Nanoestrutura de CdS: propriedades estruturais, eletrônicas e oxidação

Abstract

CdS nanostructures form different shapes, with potential applications in solar cells, optical and mechanical devices. The presence of oxygen in nanostructures of these materials can be useful for obtaining new forms such as propellers, as well as for the construction of high performance devices as regards grinding. In this work, we made initially a comparative calculation between the structural phases of the bulk material and nanostructures formed by a few layers of CdS in order to understand the formation energetically more stable. Our calculations were made via Density Functional Theory (DFT) with the GGA-PBE within the PAW method, implemented in the VASP package. The results show that the sequence of phase stability for the CdS bulk is: wurtzite, Blende zinc, graphitic, rock salt, which agrees with all results of the literature. Through the plane (0001) of the bulk phase wurtzite structure and the plane (001) of the bulk rock salt we have obtained nanostructures composed of several layers. After relaxation the wurtzite phase nanostructure undergoes a transition to graphitic phase. This is explained by calculating the surface energy of nanostructures. In these settings our results show that for nanostructures composed with less than 10 layers the graphitic phase is more stable. Due to surface states, the most nanostructures show metal features (gapless). With respect to oxygen impurity, we doped the nanostructures in two ways: by adsorption and substitutional sulfur. The doped nanostructures were monolayers and bilayers. We observed that the substitutional method as well as the adsorption occurs with the formation of chemical bonds between oxygen and the atoms of the nanostructures, and oxidation is exothermic. In all cases the reaction is more favorable to the rock salt nanostructures. With respect to double layer in the wutzite phase the oxidation by substitution is more favorable. We have observed impurity levels below the Fermi level only for the monolayer wurtzite when the oxidation is by adsorption. For the other structures oxygen is electronically inert.