Influence of oxygen-related surface adsorbates on the growth of low dimensional nanostructures and enhanced luminescence due to superoxide charge-transfer states in ZnO for application in optoelectronic devices

Abstract

Kinetically controlled growth of low dimensional zinc oxide (ZnO) nanostructures like nanorods, nanoflowers etc. have been demonstrated with enhanced photoluminescent emission by tuning of the hydroxide ion concentration in the precursor irrespective of the synthesis techniques. Change in concentration of the hydroxide ions in the precursor during the synthesis of ZnO nanostructured samples has been found to tune the aspect ratio of the nanostructures, viz. nanorods grown by electrodeposition, and morphology of nanoflowers to nanogranules synthesized by direct precipitation method. The photoluminescent emission from these samples has been found to increase synchronously with the increase in the concentration of hydroxide ions in the precursor. Observed enhancement in photoluminescence has been proposed to be due to the formation of superoxide (O2−) charge transfer (CT) states CTO2− due to the adsorption of molecular oxygen directly at grain boundaries or induced by hydroxide ions adsorbed over the surface of the nanostructures. The results are in contradiction to the established role of defect states behind photoluminescent emission from ZnO nanostructures. In this work, mathematical modelling has been used to establish the dominant role that superoxide CT states can play in enhancing the efficiency of ZnO based optoelectronic devices such as light-emitting diodes and photodiodes.