Experimental and Theoretical Investigation of Thermal, Thermoelectrical, Optical and Structural Properties in Nanostructures

Nanomaterials keep showing huge potential for diver electronic, computing, sensing, biomedical and energy conversion applications. Due to their size confinement effect, nanostructures materials (thin film, nanotubes, nanowires, two dimensional thick material and quantum dots) have very different physical properties compared to their bulk counterparts. Thus, they are promising candidates for more efficient solid state thermal, electronic, optical, biomedical devices. However it is very challenging to investigate their properties and detect their real potential.

This dissertation presents thermophysical, thermoelectric and optical characterization of nanotubes, 2D material and thin films. Three different nanostructures have been experimentally and theoretically investigated: Titania nanotubes TiO2, atomically thin molybdenum diselenide MoSe2 and nickel silicide NiSi ultrathin film. The thermal and thermoelectrical measurements for both TiO2 nanotubes and MoSe2 monolayer were performed using suspended micro fabricated devices that allows the simultaneous measurements of Seebeck coefficient S, electrical conductivity σ and thermal conductivity κ on the same nanostructure that eliminated the parasitic effects of thermal contact and electrical contact resistances from the measured properties. Also the microdevice presents a through-substrate hole under the suspended membranes that makes the structural characterization on the same sample possible using Transmission Electron Microscope (TEM) analysis TEM.

The assembly of the samples were performed using different techniques: First, the TiO2 nanotubes were assembled either by drop casting method or by use of a micromanipulator under an optical microscope the nanotube on the microdevice. Second, the PMMA transfer method that uses polymethyl methacrylate polymer as a transfer carrier for the monolayer MoSe2. In the second technique polymerization process was developed to anchor the sample to the device. For the TiO2 sample the measurements are done on both intrinsic, as synthesized, and doped samples and for both amorphous and polycrystalline anatase phase. The thermal conductivity of single nanotubes was found to be up six times lower than their bulk counterpart and very close to that of the amorphous phase. The thermoelectric properties were measured just for the doped samples and the results show semiconducting behavior which are sensitive to the doping concentration. Furthermore at high doping concentrations TiO2 nanotubes present an n-type behavior at high temperature, that switches to p-type behavior at very low temperature.

The same characterization method was used to measure the thermal conductivity of atomically thick (0.7 nm) monolayer MoSe2. The measurements are done on intrinsic and tungsten doped samples. The results (30 W/mK and 17 W/mK for intrinsic and tungsten doped samples respectively) reveal nanostructure behavior for the thermal conductivity and they are in agreement with the reported values from first principle calculations. Also, the results show more than 50% decrease in the thermal conductivity due to the phonon scattering by tungsten atoms. Furthermore, optical and electrical properties of nickel silicide NiSi thin films are investigated over a broad range of wavelengths from 300 nm to 1000 nm. Those ultrathin films (thickness of 3 to 5 nm) show wafer scale uniformity, optical transparency of 60-90% and the resistivity is 1.29×10-01 to 9.93×10-02 μΩcm.