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.