Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S12: Focus Session: Low Dimensional Thermoelectric Systems and Theory II |
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Sponsoring Units: DMP GERA FIAP DCOMP Chair: Frank Bridges, University of California, Santa Cruz Room: 007C |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S12.00001: Prediction of vibration modes and thermal conductivity for amorphous ZnO-based materials Yu-Ting Cheng, Anindya Roy, Michael L. Falk Amorphous materials, due to their distinct physical and chemical properties, have been widely used in photovoltaics, thermoelectrics and integrated circuits. Because the thermal conductivity is critical to the performance of such devices, the thermal transport in amorphous materials has received considerable attention in the last decade. So far, a number of experimental studies and theoretical models have reported the vibration modes and thermal conductivities for amorphous Si and SiO$_{2}$. However, the applicability of these vibration mode analyses and thermal conductivity models for other amorphous materials has not been studied. In this work, we employ the molecular dynamics (MD) simulations and Allen-Feldman (AF) theory [1] to investigate the vibration modes and thermal conductivity of amorphous ZnO-based materials. ZnO is basis of a promising class of n-type semiconductors for thermoelectric application. Additionally, from this work, the contribution of individual vibrational modes to the thermal conductivity can be characterized. These results are expected to guide the interpretation of thermal transport in amorphous ZnO-based materials and the optimization for their performance with different applications. [1] P. B. Allen and J. L. Feldman, Phys. Rev. B \textbf{48}, 12581 (1993). [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S12.00002: Optimization of thermoelectric properties in cross-plane superlattices - A 1D NEGF Study Mischa Thesberg, Mahdi Pourfath, Neophytos Neophytou, Hans Kosina Thermoelectric materials utilize carrier energy filtering through potential barriers to achieve improvements in the Seebeck coefficient. Barriers, however, tend to drastically reduce the electrical conductivity, and power factor improvements are difficult to be realized. In this work we present a fully quantum mechanical simulation study of thermoelectric transport in the presence of barriers for energy filtering. For this, we use the non-equilibrium Green's function (NEGF) method, including both acoustic and optical phonons. We show that power factor improvements can be achieved by properly adjusting a series of interrelated parameters: i) the position of the Fermi level, ii) the width, size and shape of the barriers as well as the separation between them, iii) the optical phonon energies. Our results provide insight on how to optimize superlattices and nanocomposite materials for enhanced thermoelectric properties. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S12.00003: Sub-Band engineering through superlattice based barrier heterostructures for higher thermoelectric efficiency Mahyar Pourghasemi, Jivtesh Garg There is a huge desire to increase operation speeds in modern integrated circuits as they get more compact. Heat generation in such a submicron devices is a key factor limiting their performances. As a solution, thermoelectric cooling in heterostructures can address heat dissipation issue in submicron devices. Performance of single barrier heterostructures depends strongly on several parameters including barrier height, barrier width and thermal conductivity of barrier. Superlattice structures have been known to have the lowest thermal conductivities reported for crystalline materials. Low thermal conductivity is beneficial for thermoelectric cooling as it reduces the heat flow from hot end to cold junction. Moreover the band offset between the barrier and base material can be easily tuned by changing the superlattice period. By optimizing the conduction band offset (barrier height), it is possible to control the Joule heating and also optimize the amount of heat absorbed due to Peltier cooling. We investigate the feasibility of using PbSe/PbSnSe superlattice in heterostructures using Monte Carlo simulation. The effect of different parameters such as barrier height, barrier width and superlattice thermal conductivity on thermoelectric cooling of such structures will be presented. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 9:12AM |
S12.00004: Development of thermal rectifier using unusual electron thermal conductivity of icosahedral quasicrystals Invited Speaker: Tsunehiro Takeuchi The bulk thermal rectifiers usable at high temperature were developed using the unusual increase of electron thermal conductivity of icosahedral quasicrystals (ICQ's) at high temperature. Our previously performed analyses in terms of linear response theory suggested that the unusual increase of electron thermal conductivity of ICQ was brought about by the synergy effect of quasiperiodicity and narrow pseudogap at the Fermi level. Since the linear response theory suggests that the unusual increase of electron thermal conductivity is coupled with the small magnitude of Seebeck coefficient, the composition of Al-Cu-Fe ICQ, where the thermal conductivity shows the most significant increase with increasing temperature, was determined with a great help of Seebeck coefficient measurements. Consequently obtained Al$_{\mathrm{61.5}}$Cu$_{\mathrm{26.5}}$Fe$_{\mathrm{12.0}}$ ICQ, which was characterized by the small magnitude of Seebeck coefficient, possessed 9 times larger value of thermal conductivity at 1000 K than that observed at 300 K. The increasing tendency of electron thermal conductivity with increasing temperature was further enhanced by means of small amount of Re substitution for Fe. This substitution definitely reduced the lattice thermal conductivity while the electron thermal conductivity was kept unchanged. The lattice thermal conductivity was reduced by 35 {\%} under the presence of 0.5 at.{\%} Re, and the thermal conductivity at 1000 K consequently became about 11 times larger than that at 300 K. The thermal rectifiers were constructed using our newly developed ICQ (Al$_{\mathrm{61.5}}$Cu$_{\mathrm{26.5}}$Fe$_{\mathrm{12.0}}$ or Al$_{\mathrm{61.0}}$Si$_{\mathrm{0.5}}$Cu$_{\mathrm{26.5}}$Fe$_{\mathrm{11.5}}$Re$_{\mathrm{0.5}})$ together with one of the selected materials (Si, Al$_{\mathrm{2}}$O$_{\mathrm{3}}$, CuGeTe$_{\mathrm{2}}$ or Ag$_{\mathrm{2}}$Te) that possess thermal conductivity decreasing with increasing temperature. The heat current flowing in the rectifiers was confirmed to show significant direction dependence. The consequently obtained \textit{TRR }$=$\textbar \textbf{\textit{J}}$_{\mathrm{large}}$\textbar / \textbar \textbf{\textit{ J}}$_{\mathrm{small}}$ \textbar \quad for the composite consisting of Al$_{\mathrm{61.0}}$Si$_{\mathrm{0.5}}$Cu$_{\mathrm{26.5}}$Fe$_{\mathrm{11.5}}$Re$_{\mathrm{0.5}}$/ CuGeTe$_{\mathrm{2}}$ reached 2.24, and that is the largest value ever reported for the bulk thermal rectifiers. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S12.00005: Thermopower measurements of atomic and molecular junctions using microheater-embedded mechanically-controllable break junctions Makusu Tsutsui, Takanori Morikawa, Akihide Arima, Masateru Taniguchi There has been growing interest in developing high-performance thermoelectric materials for realizing thermoelectric power generation. Quantum confinement effects in low-dimensional structures are expected to provide high electronic density of states for enhanced thermopower, and thus considered as a promising approach for achieving a high figure of merit (M. S. Dresselhaus et al., Adv. Mat. 19 (2007) 1043-1053). From this respect, it is interesting to study thermoelectric properties of atomic and molecular junctions and evaluate their potential as a thermoelectric material. Recently, we have developed a heater-embedded micro-fabricated mechanically-controllable break junction (MCBJ) for investigating the thermoelectric transport in single-atom and --molecule junctions. Using the MCBJ devices, we could repeatedly form stable junctions at room temperatures via a self-breaking mechanism with one side being heated by the adjacent microheater. In my presentation, I will show the results of simultaneous measurements of the thermoelectric voltage and the electrical conductance of atom-sized Au junctions and Au-benzenedithiol-Au junctions and discuss on the geometrical dependence of thermoelectric transport. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S12.00006: Phonon thermal conductivity of a nanowire attached to leads Selman Hershfield, Khandker Muttalib There is experimental evidence as well as theoretical proposals that nanowires can be made to have high thermoelectric efficiency by tuning the electronic properties; however, there is always a phonon contribution to the heat transport which reduces the thermoelectric efficiency. In the harmonic approximation we compute the transmission of phonons through a nanowire coupled to large leads. There is a finite thermal conductivity because of the restriction provided by the nanowire. The nanowire reduces the thermal transport because of the mismatch between the leads and wire modes. We examine the effect of disorder in three places: in the wire, in the leads near the wire, and in the leads far way from the wire. In some cases disorder can increase the thermal conduction because of enhanced mode coupling. We will discuss the implications of our results for thermoelectric nanowire devices. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S12.00007: Measurement of thermal boundary conductance at sintered Si-Si interface Masanori Sakata, Takuma Hori, Takafumi Oyake, Jeremie Maire, Masahiro Nomura, Junichiro Shiomi Performance of thermoelectric materials is enhanced by reducing thermal conductivity (TC) without appreciably decreasing electrical properties. Recently, nanocrystalline formed by compaction of nanopowder by sintering has been shown to be a promising solution for low TC and high scalability However, little is known about the thermal boundary conductance (TBC) of the grain boundaries, which dominantly affect the TC, because of the difficulty to directly measure the TBC of the local boundaries. We have therefore developed a process to fabricate a highly planer and uniform bonded-interface between Si thin film and Si substrate, which is suitable for measuring the TBC of the interfaces with time-domain thermoreflectance method. We have found that sintering temperature and HF removal of native oxide on the wafers can change the interface structures from uniform to local SiO$_{\mathrm{x}}$ structures, which alter the TBC from 0.1 to 1 GWm$^{-2}$K$^{-1}$ order. Moreover, crystal orientation mismatch can change the TBC by several times. Together with theoretical calculation that relates the TBC and TC of nanocrystalline Si, the measurement results identify the route to reduce the TC less than the state-of-art value. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S12.00008: Tuning nanoscale thermoelectricity with electron-electron interactions Arunima Coomar, Charles Stafford The Nonequilibrium Green's Function (NEGF) formalism is a powerful tool that provides a microscopic theory for interacting quantum systems out of equilibrium. In this talk, I will be presenting a few results obtained using the NEGF approach combined with pi-electron effective field theory to study the thermoelectric transport properties such as the thermopower (S) and the dimensionless figure of merit (ZT) across single-molecule junctions with pi-conjugated molecular systems, which exhibit destructive quantum interference of the electron waves. Some interesting results showcasing the tuning of the thermoelectric properties by embedding the junctions in a dielectric medium will be presented, along with our ongoing investigations of the transmission node spectrum in these molecular junctions, and the enhanced thermoelectricity resulting from it. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S12.00009: Thermal Transport Properties of Low Dimensional SI, GE, and SI-GE Superlattice Structures Ali Kandemir, Cem Sevik Potential low dimensional thermoelectric materials have captured considerable attention of researchers due to their possible adaptation in power generation, energy conversion, and solid-state cooling applications. Due to enhanced electrical conductivity of these systems, researchers mainly focused on to find a way of reducing thermal conductivity of these systems to enhance their thermoelectric performance. With this intention a number of theoretical and experimental studies have been carried out. Due to their extraordinary transport properties carbon based nano materials have been studied intensively and several different researchers have pointed out their highly efficient thermoelectric properties. Furthermore, as another nanostructure material family Si, Ge, and Si-Ge based nano systems have attracted attention and ZT values larger than 2.0 have been reported for some Si nanostructures. Considering the potential of these materials, we systematically investigate the thermal transport properties of bulk and nano superlattice structures of Si, Ge and SiGe by equilibrium molecular dynamics simulations. We predicted quite low lattice thermal conductivity for some specific structures of these materials. Our results show that the thermal conductivity of these structures can be suppressed up to {\%}75 percent of bulk superlattice or pure nanowire structures. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S12.00010: Thermoelectric Properties of Carbon nanohybrids Incorporated Polymer Nanocomposites Kun Zhang, Shiren Wang In this work, non-covalently functionalized graphene with fluorinated fullerene (F-C$_{60})$ by $\pi $-$\pi $ stacking was integrated into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). F-C$_{60}$ as a $p$-type organic semiconductor with deep highest occupied molecular orbital (HOMO) level modulates the band structure of reduced graphene oxide (rGO). Altering HOMO levels of rGO has been achieved by changing the ratio between rGO and F-C$_{60}$. Incorporating of rGO/F-C$_{60}$ nanohybrids into highly conductivity metallic PEDOT:PSS formed Schottky barrier to selectively scatter low-energy carriers. Enhanced thermoelectric power factor of rGO/F-C$_{60}$/PEDOT:PSS nanocomposites were observed with the optimized power factor of 83.2 $\mu $W/m.K$^{2}$, which is 19 times of that of the highly conductive PEDOT:PSS. Additionally, the F-C$_{60}$ nanoparticles on rGO surfaces hinder thermal transport by phonon scattering, resulting in the synergistic effect on enhancing thermoelectric properties. As a result, a figure of merit (\textit{ZT}) of 0.10 was achieved. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S12.00011: Non-linear thermoelectricity in disordered nanowires Khandker Muttalib, Selman Hershfield We consider non-linear thermoelectric transport in an effectively one-dimensional disordered semiconductor nanowire connected to a pair of three-dimensional perfectly conducting semi-infinite leads, where the impurity band of the disordered wire can be shifted relative to the conduction band of the leads by applying a gate voltage. We show how the gate voltage can be tuned to optimize a unique interplay between the microscopic parameters characterizing the transmission of electrons through the nanowire and the thermodynamic parameters that characterize the Fermi functions in the leads. Assuming a Lorentzian distribution of disorder in the wire, we calculate the full non-linear thermodynamic efficiency $\eta$ as well as the power output $P$. We show that for a fixed set of microscopic and thermodynamic parameters $\eta$ can be increased from zero to $\eta > 0.5 \eta_c$, where $\eta_c$ is the Carnot efficiency, by simply changing the gate voltage. The power output $P$ can then be scaled by connecting many wires in parallel. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S12.00012: Tuning the Thermoelectric Properties of a Single-Molecule Junction by Mechanical Stretching Renato Pontes, Alberto Torres, Antonio J.R. da Silva, Adalberto Fazzio We theoretically investigate, as a function of the stretching, the behaviour of the thermoelectric properties - Seebeck coefficient ($S$), the electronic heat conductance ($\kappa_{el}$) and the figure of merit (ZT) - of a molecule-based junction composed by benzene-1,4-dithiol molecule (BDT) coupled to Au(111) surfaces at room temperature. We show that the thermoelectric properties of a single molecule junction can be tuned by mechanic stretching. The Seebeck coefficient is positive, indicating that it is dominated by the HOMO. Furthermore, it increases as the HOMO level, which is associated to the sulphur atom, goes to energies close to the Fermi energy. By modelling the transmission coefficient of the system as a single lorentzian peak, we propose a scheme to obtain the maximum ZT of any molecular junction. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S12.00013: First-Principles Study on Thermoelectric Properties of Carbon Nanotubes Jounghee Lee, Eui-Sup Lee, Yong-Hyun Kim Carbon nanotubes (CNTs) have attracted much attention because of their extraordinary material properties such as strong mechanical strength, chirality- and diameter-dependent electronic structure, and high thermal conductivity. As an electronic property, the Seebeck coefficient should also sensitively depend on the charility and diameter of CNTs. In this work, we propose a way to calculate an intrinsic Seebeck coefficient of one-dimensional CNT systems based on coherent electron transport within first-principle calculations and Landauer formulation. We will also estimate a contribution from diffusive transport, comparing to experimental results. The calculated maximum Seebeck coefficient is 0.13 mV/K for (9,9) metallic CNTs, while it is 0.8 mV/K for (10,8) semiconducting CNTs with similar diameter. When the diameter is smaller, the Seebeck coefficient of semiconducting CNTs can be as big as 1.3 mV/K with appropriate doping level. [Preview Abstract] |
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