Modeling ZnS and ZnO Nanostructures: Structural, Electronic, and Optical Properties

We report the computational modeling of ZnS and ZnO nanostructures by defining realistic nanoparticle models similar to 1.5 nm sized for each material and investigating their structural, electronic, and optical properties by means of DFT/TDDFT calculations. To provide a direct comparison of calculated data to experimentally characterized nanoparticles, 3D (ZnX)(111) nanoclusters of prismatic shape have been set up starting from the bulk wurtzite (X = O, S), with two different saturation patterns of the polar surfaces. The investigated models have been optimized by means of Car-Parrinello molecular dynamics and local geometry optimization techniques. The investigated systems exhibit a well-opened HOMO-LUMO energy gap, without any artificial intraband-gap states. TDDFT calculation of the lowest excitation energies are in excellent agreement, within 0.1-0.2 eV, with the experimental absorption onsets reported for similarly sized ZnO and ZnS nanoparticles (3.70 and 4.40 eV, respectively). We have also investigated the electronic structure of the considered nanoparticles, with reference to the valence band structure, finding calculated binding energies for the Zn d-shell to be only slightly displaced toward lower values compared to experimental values, possibly due to quantum confinement effects. This work provides the required computational framework for modeling ZnX and in general II-VI semiconductor nanomaterials, opening the way to simulation of ligand/semiconductor interactions.