Single-walled carbon nanotubes (SWCNTs) are promising for use in solar cells and photodetectors because of their strong optical absorption in most of the solar spectrum. There have been many reports about the photovoltaic effect in nanoelectronic devices based on individual SWCNTs, but they have been limited by complicated fabrication and miniscule absorption. There has been a growing trend for merging SWCNTs into micro-and macroscopic devices to provide more practical applications. Here we report the photoresponse of macroscopic SWCNT films with a p-n junction at room temperature. Photovoltage (PV) and photocurrent (PC) due to the photothermoelectric (PTE) effect were observed at the junction, and they were larger by one order of magnitude as compared with their values at the metal-SWCNT interfaces. Various factors affecting PV amplitude and response time have been studied, including junction length, substrate, and doping level. The maximal responsivity we observed was 1V/W with samples on Teflon tape, while a fast response time 80 S was observed with samples on AlN substrates. Hence an optimal combination of photoresponse time and amplitude can be found by proper choice of substrate. It was found that PV increased nonlinearly with increase in n-doping concentration, indicating the existence of an optimal doping level. This result also suggests the possibility to further improve photoresponse by changing p-doping level. Finally, we checked the photoresponse in wide wavelength range (360-900 nm), and PV was observed throughout, indicating that the device could potentially be used as a broadband photodetector.

The main focus of this work is on exploratory investigation of thermoelectric (TE) materials. Thermoelectric devices are solid-state devices that convert thermal energy from a temperature gradient into electrical energy (Seebeck effect), or convert electrical energy into a temperature gradient (Peltier effect). Modifying existing materials and finding new materials with proper thermoelectric properties are the two approaches considered in this research. Good thermoelectric materials are usually narrow band gap semiconductors with large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. Early transition metal antimonides and arsenides, with unique structural features were chosen for finding high performance TE materials. During the investigation of group four antimonides, a series of new ternaries, ZrSiδSb2-δ, ZrGeδSb2-δ and HfGeδSb2-δ was developed. Single crystal X-ray diffraction was used for crystal structure determination, and energy depressives X-ray analysis (EDX) was used for compositional analysis. Metallic properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. It was revealed that Mo3Sb7 turns semiconducting by partial Sb/Te exchange. Similarly, isostructural Re3As7 was modified to become semiconducting by partial Ge/As exchange. Crystal structures were determined by single crystal X-ray and powder X-ray diffraction utilizing Rietveld method. Electronic structures were determined by using the LMTO method and confirmed the semiconducting properties of these ternary compounds. Physical property measurements showed exceptional TE properties for these compounds. It was also confirmed by the X-ray single crystal analysis that it is possible to intercalate different cations with the proper size into the existing cubic voids of the structure. The effect of cation intercalation on physical properties of these compounds were investigated and revealed the enhancement of transport properties as a result of this intercalation.

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.

Bismuth telluride based compounds is known to be the best thermoelectric materials within the low temperature regime. In this study, the P-type Bi0.5Sb1.5Te3 thermoelectric alloy was synthesized by ceramic processing method. The Bi0.5Sb1.5Te3 thermoelectric materials were prepared via the ball milling, cold pressing, and sintering processes. The effects of sintering time and temperature on the microstructures and thermoelectric properties were investigated and discussed. The X-ray diffraction patterns of Bi0.5Sb1.5Te3 reveal that the compounds have the oxides after the sintering processes and the heat treatment process causes grain growth by the increased sintering temperature and time. The results of thermoelectric properties show that the optimal Seebeck coefficient 300(£gV/K) was obtained as the sample was sintered at 350¢XC for 3h and the resistivity will reach the maximum. The figure of merit of 0.15 was obtained at room temperature as the sample was sintered at 375¢XC for 3h.

This paper presents an integrated column-type micro thermoelectric cooler (£g-TEC) constructed with serial connected p-type antimony-tellurium (Sb2Te3) and n-type bismuth-tellurium (Bi2Te3) micro pillars deposited by electrochemical deposited technology. To optimize the power factor, density and uniformity of the TE films and to enhance the reproducibility of £g-TEC device, a cathode with tunable rotary speed and with accurate current controller has been designed in the electroplating system of this research. The electroplating deposited Bi2Te3 and Sb2Te3 with an average thickness of 8 £gm, are connected using Cr/Au layers at the hot junctions and cold junctions. The measured Seebeck coefficient and electrical resistivity are -86 £gV/K and 16 £g£[-m for Bi2Te3 films after annealed at 250¢XC, and are 68 £gV/K and 30 £g£[-m for Sb2Te3 films after annealed at 200¢XC. The optimized power factors of the n-type (2.64¡Ñ10-4 W/K2m) and p-type (2.64¡Ñ10-4 W/K2m) telluride compounds have been demonstrated in this paper. Under 5 volts driven, the integrated £g-TEC device shows average cooling achieved is about 1.3 ¢XC.

Thesis (M.S.)--West Virginia University, 2002. / Title from document title page. Document formatted into pages; contains vii, 66 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 29).

An analytical transport/reaction model was developed to simulate the catalytic performance of ZnO nanowires as a catalyst support. ZnO nanowires were chosen because they have easily characterized, controllable features and a spatially uniform morphology. The analytical model couples convection in the catalyst flow channel with reaction and diffusion in the porous substrate material; it was developed to show that a simple analytical model with physics-based mass transport and empirical kinetics can be used to capture the essential physics involved in catalytic conversion of hydrocarbons. The model was effective at predicting species conversion efficiency over a range of temperature and flow rate. The model clarifies the relationship between advection, bulk diffusion, pore diffusion, and kinetics. The model was used to optimize the geometry of the experimental catalyst for which it predicted that maximum species conversion density for fixed catalyst surface occurred at a channel height of 520 [mu]m. A modeling study of thermoelectric (TE) vehicle waste heat recovery was conducted based on abundant and inexpensive Mg₂ Si[subscript 0.5] Sn[subscript 0.5] and MnSi[subscript 1.75] TE materials with consideration of performance at the system and TE device levels. The modeling study identified a critical TE design space of fill fraction, leg length, n-/p-type leg area ratio, and current; these parameters needed to be optimized simultaneously for positive TE power output. The TE power output was sensitive to this design space, and the optimal design point was sensitive to engine operating conditions. The maximum net TE power for a 29.5 L strip fin heat exchanger with an 800 K exhaust flow at 7.9 kg/min was 2.25 kW. This work also includes two generations of TE waste heat recovery systems that were built and tested in the exhaust system of a Cummins 6.7 L turbo Diesel engine. The first generation was a small scale heat exchanger intended for concept validation, and the second generation was a full scale heat exchanger that used the entire exhaust flow at high speed and torque. The second generation heat exchanger showed that the model could accurately predict heat transfer, and the maximum experimental heat transfer rate was 15.3 kW for exhaust flow at 7.0 kg/min and 740 K. / text

The objective of this work is to develop methods to investigate the thermoelectric (TE) transport in semiconducting nanowires (NWs). The thermal conductivity of degenerately doped electrochemically-etched (EE) silicon NWs was measured to be lower than silicon NWs synthesized by a vapor-liquid-solid (VLS) method without showing a clear dependence on the NW diameter. The thermoelectric figure of merit (ZT) at near room temperature obtained from the three measured TE properties on the same EE Si NW was found to be between 0.01 of a very rough NW and 0.08 of a relatively smooth NW, the latter of which is about four times higher than that reported for bulk p-type Si at the optimum doping concentration. In addition, the NW samples could be contaminated or oxidized during the device processing. Based on the TEM characterization, they have relatively thick oxide layer and small surface roughness, and are apparently different from the EE Si NWs that a Berkeley team reported. Typical rough NWs reported by the Berkeley team have thin oxide layer and are free of major structural defects. Hence, given the significant structural differences in the samples, it would be scientifically inappropriate to compare the transport properties obtained from the two studies. In addition, a five to ten fold reduction in thermal conductivity was observed in wurtzite InAs NWs compared to bulk InAs of zinc blend phase, and is mainly attributed to diffuse surface scattering of phonons. Moreover, InSb NWs have been synthesized at three different base pressures. The NWs were found to be zinc-blende structure with <110> growth direction. The ZT of the two NWs is about 10 times lower than the bulk values mainly because of the much higher doping levels in NWs than the bulk as well as mobility suppression in the NWs. The ZT of one NW grown at a high vacuum base pressure is higher than another NW grown at low vacuum. These results show that it is necessary to better control the impurity doping in order to increase the ZT of the InSb NWs. / text

This dissertation presents the development of methods based on microfabricated devices for combined structure and thermoelectric characterizations of individual nanowire and thin film materials. These nanostructured materials are being investigated for improving the thermoelectric figure of merit defined as ZT=S²[sigma]T/K, where S is the Seebeck coefficient, [sigma] is the electrical conductivity, K is the thermal conductivity, and T is the absolute temperature. The objective of the work presented in this dissertation is to address the challenges in the measurements of all the three intrinsic thermoelectric properties on the same individual nanowire sample or along the in plane direction of a thin film, and in correlating the measured properties with the crystal structure of the same nanowire or thin film sample. This objective is accomplished by the development of a four-probe thermoelectric measurement procedure based on a micro-device to measure the intrinsic K, [sigma], and S of the same nanowire or thin film and eliminate the contact thermal and electrical resistances from the measured properties. Additionally the device has an etched through hole that facilitates the structural characterization of the sample using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). This measurement method is employed to characterize individual electrodeposited Bi[subscript 1-x]Te[subscript x] nanowires. A method based on annealing the nanowire sample in a forming gas is demonstrated for making electrical contact between the nanowire and the underlying electrodes. The measurement results show that the thermoelectric propertied of the nanowires are sensitive to the crystal quality and impurity doping concentration. The highest ZT found in three nanowires is about 0.3, which is still lower than that of bulk single crystals at the optimum carrier concentration. The lower ZT found in the nanowires is attributed to the high impurity or carrier concentration and defects in the nanowires. The micro-device is further modified to extend its use to characterization of the in-plane thermoelectric properties of thin films. Existing practice for thermoelectric characterization of thin films is obtaining K in the cross plane direction using techniques such as the 3[omega] method or time domain laser thermal reflectance technique whereas the [sigma] and S are usually obtained in the in-plane direction. However, transport properties of nanostructured thin films can be highly anisotropic, making this combination of measurements along different directions unsuitable for obtaining the actual ZT value. Here, the micro-device is used to measure all three thermoelectric properties in the in-plane direction, thus obtaining the in-plane ZT. A procedure based on a nano-manipulator is developed to assemble etched thin film segments on the micro-device. Measurement results of two different types of thin films are presented in this dissertation. The first type is mis-oriented, layered thin films grown by the Modulated Elemental Reactant Technique (MERT). Three different structures of such thin films are characterized, namely WSe₂, W[subscript x](WSe₂)[subscript y] and (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] superlattice films. All three structures exhibit in-plane K values much higher than their cross-plane K values, with an increased anisotropy compared to bulk single crystals for the case of the WSe₂ film. The increased anisotropy is attributed to the in-plane ordered, cross-plane disordered nature of the mis-oriented, layered structure. While the WSe₂ film is semi-insulating and the W[subscript x](WSe₂)[subscript y] films are metallic, the (PbSe₀.₉₉)[subscript x](WSe₂)[subscript x] films are semiconducting with its power factor (S²[sigma]) greatly improved upon annealing in a Se vapor environment. The second type of thin films is semiconducting InGaAlAs films with and without embedded metallic ErAs nanoparticles. These nanoparticles are used to filter out low energy electrons with the introduction of Schottky barriers so as to increase the power factor and scatter long to mid range phonons and thus suppress K. The in-plane measurements show that both the S and [sigma] increase with increasing temperature because of the electron filtering effect. The films with the nanoparticles exhibited an increase in [sigma] by three orders of magnitude and a decrease in S by only fifty percent compared to the films without, suggesting that the nanoparticles act as dopants within the film. On the other hand, the measured in-plane K shows little difference between the films with and without nanoparticles. This finding is different from those based on published cross-plane thermal conductivity results. / text

Not available / text