Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y12: Focus Session: Themoelectrics Nanomaterials II |
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Sponsoring Units: DMP GERA FIAP Chair: Austin Minnich, CalTech Room: 314 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y12.00001: High Temperature Thermal Conductivity from First Principles Christian Carbogno, Rampi Ramprasad, Matthias Scheffler In spite of significant research efforts, a first principles determination of the thermal conductivity at high temperatures has remained elusive. Under such conditions, techniques that rely on the harmonic approximation are no longer valid, while standard non-equilibrium molecular dynamics methods require huge temperature gradients that lead to deviations from Fourier's law. The Green-Kubo method [1], which does not suffer from these shortcomings, involves the assessment of the thermal conductivity from the auto-correlation of the heat flux in equilibrium. In classical MD, the heat flux is computed from the energetic contributions of the individual atoms; we show that the Green-Kubo approach can be reformulated in terms of the energy and stress densities [2], which are directly accessible in DFT calculations. This approach leads to a unique definition of the heat flux that does not rely on any partitioning scheme for the total energy. We critically discuss the computational cost, the accuracy, and the applicability of this approach by investigating the thermal conductivity for oxides and semiconductors with low thermal conductivities.\\[4pt] [1] R. Kubo, M. Yokota, S. Nakajima, J. Phys. Soc. Jpn. 12, 1203 (1957).\\[0pt] [2] R. Ramprasad, J. Phys. Condens. Matter 14, 5497 (2002). [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y12.00002: Nonlinear thermoelectric transport in mesoscopic systems Jonathan Meair, Philippe Jacquod We construct a scattering theory of weakly nonlinear thermoelectric transport through mesoscopic conductors. To preserve gauge invariance interaction induced potentials within the conductor must be self-consistently determined. We describe how to do this and apply our theory to calculating the leading nonlinear contribution to both electrical and heat currents. We present sum rules for our nonlinear response coefficients that must hold for current conservation and gauge invariance to be satisfied. We illustrate the method by investigating the thermoelectric response of a quantum point contact and a resonant tunneling barrier. [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y12.00003: Monte Carlo Simulations of Mode Dependent Phonon Transport in Nanostructured Thermoelectric Materials Takuma Hori, Junichiro Shiomi Nanostructuring are efficient process to lower the lattice thermal conductivity and thus enhance thermoelectric performance of semiconducting materials. Here, detailed knowledge of phonon transport properties in the nanostructures is needed for prediction of performance and/or optimization of structures. The approach to solve the linearized phonon Boltzmann transport equations stochastically by Monte Carlo method has been demonstrated to be useful to obtain phonon transport properties in mesoscale and complex structures. In this study, we have performed the Monte Carlo simulations to investigate phonon transport properties in nanostructured thermoelectric materials. With the mode-dependent bulk phonon transport properties obtained by first-principles-based calculations, the Monte Carlo simulations are performed to investigate the influence of nanostructure length-scales on the mode-dependent lattice thermal conductivity and its sensitivity to interfacial phonon transmission. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y12.00004: Ab initio thermal transport properties of nanostructures from density functional perturbation theory. Thushari Jayasekera, Arrigo Calzolari, Ki Wook Kim, Marco Buongiorno Nardelli We present a comprehensive first principles study of the thermal transport properties of low-dimensional nanostructures such as polymers and nanowires. An approach is introduced where the phonon quantum conductance is computed from the combination of accurate plane-wave density functional theory electronic structure calculations, the evaluation of interatomic force constants through density functional perturbation theory for lattice dynamics and the calculation of phonon transport properties by a real space Green's function method based on the Landauer formalism. This approach is computationally very efficient, can be straight-forwardly implemented as a post-processing step in a standard electronic-structure calculation (Quantum ESPRESSO and WanT in the present implementation), and allows us to directly link the thermal transport properties of a device to the coupling, dimensionality, and atomistic structure of the system. It provides invaluable insight into the mechanisms that govern the heat flow at the nanoscale and pave the way to the fundamental understanding of phonon engineering in nanostructures. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y12.00005: Designing Graphene-based Thermoelectric materials with Chemical Functionalization Jeong Yun Kim, Jeffrey Grossman Graphene has been explored as a thermoelectric (TE) material recently due to its superior mobility and ambipolar nature. However, the extremely high thermal conductivity ($\kappa )$ and only moderate Seebeck coefficient (S) make a graphene monolayer a highly inefficient TE material. Graphene superlattices made with chemical functionalization offer the possibility of tuning both the thermal and electronic properties via nano-patterning of the graphene surface. In this work, we investigate the effects of chemical functionalization on the thermoelectric transport properties of graphene using classical and quantum mechanical calculations. Our calculations show that chemical functionalization can control the power factor by changing the width of the pure graphene region and functionalization configuration, as well as $\kappa $ depending on the functional groups and functionalization coverage. These results suggest that chemical functionalization could be an efficient route to designing graphene-based TE materials. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y12.00006: Power-efficiency trade-off due to density of states (DOS) distortion in a molecular thermoelectric system Priyanka deSouza, Bhaskaran Muralidharan The issue of how a distortion in the electronic DOS affects nanoscale thermoelectric performance is addressed within an ``electrical engineering'' perspective. This view point is based on the direct evaluation of the overall efficiency and power from device current-voltage characteristics and gives a more complete picture of the thermoelectric performance in comparison to the traditional ``figure of merit'' based material science approach. We use representative examples from molecular conduction to study the trade-off between maximum efficiency and the maximum power generated within the set up. The trade-off is maximum for the well known example of a sharply resonant molecular level which represents the ultimate distortion in the electronic density of states. As the distortion is reduced via contact induced broadening, we obtain a smaller trade-off between maximum power and efficiency. We then present the effects of self consistent charging, contact induced asymmetry and the HOMO-LUMO gap on the thermoelectric performance. In all cases we compare our non-equilibrium calculations with zT calculations, and our results depict that zT is not the sole metric for the assessment of nanoscale thermoelectric performance. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y12.00007: Solvothermal synthesis and thermoelectric property of undoped and indium doped lead telluride nanoparticles Kamal Kadel, Wenzhi Li Undoped and indium (In) doped lead telluride (PbTe) nanostructures were synthesized via solvothermal/hydrothermal route. The crystallinity of the as-prepared un-doped and In-doped PbTe sample were examined by X-ray diffraction (XRD) which indicated the formation of face centered single phase cubic PbTe. Lattice constant calculation from XRD pattern revealed the formation of un-doped and In-doped PbTe crystals with almost similar size. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examinations indicated that undoped and In-doped PbTe nanostructures were mostly cubically shaped and highly crystalline. The effect of the synthesis temperature on the structure and morphology of undoped PbTe was also investigated; it was found that the particle size increased with the synthesis temperature. Thermoelectric property of as-synthesized lead telluride sample was also investigated. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y12.00008: Temporal evolution of Seebeck coefficient in an ac driven strongly correlated quantum dot Ali Ihsan Goker, Elif Gedik We study the response of the thermopower of a quantum dot in the Kondo regime to sinusoidal displacement of the dot energy level via a gate voltage using time dependent non-crossing approximation and linear response Onsager relations. Instantaneous thermopower begins to exhibit complex fluctuations when the driving amplitude is increased at constant driving frequency. We also find that the time averaged thermopower decreases steadily until it saturates at constant driving amplitude as a function of inverse driving frequency. On the other hand, time averaged thermopower is found to be quite sensitive to ambient temperature at all driving frequencies for large driving amplitudes. We discuss the underlying microscopic mechanism for these peculiarities based on the behaviour of the dot density of states. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y12.00009: Thermopower Measurements of Highly Conducting Single-Molecule Devices Jonathan R. Widawsky, Wenbo Chen, Hector Vazquez, Taekyeong Kim, Mark S. Hybertsen, Ronald Breslow, Latha Venkataraman We measure the conductance ($G)$ and thermopower ($S)$ of highly conducting single-molecule junctions with Au electrodes. The junctions are formed and measured using a scanning tunneling microscope-based break-junction technique. The target molecules are synthesized with SnMe$_{\mathrm{3}}$ terminations that cleave off \textit{in situ}, allowing for the formation of direct Au-C covalent bonds to the electrodes[1,2]. We compare the conductance and thermopower for two families of molecules: pi-conjugated polyphenyls, which have a high conductance and thermopower, and sigma-bonded alkyl systems, where we observe a significant thermopower despite the low conductance. For these measurements, we use the most probable thermopower to determine a power factor, \textit{GS}$^{\mathrm{2}}$, for each molecular junction studied. Our results show that the molecular thermopower increases systematically and non-linearly with molecular length and also that the power factor is exceptionally large for the case of the biphenyl. [1] Z. L. Cheng, R. Skouta, H. Vazquez\textit{ et al.}, Nat. Nano. \textbf{6}, 353 (2011). [2] W. Chen, J. R. Widawsky, H. V\'{a}zquez\textit{ et al.}, J. Am. Chem. Soc. \textbf{133}, 17160 (2011). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y12.00010: Operating Characteristics of a Microfabricated Phonon Spectrometer Richard Robinson, Jared Hertzberg, Obafemi Otelaja, Mahmut Aksit Phonon scattering exhibits a strong influence on the thermal properties of nanostructures. By promoting phonon scattering at surfaces and interfaces, a nanostructured thermoelectric material may achieve reduced thermal conductivity and enhanced thermoelectric efficiency. While phonons over a wide frequency range contribute to energy transport, thermal conductivity measurements capture only their combined effect. However, a window into phonon transport in nanostructures at specific frequencies could provide unique information and also serve as a crucial test platform for phonon transport theories. To this end, we have constructed a microfabricated phonon spectrometer. At a temperature of 0.3K, a superconducting tunnel junction locally generates non-thermal distributions of phonons and transmits them through adjacent silicon micro- and nanostructures.[1] We employ modulation techniques to select narrow frequency bands of phonons at frequencies up to hundreds of GHz. This prototype phonon spectrometer achieves phonon frequency resolution as low as $\sim$10 GHz, more than an order of magnitude lower than comparable thermal methods. We describe the other key parameters of this technique: spatial resolution, frequency range, dynamic range, signal-to-noise ratio and calibration methods. This work was supported in part by the National Science Foundation under Agreement No. DMR-1149036.\\[4pt][1] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011) [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y12.00011: Nonlinear thermoelectric response of quantum dots Stefan Kirchner, Farzaneh Zamani, Enrique Munoz, Lukas Merker, Theo Costi The thermoelectric transport properties of nanostructured devices continue to attract attention from theorists and experimentalist alike as the spatial confinement allows for a controlled approach to transport properties of correlated matter. Most of the existing work, however, focuses on thermoelectric transport in the linear regime despite the fact that the nonlinear conductance of correlated quantum dots has been studied in some detail throughout the last decade. To go beyond the linear response regime, we use a recently developed scheme [1], to address the low-energy behavior near the strong-coupling fixed point at finite bias voltage and finite temperature drop at the quantum dot. We test the reliability of the method against the numerical renormalization group [2] and determine the charge, energy, and heat current through the nanostructure. This allows us to determine the nonlinear transport coefficients, the entropy production, and the fate of the Wiedemann-Franz law in the non-thermal steady-state~[3].\\[4pt] [1] E. Munoz et al, arXiv:1111.4076.\\[0pt] [2] L. Merker et al, in preparation.\\[0pt] [3] S. Kirchner, F. Zamani, and E. Munoz, in ``New Materials for Thermoelectric Applications: Theory and Experiment,'' Springer (2012). [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y12.00012: Atomistic quantum thermal conductance profile of hybrid interfaces Jeevaka Weerasinghe, Arrigo Calzolari, Marco Buongiorno Nardelli Atomistic structure at interfaces has been shown to play a critical role in quantum thermal conductance across nanoscale interfaces. In general, current models derive phonon transmission probabilities from bulk material properties. However, they do not account for the effect of atomic scale interfacial structure on thermal conductance. Here we use an ab initio approach that we have recently developed to investigate the correlation between interfacial atomic structure and quantum thermal conductance. In particular, we will discuss the electronic structure and thermal conductances in systems with hybrid metal/self-assembled monolayer (SAM) interfaces with varying chemistry in order to elucidate the role of metal-organic bonds in the thermal properties of complex assemblies. Our methodology integrates the accurate self-consistent minimization of the ground state electronic structure via first-principles density functional theory based calculations, the determination of interatomic force constants via density functional perturbation theory, and the calculation of the quantum conductance using a real space Green's function formalism based on the Landauer approach. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y12.00013: Studies of Thermal Conductivity in Hybrid Organic-Inorganic Nanocrystal Arrays Wee-Liat Ong, Sara Rupich, Dmitri Talapin, Alan McGaughey, Jonathan Malen The thermal conductivity of nanocrystal arrays (NCAs) is studied and found to be tunable through the nanocrystal diameter, and chemistry - a conclusion that is supported by our Molecular Dynamics simulation. Nanocrystal arrays self-assemble from colloidal molecule-coated nanocrystals into close-packed 3D films. It has been suggested that their electronic and thermal transport properties can be decoupled, enabling a resolution to the conflicting needs of various thermal management and solid-state energy conversion applications (e.g. high figures of merit materials for thermoelectric, high-efficiency photovoltaic materials). Although the electronic transport in NCAs has been studied extensively, little is known about their thermal transport. We herein report both experimental measurements and modeling performed to elucidate the thermal transport mechanisms in NCAs. Various factors including the geometry and chemical compositions of the NCAs will be presented. Simulation results showed good agreement with the observed experimental trends, providing a complementary computational approach for elucidating and optimizing NCA thermal properties. [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y12.00014: Probing tunable thermal properties of organic hetero-junctions Shubhaditya Majumdar, Scott N. Schiffres, Jonathan A. Malen, Alan J.H. McGaughey The ability to tune physical properties of new organic-inorganic heterojunctions is essential for their popularity in the fields of molecular electronics and energy-generation devices. Intimate associations between the organic and inorganic components at the nano-scale level lead these materials to possess unique transport properties. Here, we probe the thermal conductance of self-assembled monolayer (SAM) junctions using both computational and experimental methods. SAM junctions are ordered, periodic arrays of a single layer of organic molecules chemically bonded to two inorganic substrates. Molecular dynamics simulations are performed on the SAM junctions to study the effect of physical parameters on the junction thermal conductance. These include atomic masses of leads, junction temperature, molecular chain length, and surface coverage. Another important aspect is the contribution of the stiff C-H bonds to thermal transport, an analysis of which is also presented. Lattice dynamics calculations are employed to study the effect of molecular vibrations on the thermal coupling between the leads. The SAM junctions are prepared in the laboratory through a combination of solution immersion and transfer printing techniques. Frequency domain thermo-reflectance (FDTR) -- a laser-based non-contact measurement scheme to probe the thermal properties of thin films, is employed to study the samples. A comparison between the results obtained from these studies is thus presented. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y12.00015: Seebeck Coefficient of Manganese Oxide Nanoparticles as a Function of Ohmic Resistance Nicholas Francis, Morgan Hedden, Costel Constantin Due to the ever increasing energy demand and growing global concern over the environmental impact of CO$_{2}$ emissions, there is an urging need to seek solutions to transit from fossil fuels to sustainable energy. Thermoelectric (TE) materials show great promise for converting waste heat energy into electricity. TE systems have many unique advantages such as silent operationality, time reliability, and dimensional scalability. Most recently, researchers Song et al. [1] found that MnO$_{2}$ nanoparticles show a giant Seebeck coefficient of S $=$ 20 mV/K, which is100 times higher than bismuth telluride, one of the best TE materials. Song et al.[1] concluded the paper claiming that the giant S is related to the surface density of the electronic states (DOS). However, they provided very little information about the S as a function of Ohmic resistance [R] for different nano particle sizes which can give information about the DOS. Our preliminary results show that there is a sudden increase of S from 0.33-0.63 mV/K as R increases from 80-110 Ohms. This transition has never been seen before and it can give clues as to the existence of the Giant S observed in this material.\\[4pt] [1] F. Song, L. Wu and S. Liang, Giant Seebeck coefficient thermoelectric device of MnO$_{2}$ powder, Nano. 23, 085401 (2012). [Preview Abstract] |
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