Session B17: Focus Session: Thermoelectrics - Nanowires and Other Nanostructures

Thermoelectric Transport in Bismuth Telluride Nanoplates, Semiconductor Nanowires, and Silicide Nanocomposites: Effects of Low Dimensionality, Surface States, Interface Structures, and Crystal Complexity

This presentation will review recent measurement results of thermoelectric properties of individual bismuth telluride nanoplates, semiconductor nanowires, and silicide nanocomposites. In experiments with these realistic nanostructured materials, a number of factors influence the transport properties. For example, unintentional doping, interface roughness and impurities can often obscure the predicted effects of the low-dimensional electronic density of states and the protected surface states, the latter of which have been suggested for bismuth telluride and other thermoelectric materials, now also referred as topological insulators. Similarly, impurities and defects as well as contact thermal resistance can play an important role in phonon transport in nanostructures, making it nontrivial to quantify the actual effects of phonon-surface scattering and other intriguing low-dimensional phonon transport phenomena. Because of these experimental complications, diverse theoretical interpretations of experimental results have appeared in the literature, and will be discussed. Moreover, the effects of twin defects and crystal complexity on thermoelectric transport in nanostructures will be examined based on measurement results of III-V and silicide nanostructures.   

Universal Scaling Relations for the Thermoelectric Power Factor of Semiconducting Nanostructures

We present a model for the power factor (\textit{PF}) of wires and thin films which bridges between strongly confined and bulk behavior. Relevant scattering mechanisms are considered in the framework of the relaxation time approximation. Previous models for the transport properties of nanostructured thermoelectric materials predicted vast improvements in the \textit{PF} values over bulk due to discretization of the electron density-of-states function as the result of confinement. Using this model, we find that the \textit{PF} of nanowires and thin films in fact falls below the bulk value for most of the experimentally-accessible size range. We find a non-monotonic relationship between \textit{PF} and system size in all systems studied---regardless of the particular materials parameters and dominant scattering mechanisms. The effects of the size, dimensionality, temperature, carrier concentration and dominant scattering mechanism in single-carrier semiconductors will be discussed. In the framework of the \textit{constant} relaxation time approximation, universal scaling relations for the power factor of all single-carrier semiconductors are obtained.   

Thermal conductivity of silicon nanowires: interplay between core defects and surface roughness

Recent experiments [1] suggested that the thermal conductivity ($\kappa$) of Si nanowires may be reduced by about two orders of magnitude compared to that of bulk Si ($\kappa_{bulk}$), making them attractive materials for thermoelectric applications. Size reduction plays an important role [2] in determining such reduction but it does not fully account for recent measurements [1]. We investigated $\kappa$ in wires with 15 nm diameter (comparable to experimental sizes) using large scale molecular dynamics simulations. We show that, unlike the case of thin [3] (2-3 nm diameter) rods, the presence of an amorphous layer at the surface accounts only for a decrease by a factor of 4 in $\kappa$ with respect to that of wires with smooth surfaces. It is the combined effect of defects in the core and rippled surfaces that enables a decrease up to a factor of 90 with respect to $\kappa_{bulk}$. Work supported by DOE/SciDAC-e.\\[4pt] [1] A.I. Hochbaum et al., Nature 451, 163 (2008); A. I. Boukai et al., Nature, 451, 168 (2008).\\[0pt] [2] D.Li et al. APL 2003.\\[0pt] [3] D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009).   

Electric-Field Guided Synthesis of Standalone Nanowire Arrays for Thermoelectric Applications

Theoretical studies have suggested that figure of merits of thermoelectric materials can be improved through fabrications of nanoscaled thermoelectric materials. Thin films are expected to result in up to a seven fold improvement in efficiency over bulk materials; even greater enhancement, up to 15 times in efficiency, is expected for very thin wires. Researchers have already succeeded in increasing the efficiency by making thin-layered materials and nanowires of a non-thermoelectric material, i.e. silicone. For practical applications, however, arrays of standalone nanowires or isolated thermoelectric nanowire devices without any template will be required. Here I present an electromagnetic field guided nanostructured synthesis of an array of standalone thermoelectric nanowires. This technique utilizing electric field as a guide in building highly ordered nanostructures will be an elegant, ``bottom-up'' method for nanofabrication without the need of a template. An array of quasi-one dimensional chalcogenide nanowires has been successfully grown in between two conducting plates. Thermoelectric transport measurements including thermalconductivity, thermoelectric power and figure of merit can be easily performed in the device, without any need of complicated electron beam lithography technique.   

Thermoelectric Property Characterization of Suspended Silicon Nanowires

A key challenge in the measurement of thermal and electrical properties of suspended nanowires (NWs) is the ability to obtain clean, reliable electrical and thermal contact between the nanowire and device. We report on a technique for aligning NWs to sub-micron accuracy on a suspended device made of two SiNx membranes with the assistance of a polymethyl methacrylate (PMMA) carrier layer. Electrical and thermal contact is made by using electron beam lithography to pattern a window in the PMMA over the device electrodes, followed by oxide etching and surface passivation in wet etchant, metal deposition through a shadowmask, and lift-off. We demonstrate this technique on rough Si nanowires grown by metal-assisted chemical etching. The whole assembly is clean and without contamination. Thermoelectric properties and their correlation with crystal structure will be discussed   

Thermoelectric Properties of Low-Dimensional Si and Ge Based Nanostructures

Low-dimensional thermoelectric nanostructures based on Si and Ge are promising candidates for high performance energy conversion and generation applications. 1D nanowires (NWs) and 2D superlattices of Si, Ge and Si/SiGe have experimentally demonstrated excellent performance. In these confined systems the electrical conductivity, the thermal conductivity, and the Seebeck coefficient can be designed to some degree independently so as to achieve enhanced ZT values as compared to the related bulk material values. In this work, we calculate the thermoelectric coefficients of scaled Si and Ge NWs and thin-layers. We use the sp3d5s* tight-binding model for the electronic structure and linearized Boltzmann transport theory. Our calculations include structures of feature sizes up to 12nm containing over 5500 atoms. This study indicates that the confinement length scale can be exploited as a degree of freedom in designing the material properties. We examine n-type and p-type materials of different cross section sizes and confinement/transport orientations and provide optimization directions for power factor improvement. Finally, using measured values for the lattice thermal conductivity, the ZT is estimated.   

Gate Controlled Tuning of Seebeck Coefficient in InAs Nanowires

Here we present measurements of the Seebeck Coefficient in InAs nanowires grown by Chemical beam epitaxy. Nanowires were mechanically transferred onto a SiO2 substrate with a global metallic backgate, and Ohmic contacts to a single nanowire were made by standard electron beam lithography techniques. We were able to tune the measured thermovoltage in the nanowire by field effect gating and correlate this behavior with the conductance through the nanowire. Interestingly, large enhancements in the thermoelectric power factor were seen at low temperature for certain gate voltages. Such controllability allows for optimizing the thermoelectric response of the nanowire at different substrate temperatures.   

Phonon transmission and thermal transport in GaAs-AlAs superlattices from first-principles

Using the Green's function method and Landauer's formula, we formulate phonon transport in a 3D superlattice. The theory is harmonic and describes coherent (elastic) transport of heat through a periodic structure which may also have disorder present at its interfaces. We compute the force constants from first-principles density functional calculations and use them to compute the transmission, and the thermal conductance of a GaAs-AlAs superlattice versus length and temperature. We also investigate the effect of mass disorder at the interface and anharmonicity, to locate the transition from coherent to incoherent transport. Results are finally compared with experimental measurements.   

Phonon density of states in nanocrystalline Si1-xGex explored by inelastic neutron scattering

Recently there have been significant advances in the efficiencies of traditional thermoelectric compounds gained via the creation of thermoelectric nanocomposites possessing substantially reduced thermal conductivity relative to their bulk counterparts [1,2]. The dramatic reduction in the heat transport of these nanocomposites is often attributed to the increased interface scattering of phonons or induced surface/boundary modes; however notably little work has been put worth into exploring the detailed changes in the phonon density of states in many of these functional nanocomposite samples. Here we present inelastic neutron scattering measurements exploring the phonon density of states in a series of Si1-xGex thermoelectric nanocomposites. The evolution of the phonon spectral weight distribution and linewidths as a function of Ge-doping will be discussed and compared to the known bulk phonon density of states in this system. \\[4pt] [1] Giri Joshi et al., Nano Letters 8, 4670 (2008). \\[0pt] [2] X. Wang et al., App. Phys. Lett. 93, 193121 (2008).   

Ab-initio investigation of thermal transport in alloyed and nanostructured materials

A whole spectra of intriguing physical properties appears in conventional materials when structural features reach nanoscale. Since thermal conductivity is controlled by the heat carriers' mean free paths, it becomes of paramount importance to understand and engineer the role of alloying and nanostructuring on transport coefficients. First-principles calculations often provide accurate microscopic parameters, but at significant computational cost even for ideal, perfect systems. We present a hybrid classical-quantum method to compute thermal conductivity from both harmonic and anharmonic terms using Boltzmann transport formalism. We combine first-principles calculations of harmonic terms and force-field calculations of third-order and fourth-order force constant. Results for SiGe will be discussed to show the validity of approach. We also discuss the effects of nanostructuring by introducing boundary scattering contributions, as well as mechanisms of filler rattling in thermoelectric skutterudites.   

ZT enhancement using nanocomposite materials

The effect of interface scattering on the performance of disordered, nanocomposite thermoelectric materials is studied theoretically (within a linear response formalism), using effective medium theory, and direct numerics. The general relation between interfacial and bulk transport properties which results in an enhanced ZT is determined. Given these requirements of interfacial transport properties, a series of microscopic calculations of interface scattering are presented to assess the feasibility of using nanocomposites for ZT enhancement.   

Anderson Localization of Phonons in Random Multilayer Thin Film Thermoelectrics

Anderson localization of phonons in random multilayer thin films has been explored as a means for reducing latttice thermal conductivity in thermoelectric materials. Silicon based systems have been explored due to silicon's high crust abundance and Seebeck coefficient. Reverse non-equilibrium molecular dynamics simulations have been used to determine the thermal conductivity of silicon in which randomly selected atomic planes (20\% of lattice planes) are subject to mass increase. The simulation results indicate that the lattice thermal conductivity of silicon can be decreased by a factor of over ten thousand (to 15 $\mathrm{mW}/{m \cdot K}$). Based on models in which the charge carrier mean free path is limited by scattering from the planes with mass disorder, the mobility of silicon is expected to reach values of 10 $\mathrm{cm}^2/{V \cdot s}$. At this mobility the thermoelectric figure of merit, ZT, is found to be greater than ten when the mass ratio of the disordered planes to that of silicon approaches 10. These results indicate that the pursuit of nanostructured silicon thermoelectric materials in the form of random multilayers may provide a path to efficient and sustainable thermoelectric materials.