdf]

(126)
“Polyhedral Silver Nanocrystals with Distinct Scattering Signatures” ,

A. R. Tao, P. Sinsermsuksakul, P. Yang, Angew Chem. Int. Ed. 45, 4597, 2006.

A scattering of silver: Polyhedral silver nanocrystals display complex and distinct scattering signatures dictated by their shape and size (see picture). The ability to engineer specific plasmon modes should have profound consequences for surface-enhanced Raman spectroscopy, subwavelength optics, and plasmonic transport.

[pdf]

(125)
"Characterization of Heat Transfer Along a Silicon Nanowire Using Thermoreflectance Technique",

Y. Zhang , J. Christofferson, A. Shakouri, D. Li, A. Majumdar, Y. Wu, R. Fan, P. Yang, IEEE Transactions on Nanotechnology, 5, 67, 2006.

We studied heat transfer along a silicon nanowire suspended between two thin-film heaters using a thermoreflectance imaging technique. The thermoreflectance imaging system achieved submicrometer spatial resolution and 0.1°C temperature resolution using visible light. The temperature difference across the nanowire was measured, and then its thermal resistance was calculated. Knowing the dimension of the nanowire (115 nm in width and 3.9 μm in length), we calculated the thermal conductivity of the sample, which is 46 W/mK. Thermal conductivity decreases with decreasing wire size. For a 115-nm-wide silicon nanowire, the thermal conductivity is only one-third of the bulk value. In addition, the transient response of the thin-film heaters was also examined using three-dimensional thermal models by the ANSYS program. The simulated thermal map matches well with the experimental thermoreflectance results.

[pdf]

(124)
“Electrostatics of nanowire transistors with triangular cross sections”,

D. Vashaee, A. Shakouri, J. Goldberger, T. Kuykendall, P. Pauzauskie, P. Yang, J. Appl. Phys. 99, 054310, 2006.

The electrostatic properties of nanowire field effect transistors with triangular cross sections were investigated. The Poisson equation was solved for these structures; furthermore, two properties of the nanowire field effect transistors, the gate capacitance and current versus gate voltage, were calculated. The simulation results yielded the type, mobility, and concentration of the carriers, as well as the Ohmic contact resistance of the wire transistor. We examined how wire capacitance depends on various parameters: wire diameter, gate oxide thickness, charge density, and shape. It is shown that the capacitance of a triangular nanowire is less than that of a cylindrical nanowire of the same size, which could be significant in structures with thin gate oxides. The simulation results were compared with the previously reported experimental data on GaN nanowires.

[pdf]

(123)
“Semiconductor Nanowire Ring Resonator Laser”,

P. Pauzauskie, D. Sirbuly, P. Yang, Phys. Rev. Lett. 96, 143903, 2006.

Nanowires of the wide band-gap semiconductor gallium nitride (GaN) have been shown to act as room-temperature uv lasers. Recent advances in nanomanipulation have made it possible to modify the shape of these structures from a linear to a pseudoring conformation. Changes to the optical boundary conditions of the lasing cavity affect the structure’s photoluminescence, photon confinement, and lasing as a function of ring diameter. For a given cavity, ring-mode redshifting is observed to increase with decreasing ring diameter. Significant shifts, up to 10 nm for peak emission values, are observed during optical pumping of a ring resonator nanolaser compared to its linear counterpart. The shifting appears to result from conformational changes of the cavity rather than effects such as band-gap renormalization, allowing the mode spacing and position to be tuned with the same nanowire gain medium.

[pdf]

(122)
"Silicon Vertically Integrated Nanowire Field Effect Transistors",

J. Goldberger, A. Hochbaum, R. Fan, P. Yang, Nano Lett. 6, 973, 2006.

Silicon nanowires have received considerable attention as transistor components because they represent a facile route toward sub-100-nm single-crystalline Si features. Herein we demonstrate the direct vertical integration of Si nanowire arrays into surrounding gate field effect transistors without the need for postgrowth nanowire assembly processes. The device fabrication allows Si nanowire channel diameters to be readily reduced to the 5-nm regime. These first-generation vertically integrated nanowire field effect transistors (VINFETs) exhibit electronic properties that are comparable to other horizontal nanowire field effect transistors (FETs) and may, with further optimization, compete with advanced solid-state nanoelectronic devices.

[pdf]

(121)
"A general method for assembling single colloidal particle lines.",

J. Huang, A. Tao, S. Connor, P. Yang, Nano Lett. 6, 524, 2006.

We have developed a general method for assembling colloidal particles into one-dimensional lines of single particle thickness. Well-spaced, parallel single particle lines can be readily deposited on a substrate from a dilute Langmuir−Blodgett particle monolayer via a stick−slip motion of the water−substrate contact line. The particle density within the lines is controllable by the particle concentration in the monolayer as well as the pulling speed of the substrate. Lines of a great variety of materials and sizes, ranging from a few nanometers to a few micrometers, have been demonstrated. Multiple depositions create complex patterns such as cross lines, even of different particles. The ability of placing nanoparticles into one-dimensional arrays enables the construction of higher hierarchical device structures. For example, using gold nanoparticle seeds, vertical single nanowire arrays of silicon can be grown replicating the pattern of single particle lines.

[pdf]

(120)
“Hydrothermal growth of mesoporous SBA-15 silica in the presence of PVP-stabilized Pt nanoparticles: Synthesis, characterization and catalytic properties”,

H. Song, R. M. Rioux, J. D. Hoefelmeyer, R. Komor, K. Niesz, M. Grass, P. Yang, G. A. Somorjai, J. Am. Chem. Soc. 128, 3027, 2006.

A novel high surface area heterogeneous catalyst based on solution phase colloidal nanoparticle chemistry has been developed. Monodisperse platinum nanoparticles of 1.7−7.1 nm have been synthesized by alcohol reduction methods and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. Characterization of the Pt/SBA-15 catalysts suggests that Pt particles are located within the surfactant micelles during silica formation leading to their dispersion throughout the silica structure. After removal of the templating polymer from the nanoparticle surface, Pt particle sizes were determined from monolayer gas adsorption measurements. Infrared studies of CO adsorption revealed that CO exclusively adsorbs to atop sites and red-shifts as the particle size decreases suggesting surface roughness increases with decreasing particle size. Ethylene hydrogenation rates were invariant with particle size and consistent with a clean Pt surface. Ethane hydrogenolysis displayed significant structure sensitivity over the size range of 1−7 nm, while the apparent activation energy increased linearly up to a Pt particle size of ~4 nm and then remained constant. The observed rate dependence with particle size is attributed to a higher reactivity of coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in large particles. The most reactive of these unsaturated surface atoms are responsible for ethane decomposition to surface carbon. The ability to design catalytic structures with tunable properties by rational synthetic methods is a major advance in the field of catalyst synthesis and for the development of accurate structure−function relationships in heterogeneous reaction kinetics.

[pdf]

(119)
“Synthesis of High Density, Size Controlled Si Nanowire Arrays via Porous Anodic Alumina Mask”,

I. Lombardi, A. I. Hochbaum, P. Yang, C. Carraro, R. Maboudian, Chem. Mater. 18, 988, 2006.

Porous anodic alumina (PAA) masks are employed for the template synthesis of high density silicon nanowire (SiNW) arrays on a silicon substrate by the vapor−liquid−solid method. The uniform-sized ordered arrays of nanopores in the PAA mask anchored to Si(111) substrate are shown to enable the realization of vertically aligned epitaxial SiNWs with uniform diameter and spacing. The average diameter of the wires is 72 nm while the density is 60 wires/μm2. The high packing density and tightly controlled dimensions of SiNWs obtained by this nonlithographic method allow their effective integration into nanodevices for mass production.

[pdf]

(118)
“Optical Trapping and Integration of Semiconductor Nanowire Assemblies in Water”,

P. Pauzauskie, A. Radenovic, E. Trepagnier , H. Shroff, P. Yang, J. Liphardt, Nature. Mater. 5, 97, 2006.

Semiconductor nanowires have received much attention owing to their potential use as building blocks of miniaturized electrical1, nanofluidic2and optical devices3. Although chemical nanowire synthesis procedures have matured and now yield nanowires with specific compositions4 and growth directions5, the use of these materials in scientific, biomedical and microelectronic applications is greatly restricted owing to a lack of methods to assemble nanowires into complex heterostructures with high spatial and angular precision. Here we show that an infrared single-beam optical trap can be used to individually trap, transfer and assemble high-aspect-ratio semiconductor nanowires into arbitrary structures in a fluid environment. Nanowires with diameters as small as 20 nm and aspect ratios of more than 100 can be trapped and transported in three dimensions, enabling the construction of nanowire architectures that may function as active photonic devices. Moreover, nanowire structures can now be assembled in physiological environments, offering new forms of chemical, mechanical and optical stimulation of living cells.

[pdf]

(117)
"Inorganic Nanotubes: A Novel Platform for Nanofluidics",

J. Goldberger, R. Fan, P. Yang, Acct. Chem. Res. 39,239, 2006.

Templating approaches are being developed for the synthesis of inorganic nanotubes, a novel platform for nanofluidics. Single crystalline semiconductor GaN nanotubes have been synthesized using an epitaxial casting method. The partial thermal oxidation of silicon nanowires leads to the synthesis of silica nanotubes. The dimension of these nanotubes can be precisely controlled during the templating process. These inorganic nanotubes can be integrated into metal−oxide solution field effect transistors (MOSolFETs), which exhibit rapid field effect modulation of ionic conductance. These nanofluidic devices have been further demonstrated to be useful for single-molecule sensing, as single DNA molecules can be readily detected either by charge effect or by geometry effect. These inorganic nanotubes will have great implications in subfemtoliter analytical technology and large-scale nanofluidic integration.

[pdf]

(116)
“Transition metal doped zinc oxide nanowires”,

B. D. Yuhas, D. O. Zitoun, P. J. Pauzauskie, R. He, P. Yang, Angew. Chem. Int. Ed. 45,420, 2006.

Bring in the substitute! Zn1−xCoxO nanowires are prepared by using a solution-based synthetic route. Structural, optical, and spectroscopic characterizations indicate that the cobalt atoms substitute the zinc cations in the host lattice as a result of the cobalt doping (the picture shows prepared Co-doped ZnO nanowires).

[pdf]

2005

(115)
“Synthesis of Bifunctional Polymer Nanotubes from Silicon Nanowire Templates via Atom Transfer Radical Polymerization”,

M. J. Mulvihill, B. L. Rupert, R. He, Al. Hochbaum, J. Arnold, P. Yang, J. Am. Chem. Soc. 127, 16040, 2005.

As a way to control the surface properties of nanowires and nanotubes, we present a method for growing polymer from the surface of silicon/silica core/shell nanowires. After modification of nanowire surfaces with polymer initiators, Atom Transfer Radical Polymerization (ATRP) was used to grow methacrylate polymer chains from the surface. The resulting structures were characterized by SEM, TEM, and EELS. After etching the silicon cores, the resulting polymer-coated nanotubes will have hydrophilic silica cores with hydrophobic polymer shells.

[pdf]

(114)
“Polarized Raman Confocal Microscopy of Single Gallium Nitride Nanowires”,

P. J. Pauzauskie, D. Talaga, K. Seo, P. Yang, F. Lagugné-Labarthet, J. Am. Chem. Soc. 127(49); 17146-17147.

Polarized Raman spectra and corresponding Raman scattering intensity images of an isolated gallium nitride nanowire with a diameter of 170 nm are presented. The sensitivity of the confocal microscope combined with a high-resolution piezoelectric stage enables analysis of the crystalline phase and crystallographic orientation of an individual nanowire with an excellent spatial and spectral resolution in a short acquisition time.

[pdf]

(113)
“Spontaneous formation of nanoparticlestripe patterns through dewetting”,

J. Huang*, F. Kim*, A. Tao*, S. Conner, P. Yang, Nature Mater, 4,896, 2005.

Significant advancement has been made in nanoparticle research, with synthetic techniques extending over a wide range of materials with good control over particle size and shape1, 2, 3,4, 5, 6. A grand challenge is assembling and positioning the nanoparticles in desired locations to construct complex, higher-order functional structures. Controlled positioning of nanoparticles has been achieved in pre-defined templates fabricated by top–down approaches7, 8. A self-assembly method, however, is highly desirable because of its simplicity and compatibility with heterogeneous integration processes. Here we report on the spontaneous formation of ordered gold and silver nanoparticle stripe patterns on dewetting a dilute film of polymer-coated nanoparticles floating on a water surface. Well-aligned stripe patterns with tunable orientation, thickness and periodicity at the micrometre scale were obtained by transferring nanoparticles from a floating film onto a substrate in a dip-coating fashion. This facile technique opens up a new avenue for lithography-free patterning of nanoparticle arrays for various applications including, for example, multiplexed surface-enhanced Raman substrates and templated fabrication of higher-order nanostructures.

[pdf]

(112)
“Mechanical Elasticity of Single and Double Clamped Silicon Nanobeams”,

Alvaro San Paulo, R. He, D. Gao, C. Carraro, J. Bokor, R. T. Howe, R. Maboudian, P. Yang, Appl. Phys. Lett. 87, 053111, 2005.

Atomic force microscopy has been used to characterize the mechanical elasticity of Si nanowires synthesized by the vapor-liquid-solid method. The nanowires are horizontally grown between the two facing Si(111) sidewalls of microtrenches prefabricated on a Si(110) substrate, resulting in suspended single and double clamped nanowire-in-trench structures. The deflection of the nanowires is induced and measured by the controlled application of normal forces with the microscope tip. The observed reversibility of the nanowire deflections and the agreement between the measured deflection profiles and the theoretical behavior of single and double clamped elastic beams demonstrate the overall beamlike mechanical behavior and the mechanical rigidity of the clamping ends of the nanowire-in-trench structures. These results demonstrate the potential of the nanowire-in-trench fabrication approach for the integration of VLS grown nanostructures into functional nanomechanical devices.

[pdf]

(111)
“Polarized surface enhanced Raman Spectroscopy on coupled metallic nanowires”,

A. R. Tao, P. Yang, J. Phys. Chem. B 109, 15687, 2005.

Regular-shaped metal nanocrystals and their ensembles can serve as ideal substrates for studying surface-enhanced Raman scattering (SERS). We synthesized well-defined silver nanowires for a systematic study of SERS signal with respect to polarization and structural ordering. The observed dependence on polarization direction confirms prior theoretical predictions that large electromagnetic (EM) fields are localized in the interstitials between adjacent nanowires. We show that these modes are largely dipolar in nature and rely on short-range EM coupling between nanowires.

[pdf]

(110)
“Polarity switching and transient responses in single nanotube nanofluidic transistors”,

R. Fan, R. Karnik, M. Yue, A. Majumdar, P. Yang, Phys. Rev. Lett., 95,086607, 2005.

We report the integration of inorganic nanotubes into metal-oxide-solution field effect transistors (FETs) which exhibit rapid field effect modulation of ionic conductance. Surface functionalization, analogous to doping in semiconductors, can switch the nanofluidic transistors from p-type toambipolar and n-type field effect transistors. Transient study reveals the kinetics of field effect modulation is controlled by ion-exchange step. Nanofluidic FETs have potential implications in subfemtoliter analytical technology and large-scale nanofluidic integration.

[pdf]

(109)
“Effect of biological reactions and modifications on conductance of nanofluidic channels”,

R. Karnick, Castelino, R. Fan, P. Yang, A. Majumdar, Nano Lett. 5,1638, 2005.

Conductance characteristics of nanofluidic channels (nanochannels) fall into two regimes:  at low ionic concentrations, conductance is governed by surface charge while at high ionic concentrations it is determined by nanochannel geometry and bulk ionic concentration. We used aminosilane chemistry and streptavidin−biotin binding to study the effects of surface reactions on nanochannel conductance at different ionic concentrations. Immobilization of small molecules such as aminosilane or biotin mainly changes surface charge, affecting conductance only in the low concentration regime. However, streptavidin not only modifies surface charge but also occludes part of the channel, resulting in observable conductance changes in both regimes. Our observations reflect the interplay between the competing effects of charge and size of streptavidin on nanochannel conductance.

[pdf]

(108)
“DNA translocation in inorganic nanotubes”,

R. Fan, R. Karnik, M. Yue, D. Li, A. Majumdar, P. Yang, Nano Lett., 5, 1633, 2005.

Inorganic nanotubes were successfully integrated with microfluidic systems to create nanofluidic devices for single DNA molecule sensing. Inorganic nanotubes are unique in their high aspect ratio and exhibit translocation characteristics in which the DNA is fully stretched. Transient changes of ionic current indicate DNA translocation events. A transition from current decrease to current enhancement during translocation was observed on changing the buffer concentration, suggesting interplay between electrostatic charge and geometric blockage effects. These inorganic nanotube nanofluidic devices represent a new platform for the study of single biomolecule translocation with the potential for integration into nanofluidic circuits.

[pdf]

(107)
“General route to vertical ZnO nanowire arrays using textured ZnO seeds”,

L. Greene, M. Law, D. H. Tan, J. Goldberger, P. Yang, Nano Lett. 5, 1231, 2005.

A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A ~10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200−350 °C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod−polymer solar cells.

[pdf]

(106)
“Thermally driven interfacial dynamics of metal-oxide bilayer nanoribbons”,

X. Zhang*, M. Law*, R. Yu, T Kuykendall, P. Yang, Small, 1, 858, 2005.

Solid–solid interfacial processes greatly affect the performance of electronic and composite materials, but probing the dynamics of buried interfaces is challenging and often involves lengthy or invasive sample preparation. We show that bilayer nanoribbons—made here of tin dioxide and copper—are convenient structures for observing as-made interfaces as they respond to changing temperature in a transmission electron microscope (TEM). At low temperatures (<200 °C), differential thermal expansion causes the bilayers to bend when heated or cooled, with the motion determined by the extent of Cu–SnO2 epitaxy. At higher temperatures, we are able to watch—in real time and with nanometer resolution—a progression of grain growth, interdiffusion, island formation, solid-state chemical reactions, and melting. This novel TEM geometry is readily applicable to other nanoribbon/coating combinations and is well suited to observing interfacial phenomena driven thermally or by the application of mechanical, electrical, or magnetic forces.

[pdf]

(105)
"Optical routing and sensing with nanowire assemblies",

D. J. Sirbuly*, M. Law*, P. Pauzauskie, H. Yan, A. V. Maslov, K. Knudsen, R. J. Saykally, P. Yang, Proc. Nat. Acad. Sci., 102, 7800, 2005.

The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.

[pdf]

(104)
"Semiconductor Nanowires for Subwavelength Photonics Integration",

D. Sirbuly, M. Law, H. Yan, P. Yang, J. Phys. Chem. B. (Feature Article), 109, 15191, 2005.

This article focuses on one-dimensional (1D) semiconductor subwavelength optical elements and assesses their potential use as active and passive components in photonic devices. An updated overview of their optical properties, including spontaneous emission, ultrafast carrier dynamics, cavity resonance feedback (lasing), photodetection, and waveguiding, is provided. The ability to physically manipulate these structures on surfaces to form simple networks and assemblies is the first step toward integrating chemically synthesized nanomaterials into photonic circuitry. These high index semiconductor nanowires are capable of efficiently guiding light through liquid media, suggesting a role for such materials in microfluidics-based biosensing applications.

[pdf]

(103)
“Nanowire dye-sensitized solar cells”,

M. Law*, L. E. Greene*, J. C. Johnson, R. Saykally, P. Yang, Nature Materials, 4, 455, 2005.

Excitonic solar cells1—including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient2 and stable3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

[pdf]

(102)
“Electrostatic Control of Ions and Molecules in Nanofluidic Transistors”,

R. Karnik*, R. Fan*, M. Yue, D. Li, P. Yang, A. Majumdar, Nano Lett., 5, 943, 2005.

We report a nanofluidic transistor based on a metal-oxide-solution (MOSol) system that is similar to a metal-oxide-semiconductor field-effect transistor (MOSFET). Using a combination of fluorescence and electrical measurements, we demonstrate that gate voltage modulates the concentration of ions and molecules in the channel and controls the ionic conductance. Our results illustrate the efficacy of field-effect control in nanofluidics, which could have broad implications on integrated nanofluidic circuits for manipulation of ions and biomolecules in sub-femtoliter volumes.

[pdf]

(101)
“Si Nanowire Bridges in Trenches:Integration of Growth into Device Fabrication”,

R. He, D. Gao, R. Fan, A. I. Hochbaum, C. Carraro, R. Maboudian, P. Yang, Adv. Mater., 17, 2098, 2005.

Silicon nanowire bridges are grown in prefabricated microtrenches on (110) silicon-on-insulator wafers (see Figure). Silicon trenches are used as substrates during growth and probing electrodes after growth. This way, nanowire growth and device fabrication can be achieved simultaneously, providing a simple and rational way to realize nanowire-based integrated circuits.

[pdf]

(100)
“Single Crystalline diluted magnetic semiconductor GaN:Mn nanowires”,

H. Choi, H. Seong, J. Chang, Y. Park, J. Kim, R. He, T. Kuykendall, P. Yang, Adv. Mater., 17, 1351, 2005.

Single-crystalline diluted magnetic semiconductor GaN:Mn nanowires with controlled Mn concentrations have been successfully synthesized and incorporated into devices (see Figure). These nanowires exhibit Curie temperatures above room temperature, magnetoresistances near room temperature, and spin-dependent transport. The nanowires are used as building blocks for the fabrication of GaN:Mn/n-SiC based light-emitting diodes.

[pdf]

(99)
“Sol-gel synthesis of ordered mesoporous alumina”,

K. Niesz, P. Yang, G. A. Somorjai, Chem. Commun., 1986, 2005.

Well-ordered mesoporous alumina materials with high surface area and a narrow pore size distribution were synthesized using a sol-gel based self assembly technique.

[pdf]

(98)
“Thermal wetting of platinum nanocrystals on silica surface”,

R. Yu, H. Song, X. Zhang, P. Yang, J. Phys. Chem. B, 109, 6940, 2005.

Thermal stability of facetted Pt nanocrystals on amorphous silica support films was investigated using in situ transmission electron microscopy in a temperature range between 25 and 800 °C. The particles started to change their shapes at ~350 °C. Above 500 °C, the particles spread on the support film with increasing temperature, rather than becoming more spherical. Such temperature-induced wetting of Pt nanoparticles on silica surface can be attributed to the interfacial mixing of Pt and SiO2 and the resulting negative interface energy.

[pdf]

(97)
“The Chemistry and Physics of Semiconductor Nanowires”,

P. Yang, (Award Address) MRS Bulletin, 30, 85, Feb., 2005.

The following article is based on the Outstanding Young Investigator Award presentation given by Peidong Yang of the University of California, Berkeley, on April 14, 2004, at the Materials Research Society Spring Meeting in San Francisco.Yang was cited for “innovative synthesis of a broad range of nanowires and nanowireheterostructure materials, and the discovery of optically induced lasing in individual nanowire devices.” One-dimensional nanostructures are of both fundamental and technological interest.They not only exhibit interesting electronic and optical properties associated with their low dimensionality and the quantum confinement effect, but they also represent critical components in potential nanoscale devices. In this article, the vapor–liquid–solid crystal growth mechanism will be briefly introduced for the general synthesis of nanowires of different compositions, sizes, and orientation. Unique properties, including light-emission and thermoelectricity, will be discussed. In addition to the recent extensive studies on “single-component” nanowires, of increasing importance is incorporating different interfaces and controlling doping profiles within individual single-crystalline nanowires. Epitaxial growth plays a significant role in fabricating such nanowire heterostructures. Recent research on superlattice nanowires and other nanostructures with horizontal junctions will be presented. The implication of these heterojunction nanowires in light-emission and energy conversion will be discussed. Ways to assemble these one-dimensional nanostructures will also be presented.

[pdf]

(96)
“Controlled Growth of Si Nanowire Arrays for Device Integration”,

A. I. Hochbaum, R. He, R. Fan, P. Yang, Nano Lett. 5, 457, 2005.

Silicon nanowires were synthesized, in a controlled manner, for their practical integration into devices. Gold colloids were used for nanowire synthesis by the vapor−liquid−solid growth mechanism. Using SiCl4 as the precursor gas in a chemical vapor deposition system, nanowire arrays were grown vertically aligned with respect to the substrate. By manipulating the colloid deposition on the substrate, highly controlled growth of aligned silicon nanowires was achieved. Nanowire arrays were synthesized with narrow size distributions dictated by the seeding colloids and with average diameters down to 39 nm. The density of wire growth was successfully varied from ~0.1−1.8 wires/μm2. Patterned deposition of the colloids led to confinement of the vertical nanowire growth to selected regions. In addition, Si nanowires were grown directly into microchannels to demonstrate the flexibility of the deposition technique. By controlling various aspects of nanowire growth, these methods will enable their efficient and economical incorporation into devices.

[pdf]

(95)
“High Surface Area Catalyst Design: Synthesis, Characterization, and Reaction Studies of Platinum Nanoparticles in Mesoporous SBA-15 Silica”,

R. Rioux*, H. Song*, J. Hoefelmeyer, P. Yang, G. Somorjai, J. Phys. Chem. B, 109, 2192, 2005.

Platinum nanoparticles in the size range of 1.7−7.1 nm were produced by alcohol reduction methods. A polymer (poly(vinylpyrrolidone), PVP) was used to stabilize the particles by capping them in aqueous solution. The particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations demonstrate that the particles have a narrow size distribution. Mesoporous SBA-15 silica with 9-nm pores was synthesized by a hydrothermal process and used as a catalyst support. After incorporation into mesoporous SBA-15 silica using low-power sonication, the catalysts were calcined to remove the stabilizing polymer from the nanoparticle surface and reduced by H2. Pt particle sizes determined from selective gas adsorption measurements are larger than those determined by bulk techniques such as XRD and TEM. Room-temperature ethylene hydrogenation was chosen as a model reaction to probe the activity of the Pt/SBA-15 materials. The reaction was shown to be structure insensitive over a series of Pt/SBA-15 materials with particle sizes between 1.7 and 3.6 nm. The hydrogenolysis of ethane on Pt particles from 1.7 to 7.1 nm was weakly structure sensitive with smaller particles demonstrating higher specific activity. Turnover rates for ethane hydrogenolysis increased monotonically with increasing metal dispersion, suggesting that coordinatively unsaturated metal atoms present in small particles are more active for C2H6 hydrogenolysis than the low index planes that dominate in large particles. An explanation for the structure sensitivity is suggested, and the potential applications of these novel supported nanocatalysts for further studies of structure−activity and structure−selectivity relationships are discussed.

[pdf]

(94)
"Selective Growth of Si Nanowire Arrays via Galvanic Displacement Processes in Water-in-Oil Microemulsions",

D. Gao, R. He, C. Carraro, R. T. Howe, P. Yang, R. Maboudian, J. Am. Chem. Soc., 127, 4574, 2005.

Galvanic displacement processes are employed in water-in-oil microemulsions to deposit gold nanoclusters selectively on Si surfaces and sidewalls. The gold clusters then serve as catalysts to achieve selective growth of vertically and laterally aligned Si nanowire arrays by chemical vapor deposition via the vapor−liquid−solid growth mechanism. The size of the gold clusters is shown to have a good correlation with the microemulsion parameters, which in turn controls the size of the synthesized nanowires.

[pdf]

(93)
“ZnO Nanowire Transistors”,

J. Goldberger, D. J. Sirbuly, M. Law, P. Yang, J. Phys. Chem. B109, 9, 2005.

ZnO nanowire field-effect transistors (FETs) were fabricated and studied in vacuum and a variety of ambient gases from 5 to 300 K. In air, these n-type nanowire transistors have among the highest mobilities yet reported for ZnO FETs (μe = 13 ± 5 cm2 V-1 s-1), with carrier concentrations averaging 5.2 ± 2.5 × 1017 cm-3 and on−off current ratios ranging from 105 to 107. Four probe measurements show that the resistivity of the Ti/Au−ZnO contacts is 0.002−0.02 Ω·cm. The performance characteristics of the nanowire transistors are intimately tied to the presence and nature of adsorbed surface species. In addition, we describe a dynamic gate effect that seems to involve mobile surface charges and causes hysteresis in the transconductance, among other effects.

[pdf]

(92)
"Pt Nanocrystals: Shape Control and Langmuir−Blodgett Monolayer Formation",

H. Song, F. Kim, S. Connor, P. Yang, J. Phys. Chem. B. 109, 188, 2005.

We report the synthesis of monodisperse Pt nanocrystals with three different shapes-cubes, cuboctahedra, and octahedra, selectively, with similar sizes of 9−10 nm by a modified polyol process. We found that addition of silver ion enhances the crystal growth rate along <100>, and essentially determines the shape and surface structure of the Pt nanocrystals. After the reaction, the silver species can be easily removed by repetitive precipitation giving pure Pt nanoparticles. Two-dimensional arrays of the Pt nanocrystals were assembled by using the Langmuir−Blodgett (LB) method. The particles were evenly distributed on the entire substrate, and their surface coverage and density can be precisely controlled by tuning the surface pressure. The resulting Pt LB layers are potential candidates for 2-D model catalysts as a result of their high surface area and the structural uniformity of the metal nanocrystals.

[pdf]

(91)
"Crystal overgrowth on gold nanorods: tuning the shape, facet, aspect ratio, and composition of the nanorods",

J. H. Song*, F. Kim*, D. Kim, P. Yang, Chem. Euro. J., 11, 910, 2005.

Electrochemically prepared Au nanorods were used as seeds for the overgrowth of thin shells of gold, silver, and palladium by using a mild reducing agent, ascorbic acid, in the presence of surfactants at ambient condition. The unique crystal facets of the starting nanorods results in anisotropic crystal overgrowth. The overgrowth rates along different crystallographical directions can be further regulated by adding foreign ions or by using different metal reduction methods. This overgrowth study provides insights on how different metal ions could be reduced preferentially on different Au nanorod surfaces, so that the composition, aspect ratio, shape, and facet of the resulting nanostructures can be rationally tuned. These surfactant-stabilized bimetallic AucoreMshell (M=Au, Ag, Pd) nanorod colloids might serve as better substrates in surface-enhanced Raman spectroscopy as well as exhibiting enhanced catalytic properties.

[pdf]

2004

(90)
"Electrochemomechanical Energy Conversion in Nanofluidic Channels",

H. Daiguji, P. Yang, A. Szeri, A. Majumdar, Nano Lett. 4, 2315, 2004.

When the Debye length is on the order of or larger than the height of a nanofluidic channel containing surface charge, a unipolar solution of counterions is generated to maintain electrical neutrality. A pressure-gradient-driven flow under such conditions can be used for ion separation, which forms the basis for electrochemomechanical energy conversion. The current−potential (I−φ) characteristics of such a battery were calculated using continuum dynamics. When the bulk concentration is large and the channel does not become a unipolar solution of counterions, both the current and potential become small. On the other hand, when bulk concentration is so much smaller, the mass diffusion becomes the rate-controlling step and the potential drops rapidly in the high current density region. When the Debye length of the solution is about half of the channel height, the efficiency is maximized.

[pdf]

(89)
“Rapid prototyping of site-specific nanocontacts by electron and ion beam assisted direct-write nanolithography”,

V. Gopal, C. Daralo, V. R. Radmilovic, S. Jin, P. Yang, E. A. Stach, Nano Lett. 4, 2059, 2004.

Rapid prototyping of bottom-up nanostructure circuits is demonstrated, utilizing metal deposition and patterning methodology based on combined focused ion and electron beam induced decomposition of a metal−organic precursor gas. Ohmic contacts were fabricated using electron beam deposition, followed by the faster process of ion beam deposition for interconnect formation. Two applications of this method are demonstrated:  three-terminal transport measurements of Y-junction carbon nanotubes and fabrication of nanocircuits for determination of electromechanical degradation of silver nanowires.

[pdf]

(88)
“Fabrication and Characterization of a Nanowire/Polymer-based Nanocomposite for a Prototype Thermoelectric Device”,

A. R. Abramson, W. C. Kim, S. T. Huxtable, H. Yan, Y. Wu, A. Majumdar, C.-L. Tien, P. Yang, J. Microelectromech. Syst., 13, 505, 2004.

This paper discusses the design, fabrication and testing of a novel thermoelectric device comprised of arrays of silicon nanowires embedded in a polymer matrix. By exploiting the low-thermal conductivity of the composite and presumably high-power factor of the nanowires, a thermoelectric figure of merit, higher than the corresponding bulk value, should result. Arrays were first synthesized using a vapor-liquid-solid (VLS) process leading to one-dimensional (1-D) growth of single-crystalline nanowires. To provide structural support while maintaining thermal isolation between nanowires, parylene, a low thermal conductivity and extremely conformal polymer, was embedded within the arrays. Mechanical polishing and oxygen plasma etching techniques were used to expose the nanowire tips and a metal contact was deposited on the top surface. Scanning electron micrographs (SEMs) illustrate the results of the fabrication processes. Using a modification of the 3ω technique, the effective thermal conductivity of the nanowire matrix was measured and 1 V characteristics were also demonstrated. An assessment of the suitability of this nanocomposite for high thermoelectric performance devices is given.

[pdf]

(87)
"Nanoribbon Waveguides for Subwavelength Photonics Integration",

M. Law*, D. Sirbuly*, J. Johnson, J. Goldberger, R. Saykally, P. Yang, Science, 305, 1269, 2004.

Although the electrical integration of chemically synthesized nanowires has been achieved with lithography, optical integration, which promises high speeds and greater device versatility, remains unexplored. We describe the properties and functions of individual crystalline oxide nanoribbons that act as subwavelength optical waveguides and assess their applicability as nanoscale photonic elements. The length, flexibility, and strength of these structures enable their manipulation on surfaces, including the optical linking of nanoribbon waveguides and other nanowire elements to form networks and device components. We demonstrate the assembly of ribbon waveguides with nanowire light sources and detectors as a first step toward building nanowire photonic circuitry.

[pdf]

(86)
“Platonic Gold Nanocrystals”,

F. Kim, S. Connor, H. Song, T. Kuykendall, P. Yang, Angew. Chem. Int. Ed. 43, 3673, 2004.

Systematisch sich entwickelnde Formen von Goldnanokristallen mit Größen zwischen 100 und 300 nm traten bei einem modifizierten Polyolprozess auf. Durch Zugabe eines Oberflächen regulierenden Polymers und von Fremdionen sind die Formen Tetraeder, Würfel, Oktaeder und Ikosaeder („platonische Nanokristalle“, siehe Bild) einfach in hohen Ausbeuten und großer Einheitlichkeit zugänglich.

[pdf] [Cover Highlight]

(85)
“Crystallographic Alignment of High Density Gallium Nitride Nanowire Arrays”,

T. Kuykendall*, P. J. Pauzauskie*, Y. Zhang, J. Goldberger, D. Sirbuly, J. Denlinger, P. Yang, Nature Materials3, 524, 2004.

Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics1. Elucidation of the vapour–liquid–solid growth mechanism2 has already enabled precise control over nanowire position and size1, 3, 4, 5, 6, 7, 8, yet to date, no reports have demonstrated the ability to choose from different crystallographic growth directions of a nanowire array. Control over the nanowire growth direction is extremely desirable, in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and bandgap may be used to tune the physical properties of nanowires made from a given material. Here we demonstrate the use of metal–organic chemical vapour deposition (MOCVD) and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100) γ-LiAlO2 and (111) MgO single-crystal substrates resulted in the selective growth of nanowires in the orthogonal [1-10] and [001] directions, exhibiting triangular and hexagonal cross-sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin-film technology, which would lead to easy scale-up and device integration.

[pdf] [Cover Highlight]

(84)
"Semiconductor nanowires and nanotubes",

M. Law, J. Goldberger, P. Yang, Annu. Rev. Mater. Sci. 34, 83, 2004.

Semiconductor nanowires and nanotubes exhibit novel electronic and optical properties owing to their unique structural one-dimensionality and possible quantum confinement effects in two dimensions. With a broad selection of compositions and band structures, these one-dimensional semiconductor nanostructures are considered to be the critical components in a wide range of potential nanoscale device applications. To fully exploit these one-dimensional nanostructures, current research has focused on rational synthetic control of one-dimensional nanoscale building blocks, novel properties characterization and device fabrication based on nanowire building blocks, and integration of nanowire elements into complex functional architectures. Significant progress has been made in a few short years. This review highlights the recent advances in the field, using work from this laboratory for illustration. The understanding of general nanocrystal growth mechanisms serves as the foundation for the rational synthesis of semiconductor heterostructures in one dimension. Availability of these high-quality semiconductor nanostructures allows systematic structural-property correlation investigations, particularly of a size- and dimensionality-controlled nature. Novel properties including nanowire microcavity lasing, phonon transport, interfacial stability and chemical sensing are surveyed.

[pdf]

(83)
"Solution-phase synthesis of single-crystalline iron phosphide nanorods/nanowires",

C. Qian, F. Kim, L. Ma, F. Tsui, P. Yang, J. Lie, J. Am. Chem. Soc. 126, 1195, 2004.

A solution-phase route for the preparation of single-crystalline iron phosphide nanorods and nanowires is reported. We have shown that the mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP), which are commonly used as the solvents for semiconductor nanocrystal synthesis, is not entirely inert. In the current process, TOP, serving as phosphor source, reacts with Fe precursors to form FeP nanostructures with large aspect ratios. In addition, the experimental results show that both TOP and TOPO are necessary for the formation of FeP nanowires and their ratio appears to control the morphology of the produced FeP structures. A possible growth mechanism is discussed.

[pdf]

(82)
"Ultrafast carrier dynamics in single ZnO nanowire and nanoribbon lasers",

J. Johnson, Kelly P. Knutsen, H. Yan, M. Law, P. Yang, R. Saykally, Nano Lett. 4, 197, 2004.

Time-resolved second-harmonic generation (TRSHG) and transient photoluminescence (PL) spectroscopy are utilized to probe the ultrafast creation and subsequent relaxation of excited carriers immediately following band-gap excitation in single ZnO nanowire and nanoribbon lasers. The TRSHG signal consists of a 1−5 ps recovery present only during strong lasing and a 10−80 ps intensity-dependent component. The transient PL response from single structures exhibits an 80 ps decay component independent of pump power (free exciton PL), and a < 10 ps power-dependent component (stimulated emission) that shifts to earlier delay by ca. 10 ps at high pump fluence.

[pdf] [Cover Highlight]

(81)
"Ion transport in nanofluidic channels",

H. Daiguji, P. Yang, A. Majumdar, Nano Lett. 4, 137, 2004.

Theoretical modeling of ionic distribution and transport in silica nanotubes, 30 nm in diameter and 5 μm long, suggest that when the diameter is smaller than the Debye length, a unipolar solution of counterions is created within the nanotube and the coions are electrostatically repelled. By locally modifying the surface charge density through a gate electrode, the ion concentration can be depleted under the gate and the ionic current can be significantly suppressed. It is proposed that this could form the basis of a unipolar ionic field-effect transistor.

[pdf]

2003

(80)
"Nanotechnology: Wires on water",

P. Yang, Nature, 425, 243, 2003.

A centuries-old technique for transporting timber is the inspiration for a new method of assembling nanowires into large-scale, ordered patterns that could form the basis of a new generation of electronic devices.

[pdf]

(79)
"Metalorganic chemical vapor deposition route to GaN nanowires with triangular cross sections",

T. Kuykendall, P. Pauzauskie, S. K. Lee, Y. Zhang, P. Yang, Nano Lett. 3, 1063, 2003.

High-quality gallium nitride nanowires have been synthesized via metal-initiated metalorganic chemical vapor deposition for the first time. Excellent substrate coverage was observed for wires prepared on silicon, c-plane, and a-plane sapphire substrates. The wires were formed via the vapor−liquid−solid mechanism with gold, iron, or nickel as growth initiators and were found to have widths of 15-200 nm. Transmission electron microscopy confirmed that the wires were single-crystalline and were oriented predominantly along the [210] or [110] direction. Wires growing along the [210] orientation were found to have triangular cross-sections. Transport measurements confirmed that the wires were n-type and had electron mobilities of 65 cm2/V·s. Photoluminescence measurements showed band edge emission at 3.35 eV (at 5 K), with a marked absence of low-energy emission from impurity defects.

[pdf]

(78)
"ZnO Nanoribbon Microcavity Lasers",

H. Yan, J. Justin, M. Law, R. Saykally, P. Yang, Adv. Mater. 15, 1907, 2003.

ZnO nanoribbons with pseudo-rectangular cross-sections (see Figure) are demonstarted to be excellent microcavities with a high quality factor (∼ 3000). The lasing threshold is shown to be inversely proportional to the length of the ribbon for pumping intensities lower than the saturation region. Analysis of the emission spectra points to the possibility of the existence of both pure axial modes and “bow-tie” cavity modes.

[pdf]

(77)
"Langmuir−Blodgett Silver Nanowire Monolayers for Molecular Sensing Using Surface-Enhanced Raman Spectroscopy",

A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia, P. Yang, Nano Lett. 3, 1229, 2003.

Langmuir−Blodgett technique was used to assemble monolayers (with areas over 20 cm2) of aligned silver nanowires that are 50 nm in diameter and 2−3 μm in length. These nanowires possess pentagonal cross-sections and pyramidal tips. They are close-packed and are aligned parallel to each other. The resulting nanowire monolayers serve as excellent substrates for surface-enhanced Raman spectroscopy (SERS) with large electromagnetic field enhancement factors (2 × 105 for thiol and 2,4-dinitrotoluene, and 2 × 109 for Rhodamine 6G) and can readily be used in ultrasensitive, molecule-specific sensing utilizing vibrational signatures.

[pdf]

(76)
"Watching GaN nanowires grow",

E. Stach, P. Pauzauskie, T. Kuykendall, J. Goldberger, P. Yang, Nano Lett. 3, 867,2003.

We report real-time high temperature transmission electron microscopy observations of the growth of GaN nanowires via a self-catalytic vapor−liquid−solid (VLS) mechanism. High temperature thermal decomposition of GaN in a vacuum yields nanoscale Ga liquid droplets and gallium/nitrogen vapor species for the subsequent GaN nanowire nucleation and growth. This is the first direct observation of self-catalytic growth of nanowires via the VLS mechanism and suggests new strategies for synthesizing electronically pure single-crystalline semiconductor nanowires.

[pdf]

(75)
"Low-temperature wafer scale production of ZnO nanowire arrays",

L. Greene, M. Law, J. Goldberger, F. Kim, J. Johnson, Y. Zhang, R. Saykally, P. Yang, Angew. Chem. Int. Ed. 42, 3031, 2003.

Homogeneous and dense arrays of ZnO nanowires were synthesized on silicon wafers (and many other substrates) using a mild solution process at 90 °C. Uniform ZnO nanocrystals were deposited to act as seeds for subsequent hydrothermal nanowire growth, which yielded single-crystalline ZnO nanowires grown along the [0001] direction and oriented perpendicular to the wafer surface (see picture; scale bar=1μm). The photoluminescence and lasing behavior of the arrays has been studied as a function of annealing treatment conditions.

[pdf]

(74)
"Optical Cavity Effects in ZnO Nanowire Lasers and Waveguides",

J. Johnson, H. Yan, P. Yang, R. Saykally, J. Phys. Chem. B, 107, 8816, 2003.

Wide band gap semiconductor nanostructures with near-cylindrical geometry and large dielectric constants exhibit two-dimensional ultraviolet and visible photonic confinement (i.e., waveguiding). Combined with optical gain and suitable resonant feedback, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires. We have characterized the nanowire emission in detail with high-resolution optical microscopy. The waveguiding behavior of individual zinc oxide (ZnO) nanowires depends on the wavelength of the emitted light and the directional coupling of the photoluminescence (PL) to the emission dipoles of the nanowire. Polarization studies reveal two distinct regimes of PL characterized by coupling to either guided (bound) or radiation modes of the waveguide, the extent of which depends on wire dimensions. Pumping with high pulse energy engenders the transition from spontaneous to stimulated emission, and analysis of the polarization, line width, and line spacing of the laser radiation facilitates identification of the transverse and longitudinal cavity modes and their gain properties. Interpretation of the lasing spectra as a function of pump fluence, with consideration of ZnO material properties and ultrafast excitation dynamics, demonstrates a transition from exciton (fluence < 1 μJ/cm2) to electron−hole plasma dynamics (fluence > 1 μJ/cm2) and gain saturation behavior (fluence > 3 μJ/cm2) modified by the constraints of the nanoscale cylindrical cavity.

[pdf]

(73)
"Self-organized GaN quantum wire UV lasers",

H. Choi, J. Johnson, R. He, S. Lee, F. Kim, P. Pauzauskie, J. Goldberger, R. Saykally, P. Yang, J. Phys. Chem. B, 107, 8721, 2003.

Quantum wire lasers are generally fabricated through complex overgrowth processes with molecular beam epitaxy. The material systems of such overgrown quantum wires have been limited to Al−Ga−As−P, which leads to emission largely in the visible region. We describe a simple, one-step chemical vapor deposition process for making quantum wire lasers based on the Al−Ga−N system. A novel quantum-wire-in-optical-fiber (Qwof) nanostructure was obtained as a result of spontaneous Al−Ga−N phase separation at the nanometer scale in one dimension. The simultaneous excitonic and photonic confinement within these coaxial Qwof nanostructures leads to the first GaN-based quantum wire UV lasers with a relatively low threshold.

[pdf]

(72)
"Thermal conductivity of Si/SiGe superlattice nanowires",

D. Li, Y. Wu, P. Kim, L. Shi, N. Mingo, Y. Liu, P. Yang, A. Majumdar, Appl. Phys. Lett. 83, 3186. 2003.

The thermal conductivities of individual single crystalline Si/SiGe superlattice nanowires with diameters of 58 and 83 nm were measured over a temperature range from 20 to 320 K. The observed thermal conductivity shows similar temperature dependence as that of two-dimensional Si/SiGe superlattice films. Comparison with the thermal conductivity data of intrinsic Si nanowires suggests that alloy scattering of phonons in the Si–Ge segments is the dominant scattering mechanism in these superlattice nanowires. In addition, boundary scattering also contributes to thermal conductivity reduction.

[pdf]

(71)
"SnO2 nanoribbons as NO2 sensors: insights from first-principles calculations",

A. Maiti, J. A. Rodriguez, M. Law, P. Kung, J. R. McKinney, P. Yang, Nano Lett. 3, 1025, 2003.

SnO2 nanoribbons with exposed (1 0 1̄) and (0 1 0) surfaces have recently been demonstrated to be highly effective NO2 sensors even at room temperature. The sensing mechanism is examined here through first principles density functional theory (DFT) calculations. We show that the most stable adsorbed species involve an unexpected NO3 group doubly bonded to Sn centers. Significant electron transfer to the adatoms explains an orders-of-magnitude drop in electrical conductance. X-ray absorption spectroscopy indicates predominantly NO3 species on the surface, and computed binding energies are consistent with adsorbate stability up to 700 K. Nanoribbon responses to O2 and CO sensing are also investigated.

[pdf]

(70)
"Thermal conductivity of individual silicon nanowires",

D. Li, Y. Wu, P. Kim, L. Shi, N. Mingo, Y. Liu, P. Yang, A. Majumdar, Appl. Phys. Lett. 83, 2934, 2003.

The thermal conductivities of individual single crystalline intrinsic Si nanowires with diameters of 22, 37, 56, and 115 nm were measured using a microfabricated suspended device over a temperature range of 20–320 K. Although the nanowires had well-defined crystalline order, the thermal conductivity observed was more than two orders of magnitude lower than the bulk value. The strong diameter dependence of thermal conductivity in nanowires was ascribed to the increased phonon-boundary scattering and possible phonon spectrum modification.

[pdf]

(69)
"Single crystal gallium nitride nanotubes",

J. Goldberger, R. He, S. Lee, Y. Zhang, H. Yan, H. Choi, P. Yang, Nature, 422, 599, 2003.

Since the discovery of carbon nanotubes in 1991 (ref. 1), there have been significant research efforts to synthesize nanometre-scale tubular forms of various solids2, 3, 4, 5, 6, 7, 8, 9, 10. The formation of tubular nanostructure generally requires a layered or anisotropic crystal structure2, 3, 4. There are reports5, 6, 11 of nanotubes made from silica, alumina, silicon and metals that do not have a layered crystal structure; they are synthesized by using carbon nanotubes and porous membranes as templates, or by thin-film rolling. These nanotubes, however, are either amorphous, polycrystalline or exist only in ultrahigh vacuum8. The growth of single-crystal semiconductor hollow nanotubes would be advantageous in potential nanoscale electronics, optoelectronics and biochemical-sensing applications. Here we report an ‘epitaxial casting’ approach for the synthesis of single-crystal GaN nanotubes with inner diameters of 30–200 nm and wall thicknesses of 5–50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapour deposition system. The ZnO nanowire templates were subsequently removed by thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This templating process should be applicable to many other semiconductor systems.

[pdf]

(68)
"Dendritic Nanowire Ultraviolet Laser Array",

H. Yan, R. He, J. Johnson, M. Law, R. J. Saykally, P. Yang, J. Am. Chem. Soc., 125, 4728, 2003.

Self-organized dendritic crystal growth is explored to assemble uniform semiconductor nanowires into highly ordered one-dimensional microscale arrays that resemble comb structures. The individual ZnO nanowires have uniform diameters ranging from 10 to 300 nm. They are evenly spaced on a stem with a regular periodicity of 0.1−2 μm. Under optical excitation, each individual ZnO nanowire serves as a Fabry−Perot optical cavity, and together they form a highly ordered nanowire ultraviolet laser array.

[pdf]

(67)
"Fabrication of Silica Nanotube Arrays from Vertical Silicon Nanowire Templates",

R. Fan, Y. Wu, D. Li, M. Yue, A. Majumdar, P. Yang, J. Am. Chem. Soc. 125, 5254, 2003.

A simple thermal oxidation-etching process was developed to translate vertical silicon nanowire arrays into silica nanotube arrays. The obtained nanotubes perfectly retain the orientation of original silicon nanowire arrays. The inner tube diameter ranges from 10 to 200 nm. High-temperature oxidation produces relative thick, rigid, and pinhole-free walls that are made of condensed silica. This method could be useful for fabrication of single nanotube sensors and nanofluidic systems.

[pdf]

(66)
"One-dimensional nanostructures: synthesis, characterization and applications",

Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15, 353, 2003.

This article provides a comprehensive review of current research activities that concentrate on one-dimensional (1D) nanostructures—wires, rods, belts, and tubes—whose lateral dimensions fall anywhere in the range of 1 to 100 nm. We devote the most attention to 1D nanostructures that have been synthesized in relatively copious quantities using chemical methods. We begin this article with an overview of synthetic strategies that have been exploited to achieve 1D growth. We then elaborate on these approaches in the following four sections: i) anisotropic growth dictated by the crystallographic structure of a solid material; ii) anisotropic growth confined and directed by various templates; iii) anisotropic growth kinetically controlled by supersaturation or through the use of an appropriate capping reagent; and iv) new concepts not yet fully demonstrated, but with long-term potential in generating 1D nanostructures. Following is a discussion of techniques for generating various types of important heterostructured nanowires. By the end of this article, we highlight a range of unique properties (e.g., thermal, mechanical, electronic, optoelectronic, optical, nonlinear optical, and field emission) associated with different types of 1D nanostructures. We also briefly discuss a number of methods potentially useful for assembling 1D nanostructures into functional devices based on crossbar junctions, and complex architectures such as 2D and 3D periodic lattices. We conclude this review with personal perspectives on the directions towards which future research on this new class of nanostructured materials might be directed.

[pdf]

(65)
"Chemistry and physics of nanowires",

Y. Xia, P. Yang, Adv. Mater. 15, 351, 2003.

In this Editorial for the Nanowires Special Issue of Advanced Materials, Younan Xia and Peidong Yang welcome you to this survey of the fast-moving area of nanowire research. Communications and Research News articles covering vapor-phase, solution-based, and template-directed routes to the synthesis of nanowires, self-assembly with nanowires as the building blocks, and new physics associated with 1D nanostructures can all be found within.

[pdf]

(64)
"Morphogenesis of One-Dimensional ZnO Nano- and Microcrystals",

H. Yan, R. He, J. Pham, P. Yang, Adv. Mater. 15, 402, 2003.

The deterministic growth of different shapes of ZnO crystals from nanometer to micrometer scale is reported. Tetrapods (see Figure) and dendrites have been synthesized by simply adjusting the reaction temperature and the partial pressure of oxygen within the system. Size control of these structures can be achieved. ZnO nanoribbons are also easily accessible.

[pdf]

2002

(63)
"Photochemical synthesis of gold nanorods",

F. Kim, J. Song, P. Yang, J. Am. Chem. Soc. 124, 14316, 2002.

Gold nanorods have been synthesized by photochemically reducing gold ions within a micellar solution. The aspect ratio of the rods can be controlled with the addition of silver ions. This process reported here is highly promising for producing uniform nanorods, and more importantly it will be useful in resolving the growth mechanism of anisotropic metal nanoparticles due to its simplicity and the relatively slow growth rate of the nanorods.

[pdf]

(62)
"Functional Bimorph Composite Nanotapes",

R. He, M. Law, R. Fan, F. Kim, P. Yang, Nano Lett., 2, 1109, 2002.

Single-crystalline nanoribbons were used as substrates for the epitaxial growth of different functional thin films deposited by laser ablation techniques. This simple method yields highly crystalline bilayer nanotapes with sharp structural and compositional interfaces. As an example, Co0.05Ti0.95O2@SnO2 nanotapes are shown to be ferromagnetic at room temperature. These composite nanotapes, with their various possible functionalities, represent an important new class of nanoscale building blocks for optoelectronic applications.

[pdf]

(61)
"Synthesis and Characterization of Crystalline Ag2Se Nanowires Through a Template-Engaged Reaction at Room Temperature",

B. Gates, B. Mayers, Y. Wu, Y. Sun, B. Cattle, P.Yang, Y. Xia, Adv. Func. Mater. 12, 679, 2002.

Single-crystalline Ag2Se nanowires have been successfully synthesized through a template-engaged topotactic reaction in which nanowires of trigonal selenium were transformed into Ag2Se by reacting with aqueous AgNO3 solutions at room temperature (RT). An interesting size-dependent transition between two crystal structures has also been observed for this newly synthesized one-dimensional system: The Ag2Se nanowires adopted a tetragonal structure when their diameters were less than ∼40 nm; an orthorhombic structure was found to be more favorable as the diameter of these nanowires was increased beyond 40 nm. Since this reaction can be carried out at ambient pressure and temperature, it should be straightforward to scale up the entire process for the high-volume production of Ag2Se nanowires with well-controlled sizes and crystal structures. These highly uniform nanowires of single-crystalline Ag2Se are potentially useful as photosensitizers, superionic conductors, magnetoresistive compounds, or thermoelectric materials. This work also represents the first demonstration of a template-engaged process capable of generating single-crystalline nanowires from the solution-phase and at RT.

[pdf]

(60)
"Single gallium nitride nanowire lasers",

J. Johnson, H. J. Choi, K. P. Knutsen, R. D. Schaller, P. Yang, R. J. Saykally, Nature Materials, 1, 106, 2002.

There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices1, 2, including lasers3,4and nonlinear optical frequency converters5. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet–blue light-emitting diodes, lasers and photodetectors6, 7. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films8, 9. Here we report the observation of ultraviolet–blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry–Perot modes (Q approximately 103) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV muJ cm-2) supports the idea that the electron–hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet–blue coherent light sources.

[pdf]

(59)
"Miniaturised ultraviolet lasers",

P. Yang, Global Photonics Applications and Technology, World Markets Series, Business Briefing, 42-47, 2002.

(58)
"Photochemical Sensing of NO2 with SnO2 Nanoribbon Nanosensors at Room Temperature",

M. Law, H. Kind, F. Kim, B. Messer, P. Yang, Angew. Chem. Int. Ed. 41, 2405, 2002.

Good candidates for miniaturized, ultrasensitive gas sensors in many applications are individual single-crystalline SnO2 nanoribbons. Here it is shown that they can be used to detect ppm-level concentrations of NO2 at room temperature under UV illumination. The picture illustrates that they work reliably even near their resolution limit under 365-nm light.

[pdf]

(57)
"Inorganic semiconductor nanowires",

Y. Wu, R. Fan, P. Yang, Int. J. Nano. (Invited Review, Inaugural issue), 1, 1, 2002.

(56)
"Controlled Growth of ZnO Nanowires and Their Optical Properties",

P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H. Choi, Adv. Func. Mater. (Invited Feature Article), 12, 323,2002.

This article surveys recent developments in the rational synthesis of single-crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power-dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short-wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 1010cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single-crystalline, well-facetted nanowires can function as self-contained optical resonance cavities. This argument is further supported by our recent near-field scanning optical microscopy (NSOM) studies on single nanowires.

[pdf]

(55)
"Synthesis of Ultra-Long and Highly Oriented Silicon Oxide Nanowires from Liquid Alloys",

B. Zheng, Y. Wu, P. Yang, J. Liu, Adv. Mater. 14, 122, 2002.

Long and highly ordered amorphous silicon oxide nanowires (see Figure) have been prepared in bulk quantities by these authors. The wires formed on a Ga ball placed on top of a Si wafer, which acted as a source for the nanowires, in a quartz tube. The growth can be explained by dissolution of Si in molten Ga, followed by oxidation to SiO2, which induces the precipitation of the wires.

[pdf]

(54)
"Block-by-Block Growth of Single-Crystalline Si/SiGe Superlattice Nanowires",

Y. Wu, R. Fan, P. Yang, Nano Lett, 2, 83, 2002.

Heterojunction and superlattice formation is essential for many potential applications of semiconductor nanowires in nanoscale optoelectronics. We have developed a hybrid pulsed laser ablation/chemical capor deposition (PLA-CVD) process for the synthesis of semiconductor nanowires with longitudinal ordered heterostructures. The laser ablation process generates a programmable pulsed vapor source, which enables the nanowire growth in a block-by-block fashion with a well-defined compositional profile along the wire axis. Single-crystalline nanowires with longitudinal Si/SiGe superlattice structure have been successfully synthesized. This unique class of heterostructured one-dimensional nanostructures holds great potential in applications such as light emitting devices and thermoelectrics.

[pdf]

(53)
"Near-Field Imaging of Nonlinear Optical Mixing in Single Zinc Oxide Nanowires",

J. C. Johnson, H. Yan, R. D. Schaller, P. B. Peterson, P. Yang, R. J. Saykally, Nano Lett, 2, 279, 2002.

The nonlinear optical response of semiconductor nanowires has potential application for frequency conversion in nanoscale optical circuitry. Here, second- and third-harmonic generation (SHG, THG) are imaged on single zinc oxide (ZnO) nanowires using near-field scanning optical microscopy (NSOM). The absolute magnitudes of the two independent χ(2)elements of a single wire are determined, and the nanowire SHG and THG emission patterns as a function of incident polarization are attributed to the hexagonal nanowire geometry and χ(2) tensor symmetry.

[pdf]

(52)
"Langmuir–Blodgett Assembly of One-Dimensional Nanostructures",

F. Kim, P. Yang, ChemPhysChem, (Invited Concept Article), 3, 503, 2002.

The Langmuir–Blodgett technique has been used to assemble one-dimensional nanoscale building blocks. Various superstructures can be obtained as a result of different interactions between the individual nanostructures and different surface pressure applied. The general assembly behavior is exemplified here with BaCrO4, BaWO4, Au nanorods, and Mo3Seequation image nanowires.

[pdf]

(50)
"Semiconductor Nanowire Array: Potential Substrates for Photocatalysis and Photovoltaics",

Y. Wu, H. Yan, P. Yang, Topics in Catalysis, 19(2), 197, 2002.

A novel vapor-liquid-solid epitaxy (VLSE) process has been developed to synthesize high-density semiconductor nanowire arrays. The nanowires generally are single crystalline and have diameters of 10-200 nm and aspect ratios of 10-100. The areal density of the array can readily approach 1010cm-2. Results based on Si and ZnO nanowire systems are reported here. Because of their single crystallinity and high surface area, these nanowire arrays could find unique applications in photocatalysis and photovoltaics.

[pdf]

(51)
"Nanowire Ultraviolet Photodetectors and Optical Switches",

H. Kind, H. Yan, M. Law, B. Messer, P. Yang, Adv. Mater. 14, 158, 2002.

Highly sensitive nanowire switches have been created using ZnO nanowires. The light- induced conductivity increase, which is extremely sensitive to ultraviolet light, allows the reversibly switching of the nanowires between the “OFF” and “ON” states by an optical gating phenomenon analogous to the commonly used electrical gating. The Figure shows a field-emission SEM image of a 60 nm ZnO nanowire bridging four Au electrodes.

[pdf]

(49)
"Inorganic Semiconductor Nanowires: Rational Growth, Assembly, and Novel Properties",

Y. Wu, H. Yan, M. Huang, B. Messer, J. Song, P. Yang, Chemistry, Euro. J. (Invited Concept Article) 8, 1260, 2002.

Rationally controlled growth of inorganic semiconductor nanowires is important for their applications in nanoscale electronics and photonics. In this article, we discuss the rational growth, physical properties, and integration of nanowires based on the results from the authors’ laboratory. The composition, diameter, growth position, and orientation of the nanowires are controlled based on the vapor–solid–liquid (VLS) crystal growth mechanism. The thermal stability and optical properties of these semiconductor nanowires are investigated. Particularly, ZnO nanowires with well-defined end surfaces can function as room-temperature ultraviolet nanolasers. In addition, a novel microfluidic-assisted nanowire integration (MANI) process was developed for the hierarchical assembly of nanowire building blocks into functional devices and systems.

[pdf]

2001

(48)
"Mesostructured materials for optical applications: from low-k dielectrics to sensors and lasers",

G. Wirnsberger, P. Yang, B. Scott, G. Stucky, Spectrochim. Acta A57, 2049 (2001).

Recent advances on the use of mesoporous and mesostructured materials for electronic and optical applications are reported. The focus is on materials which are processed by block-copolymer templating of silica under weakly acidic conditions and by employing dip- and spin-coating as well as soft lithographic methods to bring them into a well-defined macroscopic shape. Several chemical strategies allow the mesostructure architecture to be used for electronic/optical applications: Removal of the block-copolymers results in highly porous, mechanically and thermally robust materials which are promising candidates for low dielectric constant materials. Since the pores are easily accessible, these structures are also ideal hosts for optical sensors, when suitable are incorporated during synthesis. For example, a fast response optical pH sensor was implemented on this principle. As-synthesized mesostructured silica/block-copolymer composites, on the other hand, are excellently suited as host systems for laser dyes and photochromic molecules. Laser dyes like rhodamine 6G can be incorporated during synthesis in high concentrations with reduced dimerization. This leads to very-low-threshold laser materials which also show a good photostability of the occluded dye. In the case of photochromic molecules, the inorganic–organic nanoseparation enables a fast switching between the colorless and colored form of a spirooxazine molecule, attributed to a partitioning of the dye between the block-copolymer chains. The spectroscopic properties of these dye-doped nanocomposite materials suggest a silica/block-copolymer/dye co-assembly process, whereby the block-copolymers help to highly disperse the organic dye molecules.

[pdf]

(47)
"Bismuth nanotubes: a rational low-temperature synthetic route",

Y. Li, J. Wang, Z. Deng, Y. Wu, X. Sun, S. Fan, D. Yu, P. Yang, J. Am. Chem. Soc. 123,9904, 2001.

[pdf]

(46)
"Single-Crystalline Nanowires of Ag2Se Can Be Synthesized by Templating against Nanowires of Trigonal Se",

B. Gates, Y. Wu, Y. Yin, P. Yang, Y. Xia, J. Am. Chem. Soc. 123, 11500, 2001.

[pdf]

(45)
"Single Nanowire Lasers",

J. Johnson, H. Yan, R. Schaller, L. Haber, R. Saykally, P. Yang, J. Phys. Chem. B, 105, 11387, 2001.

Ultraviolet lasing from single zinc oxide nanowires is demonstrated at room temperature. Near-field optical microscopy images quantify the localization and the divergence of the laser beam. The linewidths, wavelengths, and power dependence of the nanowire emission characterize the nanowire as an active optical cavity. These individual nanolasers could serve as miniaturized light sources for microanalysis, information storage, and optical computing.

[pdf]

(44)
"MMo3Se3 (M = Li+, Na+, Rb+, Cs+, NMe4+) Nanowire Formation via Cation Exchange in Organic Solution",

J. Song, B. Messer, Y. Wu, H. Kind P. Yang, J. Am. Chem. Soc. 123, 9714, 2001.

[pdf]

(43)
"Metal Nanowire Formation Using Mo3Se-3 as Reducing and Sacrificing Templates",

J. Song, Y. Wu, B. Messer, H. Kind, P. Yang, J. Am. Chem. Soc. 123, 10397, 2001.

Gold nanowires are synthesized from aqueous solutions of AuCl4 − using LiMo3Se3 nanowires in DMSO as both reducing agent and sacrificial template. The samples are characterized by TEM. The nanowires have diameters of 10–100 nm and lengths of several micrometers. They display small ohmic resistances at room temperature, indicating that these wires could prove useful as interconnects in nanoelectronic circuits. The method can also be used to prepare nanowires of Ag, Pt, and Pd.

[pdf]

(42)
"Synthesis of mesocellular silica foams with tunable window and cell dimensions",

W. Lukens, P. Yang, G. Stucky, Chem. Mater. 13, 28, 2001.

Polystyrene microspheres coated with cationic surfactants are easily prepared by microemulsion polymerization. These microspheres can be used in place of surfactants to synthesize porous silica foams. The nature of the foams depends strongly upon the synthesis conditions. Under basic catalysis, “closed-cell” foams are obtained. Under acidic catalysis, open-cell foams are obtained. The windows that connect the cells of the open-cell foams are believed to arise from direct contact between adjacent spherical templates. These silica foams resemble dense aerogels.

[pdf]

(41)
"Superconducting MgB2 nanowires",

Y. Wu, B. Messer, P. Yang, Adv. Mater. 13, 1487, 2001.

The novel superconductor MgB2 (TC = 39 K) is available as nanowires! Their synthesis has been achieved by a two-step protocol: Slightly Si-doped boron nanowires were generated by transport reaction, and treated with Mg vapor to form polycrystalline nanowires of MgB2 with diameters of 50–400 nm and lengths of several tens of micrometers (see Figure). Superconductivity was proven by a strong Meissner effect at 33 K.

[pdf]

(40)
"Room-temperature ultraviolet nanowire nanolasers",

M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science, 292, 1897, 2001.

Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated. The self-organized, <0001> oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process. These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers. Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 0.3 nanometer. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis.

[pdf]

(39)
"Synthesis and assembly of BaWO4 nanorods",

S. Kwan, F. Kim, J. Arkana, P. Yang, Chem. Commun., 5, 447, 2001.

Uniform, large aspect-ratio, monocrystalline BaWO4 nanorods were synthesized using a reversed micelle templating method; novel nanorod superstructures were observed in both as-made materials and Langmuir–Blodgett monolayer assemblies.

[pdf]

(38)
"Langmuir−Blodgett Nanorod Assembly",

F. Kim, S. Kwan, J. Arkana, P. Yang, J. Am. Chem. Soc. 123, 4360, 2001.

[pdf]

(37)
"Direct Observation of Vapor-Liquid-Solid Nanowire Growth",

Y. Wu, P. Yang, J. Am. Chem. Soc. 123, 3165, 2001.

[pdf]

(36)
"Melting and welding semiconductor nanowires in nanotubes",

Y. Wu, P. Yang, Adv. Mater. 13, 520, 2001.

Significant melting point depression and large hysteresis during the melting–recrystallization cycle are observed for semiconductor Ge nanowires encapsulated within carbon nanotubes. The capability of cutting, linking (see cover), and welding (see Figure) nanowires at relatively modest temperatures may provide a new approach to integrating these 1D nanostructures into functional devices and circuitry.

[pdf]

(35)
"Patterning porous oxides within microchannel networks",

P. Yang, A. Rizvi, B. Messer, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Adv. Mater. 13(6), 427 , 2001.

A continuing challenge for materials chemists and engineers is the ability to create multifunctional composite structures with well-defined superimposed structural order from nanometer to micrometer length scales. Materials with three-dimensional structures ordered over multiple length scales can be prepared by carrying out colloidal crystallization and inorganic/organic cooperative self-assembly within microchannel networks. The resulting materials show hierarchical ordering over several discrete and tunable length scales ranging from several nanometers to micrometers. These patterned porous materials hold promise for use as advanced catalysts, sensors, low-kdielectrics, optoelectronic and integrated photonic crystal devices.

[pdf]

(34)
"Catalytic Growth of Zinc Oxide Nanowires by Vapor Transport",

M. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater.13(2), 113,2001.

Patterned nanowire networks of photoluminescent highly crystalline ZnO nanowires (see Figure) have been produced via the vapor–liquid–solid mechanism using gold as catalyst. The diameters of the nanowires can be controlled by varying the thickness of the gold layer. The size of the wires has an influence on their emission characteristics (see also cover).

[pdf] [Cover Highlight]

(33)
"Patterned block-copolymer-silica mesostructures as host media for the laser dye Rhodamine 6G",

G. Wirnsberger, P. Yang, H. C. Huang, B. Scott, T. Deng, G. M. Whitesides, B. F. Chmelka, G. D. Stucky, J. Phys. Chem. B, 105, 6307, 2001.

Rhodamine 6G-doped mesostructured silica is prepared by an acidic sol−gel route using poly-b-poly(propylene oxide)-b-poly(ethylene oxide) (EOx−POy−EOx) block copolymer surfactants. Using low-refractive-index (n  1.2) mesoporous SiO2 as a support, the synthesis is combined with soft lithography to produce high-quality waveguides. This enables efficient waveguiding in the line-patterned rhodamine 6G-doped mesostructured domains, which have a higher refractive index than both the mesoporous support and cladding. For the structure-directing block copolymer surfactants used, (EO)20(PO)70(EO)20 (P123) and (EO)106(PO)70(EO)106 (F127), X-ray diffraction patterns and transmission electron microscopy reveal hexagonal mesophases, whose longitudinal cylinder axes are aligned predominantly parallel to the substrate plane. For samples made by micromolding-in-capillaries (MIMIC), the longitudinal axes are also aligned along the longitudinal waveguide axes. Samples made by micromolding also possess a high mesostructural order, though in the absence of an aligning flow field, their long-range order (ca. several hundred nanometers) is lower than for samples processed using the MIMIC technique. When optically pumped, the rhodamine 6G-doped waveguides exhibit amplified spontaneous emission with thresholds as low as 6 kW cm-2, substantially lower than rhodamine 6G-doped sol−gel glasses. This is attributed to the ability of the polymeric surfactant to co-assemble with the dye molecules, thereby leading to high dye dispersions and reduced dye dimerization. Additionally, rhodamine 6G shows good photostablility in the mesostructured waveguides, similar to that of rhodamine 6G in organically modified silicates.

[pdf]

2000

(32)
"Microchannel Networks for Nanowire Patterning",

B. Messer, J. H. Song, P. Yang, J. Am. Chem. Soc. 122, 10232-33, 2000.

[pdf]

(31)
"Germanium/carbon core-sheath nanostructures",

Y. Wu, P. Yang, Appl. Phys. Lett. 77, 43, 2000.

Germanium/carbon core–sheath nanostructures and junctions are produced when Ge nanowires are subject to a thermal treatment in an organic vapor doped vacuum. The organic molecules pyrolyze on the surface of the Ge nanowires and form a continuous graphitic coating. The carbon-sheathed Ge nanowires undergo melting and evaporation at high temperature, which results in the formation of germanium/carbon junctions. These core–sheath nanostructures and junctions generally have diameters of 5–100 nm, sheath thickness of 1–5 nm, and lengths up to several micrometers. This process may prove to be an effective approach to prevent the nanowire surface oxidation and create nanowires with chemically inert surface.

[pdf]

(30)
"Ag nanowire formation within mesoporous silica",

M. Huang, A. Choudrey, P. Yang, Chem. Commun. 12,1063, 2000.

Uniform Ag nanowires have been synthesized within nanoscale channels of mesoporous silica SBA-15 by a simple chemical approach, which involves AgNO3 impregnation, followed by thermal decomposition.

[pdf]

(29)
"Surfactant-Induced Mesoscopic Assemblies of Inorganic Molecular Chains",

B. Messer, J. H. Song, M. Huang, Y. Wu, F. Kim, P. Yang, Adv. Mater. 12,1526, 2000.

A disassembly–reassembly approach is used here in the self-organization of [Mo3Se3] chains—possible molecular wires. Either lamellar or hexagonal mesostructures (see Figure) are formed in the presence of oppositely charged surfactants and the spacing between these inorganic chains can be varied (20–40 Å) by using surfactants with different alkane lengths.

[pdf]

(28)
"Germanium Nanowire Growth via Simple Vapor Transport",

Y. Wu, P. Yang, Chem. Mater. 12,605, 2000.

[pdf]

(27)
"Hexagonal to Mesocellular Foam Phase Transition in Polymer-Templated Mesoporous Silicas",

J. S. Lettow, Y. J. Han, P. Schmidt-Winkel, P. Yang, D. Zhao, A. Butler, G. D. Stucky, J. Y. Ying, Langmuir, 16,8291, 2000.

We have investigated the phase transition between two distinct mesoporous silicas:  SBA-15, comprising a hexagonally packed arrangement of cylindrical pores (6−12 nm in diameter), and mesocellular silica foams (MCF), consisting of spherical voids (22−42 nm in diameter) interconnected by “windows” of 10 nm. Both SBA-15 and MCF are formed using an amphiphilic triblock copolymer (Pluronic P123) as a template. The synthesis conditions for the two materials are identical, except substantial trimethylbenzene is added to form MCF. We find that the phase transition occurs at an oil−polymer mass ratio of 0.2−0.3. Although the pore structures and pore sizes change dramatically, the mean surface curvature of the system remains essentially the same throughout the transition.

[pdf]

(26)
"Microemulsion templating of siliceous mesostructured cellular foms with well-defined ultralarge mesopores",

P. Schmidt-Winkel, W. W. Lukens, P. Yang, D. I. Margolese, J. S. Lettow, J. Y. Ying, G. D. Stucky, Chem. Mater. 12,686, 2000.

Siliceous mesostructured cellular foams (MCFs) with well-defined ultralarge mesopores and hydrothermally robust frameworks are described. The MCFs are templated by oil-in-water microemulsions and are characterized by small-angle X-ray scattering, nitrogen sorption, transmission electron microscopy, scanning electron microscopy, thermogravimetry, and differential thermal analysis. The MCFs consist of uniform spherical cells measuring 24−42 nm in diameter, possess BET surface areas up to 1000 m2/g and porosities of 80−84%, and give, because of their pores with small size distributions, higher-order scattering peaks even in the absence of long-range order. Windows with diameters of 9−22 nm and narrow size distribution interconnect the cells. The pore size can be controlled by adjusting the amount of the organic swelling agent that is added and by varying the aging temperature. Adding ammonium fluoride selectively enlarges the windows by 50−80%. In addition, the windows can be enlarged by postsynthesis treatment in hot water. The MCF materials resemble aerogels, but offer the benefits of a facilitated synthesis in combination with well-defined pore and wall structure, thick walls, and high hydrothermal stability. The open system of large pores give MCFs unique advantages as catalyst supports and separation media for processes involving large molecules, and the high porosities make them of interest for electrical and thermal insulation applications.

[pdf]

(25)
"Mirrorless Lasing from Mesostructured Waveguides Patterned by Soft Lithography",

P. Yang , G. Wirnsberger, H. C. Huang, S. R. Cordero, M. D. McGehee, B. Scott, T. Deng, G. M. Whitesides, B. F. Chmelka, S. K. Buratto, G. D. Stucky, Science, 287, 465, 2000.

Mesostructured silica waveguide arrays were fabricated with a combination of acidic sol-gel block copolymer templating chemistry and soft lithography. Waveguiding was enabled by the use of a low–refractive index (1.15) mesoporous silica thin film support. When the mesostructure was doped with the laser dye rhodamine 6G, amplified spontaneous emission was observed with a low pumping threshold of 10 kilowatts per square centimeter, attributed to the mesostructure’s ability to prevent aggregation of the dye molecules even at relatively high loadings within the organized high–surface area mesochannels of the waveguides. These highly processible, self-assembling mesostructured host media and claddings may have potential for the fabrication of integrated optical circuits.

[pdf]

1999 and earlier

(24)
"Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires",

J. Hu, M. Ouyang, P. Yang, C. M. Lieber, Nature, 399, 48,1999.

Nanometre-scale electronic structures are of both fundamental and technological interest: they provide a link between molecular and solid state physics, and have the potential to reach far higher device densities than is possible with conventional semiconductor technology1,2. Examples of such structures include quantum dots,which can function as single-electron transistors3,4 (although theirsensitivity to individual stray charges might make them unsuitable for large-scale devices) and semiconducting carbon nanotubes several hundred nanometres in length, which have been used to create a field-effect transistor5. Much smaller devices could be made by joining two nanotubes or nanowires to create, for example, metal–semiconductor junctions, in which the junction area would be about 1 nm2 for single-walled carbon nanotubes. Electrical measurements of nanotube ‘mats’ have shown the behaviour expected for a metal–semiconductor junction6. However, proposed nanotube junction structures7 have not been explicitly observed, nor have methods been developed to prepare them. Here we report controlled, catalytic growth of metal–semiconductor junctions between carbon nanotubes and silicon nanowires, and show that these junctions exhibit reproducible rectifying behaviour.

[pdf]

(23)
"Block Copolymer Templating Syntheses of Mesoporous Metal Oxides with Large Ordering Lengths and Semicrystalline Framework",

P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Chem. Mater. 11, 2813, 1999.

A simple and general procedure has been developed for the syntheses of ordered large-pore (up to 14 nm) mesoporous metal oxides, including TiO2, ZrO2, Nb2O5, Ta2O5, Al2O3, SiO2, SnO2, WO3, HfO2, and mixed oxides SiAlOy, Al2TiOy, ZrTiOy, SiTiOy, ZrW2Oy. Amphiphilic poly(alkylene oxide) block copolymers were used as structure-directing agents in nonaqueous solutions for organizing the network-forming metal oxide species. Inorganic salts, rather than alkoxides or organic metal complexes, were used as soluble and hydrolyzable precursors to the polymerized metal oxide framework. These thermally stable mesoporous oxides have robust inorganic frameworks and thick channel walls, within which high densities of nanocrystallites can be nucleated. These novel mesoporous metal oxides are believed to be formed through a mechanism that combines block copolymer self-assembly with alkylene oxide complexation of the inorganic metal species.

[pdf]

(22)
"Multiphase Assembly of Mesoporous−Macroporous Membranes",

D. Zhao, P. Yang, B. F. Chmelka, G. D. Stucky, Chem. Mater. 11, 1174, 1999.

[pdf]

(21)
"Fluoride-Induced Hierarchical Ordering of Mesoporous Silica in Aqueous Acid-Syntheses",

P. Schmidt-Winkel, P. Yang, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Adv. Mater. 11, 303, 1999.

Hierarchical ordering of mesoporous silica induced by fluoride addition is described for aqueous acid-syntheses. The authors report that small amounts of fluoride make it possible to control the organization of the mesoporous materials on different length scales. At low pH, fluoride addition brings about the formation of large hydrothermally stable silica rods (see Figure) consisting of aligned bundles of mesoporous silica fibers.

[pdf]

(20)
"Hierarchically Ordered Oxides",

P. Yang, T. Deng, D. Zhao, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Science, 282, 2244, 1998.

Porous silica, niobia, and titania with three-dimensional structures patterned over multiple length scales were prepared by combining micromolding, polystyrene sphere templating, and cooperative assembly of inorganic sol-gel species with amphiphilic triblock copolymers. The resulting materials show hierarchical ordering over several discrete and tunable length scales ranging from 10 nanometers to several micrometers. The respective ordered structures can be independently modified by choosing different mold patterns, latex spheres, and block copolymers. The examples presented demonstrate the compositional and structural diversities that are possible with this simple approach.

[pdf]

(19)
"Mesocellular Siliceous Foams with Uniformly Sized Cells and Windows",

P. Schmidt-Winkel, W. Luckens, P. Yang, D. Zhao, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc., 121, 254, 1998.

[pdf]

(18)
"Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks",

P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature, 396, 152,1998.

Surfactants have been shown to organize silica into a variety of mesoporous forms, through the mediation of electrostatic, hydrogen-bonding, covalent and van der Waals interactions1, 2, 3, 4, 5, 6, 7, 8. This approach to mesostructured materials has been extended, with sporadic success, to non-silica oxides5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, which might promise applications involving electron transfer or magnetic interactions. Here we report a simple and versatile procedure for the synthesis of thermally stable, ordered, large-pore (up to 140 Å) mesoporous metal oxides, including TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, and mixed oxides SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5 and ZrW2O8. We used amphiphilic poly(alkylene oxide) block copolymers as structure-directing agents in non-aqueous solutions for organizing the network-forming metal-oxide species, for which inorganic salts serve as precursors. Whereas the pore walls of surfactant-templated mesoporous silica1 are amorphous, our mesoporous oxides contain nanocrystalline domains within relatively thick amorphous walls. We believe that these materials are formed through a mechanism that combines block copolymer self-assembly with complexation of the inorganic species.

[pdf]

(17)
"Synthesis of continuous mesoporous silica thin films with three-dimensional accessible pore structures",

D. Zhao, P. Yang, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Chem. Commun. 22, 2499, 1998.

Continuous mesoporous silica thin films with three-dimensional (3-D) accessible pore structures (Pm3nP63/mmc space groups) have been prepared by a dip-coating technique using cationic surfactants as the structure-directing agents in nonaqueous media under acidic conditions.

[pdf]

(16)
"Continuous Mesoporous Silica Films with Highly Ordered Large Pore Structures",

D. Zhao, P. Yang, N. Melosh, J. Feng, B. F. Chmelka, G. D. Stucky, Adv. Mater. 10, 1380, 1998.

The formation of continuous mesoporous silica films with large periodic cage and pore structures is reported here. The authors use low-cost commercially available triblock copolymers and poly(ethylene oxide) non-ionic surfactants as the structure-directing agents in conjunction with dip-coat processing. In the Figure a transmission electron microscope image of a calcined hexagonal mesoporous silica film is shown.

[pdf]

(15)
"Triblock-Copolymer-Directed Syntheses of Large-Pore Mesoporous Silica Fibers",

P. Yang, D. Zhao, N. Melosh, B. F. Chmelka, G. D. Stucky, Chem. Mater. 10, 2033, 1998.

[pdf]

(14)
"Using the organic-inorganic interface to define pore and macroscale structure",

G. D. Stucky, D. Zhao, P. Yang, W. Lukens, N. Melosh, B. F. Chmelka, Stud. in Surf. Sci. Cat. 117, 1,1998.

This chapter presents a selected review of some of the current research in the area of mesoporous materials. There is a continuum of nearly monodispersed porosities that can be created for almost any mean pore size for a variety of compositions and structural phases through the mesoscale regime. The evolution of the development of these materials and their applications will be an exciting part of the future. One of the most fascinating underlying aspects of the biogenesis of materials is the space/time definition of structure, function, and morphology at multiple length scales from complex mixtures of reactants and accessible processes. Paradoxically, spatial, and kinetic incompatibilities during assembly result in instabilities that make it easy to temporally modify product composition, assembly, and macroscale form. In the same manner, in the synthesis of ordered mesoporous materials, competing spatial sequences in the solvent media as well as the reactants are useful in defining domain separation and ultimately pore structure and morphology in mesoporous phases.

(13)
"Nitrogen Driven structural transformation in carbon nitride materials",

J. Hu, P. Yang, C. M. Lieber, Appl. Surf. Sci. 127, 569, 1998.

The variation of local bonding as a function of nitrogen concentration in plasma-assisted pulsed-laser deposited carbon nitride films has been systematically studied. Time-of-flight (TOF) mass spectroscopy and electron energy loss spectroscopy (EELS) were combined to identify ablation conditions that produce highly sp3-hybridized diamond-like-carbon (DLC) for typical carbon nitride growth pressures. EELS studies of carbon nitride films grown using these optimal conditions demonstrate that there is a structural transformation from ∼70 to 0% sp3-bonded carbon as the nitrogen concentration increases from 12 to 17%. Density measurements show that this transformation is accompanied by a density decrease from 3.3 to 2.1 g/cm3. Hartree–Fock and density functional calculations on nitrogen substituted diamond clusters show that there is a strong preference to form sp2-bonded carbon when the local nitrogen concentration is larger than 12 atomic percent. These experimental results and calculations suggest that amorphous carbon nitride structures with highly sp3-hybridized carbon are unstable.

[pdf]

(12)
"Topological construction of mesoporous materials",

D. Zhao, P. Yang, Q. Huo, B. F. Chmelka, G. D. Stucky, Curr. Opin. Solid State & Mater. Sci. V3, 111, 1998.

Major advances in the field of ordered mesoporous materials have been achieved in topological structure definition at the meso phase, and macroscale levels (shape and morphology) using molecular control during mesoporous materials synthesis. Examples include the use of block copolymers for the preparation of mesoporous materials with large pore sizes (View the MathML source30 nm), the formation of mesoporous silica with 3D periodically ordered cage-structures, and the fabrication of selected mesoporous silica having designated macrostructures, including fibers, thin films and monoliths along with hollow and transparent hard spheres. The judicious integration of hydrogen—bonding interactions at the organic/inorganic interface with organic/inorganic domain assembly and the use of sol-gel and emulsion chemistry in acidic media proves to be a general route for the syntheses of mesoporous materials with potential applications in catalysis, sensors, separations, optoelectronics, and functional nanomaterial fabrication.

[pdf]

(11)
"Nitrogen-driven sp3 to sp2 transformation in carbon nitride materials",

J. Hu, P. Yang, C. M. Lieber, Phys. Rev. B, 57, R3185, 1998.

The coordination of carbon as a function of nitrogen concentration in energetically deposited carbon nitride films has been systematically studied. A structural transformation from primarily sp3-bonded to sp2-bonded carbon and a density decrease from 3.3 to 2.1 g/cm3 were observed as the nitrogen concentration increases from 11 to 17 %. Calculations on nitrogen-substituted carbon clusters indicate that there is a preference to form sp2-bonded carbon when the nitrogen concentration is larger than 12%. The implications for these results to the synthesis of superhard carbon nitride materials are discussed.

[pdf]

(10)
"High-Temperature Superconductors",

C. M. Lieber, P. Yang, Science, 277,1909, 1997.

(9)
"Nanostructured high-temperature superconductors: Creation of strong-pinning columnar defects in nanorod/superconductor composites",

P. Yang, C. M. Lieber, J. Mater. Res. 12, 2981, 1997.

A chemical approach to the formation of columnar defects involving the growth and incorporation of MgO nanorods into high temperature superconductors (HTS’s) has been developed. MgO nanorods were incorporated into Bi2Sr2CaCu2Oz, Bi2Sr2Ca2Cu3Oz, and Tl2Ba2Ca2Cu3Oz superconductors at areal densities up to 2 × 1010/cm2. Microstructural analyses of the composites demonstrate that the MgO nanorods create a columnar defect structure in the HTS matrices, form a compositionally sharp interface with the matrix, and self-organize into orientations perpendicular and parallel to the copper oxide planes. Measurements of the critical current density demonstrate significant enhancements in the MgO nanorod/HTS composites at elevated temperatures and magnetic fields compared with reference samples.

[pdf]

(8)
"Columnar defect formation in nanorod/Tl2Ba2Ca2Cu3Oz superconducting composites",

P. Yang, C. M. Lieber, Appl. Phys. Lett., 70(23), 3158, 1997.

Nanorod/superconductor composites were formed by depositing Tl2Ba2Ca2Cu3Oz (Tl-2223) thick films on high density MgO nanorod arrays that were grown on MgO single crystal substrates. Electron microscopy studies show that this approach creates a high density of columnar defects normal to the CuO2 planes within crystal grains of the composites. The nanorod/superconductor composites exhibited enhanced critical current densities and an upward shift in the irreversibility line compared with reference samples. These results demonstrate that a nanorod-composite approach represents an effective strategy for introducing correlated defects into high-Tc superconductors, and thus may be useful for applications.

[pdf]

(7)
"Nanorod-Superconductor Composites: A Pathway to Materials with High Critical Current Densities",

P. Yang, C. M. Lieber, Science, 273, 1836, 1996.

Most large-scale applications of the high-temperature copper oxide superconductors (HTSCs) require high critical current densities (Jc‘s) at temperatures near the boiling point of liquid nitrogen to be technologically useful, although thermally activated flux flow reduces Jc dramatically at these temperatures. This intrinsic limitation can be overcome by introducing nanometer-sized columnar defects into an HTSC. Nanorods of magnesium oxide were grown and incorporated into HTSCs to form nanorod-HTSC composites. In this way, a high density of nanorod columnar defects can be created with orientations perpendicular and parallel to the copper oxide planes. The Jc‘s of the nanorod-HTSC composites are enhanced dramatically at high temperatures and magnetic fields as compared with reference samples; these composites may thus represent a technologically viable strategy for overcoming thermally activated flux flow in large-scale applications.

[pdf]

(6)
"One-dimensional nanostructural materials: rational syntheses, novel properties and application",

C. M. Lieber, A. M. Morales, P. E. Sheehan, E. W. Wong, P. Yang, The Robert A. Welch Foundation 40th Conference on Chemical Research, Chemistry on the Nanometer Scale, 1996.

(5)
"Pulsed Laser Deposition of Diamond-Like Carbon Thin Films: Ablation Dynamics and Growth",

P. Yang, Z. J. Zhang, J. Hu, C. M. Lieber, Mat. Res. Soc. Symp. Proc. 438, 593, 1996.

Thin films of diamond-like carbon have been grown by pulsed laser deposition using a Nd:YAG laser at 532 nm. Time-of-flight mass spectroscopy was used to investigate the effects of laser power density and background gas pressure on the plume characteristics including the species in the plume and the kinetic energy distribution of each species. We found that with increasing laser power density (1) the relative amount of C+ ions increases, (2) the kinetic energy distributions of C+ get broader and can be deconvoluted into fast and slow components, and (3) the kinetic energy of the fast component of C+ ions increases from several to 40 eV. The resistivity and the local carbon bonding in films grown under these same conditions were also characterized. It was found that there is direct correlation between the characteristics of fast part of C+ ions in the plume and the diamond-like properties of the thin films. Under optimal growth conditions diamond-like carbon films with a large fraction of sp3 bonding can be prepared, although the maximum fraction appears to saturate at 70%. The implications of these results are discussed.

[pdf]

(4)
"Growth and Properties of Carbon Nitride Thin Films",

Z. J. Zhang, P. Yang, C. M. Lieber, Mat. Res. Soc. Symp. Proc. 388, 271,1995.

Recent research on carbon nitride thin films grown using pulsed laser deposition combined with atomic beam techniques is reviewed. the composition, growth mechanism and phases of these films have been systematically investigated. the nitrogen composition was found to increase to a limiting value of 50% as the fluence was decreased for laser ablation at both 532 nm and 248 nm wavelengths. Time of flight mass spectroscopy investigations of the ablation products have shown that the fluence variations affect primarily the yield of the carbon reactant. these experiments demonstrate that the overall film growth rate determines the average nitrogen composition, and furthermore, suggest that a key step in the growth mechanism involves a surface reaction between carbon and nitrogen. INfrared spectroscopy has been used to assess the phases present in the carbon nitride thin films as a function of the overall nitrogen content. these measurements have shown that a cyanogen-like impurity occurs in films with nitrogen compositions greater than 30%. Studies of thermal annealing have shown, however, that this impurity phase can be eliminated to yield a single phase C2N material. In addition, systematic studies of the electrical resistivity and thermal conductivity of the carbon nitride films are discussed.

[pdf]

(3)
"Preparation of Layered Sr2CuO3+.delta. by Pulsed Laser Deposition: Rational Synthesis and Doping of a Metastable Copper Oxide Material",

A. M. Morales, P. Yang, C. M. Lieber, J. Am. Chem. Soc. 116, 8360, 1994.

[pdf]

(2)
"A new type of layered cuprates with 1222 structure. (CdCe)Sr2(LnCeSr)2Cu2Oy (Ln=Nd, Y)",

S. Wang, K. Tang, Y. Qian, P. Yang, Z. Chen, W. Yu, G. Zhou, Physica C, 226, 121, 1994.

A new family of layered cuprates with the 1222 structure (CdCe)Sr2(LnCeSr)2Cu2Oy (Ln=Nd, Y) has been succesfully synthesized and identified. The diffraction patterns can be indexed by the tetragonal lattice parameters View the MathML sourceView the MathML source (Ln=Y) and View the MathML sourceView the MathML source (Ln=Nd), respectively. The space group is 14/mmm which is consistent with that of Pb-1222. The resistivity measurement showed that the two compounds were narrow-gap semiconductors.

[pdf]

(1)
"The synthesis and superconductivity of a new type of Bi-1212 phase (Bi, Cd)Sr2(Y, Ca)Cu2Oz",

Y. Qian, K. Tang, P. Yang, Z. Chen, R. Li, G. Zhou, Y. Zhang, N. Wang, Physica C, 209, 516 , 1993.

A novel series of Bi-1212 superconducting cuprates (Bi1-xCdx)Sr2(Y1-yCay)Cu2Oz has been discovered in this work. The structure was suggested to be that of (Bi, Cu)Sr2YCu2Oz with Cd2+ ions substituting into the Cu2+ sites in the rock-salt-type (Bi, Cu)O layers. The superconductivity was induced in the as-prepared samples by treatment under oxygen pressure of 10 atm at 800°C with a composition of (Bi0.27Cd0.73)Sr2(Y0.7Ca0.3)Cu2Oz. The zero-resistivity temperature is 26 K.

[pdf]

()
nl502961e_si_001,

[pdf]