Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F34: Focus Session: Thermoelectrics  Novel Approaches 
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Sponsoring Units: GERA DMP Chair: Alex Zevalkink, Jet Propulsion Laboratory Room: 704 
Tuesday, March 4, 2014 8:00AM  8:12AM 
F34.00001: Profiling the Local Seebeck Coefficient of InAsGaAs Quantum Dots Using Scanning Thermoelectric Microscopy YenHsiang Lin, Jenna Walrath, Simon Huang, Rachel Goldman Thermoelectric (TE) devices offer a method of recovering waste heat through solid state conversion of heat to electricity. However, the typical efficiencies of TE devices are 510{\%} which constitutes a barrier to wide spread use. There have recently been a number of reports of an increase in the bulk thermopower due to nanostructuring. In addition to our recent report of enhanced thermopower for GaAs embedded with indium nanocrystals [1], a theoretical study by Mahan and Sofo suggested that the best thermoelectric materials have a delta function density of states [2]. Quantum dots fit ideally into such a picture. To date, the influence of nanostructuring on the electronic LDOS and thermopower has been studied using spatially averaged measurements; a nanoscale investigation of the effects of nanostructures on thermopower has yet to be presented. To investigate the link between dimensionality and TE properties, we are examining structures ranging from QDs to bulklike layers, comparing SThEM measurements of the local Seebeck coefficient, S, with STS measurements of the local density of states (LDOS). STM, STS, and SThEM performed on InAs quantum dots (QDs) grown on GaAs. SThEM reveals enhanced Svalues near the QD edge; STS reveals bandbending at the QD/GaAs interface, suggesting that the S enhancement is due to interfacial charge accumulation. \\[4pt] [1] M. V. Warren, et. al, J. Appl. Phys. 114, 043704 (2013).\\[0pt] [2] Mahan and Sofo. Proc. Natl. Acad. Sci. USA 93:7436(1996). [Preview Abstract] 
Tuesday, March 4, 2014 8:12AM  8:24AM 
F34.00002: Experimental determination of the valence band of Bi$_2$Se$_3$ YiBin Gao, Bin He, Ioannis Androulakis, Joseph P. Heremans Ptype Bi$_2$Se$_3$ is predicted theoretically to have good thermoelectric properties[1], because its valence band has a high calculated density of states (DOS). In this presentation, ptype Bi$_2$Se$_3$ samples are prepared both as single crystals and as polycrystals. Shubnikov  de Haas (SdH) measurements are carried out in a rotating stage on single crystals to obtain the Fermi surface crosssections and the cyclotron effective masses. Thermoelectric transport measurements are done on polycrystals, and used to construct Pisarenko plots of Seebeck coefficient versus hole concentration. The Fermi surface crosssection measurements confirm the theoretically predicted [1] shape of the Fermi surface.Both cyclotron masses and Pisarenko plotsare in good agreement and show that ptype Bi$_2$Se$_3$ has a hole effective mass smaller than the theoretically predicted value. The reason for the discrepancy is not yet understood at this time. Reference: [1] Phys. Rev. X 1, 021005 (2011) [Preview Abstract] 
Tuesday, March 4, 2014 8:24AM  8:36AM 
F34.00003: Correlation between defect transition levels and thermoelectric operational temperature of doped CrSi$_{2}$ Abhishek Singh, Tribhuwan Pandey The performance of a thermoelectric material is quantified by figure of merit ZT. The challenge in achieving high ZT value requires simultaneously high thermopower, high electrical conductivity and low thermal conductivity at optimal carrier concentration. So far doping is the most versatile approach used for modifying thermoelectric properties. Previous studies have shown that doping can significantly improve the thermoelectric performance, however the tuning the operating temperature of a thermoelectric device is a main issue. Using first principles density functional theory, we report for CrSi$_{2}$, a linear relationship between thermodynamic charge state transition levels of defects and temperature at which thermopower peaks. We show for doped CrSi$_{2}$ that the peak of thermopower occurs at the temperature T$_{m}$, which corresponds to the position of defect transition level. Therefore, by modifying the defect transition level, a thermoelectric material with a given operational temperature can be designed. [Preview Abstract] 
Tuesday, March 4, 2014 8:36AM  8:48AM 
F34.00004: Firstprinciples study of thermoelectric properties of pyrite Yi Xia, Fei Zhou, Vidvuds Ozolins Due to its natural abundance, moderate band gap and good light absorption properties, pyrite (FeS$_2$) is being considered for use in nanocrystalline solar cells. Highquality ntype samples show high electron mobility, but their adoption in solar cells is hampered by low open circuit voltages. Here, using densityfunctional theory (DFT), we study charge and thermal transport properties of FeS$_2$. Using the DebyeCallaway model, we obtain lattice thermal conductivity in good agreement with experimental data, suggesting that significant reduction of lattice thermal conductivity would be needed for thermoelectric applications. In addition, we find that holes in ptype pyrite form localized small polaron states, which naturally explains low hole mobilities observed experimentally. [Preview Abstract] 
Tuesday, March 4, 2014 8:48AM  9:00AM 
F34.00005: Efficient simulation of quasiballistic heat transport in nanostructured materials Giuseppe Romano, Jeffrey Grossman Modeling nanoscale heat transport is challenging because of the presence of phonon size effects, which cannot be captured by Fourier's law. Furthermore, accurate phonon transport calculations require the knowledge of phonon dispersion curves and scattering times, which are unknown for most promising thermoelectric materials. We introduce a model based on the Boltzmann Transport Equation that computes heat transport in nanostructured materials by only using the bulk thermal accumulation function, which is a material property that can be directly obtained by experiments. Furthermore, our model is computationally convenient compared with other frequencydependent approaches. We apply this method to nanoporous Silicon and find good agreement with experiments. The presented method could be useful in the design of highefficiency thermoelectric materials. [Preview Abstract] 
Tuesday, March 4, 2014 9:00AM  9:12AM 
F34.00006: Experimental and Theoretical Studies of Thermoelectric Properties of Manganese (IV) Oxide Particles as a Function of Electrical Resistance Morgan Hedden, Nicholas Francis, Jason Haraldsen, Costel Constantin Thermoelectric (TE) materials show great promise for converting waste heat energy into electricity. TE systems have many unique advantages such as silent operation, time reliability, and dimensional scalability. Recently, researchers have found that MnO$_{\mathrm{2\thinspace }}$nanoparticles show a giant Seebeck coefficient of S $=$ 20 mV/K, which is 100 times higher than that of bismuth tellurideone of the best TE materials. However, no figureofmerit measurements (ZT) have been reported so far. In this project, we present preliminary results of ZT, Seebeck coefficient, thermal and electrical conductivities as a function of particle electrical resistance in the range of 1080 $\Omega $ for particle sizes in the range of 5 nm  150 $\mu $m. ZT values ranged between 0.120.18. The samples with the smallest particle size show the greatest promise for further increasing the ZT. For comparison to experiment, we also present density functional theory results for conductance and other transport properties. [Preview Abstract] 

F34.00007: ABSTRACT WITHDRAWN 
Tuesday, March 4, 2014 9:24AM  9:36AM 
F34.00008: Thermoelectric properties of electrondoped SrTiO3 thin films Elias FerreiroVila, Alexandros Sarantopoulos, Victor Leboran, CongTinh Bui, Francisco Rivadulla Two dimensional conductors are expected to show an improved thermoelectric performance due the positive effect of quantum confinement on the thermoelectric power, and the decrease of thermal conductivity by interface boundary scattering. The recent report of a large increase of the thermoelectric power in quantum wells of Nbdoped SrTiO3 (STO) seems to be in agreement with this hypothesis. However, extrinsic effects like the existence of oxygen vacancies that propagate away from the interface cannot be ruled out, and the results are far from clear. Here we will show the thermoelectric properties (electrical conductivity, Seebeck coefficient, and Hall effect), of epitaxial thinfilms of (La,Nb)doped STO. The films have been deposited by PLD on different substrates (STO, LAO...) to study the effect of tensile/compressive stress on the thermoelectric properties of the system. The oxygen pressure during the deposition was carefully controlled to tune the amount of oxygen vacancies and to compare with the cation doping. We have performed a systematic study of the transport properties as a function of thickness and doping, which along with the effect of stress, allows to understand the effect of charge density and dimensionality in an oxide system with promising thermoelectric properties. [1] H. Ohta et al. Nat. Mat. \textbf{6}, (2007) 129. [Preview Abstract] 
Tuesday, March 4, 2014 9:36AM  9:48AM 
F34.00009: Large transverse thermoelectric effects in single crystals of the quasionedimensional metal Li$_{0.9}$Mo$_6$O$_{17}$ Saeed Moshfeghyeganeh, Joshua Cohn, Carlos A.M. dos Santos, John J. Neumeier We present measurements of transverse thermoelectric (TE) effects in the temperature range 300500 K for single crystals of the quasionedimensional (q1D) metal Li$_{0.9}$Mo$_6$O$_{17}$ (lithium purple bronze). Prior work demonstrates a highly anisotropic Seebeck coefficient (S), with metallic $n$type behavior along the q1D chains (crystallographic \emph{b} axis), $p$type semiconductor behavior in the perpendicular, interchain direction (\emph{c} axis), and a difference $\Delta S\simeq 200 \mu$V/K near $T=450$~K. Significant transverse TE voltages, induced by applied temperature differences, and Peltier cooling, induced by applied currents, in specimens with body axes misaligned with the \emph{b} and \emph{c} axes will be discussed. [Preview Abstract] 
Tuesday, March 4, 2014 9:48AM  10:00AM 
F34.00010: Photonassisted thermoelectric properties of noncollinear spinvalves Xiaobin Chen, Dongping Liu, Wenhui Duan, Hong Guo We report theoretical analysis of thermalspin and thermoelectric properties of noncollinear spinvalves driven by a high frequency AC voltage bias. The spinvalve consists of two ferromagnetic contacts sandwiching a singlelevel or multilevel quantum dot (QD). A general formulation for the timeaveraged thermalspin and thermoelectric properties of spinvalves is derived within the nonequilibrium Green's function theory, which provides a starting point for further numerical calculations of these properties. Numerical results of a spinvalve having a spindegenerate singlelevel QD are given as an example. The AC bias induces various photonassisted transmission peaks which can greatly enhance the Seebeck coefficients and the figures of merit, and offer a new possibility to tune both the spindependent and normal thermoelectric properties of the spinvalve. Details of these properties and how they depend on the noncollinearity of the spinvalve, magnetic polarization, temperature, AC bias, and other control parameters are reported. A particularly interesting result is the opposite dependency of the thermoelectric properties on the magnetic polarization and noncollinearity for contacts with or without spin accumulation. [Preview Abstract] 
Tuesday, March 4, 2014 10:00AM  10:12AM 
F34.00011: Extreme Seebeck anisotropy in the quasionedimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$ Joshua Cohn, Saeed Moshfeghyeganeh, Carlos A.M. dos Santos, John J. Neumeier We present resistivity and thermopower measurements in the range $300~K\leq T\leq 500$~K on single crystals of the quasionedimensional (q1D) metal, Li$_{0.9}$Mo$_6$O$_{17}$ (LiPB) transverse to the q1D metallic chains. Direct electron transfer between the chains of this material is sufficiently weak that interchain transport above 400 K is predominated by thermal activation of valenceband states ($\sim 0.14$~eV below $E_F$), yielding a large, {\emph p}type interchain Seebeck coefficient that coexists with {\emph n}type metallic behavior confined along the q1D chains. A significant Seebeck anisotropy, $\Delta S\simeq 200\ \mu$V/K, along mutually perpendicular directions gives LiPB potential as a transverse thermoelectric. This anisotropy along with a relatively low interchain thermal conductivity $(\kappa\simeq 2$W/mK) results in a substantial transverse Peltier effect. [Preview Abstract] 
Tuesday, March 4, 2014 10:12AM  10:24AM 
F34.00012: PhotoSeebeck effect of ZnO single crystals and thin films Ryuji Okazaki, Ayaka Horikawa, Masaru Fujita, Hiroki Taniguchi, Ichiro Terasaki, Hiromichi Ohta We have investigated the thermoelectric properties of ZnO single crystals and thin films under illumination. In both samples, the electrical conductivity and the Seebeck coefficient are varied under ultraviolet light ($h\nu$ = 3.4 eV) while a negligible change is observed under visible green light ($h\nu$ = 2.4 eV), indicating a carrier excitation across the band gap of ZnO ($E_g\sim$ 3.3 eV) by the ultraviolet illumination. This phenomenon thus can be ascribed to a photo doping effect into thermoelectric materials [1]. The carrier concentration doped by illumination is estimated to be about 10$^{19}$ cm$^{3}$, which is close to the optimal value for conventional thermoelectrics, suggesting a possible optical control of the thermoelectric efficiency. We also investigate the sample thickness dependence of the photoSeebeck effect in ZnO thin films, whose thickness is comparable to the absorption length of ultraviolet light. These results are compared with the bulk sample results in terms of a parallelcircuit model consisting of photoexcited metallic and unexcited insulating layers. \\[4pt] [1] R. Okazaki {\it et al}., J. Phys. Soc. Jpn. {\bf 81}, 114722 (2012). [Preview Abstract] 
Tuesday, March 4, 2014 10:24AM  10:36AM 
F34.00013: Infrared power generation in an insulated environment Yosyp Schwab, Harkirat Mann, Brian Lang, Jarrett Lancaster, Ronald Parise, Anita VincentJohnson, Giovanna Scarel Alternative energy sources are an increasingly popular field of research, in particular energy harvesting through solar radiation, focusing on infrared (IR) radiation. Exploitation of readily available thermal energy is particularly interesting due to possible widespread applications. This work examines the behavior of thermoelectric devices exposed to infrared radiation in a controlled environment. Thermoelectric power generators work according to the Seebeck effect, where the temperature difference $\Delta $V induced between the two junctions is linearly proportional to the voltage difference $\Delta $T across the two contacts. Our experimental results show [1] that heat and radiation do not activate the same mechanisms in the thermoelectric power generator. Analysis and simulation further support the distinction between the IR and heat power generation. In particular, $\Delta $T and $\Delta $V have a linear and exponential behavior versus time with heat and IR, respectively. Our work is of significant importance for designing IR sensors, detectors, and radiation harvesting devices. \\[4pt] [1] Y. Schwab, H. S. Mann, B. N. Lang, J. L. Lancaster, R. J. Parise, A. J. VincentJohnson, and G. Scarel, ``Infrared power generation in an insulated compartment,'' Complexity, in press (2013). [Preview Abstract] 
Tuesday, March 4, 2014 10:36AM  10:48AM 
F34.00014: Modulation of thermopower in heater integrated fieldeffect molecular devices Youngsang Kim, Wonho Jeong, Kyeongtae Kim, Woochul Lee, Pramod Reddy Study of thermopower in molecular junctions is of great importance for understanding charge transport mechanisms as well as for making efficient energy conversion devices. In order to achieve a large thermoelectric efficiency (figure of merit, \textit{ZT}), it is crucial to simultaneously increase the electrical conductance and Seebeck coefficient of junctions. The electrical conductance of molecular junctions is directly proportional to the transmission ($T(E))$ at the Fermi level ($E_{F})$, while the Seebeck coefficient is proportional to the energy derivative of $T(E)$ at $E_{F}$. In this study, we successfully fabricated electromigrated break junction with integrated heater devices to establish temperature differentials across molecular junctions and study the possibility of tuning the electronic structure to simultaneously increase $T(E)$ and its energy derivative at $E_{F}$. Further, using this platform, we studied the thermopower and the IV characteristics of molecular junctions while modulating the electronic structure using a gate electrode. Our results unambiguously show that the thermopower and the electrical conductance of molecular junctions can be simultaneously enhanced by tuning the electronic structure. These results could pave the way for both understanding energy conversion at the molecular scale and the development of novel thermoelectric devices. [Preview Abstract] 
Tuesday, March 4, 2014 10:48AM  11:00AM 
F34.00015: Manipulating thermal conductance across 3D/1D interface by impedance matching Jingjie Zhang, Carlos Polanco, Avik Ghosh Selfassembled monolayers (SAMs) are of special interest to nanoelectronic and thermal devices, because we can tune their properties by changing the bonding strength that links the SAMs to a thin film layer. We explain how this bonding strength influence heat across this 3D1D interfaces based on a frequency dependent broadening matrix that acts as a generalization of acoustic impedance. We demonstrate both how to build an equivalent ``impedance'' broadening matrix that captures the dimensionality mismatch at the 3D1D transition and the ``matching'' effect of the end group on an equivalent 1D1D interface. We calculate thermal boundary conductance (TBC) at metal/polymer interfaces with different terminal groups and polymers. The calculations are done with nonequilibrium Green's function formalism coupled with abinitio parameters for the chemical group functionalized systems. Our results confirm that in the low frequency spectrum, the stronger the bonding the larger the TBC. Nevertheless, when we consider the whole phonon spectrum, there is a sweet spot in the bonding strength that maximizes TBC. [Preview Abstract] 
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