### Session P29: Focus Session: Thermoelectrics III: IV-VI's & Nanostructures

 Wednesday, March 17, 2010 8:00AM - 8:12AM P29.00001: Enhancement of Thermoelectric Properties of Lead Telluride by Doping Vladislav O. Kontsevoi , Jung-Hwan Song , A.J. Freeman We investigated the effects of doping a promising thermoelectric alloy, PbTe, with a number of mono-, bi- and trivalent dopants, such as Li, Na, In, Tl, As, Sb, Cu, and Zn, by means of first principles calculations using the full-potential linearized augmented plane wave method (FLAPW) with GGA and spin-orbit coupling included. The calculations were performed for 64-atom supercells corresponding to 1.56 at.\% doping, and the site preference for dopant atoms was established. We showed that dopant atoms create electronic levels near the band edges that can be beneficial for the improvement of thermoelectric properties. To determine the Seebeck coefficient and the electrical conductivity, we calculated the transport coefficients based on the Boltzmann distribution function in the constant relaxation time approximation, with full intraband optical matrix elements included in the calculation of the group velocity. To calculate the thermoelectric coefficients for different carrier concentrations, the rigid band model was employed. We then explored possible enhancement of the Seebeck coefficient of PbTe in terms of band structure and the electronic density of states at optimal carrier concentrations for certain doping schemes. Wednesday, March 17, 2010 8:12AM - 8:24AM P29.00002: Impurity clustering and impurity-induced bands in PbTe-based bulk thermoelectrics S.D. Mahanti , Khang Hoang , M.G. Kanatzidis Complex multicomponent systems based on PbTe are of great current interest for high-temperature thermoelectric applications. A deeper understanding of the atomic and electronic structures of these materials is crucial for explaining, predicting, and optimizing their properties, and to suggest new materials for better performance. In this talk, we present our first-principles studies of the energy bands associated with monovalent (Ag) and trivalent (Sb, Bi) impurities and impurity clusters in PbTe. We find that monovalent and trivalent impurity atoms attract one another and tend to form impurity-rich clusters. The electronic structure of the host material is strongly perturbed by these impurities. Based on the calculated band structures, we suggest how to tailor the band gap and band structure near the band gap (hence transport properties) by choosing the type of impurity and its concentration or tuning the monovalent/trivalent ratio [1]. We are also able to explain qualitatively the measured transport properties of a large class of PbTe-based bulk thermoelectrics. [1] K. Hoang, S. D. Mahanti, and M. G. Kanatzidis, arXiv:0911.2685. Wednesday, March 17, 2010 8:24AM - 8:36AM P29.00003: Synthesis and Transport properties of Dimensional Nanocomposite Chalcogenides A. Datta , J. Paul , A. Popescu , L. Woods , G.S. Nolas Bulk polycrystalline chalcogenides were prepared from chalcogenide nanocrystals synthesized in high yields by a variety of chemical processes. Doping of the nanocrystals allowed for the modification of the carrier concentration in the bulk composites. Densification employing spark plasma sintering dimensionally integrated nanoscale grains within a bulk matrix, resulting in a uniform dispersion of nonconglomerated nanocrystals. Transport properties of the resulting dimensional nanocomposites were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. These properties were investigated as a function of nanocrystal size and composition. The results are discussed in terms of the potential for thermoelectric applications. Wednesday, March 17, 2010 8:36AM - 9:12AM P29.00004: Recent Advances in Thermoelectric Nanocomposite Materials Invited Speaker: Terry M. Tritt A short review of some of the important results over the last several years will be given with an emphasis on non-traditional materials such as skutterudites, which employ cage structures and thin film superlattices grown by MOCVD and MBE methods. Recently, there has been an ever-increasing research effort on thermoelectric nanocomposite materials. Composites using a mixture of bulk thermoelectric materials with nanoparticles incorporated within the bulk are of specific interest. One of the main goals is to have additional design or tuning parameters for materials (such as dimensionality) in order to manipulate and control the phonon scattering mechanisms, without significantly deteriorating the electrical transport properties. This means that one must deal with different length scales for the scattering processes. The role of phonons may be one of the most important parameters to understand in these novel materials. The ability to decouple the electron and phonon scattering mechanisms is very important in the development of higher efficiency thermoelectric (TE) materials, wherein the figure of merit, ZT, can be greater than unity. Wednesday, March 17, 2010 9:12AM - 9:24AM P29.00005: Sb-doped PbTe: An NMR Perspective E.M. Levin , K. Schmidt-Rohr , C.M. Jaworski , J.P. Heremans In PbTe, Sb as a dopant can occupy either Pb or Te sites. To understand the effect of Sb on the local charge-carrier concentration in both cases, we have studied high-resolution $^{125}$Te and $^{207}$Pb NMR spectra of Pb$_{1-x}$Sb$_{x}$Te, PbSb$_{x}$Te$_{1-x}$, and $n$- and $p$-type PbTe samples. The spectra of Pb$_{0.9975}$Sb$_{0.0025}$Te and PbSb$_{0.0025}$Te$_{0.9975}$ have distinctly different resonance frequencies due to Knight shifts and chemical shifts produced by Sb at Pb or Te sites. Pb$_{0.9975}$Sb$_{0.0025}$Te is $n$-type while in PbSb$_{0.0025}$Te$_{0.9975}$ both $n$- and $p$-type are found. NMR spectra and spin-lattice $T_{1}$ relaxation of $^{207}$Pb nuclei in PbSb$_{0.0025}$Te$_{0.9975}$, which are sensitive to the hyperfine interaction between charge carriers and NMR nuclei, reveal at least four components, which reflect electronic inhomogeneity of the sample. The local carrier concentrations estimated from $T_{1}$ NMR varies from $n<$3x10$^{17}$ to $p\sim$10$^{19}$ cm$^{-3}$. These multiple components help rationalize the complex temperature dependence of the thermopower of PbSb$_{0.0025}$Te$_{0.9975}$. However, comparison with Hall and Seebeck effects data indicates that some NMR signals are due to localized electron states, which do not directly contribute to transport. Wednesday, March 17, 2010 9:24AM - 9:36AM P29.00006: Investigating the coherent phase stability of the PbS-PbTe system using first principles calculations William Counts , Chris Wolverton Nanoscale inhomogeneities in PbS-PbTe alloys may enhance thermoelectric properties of these materials. The phase diagram and coherent spinodal are key factors controlling these nanostructured morphologies. We study the coherent phase stability of PbS-PbTe using density functional theory based calculations, a mixed-space cluster expansion, and Monte Carlo simulations. (i) Our calculations correctly reproduce the phase separating tendency of the PbS-PbTe phase diagram. (ii) Strain energy calculations show that for this rocksalt-based system (100) is the elastically hardest direction and that (111) is the softest. (iii) The formation energies for the PbS-PbTe structures are of the same order as the strain energies suggesting that strain energy will significantly depress spinodal decomposition in this system. Wednesday, March 17, 2010 9:36AM - 9:48AM P29.00007: Thermomagnetic and Galvanomagnetic properties of Tl doped PbTe-PbS Christopher Jaworski , Steven Girard , Mercouri Kanatzidis , Joseph Heremans It was recently shown that p-type PbTe-Tl has a significant increase in zT over that PbTe:Na\footnote{\textit{Science} 25 July 2008:Vol. 321. pp. 554 - 557} due to an increase in power factor. Also, a large increase in zT in n-type PbTe was reported when alloyed with PbS due to the reduction in thermal conductivity.\footnote{\textit{J. Am. Chem. Soc.}, 2007, 129 (31), pp 9780--9788} This work is an attempt at combining those two effects to further increase zT. We synthesis Pb$_{0.98}$Tl$_{0.02}$Te$_{1-x}$S$_{x}$ with 4{\%}$<$x$<$16{\%} and report here on measurements of thermopower, electrical resistivity, thermal conductivity, Hall and Nernst coefficients from 80-600K, and report zT. Initial thermopower and carrier density measurements indicate that Tl remains resonant in PbTe-PbS. Wednesday, March 17, 2010 9:48AM - 10:00AM P29.00008: Enhancement of Thermoelectric Properties of Lead Selenide by Doping Haowei Peng , Jung-Hwan Song , A. J. Freeman Lead chalcogenide materials are very important in thermoelectric investigations. Recently, it is reported that Tl doping in PbTe can greatly enhance the figure of merit (ZT) of PbTe,\footnote{J. P. Heremans, et al, Science, {\bf 321}, 554 (2008)} which can be mainly attributed to resonance states near the Fermi level. PbSe is also a good candidate as a thermoelectric material since it has a high melting temperature and low thermal conductivity (1.6 W/mK).\footnote{S. Ahmad, et al., Phys. Rev. B {\bf 74}, 155205 (2006) } Here we present a theoretical study of the electronic structures, formation energies, and transport properties of PbSe doped with various impurities such as Ga, In, Tl, As, Sb, and Cd using the highly precise FLAPW method.\footnote{Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)} Our calculated formation energies indicate that As prefers Se sites rather than Pb sites due to atom size mismatch, but Sb has no apparent preferable sites. Based on the analysis of the densities of states, PbSe with Tl and Cd doped on the cation sites are good candidates for thermoelectric applications as p-, and n-type materials. We also discuss the possible enhancement of thermoelectric properties in terms of optimal carrier concentrations. Wednesday, March 17, 2010 10:00AM - 10:12AM P29.00009: Thermal conductivity reduction in self-assembled lamellar materials with randomly oriented grains Chris Dames , Fan Yang Bulk-scale thermoelectric materials with nanometer-scale lamellae (e.g. PbTe / Sb2Te3) can be prepared by rapid solidification [Ikeda et al., Chem. Mater. 19, 763 (2007)]. These materials contain numerous randomly oriented grains, each of which contains multiple lamellar periods and therefore can be modeled as a small superlattice. We develop a frequency-dependent Boltzmann Transport Equation (BTE) to calculate the anisotropic thermal conductivity tensor of a single grain. Then the effective isotropic thermal conductivity of the macroscopic material is obtained using an analytical averaging rule that is verified by finite element methods. The results reveal that the macroscopic thermal conductivity is best understood as dominated by the physics of heat conduction in parallel (rather than series), and that further efforts to reduce the effective conductivity should focus on reducing the in-plane (rather than cross-plane) component of the superlattice thermal conductivity tensor within each grain. Wednesday, March 17, 2010 10:12AM - 10:24AM P29.00010: Understanding local heat and electron transport across nano-scale interfaces Bhaskaran Muralidharan , Gang Chen The issue of local transport properties across nano-interfaces is addressed in this talk. Local thermoelectric transport coefficients are calculated directly from local current distributions using the non-equilibrium Green function (NEGF) technique. A salient feature of our approach is that both thermoelectric (quasi-equilibrium) and thermionic (non-equilibrium) devices are handled on a similar footing. Most importantly, we capture the local variations of Peltier coefficient, chemical potential and power density that depend strongly on the nature and shape of the nano barrier. We point out the non-trivial result that the spatial extent of Peltier cooling and Joule heating near an interfacial region, in the limit of strong scattering depends on the potential profile and shape of the barrier. Wednesday, March 17, 2010 10:24AM - 10:36AM P29.00011: Thermoelectric effect due to coupled flow of electrons and phonons: a Landauer approach Lutfe Siddiqui , Supriyo Datta Performance of thermoelectric materials are determined by what is known as the thermoelectric figure of merit $ZT= S^2GT/K$, where $S$ is the Seebeck coefficient, $G$ is the electronic conductance, and $K$ is the thermal conductance. In order to compete with conventional generators and coolers the thermoelectric devices need to use materials that has a $ZT>3$. Yet the materials that have been employed in thermoelectric devices so far have not shown a $ZT$ value greater than $1$. Current research direction involves tailoring the electronic and phonon transport properties using nanostructured materials and also using coupled flow of electrons and phonons, namely, phonon drag to improve Seebeck coefficient. Consequently, a theoretical model that treats electrons and phonons in the common framework can, in the least, assist creative use of these different electronic and phonon transport engineering approaches. With this in mind we present a unified model based on Landauer approach that treats electrons and phonons on an equal footing irrespective of whether the flows are coupled or decoupled. Wednesday, March 17, 2010 10:36AM - 10:48AM P29.00012: Concurrent Measurement of Thermopower and Current-Voltage Characteristics of Molecular Junctions Pramod Reddy , Aaron Tan , Seid Sadat The Seebeck coefficient and the current-voltage characteristics of metal-molecule-metal junctions (MMMJs) are concurrently measured using a new atomic force microscope based technique. This enables the determination of the identity and the energetic separation of the molecular orbital that is closest to the Fermi level of the metal electrodes. Molecular junctions created by contacting a gold-coated atomic force microscope tip with four different self assembled monolayers (made from benzenethiol, dibenzenethiol, tribenzenethiol, tetrabenzenethiol) on gold substrates were found to have positive Seebeck coefficients ranging from 8 - 20 microvolts/K. The positive sign of the Seebeck coefficient unambiguously shows that charge transport in these junctions is dominated by the highest occupied molecular orbital (HOMO). Moreover, the Seebeck coefficient of the molecular junctions is found to increase monotonically with the length of molecular junctions. Further, by analyzing the current-voltage characteristics, the energetic separation of the HOMO level with respect to the Fermi level of the electrodes is also determined and is found to decrease with increasing lengths of the molecular junctions.