Jaeyun Moon

Gate-Modulated Thermoelectric Power Factor of Hole Gas in Ge–Si Core–Shell Nanowires

We experimentally studied the thermoelectric power factor of hole gas in individual Ge-Si core-shell nanowires with Ge core diameters ranging from 11 to 25 nm. The Ge cores are dopant-free, but the Fermi level in the cores is pinned by surface and defect states in the epitaxial Si shell thereby doping the cores into the degenerate regime. This doping mechanism avoids the high concentration of dopants usually encountered in bulk thermoelectric materials and provides a unique opportunity to enhance the carrier mobility with suppressed ionized impurity scattering. Moreover, the carrier concentration in small diameter nanowires has also been effectively modulated by field effect, allowing one to probe the electrical conductivity and thermopower within a wide range of carrier concentrations, which is crucial to understand the thermoelectric transport behavior. We found that the thermopower of nanowires with Ge core diameters down to 11 nm still follows the behavior of bulk Ge. As a result, the power factor is found to be closely correlated with the carrier mobility, which is higher than that of bulk Ge in one of the core-shell nanowires studied here.

Ultralow Thermal Conductivity of Multilayers with Highly Dissimilar Debye Temperatures

Thermal transport in multilayers (MLs) has attracted significant interest and shows promising applications. Unlike their single-component counterparts, MLs exhibit a thermal conductivity that can be effectively engineered by both the number density of the layers and the interfacial thermal resistance between layers, with the latter being highly tunable via the contrast of acoustic properties of each layer. In this work, we experimentally demonstrated an ultralow thermal conductivity of 0.33 ± 0.04 W m(-1) K(-1) at room temperature in MLs made of Au and Si with a high interfacial density of ∼0.2 interface nm(-1). The measured thermal conductivity is significantly lower than the amorphous limit of either Si or Au and is also much lower than previously measured MLs with a similar interfacial density. With a Debye temperature ratio of ∼3.9 for Au and Si, the Au/Si MLs represent the highest mismatched system in inorganic MLs measured to date. In addition, we explore the prior theoretical prediction that full phonon dispersion could better model the interfacial thermal resistance involving materials with low Debye temperatures. Our results demonstrate that MLs with highly dissimilar Debye temperatures represent a rational approach to achieve ultralow thermal conductivity in inorganic materials and can also serve as a platform for investigating interfacial thermal transport.

Simultaneous specific heat and thermal conductivity measurement of individual nanostructures

Fundamental phonon transport properties in semiconductor nanostructures are important for their applications in energy conversion and storage, such as thermoelectrics and photovoltaics. Thermal conductivity measurements of semiconductor nanostructures have been extensively pursued and have enhanced our understanding of phonon transport physics. Specific heat of individual nanostructures, despite being an important thermophysical parameter that reflects the thermodynamics of solids, has remained difficult to characterize. Prior measurements were limited to ensembles of nanostructures in which coupling and sample inhomogeneity could play a role. Herein we report the first simultaneous specific heat and thermal conductivity measurements of individual rod-like nanostructures such as nanowires and nanofibers. This technique is demonstrated by measuring the specific heat and thermal conductivity of single ~600–700 nm diameter Nylon-11 nanofibers (NFs). The results show that the thermal conductivity of the NF is increased by 50% over the bulk value, while the specific heat of the NFs exhibits bulk-like behavior. We find that the thermal diffusivity obtained from the measurement, which is related to the phonon mean free path (MFP), decreases with temperature, indicating that the intrinsic phonon Umklapp scattering plays a role in the NFs. This platform can also be applied to one- and two- dimensional semiconductor nanostructures to probe size effects on the phonon spectra and other transport physics.

Chapter 9:Phononic and Electronic Engineering in Nanowires for Enhanced Thermoelectric Performance

In this chapter, we review recent developments pertaining to “nanowire thermoelectrics.” In particular, we focus on the fundamental aspects of engineering charge and heat transport properties in nanowires and its implications for thermoelectric applications. Specifically, we discuss the following topics in this chapter: general background of thermoelectrics and the relevant length scales related to thermoelectric transport; brief overview of main synthesis techniques for thermoelectric nanowires; thermal conductivity of semiconductor nanowires, including characterization techniques and measurement results; thermoelectric power factor measurements and results of semiconductor nanowires; approaches to assemble nanowires into bulk thermoelectric materials and devices; future outlook of possible strategies pertaining to nanowire thermoelectrics.

Thermal Conductivity Measurement of Thin Nanowires

Semiconductor nanowires hold great promise for applications such as nano-electronics and energy conversion. A detailed knowledge of the thermal properties of the nanowire materials is essential for proper thermal management in nano-devices and thermal energy conversion. Prior thermal measurements on individual nanowires have shown that nanowires have reduced lattice thermal conductivity and, in some cases, enhanced thermoelectric properties. However, such thermal measurements are limited to nanowire thermal conductance of the order of 1 nW/K and are typically limited to nanowire diameters greater than 20 nm. Measurements to obtain the thermal conductivities of single nanowires with smaller diameter nanowires, which may exhibit even lower thermal conductivity and possibly quantum confinement effect at low temperature, have proven elusive. Herein, we demonstrated an experimental technique with improved measurement sensitivity that is capable of measuring the thermal conductance of 10 pW/K. This more sensitive technique overcomes several issues with current instrumentations and provides a tool for characterizing the properties of much smaller diameter nanowires, such as nanowires with 1 W/m-K thermal conductivity, 10 nm diameter and 1 μm length. Measurement enabled by this measurement platform will improve our understanding of thermal transport in confined nanostructures.Copyright