Abstract
ZnO nanostructures with a variety of morphologies and sizes, such as urchinlike structures composed of small-head nanonails, vertically aligned nanonails and nanocones, comblike nanostructures, hierarchical nanostructures, etc., have been synthesized via the simple thermal evaporation process onto the steel alloy substrate without the use of any metal catalyst or additives. It was observed from the detailed morphological and structural studies that substrate temperature, distances between source material and substrates, concentration of zinc vapors, zinc partial pressure, concentrations of reactant gases, and choices of substrates have serious impact on the morphologies and structural properties of as-grown products. Therefore, within certain reaction parameters, specific morphologies can be obtained. Detailed structural observations confirmed that the as-grown products are single-crystalline with the wurtzite hexagonal phase and grown along the (0001) directions. Raman-scattering and room-temperature photoluminescence (PL) studies revealed that the as-grown products have good crystallinity with excellent optical properties. The low-temperature PL (LTPL) studies, measured in the temperature range from 11-300 K, of the vertically aligned ZnO nanonails grown at position "B" revealed the presence of excitonic emissions and their multiple-phonon replicas in the ultraviolet region. Additionally, it has also been found that the temperature dependence of free exciton peak position can be successfully described by the standard Varshini's expression.