Thermal transport in individual nanowires and nanotubes

This dissertation presents an experimental study of thermal transport in individual nanowires and nanotubes. All measurements are made by applying a microfabricated device consisting of two adjacent suspended SiN_x membranes with integrated Pt resistance heaters and thermometers suspended by five 420 μm long SiN_x beams. A batch fabrication process has been developed to fabricate more than 1500 suspended micro-heater structures on a 100 mm wafer. Individual nanowires or nanotubes are placed between the two adjacent suspended SiN_x membranes with amorphous carbon or platinum deposits at the nanowire-membrane contacts to reduce the contact thermal resistance. Thermal conductivities and Seebeck coefficients of different diameter single crystalline Si, Si/SiGe superlattice nanowires, polycrystalline Bi₂Te₃ nanowires, and a single wall carbon nanotube (SWCNT) bundle have been measured from 20 to 320 K.; The thermal conductivity of individual single crystalline intrinsic Si nanowires with diameters of 22, 37, 56, and 115 nm is found to be more than two orders of magnitude smaller than the bulk value. Strong size-dependent thermal conductivity in these wires is observed and ascribed to the increased role of boundary phonon scattering. In addition, further suppression of thermal conductivity is found in the smallest nanowire measured (22 nm), which suggests changes in phonon dispersion due to confinement effects. The thermal conductivity of 58 nm and 83 nm Si/SiGe superlattice nanowires shows quite similar temperature dependence to that of two dimensional (2D) Si/SiGe superlattice films, indicating that alloy scattering in the SiGe segments dominates phonon transport in these Si/SiGe superlattice nanowires. However, the nanowire boundary provides additional scattering mechanism and results in lower thermal conductivity than 2D superlattice.; The measured thermal conductance of a SWCNT bundle peaks at about 310 K, showing the signature of Umklapp phonon scattering. Between 14.5 and 50 K, the thermal conductance exhibits a T1.6 temperature dependence, which shows a lower dimensionality than the individual MWCNT. The electrical conductance and Seebeck coefficient are also measured and the thermoelectric figure-of-merit has been derived based on measured parameters with one single measurement. Thermal conductivities and Seebeck coefficients of polycrystalline Bi₂Te₃ nanowires synthesized by electrodeposition have been measured. The thermal conductivities of different wires show different values and varying temperature dependence, which are likely due to different compositions and crystalline structures of the samples.