Field Emission and Optical Properties of Wide Band Gap Semiconductor Nanostructures

Abstract

The work presented in this thesis establishes scientific understanding of the vapor liquid solid (VLS) mode of growth for the synthesis of wide band gap semiconductor nanostructures. The focus of the thesis was to understand the physical mechanisms responsible for rates of the growth and self doping as functions of catalyst’s surface tension and liquid solid interface. The effect of different droplet densities and radii of metal catalysts were studied on the ultimate morphologies of ZnS nanostructures. Different metal catalysts have different accommodation coefficients, diffusion coefficients and different bonding energies with the host lattice. These directly affected the growth rate and the site of dopant, i.e., either in the bulk or on the surface. This concept was employed to demonstrate the growth rate for Sn, Au and Mn catalyzed ZnS nanostructures. It has also been justified theoretically and experimentally that suitable choice of the catalyst was very important for the growth of nanostructures towards device fabrication. The optical properties of the doped nanomaterials make them an interesting category of material for optoelectronic applications. Optical properties of self doped Au, Mn and Sn catalyzed ZnS nanostructures grown with varied thicknesses of catalysts were studied. Photoluminescence spectroscopy results of ZnS nanostructures showed the activation of all major types of defects and their dependence on catalysts, i.e., Zn and S vacancies and interstitials, catalyst related defects. Surface optical phonons were observed in Raman spectra were due to symmetry breaking at the surface. Raman spectra for each type of catalysts showed variation in the surface potential modulation created as a result of self doping of the catalyst on the surface. Dielectric continuum (DC) model was used to calculate the wavelength of surface potential modulations. The calculated modulation wavelength for symmetry breaking for the tapered nanobelts was in good agreement with the experimentally observed in TEM analysis. Morphology plays a crucial role in the physical properties of materials, e.g., optical, electrical, field emission (FE) and sensing. Different approaches were used to synthesize ZnS tapered and In 2 O 3 pyramidal beaded nanowires. Possible growth mechanism for the beaded nanowires was discussed in detail and explained with the help of visualization of xelectrical and structural analysis (VESTA) software. Finally, the FE measurements of the tapered ZnS nanostructures and beaded In 2 O 3 nanowires were undertaken. These FE results were compared with other published FE results by different groups and found comparably good FE properties.