We have demonstrated a quantum confinement effect giving rise to two dimensional electron gas (2DEG) in a 2D superlattice, STO/STO:Nb, which could generate giant thermopower while keeping high electrical conductivity. Then, a “synergistic nanostructuring” concept incorporating 2DEG grain boundaries as well as nanosizing of grains has been applied to our STO material and 3D superlattice ceramics was designed and proposed. This 3D superlattice ceramics was verified by numerical simulation to be capable of showing ZT>0.8 @300K. We, then, have attempted to develop a new process to realize 3D superlattice ceramics through hydrothermal synthesis of La-STO nanocubes, Nb attachment to nanocube surfaces, self-assembly of these nanocubes into 3D superlattices, followed by sintering in a reducing atmosphere.

The thermoelectric figure of merit is determined by ZT=S2T/ρκ, in which S2/ρ is the power factor, S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is the temperature. It has been known that tensile strained n-type silicon exhibits splitting of the six-fold degenerate conduction band [1], which leads to decreased inter-valley scattering and increased electron mobility [2]. Silicon nanomesh materials also result in a decrease of the thermal conductivity due to increased phonon scattering. Thus, ZT could be potentially increased by using n-type tensile strained silicon nanomesh thin films.
Here we report a unique method of patterning nanomesh features with self-assembled block copolymers on tensile strained SOI that yields an enhacement of the power factor over unstrained silicon[3]. The nanoscale features are obtained by transferring the self-assembled block copolymer pattern to the underlying silicon device layer by reactive ion etching. Seebeck coefficient and electrical resistivity measurements are performed on the tensile strained silicon nanomesh devices in an evacuated cryostat over a wide temperature range.
[1] C. Euaruksakul, et al. Influence of Strain on the Conduction Band Structure of Strained Silicon Nanomembranes. Phys. Rev 101, 147403 (2008)
[2] F. Schäffler. High-mobility Si and Ge structures. Semicond. Sci. Technol 12, 1515-1549 (1997)
[3] R. Ruiz, et al. Density multiplication and improved lithography by directed block copolymer assembly. Science 321, 936-939 (2008)

Oxide materials have attracted much attention as potential thermoelectric materials, especially since the discovery of a high thermoelectric power factor of 50 mu;W/K²cm in Na_xCoO₂ single crystals.[1] However, the overall thermoelectric performance remains limited, because of the relatively high thermal conductivity of Na_xCoO₂ and other cobaltites. Significant reduction of the thermal conductivity of Na_xCoO₂ remains a challenge and no successful attempts have been reported. We present a study that aims at suppressing the thermal conductivity in Na_xCoO₂ by confinement in thin films and superlattices.
Previously it is shown that single-phase Na_xCoO₂ thin films can be deposited by pulsed laser deposition and that chemical stability can be achieved by deposition of an amorphous AlO_x capping layer. [2] These thin films of Na_xCoO₂ are shown to have thermoelectric potential based on their room temperature properties, which are approaching the properties of single crystals. [1, 2]
We demonstrate structural and chemical stability of these capped Na_xCoO₂ thin films up to approximately 800K, in addition to the previously reported room temperature stability [2]. Furthermore, we present high-temperature thermoelectric properties of various Na_xCoO₂ capped thin films, focusing on the effect of layer thickness on the properties to demonstrate the effect of confining Na_xCoO₂ in thin films. These high-temperature measurements reveal the thermoelectric potential of Na_xCoO₂ thin films.
Additionally, we present the growth and characterization of thermoelectric superlattices, with Na_xCoO₂ layers as thermoelectric building blocks. Various insulating materials are used as barrier layer and their effect on the thermoelectric properties of these cobaltite superlattices is shown. Additionally, the effect of the layer thickness within these samples is shown and the high-temperature thermoelectric properties of these Na_xCoO₂ based superlattices are presented.
[1] I. Terasaki, Y. Sasago and K. Uchinokura, Phys. Rev. B, 1997, 56, R12685-R12687
[2] P. Brinks, H. Heijmerikx, T.A. Hendriks, G. Rijnders, M. Huijben, RSC Advances, 2012, 2, 6023-6027

Since the observation that nanostructured single-crystalline silicon can achieve high zT through large reductions in thermal conductivity, considerable effort has been placed into understanding the viability of silicon as a thermoelectric material. As the thermal conductivity of nanostructured Si approaches the amorphous limit, further increases in zT will need to be made with strategies that improve power factor. In optimally doped n-type Si, the high density of ionized impurities strongly scatters charge carriers, which results in a relatively low electron mobility. This presents a unique opportunity for improving the power factor of Si nanowires (NWs), where an electrical gate-all-around structure can modulate a conductive channel inside an intrinsic NW. Similar to field-effect transistors, the electrical gate is used to induce mobile charge within the Si NW while maintaining high mobility due to the absence of ionized impurities. In this work, we model the thermoelectric properties of gated Si NWs with the 1-D Boltzmann transport equation (BTE) and by self-consistently solving the Poisson and Schrödinger equations for various NW geometries and gate biases. We first validated our BTE solver and scattering rates with n-type bulk Si and found good agreement between simulated and published optimal power factor, which was ~4 x 10^-3 W/m-K² at 300 K for a doping density of ~5 x 10¹⁹ cm^-3. For gated Si NWs, we find that the room temperature power factor approaches 1 x 10^-2 W/m-K² for Si NWs with diameters < 10 nm and positive gate bias. As the NW diameter increases, the gated power factor decreases below the optimal n-type bulk Si value because the conductive area relative to the cross-sectional area of the NW decreases. We are currently implementing surface roughness scattering in these calculations, which will likely reduce the power factor of both gated and highly doped Si NWs with small diameters. A discussion of gated Si NW thermoelectrics will be presented along with an analysis of our modeling results.
We are also processing gated Si NW devices on silicon-on-insulator (SOI) substrates to experimentally determine power factor enhancement. Si NWs with diameters of ~25 nm are fabricated using electron-beam lithography to define the NWs, which are then etched into SOI substrates and thermally oxidized to both reduce their diameter and grow a thin gate oxide. An aluminum gate is then deposited around the Si NWs, and Ti/Pt electrodes, heaters, and thermometers are patterned on top of the NWs for Seebeck and electrical conductivity measurements. Process development is currently underway and recent progress will be presented.

There are exciting emerging prospects for understanding and tailoring microstructure of bulk thermoelectrics with due attention to the hierarchical length-scale influence across atomic-, nano- and micro-meter dimensions. Utilizing this core idea, we have recently demonstrated that nanostructured PbTe- and PbSe-based bulk thermoelectric systems exhibit high figure of merit (ZT) that reduce thermal conductivity through multiple means, and does not appreciably compromise the power factor. In the presentation, we will present this intricate but tractable relationship between various microstructural attributes (point, line and interfacial defects) and lattice thermal conductivity in important nanostructured thermoelectric systems based on PbTe and PbSe matrices. Strategies for improvement in power factor through interfacial mediation and dual-nanostructuring will be discussed. The need for overarching approach will be emphasized for understanding the hierarchical length-scale influence to enable specific design strategies for the next generation thermoelectric materials.

Despite the great promise offered by thermoelectric devices for solid-state electricity generation, heating, and cooling, there has been only limited progress in improving the efficiency of thermoelectric devices. Improvements in device efficiencies and material ZT have been difficult due to the correlations that link the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity. Many recent thermoelectric strategies focus on methods of breaking these relationships in order to improve ZT. A new strategy is demonstrated in this study which uses the exceptionally large interfacial polarization charges present in III-Nitride materials to confine electrons to high mobility channels near interfaces, thus selectively reducing electron scattering and breaking the classic relationships between electrical conductivity, Seebeck coefficient, and thermal conductivity and allowing improvements in ZT. Previous reports have shown ZT < 1 for III-Nitride materials due either to high thermal conductivity in binary GaN, or low power factors (S²*σ) in ternary III-Nitrides. This new strategy allows a combination of the low thermal conductivities present in ternary materials with the high power factors of binary III-Nitrides for improved ZT.
Enhanced power factors are demonstrated in metal organic chemical vapor deposition (MOCVD) grown thin film GaN/InAlN heterostructures, GaN/AlN/InAlN heterostructures and GaN/AlN/AlGaN superlattices. GaN/InAlN samples with InAlN thicknesses of 34 nm have shown power factors as high as 2×10^-4 W/mK², which is three times higher than InAlN alone. The inclusion of a 1 nm AlN interlayer greatly improves electron mobility while leaving the Seebeck coefficient unchanged and results in power factors as high as 8.4×10^-4 W/mK², which is an order of magnitude improvement over bulk InAlN. Finally, high mobility GaN/AlN/AlGaN (7 nm/1 nm/2 nm) superlattices are shown to display power factors as high as 2.3×10^-3 W/mK², which is even higher than GaN without alloying or nanostructures. These power factor enhancements are due to the spatial confinement of electrons on the GaN side of interfaces caused by the interfacial polarization charges present in this material system. This confinement largely isolates the electrons from both alloy and ionized impurity scattering centers, resulting in improved electron mobilities with relatively unchanged Seebeck coefficients and thus realizing power factor improvements. Enhanced power factors have been demonstrated to extend through temperatures as high as 815 K. In-plane thermal conductivity measurements of GaN/AlN/AlGaN superlattices are currently underway.

Although theoretical calculation indicated that the thermoelectric figure of merit, ZT, of carbon nanotubes (CNTs) could reach >2, typical experimentally reported ZT values of CNTs are in the range of 0.001 to 0.01, which is too low for thermal energy conversion applications. Herein, we modified flexible CNT sheets via plasma irradiation for thermoelectric applications. The ZT values of the CNT sheets could be significantly enhanced after enough plasma irradiation. A maximum ZT value of plasma treated CNT sheet is around 40 times than the pristine one. The improved thermoelectric properties were mainly due to the great increased Seebeck coefficient and reduced thermal conductivity. It was found that the plasma irradiation have the effects on tuning carrier concentration and change electrical conduction behaviour. In an attempt to further enhance the power factor of the system and shift the Seebeck coefficient peak position, plasma treated CNTs can be decorated by some dopants to further tuning the carrier concentration and modify the electronic structure. Such improvement makes the plasma treated CNT sheets promising as a new type of thermoelectric materials for certain niche applications as they are easily processed, mechanically flexible and durable, and chemically stable.

Si-Ge superlattices are expected to have an increased figure of merit due to the two- dimensional structure. A high ZT can be related to a decrease in the cross-plane thermal conductivity as a result of the phonon scattering at the interfaces. Sim-Gen superlattices with stacks of m Si and n Ge layers of different thicknesses and doping levels were grown by molecular beam epitaxy on (001) or (111) Si substrates. The structures were characterized by cross-section transmission electron microscopy. The lithographic preparation and measurement schemes for the cross plane transport properties in dependence of temperature are outlined, and the results are presented.
In a further approach, we prepared Si and Si-Ge nanopillars. They were produced in a top-down fabrication scheme by metal-assisted chemical wet etching [1]. The understanding of the etching mechanism [2] is the prerequisite of the precise fabrication of nanopillars with hexagonal symmetry and adjustable diameters ranging from about 10 nm to several micrometers over large areas. High area densities and the control of diameter, length, position, and the internal structure of the nanowires are possible. Well-defined nanopillars with diameters below 10 nm exhibit important size-dependent quantum effects, which account for the decoupling of the electrical conductivity, the Seebeck coefficient, and the thermal conductivity. Furthermore, Si-Ge superlattice nanopillars are promising candidates to reduce further the thermal conductivity.
In a third approach, we investigate nanocrystalline silicon particles embedded in an oxide matrix of SiO2 as effective thermoelectric hybrid materials. These quantum dots are formed in a phase-separation process in thin films deposited by chemical or physical vapor deposition. Alternatively, a solid-state transformation is used in a quartz-aluminum system to form Si nanoparticles in an Al2O3 layer. The formation of the nanocrystals can be tuned by rapid thermal annealing with respect to the uniformity in size, distribution, and surface structure. In order to maximize the thermoelectric power factor, a high doping level of the particles is required. With the low thermal conductivity of the amorphous matrix, a figure of merit close to 1 may be achieved at room temperature.
[1] N. Geyer, Z. Huang, B. Fuhrmann, S. Grimm, M. Reiche, T.-K. Nguyen-Duc, J. de Boor, H. S. Leipner, P. Werner, U. Gösele, Nano Lett. 2009, 9, 3106-3110.
[2] N. Geyer, B. Fuhrmann, Z. Huang, J. de Boor, H. S. Leipner, P. Werner, J. Phys. Chem. C 2012, 116 (2012) 13446-13451.

The thermoelectric field has advanced significantly since its renaissance in 1992 with the concept of nano-thermoelectricity. Once this concept gained favor by the research community, interest in this research field increased as progress was made in increasing ZT through the introduction of nanostructures to allow some independent control of the electrical and thermal conductivity. Twenty years have passed since the Hicks-Dresselhaus papers were published and now new concepts for increasing ZT have been introduced and some have even been demonstrated. A review of the status of progress will be given.

There has been a high activity to find viable thermoelectric (TE) materials. One need exists to develop materials which can function at high temperature, T, for applications utilizing high temperature waste heat, such as focused solar power, RTG, etc. Boron-rich compounds are attractive materials for their stability, typically exhibiting melting points above 2200 K, and they have been found to possess intrinsic low thermal conductivity, κ, even for single crystals [1]. Novel borides promising for TE were found, such as p-type REB₄₄Si₂ [2], and REB₁₇CN, REB₂₂C₂N, and REB_28.5C₄, the long awaited n-type counterparts to p-type boron carbide [3], which is one of the few previously commercialized TE materials [4]. In a further recent striking development, YAlB₁₄ was found to be able to be controlled and strong p and n characteristics were freely controlled in a compound with the same crystal structure and same composing elements [5].
In this work, a series of icosahedral boron rich solids, with nominal compositions B₆S_1-x (0.37 lE;x lE;0.40) have been synthesized from the reaction of amorphous boron and excess of sulfur at 1473 -1573 K in a BN crucible using a RF furnace. Rietveld analysis of intensity data collected with synchrotron and conventional X-ray sources revealed a rhombohedral structure with the alpha-rhombohedral boron framework of B₁₂ icosahedra and a narrow filling fraction range for sulfur atoms in the octahedral voids. Although obtained samples had high resistivities due to low densities, B₆S_1-x exhibited p-type semiconducting behavior similar to boron carbide with large Seebeck coefficients reaching a maximum of ~220 mu;VK-1 at the highest measured temperature 810 K.
[1] T. Mori, “Higher Borides” in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 38, (North-Holland, Amsterdam, 2008) pp. 105-173 (2008).
[2] T. Mori et al., J. Appl. Phys. 97 (2005) 093703, Dalton Trans. 39 (2010) 1027 (Hot Article).
[3] T. Mori et al., J. Solid State Chem. 179 (2006) 2908, J. Appl. Phys. 101 (2007) 093714.
[4] C. Wood et al., Phys. Rev. B 29 (1984) 4582.
[5] S. Maruyama et al., Appl. Phys. Lett. 101 (2012) 152101.

Thermoelectric devices directly convert heat into electric energy and thus can be used for the waste heat recovery in automotive engines. Efficient thermoelectric material should combine high electrical conductivity and Seebeck coefficient with low thermal conductivity, thus making figure-of-merit zT reasonably high. Other requirements for the material include high stability at the temperature range of the exhaust manifold of automobile engines, low cost and light weight. Magnesium silicide Mg₂Si a promising material, since it is stable at high temperatures and consists of earth abundant, light weight elements. Electrical conductivity of pristine Mg₂Si can be tuned by changing the carrier concentration with dopants to either p- or n-type. However thermal conductivity of Mg₂Si remains fairly high (above 40 mW/cmK). Nanostructuring of Mg₂Si was recently suggested as a new approach to reduce thermal conductivity and improve zT. It was shown that thermal conductivity of Mg₂Si can be reduced by introducing Si nanoparticles, which act as phonon scattering centers. This approach can be further extended by introducing Ge in the form of nanoparticles. Mg₂Si composites with embedded Ge nanoparticles were prepared by two approaches. In the first one, Ge nanoparticles were produced by solution assisted method developed in our group. In the second one, Ge was ball milled together with Si in order to obtain micron size powders of Si_1-xGe_x. Mg₂Si nanocomposites with different concentrations of Ge were successfully synthesized utilizing reaction of MgH₂ with Si/Ge and densified by means of spark plasma sintering (SPS) technique. The presence of hydride provides the reducing atmosphere and minimizes magnesium oxide impurity according to powder X-ray diffraction of the products. Microprobe analysis showed that Ge is distributed over both phases, Mg₂Si and Si, thus leading to the Mg₂Si_1-xGe_x and Si_1-xGe_x phases. The addition of Ge either in the form of nanoparticles prepared by solution assisted method or by alloying of Si with Ge leads to the considerable lowering of thermal conductivity. Further optimization of the electrical properties of the Mg₂Si/Ge nanocomposites by introducing different concentrations of dopants is currently underway. The results of the thermoelectric performance optimization of Mg₂Si nanocomposites will be discussed.

The paper reports thermal diffusivity measurements of porous anodic alumina (AAO) templates and bismuth telluride nanowires embedded in AAO matrix. The technique combines infrared (IR) lock-in thermography and a simple data evaluation procedure for a fast noncontact measurement of thermal diffusivity. AAO templates are extensively used for electrodeposition of bismuth telluride nanowires for thermoelectric applications etc. Thermal characterization of individual thermoelectric nanowire is a big challenge. So one way of estimating the value is to perform measurements on empty AAO templates and nanowires embedded template matrix. The reported methods of thermal diffusivity measurements for AAO templates are the 3omega; method and the photothermoelectric technique which are both contact methods. Till date Infrared thermography based study of AAO samples are not reported.
Our study reports application of a noncontact process similar to the Armstrong IR Thermography method in measuring the thermal diffusivity of AAO templates. The study includes measurement of perpendicular channel thermal diffusivity values of 20nm, 100nm and 200nm pore AAO templates. Also included is the thermal diffusivity of bismuth telluride nanowires embedded AAO matix. The technique is based on the non contact IR Lock-in thermography [18,19] detection of oscillating temperature distribution, i. e., thermal wave, produced by the absorption of an intensity modulated optical laser beam. The sample is subjected to a periodic heat flux by a semiconductor laser and an infrared image sequence is recorded as a function of time with an infrared camera. In our work, we have used both reflective and transmissive geometries, and we give a detailed comparison of the benefits and disadvantages of both. We describe the equipment needed as well as the mathematical modelling of the dynamic equations of heat propagation through the thermoelectric nanocomposite material. In the end, we present results to show that our technique can be used to measure thermal diffusivity of thermoelectric nanowires to a high degree of accuracy. The technique lends itself to use for probing any three-dimensional conductiing material at the micron scale.

Ultra-thin silicon membranes and nanowires have been the subject of extensive studies due to their low dimensionality that leads to enhanced thermo-electric properties and improved figure of merit, ZT. This increase of ZT is attributed to the decrease of the thermal conductivity, which is predicted to be related in part to the modification of the acoustic dispersion relation due to periodicity, e.g. phononic crystal, or spatial confinement of the phonon modes, e.g. nanowires.
We investigate the acoustic phonon dispersion in ultra-thin free-standing silicon membranes and nanowires based on the elastic continuum model. The thermal properties are calculated comparing both, the modified dispersion relation and Debye dispersion relation approximation. In addition, we explicitly calculate the relative contribution of each scattering processes to the total relaxation time.
The theoretical predictions are compared with other reported theoretical and experimental results as well as with our measurements.

A scheme of normal mode analysis based on anharmonic phonon eigenvectors to study the thermal properties is proposed. Compared to the traditional normal mode analysis, our scheme has several advantages of more convenience, stronger capability, and higher accuracy. As an application, the phonon properties such as the phonon dispersion, relaxation time, mean free path and thermal conductivity of bulk argon and bulk germanium at different temperatures have been investigated using this scheme based on classical molecular dynamics (MD). We found the dependence of phonon relaxation time to frequency omega; and temperature T for different temperatures and different modes vary, respectively, from ~omega;^(-1.3) to ~omega;^(-1.8) and ~T^(-0.8) to ~T^(-1.8) for argon, and from ~omega;^(-0.6) to ~omega;^(-2.8) and ~T^(-0.4) to ~T^(-2.5) for germanium. The predicted thermal conductivities are in reasonable agreement with these obtained from classical Green-Kubo method and more accurate than other Boltzmann Transport Equation based MD method in high temperature range. The effective mean free path for argon and germanium at 20K and room temperature (300K) are approximately 2.5nm and 39nm after correction, respectively. The most promising application of our scheme is to calculate the thermal properties of materials based on First principle MD or Tight Binding MD with atomic potential unknown, which should produce a bright prospect and a fairly competitive area in the future.

The thermoelectric properties of the layered oxides KxRhO2 (x = 1/2 and 7/8) are investigated by means of the electronic structure, as determined by ab inito calculations and Boltzmann transport theory. In general, the electronic structure of KxRhO2 is similar to NaxCoO2, but with strongly enhanced transport. K7/8RhO2 exceeds the ultrahigh power factor of Na0.88CoO2 reported previously by more than 50%. The roles of the cation concentration and the lattice parameters in the transport properties in this class of compounds are explained.

Micro/nanosensors have attracted much attention due to their potential applications in detecting the micro/nano-objects such as a particle, cell, DNA, and so on. Micro/nanomaterials are important for fabricating these small scale sensors. The self-powered nanotechnology is based on driving a nanodevice by harvesting energy from its working environment instead of a conventional battery or any other energy storage/supply system. There is an urgent need to develop nanotechnology that harvests energy from the environment to power these sensors. It means that we can use the pyroelectric nanogenerator as a self-powered temperature sensor that automatically detects temperature without using a battery as the power source. This approach can greatly enhance the adaptability and mobility of such devices. Although some temperature sensors have been reported, there are few studies about using a single PZT micro/nanowire pyroelectric nanogenerator as a self-powered temperature sensor. Here, a single PZT micro/nanowire pyroelectric nanogenerator was fabricated, which was used as a self-powered temperature sensor. The output voltage of the sensor was found to linearly increase with an increasing rate of change in temperature of the detected heat sources. The response time and reset time of the fabricated sensor are about 0.9 and 3 s, respectively. It can be used to detect the minimum temperature change of about 0.4 K at room temperature. We also demonstrated that the temperature sensor can be used to detect the temperature of the finger surfaces and light a LCD under a heated temperature of 473 K. The self-powered temperature sensors developed here have potential applications in temperature measurements in environmental sciences, safety monitoring, medical diagnostics, and more.
Refs:
[1] Ya Yang, Jong Hoon Jung, Byung Kil Yun, Fang Zhang, Ken C. Pradel, Wenxi Guo, and Zhong Lin Wang. Flexible pyroelectric nanogenerators using a composite structure of lead-free KNbO3 nanowires. Advanced Materials. 2012, 24, 5357-5362.
[2] Ya Yang, Yusheng Zhou, Jyh Ming Wu, and Zhong Lin Wang. Single micro/nanowire pyroelectric nanogenerators as self-powered temperature sensors. ACS Nano. 2012, 6, 8456-8461.

Phonon transport in a low dimensional nanostructure has played a key role in determining the lattice thermal conductivity. In most semiconductors and insulators where the electron conduction is relatively low, phonons are the dominant energy carriers. Thus the quantitative understanding of their interactions has become crucial for micro/nanoscale applications. It normally requires a material with the reduced phonon thermal conductivity to enhance the performance of thermoelectric materials. The incorporation of structure complexity and phonon engineering can be employed to design a novel material with the favorable thermal conductivity. In addition, the stress field in a confined structure could be a parameter for future phonon engineering since varying mechanical stress can alter the velocity of phonons. However, it still lacks of knowledge on the influence of phonon tailoring by mechanical stresses. We consider an initially twisted nanowire made of a conductive polymers and investigate the influence of mechanical stress due to torsion on the phonon transport using the continuum approach. Phonon dispersion relation, phonon group velocity, and lattice thermal conductivity are computed to provide a complete picture of the transport mechanism. This study can suggest a phonon engineering approach to tune the conductivity of nanomaterials.

In search for non-toxic thermoelectric materials that are stable in air at elevated temperatures, zinc oxide has been shown to be one of only few efficient n-type oxidic materials. Our approach starts with very small (<10 nm) aluminum-doped ZnO nanoparticles prepared by chemical vapor synthesis in a hot wall reactor. The nominal doping concentration was varied between 0.5 % and 8 %. In order to obtain bulk nanostructured solid bodies, the powders were compacted in a current-activated pressure-assisted densification (CAPAD) process at 900 °C. Electrical transport coefficients and thermal conductivity were characterized in an Ulvac ZEM-3 system and in a Netzsch LFA 457 laserflash apparatus, respectively.
Very high electrical conductivities with values considerably above 10⁵ S/m can be observed over a broad temperature range, independent of the doping concentration. The high conductivity values indicate a highly efficient doping mechanism during the synthesis and densification, but the high charge carrier concentration adversely affects the Seebeck coefficient. Using modified synthesis conditions, the Seebeck coefficient can be increased to minus;110 µV/K at 700 °C resulting in a power factor of 4×10^-4 W/m/K².
Microstructural analysis of the sintered samples clearly shows that addition of aluminum beyond its solubility limit in ZnO hinders grain growth and leads to the formation of ZnAl₂O₄ precipitates. This is confirmed by the thermal conductivity values which strongly decrease with increasing aluminum content. The sample with the highest figure of merit zT=0.09 at 700 °C shows a low thermal conductivity of 4 W/m/K.

Latest nanotechnology concepts applied in TE research have opened many new avenues to improve the zT value. Low dimensional structures can improve the ZT value as compared to bulk materials by substantial reduction in the κL. However, the materials were not feasible for the industrial scale production of macroscopic devices because of complicated and costly manufacturing processes involved. Therefore, demands for enhanced zT of bulk NS materials have expanded the research on large scale production of TE materials. Bulk NS TEs, on the other hand, are normally fabricated using a bulk process rather than a nano-fabrication process, which has the important advantage of producing in large quantities and in a form that is compatible with commercially available TE devices.
We developed fabrication strategies for bulk nanostructured skutterudite materials based on CoSb3. The process is based on precipitation of a precursor material with the desired metal atom composition, which is then exposed to thermochemical processing of calcination followed by reduction. The resultant material thus formed maintains nanostructured particles which are then compacted using Spark Plasma Sintering (SPS) by utilizing previously optimized process parameters. Microstructure, crystallinity, phase composition, thermal stability and temperature dependent transport property evaluation has been performed for compacted NS CoSb3. Evaluation results are presented in detail, suggesting the feasibility of devised strategy for bulk bulk quantities TE nanopowder fabrication.
Acknowledgments
This work has been funded by EC-FP7 program under NEXTEC project and in part by the Swedish Foundation of Strategic Research - SSF.

Materials having rocksalt-like structures and narrow band gaps have great importance for thermoelectrics and phase change materials. For example, PbTe, Bi2Te3, and Bi-Sb alloy are the best thermoelectric materials from sub-room temperature to high temperature. In addition, pseudo binary solutions of GeTe and Sb2Te3 are considered the most promising candidate for phase change materials due to their large optical contrast and fast phase transition. For both thermoelectrics and phase change materials, it is essential to have better understanding on lattice dynamics to reduce thermal conductivity and enhance phase transition characteristics. In this presentation, we will show using first principles that fourth neighbors in the rocksalt-like structures (PbTe, Bi2Te3, Bi, and Sb) commonly exhibit stronger interatomic force constants than second and third neighbors even though the atomic distance is longer. The reasons of this anomalous behavior are collinear bonding in rocksalt-like structures and large electronic polarizability. The large electronic polarizability in PbTe and Bi2Te3 can be understood within the context of the resonant bonding theory. In Bi and Sb, the large electronic polarizability is due to their semi-metallic behavior. This observation will lead to the better understanding on lattice thermal transport and phase change characteristics.
Acknowledgment: This work is support by AFOSR MURI through OSU (S.L. and G.C.) and S3TEC, a DOE EFRC (K.E. and T.L.).

ZnO is a semiconductor with large Seebeck coefficient (S) while very low electrical conductivity (σ). It is found σ can be improved without too much decrease of S by doping with metallic elements and this made it to be a possible thermoelectric candidate. In this paper, we are trying to improve thermoelectric performance of ZnO with the doping of Al and Cu. We studied electronic and thermoelectric properties of ZnO, AlxZn1-xO and CuxZn1-xO (x=0.05-0.2) by using the density functional theory and the Boltzmann semi-classical equations. It is found that pure ZnO has a high Seebeck coefficient while the electrical conductivity remains very low. With the doping of aluminum, σ increases dramatically (by three orders of magnitude at most), while still presenting a large Seebeck coefficient (80-140 mu;V/K). Those trends lead to a significant increase of the power factor. However, Cu doping does not help a lot to increase the power factor as the Seebeck coefficient decreases substantially even though the electrical conductivity increases. Current improvements of the power factor for AlxZn1-xO are still not satisfactory for targeting applications and further studies should be performed.

We study the effect of piezoelectric stress on the thermal conductivity in dielectric material from the point of view of a bulk crystal. Equilibrium MD simulations are used to calculate the piezoelectric coefficients and thermal conductivity of a perfect ZnO crystal, and the connection between the two is made initially through the classical picture of phonons and kinetic theory. The model used for ZnO in the study is a core only one, i.e. the effect of electron shell delocalization on the polarization is not considered. We report the calculation of piezoelectric coefficients such as e33 and e31 using fluctuation-dissipation theorem and of the thermal conductivity using Green-Kubo formulation i.e. equilibrium molecular dynamics, and compare our results with those published in other papers. As such, the effect of the induced strain on the bulk thermal conductivity is studied in hopes of locating a suitable avenue for optimization.

Abstract: Improving energy efficiency by converting waste heat into electricity based on thermoelectric is of great interest, and the key to the fulfillment of thermoelectric waste heat recovery lies in the design and fabrication of high-performance thermoelectric materials. Here, we present a pioneering design and synthesis of dumbbell-like PbTe-Ag2Te-PbTe heterostructures and demonstrate the fabrication of these nano building blocks into “nano-bulk” thermoelectric composites. The research is motivated by the previous experimental observation of the high thermoelectric figure of merit (ZT) in thin film superlattice structures and nano-inclusions-in-bulk structures as well as the theoretical prediction of significantly reduced thermal conductivity due to phonon scattering at nanoscale surface/interface and enhanced power factor due to energy filtering. Experimental studies showed 35% increase of ZT in our nano-bulk composite comparing to ZT in the bulk counterparts primarily due to the enhancement in Seebeck coefficient. Theoretical modeling showed that the exceptionally high Seebeck coefficient approaching 400 mu;V/K at 400 K could be explained by energy filtering effect. Our synthesis approach of the dumbbell-like heterostructures can be readily modified for a serial of telluride cation heterostructures such as PbTe-Bi2Te3-PbTe, Bi2Te3-Ag2Te-Bi2Te3, etc, which facilitates the design of nano-composites with optimal band alignments in order to achieve even higher ZT. More importantly, unlike the other nanotechnology-based thermoelectric research, our synthesis approach based on wet chemistry is facile and scalable, which enables the fabrication of nanostructured bulk materials for the real production of thermoelectric modules with superior performance compared to the state of the art.

Recently it has been shown that filling the cages in CoSb3 skutterudite crystal lowers lattice conductivity. Since the vibration modes of pnicogen rings in the skuttterudites dominate the heat transport, we show configuring the pnicogen rings also reduces lattice conductivity. We present an analysis of pnicogen ring configuration role in reducing of lattice conductivity double-substituted skutterudites CoSb3-x-yGexTey. Based on ab-initio lattice dynamics calculations, the substituted Ge atoms form the softest bonds acting as a pseudo-rattler with distinct mode-flattening feature and acting as a local phonon softener (comparable to common rattler in filled skutterudites). We also show the collective modes of this lattice configuration induce breathing mode in the cage which is highly correlated with the reduction in lattice conductivity. The lattice conductivity predicted with non-equilibrium ab-initio molecular dynamics is in good agreement with the experimental results and the point-defect scattering model. Our lattice dynamics and thermal transport analyses can guide design of high thermoelectric figure-of-merit skutterudites.

Phonon-glass electron-crystal materials were firstly named by G. A. Slack, which require material properties like a large effective mass, a high mobility for carriers and a low lattice thermal conductivity. However, actually a high mobility needs a low mobility effective mass but a high density of stated demands a large density of stated effective mass. Therefore to achieve ideal phonon-glass electron-crystal property in a single compound bulk device requires judicious selection of alloy components or complex superlattice structure. Here we introduce a facile solution to achieve disassociated Electrical Conductivity and Thermal Conductivity by building bulk device from PbTe and TiO2 nanocrystals. Nanocrystals were prepared from one step solvothermal synthesis following by homogeneous mixing and ligands removing process. Utilizing spark plasma sintering process, PbTe and TiO2 nanocomposites were sintered into densified tablet with 1 inch in diameter which have shown almost 90% of theoretical density. And transport properties were tested that electrical conductivity and thermal conductivity has been reduced to 1/30 and 1/4, respectively. After sintering, PbTe nanocrystal forged into a network-like structure and supplied thermoelectric property for as-prepared bulk device, while the interfaces at PbTe-TiO2 nanocrystals and TiO2-TiO2 nanocrystals scattered transportation of phonon, therefore reduced thermal conductivity. We believe that this is a facile solution to prepare economical and practical phonon-glass electron-crystal materials in real application and further improving electrical conductivity is able to achieve advanced ZT.

We present a detailed theoretical investigation and comparison of the thermal conductivities of n-type (Bi₂Te₃)_0.85(Bi₂Se₃)_0.15 single crystal doped with 0.1 wt.% CuBr and p-type (Bi₂Te₃)_0.20(Sb₂Te₃)_0.80 single crystal doped with 3 wt.% Te. The thermal conductivity contributions from carriers (electrons or holes) (κ_c), electron-hole pairs (κ_bp), phonons (κ_ph) are computed to explain available experimental results [1,2]. For the theoretical calculations of κ_c and κ_bp, Wiedemann-Franz law [3] and Price&’s theory [4] are employed, respectively. The phonon thermal conductivity (κ_ph) is calculated by using the Debye model within the single-mode relaxation-time approximation. Boundary, alloy, isotope, electron-phonon (or hole-phonon) and phonon-phonon interactions are included rigorously. For anharmonic phonon interaction we restrict ourselves to only three-phonon scattering events by following Srivastava&’s scheme [5]. The frequency dependence of phonon thermal conductivity is also studied for the p-type alloy and compared with the results for the n-type alloy [6,7]. The calculated total thermal conductivities (κ_total = κ_c+ κ_bp + κ_ph) of n- and p-type Bi₂Te₃ based alloys successfully explain the experimental results obtained by Hyun et al. [1] and Li et al. [2]. The value of κ_total is found to be 3.15 W/(K.m) at 380 K for the n-type alloy and 1.145 W/(K.m) at 400 K for the p-type alloy. From the theoretical calculations, it is found that the most dominant effect to have a smaller value of κ_total for the p-type alloy throughout the temperature range 290 K le; T le; 500 K is the bipolar contribution on thermal conductivity. Narrower indirect band gap of the p-type alloy leads to eight times smaller value of κ_bp compared with the result of the n-type alloy. Moreover, nearly four times smaller value of κ_ph for the p-type alloy is obtained since it has larger mass defect and anharmonic scatterings. These results indicate that using p-type Bi₂Te₃ based alloy rather than n-type alloy is likely to give rise to a higher value of thermoelectric figure of merit (ZT) for Bi₂Te₃ based alloys.
[1] D. B. Hyun et al., J.Mat.Sci. 33, 5595 (1998).
[2] D. Li et al., Intermet.19, 2002 (2011).
[3] C. Kittel, ‘Introduction to Solid State Physics&’ (John Wiley and Sons Inc, USA, 2005).
[4] P. J. Price, Phil. Mag. 46, 1252 (1955).
[5] G. P. Srivastava, ‘The Physics of Phonons&’ (Taylor and Francis Group, New York, 1990).
[6] Ö. C. Yelgel, G. P. Srivastava, Phys. Rev. B 85, 125207 (2012).
[7] Ö. C. Yelgel, G. P. Srivastava, Mater. Res. Soc. Symp. Proc. 1404, mrsf11-1404-w03-02 (2012). 449

Higher manganese silicides (HMS), MnSi_1.73, are attractive thermoelectric materials with a reported maximum ZT ~0.6 at 800 K, close to the ZT values of pure PbTe or SiGe, but without expensive or toxic elements. However, HMS are notoriously difficult to synthesize in pure form without significant MnSi or Si impurities. Stoichiometric HMS (nominal composition MnSi_1.73) undergo peritectic solidification from the liquid to solid phases, resulting in the initial formation of metallic MnSi lamellae that self-assemble along the [001] axis of HMS. It has been suggested that these MnSi striations deleteriously affect the thermoelectric transport. Conversely, Si-rich HMS (nominal MnSi_1.98) solidify by a eutectic transformation, resulting in a considerable concentration of embedded Si particles. Herein we report the synthesis of extremely high purity HMS crystals by chemical vapor transport. By reacting HMS powders and differing metal halides in fused silica ampoules kept at static vacuum over a temperature gradient, crystals of impurity-free HMS crystals between 0.1 - 1 mm in size can be produced. SEM/EDS, HRTEM, microprobe, ICP, and synchrotron PXRD analysis all confirm these HMS crystals to be extremely pure with little MnSi impurity present. Based on these findings, the potential impact that embedded MnSi impurities may have on the thermoelectric properties of HMS will be discussed.

Topological insulators (TI) are novel quantum materials with insulating bulk and topologically protected metallic conducting surfaces with Dirac-like band structure. I will talk about a new proposal on how to increase thermoelectric efficiency originating from the topological properties of the band structure imparted by the strong spin-orbit interaction in TIs. The thermoelectric properties of 3D TIs with many holes (or pores) propagating through the bulk will be discussed. I will show that at high density of these holes the thermoelectric efficiency, called ZT, can be large due to high ratio of perfectly conducting surface states to the bulk and the suppressed phonon thermal conductivity. These large values of ZT, much higher than unity, make this system an ideal candidate for applications in heat management of nanodevices, especially at low temperatures. Then I will discuss how to extend this idea to the entire class of TIs and wider range of temperatures. The proposal is based on the fact that the dislocations in certain 3D TIs have topologically protected 1D conducting channels. We predict that at high densities of the dislocations ZT can be dominated by these 1D states which can reduce the thermal conductivity on one hand and increase the conductivity and thermopower on the other. I will show that in principle this system can have very high ZT of order 10.

Phase change memory (PCM) is a recently commercialized high-speed and nonvolatile memory [1], but considerable improvements in energy efficiency, endurance, and density are still required before widespread adoption [2]. A nanoscale volume of a phase-change material (Ge2Sb5Te5, or GST, is the most commonly used) is rapidly and repeatedly switched between the high-resistance amorphous state and the low-resistance crystalline states by self-heating above melting or crystallization temperatures. The state can be detected with a relatively small read pulse, yielding a 0 or 1 for binary logic. It has been shown that thermoelectric transport can have a large impact on PCM device operation due to the large current densities and temperature gradients involved [3]. In this study, 2D rotationally symmetric PCM mushroom cells are simulated using COMSOL Multiphysics to study the thermoelectric effects and its implications for PCM structures. In these simulations, a 100 nm thick layer of GST is sandwiched between a wide TiN top contact and a 10 nm wide TiN heater along with a series nFET access device. The material parameters of the active materials are modeled with full temperature dependency from 300 to 1000 K, including electrical resistivity, thermal conductivity, and Seebeck coefficient; the thermoelectric terms are included in the current continuity and heat equations [4]. Thermal boundary resistance between GST and TiN is accounted for. The separate effects of Thomson heat, resulting from a Seebeck gradient along a current carrying homogeneous material, and Peltier heat, resulting from a Seebeck difference at an interface between two materials, are analyzed. Simulation results show favorable thermoelectric heating for one polarity.
References
[1] J. Rice, Micron Announces Availability of Phase Change Memory for Mobile Devices: First PCM Solution in the World in Volume Production. 2012. Available:http://investors.micron.com/releasedetail.cfm?ReleaseID=692563.
[2] H. -. P. Wong, S. Raoux, S. B. Kim, J. Liang, J. P. Reifenberg, B. Rajendran, M. Asheghi and K. E. Goodson, "Phase Change Memory," Proc. IEEE, vol. 98, pp. 2201-2227, 2010.
[3] A. Padilla, G. W. Burr, C. T. Rettner, T. Topuria, P. M. Rice, B. Jackson, K. Virwani, A. J. Kellock, D. Dupouy and A. Debunne, "Voltage polarity effects in Ge2Sb2Te5-based phase change memory devices," J. Appl. Phys., vol. 110, pp. 054501, 2011.
[4] L. D. Landau, E. M. Lifshitz and L. P. Pitaevskii, Electrodynamics of Continuous Media. 2nd ed., Burlington, MA: Butterworth-Heinemann, 1984.

Thermionic energy converters (TECs), which convert heat to electricity via thermionic emission from a hot cathode in a vacuum diode, are promising devices for solar energy conversion in concentrator systems owing to their high theoretical conversion efficiencies. While TECs have been investigated for nearly a century, conversion efficiencies have been severely limited by the effects of space charge in the gap between the cathode and anode. We present a path to higher energy conversion efficiencies from TECs through the reduction of the inter-electrode gap and the collector work function. We describe the design and construction of a novel, easy-to-assemble thermionic converter with micro-scale gaps using size selected ceramic microbeads. We test this architecture under concentrated solar illumination to characterize performance in real-world conditions. We demonstrate that the space charge effect can be minimized considerably using micron-scale gaps.

It has been recently reported that thermal conductivity decreases greatly without damaging electrical properties in single crystalline silicon nanostructures due to their nanoscopic features : surface roughness and nano-hole arrays. There have been many attempts to understand the mechanism of suppressing the phonon transport in these nanostructures in terms of thermal phonon wavelength and mean free path (mfp) as well as the phonon dispersion relation. However, the lack of the experimental evidence has made difficult to reach a meaningful conclusion. In this report, silicon nanowires were controllably roughened and the surface roughness near the characteristic length scale of thermal phonon wavelength is shown to interfere with phonon transport. Furthermore, silicon membranes with periodic nano-holes, where both electrical and thermal properties can be measured simultaneously, were prepared to study decoupling of electron and phonon transport associated with their mfp.
First, roughened vapor-liquid-solid (VLS) grown nanowires were thoroughly characterized with transmission electron microscopy to obtain detailed surface profiles. Once the roughness information was extracted from the surface profile of a specific nanowire, the thermal conductivity of the same nanowire was measured. The thermal conductivity correlated well with the power spectra of surface roughness, which varies as a power law in the 1-100nm length scale range. We also emphasize the importance of the ‘a_p&’ parameter to characterize the roughness and show that this roughness parameter captures the physics of phonon scattering better than conventionally used roughness parameters: the root-mean-square and the correlation length,
Secondly, a platform to simultaneously measure thermal conductivity, electrical conductivity, and the Seebeck coefficient for a silicon membrane with nano-hole arrays (pitch distance: 50 ~ 70 nm) has been developed to extract ZT directly from a single device. Using this measurement platform, we have found that phonon transport was more sensitive to the neck width in the narrow range from 20nm to 30nm than electron transport, causing optimized ZT. This result has identified the characteristic mfp and boundary scattering cross-section of phonons in silicon.

A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines phonon absorption in a space charge region with the Seebeck effect by applying a temperature gradient parallel to the PN junction. The PN junction is produced by stacking n-type and p-type nanoparticles powder prior to a densification process. The nanoparticulate nature of the densified PN junction enhances the thermoelectric properties and increases the intraband traps density which we propose is beneficial for transport across the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator.

We report on the thermoelectric properties at room temperature for glass and glass-ceramics with different crystalline fraction in the Cu-As-Te system. Glass-ceramics including metastable β-As2Te3 crystalline phase have been obtained through Spark Plasma Sintering (SPS) experiments from amorphous powder. Preliminary results show that the thermoelectric performances increase as the crystallization proceeds. The highest power factor of 260µW.K-2.m-1 at T=300K has been found for the highest crystalline fraction leading to a maximum dimensionless thermoelectric figure of merit ZT~0.1.

The thermal conductivity of silicon (Si) is too high to be well suited for thermoelectric applications. Recently we demonstrated that the thermal conductivity of Si can be effectively reduced by alternating layers of highly enriched 28Si and 29Si [1]. A 20 bilayer structure of (28Si/29Si)20 yields a reduction of almost a factor of three compared to the thermal conductivity of natural Si. This strategy to reduce the thermal conductivity of Si without affecting the electronic properties is very desirable for the optimization of thermoelectric materials. In order to investigate the potential of our approach, we prepared alternating epitaxial layers of 28Si and 30Si on Si wafer substrates by means of molecular beam epitaxy. Isotope multilayer structures with both periodic and aperiodic sequences in layer thickness were grown. Moreover, isotopically modulated Si nanopillars were prepared by reactive ion etching. The thermal conductivity of the multilayers and nanopillars was investigated with time-resolved X-ray scattering (TRXS). For these measurements the isotope structures and appropriate reference samples were covered by a 2 nm thick chromium layer followed by a 28 nm gold film. In pump-and-probe experiments conducted at the beamline ID09B of the European Sychrotron Radiation Facility (ESRF) in Grenoble (France) the cooling of the Au layer after pulse-heating with a femtosecond laser was followed by measuring the Au lattice constant with X-ray pulses synchronized with the laser beam. The Au lattice expansion is a direct measure for the temperature of the Au layer [2]. The effective thermal conductivity of the isotopically modulated structures is deduced from numerical simulations of the heat transport problem. To understand the nature of the phonon scattering process, the effective thermal conductivity of the isotope structures is compared to predictions on phonon transport in isotopically modulated structures deduced from solutions of the Boltzmann transport equation and from molecular dynamics simulations.
[1] H. Bracht, N. Wehmeier, S. Eon, A. Plech, D. Issenmann, J. Lundsgaard Hansen, A. Nylandsted Larsen, J.W. Ager III, and E.E. Haller, Appl. Phys, Lett. 101, 064103 (2012).
[2] D. Issenmann, N. Wehmeier, S. Eon, H. Bracht, G. Buth, S. Ibrahimkutty, and A. Plech, Thin Solid Films (2012), accepted.

Boron-rich compounds possess atomic networks which are formed through the covalent bonding of boron. Through incorporation of rare earth or other metal atoms and third elements like C, N and Si in the framework, network structures and physical properties can be controlled to some degree [1]. In addition to this, these compounds generally have attractive properties such as thermal and chemical stability, high hardness and the framework is composed of abundant elements. We focused on YAlB₁₄ [2] and YB₂₅ [3] which have boron icosahedra clusters forming similar network structures. Korsukova reported a single crystal sample of YAlB₁₄ which was prepared using high temperature molten Al flux method, with a refined composition of Y_0.62Al_0.71B₁₄. However, details on the thermoelectric properties haven&’t been fully investigated, and only a small range of composition for YAlB₁₄ has been reported. Through synthesis processes, we were successful in synthesizing Y_xAl_yB₁₄ (x sim; 0.57) with different Al occupancy y (0.41 le; y le; 0.63) and found striking thermoelectric properties [4].
Positive Seebeck coefficients were obtained for Al-poor samples which were shifted in the negative direction with increase of y. Maximum Seebeck coefficient values were approximately 400 mu;VK^-1 at 850 K and -200 mu;VK^-1 at 1000 K, for p-type and n-type, respectively. Although ZT values are still not high due to the poor electrical resistivity which can be partly attributed to non-optimal densities of the samples, excellent control of p-n characteristics were achieved in a system with the same crystal structure and consisting of the same elements.
[1] T. Mori, Handbook on the Physics and Chemistry of Rare-earths, Vol. 38, ed. K. A. Gschneidner Jr., J. -C. Bunzli and V. Pecharsky, North-Holland, Amsterdam (2008) 105.
[2] M. M. Korsukova, T. Lundstrom and L.-E. Tergenius, J. Alloy Comp. 187 (1992) 39.
[3] T. Mori, F.X. Zhang and T. Tanaka, J. Phys.: Condens. Matter. 13 (2002) L423.
[4] S. Maruyama, Y. Miyazaki, K. Hayashi, T. Kajitani and T. Mori, Appl. Phys. Lett. 101 (2012) 152101.

Thanks to their ability to significantly suppress phonon transport, nanostructured materials offer a unique platform for designing high-efficiency thermoelectric devices. One of the key challenges behind tuning the thermal conductivity in some semiconductor materials is that the phonon mean free paths span many orders of magnitude, revealing that engineering heat transport is intrinsically a multi-scale problem. To this end, we introduce a new computational technique based on the coupling of the Fourier model with the frequency-dependent Boltzmann transport equation (FDBTE). The model, which is based on the Discontinuous Galerkin discretization, ensures energy conservation and it is computationally affordable. We apply the developed method to compute heat transport across porous-Si with arbitrary pore arrangements, a situation for which the standard FDBTE would be very computationally expensive, and find, in agreement with recent nanoscale heat measurements, that classical size effects occur even at the microscale. We demonstrate that pores disorder is a crucial degree of freedom for optimizing ZT, predicting improvements in ZT of up to 5-10 times that found for the case of aligned pores. Although the presented method has been applied to porous-Silicon, it can be readily applied to any nanostructured material.

A detailed investigation of the influence of the Sn-content at different Ni/Zn-ratios on the thermoelectric behaviour of the quinary type I clathrate Ba8NixZnyGe46-x-y-zSnz is presented. Varying the Ni-content, the band gap can be tuned in a wide range and hence the maximum in the thermoelectric figure of merit, ZT, can be shifted in a large temperature range from 530 to 830 K. Whereas for Ba8Zn7.78Ge38.22 a maximum ZT=0.4 is observed at 800 K, Ni-doping rises the figure of merit to ZT=0.65 and additions of tin yield ZT=0.8 for Ba8.2Zn7.66Ge36.35Sn1.79. The highest ZT-value of 0.9 at 830 K was observed for Ni/Sn doped Ba8Ni0.22Zn7.22Ge37.03Sn1.53. This is one of the highest ZT-values achieved for clathrate type-I polycrystalline bulk samples.

The research of oxides as possible thermoelectric materials was triggered by the discovery that metallic layered cobaltate NaxCoO2 exhibits a large Seebeck coefficient combined with a high electrical conductivity and a low thermal conductivity which was attributed to its layered crystal structure consisting of two dimensional sheets of edge sharing CoO6 octahedra intercalated by Na ions. The highest reported zT values of NaxCoO2 are sim;1.0 for single crystalline and sim;0.8 for polycrystalline material at the temperatures in the vicinity of 800oC. With such properties it was considered to be a good candidate for the high-temperature p-type thermoelectric material. However the chemistry of layered sodium cobaltates is governed by the high mobility of interlayer sodium, which reacts with atmospheric moisture and carbon dioxide. Furthermore layered crystal structure of NaxCo2O4 enables intercalation of molecules such as water, which can lead to exfoliation and thus degradation of the material. Because of this the focus of the research turned to a semiconducting misfit-layered cobaltate Ca3Co4O9 the structrure of which consist of triple Ca2CoO3 layers and single layers of CoO2 analogous to CoO6 sheets of NaxCoO2 compounds. The highest zT reported for this structural type was sim;0.6. We found that the sheets of octahedrally coordinated Co ions, which are the common structural element of NaxCoO2 and Ca3Co4O9 phases allow spontaneous intergrowth of the two structures leading to significant improvement of environmental stability. Furthermore coherent intergrowth of the two structural types results in effective texturing in polycrystalline material with the preferred grain growth aligned in-plane with common CoO6 layers thus allowing high electrical conductivity. The nanostructured integrowths also result in a significant reduction of thermal conductivity, which was at 700oC measured to be sim;0.3 W/mK for “out-of plane” and sim;0.6 W/mK for “in-plane” direction. With the measured power factor of sim;6.5*10-4 W/mK2 the calculated “in-plane” zT of intergrowth structure material with nominal composition Ca2.2Na0.8Co4O9 is sim;1.0 at 700oC.

Bi2Te3, Bi2Se3, and Sb2Te3 have been studied for decades as some of the best bulk thermoelectric materials. Various phonon engineering approaches (mostly through nano-structuring) have been employed in the past few decades to raise the thermoelectric figure of merit to the current state of the art values. Recently, these materials have been revealed to be "topological insulators" (TI) that possess novel “topologically protected” highly conducting surface states with spin-polarized Dirac electrons. Can such topological surface states open new "electronic" degrees of freedom that may be used to further enhance the thermoelectric figure of merit? I will discuss our recent experiments on thermoelectric transport in both bulk topological insulators and gated-tunable thermoelectric devices based on TI thin film field effect transistors (TIFET). I will discuss the role of topological surface states in the thermoelectric transport and strategies to develop new kinds of high-performance thermoelectric devices harnessing the unique properties of topological insulators.

Building on studies of ErAs:In_0.53Ga_0.47As composites for thermoelectric applications, we have explored the incorporation of other rare earth elements in III-V semiconductors, in conjunction with conventional dopants such as Si. In this study, the Ce-containing nanostructures approximately 12 x 3 x 3 nm in size are embedded in In_0.53Ga_0.47As thin films by molecular beam epitaxy. The incorporation of these Ce-containing nanostructures reduces the thermal conductivity of the composite to ~3 W/m-K as compared to ~5 W/m-K for undoped In_0.53Ga_0.47As as measured at room temperature, while not impacting the conduction electron concentration. This is in strong contrast to Er in In_0.53Ga_0.47As, which reduces the thermal conductivity but also increases the conduction electron concentration. We examine why Ce forms elongated particles and why these particles are less electrically active in In_0.53Ga_0.47As than ErAs particles. Ce provides a means to effect the thermal conductivity much more strongly than the electrical conductivity. With the combination of Ce and Si dopants in In_0.53Ga_0.47As, we can independently control the thermal conductivity and the conduction electron concentration. We present measurements of the thermoelectric figure of merit of the (Ce,Si):In_0.53Ga_0.47As composites up to temperatures above 500°C.

The hollandite family of compounds has drawn growing interest in recent years as they are very appealing functional materials in view of potential technological applications, such as cathode electrode for lithium batteries, ion sieving sorption, catalysis, etc. The hollandite structure was reported for the first time by Byström and Byström in 1950 for oxides which have a composition of AXB8O16 (A = Ba, K, Sr, Li, or Bi; B = Mn, V, Ru, Rh, Cr, Ti, or Mo) and a crystal structure characterized by one dimensional tunnels formed by corner shared chains of double edge-sharing BO6 octahedra with A ions placed in the tunnels of the BO2 framework6. Manganese oxides with hollandite structure present fascinating physical and chemical properties since a manganese ion may adopt a variety of oxidation states (+2,+3,+4,+6,+7) leading to different magnetic properties. Furthermore, it is possible to obtain mixed-valence compounds and the redox property of the manganese oxides is often related to their electrical properties.
Ba1.2Mn8O16 hollandite rods were directly prepared in few minutes by using the magnetic component of the microwave in a single-mode microwave set-up. This semiconductor material presents transport properties -Seebeck coefficient, resistivity and thermal conductivity- strongly related with a first-order (monoclinic-monoclinic) structural transition that appears at asymp; 400 K. Magnetic measurements reveal antiferromagnetic interactions and magnetic frustation due to the triangular framework because of the hollandite structure.

It is well known that an inherent connection between all the thermoelectric properties (Seebeck coefficient α, electrical conductivity σ and thermal conductivity κ) has been a bottleneck in realizing high ZT. For example, improving electric conductivity often results in decreased Seebeck coefficient and thus limits the power factor (PF=α2σ) in one conventional three dimensional (3D) crystalline systems. However, nanocomposites became a new paradigm in thermoelectric materials research and led to significant improvement in many thermoelectric materials, including the half-Heusler compounds. In the work presented herein, we adopted an inductive-melting-spark-plasma-sintering synthesis process to prepare n- and p-type TiCoSb (and ZrNiSn) based nanoscomposites in which InSb and GaSb nanoinclusions were formed in situ during the synthesis process. The results of electrical conductivity, Seebeck coefficient, thermal conductivity, and Hall coefficient measurements indicate that the combined high mobility electron injection, low energy electron filtering, and boundary scattering, lead to a simultaneous improvement of all three individual thermoelectric properties: enhanced Seebeck coefficient and electrical conductivity as well as reduced lattice thermal conductivity. This represents a rare case that the same nanostructuring approach successfully works for both p-type and n-type thermoelectric materials of the same class, hence pointing to a promising materials design route for higher performance half-Heusler materials in the future and hopefully will realize similar improvement in TE devices based on such half Heusler alloys.

Chalcogenide nanowires based on Bi₂Te₃ and related materials are of significant interest for two scientific fields: nanostructured thermoelectrics and topological insulators. In the presentation, we will describe two important chemical synthesis approaches for nanostructured thermoelectric materials on the way towards optimized physical model systems. We will present the thermoelectric properties of nanostructured objects which have been synthesized by the following two different approaches:
Growth by the Vapour Liquid Solid (VLS) mode of single-crystalline and binary semiconductor nanowires and nanobelts is a widespread technique. The resulting Bi₂Te₃ nanowires exhibit a reduced tellurium content at the nanowire surface. After annealing in a Te atmosphere, single-crystalline Bi₂Te₃ nanowires have been obtained, which show reproducible electronic transport properties (electrical conductivity and Seebeck coefficient) close to those of intrinsic bulk Bi₂Te₃.
Millisecond-Pulsed Electrochemical Deposition is a quite flexible approach for achieving nanowires of ternary chalcogenide compounds, which have been grown in nanoscale confined spaces. After annealing in Te, enhanced transport properties close to those of bulk materials have been observed: Single Bi₂(Te_1-xSe_x)₃ and (Bi_x-1Sb_x)₂Te₃ nanowires exhibit power factors of 3100 mu;W/K²m and 1600 mu;W/K²m, respectively.
Both combined approaches, each based on a chemical synthesis technique and subsequent balancing of the stoichiometry by annealing under Te atmosphere, have resulted in bulk-like power-factors for Bi₂Te₃-based nanowires, which are very promising for systematic investigations of the thermal conductivity as a function of the nanowire diameter and the determination of the figure of merit ZT. Furthermore, we present the concept of tuning of the thermoelectric transport properties by modifications of the nanowire surfaces by Atomic Layer Deposition (ALD) of chalcogenide layers, resulting in core-shell nanowires.
The financial support by the German Priority Program DFG-SPP 1386 on Thermoelectric Nanostructures is gratefully acknowledged:www.spp1386thermoelectrics.de

Control of crystallinity and composition within thermoelectric nanowire materials is essential to realize the predicted enhanced power factor due to confinement. While electrochemical deposition can control the composition of Bi₂Te₃ nanowires formed in nanopore templates, the poor crystallinity of as-deposited nanowire materials leads to low electrical conductivity and, hence, low zT. Thermal annealing shows promise for improving nanowire conductivity raising the question of how the nanostructure evolves during this process. In this presentation we address this question through a systematic microscopy study of the evolution of Bi₂Te₃ nanowire structure during annealing.
We electrodeposit Bi₂Te₃ nanowires into anodized aluminum oxide (AAO) templates with ~ 75 nm diameter pores. The AAO is then chemically removed to reveal a high-density nanowire array on Si. The entire array is then thermally annealed between 200 °C and 350 °C to improve the crystallinity of the individual nanowires. We then analyze the nanostructure of these nanowires using a combined STEM/CBED mapping technique.
With this approach, large angle grain boundaries are observed and used to create nanowire grain maps as well as to determine the average grain sizes for different temperature anneals. Furthermore, small changes to the orientation of zones within a grain are plotted, correlating to the numerous small angle grain boundaries within individual grains. Combined together, this study provides a thorough investigation of the evolution from polycrystalline to nearly single crystalline nanowires. The atomic ratios are also monitored over the series using EDS, indicating a maintained Bi₂Te₃ stoichiometry. The remaining defects within the nearly single grain materials are imaged using atomic resolution HAADF, and the potential effect of these defects on the resultant transport properties are discussed.
Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Since Bi is a semimetal, it has a very low Seebeck coefficient, requiring the use of magnetic fields to make it an interesting thermoelectric (TE) material. However, by confining the carrier paths to nanostructured geometries, a bandgap can be induced that under optimal doping conditions could yield high ZT values. Nevertheless, two problems prevent taking advantage of this TE material in nanowire geometries: (1) the buildup of a thick Bi-oxide surface layer that greatly hinders the ability to make good electrical contacts, and (2) control of the appropriate crystalline phase and orientation along the nanowire lengths.
Taking advantage of the first problem can provide a solution to the second. We start by growing ~ 5 µm long Bi nanowires with 75 nm diameters via electrochemical deposition under nitrogen into anodized aluminum oxide (AAO) templates grown directly onto ~ 100 nm W coated Si(100). Next, the AAO templates are chemically removed, leaving behind an array of vertically-aligned Bi nanowires on a Si substrate. We study the effect on microstructure of annealing these arrays in a 3% H2/Ar reducing atmosphere (to minimize Bi oxidation) to temperatures ranging from 200 - 400°C. Scanning electron microscopy finds that the nanowire arrays remain intact even for temperatures well above the 271°C melting point of Bi. X-ray diffraction and transmission electron microscopy find negligible change in the microstructure of the as-deposited nanowires with all anneals below the melting point. Only above the melting point does annealing cause a dramatic change in the crystallinity of the Bi nanowires. TEM shows that the as-deposited nanowires have a few nm&’s thick Bi-oxide shell, likely formed immediately upon exposure to air. This oxide shell is stable to high temperatures and provides a stiff protective structure that allows Bi to melt within it and maintain its nanowire geometry. In-situ TEM heating experiments confirm that melting occurs within the oxide shell structure. Upon cooling, the Bi re-solidifies inside the shell.
The crystalline quality and orientation control of the Bi nanowires can be controlled via the cooling rate as the array temperature drops below the melting point and the individual nanowires re-solidify. Using XRD and TEM, we will compare the microstructures of the as-deposited Bi nanowires to those that are melted and then re-solidified using fast and slow cooling rates. As expected, slower recrystallization enables larger grain growth with improved crystallinity, and most intriguingly, primarily with the preferred trigonal orientation for TE properties.
Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Tellurium alloys have attracted significant attention due to their high performance thermoelectric properties [1]. In particular, nanostructured bulk lanthanum telluride (La3-xTe4) by mechanical ball-milling was reported to exceed figure of merit (ZT) of 1 at high temperatures near 1300K [2-3]. Since the increased thermoelectric efficiency of nanostructured materials are due to the enhancement of phonon scattering introduced by quantum confinement, thin films (2-D systems) and nanowires (1-D systems) also are generated significant scientific and technological interests [4-6]. Here, we report on the electrochemical synthesis of lanthanum telluride thin films. The thickness of thin films can be controlled by deposition duration. We also report on the process of nanowire fabrication involves first electrodepositing lanthanum telluride arrays into anodic aluminum oxide (AAO) nanoporous membranes. After dissolving the membranes, the ordered and vertically-aligned nanowire arrays are revealed on the substrate [7]. These novel procedures can serve as an alternative means of simple, inexpensive and laboratory-environment friendly methods to synthesize nanostructured thermoelectric materials. The in-plane Seebeck coefficient, in-plane electrical resistivity, in-plane thermal conductivity and the calculated ZT of lanthanum telluride thin films will be presented along with the thermoelectric properties of lanthanum telluride nanowires to compare to current state-of-the-art thermoelectric materials. The morphologies and chemical compositions of the deposited films and nanowires are characterized using SEM, XRD and EDAX analysis.
References:
[1] D. M. Rowe, CRC Handbook of Thermoelectrics, CRC Press (1995).
[2] A. May, J-P. Fleurial and G. J. Snyder, Phys. Rev. B 78, 125205 (2008).
[3] O. Delaire et al., Phys. Rev. B 80, 184302 (2009).
[4] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 (1993).
[5] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 16631 (1993).
[6] M. S. Dresselhaus et al., Adv. Mater. 19 (2007).
[7] S. C. Chi, S. L. Farias and R. C. Cammarata, “Synthesis of Vertically Aligned Gold Nanowire-Ferromagnetic Metal Matrix Composites,” 220th ECS Meeting eds. P. Vereecken, G. Oskam, I. Shao, and J. Fransaer, ECS Trans. 41, 119 (2012).

Non-toxic, temperature stable and efficient thermoelectric materials are developed for the construction of thermoelectric modules for renewable energy technologies. Tailor made nanostructured perovskite-type titanates, manganates and cobaltates with strongly correlated electrons are used as prospective thermoelectric oxides. Calcium manganates zinc oxides and strontium titanates are most promising n-type thermoelec-tric materials, while cobaltates are known to present a large spin-orbit entropy factor enhancing the ther-mopower. From the pressed sintered powders and nanocomposites p- and n-type legs are fabricated to be tested at elevated temperatures for high energy density converters. The low temperature side of the converters consists of thermoelectric p- and n-type Heusler compounds.

The work to be presented is focussed on the nano-scale transport of electrons across a Schottky, metal-semiconductor interface, and the development of a model that better relates experimentally observed current-voltage results to the theory of laterally varying barrier heights, and their respective activation.

Several publications [1-3] advocate modifications to the basic diode equation to take into account lateral fluctuations in the barrier height that result from interfacial inhomogeneity (dirt, surface roughness, uneven doping, crystal defects, etc.). Of these, only the Tung [1] model is able to relate this phenomena back to common non-linearities in the turn-on characteristics of log(I)-V profiles, such as double bumps, ideality factors and barrier energies that vary with temperature, and ideality factors with values that far exceed 1. In light of experimental results over several Si Schottky diodes of varying anode metal (Ti, NiV, Cr and Nb) and homogeneity, we have recently shown [4] that the fundamental fitting parameters within the Tung Model are temperature dependent. This necessitates a significant modification to the existing model if real current-voltage data over a wide temperature range (77-300 K), is to be accurately replicated.
In this paper we will present these very recent findings as well as further verification of our model, in which data taken from SiC diodes and alternative Si diodes are accurately reproduced.
1. R. T. Tung, Phys. Rev. B, 45, 13509 (1992).
2. J. H. Werner and H. H. Guttler, J. Appl. Phys. 69, 1522 (1991)
3. F. Roccaforte et al, J. Appl. Phys., 93, 9137 (2003).
4. P. M. Gammon et al, In Press (2012)

Wide band-gap semiconductors, such as the group-III nitrides,
are currently regarded as viable solutions for the next generation
of electronic/optoelectronic devices. Small diameter nanowires,
down to a few lattice constants, are structurally and electronically
different from bulk, due to the large surface-to-volume ratio and
the effects of the surface states, which has consequences in the
optical absorption and in the electrical/thermal transport.
It has been established that AlN nanowires can suffer a stress
induced phase transition from a wurtzite to a graphite-like phase [1].
Ab initio calculations based on the density functional theory (DFT)
have been performed and the phonon spectrum has been obtained for each
structural configuration. The thermal conductance and heat capacity
are then extracted and they prove to be consistent with existing
experimental data.
The thermopower of atomic-sized wurtzite AlN wires coupled to Al(111)
bulk contacts is investigated
at low temperatures using Green-Keldysh formalism.
We find that the conduction of the wide bandgap semiconductor
wire is essentially enhanced by the presence of surface states.
We show that the
evanescent coupling to the surface states is strong enough to
render thermopower of a few tens of
micro-V/K, which may be enhanced by controlling the position of the surface states.
[2].
We also investigate the changes in the thermopower under applied axial stress,
comparatively analyzing the nanowires in the wurtzite and graphite-like
configurations.

[1] T.L. Mitran, Adela Nicolaev, G.A. Nemnes, L. Ion, S. Antohe,
Comput. Mat. Sci. 50, 2955 (2011)
[2] G.A. Nemnes, C. Visan, S. Antohe,
Physica E 44, 1092 (2012)

Thermoelectric devices, which convert electricity to heat and vice versa, have attracted great interest for waste-heat recovery and non-vibration anti-noise cooling system. During the last decade, great progress on thermoelectric semiconductors has been made toward increasing figure of merit zT through the reduction of the lattice thermal conductivity. Recently, the enhancement of thermopower by the distortions of the electronic density of states is reported. Engineering of the density of states near the Fermi level by doping a proper element achieved the thermopower enhancement despite higher carrier concentration. In this study, thermoelectric properties of Bi2Te3 films by doping I ions for the generation of resonant impurity level was investigated. I-doped Bi2Te3 films of high quality were successfully grown on 4o off GaAs (100) substrate by metal organic chemical vapor deposition. In the presentation, structural and electrical properties of the I-doped Bi2Te3 films will be discussed in terms of I doping concentration. Recent results on the band engineering of Bi2Te3 films through I doping will also be discussed.

Recently, a percolation effect has been introduced to tune thermoelectric transport properties. According to the principle of the percolation effect, a high coarse/fine size ratio in a thermoelectric powder would generate a more significant enhancement of figure of merit (ZT). In this study, two kinds of Bi0.4Sb1.6Te3 powders with large particles size difference (coarse, ca. 1 mu;m; fine, ca. 100 nm) were fabricated by mechanical alloying for different times, respectively. Their mixtures at different ratios were consolidated by vacuum hot pressing to produce nano-/microstructured composites with the same chemical compositions. From the measurements of Seebeck coefficient, electrical resistivity and thermal conductivity, a ZT value up to 1.19 was achieved at 373 K for the sample containing 40% fine particles. The achieved higher ZT value is attributed to the unique micro/nanostructures which reduce the thermal conductivity effectively by increasing the density of grain boundaries and interfaces. It indicated that the micro/nanostructured Bi0.4Sb1.6Te3 alloy could be served as a high-performance material for the application on thermoelectric devices. The results also demonstrated the possibility to enhance ZT value of Bi0.4Sb1.6Te3 alloy by introducing nonuniform microstructures even without changing chemical compositions.

A variety of oxides have been proposed as affordable thermoelectric materials, and nanostructured oxides including nanowires may be one route to increase the figure-of-merit to competitive values for applications including waste heat recovery. Here, we report the Seebeck coefficient and electrical conductivity of zinc oxide nanowire films, measured using a hybrid infrared imaging approach with a resulting room temperature value of -67 µV/K at 300 K. The samples include solution-synthesized nanowires with varied levels of aluminum and gallium doping suspended in ethanol and spray-coated onto glass substrates patterned with silver electrodes. The measurement technique determines the voltage developed in response to an applied temperature gradient measured using infrared thermometry. The porosity of the film is determined with image analysis of scanning electron micrographs. Using an estimate of the thermal conductivity based on effective medium theory and electrical conductivity measured with a 4-point probe technique, we estimate a room temperature figure-of-merit near 10-2. While this relatively low value is not competitive with state-of-the-art chalcogenide thermoelectric materials, it could be promising for cost-effective thermoelectrics which must operate at high temperatures or harsh environments where oxidation of typical materials is a challenge.

Sb₂Te₃ are narrow band-gap p-type semiconductors. These materials are used as thermoelectric materials partly due to their low thermal conductivity. In this study, Sb₂Te₃ was deposited on BaF₂ (111) substrates by pulsed-plasma-enhanced metallorganic chemical vapor deposition at temperatures ranging from 100 to 300 °C using triisopropyl antimony and diisopropyl tellurium as Sb and Te precursors, respectively. The chemical composition of the films was controlled by varying the precursor injection times of Sb and Te. We have investigated the crystal structure change that accompanies the variation in the orientation of Sb₂Te₃ phase and its effect on the electrical properties. The quality of the deposited films were characterized using X-ray diffractometry (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM). The electrical properties, i.e. carrier concentrations, carrier mobilities and electrical resistivity were determined by Hall measurement at room temperature.

Nanostructure engineering been theoretically and experimentally proven as a practical strategy for thermoelectric materials for effectively enhancing thermoelectric figure of merit, ZT (defined as σS²T /κ). By nanostructuring, not only could the largely created surface or interfaces inhibit the thermal conductivity (κ), but the power factor (σS²) could be improved through the induced quantum confinement. Antimony selenide (Sb₂Se₃) has a higher Seebeck coefficient (1800 mu;VK^-1) and lower thermal conductivity (~2.7 Wm^-1K^-1) compared with the widely studied bismuth telluride (Bi₂Te₃) bulk and is thus considered as a promising alternate of the room-temperature thermoelectric materials for the next generation. However, to date, thermoelectric data related to the Sb₂Se₃ nanostructures have less been reported. Therefore, growth of nanostructured Sb₂Se₃ becomes a very attractive and important research topic for approaching potentially outstanding thermoelectric performance. In this work, by using pulsed laser deposition (PLD) techniques, a series of well-aligned nanostructured Sb₂Se₃ films was successfully prepared on insulated SiO₂/Si substrates without prebuilt catalysts and templates. At least seven types of previously unreported Sb₂Se₃ nanostructures including nanoteeth, nanowings, nanorods, nanodecks, nanotweezers, nanocolumns, and nanotubes were reproducibly obtained via precisely controlling the substrate temperature and ambient pressures. The temperature-dependent growth seems to be reasonably explained by the well-known structure zone model (SZM) and self-catalyst enhanced vapor-liquid-solid (VLS) growth. Among these specimens, the Sb₂Se₃ nanowings show an excellent electrical conductivity of 750 Sm^-1 at 400 °C, which is comparable to the reported value of an isolated nanowire (852 Sm^-1). In addition, the room-temperature electrical conductivity of the Sb₂Se₃ nanoteeths is 4 to 5 orders higher that that of the non-nanostructured Sb₂Se₃ films, indicating the great success in improving the electrical conductivity of the nanostructured Sb₂Se₃ films.

Free carrier concentration is a key parameter in thermoelectric materials, as it monitors the doping level. Being usually determined as a global (bulk) property by Hall effect measurements, it generally hinders the sample in-homogeneities caused by compositional (or) doping gradient, or local microstructure. The current work describes the application of a non-destructive, reflection-based, mid infrared micro-spectroscopic mapping analysis into thermoelectric materials towards identification of position-sensitive structural in-homogeneities. This is done through the coupling of an infrared spectrometer to an infrared microscope and offers the unique opportunity of studying larger samples with extended spatial resolution.
Room temperature mid infrared reflectivity measurements at near normal incidence, were performed using a Perkin Elmer micro-spectrometer, equipped with an infrared optical microscope. The spatially resolved spectral information was obtained by scanning the sample spot by spot over the surface area using a stationary detector and collecting at each spot a complete infrared spectrum in the spectral range of 700-4000cm-1. The sample could be moved by x-y-computer controlled stage at a spatial resolution of about 100mu;m. Detailed analysis of the collected IR spectra yields the plasmon frequency, the free carrier damping factor and the high frequency dielectric constant, which are related to the free carrier concentration, the optical mobility and conductivity. The method has been tested in PbTe, PbSe and Mg2Si based thermoelectric materials. Dopant in-homogeneities were also confirmed by SEM-EDS. The carrier-mapping technique may be useful in the study of segmented TE legs and/or in (intentionally made in-homogeneous) functionally graded thermoelectric materials.

Perovskite-type oxides and their derivatives provide useful possibilities for the research for better thermoelectric materials. Particular advantage is the flexibility of the perovskite crystal structure over gradually changing chemical composition, which often enables rather analytical observation on the effects provided by a particular element implemented into the structure. While the focus has dominantly been on modifications made into the cation sublattice, the anion sublattice of the perovskites has been much less charted.
N3- (146 Å), O2- (140 Å) and F- (133 Å) anions have similar ionic radii, and therefore can easily be replaced by each other in a perovskite lattice. Different bonding characters of the anions affect e.g. on the band-structures, transport properties and chemical stability.[Different anion oxidation numbers, on the other hand, result into respective changes in the cation valencies and/or the crystal structure. An alternative for the anion substitution is the anion elimination resulting e.g. at high anion-deficiency into interesting multitude of vacancy structures, often with a profound low-dimensionality. Thermogravimetric methods have pointed out to be useful in studying these materials as anions tend to show a reasonable volatility in many of their perovskite compounds. The Study combines in-situ observations at reaction temperatures with ex-situ characterizations of the reduction end-products at room-temperature. Phase composition (XRD) and oxygen content (TG).

Mg ₂Si is a promising thermoelectric material since both of elements Mg and Si are abundant and environmentally friendly. However, its thermoelectric figure of merit (ZT ~ 0.5 at 700K) is not yet high enough to efficiently convert heat energy into electricity, or vice versa. Recently, we find that Mg₂Si thin film deposited at 300K and 500K shows p-type and n-type semiconductor behavior, respectively. The charge carrier type remains the same after the thin film went through a 600K annealing. XRD shows there is no phase difference between p-type and n-type thin film, which suggests that the different electrical property may arise from the intrinsic defect of Mg₂Si. In this work, we use a theoretical approach to investigate the impact of intrinsic defects (i.e., interstitials and vacancies) on thermoelectric properties of Mg₂Si. It is found that either n-type or p-type Mg₂Si can be realized by introducing different intrinsic defects in Mg₂Si bulk crystal. Using the Boltzmann transport theory, we have calculated transport properties, such as electron conductivity, Seebeck coefficient and thermal conductivity from electron contribution. To compute phonon contribution to the thermal conductivity, we have developed modified embedded atom method (MEAM) potential for Mg-Si such that thermal conductivity of Mg₂Si can be determined using non-equilibrium molecular dynamics simulation method. With all these computed transport properties, we have calculated ZT values for Mg₂Si with different intrinsic defects. Our result provides estimation on the upper limit of ZT values that can be achieved through engineering the intrinsic defects in bulk Mg₂Si and an insight on the contributing factors in ZT enhancement.

Highly uniform Bi₂Te₃ and Cu doped Bi₂Te₃ 1D nanostructures with a length of 1 mu;m and a diameter of 40 nm were synthesized through a simple and fast solution process by using ultrathin Te nanowires as sacrificial templates. The synthesized nanostructures were investigated with HR-TEM, XRD, FT-IR spectroscopy, SEM-EDS, TGA analysis. Nanostructured bulk pellets were prepared by spark plasma sintering of chemically synthesized Bi₂Te₃ nanotubes and Cu doped Bi₂Te₃ nanowires. The densification behavior, microstructure, and phase formation of sintered samples were characterized. The electrical conductivity, Seebeck coefficient, and thermal conductivity were investigated and the resulting thermoelectric figure of merit was determined. Unlike pure Bi₂Te₃, Cu doped Bi₂Te₃ are contained Cu which can be induced formation of heterogeneous nanostructure on the surface. The figure of merit (ZT) of the Cu doped Bi₂Te₃ nanostructured bulk shows significantly enhanced value compared to that of pure Bi₂Te₃ and maximum ZT of 0.68 was obtained at 418 K, which is one of the high value among the reported values of n-type nanostructured materials based on chemically synthesized nanoparticles. It is considered that the improved thermoelectric performance of the nanostructured bulk mainly originated from thermal conductivity that was reduced by active phonon-scattering at the heterogeneous nanostructured interface.

Bi₂Te₃ is the most suitable thermoelectric materials for use near room temperature but need more enhancement of efficiency to reach commercialization. It has been known that the intercalation of small amounts of 3d transition metal between the van der Walls layers of Bi₂Te₃ is expected to increase the thermoelectric figure-of-merit (ZT) of Bi₂Te₃-based compounds.
We prepare Cr-intercalated Bi₂Te₃ with composition Cr_xBi₂Te₃ (x = 0.01, 0.03, 0.05) and Cr-substituted Bi₂Te₃ with composition Cr_yBi_2-yTe₃(y = 0.01, 0.03, 0.05) using Bridgman method, and study thermoelectric properties as a function of Cr contents in Bi₂Te₃. The structure and composition of the samples are characterized by using XRD and EDS. The temperature dependence of electrical conductivity, thermal conductivity and Seebeck coefficient are measured. Cr-intercalated Bi₂Te₃ samples show more enhanced thermoelectric property than Cr-substituted samples. A peak ZT value of 0.7 at 300K was achieved in Cr_0.01Bi₂Te₃ samples.

We have prepared thermoelectric devices from alternating layers of Si/Si+Sb superlattice films using ion beam assisted deposition (IBAD). In order to determine the stoichiometry of the elements and the thickness of the grown multi-layer film, Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used. SEM and EDS have been used to analyze the surface and composition of the thin films. The 5 MeV Si ion bombardments have been performed using the AAMU Pelletron ion beam accelerator, to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and increase the cross plane electrical conductivity to increase the figure of merit. Some optical instrumentations have been used addition to RBS and SEM. We will be showing our findings.
Keywords: Ion bombardment, thermoelectric properties, multi-nanolayers, figure of merit.
Acknowledgement
Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF-12-1-0063.

We prepared a thermoelectric generator device from 36 alternating layers of SiO2/SiO2+Cu superlattice films using Magnetron DC/RF Sputtering. Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used to determine the stoichiometry of Si and Cu in the grown multilayer films and the thickness of the grown multi-layer films. SEM and EDS have been used to analyze the surface and composition of the thin films. The thin films have been annealed at the different temperatures to make quantum clusters in the multi-layer superlattice thin films to tailor the thermoelectric and optical properties of the thin film systems. We will be showing our findings.

Keywords: Ion bombardment, thermoelectric properties, multi-nanolayers, figure of merit.
Acknowledgement
Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF-12-1-0063.

We prepared thermoelectric generator devices from 10 and 40 alternating layers of Si/Si+Ge superlattice films using Magnetron DC/RF Sputtering. Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used to determine the stoichiometry of Si and Ge in the grown multilayer films and the thickness of the grown multi-layer films. SEM and EDS have been used to analyze the surface and composition of the thin films. The 5 MeV Si ion bombardments have been performed using the AAMU Pelletron ion beam accelerator, to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and increase the cross plane electrical conductivity to increase the figure of merit. We will be showing our findings.

Keywords: Ion bombardment, thermoelectric properties, multi-nanolayers, figure of merit.
Acknowledgement
Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF-12-1-0063.

The chalcopyrite-type semiconductor compounds are known to have a potential for application in photovoltaic cells, light-emitting diodes and nonlinear optical devices as well as thermoelectric devices. The thermoelectric devices, allowing the solid-state conversion between thermal and electrical energy, have long been considered a very attractive technology for cooling and waste heat recovery.
The ternary and quaternary chalcogenide compounds can be designed by various combinations of elements in the vicinity of the group IV element. Such tetrahedrally coordinated semiconductors, for example, defect chalcopyrite, spinel, stannite and famatinite, have been suggested. To examine the possibility of the above semiconductors for useful devices is very important for newer material developments. However, the physical properties of these semiconductor compounds, especially of many quaternary compounds, have been scarcely investigated, because it is very difficult to obtain the crystals with suitable size and quality as compared with elemental and binary compounds.
In this paper, we have synthesized Cu2ZnSnSe4 (CZTSe) materials using simple and cost-effective solid state reaction method from elemental Cu, ZnO, SnO and elemental Se powders. The SEM images show uniform size with aggregation in powder. The phase separation and thermal analysis of the milled powders suggested that most of the starting powders reacted because of a mechanical alloying effect during milling process. After the solid state reaction at above 500 oC, the powder crystallized into stannite single phase, which confirm by XRD spectra. The thermoelectric properties of synthesized powder will be characterized.

Mixed ion-electron conductors provide an alternative to typical semiconductor materials. In the high temperature phase of Copper(I) Selenide, the Cu+ ions are mobile, while the Se forms an FCC cage. The Se structure allows for conduction of holes via crystalline pathways. The high conductivity of the mobile Cu ions results in liquid-like behavior and unusually high phonon scattering. At 1,000 K zT = 1.5 has been reported in Cu2Se.1 By examining the behavior of transport variables such as Hall mobility, thermal diffusivity, heat capacity and thermopower, understanding of how to engineer this new class of materials can be gained. Of particular interest is the behavior of these variables near the transition from non-liquid-like to liquid-like ion conduction. The results indicate that mixed conductors are a promising new class of thermoelectric materials.

Nano-structured thermoelectric materials, such as superlattice films including nano-particles, nanowires, nanoporous and nanocomposites, have been investigated to enhance the thermoelectric figure of merit ZT=(S2σ/κ), because high density of interfaces in the materials reduce their thermal conductivity attributing to the phonon scattering [1]. Previously, we have reported the reduction of thermal conductivity of ZnSe crystals with high-density of twin defects due to the nano-sized boundary effect of twins [2]. In this paper, we report the influence of the direction of twin boundary to the thermal conductivity (phonon transport) in the ZnSe crystals. Single crystalline ZnSe (phi;10mm x 50mm) with high-density of twin defects were grown by the vertical Bridgman method using Zn-partial pressure [3]. The defect type was 120° rotational twin along <111> direction. Periods of the twin boundaries were between 10 nm and 60 nm. Bulk samples (2 x 2 x 10 mm) of two different crystal directions were prepared for the thermal conductivity measurements. Both samples were cut from a same ZnSe crystal with high-density of twins. One was the longitudinal axis parallel to <111> (sample #A), and the other was that perpendicular to <111> (sample #B). Thermal conductivity measurement was carried out by a conventional static method between 2 and 300 K using PPMS (Quantum Co.). The measured thermal conductivity of both the twinned samples was approximately 17 W/m-K at 300 K, which is slightly lower than that of non-twinned ZnSe bulk crystal (19-20 W/m-K). The thermal conductivities increased with decreasing the temperature and reached maximum values of approximately 600 W/m-K for #A and 390 W/m-K for #B at around 12 K. Both the maximum values are lower than that of the non-twinned ZnSe (670 W/m-K) due to the boundary effect of high-density twins. The difference of the thermal conductivity between #A and #B indicates that the direction of twin boundary affects the phonon transport, that is, the scattering of phonon is more enhanced for the direction parallel to the twin boundary than for that perpendicular to one.
[1] W. Kim et al., Nanotoday 2(2007)40.
[2] H. Udono et al., MRS 2010 Spring meeting, DD10.19.
[3] H. Udono et al., J.Cryst.Growth,197(1999)466.

Electronic and thermoelectric properties of V2O5, MgV2O5 and CaV2O5 have been calculated using density functional theory and Boltzmann semi-classical equations. MgV2O5 and CaV2O5 are proposed to be good candidates for thermoelectric materials at high temperatures as 1), Reasonable thermal power at high temperatures is obtained for them. 2), Very narrow band gaps are found for these two compounds, indicating high electrical conductivity may possess. 3), low thermal conductivities were measured experimentally. Besides, high anisotropy for thermal power and electrical conductivity are observed in MgV2O5 and CaV2O5. The anisotropy of the properties may lead us to design low-dimensionnal highly performant thermoelectric materials, such as thin films, superlattices and nanowires.

The increasing world's demand for ubiquitous power generation is driving the development of new materials and technologies able to extract energy from the environment. For ambient energy sources such as vibrational, solar or thermal, the power density that can be harvested is in the range of tenths of mu;W to mW/cm3, i.e. high enough for powering small electronics, nodes in wireless sensor networks or remote actuators. However, these applications usually involve scaling down the generators to the microscale by using MEMS technology, therefore limiting the materials allowed for their fabrication. Furthermore, recently discovered nanodevices fabricated with one-dimensional nanostructures, regardless of having low-power requirements, call for power sources compatible to nano- or micro-electronic technologies in order to take advantage of their size.
Thermoelectric generators (TEGs) are devices able to harvest waste heat directly converting it into electricity. Due to their adaptability to different thermal gradients and energy densities as well as good scalability from mu;W to hundreds of kW, TEGs are particularly interesting for powering portable devices. However, the poor thermoelectric properties of materials traditionally used in microelectronics (e.g. silicon with ZTsim;0.01) and the poor compatibility of good thermoelectric materials (e.g. Bi2Te3) with mainstream microelectronics have limited their integration.
One-dimensional (1D) nanowire structures have been shown to be promising candidates for enhancing the thermoelectric properties of semiconductor materials. This work reports on the implementation of multiple electrically connected dense arrays of well-oriented and size-controlled silicon nanowires (Si NWs) grown by the CVD-VLS mechanism into microfabricated structures to develop thermoelectric microgenerators. Low thermal mass suspended silicon structures have been designed and microfabricated to naturally generate thermal gradients in planar microthermoelements (mu;TE). The hot and cold parts of the device are linked with horizontal arrays of Si NWs growth by a single bottom-up process. In order to improve the performance of the device as energy harvester, the successive linkage of multiple Si NW arrays has been developed to generate larger temperature differences while preserving a good electrical contact that allows keeping small internal mu;TE resistances. The fabricated mu;TE have shown Seebeck voltages up to 60 mV and generated power densities up to 1.44 mW/cm2 for ΔT=300C and, working as energy harvesters, a maximum Seebeck voltage of 4.4 mV and a generated power density of 9 mu;W/cm2 for ΔT=27C (across the nanowires) in a single mu;TE. The fabricated microgenerator, taking advantage of the simple planar geometry and compatibility with silicon technology, provides an alternative to the state-of-the-art mu;TEGs based on non-integrable and scarce V-VI semiconductor materials and a promising energy harvester for advanced micro/nanosystems.

The performances of thermoelectric energy conversion is shown by the dimensionless figure of merit ZT = (S2σ/k)T, where S is the Seebeck coefficient, σ is the electrical conductivity of carrier, T is the absolute temperature, and k is the thermal conductivity. Bulk silicon has a poor thermoelectric performance due to high k of 142 Wm-1K-1 at room temperature (RT). However, it is possible that the k is reduced by enhancement of phonon scattering at interfaces of Si nanostructures, such as nanowires and nanocrystals (NCs). Bux et al. fabricated nanostructured bulk Si assembled from Si NCs with diameters of 5-20 nm and measured thermoelectric properties. They observed k of 6.3 W/mK, which is much lower than the value of 87.3 W/mK for heavily doped Si at RT. The ZT of nanostructured bulk Si was 0.023 at RT, while single crystal Si had ZT of ~0.01 [Adv. Funct. Mater. 19, 2445 (2009)]. Recently, we have demonstrated semiconducting composite films Si and Ni silicide NCs (Ni-Si composite film), having much lower k of ~5 Wm-1K-1 at room temperature than that of bulk Si. Reduction of thermal conductivity suggests that the composite film is promising as high-efficiency Si-based thermoelectric materials.
The composite films were synthesized by phase separation from amorphous Ni-Si alloy films, which have composition of Si/Ni~20, followed by thermal annealing at 600-1200 degrees C. Scanning transmission electron microscope images shows that Si and Ni silicide NCs were grown to more than 50nm in diameters. In this work, we investigated the carrier doping to SiNCs in the composite films. Boron and phosphorus atoms of 2 mol % were doped into Ni-Si alloy films. Formation of SiNCs and the dopant activations were carried out by thermal annealing at 1000-1200 degrees C. Electrical properties were further analyzed by Hall measurements at 300K for B- and P- doped Ni-Si composite films. The electron and hole mobility and carrier concentration were estimated to be 10-20 cm2/Vsec and 1019-20 cm-3. The σ of the films was 105-10 6/Omega;m. Seebeck coefficients of the films were estimated to be -100 --160 uV/K for P doped films and 100-130 uV/K for B doped films at 300K by the 2-probe method. The power factor values were up to 1.7 mW/mK2 and 0.8 mW/mK2 for p-type and n-type Ni-Si composite films. We observed the resulting maximum the ZT values of 0.11 and 0.05 for p-type and n-type films. This ZT is higher than that of the single crystal Si and also the nanostructured bulk Si at RT.
In summary, we prepared both p and n type Ni-Si composite films by B and P doping. The Ni-Si composite film has much higher ZT values for than that of bulk Si. We believe that the Ni-Si composite films are useful high-efficiency Si-based thermoelectric materials.

In addition to the well-established need for highly reliable space power systems, there has been recently renewed interest in developing practical new materials capable of efficient thermal-to-electric conversion of high grade heat sources (> 1200 K), generated through fossil fuel combustion or as a waste exhaust stream. Proven state-of-practice Si0.8Ge0.2 alloys have a combined dimensionless figure of merit (ZT) value lower than 0.5 when averaged over operating temperatures of 1275 K to 300 K. In addition, the significant Ge content of these materials precludes their use for large scale terrestrial applications due to cost considerations.
We present an overview of NASA-funded collaborative research efforts to identify and characterize advanced bulk thermoelectric materials capable of quadrupling average ZT values while maintaining reliable operation for more than 15 years at temperatures up to 1300 K. Some of the materials being investigated may have promising potential for larger scale terrestrial applications.
The research areas include structurally complex refractory rare earth compounds, electronic band-engineered advanced PbTe and bulk 3-D nanostructures that emulate results obtained on low dimensional superlattices through “force engineering” and “self-assembling” techniques. This latter research area, perhaps of most interest to terrestrial applications, focuses on engineering bulk homogenous and composite 3-D nanostructures that effectively decouple electrical and thermal transport effects to enable independent optimization strategies. Silicon and silicide nanostructured materials have been predicted to have the potential for large gains in ZT values through a combination of low lattice thermal conductivity brought by effective interface scattering and tuning of electrical transport through the introduction of suitable nanoscale inclusions. Strategies to achieve the necessary grain size, size distribution and nanoscale inclusion “seeding” in thermally stable bulk structures are presented. Recent results on silicon/silicide nanocomposites prepared through a bottoms-up synthetic process will also be discussed.

One of the major roadblocks to large improvements in the thermoelectric figures of merit (ZT) of leading candidate thermoelectric materials such as the Bi2Te3, PbTe, CoSb3 and half-Heusler (HH) based systems remains the difficulty in making meaningful simultaneous improvements in both the electrical conductivity (σ) and thermopower (S) of these materials through doping and/or substitutional chemistry. In conventional semiconductors, both materials parameters (S and σ) are fundamentally coupled adversely through the concentration, n, of charge carriers. Therefore, the maximization of one parameter by tuning the carrier concentration (n) via doping and/or substitutional chemistry inevitably results in the minimization of the other. Here, we show that by coherently embedding sub-ten nanometer scale inclusions (quantum dot) within a semiconducting half-Heusler matrix, large enhancements of the thermopower (S) and the mobility (mu;) can be achieved simultaneously in both n-type and p-type nanocomposites1-3. The enhancement in thermopower originates from large reductions in the effective carrier density (n) coupled presumably with an increase in the carrier effective mass (m*). The surprising enhancement in the mobility is attributed to an increase in the mean-free time (tau;) between scattering events (phonon-electron scattering, ionized-impurity scattering and electron - electron scattering). Using X-ray powder diffraction, electron microscopy, and electronic transports data, we will discussed the mechanism of phase formation and transformation, at the sub-ten nanometer scale, in bulk half-Heusler (HH) matrix and the mechanism by which the embedded nanostructures regulate electronic charge transport within the semiconducting HH matrix to achieve unprecedented combinations of physical properties such as, large enhancements in the carrier mobility (mu;), thermopower (S) and electrical conductivity (σ) simultaneously with drastic decrease in thermal conductivity (κ) at high temperatures. Emphasis will be placed on the n-type Zr0.25Hf0.75Ni1+xSn1-yBiy and Ti0.1Zr0.9Ni1+xSn, and the p-type Ti0.5Zr0.5Co1+xSb nanocomposites.
(1) Makongo, J. P. A.; Misra, D. K.; Zhou, X.; Pant, A.; Shabetai, M. R.; Su, X.; Uher, C.; Stokes, K. L.; Poudeu, P. F. P. J. Am. Chem. Soc. 2011, 133, 18843.
(2) Liu, Y.; Makongo, J. P. A.; Zhou, X.; Uher, C.; Poudeu, P. P. F. in preparation 2012.
(3) Sahoo, P.; Makongo, J. P. A.; Zhou, X.; Uher, C.; Poudeu, P. P. F. in preparation 2012.

Because of the continuously declining global energy resources, sustainable methods of power generation, such as the thermoelectric energy conversion, become increasingly important.
Our first report on the thermoelectric properties of the Te-doped intermetallics Mo₃Sb₇ dates back to 2002, where we demonstrated that this metallic antimonide turns into a semiconductor by substituting Sb in part with Te. Subsequently we succeeded in adding small metal atoms into the cubic Sb₈ cages, thereby increasing the unit cell volume. Gascoin, Snyder et al. determined a ZT_max(1050 K) = 0.76 for Mo₃Sb_5.4Te_1.6, which we improved to 0.93 after adding small amounts of nickel into the Sb₈ cubes.
Since the relatively high lattice thermal conductivity of these materials constitutes a challenge, we successfully attempted to lower it by forming nanocomposites with fullerene. The local structure of these composites as well as the key thermoelectric properties will be presented with this contribution.

From the viewpoint of thermoelectrics, diamond is a very inefficient material since it exhibits the highest thermal conductivity of all solids and is an almost perfect electrical insulator. Nevertheless, it is shown that boron doped nanocrystalline diamond is a promising candidate as a thermoelectric material.
Nanocrystalline diamond films were deposited by microwave-plasma CVD from an Ar/H₂/CH₄ plasma with admixtures of trialkylborane. The in-plane electrical transport properties were characterized in a commercial Ulvac ZEM-3 setup using as-deposited films on alumina substrates. p-type semiconducting behavior was observed with Seebeck coefficients greater than 200 µV/K and electrical conductivities in the range of 3×10³ S/m at 800 °C, resulting in power factors higher than 1.3×10^-4 W/m/K². An evaluation of the linear slope of the Seebeck coefficient over the temperature reveals high charge carrier concentrations in the range of 10¹⁹ to 10²⁰ cm^-3 indicating a successful incorporation of the dopant atoms at substitutional lattice sites during the deposition process.
For the characterization of the in-plane thermal diffusivity using a laserflash method (Netzsch LFA 457), the diamond films were deposited on silicon wafers, and the wafers were subsequently etched resulting in self-supporting films. The thermal conductivity is 60 W/m/K at 200 °C, decreasing with increasing temperature. Defensively assuming this value as constant with temperature, the figure of merit zT calculates to 0.002 at 800 °C. Since the microstructure reveals optimization potential with respect to the crystallite size, the thermal conductivity is expected to be further reducible.
An investigation of the thermal stability of the films showed that the electrical transport characteristics do not change up to temperatures of 800 °C. Furthermore, the material exhibits high mechanical stability allowing self-supporting films of only a few micrometers thickness.

Thermoelectric (TE) conversion of waste heat into useful electricity faces a number of challenges. It demands not only optimised thermal and electrical transport properties resulting in a high figure of merit ZT and a high thermal-electric conversion efficiency eta; over a wide temperature range, but also sufficient mechanical integrity to survive continuous heating / cooling cycles. Thermal expansion of the material as well as mechanical properties play an important role; their values should be as similar as possible for p- and n-type alloys to avoid stresses when used in a TE device. In this paper multifilled (Ba, Sr, DD, Yb) Fe/Ni substituted Sb-based p-and n-type skutterudites with ZT>1 and eta;>13% are presented, showing, in contrary to hitherto investigated skutterudites, for the first time practically identical thermal expansion coefficients and elastic moduli. The ZT values of these skutterudites could be further enhanced by more than 20% after severe plastic deformation via high-pressure torsion.

The wide-scale utilization of thermoelectric materials for waste heat-to-power generation will depend on earth-abundant and inexpensive materials with good ZT. MnSi_1.73, or higher manganese silicides (HMS), are intriguing earth-abundant thermoelectric materials with reported ZT ~0.6 at 800 K. However, if HMS is to be used for commercially, the ZT must be increased to values comparable to state-of-the-art PbTe or SiGe, with ZT > 1 at 700 K and 1200 K, respectively. For both PbTe and SiGe, the ZT has been successfully increased through nanostructuring to reduce the lattice thermal conductivity. Our preliminary investigation of HMS nanowires suggest similar reduction in thermal conductivity, but this has yet to be demonstrated on bulk HMS. Herein we report nanostructured HMS composites prepared using a matrix encapsulation method. In this approach, a nanoscale minor phase is effectively trapped by rapidly quenching a molten mixture of the parent (matrix) and minor (encapsulated) phases. We have investigated the solid-state immiscibility between HMS with several metal (Fe, Cu, Ni, and Co) silicides. X-ray diffraction and electron microscopy analysis showed that among the silicides studied, Cu₃Si can form stable pseudo-binary mixtures with HMS. By rapidly quenching the molten mixtures of HMS and Cu₃Si, we observed naturally phase-separated, nanoscale Cu₃Si particles and lamellae embedded throughout the HMS matrix. The size of the Cu₃Si structures may be tuned by adjusting the rate of cooling. We will also report the preliminary thermoelectric properties of these nanostructured HMS composites.

Phonon scattering from nanoscale interfaces is an important physical process for many applications such as thermoelectrics, yet this process is poorly understood. Recent experimental measurements contradict the typical frequency independent phonon grain boundary model and indicate that the interface scattering rate may decrease with the phonon frequency, allowing low frequency modes to travel relatively unimpeded and reducing the thermoelectric figure of merit ZT. In this work, we use variance-reduced Monte Carlo simulations to study the thermal conductivity of doped nanocrystalline silicon. The phonon transmissivity through the disordered grain boundary region is computed using atomistic Green&’s functions, yielding a more accurate result than the acoustic or diffuse mismatch models. Our results show that low-frequency phonons contribute a significant amount to the thermal conductivity, substantially more than predicted by the gray model. We expect that this new knowledge of phonon scattering at interfaces will prove useful to designing nanostructures that can effectively scatter all phonon modes and therefore enhance ZT.

In thermolectricity, SiGe is one of the best materials for high temperature applications. n and p-type SiGe granulated powders have been prepared by mechanical alloying. To determine the best compositions, a study of the doping ratio is performed and then to improve the thermoelectrical properties, nano inclusions have been also homogeneously dispersed in the raw powder. Compacts have been sintered by spark plasma sintering. The thermoelectrical properties (electrical conductivity, Seebeck coefficient and thermal conductivity) have been measured from room temperature to 700°C. The sintered microstructure has been observed by transmission electron microscopy and scanning electron microscopy. They are used to understand the differences between the thermoelectric properties of the various studied compositions of SiGe. Then directions are given to optimize the ZT parameter in the temperature range of interest.

Grain boundaries are known to be able to impede phonon transport in the material. In the thermoelectric application, this phenomenon could help improve the figure-of-merit (ZT) and enhance the thermal to electrical conversion. Bi₂Te₃ based alloys are renowned for their high ZT around room temperature but still need improvements, in both n- and p-type materials, for the resulting power generation devices to be more competitive. To implement high density of grain boundaries into the bulk materials, a bottom-up approach is employed in this work: consolidations of nanocrystalline powders into bulk disks. Nanocrystalline powders are developed by high energy cryogenic mechanical alloying that produces Bi(Sb)Te(Se) alloy powders with grain size in the range of 7 to 14 nm, which is about 25% finer compared to room temperature mechanical alloying. High density of grain boundaries are preserved from the powders to the bulk materials through optimized high pressure hot pressing. The consolidated bulk materials have been characterized by X-ray diffraction and transmission electron microscope for their composition and microstructure. They mainly consist of grains in the scale of 100 nm with some distributions of finer grains in both types of materials. Preliminary transport property measurements show that the thermal conductivity is significantly reduced at and around room temperature: about 0.65 W/m-K for the n-type BiTe(Se) and 0.95 W/m-K for the p-type Bi(Sb)Te, which are attributed to increased phonon scattering provided by the nanostructure and therefore enhanced ZT about 1.5 for the n-type and 1.25 for the p-type are observed. Detailed transport properties, such as the electrical resistivity, Seebeck coefficient and power factor as well as the resulting ZT as a function of temperature will be described.

Gallium telluride (Ga₂Te₃), one of the typical A₂^IIIB₃VI semiconductors, has a crystal structure of zinc blende (ZnS). To keep the stoichiometry of the Ga³⁺ and Te^2- in the zinc blende structure, a large number of periodically assembled two-dimensional vacancy planes spontaneously form, which could greatly scatter the phonons and thus effectively decrease the thermal conductivity (κ). In addition, the Seebeck coefficient of Ga₂Te₃ found from the very few thermoelectric data available can reach a very high level of ~800 mu;VK^-1, which is four times higher than that of the well-studied bismuth telluride (Bi₂Te₃, ~200 mu;VK^-1). However, according to the relevant studies, the electrical conductivity of all kinds of Ga₂Te₃ structures is rather low probably due to the great presence of the 2D vacancy planes, which also leads extremely low values of thermoelectric figure of merit (ZT). Nanostructuring is recognized as one of the most effective strategies for improving the ZT values since not only could the largely created surfaces or interfaces inhibit the thermal conductivity but the Seebeck coefficient could be improved through the induced quantum confinement. In this work, in addition to the above mentioned concepts, we introduce a completely new concept, that is, the nanocomposites for fundamentally resolving the problem of interface resistance. By precipitating the conductive second phase, each separated nano-domains of the main phase can be well linked by point-to-point or face-to-face approaches. By changing the amount of precipitations, we could further control the Seebeck coefficient of the nanocomposites to achieve the goal of ZT enhancements.
In this work, by using the pulsed laser deposition (PLD) technique, the Ga₂Te₃ nanoparticles were deposited on the insulated SiO₂/Si substrates (1.5×1.5 cm²). Through the post annealing, the Te second phase could precipitate between the Ga₂Te₃ nano-domains and thus formed unusual periodically-aligned Ga₂Te₃/Te nanocomposites. By precisely controlling the annealing temperature and time, the morphology of the Te second phase varies from the point-to-point single-crystal bridge (200 °C) to fully (225 °C) or partially (250 °C) filled river-like nanostructures for providing brand-new pathways for carriers. As a result, the carrier mobility is greatly improved and is comparable to the value found from the single crystalline Ga₂Te₃ (~28 cm²V^-1s^-1). Interestingly, the carrier concentration (~3×10¹⁷ cm^-3) also dramatically increases up to 5 orders compared to that of the single crystalline Ga₂Te₃. The present Ga₂Te₃/Te nanocomposites show an extremely high electrical conductivity of 177.7 Sm^-1 at room temperature. The obtained power factor of 19.6 mu;Wm^-1K^-2 is about 87 times higher than that of the best value been reported up to date.

Nanocomposite materials have been identified as promising candidates for high figure-of-merit thermoelectric materials. Due to the increased control of the density of states and hence, the energies of charge carriers, nanocomposite materials are predicted to have a significantly higher thermoelectric power factor (S²σ, where S is the Seebeck coefficient and σ is the electrical conductivity) compared to bulk materials. Nanoscale inclusions of metallic and semi-metallic particles are predicted to enhance the Seebeck coefficient via electron energy filtering. Here, we discuss the influence of embedded metallic In and Bi nanocrystals (NC) on the GaAs thermoelectric properties. In both cases, we fabricate nanocomposites using matrix-seeded growth, which consists of ion irradiation followed by rapid-thermal annealing (RTA). To maximize the retained ion dose, we use a sputter-mask method to preventing sputtering during irradiation.¹ For the case of In+ implantation into GaAs at fluences in the range of 3.8x10¹⁵ to 3.8x10¹⁷ cm^-2, |S| increases with ion fluence, with an enormous Seebeck coefficient of -12 mV/K at 4 K for the highest ion fluence. For the medium fluence film, a high retained In concentration was observed and the nucleation of metallic In NC within a polycrystalline GaAs matrix was observed for post-irradiation RTA at 450°C. Interestingly, the presence of In NC leads to an increase in |S| of 50 mu;V/K at 300 K in comparison to an unimplanted film, as well as an increase in free carrier concentration in comparison to GaAs:In films without In NC.² For the case of Bi+ implantation into GaAs, an amorphous layer containing crystalline remnants is observed. We will discuss the influence of Bi dose and annealing temperature on Bi NC formation and the resulting thermoelectric properties of GaAs:Bi.
1. M.V. Warren, A.W. Wood, J.C. Canniff, F. Naab, C. Uher, and R.S. Goldman, Appl. Phys. Lett. 100, 102101 (2012).
2. M.V. Warren, J.C. Canniff, H. Chi, E. Morag, F. Naab, C. Uher, and R.S. Goldman, “Influence of Embedded Indium Nanocrystals on GaAs Thermoelectric Properties,” submitted for publication.

Thin films of bismuth telluride, Bi2Te3, engineered at the nanoscale exhibit profound thermoelectric properties. Recently, the topological insulating properties of Bi2Te3 have lead to exciting prospects for these materials in the fields of spintronics and quantum computing. As film thickness decreases, properties become dominated by interface characteristics. Thus, the interface structure of these materials can greatly influence both thermoelectric and topologically insulating properties. Moreover, the integration of Bi2Te3 thin-film functionality with electronic devices requires epitaxial growth on typical semiconductor substrates. Understanding the atomic scale structure at the nanoscale thin film / substrate interface is therefore essential.
In this talk, we will address the atomic structure and chemistry of (001) Bi2Te3 thin films grown on (001) GaAs substrates by van der Waals epitaxy. We will demonstrate that aberration corrected STEM enables the determination fine interface details that were inaccessible with previous generation microscopes. Epitaxial growth will be confirmed with plan-view and cross sectional imaging. Using HAADF STEM imaging, we will show that an anomalously bright column of atoms is present throughout the Bi2Te3/GaAs interface. An intuitive HAADF Z2 interpretation leads to the conclusion that these columns are composed of high Z elements, namely bismuth.
We will show that state-of-the-art energy dispersive x-ray spectroscopy (EDS) enables chemical mapping across these interfaces on an atom-by-atom column basis. Using atomic resolution EDS, we will demonstrate that the anomalous atom columns at the interface are composed of Te, forming a quarter unit cell of Ga2Te3. We will discuss the implications of the interfacial layer on controlling van der Waals epitaxy. Further, the local chemistry and structure of interface kink sites will be explored along with twin defects. From this investigation, we will highlight that the combination of atomic resolution EDS with STEM imaging makes possible an unambiguous interpretation of atomic scale structure and chemistry, even for samples containing heavy elements.

To date, studies of electron transport through single molecules tethered between metal electrodes still rely on statistical analyses due to poorly defined molecule-electrode contacts. Acquiring large amount of data with high efficiency is therefore critical to the field. The scanning tunneling microscope break junction (STM-BJ) technique1 has been proven useful in statistically studying the conductance of a large variety of molecules and thus is one of the most commonly used techniques in the field.
Herein, based on the STM-BJ technique, we report a new technique in attempt to quickly acquire thousands of current-voltage (I-V) characteristic curves of repeatedly formed single-molecule junctions.2 Such technique enables statistical studies of more transport properties that are important in single-molecule junctions besides the conductance. First, in addition to the current-distance traces, this method introduces one more dimension to the previous statistics, and allows one to obtain 2-dimensional current-voltage (I-V) and conductance-voltage (G-V) histograms. Furthermore, such statistics also allows for in-depth analyses such as the energy alignment of the frontier molecular orbital with respect to the electrode Fermi level (characterized by single molecule transition voltage spectroscopy3 (SM-TVS)), as well as its correlation with low-bias conductance. Finally, single-molecule thermoelectric effects can also be studied by extracting either the zero-current voltage4 or the zero-bias offset current5 from the I-V data in the presence of a temperature gradient across the STM tip and the substrate. The correlation study with the TVS data shows that the thermopower of single molecule junctions is indeed an alternative measure of the energy gap between the nearest molecular orbital and the electrode Fermi level.6
In short, our new rapid I-V technique makes possible many interesting studies that were difficult in the past and shows great potential in our further research.
(1) Xu, B. Q.; Tao, N. J. Science 2003, 301, 1221.
(2) Guo, S.; Hihath, J.; Díez-Pérez, I.; Tao, N. Journal of the American Chemical Society 2011, 133, 19189.
(3) Beebe, J. M.; Kim, B.; Gadzuk, J. W.; Frisbie, C. D.; Kushmerick, J. G. Phys Rev Lett 2006, 97, 026801.
(4) Reddy, P., ;; Jang, S. Y.; Segalman, R. A.; Majumdar, A. Science 2007, 315, 1568.
(5) Widawsky, J. R.; Darancet, P.; Neaton, J. B.; Venkataraman, L. Nano Letters 2011, 12, 354.
(6) Baheti, K.; Malen, J. A.; Doak, P.; Reddy, P.; Jang, S.-Y.; Tilley, T. D.; Majumdar, A.; Segalman, R. A. Nano Letters 2008, 8, 715.

We describe an apparatus which concurrently and independently measures the parameters determining thermoelectric material conversion efficiency: the Seebeck coefficient, thermal conductivity, and electrical resistivity. The apparatus is designed to characterize thermoelectric materials which are technologically relevant for waste heat energy conversion, and may operate from room temperature to 400 °C. It is configured as a one-dimensional heat pipe of known thermal conductance, and is designed to have negligible heat loss. The Seebeck coefficient and thermal conductivity are obtained in steady-state using a differential technique, while the electrical resistivity is obtained using a four-point lock-in amplification method. Measurements on the newly developed NIST Seebeck Standard Reference Material are presented in the temperature range from 50 C to 250 C. Finally, we present measurements on composite metal-PbTe structures characterized by unusual magneto-transport properties and enhanced thermopower.

Extended crystallographic defects, such as dislocations, grain boundaries, and stacking faults, can strongly affect the thermal and electronic transport properties of thermoelectric materials. At present, however, our understanding of the atomic-scale structural details of such defects in thermoelectrics is in its infancy, even for such widely used materials as bismuth telluride. In this presentation, we discuss our analyses of grain boundary structure in Bi2Te3, contrasting the structure of low-angle boundaries composed of individual dislocations with that of high angle-angle boundaries. Our HAADF-STEM observations of a low-angle <2-1-1 0> tilt boundary help to clarify the structure of the relevant defects in bismuth telluride, including their Burgers vectors, relationship to intergranular misorientation, and dislocation core configurations. We also consider the (0001) basal twin in bismuth telluride and show how defects in interfaces vicinal to this relatively simple and low energy interface can be interpreted in terms of interfacial disconnections (i.e. step defects possessing dislocation content). For instance, our ab initio, density functional theory calculations indicate a strong energetic preference for terminating (0001) twins at the Te(1)-Te(1) sites of the crystal structure, a result that is consistent with our HAADF-STEM observations. This energetic preference also imposes strong constraints on the possible disconnection arrangements. Our observations of more complex high angle boundaries also identify faceted step formation on tellurium-terminated planes, suggesting such analyses can be extended more generally in this and related layered tetradymite-type materials.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Bi₂Se₃ is both a 3D topological insulator (TI) and a thermoelectric material. Thermoelectric properties are traditionally controlled by bulk semiconducting electronic states; meanwhile, in many thermoelectric devices, geometries with large surface area are desirable. Therefore, the interplay between the surface state and the bulk is critical to thermoelectric device performance. Bi₂Se₃&’s TI surface state is gapless and supports exotic quasiparticle carriers, complicating the interaction between the surface and the bulk.
Quasiparticle scattering measurements of Bi₂Se₃ obtained in the past by scanning tunneling microscopy (STM) have revealed surface scattering states, but they were attributed exclusively to the topological surface state.[1] In particular, the Dirac cone is known to warp above the Dirac point, allowing new scattering pathways with different properties.[2] However, nearly identical quasiparticle scattering patterns have also been observed for Bi₂Te₃,[1] which has a very different warping parameter from Bi₂Se₃.[3,4] The similarities in the quasiparticle scattering, and the dissimilarities in the surface state geometries, suggest that additional physics is involved.
We report STM studies of quasiparticle scattering in Bi₂Se₃ and Cu-doped Bi₂Se₃ well above the Dirac point which appear to be consistent with a bulk-mediated scattering process. Our detailed analysis of the energy and k-dependence of quasiparticle scattering patterns at defect sites further supports the notion that other scattering channels are accessible. The direct observation of these scattering states indicates that, while the bulk is gapped, the surface is coupled to the conduction bands through these scattering channels. Consequently, we expect the thermoelectric properties of Bi₂Se₃ to involve a surface state contribution to both the conductivity and the conduction band-valence band interaction.
1. H. Beidenkopf, P. Roushan, J. Seo, L. Gorman, I. Drozdov, Y. S. Hor, R. J. Cava and A. Yazdani, Nat Phys 7 (12), 939-943 (2011).
2. J. Wang, W. Li, P. Cheng, C. Song, T. Zhang, P. Deng, X. Chen, X. Ma, K. He and J. F. Jia, Physical Review B 84 (23), 235447 (2011).
3. Y. Chen, J. Analytis, J. H. Chu, Z. Liu, S. K. Mo, X. L. Qi, H. Zhang, D. Lu, X. Dai and Z. Fang, Science 325 (5937), 178-181 (2009).
4. K. Kuroda, M. Arita, K. Miyamoto, M. Ye, J. Jiang, A. Kimura, E. Krasovskii, E. Chulkov, H. Iwasawa and T. Okuda, Physical Review Letters 105 (7), 76802 (2010).

An outstanding challenge for nanoscale thermoelectrics is to probe thermoelectric energy conversion with the spatial resolution comparable to dopants and inhomogeneities, from sub-50 nm to few nanometer length-scales. There is a growing number of cases where scanning probe microscopy is used to probe thermoelectric response locally, but the experiments are almost exclusively carried out with the tip in mechanical contact to the surface. Though attractive, this methodology may introduce significant sample damage and it almost always has to deal with the unknown thermal resistance in the contact area.
We will present our recent efforts in developing thermoelectric characterization in the non-contact mode, where the tip is separated from the surface by a tunneling gap. The corresponding technique, termed Scannning Thermovoltage Microscopy has a comparatively long history, but also a number of open questions. Chief among them is whether there exists a well-defined relationship between local tunneling observables and macroscopically measured theremoelectric performance. According to early theoretical model by Stovneng-Lipavsky (SL), the net thermovoltage measured in the tunneling gap has a significant and distance dependent component due to tunneling itself. We have verified this prediction directly on both metal and Bi₂Te₃ surfaces in ultrahigh vacuum. Moreover, not only the magnitude but also the sign of tunneling thermovoltage were found to depend on the tip-surface distance, which does indeed require a systematic decoupling of the distance dependence if one is interested in surface-specific properties. The decoupled surface terms, however were almost in complete contradiction to the bulk values, ~ -20 mu;V/K for Ag(111), an order of magnitude larger than for bulk silver (and of opposite sign), and <50 mu;V/K, at least a factor of 4 smaller than the bulk value of 236 mu;V/K for our samples. But, we will show that these terms can, in fact, be explained by the electronic structure of the surfaces, involving the resonant enhancement of thermopower by the surface-state band on silver, and band-bending on the surface of Bi₂Te₃. Moreover, we will demonstrate how to apply Landauer formalism to interpret tunneling measurements systematically and more accurately than using the SL model, which is not valid for strongly energy-dependent density of states. Overall, we believe this technique will find interesting applications in layered, 2D and nanostructured materials where the surface effects will dominate thermoelectric performance.
SJ, PM: Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. MM: Supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.
[1] S. J. Kelly, M. A. McGuire, and P. Maksymovych, submitted (2012).

The unique thermal properties exhibited by free-standing semiconductor thin films (i.e., membranes) has recently triggered considerable amount of research activities in this filed. A precise knowledge on the influence of low dimensionality, detailed chemical composition, degree of crystallinity, and surface roughness results essential to tailor the thermal properties of the membranes. Recently, we have shown that the phase and group velocities of the fundamental flexural mode in ultrathin Si membranes are reduced by one order of magnitude for the smaller membrane thicknesses (8 nm) with respect to their bulk value [1]. Herein, we present a comparative study on the thermal conductivity and heat capacity of Si, Ge, and SiNx membranes in the temperature range from 4 to 300 K and using a variety of optical and electrical characterization techniques such as Raman thermometry and a modified version of 3-omega method as recently introduced by Jain and Goodson [2]. A deeper insight into the reduction of the thermal conductivity in membranes is discussed in terms of a modified dispersion relation experimentally investigated using angle-resolved Brillouin scattering. We comparatively discuss the advantages and disadvantages offered by each technique emphasizing on their limitations and experimental accuracy. In particular, we show the great advantages offered by the 3-omega method to determine the thermal conductivity and heat capacity of this kind of systems for cases where optical techniques result unsuitable. We also discuss the different temperature behaviour and thickness dependence exhibited by the crystalline Si and Ge membranes as compared to the amorphous case of SiNx . Different approaches to tailor the thermal conductivity of the membranes by nano/micro-structuration will also be briefly discussed.

The impact of vertical and lateral confinement of nanoscale structures on their thermal properties and heat conductivity is of fundamental importance for many applications. To study these effects, we use the self-organized formation of well-defined Ge hut and dome clusters on Si(001) as a robust model system.
Due to a lattice mismatch of 4.2%, Ge grows on Si(001) in a Stranski-Krastanov mode. First, a strained wetting layer is formed, followed by the kinetically self-limited epitaxial growth of so-called hut and dome clusters. These islands have a very narrow size distribution, typically with dimensions smaller than the mean free path lambda;phonon > 100 nm of phonons in Ge at 80 K: the huts are 2 nm high and 20 nm wide, while the domes are 6 nm high and 50 nm in diameter. Within very robust growth parameters of 400°C < Tgrowth < 600°C both cluster types are mono-crystalline, free of defects or dislocations. The hut-clusters are composed of four reconstructed {105}-type facets, whereas dome-clusters have a more complex faceting.
The thermal transport properties from the in-situ grown clusters to the substrate were determined by the dynamics of the cooling process upon heating by femtosecond laser pulses. The transient temperature rise was measured by the Debye-Waller effect through ultrafast high energy electron diffraction at 20 keV in a pump-probe setup under ultra high vacuum conditions. Grazing incidence at 3° ensures surface sensitivity. The recorded diffraction patterns exhibit contributions from volume diffraction as well as surface diffraction. We were able to clearly distinguish between the transient response of the two cluster types through a spot profile analysis.
The Ge clusters were excited by 50 fs laser pulses at a wavelength of 800 nm and a fluence of 8 mJ/cm2 at a base temperature of 25 K. Diffraction patterns were taken as a function of the time delay between the pump and probe pulse. Excitation by the laser pulse results in immediate heating by 100 K. Different cooling time constants for both clusters have been determined. Hut-clusters cool in 50 ps while dome-clusters cool three times slower in 150 ps.
With the bulk values for the Ge heat capacity, we estimate a thermal boundary conductance of σTBC = 1500 Wcm-2K-1. This thermal boundary conductance of the Ge clusters is a factor of two to three smaller than predictions from acoustic mismatch model and diffuse mismatch model for a homogeneous Ge heterofilm of an equivalent thickness. Such a strong reduction in heat conductivity clearly demonstrates that size effects are significant for the thermal properties of nanoscale heterosystems.

Organic semiconductors have a wide range of electronic and optoelectronic applications, in part due to the potential advantages of cost, low weight, flexibility, and scalability. However, the low thermoelectric (TE) figure of merit (ZT=S^2 σe/kth, kth: thermal conductivity; σe: electrical conductivity; S: Seebeck coefficient) makes organic compounds much less competitive in thermoelectric applications. In homogeneous materials, kth and σe often scale proportionately, and much of the previous work on thermoelectric properties of organic materials focused on optimizing the trade-off between S and σe. While doping is often used to enhance σe and S of the organics, we find that introducing conductive filler particles into an organic matrix can break the trade-off between kth and σe by virtue of boundary effects, namely, the additional transport resistance arising at the interface between the filler and the matrix. This novel principle for tuning the overall TE behavior of the composite is demonstrated in experiments and in simulations for the composites comprising archetypal organic semiconductor copper phthalocyanine (CuPc) and silver (Ag) nanoparticles. The resistive nature of the thermal boundary between CuPc and Ag [Jin, et al. (2011)] shifts the percolation threshold for thermal transport to higher concentrations (xc-th%) compared to that for electrical transport (xc-e%) for the same samples, resulting in a maximum σe/kth ratio at a certain concentration between xc-e% and xc-th%. This maximum ratio is 3 times larger than that of the pure CuPc, potentially increasing ZT by more than a factor of 10 times for organic TE materials. This dramatically increased performance potential then calls for a composite system with high thermal boundary resistance but low electrical contact resistance, which is more easily achievable, and can allow σe/kth to be maximized in the regime where S is relatively stable.

Nanowires have been shown to be promising thermoelectric materials. The enhanced thermoelectric figure of merit of nanowires is primarily due to the suppressed thermal conductivity that results from phonon boundary scattering. However, the exact picture of phonon transport in nanowires is far from complete. Here we present experimental study probing thermal and thermoelectric transport mechanism in nanostructures with ultra-small diameter. .

Silicon&’s economy of scale makes it an attractive material for waste heat harvesting using nanostructured thermoelectrics. However, enhancing the figure of merit for energy conversion, ZT, in silicon is especially challenging given its high thermal conductivity. Recent research [1] shows that a dramatic reduction in the thermal conductivity of silicon is possible through metal assisted etching of silicon. We discuss recent experiments [2] and theory [3] that aim to elucidate heat and charge transport processes in such electrolessly-etched silicon. In particular, we show that while dramatic reduction in thermal conductivity is indeed possible, it does not necessarily lead to ZT enhancement. We also discuss preliminary investigations into the thermoelectric properties of silicon inverse opals [4].
1. Hochbaum et al., Nature, 451, 163 (2008).
2. J. Feser et al, J. Appl. Phys., In press (2012); M.G. Ghossoub et al., In review (2012); J.S. Sadhu et al., In review (2012); K. Balasundaram et al., Nanotechnology , vol. 23, 305304 (2012).
3. J.S. Sadhu and S. Sinha, Phys. Rev. B, vol. 84, 115450 (2011); M. Seong et al., J. App. Phys., vol. 111, 124319 (2012).
4. J. Ma and S. Sinha, J. Appl. Physics, vol. 112, 073719 (2012); J. Ma et al., In review (2012).

We prepared a thermoelectric generator device from 100 alternating layers of SiO2/SiO2+Ge superlattice films using Magnetron DC/RF Sputtering. Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package were used to determine the stoichiometry of Si and Ge in the grown multilayer films and the thickness of the grown multi-layer films. SEM and EDS have been used to analyze the surface and composition of the thin films. The 5 MeV Si ion bombardments have been performed using the AAMU Pelletron ion beam accelerator, to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and increase the cross plane electrical conductivity to increase the figure of merit. We will be showing our findings.

Keywords: Ion bombardment, thermoelectric properties, multi-nanolayers, figure of merit.
Acknowledgement
Research sponsored by the Center for Irradiation of Materials (CIM), National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, DOD under Nanotechnology Infrastructure Development for Education and Research through the Army Research Office # W911 NF-08-1-0425, and DOD Army Research Office # W911 NF-12-1-0063.

Thermoelectric device interconverts thermal gradient and electricity for power generation or cooling. Traditionally, Bi2Te3 semiconductor has been widely used as thermoelectric material. On the contrary, silicon has been considered as the impropriate material due to high thermal conductivity property. However, recent research revealed the possibility of silicon as thermoelectric material by incorporating nanotechnology. One-dimensional silicon nanowire can dramatically reduce the phonon propagation through the nanowire while maintaining the electron/hole propagation property.
In this work, silicon manufacturing process based top-down approach is adopted to implement the n-/p-leg included silicon thermoelectric device. The 50 nm width n- and p-type silicon nanowires (SiNWs) are manufactured using a conventional photolithography and ion implantation methods on 8 inch silicon wafer. For the evaluation of the Seebeck coefficients of the silicon nanowires, heaters and temperature sensors embedded test pattern is fabricated. Moreover, for the elimination of electrical and thermal contact resistance issues, the SiNWs, heaters and temperature sensors are fabricated monolithically using a CMOS process. For the validation of temperature measurement by an electrical method, scanning thermal microscopy analysis is carried out. The highest Seebeck coefficients are -170 uV/K and 153 uV/K and the highest power factors are 2.77 mW/mK^2 and 0.65 mW/mK^2 for n- and p-type SiNWs, respectively, in the temperature range from 200 to 300 K. The larger power factor value for n-type SiNW is due to the higher electrical conductivity. The total Seebeck coefficient and total power factor for the n-/p-leg included unit device are 158 uV/K and 9.30 mW/mK^2 at 300 K, respectively.

The rapid development of thermoelectric materials in the past decade has brought a new hope to the possibility of directly converting waste heat back to electricity based on the Seebeck effect. However, critical gaps still remain that prevent scalable, practical manufacture and wide deployment of thermoelectric devices. First, most conventional thermoelectric materials, including both bulk crystals and nanostructured materials, are based on tellurides, antimonides, germanium, and rare earth element doped compounds. The bulk crystals or the composite disks fabricated by compressing/sintering nanomaterials are micro-machined into millimeter-thick pillar structures; such processes impose a high demand on these expensive, scarce, and sometimes toxic materials. Further, during manufacture, much material is wasted, causing adverse environmental impact and high recycling costs. Second, the majority of nanostructure-based thermoelectric research is limited to lab-scale device fabrication and measurements performed on a single nanowire or a thin film of nanocrystals or heterostructures with maximum dimensions of hundreds micro-meters; this is mainly due to the lack of scalable and reproducible synthesis. In most cases, only milligrams of these nanomaterials can be obtained and there are large variations between different batches of samples. Third, nearly all-current thermoelectric modules are based on a design by assembling these rigid pillar structures with metal interconnects. Such a design leads to numerous intrinsic problems including: the mismatch of thermal expansion coefficients among thermoelectric materials, interconnects, and heat exchangers (leading to a shorter life cycle), and the restriction to installation on flat surfaces with regular shapes.
In the past three years, we have developed a solution-phase production of ultrathin nanowires and nanowire heterostructures with uniform morphology, size, and properties at industrial scale (kilogram level in relatively short period of time) using industrial standard reactors. We have been able to investigate the bulk thermoelectric properties on the bulk nanocomposite disks fabricated by spark plasma sintering or hot pressing the nanopowders. A 13% improvement in thermoelectric figure of merit (ZT) has been observed in our n-type Bi2Te3 nanowires compared to the best commercially-available n-type Bi2Te3-xSex and a 40% improvement has been observed in our PbTe-Ag2Te-PbTe nanowire heterostructure system compared to the bulk crystals. These improvement are mainly due to the quantum confinement due to the finite size as well as the energy filtering.

Our nation discards more than 50% of the total input energy as waste heat in various industrial processes such as metal refining, heat engines, and cooling. If we could harness a small fraction of the waste heat through the use of thermoelectric (TE) devices while satisfying the economic demands of cost versus performance, then TE power generation could bring substantial positive impacts to our society in the forms of reduced carbon emissions and additional energy. To increase the unit-less figure of merit, ZT, single-crystal semiconductor nanowires have been extensively studied as a building block for advanced TE devices because of their predicted large reduction in thermal conductivity and large increase in power factor. In contrast, polycrystalline bulk semiconductors also indicate their potential in improving overall efficiency of thermal-to-electric conversion despite their large number of grain boundaries. To further our goal of developing practical and economical TE devices, we designed a material platform that combines nanowires and polycrystalline semiconductors and have it integrated directly on a metallic surface. We will assess the potential of polycrystalline group III-V compound semiconductor nanowires directly grown on low-cost copper sheets that have ideal electrical/thermal properties for TE devices. We chose indium phosphide (InP) from group III-V compound semiconductors because of its inherent characteristics of having low surface states density in comparison to others, which is expected to be important for polycrystalline nanowires that contain numerous grain boundaries. Using metal organic chemical vapor deposition (MOCVD) polycrystalline InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. We studied the growth mechanisms of polycrystalline InP nanowires on copper surfaces by analyzing their chemical, optical, and structural properties in comparison to those of single-crystal InP nanowires formed on single-crystal surfaces. We also assessed the potential of polycrystalline InP nanowires on copper surfaces as a TE material by modeling based on finite-element analysis to obtain physical insights of three-dimensional networks made of polycrystalline InP nanowires. Our discussion will focus on the synthesis of polycrystalline InP nanowires on copper surfaces and structural properties of the nanowires analyzed by transmission electron microscopy that provides insight into possible nucleation mechanisms, growth mechanisms, and nature of grain boundaries of the nanowires.

A series of recent experiments on thermoelectric in low-dimensional systems will be reported. These will include: (i) the observation of a transverse thermovoltage (perpendicular to the applied thermal gradient) in ballistic, asymmetric structures defined in a two-dimensional electron gas; (ii) experimental studies of the magnetic-field symmetry of combined thermal and electric transport in multi-terminal devices, allowing us to test fundamental, quantum mechanical symmetry relations and their breakdown in the transition to classical transport; (iii) Finally, I will report on the observation of a power-factor enhancement in nanowires due to density-of-states effects. From studies as a function of Fermi energy, using a gate voltage, we can conclude that the enhancement is not due to 1D effects, but tentatively due to quantum-dot like states that form along the nanowire.

Nanoengineered thermoelectric (TE) materials have received a great attention because of the potential improvements in the thermoelectric figure of merit (ZT), due to the classical and quantum mechanical size effects on electrons and phonons that provide additional mechanisms to enhance TE properties. The achievement of thermoelectric ZT of ~2-3 in painstakingly grown two-dimensional (2-D) nanostructures has been experimentally proved while a ZT exceeding 5 was theoretically predicted in 1-D nanostructures. In the design of TE materials, nanotubes offer an additional degree of freedom compared to other 1-D nanostructures because the wall-thickness can be controlled in addition to length and diameter. Changes in wall-thickness are expected to strongly alter the electrical and phonon transport properties and thereby enhance the overall TE properties.
In this work, we demonstrated high scalability and cost-effective nanofabrication to synthesize ultra-long hollow PbSe nanofibers by combining electrospinning (ES) and a galvanic displacement reaction (GDR). Control over the diameter, wall-thickness, morphology, composition and crystallinity of the nanofibers was achieved by tuning the shape and dimension of the sacrificial material as well as GDR conditions. Electrical and thermoelectric properties of the nanofibers were correlated to dimension, crystallinity and composition.

Semiconducting nanowires (NWs) are promising materials for thermoelectric devices as they can achieve a much higher figure of merit ZT than the respective bulk materials. Here, we report on the thermoelectric properties of indium arsenide (InAs) NWs. InAs is a low band-gap semiconductor with high electron mobility. The ZT value of bulk InAs single crystals is typically below 0.1. However, for InAs NW geometries, ZT values well above unity are predicted.
In this study, we investigate in-situ doped single-crystalline InAs NWs grown from vapor phase epitaxy with diameters between 50nm and 150nm. As the control of the electrical mobility is a well-known challenge for III-V nanowires, special attention was paid to surface passivation. Various approaches were tested upon their influence on the charge transport.
To analyze thermoelectric properties, test structures were fabricated using NWs transferred to SiO ₂ or polyimide substrates. Platinum and nickel leads comprising resistive heaters were structured by electron beam lithography and lift-off technique.
Both, Seebeck coefficient and electrical resistance were measured for various doping levels of the NWs. The electrical resistivity of the InAs NW could be varied by sulfur doping from 0.6 mOmega;cm to 30 mOmega;cm. Over a large doping range, the Seebeck coefficient ranging from 10 to 180 mu;V/K correlated well with the conductivity. This was used to determine the charge carrier concentration and mobility. The thermal conductivity was determined using the self heating technique. This method is experimentally much simpler to realize than the typically employed direct method using MEMS-based heater structures. Accuracy and sensitivity of both methods are discussed and compared. For the InAs NWs, a thermal conductivity as low as 1.8W/(Km) was obtained which is about 20-30 times smaller than in bulk InAs.
[1] S. Karg, et al., J. Electron Mat. (2012) in press

The thermoelectric power of carbon nanotubes has been under investigation since as early as the discovery of the nanotube itself. Typically, the observed temperature dependent thermoelectric power behavior exhibits p-type conduction with a positive, but decreasing, slope with temperature. It was also determined that this p-type conduction is due to oxygen doping of the nanotubes during synthesis and subsequent exposure to air, and could be reversed by subjecting the nanotbues to vacuum, resulting in n-type thermoelectric power. In this study, we investigate the thermoelectric power of carbon nanotubes (and nanofibers) grown at varying chemical vapor deposition temperatures. We demonstrate that above a growth temperature of 750 C, the ambient atmosphere thermoelectric power switches from p-type to n-type without any intentional doping or removal of oxygen. We attribute this result to a change in the growth product from high quality small diameter (~50 nm) multiwalled nanotubes to large diameter (~200 nm) amorphous nanotubes, or more precisely, nanofibers above 750 C. These nanofibers are resistant to oxygen doping due to their composition, resulting in their intrinsic n-type thermoelectric behavior. Further, we show that these nanofibers can become p-type by probe sonication where longer sonication periods result in larger magnitude p-type behavior. This is due to the increased surface area available for oxygen doping when the nanofibers become shortened during sonication. The thermoelectric power can be altered from -4 uV/K for the as synthesized fibers to 12 uV/K after 12 hours of probe sonication. This technique could potentially be used to tailor the thermoelectric power of carbon nanofiber based composites to fit specific output requirements.

In last two decades, a great effort has been made to enhance the range of high-performance thermoelectric materials for industrial applications [1,2]. Thermoelectric devices can convert heat into electricity with no moving parts. The key ideas for improving the efficiency of these devices are connected with the enhancement of the ZT= ( S2σTau;/ Κ) (σ- electrical conductivity, -thermopower or Seebeck coefficient) and reduction in the thermal conductivity where is the contribution of the free charge carriers and KL is the lattice thermal conductivity (LTC). The latter describes phenomenologically the ability of a crystal lattice to conduct heat.
Recent theoretical and experimental results show that the LTCs of thin nanocomposite films can be reduced by orders of magnitude with respect to bulk material values. At present, semiconductor based quantum well superlattice structures and nanowires exhibit highest power factor and lowest LTC which leads to the significantly higher thermoelectric figure of merit ΖTau; than that of the bulk materials. However, in technology of thermoelectric materials for general applications, the devices based on the bulk materials are more preferable [2]. A physical model for the reduction in lattice thermal conductivity KL based on the diffuse scattering of phonons at the pore-medium interfaces is developed in case of inhomogeneous porosity-graded thermoelectrics. In “gray-medium” approximation, a theoretically sound expression is derived for the KL , σ and S and the variation of that as a function of porosity, pore sizes and the number of pore groups with different sizes is studied leading to giant ZT values. Examples will be presented confirming our model.
References
] M.S. Dresselhaus, G. Chen, M.Y. Tang, R.G. Yang, H. Lee, D.Z. Wang, Z.F. Ren,
J. P. Fleurial, and P. Gogna, Advanced Materials 19, 1043-1052 (2007).
[2] J.F. Li, W.S. Liu, L.D. Zhao and M. Zhou, .Nature Asia Mater. 2, 152-158 (2010).

Bi2Te3 is a promising compound for application as a low temperature thermoelectric material. One approach to improving the figure of merit, ZT, is to create a nanostructured device that will act to scatter phonons, while leaving a coherent path for electron conduction. In this work theoretical methods are used to investigate the thermal and electrical conductivities Bi2Te3, which has been modified to contain nanostructures to enhance phonon scattering.
Ab initio, first-principles methods are used to determine the atomic and electronic structure of Bi2Te3 that is delta-doped with Se. The electrical conductivity, Seebeck, and Hall coefficients are approximated using the semi-classical Boltzmann theory. The integration across the Brillouin zone is performed using the BoltzTraP algorithm. The effect of doping on the phonon dispersion is investigated. Using continuum mechanics the scattering of elastic waves in Bi2Te3 due to embedded crystallites is investigated. The impact of crystallite morphology on thermal transport is studied and in particular asymmetric structures are examined for possible application as a thermal rectifier.

Transport processes contributing to the thermoelectric energy conversion involve both electrons as well as phonons, and their intricate interactions. The challenging part in computing the transport coefficients that characterize the efficiency of the conversion process is the determination of the energy-dependent relaxation time functions that govern the fundamental characteristics of energy transmission. Nonetheless, due to the complexity of dealing with energy dependence of the relaxation time functions, often, a constant relaxation time approximation is used in theoretical or computational studies. In this work, using a combination of first-principles calculations and Boltzmann transport formalism, the role of energy dependence of the transmission function is examined for a range of materials systems - crystalline, polycrystalline, alloys and hetero-structures. The talk will discuss how energy dependence affects transport properties of a few materials that belong to these material systems.

Engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices, since the energy conversion performance of thermoelectrics is characterized by the dimensionless coefficient: ZT=S^2σT/κ. Due to the strong interdependence of the key physical parameters involved in electronic and phononic transport, optimization of one parameter adversely affects another, making maximizing ZT very challenging. Nevertheless, many improvements in thermoelectric performance have been demonstrated in bulk materials with embedded nanostructures [Poudel, et al., Science 320, 634 (2008)] or multilayer thin-film geometries [Caylor, et al., Appl. Phys. Lett. 87, 023105 (2005)]. Along the pathway of employing individual nanostructures, semiconductor nanowires (NWs) receive exceptional attention, due to their inherent one-dimensionality resulting in low thermal conductivity and simultaneous high carrier mobility, leading to a behavior that is drastically different from that of their corresponding bulk material. Recent progress in this direction, either by experimental fabrication and measurements or through theoretical modeling, includes investigations of smooth and rough Si nanowires [Hochbaum, et al., Nature 451, 163 (2008)], Si/Ge core-shell nanowires [Hu, et al., Nano Lett. (DOI: 10.1021/nl301971k); Nano Lett. 11, 618 (2011); Phys. Rev. B 84, 085442 (2011); J. Heat Transfer 134, 102402 (2012)], amorphous layers on the surface of Si nanowires [Donadio, et al., Phys. Rev. Lett. 102, 195901 (2009)], and Si nanotubes by conceptually drilling a small hole at the center of Si nanowires [Chen, et al., Nano Lett. 10, 3978 (2010); Wingert, et al., Nano Lett. 11, 5507 (2011)].
By employing non-equilibrium molecular dynamics (NEMD) simulations, we proposed three schemes and mechanisms responsible for the reduction in thermal conductivity of thermoelectric nanowires. The approaches include: 1) interfacial interference by covering the nanowire surface with other coatings, 2) incorporating nano-particles onto nanowire surface or inclusions into nanowires, and 3) modulating nanowire with superlattice structures. Compared to pristine nanowires, our simulation results show that the schemes proposed above can lead to nanocomposite structures with considerably lower thermal conductivity (up to 92% reduction), implying one order of magnitude enhancement in the ZT coefficient. Our calculations also revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies non-monotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for the non-monotonic behavior.

The transport properties across atomic junctions under the influence of time-dependent external perturbations are calculated using the Landauer formalism and the non-equilibrium Green function technique. It is found that the response of these systems is affected by the temporal phase coherence effects. Our results suggest that controlling the characteristics of these external perturbations may lead to an enhancement of the thermopower. The effects of other dissipative phase breaking processes on the overall thermoelectric performance are also explored.

In this work, we apply first-principles calculations to investigate both electron and phonon transport in silicon. Electron-phonon scattering rates are computed from first principles calculations to obtain electron relaxation times due to phonon scattering. The energy dependent relaxation times are then used in Boltzmann transport theory to obtain the electrical conductivity, Seebeck coefficient and electronic thermal conductivity. The anharmonic force constants are derived from first-principles calculations and used to make classical potentials that are of ab initio accuracy. Molecular dynamics simulations are then done to obtain the lattice thermal conductivity. In combination, we predict the full thermoelectric properties of silicon, including thermoelectric figure of merit ZT, without any fitting parameter and a priori experimental knowledge over a broad range of temperatures and doping levels.
This material is based on work supported as part of the Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001299/DE-FG02-09ER46577

Enhancement in the efficiency of thermoelectric materials has been a challenging issue of great scientific and technological concern for realizing thermoelectric heat-to-electrical-energy conversion system such as high-efficient power generation and eco-friendly refrigerating systems. In particular, bismuth (Bi)-based nanostructured thermoelectric materials have been extensively researched for enhancement of thermoelectric figure of merit (ZT), revealing the lattice thermal conductivity is dramatically decreased due to the scattering of long and mid-wavelength phonon employing the high density of the nano-sized grain boundaries without affecting the carrier transport significantly. Additionally, nano-grain approach could be effective for improving the power factor (S²σ) due to modified carrier transport phenomena such as carrier filtering effect by nano-interfaces, resulting there are additional room for further ZT improvement in nanostructured thermoelectric materials.
In this report, surfactant-free nanoplates of Bi₂Te₃ and Bi₂Se₃ have been synthesized using bottom-up synthesis in a fast and scalable amount, which have an advantage of the precise control in composition of nanocomposites. Through the micro-structural study, we found that the nanocomposite of Bi₂Te₃ and Bi₂Se₃ have an alternating nano-grains, which are significantly effective for scattering of long- and mid- wavelength phonons or low energy charge carriers. The maximum ZT of Bi₂Te₃ and Bi₂Se₃ nanocomposites was achieved to be ~ 0.7 at 400 K in the 10 ~ 15% Bi₂Se₃ composition, suggesting nanoscale heterostructure of the surfactant-free nanoplates can be effective for enhancement of thermoelectric efficiency.

The conversion of thermal energy into electricity by thermoelectric effect has been center of attraction for science and technology community. The efficiency of thermoelectric materials can be described by the figure of merit ZT = S²σT/κ, where S, σ, T, and κ are, respectively, the Seebeck coefficient, the electrical conductivity, the absolute temperature, and thermal conductivity. The thermal conductivity defines the contributions of both electrons (κ_e) and phonons (κ_p). The value of ZT can be increased by decreasing thermal conductivity and improving power factor PF (S²σ). In particular, nanodoping is expected to enhance phonon boundary scattering, lowering κ_p.
In this contribution, we introduce preliminary results for 2% Al-doped ZnO (AZO) thin films for thermoelectric application. We fabricated the films by means of Pulsed Laser Deposition. Dense AZO target was irradiated by Nd:YAG laser (266 nm, 10 Hz) which energy density is about 4.2 J/cm² for deposition period of 30 min. Thin films were deposited on SrTiO₃ (STO 100) substrates at 400 °C, 500 °C and 600 °C keeping an oxygen pressure of 27 Pa. We measured structural, electrical and thermal properties of as grown thin films. From the out-of plane (rocking curve) and in-plane X-ray diffraction, we found that the AZO films at 600 °C have better crystallinity than the other two. The value of electrical conductivity of thin film deposited at 400 °C was 100 S/cm at 300 K, higher than for thin films deposited at 500 °C and 600 °C. The absolute value of S for AZO thin films was in the range of 90 mu;V/K to 244 mu;V/K and the value of PF was in range of 0.06 ~ 0.15 × 10^-3 W/m.K². In the whole temperature range (300 K ~ 600 K), we have observed that the film deposited at 500 °C shows the highest value of power factor. Overall, the best performance was observed in thin films deposited at 500 °C with S = -151 µV/K, PF =0.15 × 10 W/m.K², and ZT = 0.2 at 300 K, showing big enhancement in comparison with bulk material of the same composition. Detailed results and further analysis will be reported at the conference.

Electrodeposition of thermoelectric materials, including binary and ternary compounds, have been attracting much attention because of its many advantages, including low-cost, rapid deposition rate, and ease in controlling their microstructure and crystallinity by adjusting electrodeposition parameters. Although many works demonstrated the feasibility of thin film synthesis, electrodeposited films showed the poor thermoelectric properties compared to the bulk materials due to poor morphology and porous structures. Many approaches were studied to increase the thermoelectric properties of electrodeposits. Among them, the use of a surfactant was utilized to obtain a smooth and dense structure. However, the Seebeck coefficient of electrodeposited Bi2Te3 thin films degraded severely despite showing very smooth surfaces with dense structures. In this work, we investigated the effect of a surfactant on the electrodeposition of Bi2Te3 and Sb2Te3 films, including their morphology and their structural, electrical, and thermoelectric properties. By adding cationic surfactant cetyltrimethylammonium bromide (CTAB), electrodeposition of the films showed improved the morphology and density. Annealing also played an important role in controlling multiple phases and crystallinity that are all critical to TE performance. A 50% enhancement of thermoelectric properties was achieved with the help of annealing due to the formation of Te nanodots. This result will be discussed based on a theoretical model of the carrier energy filtering effect.

Semiconducting polymers offer potential for thermoelectric applications because they typically exhibit low thermal conductivities (~0.1 W/m-K), yet with proper processing and extrinsic doping can have electrical conductivities as high as 10^4 S/cm. Furthermore, the mechanical flexibility and solution processability of polymers could enable conformal coatings of thermoelectric energy harvesting or cooling devices. Most conjugated polymers are dominantly hole-conducting and most work on the thermoelectric properties of polymers has previously focused on hole conduction. However, if all-organic thermoelectrics are to be realized, an electron-conducting leg is necessary. Here, we present a study of the thermoelectric properties of a high performance, n-type semiconducting polymer, N,N&’-dialkylnapthalenedicarboximide-dithiophene, P(NDI2OD-T2), that has a high electron mobility ~ 0.1 cm^2/V-s in thin film transistors. It has been difficult previously to stably dope n-type organic semiconductors to high carrier concentrations, so we have developed a series of novel dihydro-1H-benzoimidazol-2-yl derivatives that act as electron donors. We have used these materials to extrinsically n-dope P(NDI2OD-T2), enabling us to controllably modify the carrier density and study the Seebeck coefficient as a function of carrier density, obtaining values as high as 2 mV/K. Importantly, we have also studied the impact of processing methods that can tune the carrier mobility, and thus the electronic density of states, of the neat polymer and will report the impact on the Seebeck coefficient. We conclude that improvements in carrier mobility offer a feasible route to engineering polymers for thermoelectric applications.

Thermoelectric (TE) materials have attracted much attention in solid-cooling and power generation. Recently, the TE efficiency has been greatly improved, however, these conventional inorganic TE materials are often expensive, heavy, toxic and brittle. It is necessary to develop environmentally friendly and inexpensive next generation thermoelectric materials. Strong potential candidates are polymer-based thermoelectric materials. Typical polymers intrinsically have low thermal conductivity, which offers a great opportunity to have high TE figure-of-merit (ZT), compared to conventional inorganic thermoelectric materials. When their electrical properties (electrical conductivity and thermopower) are significantly improved, a large ZT can be obtained. In our work, a large improvement in the electrical properties (so-called power factor) has been achieved by using an ex-situ approach during the synthesis of composites containing polyaniline (PANI) and double wall carbon nanotubes (DWCNTs). More importantly, electrical conductivity and thermopower were simultaneously increased due to the high carrier mobility of DWCNTs and intertube energy barriers. A high power factor, up to ~220 µW/m-K2, has been observed, which is at least 10 times higher than that of the reported PANI composites.

Organic semiconductors (OSCs) offer numerous advantages over inorganic semiconductors (ISCs) such as low cost, mechanical flexibility, large-area deposition and low weight. While OSCs have low thermal conductivity (κ), which is important for high ZT, they traditionally suffer from low thermoelectric power factor (S²σ). As for any thermoelectric material, optimizing doping is critical to maximizing ZT in OSCs,¹ yet the thermoelectric properties of OSCs often do not obey the same relationships or tradeoffs with doping as they do in ISCs (for example, OSCs do not typically obey the Wiedemann-Franz law, as the correlation between κ and σ is weak). Dopants not only determine free carrier concentration (and hence S) in OSCs but also often affect host material morphology and hence mobility to a much greater extent than in ISCs. Furthermore, the weak van der Waals bonded structure of OSCs often leads to a very low dopant ionization fraction.²
While poly(3,4-ethylenedioxythiophene) (PEDOT) doped by poly(styrenesulfonate) (PSS) is promising as an organic-based thermoelectric material due to its stability in air and potential for very high σ, optimization of carrier concentration for high S²σ has not previously been accomplished in PEDOT:PSS, since control of PSS concentration has proven challenging due to its large molecular weight (long chain length). Both this and its small PSS ionization fraction have resulted in low S²σ in previous PEDOT:PSS samples. Recently, an ethylene glycol (EG) treatment strategy was developed³ to remove PSS from PEDOT:PSS selectively, resulting in a doubling of σ. However, the extent to which free carrier concentration (or S) is changed by this method has been unclear, since reducing free carrier concentration by removing ionized dopant species is expected to reduce σ (in the absence of other effects).
Here we study the impact of dedoping by EG treatment on the thermoelectric properties of PEDOT:PSS. We find that this dedoping method leads to simultaneous increases in both S and σ, indicating both a large reduction in free carrier concentration and a large increase in mobility. Furthermore, we show that PSS dedoping leads to a decrease in thermal conductivity. Therefore, we find that all three parameters (S, σ, and κ) change in a manner to increase ZT as PSS is dedoped, a phenomenon unknown in ISCs, where tradeoffs between the parameters exist. By optimizing PSS dedoping through this method and making careful property measurements, we derive a room-temperature ZT value of 0.42, the highest value yet found for an organic semiconductor and approaching values of conventional ISC thermoelectric materials such as bismuth telluride.
[1] O. Bubnova, Z. U. Khan, A. Malti, S. Braun, M. Fahlman, M. Berggren, and X. Crispin, Nat. Mater.10, 429 (2011).
[2] B. A. Gregg, S. Chen, and R. A. Cormier, Chem. Mater.16, 4586 (2004).
[3] Y. Kim, C. Sachse, M. L. Machala, C. May, L. Müller-Meskamp, and K. Leo, Adv. Funct. Mater.21, 1076 (2011).