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* = α2*T*/[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.

#### Research Milestones

**2002**– Growth of Si/SiGe Supperlattice Nanowires for potential thermoelectrics applications. “Block-by-Block Growth of Single-Crystalline Si/SiGe Superlattice Nanowires”, Y. Wu, R. Fan, P. Yang,**Nano Lett.****2003**– First measurement of thermal conductivity of silicon nanowires. “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***.****2003**– First measurement of thermal conductivity of superlattice nanowires. “Thermal conductivity of individual Si/SiGe superlattice nanowires” D. Li, Y. Wu, P. Kim, L. Shi, N. Mingo, Y. Liu, P. Yang, A. Majumdar,*Appl. Phys. Lett***.****2008**– First demonstration of using rough nanowires for enhanced thermoelectric performance. “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****2008**– Examination of heat transport in a quasi-1D structure. “Thermal Conductance of Thin Silicon Nanowires”, R. Chen, A. I. Hochbaum, P. Murphy, J. Moore, P. Yang, A. Majumdar,**Phys. Rev. Lett.****2010**– Demonstration of an efficient thermoelectric from a 2D structure of silicon. “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.***2011**– Using facile conversion chemistry leading to enhanced thermoelectric performance. “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.***2012**– Development of a methodology to quantify the effect of surface roughness on heat transport in Si nanowires. “Quantifying Surface Roughness Effects on Phonon Transport in Silicon Nanowires”, J. Lim*, K. Hippalgaonkar*, S. Andrews, A. Majumdar, P. Yang,*Nano Lett.*

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