Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J29: Focus Session: Thermoelectrics II: Dirac, Bi2Te3 & Nanostructures |
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Sponsoring Units: DMP FIAP GERA Chair: Joseph Heremans, Ohio State University Room: C123 |
Tuesday, March 16, 2010 11:15AM - 11:27AM |
J29.00001: Thermal transport in graphene junctions and quantum dots Yong Xu, Xiaobin Chen, Bing-Lin Gu, Wenhui Duan, Jian-Sheng Wang Thermal design for individual nanodevice is indispensable to nanoelectronics due to the more and more serious heat dissipation. As a significant support to the development of graphene-based nanoelectronics, we systematically investigate thermal transport in various graphene junctions and quantum dots by nonequilibrium Green's function method. We find that thermal transport shows quite different characteristics with respect to electronic transport. Thermal conductance of graphene junctions is insensitive to the detailed structure of the contact region and the width of wide part, while decreasing the width of narrow part will dramatically reduce thermal conductance. Moreover, graphene junctions with small connection angle and zigzag edge have better thermal transport properties. Thermal conductance of graphene quantum dots is extremely low in most cases due to the existence of the narrow constrictions. Our research provides guidance to thermal design for nanodevices and could eventually find applications in nanoelectronics and thermoelectricity. [Preview Abstract] |
Tuesday, March 16, 2010 11:27AM - 11:39AM |
J29.00002: Effect of Schottky barriers in quantum-engineered thermoelectric devices Raseong Kim, Mark Lundstrom Recent advancements in thermoelectric figure of merit mostly come from the substantially lowered lattice heat conduction. A question now is whether quantum engineered devices such as superlattices can provide improved electronic performance while still suppressing phonon transport. To address this question, we run quantum transport simulations using the non-equilibrium Green's function (NEGF) method. As a first step, we examine a simple one-dimensional wire and explore the effects of Schottky barrier on thermoelectric coefficients. For ballistic nanowires with no Schottky barrier, simulation results are consistent with those from the conventional analytical model. When there are Schottky barriers at the source/drain contracts, carriers must tunnel through the barrier to be injected to the channel, so the transmission decreases while the average energy of injected carriers increases. As a result, electrical and heat currents decrease while the Seebeck coefficient increases. The effects of momentum and energy relaxation scattering are also examined in the NEGF framework. Finally, the extension of this work to superlattices is discussed. [Preview Abstract] |
Tuesday, March 16, 2010 11:39AM - 11:51AM |
J29.00003: On Seebeck Coefficients in Nanostrucutred Graphene and Molecules Changwook Jeong, Mark Lundstrom Recently, there has been great interest in enhancing the Seebeck coefficient by a variety of techniques such as nanostructuring graphene, and appropriately engineering molecules. This raises the question of how much the Seebeck coefficient can be changed through bandstructure engineering. In this paper, we use a simple one level model for thermoelectrics as a baseline for comparison and show that within a single particle framework, all materials display a Seebeck coefficient that is close to the single level model. Under some conditions, much lower values can be obtained, but it does not appear possible within this framework to achieve substantially larger values. The conclusion is that familiar thermoelectric concepts apply to novel materials such as nanostructured graphene and to molecules and that to obtain substantially larger Seebeck coefficients, we will need to go beyond the single particle, independent electron picture and explore strongly correlated materials , electron-phonon coupling, etc. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:27PM |
J29.00004: Nernst effect in bismuth and graphite across the quantum limit Invited Speaker: Kamran Behnia Bismuth and graphite are elemental semimetals, which host a dilute liquid of highly mobile carriers of both signs. These features conspire to generate a very large Nernst coefficient. The quantum limit is attained when the magnetic field puts all electrons in their lowest Landau level and can be crossed in bismuth and graphite for particular orientations of the magnetic field. The fate of a three-dimensional electron gas pushed to this ultraquantum regime has been barely explored. According to recent studies on bismuth and graphite in the vicinity of the quantum limit, whenever a Landau level intersects the Fermi level, the Nernst signal sharply peaks and the oscillating signal exceeds by far the monotonous background. Both these features are absent in two-dimensional systems. Beyond the quantum limit, Nernst effect in bismuth detects field scales unexpected in the one-particle picture. Our recent angular-dependent Nernst measurements find that the band picture, quite successful in explaining the complex electronic spectrum of bismuth up to 9 T, is inadequate as the quantum limit is crossed, An enigmatic reorganization of electrons, most probably due to collective effects, occurs far beyond the quantum limit around B=40 T. \textbf{Collaborators:} Zengwei Zhu, Benoit Fauqu\'{e}, Huan Yang, Baptiste Vignolle, Cyril Proust, Arlei Antunes, Liam Malone, Tim Murphy, Luis Balicas, Yakov Kopelevich and Jean-Paul Issi. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J29.00005: P-type Bi$_{1-x}$Sb$_{x}$ above 300K Hyungyu Jin, Christopher Jaworski, Joseph Heremans Heavily-doped p-type bismuth was theoretically predicted to be a good thermoelectric material. Tin is a known monovalent acceptor in the semimetal bismuth, but even at moderate Sn concentrations, the material becomes compensated and even switches n-type as temperature increases. Antimony, on the other hand, is always p-type; this work aims at finding the concentration in Bi$_{1-x}$Sb$_{x}$ alloys where acceptor behavior on Sn switches. Historically, the regime where Bi$_{1-x}$Sb$_{x}$ exhibits p-type conduction has been limited to cryogenic temperatures, with Seebeck coefficients switching negative below 300K Tin-doped single crystals of composition Bi$_{1-x}$Sb$_{x }$with 0.16$<$x$<$0.5 have been synthesized using the Bridgeman method followed by a lengthy anneal. We measure and report here thermopower, electrical resistivity, thermal conductivity and thus zT measured from 2-400K and 2K carrier density as calculated from the Shubnikov-de Haas effect. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J29.00006: Towards a First-Principles Method for Calculating Thermal Conductivity Asegun Henry, David J. Singh There are a number of areas where direct calculations of thermal conductivity based on first principles would be useful, particularly thermoelectrics. This could also be important in other areas, such as planetary science where thermal models for planet interiors often employ assumptions about properties of materials at extreme temperatures and pressures. Here we describe a framework for computing the instantaneous heat flux of a material in a first principles molecular dynamics method. The time history of the heat flux is then used to compute the thermal conductivity via the Green- Kubo formalism. Our formalism is different from that used in classical molecular dynamics because in first principles dynamics the energy is not uniquely decomposed into a sum of energies of individual atoms and the forces on an atom are not normally obtained as a sum of contributions from specific other atoms. The approach presented here has the benefit of generality, as it can be applied to any phase of matter in the limit that it is subjected to reasonable thermal gradients, where the system response is linear. [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J29.00007: Low phonon thermal conductivity of the layered Bi-Te intermetallic alloys Peter Sharma, Alf Morales, Ana Lima Sharma, Monica Barney, Fivos Drymiotis, Jian He, Terry Tritt, James Turner Good thermoelectric materials should have as low a thermal conductivity as possible in order to increase the figure of merit \textit{zT}=\textit{TS}$^{2}$/\textit{$\rho \kappa $}. The thermal conductivity of a material is often tuned using microstructure, chemistry, and/or crystal structure. For example, alloy disorder combined with high atomic weights plausibly lead to the low thermal conductivity of commercial Bi$_{2}$Te$_{3}$-based thermoelectrics. The crystal structure of the Bi-Te intermetallic phases is composed of layers of Bi and Bi$_{2}$Te$_{3}$ structural units, which can be varied nearly continuously by adjusting the Bi/Te ratio. Compared to both Bi$_{2}$Te$_{3}$ and elemental Bi, we found that these materials have a strongly reduced thermal conductivity below room temperature. However, this reduction did not depend on the Bi/Te ratio. The Debye-Calloway model was used to explore the origin of the low thermal conductivity in these materials. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:15PM |
J29.00008: Atomically Thin Films of Bismuth Telluride: ``Graphene-Like'' Exfoliation and Thermoelectric Applications Desalegne Teweldebrhan, Vivek Goyal, Alexander Balandin It follows from theoretical predictions that a drastic improvement in ZT can be achieved in low-dimensional structures where electrons and phonons are strongly confined in one or two dimensions. The latter would require carrier confinement in a quantum well with a width on the order of $\sim $1 nm and very high potential barriers. Conventional materials growth techniques are not capable of producing such crystalline structures. In this talk, we show that separate Bi-Te atomic layers can be mechanically exfoliated from bulk bismuth telluride crystal following a procedure similar to the exfoliation of graphene. The presence of the van der Waals bonds between five-fold layers allowed us to disassemble bismuth telluride crystals into films with the thickness of five atomic layers and less [1]. The resulting quasi-2-D crystals retain their good electrical conduction and poor thermal conduction properties important for thermoelectric applications. [1] Supplemental Info at: \underline {http://ndl.ee.ucr.edu} [Preview Abstract] |
Tuesday, March 16, 2010 1:15PM - 1:27PM |
J29.00009: Manipulating Thermal Conduction in Nanostructured Bismuth Antimony Telluride via Bandstructure Modification Daryoosh Vashaee, Arash Mehdizadeh Dehkordi Nanostructured (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ alloy has been recently investigated for its improved thermoelectric properties mainly caused by the reduction in its thermal conductivity. Lattice and bipolar diffusion are the main components of thermal conduction in this alloy, the latter being dominant as the temperature increases above $\sim $150C. Nanostructuring provides means to reduce lattice thermal conduction by scattering phonons at grain boundaries, and to reduce bipolar diffusion by scattering electrons more than the holes due to the difference in their energy. This provides means to further enhance the thermoelectric properties of this alloy by adjusting both the grain size and the bandstructure of the alloy through different x-values. Our model calculations show that the optimum x-value can be different for the nanostructured alloy than the crystalline structure that further depends on the grain size. Our experimental measurement of the grown samples further confirms this fact. Our theoretical analysis is not only important for understanding different nanostructured alloys of (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$, but to identify means to reduce the bipolar thermal conduction through adjusting the grain size and the bandstructure parameters in other material systems. [Preview Abstract] |
Tuesday, March 16, 2010 1:27PM - 1:39PM |
J29.00010: Thermoelectric Properties of Nanostructured Bismuth Antimony Telluride with Different Alloy Compositions Arash Mehdizadeh Dehkordi, Daryoosh Vashaee We have investigated the thermoelectric (TE) properties of nanostructured (BixSb1-x)2Te3 alloys with different x values of 0.25,0.20,0.16, and 0.14. The nanostructured alloys were prepared via two different methods: mechanical alloying and melt alloying. In both methods we have started with pure elements and synthesized samples with different compositions. The materials prepared in both methods have undergone a consequent grinding process in order to make nanoscale crystallites. Powders were pressed into rod-like samples using a hot-press technique. We investigated the effect of different growth process parameters such as pressure, heating rate, and holding time on the TE material properties. Our results indicate that the melt alloying method is significantly less time-consuming in achieving the desired material composition and with higher compositional precision in comparison to the mechanical alloying approach. EDS and XRD analysis data show that the final composition of powders which were prepared by mechanical alloying deviates from the starting materials. Furthermore, melt alloying offers significant degrees of freedom to manipulate the composition of materials to optimize the TE properties. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J29.00011: Twin Boundary Structure in Bi$_{2}$Te$_{3}$: Experiment and Theory D.L. Medlin, Q.M. Ramasse, C.D. Spataru, N.Y. Yang Establishing the atomic structure and composition of interfaces in thermoelectric materials is important to understanding how these defects mediate thermal and electronic transport. Here, we discuss our experimental observations and theoretical calculations of the Bi$_{2}$Te$_{3}$ (0001) basal twin in nanocrystalline Bi$_{2}$Te$_{3}$. This interface is important both because it is common in tetradymite-structured thermoelectric compounds and because it serves as a useful model system for more complex interfaces. Macroscopically, the (0001) twin corresponds to a 180\r{ } rotation of the crystal about the [0001] axis, which reverses the stacking of the basal planes. The basal planes of Bi$_{2}$Te$_{3}$ are arranged in 5-plane groupings of alternating Bi and Te layers. Microscopically, one envisions three possible interface terminations: at the Te layer in the middle of the 5-layer packet, at a Bi layer, or at the Te-double layer at the junction of the 5-layer packet. Using aberration-corrected HAADF-STEM imaging, we have established that the twin boundary terminates at the Te-double layer. This result is consistent with \textit{ab initio} calculations, which predict that the lowest energy for the three candidate structures is for this termination. [Preview Abstract] |
Tuesday, March 16, 2010 1:51PM - 2:03PM |
J29.00012: Solvothermal synthesis and thermoelectric property of nanostructured bismuth selenide Kamal Kadel, Latha Kumari, Wenzhi Li, Jianu Huang, Paula P Provencio, Zhifeng Ren Bismuth selenide (Bi$_{2}$Se$_{3})$ nanostructures were synthesized via solvothermal method. The crystallinity of the as-prepared sample has been analyzed by X-ray diffraction, which shows the formation of rhombohedral Bi$_{2}$Se$_{3}$. Electron microscopy examination shows that the Bi$_{2}$Se$_{3}$ nanostructures have hexagonal plate like structure. The effect of the synthesis temperatures on the structure and morphology of the nanostructures has also been investigated. It is found that the particle size increases with the synthesis temperature. Thermoelectric properties of Bi$_{2}$Se$_{3}$ nanostructures have also been measured. A maximum figure-of-merit (ZT) of 0.096 was obtained at 523K and a minimum thermal conductivity of 0.751Wm$^{-1}$K$^{-1}$ at 300K. [Preview Abstract] |
Tuesday, March 16, 2010 2:03PM - 2:15PM |
J29.00013: Computational Study of Vibrational Thermal Conductivity - Effects beyond the Peierls-Boltzmann (PB) equation Tao Sun, P.B. Allen The heat flux j(ph) of phonons is the sum over modes of e(Q)v(Q)N(Q) (energy, velocity, and occupation). The PB equation relates N(Q) to the temperature gradient, and gives a theory, exact to second order in anharmonicity, for the high T form k=C/T of the thermal conductivity. We use classical molecular dynamics to evaluate the exact classical k(T) from the heat-current correlation [j(t)j(0)] for a 2-D Lennard-Jones triangular lattice. This keeps three corrections to PB theory: (1) a fully anharmonic potential V(LJ); (2) exact treatment of phonon-phonon interactions, not limited to low order; (3) an ``exact'' heat current operator j, with anharmonic terms beyond the quasiparticle version j(ph). Our work, which follows Ladd, Moran, and Hoover (Phys. Rev. B34, 5088 (1986)), finds large corrections to the PB form k=C/T, even though phonon quasiparticles are fairly well-defined. Restriction to 2-D enhances statistical and finite-size accuracy with little loss of realism. Our findings are (a) truncation of the Taylor expansion of V(LJ) gives the correct k(T), but terms up to 8th order are needed at higher T; (b) anharmonic terms in j are very important even at quite low T; (c) the computed contributions to k(T) from terms of j beyond j(ph) are qualitatively explained using the lowest non-diverging contribution of a perturbation expansion. [Preview Abstract] |
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