Structural and Optoelectronic Properties of Unsaturated ZnO and ZnS Nanoclusters

We report a systematic computational study of the structural and optoelectronic properties of unsaturated ZnO and ZnS nanoclusters with hexagonal prism structure, as a function of length and diameter. We computed the fundamental gap using density functional theory (DFT) in the framework of the Delta SCF scheme and the optical gap by means of time-dependent DFT (TDDFT). We found that all ZnO nanostructures transform from wurtzite to graphitic phase. On the contrary, ZnS nanocrystals with diameters above similar to 1 nm are found to transform to a zeolite BCT phase. These different structural properties reflect in a very different size dependence of the electronic and optical properties, with a strong discontinuity for ZnS particles. The correlation between morphology and optoelectronic properties is demonstrated by considering models of saturated clusters preserving the wurtzite phase. We additionally compared DFT/TDDFT results with many-body perturbation theory methods showing a general good agreement among the two techniques for this class of nanocrystals of the two materials.