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Thermoelectric and Mechanical Characterization of Synthesized One-Dimensional Nanostructures

Thermoelectric and Mechanical Characterization of Synthesized One-Dimensional Nanostructures
Davami, Keivan
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The relatively high cost and limited supply of fossil fuels has caused increased attention to cleaner and cheaper alternative energy sources such as solar, wind, and, as discussed here, thermoelectric energy. A new way of energy production via the conversion of thermal energy to electricity is now achievable through the application of thermoelectric effects. One-dimensional nanostructured materials have outstanding potential to enhance existing conversion devices. As tellurides of Zn, Cd and Pb have become popular for thermoelectric applications, a thorough characterization of their thermoelectric and mechanical properties is needed. In this research, the thermoelectric and mechanical properties of ZnTe nanowires (NWs) are investigated. We produced ZnTe NWs with diameters less than 30 nm via the bottom-up vapor-liquid-solid method. Bandgap engineering of single-crystalline alloy CdxZn1-xTe (0 ≤ x ≤ 1) NWs was achieved successfully through control of growth temperature and a two zone source system in a vapor-liquid-solid process. The effective process parameters on the morphologies of NWs were investigated, and the nanostructures were characterized by SEM and TEM. For the mechanical characterization, an experimental and a computational approach (molecular dynamic simulation) were used to investigate the size effects on Young’s modulus of ZnTe NWs. The mechanical properties of individual ZnTe NWs in a wide diameter range (50-230 nm) were experimentally measured inside a high resolution transmission electron microscope using an atomic force microscope probe with the ability to record in situ continuous force-displacement curves. For the smaller NWs, the computational approach was used. Mechanical characterization showed that neither molecular dynamic simulation nor experimental measurement illustrated evidence of strong size dependency of Young’s modulus of ZnTe NWs. For thermoelectric characterization, a MEMS device with two suspended thermometers was made to measure the thermal conductivity, Seebeck coefficient and electrical conductivity of the individual NWs. After positioning the NWs on the device, the aforementioned parameters were measured for NWs with different diameters. Thermal measurements concluded that the thermal conductivity of ZnTe NWs was significantly lower than that of bulk ZnTe. However, the electrical properties need to be modified in order to obtain a figure a merit comparable with or higher than bulk. The limiting factor for the use of ZnTe NWs in a wider range of applications is their high resistance. This can be removed by optimal doping or any other methods for the modification of their electrical conductivity. Also, more studies need to be done in order to achieve Ohmic electrical contacts with ZnTe NWs for a better investigation of their thermoelectric potential. Overall, it was concluded that ZnTe nanowires have the potential to be used in thermoelectric applications because of their relatively high strength and low and size dependent thermal conductance.
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