Perovskite Nanostructures

perovskite_1    CsPbBr3 NW laser    Screen Shot 2017-03-21 at 11.25.51 PM    2D NS

Recently, there is a renaissance of halide perovskites as a class of semiconductor materials for a variety of photovoltaics and optoelectronics. Particularly, the inorganic halide perovskites draw more and more attention, owing to their enhanced stability toward moisture, oxygen, and heat, compared to the organic-inorganic hybrid perovskites (e.g. methylammonium lead iodide). The controlled synthesis, detailed structural analysis, optical, and electronic properties are of great fundamental interest. This project currently focuses on the synthesis of inorganic halide perovskite nanostructures and the characterization of their physical properties to 1) advance synthetic methodology of 0D, 1D, and 2D nanostructures, 2) establish and advance technology and instrumentation to study fundamental nanomaterial properties as well as the physical, chemical, and electronic interactions between them, and 3) apply the extracted knowledge to both develop integrated optoelectronic and photonic devices from these building blocks and to feed this knowledge back into the virtuous cycle of design, synthesis, measurement and application.

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Nanowire Photonics

The manipulation of optical energy in structures smaller than the wavelength of light is key to the development of integrated photonic devices for computing, communications and sensing. 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, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires with suitable resonant feedback. This concept of using well-cleaved nanowires as natural optical cavities may be extendable to many other different semiconductor systems. We have further explored the properties and functions of individual ultralong 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 have demonstrated the assembly of ribbon waveguides with nanowire light sources and detectors as a first step toward building nanowire photonic circuitry.

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Nanowire-based Solar Cells

Excitonic solar cells – including organic, hybrid organic-inorganic and dye-sensitised cells (DSCs) – are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient and stable 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. We have introduced a new version of the dye-sensitised cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 3.5% has been demonstrated, limited primarily by the surface area of the nanowire array. We are now extending our synthetic strategy to design nanowire electrodes with much larger areas available for dye adsorption. It is worth noting that the advantages of the nanowire geometry are even more compelling for other types of excitonic photocells, such as inorganic-polymer, inorganic composite hybrid devices, in which an oriented, continuous and crystalline inorganic phase could greatly improve charge collection.

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Nanowire for Solar-to-Fuel Conversion

Direct solar energy conversion to storable fuels such as hydrogen offers a promising route toward less reliance on fossil fuels. For example, photoelectrolysis of water to generate H2 on a semiconductor/electrolyte interface has the attractive advantages of clean processing and energy savings over steam reforming of natural gas. One of the most critical issues in solar water splitting is the development of a photoanode with high efficiency and long-term durability in an aqueous environment. TiO2 has been extensively studied as a photoanode due to its high resistance to photocorrosion. However, its conversion efficiency of solar energy to hydrogen is still low due to its large bandgap. Semiconductor heterojunctions can absorb a different region of the solar spectrum. The advantage of composite structures is that each semiconductor needs to satisfy one energetic requirement: matching the conduction band minimum (CBM) or VBM with either the H2 reduction or O2 oxidation potential. Single semiconductor materials typically cannot satisfy the requirements of suitable bandgap energies for efficient solar absorption and meantime with band-edges aligned with both the H2 and O2 redox potential of water. We are currently exploring semiconductor nanowire heterojunctions for the direct solar-to-fuel conversion.

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Nanowire Thermoelectrics

Phonon transport is expected to be greatly impeded in thin (i.e., d<Λ, where d is the diameter and Λ is the phonon mean free path) 1D nanostructures as a result of increased boundary scattering and reduced phonon group velocities stemming from phonon confinement. Detailed models of phonon heat conduction in cylindrical semiconducting nanowires that consider modified dispersion relations and all important scattering processes predict a large decrease in the lattice thermal conductivity of wires tens of nanometers in diameter. Size-dependent thermal conductivity in nanostructures presents a major hurdle in the drive toward miniaturization in the semiconductor industry. Yet poor heat transport is advantageous for thermoelectric materials, which are characterized by a figure of merit (ZT = α2T/[ρ(κp + κe)], with α, T, ρ, κp and κe the Seebeck coefficient, absolute temperature, electronic resistivity, lattice thermal conductivity and electronic thermal conductivity, respectively) that improves as phonon transport worsens. A decade ago, the Dresselhaus group predicted that ZT can be increased above bulk values in thin nanowires by carefully tailoring their diameters, compositions and carrier concentrations.  Recent work in our laboratories has focused on understanding the thermal transport properties of pure silicon and Si/SiGe superlattice nanowires as the first step in the experimental verification of enhanced ZT values in these complex nanostructures.

 

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Nanowire-Cell Interface

We are in the process of developing a fully-integrated and highly-sensitive nanowire probe platform for single cell endoscopy. Developing of such flexible nanowire probes would enable us to monitor in-vivo biological processes within single living cells and will greatly improve our fundamental understanding of cell functions, intracellular physiological processes, and cellular signal pathway. There are several key features associated with these cell endoscopy nanowire probes: Minimal invasiveness; high flexibility; high refractive index; evanescent wave optical sensing principle with highly localized excitation and detection scheme; and nonlinear optical conversion capability. Such novel nanowire probes promise intracellular imaging with greatly enhanced 3-dimensional spatial resolution as well as temporal resolution. In addition, these nanowire probes could also be used to spot-delivery or extraction of chemicals (proteins/DNAs) from single living cells with much improved spatial resolution as compared to conventional delivery/extraction methods.

 

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Nanotube Nanofluidics

Ionic transport through nanoscale channels is receiving increasing attention due to recent experiments that report modulation of ion currents during the passage of single molecules of DNA or protein through the protein ion channel ?-hemolysin. The possibility of rapid DNA sequencing by monitoring the ionic conductance signatures of passing nucleotide oligomers has prompted the synthesis of artificial nanopores and the study of biomolecular transport through them. Nanotubes provide a unique high aspect ratio channel in which to study ion transport and fluid flow. A theoretical treatment of ion behavior in gated silica nanotubes suggests that when the tube diameter is smaller than the Debye length, an applied gate bias can completely expel ions of like charge and produce a unipolar solution of counter-ions within the channel. Modifying the surface charge on the nanotube with the gate electrode modulates the ionic current through the tube – the basis for a unipolar ionic field-effect transistor. Also, the 5-20 ?m length of the nanotube channels now being fabricated in this lab opens up the possibility of imaging and manipulating single molecules as they pass through a tube.

 

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Plasmonics

Known to the ancient Greeks, there are five Platonic solids that can be constructed by selecting a regular convex polygon and having the same number of them meet at each corner: tetrahedron, octahedron, hexahedron (cube), icosahedron, dodecahedron. The beauty in their symmetry and their apparent simplicity continue to inspire generations of mathematicians and scientists. In nature, certain viruses and radiolaria also routinely take the form of these polyhedral shapes. Recently, the concept of shape control has started to revitalize the centuries-old metal colloidal synthesis. Nanoparticles of various shapes, rods, wires, prisms, cubes, particularly those of silver and platinum, have been prepared using a variety of different methodologies. We recently demonstrated a systematic shape-evolution of metal nanocrystals with sizes of 5-300 nm in a modified polyol process. By adding surface-regulating polymer and foreign ions, we can readily access the distinct shapes of tetrahedron, cube, octahedron, and icosahedron with high yield and good uniformity. These nanocrystals have the perfect symmetry for 2- and 3-dimensional packing and therefore could enable the rational tuning of their optical (surface plasmon) and catalytic properties.

 

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Nanocrystal Catalysis

The ultimate goal of catalysis research is to achieve 100% selectivity for the desired products at maximum turnover rate (activity) without generating undesired byproducts. Ways to optimize activity and selectivity have been investigated on the atomic and molecular level using two-dimensional model catalysts (e.g. metal single crystals, nanoparticle arrays). It was found that key characteristics affecting both activity and selectivity are surface structure and particle size. Other features important to catalysis are site-blocking or bonding modifier additives and the metal-oxide interface. Development of a high surface area model catalyst will enable the study of the molecular ingredients which control activity and more importantly selectivity in the multifunctional (oxide / metal) composite systems. We have been developing methods for making metal nanocrystals with very narrow particle size distribution and well-defined shape. These nanoparticles are then assembled into 2-dimensional arrays using Langmuir-Blodgett technique or embedded in mesoporous oxide supports. These composites are considered as a high surface area model catalyst system as the precise control of nanocrystal size, shape, surface and interface should impart desired reaction selectivity and activity.

 

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