Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T25: Focus Session: Thermoelectrics - Nanomaterials |
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Sponsoring Units: DMP GERA FIAP Chair: Georg Madsen Room: 503 |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T25.00001: Correlated Evolution of Colossal Thermoelectric Effect and Kondo Insulating Behavior Invited Speaker: Robert Cava FeSb$_{\mathrm{2}}$ is a widely studied thermoelectric material with an unprecedentedly large low-temperature Seebeck coefficient whose origin is still being investigated. Its other electronic and magnetic properties show signatures of electronic correlations that suggest that the material is a Kondo insulator. A better understanding of the physics underlying the exceptional thermoelectric behavior is strongly needed. Even before a comprehensive understanding is attained, however, the principles already discovered are enough to warrant more attention in the search for advanced thermoelectrics. Towards this end, here we describe our work on how the Seebeck coefficient, electrical resistivity, and magnetic susceptibility of FeSb$_{\mathrm{2}}$ evolve together as the material is chemically tuned through varying degrees of electronic correlation. This was done by forming alloys with the conventional semiconductor RuSb$_{\mathrm{2}}$, whose more delocalized $d$ orbitals provided the key tuning parameter. The systematic development of the properties resulting from this straightforward chemical change enable us to construct a phase diagram that demonstrates how the colossal thermoelectric performance of FeSb$_{\mathrm{2}}$ emerges as the electronic correlation is increased and implies new principles for directing the continual search for advanced thermoelectric materials. This work is based on the Ph.D thesis of Michael K. Fuccillo, with collaborators Quinn D. Gibson, Mazhar N. Ali, and Leslie M. Schoop. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T25.00002: Giant enhancements of thermoelectric power factor in strained CoAs2 thin films Sunglae Cho, Yooleemi Shin, Anh Tuan Duong, Soyoung Jekal, Jongphil Kim, Younghun Hwang, S.C. Hong The performance of a thermoelectric material is estimated via the relation of the Seebeck coefficient (S), electrical conductivity ($\sigma )$ and thermal conductivity ($\kappa )$ at a temperature (T), which is called the thermoelectric figure of merit, ZT$=$S2$\sigma $T/$\kappa $. The achievement of a ZT above 1 is a historic mission assigned to the thermoelectric community. To date, the majority of research has focused on increasing $\mu $/$\kappa $. Heremans et al. emphasized the importance of the factor, S2n where n is a carrier density, on increasing ZT. They predicted that distortions of the electronic density of states (DOS) would induce a higher Seebeck coefficient in the thermoelectric semiconductor, resulting in an increased thermoelectric power factor (S2$\sigma )$. Here, we report that thermal stress due to thermal expansion coefficient difference between Si and CoAs2 film induces structural deformation, which modify the electronic structure for high carrier mobility and high Seebeck coefficient, resulting in huge thermoelectric power factor. We observed the Seebeck coefficient of -1038 $\mu $V/K and high electron mobility of 1885 cm2/Vs in CoAs2 films grown on Si substrate, resulting in the power factor of 545 mW/K2m. Note that monoclinic CoAs2 is semiconductor with a 0.2 eV band gap. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T25.00003: The Effects Nano-Structuring, Form of Band Structure, Asymmetry of Band-Edges, and Scattering Mechanism for Enhancement on ZT Shuang Tang, Mildred Dresselhaus Since 1993 when Hicks and Dresselhaus proposed that the low dimensional materials have enhanced ZT relative to their bulk counterparts, intensive research attention has been focused on enhancing the ZT in different materials, such as thin films, nanowires, nano-composites, etc. On the other hand, the proposal of bismuth antimony thin films in 2012, has provided a materials system with anisotropic and asymmetrical band edges, where both parabolic and non-parabolic forms of band structure exist. This raises a question on how can we enhance the figure of merit of thermoelectrics by using the special properties of these novel materials. This work will focus on exploring how the dimension, the form of band structure, the asymmetry and anisotropy of the band edges, and the electron scattering mechanism will influence the ZT. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T25.00004: Thermoelectric transport properties of Mn4Si7 thin films Yooleemi Shin, Anh Tuan Duong, Jeongyong Choi, Sunglae Cho The deposition of transition metal layers on silicon and their reaction with substrate are important issues in semiconductor device technology. The interface between metal and semiconductor determines the device performance. The 3d transition metal monosilicides such as FeSi, CoSi, MnSi and CrSi have attracted much attention because they are easily formed in the interface between transition metal and Si. On the other hand, the Mn4Si7 compound is well known a pseudo-direct band gap semiconductor (0.42 $\sim$ 0.98 eV) with a fundamental gap increasing linearly with the compression along c- or a-axis. We have grown Mn thin films on Si (111) substrates at 600 $^{\circ}$C using MBE, resulting in the formation of Mn4Si7. In order to investigate the correlation between magnetization and charge carrier transport, we performed magnetoresistance and Hall resistance measurements by using a physical property measurement system. Interestingly, we observed the Seebeck coefficient of -565 $\mu $V/K and electrical resistivity of 2.26 m$\Omega $ cm in Mn4Si7 films grown on Si substrate, resulting in the power factor of 14 mW/K2m. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T25.00005: Power-efficiency trade-off in low-dimensional thermoelectrics- An RTD study Akshay Agarwal, Bhaskaran Muralidharan A distortion in the quantum mechanical transmission due to confinement in a low-dimensional thermoelectric often points to a better figure of merit (zT). While an enhancement in zT is a highly desirable feature for a good thermoelectric, it does not provide a complete picture of the thermoelectric operation. One such aspect not apparent with a zT-based anaylsis is the trade-off between the power generated and the efficiency resulting from such a distortion. Another aspect is the role of Coulomb charging resulting from the confinement. In this talk, we elucidate the role of charging as well as the distortion in the transmission function on the thermoelectric performance using a double barrier resonant tunneling diode (RTD) set up. Transport simulations are performed using the non-equilibrium Green's function (NEGF) formalism coupled self-consistently with the Poisson equation. Various levels of transmission distortion and charging scenarios are achieved by tailoring the physical parameter space of the RTD device. The resulting set of physical situations in the simulated RTD device will provide a detailed insight into the power-efficiency trade-off trend that should result from a generic quantum confinement scenario. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T25.00006: Optimization of thermoelectric power factor in defect-engineered Bi$_{2}$Te$_{3}$ thin films Joonki Suh, Kin Man Yu, Deyi Fu, Xinyu Liu, Wladek Walukiewicz, Junqiao Wu The figure-of-merit ZT, which is related to thermoelectric energy conversion, is largely dependent on the power factor ($S^{2}\sigma)$, the electronic part of ZT. Optimizing power factor has been technically challenging due to unfavorable coupling between electrical conductivity and Seebeck coefficient, hence ZT has been commonly improved by reducing lattice thermal conductivity. In this work, we optimize the power factor with simultaneous enhancement in the in-plane electrical conductivity and Seebeck coefficient by manipulating native defects (NDs) in Bi$_{2}$Te$_{3}$ thin films using energetic alpha particles irradiation. This nontrivial optimization leads to a high power factor and potentially improves ZT by reducing the thermal conductivity. The microscopic mechanisms achieved by the multiple roles of NDs will be discussed and our work will provide a new route to improve ZT of Bi$_{2}$Te$_{3}$-related thermoelectric materials. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T25.00007: Magnetoresistence Measurements of Textured and Non-Textured Bismuth Thin Films Albert Liao, Mengliang Yao, Ferhat Katmis, Shuang Tang, Jagadeesh Moodera, Cyril Opeil, Mildred Dresselhaus Bismuth has recently received renewed interest because it is a key ingredient of many thermoelectric materials. Previous studies focus on bulk and/or single crystalline samples. However for thermoelectrics, it is desirable to assemble nano-structures to create a high ZT material. The way these nano-elements are assembled can be tuned to develop desirable properties. We control the texture of Bi films during thermal evaporation or molecular beam epitaxy, by using different growth substrates. Films deposited on mica, create a mosaic texture with the trigonal axis pointing out of plane. Films made on SiO$_{\mathrm{2}}$ are polycrystalline with grains oriented in random crystallographic direction. We measure magnetoresistance (MR) from 3-300~K while rotating our films in a magnetic field in two configurations. One where the current rotates with the plane of the film, and one where the current flows is always perpendicular to the field. We observe large discrepancies in MR behavior between the different samples at \textless ~100~K. Most surprisingly, we detect a MR when the current is supposedly parallel to the field in the non-textured film, inferring the current is not always traveling along the plane of the film. This may indicate the existence of planes within grains in which the carriers prefer to move. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T25.00008: Absolute Seebeck Coefficient Measurements of Thermoelectric Thin Films Sarah Mason, Azure Avery, Dain Basset, Barry Zink Significant advancements in thermoelectric device efficiencies are possible through size reduction to the nanoscale. Quantities that determine a material's efficiency, such as thermopower, or Seebeck coefficient, $S$, are influenced by the measurement apparatus, so that measuring a thermally generated voltage gives,$\frac{dV}{dT}= S_{sample}-S_{lead}$. If accurate values of, $S_{lead}$, are available, simple subtraction provides $S_{sample}$. This is rarely the case in measurements using micromachined devices, with leads exclusively made from thin film materials that do not have well known bulk-like thermopower values. We have developed a technique to directly measure $S$ as a function of $T$ using a micromachined thermal isolation platform consisting of a suspended, patterned SiN membrane. By measuring a series of thicknesses of metallic films up to the infinitely thick thin film limit, in which the thermopower is no longer increasing with thickness, but still not at bulk values, we are able to show the contribution of the leads needed to measure this property. Having a thorough understanding of the background contribution we are able to determine the absolute thermopower of a wide variety of thin films, as well as their thermal and electrical conductivities, on the same sample. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T25.00009: Thermoelectric Properties of Gated Silicon Nanowires Neophytos Neophytou, Hans Kosina Silicon nanostructures exhibit thermal conductivities close to the amorphous limit, which make them very promising thermoelectric materials. Room temperature figure of merit ZT$=$0.5 was recently demonstrated in Si nanowires (NWs) and nanomeshes. With the thermal conductivity, however, reaching its limits, additional benefits resulting from the electronic power factor need be investigated. In this work we theoretically investigate the thermoelectric performance of gated p-type Si NWs of diameters from D$=$5nm to D$=$20nm using atomistic calculations for electrons and phonons and linearized Boltzmann transport theory. We examine NWs in the [100], [110] and [111] transport orientations. The thermoelectric performance is found to be strongly anisotropic, with the [111] NWs having the highest and the [100] NWs the lower power factor. We demonstrate that field modulation of the carrier density can provide 4-5x higher power factors than what can be achieved in doped NWs. This is a result of the much higher electrical conductivity achieved by electric field modulation. The Seebeck coefficient is lower in field modulated channels, but the overall power factor is higher. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T25.00010: Thermoelectric power factor enhancement with electrically gated silicon nanowires Benjamin Curtin, Emilio Codecido, John Bowers We present both an experimental and theoretical study of the thermoelectric properties of electrically gated silicon nanowires. In this work, conduction electrons are induced in Si nanostructures using an electrical gate instead of the typical ionized impurities, which strongly scatter charge carriers at doping densities necessary for optimal power factor. Eliminating ionized impurities results in increased mobility and is expected to improve the thermoelectric power factor. We explore the gate and geometry dependence of thermoelectric properties with a semi-classical, multi-subband Boltzmann transport model. A maximum power factor of $\sim$ 7 $\times$ 10$^3$ W/m-K$^{2}$ was calculated for Si NWs with cross-sectional areas between 6 nm and 8 nm, which corresponds to a 2x enhancement over bulk Si. We also discuss the effects of surface roughness, quantum confinement, and transport orientation on power factor. Tri-gated Si NWs with dimensions of 25 nm x 35 nm were also fabricated from silicon-on-insulator substrates and their power factor was determined for various gate biases. Power factor was found to increase monotonically with gate bias and reached a maximum value of $\sim$ 2.2 $\times$ 10$^3$ W/m-K$^{2}$, which slightly larger than a 40 nm thick and optimally doped n-type Si thin-film. We present the thermoelectric characterization of these gated Si NWs and provide physical explanations for several effects observed during measurements. We also discuss the potential for further power factor enhancement with smaller diameter Si NWs. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T25.00011: Thermoelectric properties of individual Bi$_{1-x}$Sb$_{x}$Te$_{3-y}$ nanowires Yang-Yuan Chen, P.C. Lee, G.P. Dong, W.H. Tsai, C.H. Chien, M.N. Ou, F.Y. Chiu The low-dimensional materials exhibit innovative behaviors different from the bulk materials. The tuning of phonon-electron interactions could enhance the energy conversion efficiency of the one-dimensional thermoelectric materials. In order to study the intrinsic thermoelectric properties of an individual nanowire without external interferences, a measurement platform for such a purpose was successfully designed. A single crystalline Bi$_{1.75}$Sb$_{0.25}$Te$_{2.02}$ nanowire having thickness 250 nm was grown from a Bi$_{1.5}$Sb$_{0.5}$Te$_{3}$ film via thermal annealing method. The growth direction along [110] and composition of Bi$_{1.75}$Sb$_{0.25}$Te$_{2.02}$ for this nanowire were confirmed by TEM results. The self-heating 3$\omega $ technique was employed to characterize the thermal conductivity of this nanowire. The thermal conductivity increases from 0.5 W/m-K at 10 K to 1.4 W/m-K at 300 K. It is observed that the phonon drag at 20 K is about 6 times lower than that of Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ bulk. This enormous thermal conductivity reduction is mainly attributed to the enhanced phonon-boundary scattering of nanosized geometric effects. In the meantime the electrical resistivity and Seebeck coefficient were also measured by the heaters and electrodes built in the platform. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T25.00012: Thermal conductance in Si/Ge core-shell nanowires Jaime Bohorquez, Masoud Babaeian, Michael Ontl, Thushari Jayasekera We have studied the thermal conductance of Si/Ge core-shell $[111]$-oriented nanowires with diameters from 0.55 nm to 1.36 nm using ab initio calculations. In order to fundamentally understand the effect of atomic arrangements, we calculated the phonon conductance in a ballistic approach. Detailed analysis of phonon modes shows that thermal conductance due to selective phonon modes of Si/Ge nanowires can be suppressed by engineering the ratio of core/shell atoms. Our results suggest that, Si/Ge nanowire configurations can be engineered for optimized thermoelectric performance. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T25.00013: Comparing the Transition from Diffusive to Ballistic Heat Transport for 1D and 2D Nanoscale Interfaces J. Hernandez-Charpak, K. Hoogeboom-Pot, E. Anderson, M. Murnane, H. Kapteyn, D. Nardi How is thermal transport affected by spatial confinement in nanoscale systems? In past work we and others demonstrated that the Fourier Law of heat diffusion fails for length scales smaller than the mean free path of the energy carriers in a material. Here we probe how the transition from macroscopic diffusive behavior of phonons through the quasi-ballistic regime is different for 1D and 2D nano-confined hot spots. We study a series of periodic nickel lines (1D) and dots (2D) with linewidths varying from 750 to 30 nm deposited on both sapphire and silicon substrates. The thermal relaxation of these femtosecond-laser-excited nanostructures is monitored by the diffraction of extreme ultraviolet (EUV) light obtained from tabletop high harmonic generation. The short wavelength of EUV light, combined with the coherence and ultrashort pulses of high harmonic sources, provides a unique and powerful probe for nanostructured materials on their intrinsic length and time scales. The relaxation dynamics are linked to an effective thermal boundary resistivity with the assistance of multi-physics finite element analysis to quantify the stronger deviation from macroscopic diffusive behavior as a function of nanostructure linewidth in 2D hot spots compared to 1D. [Preview Abstract] |
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