Band gap modulation and indirect to direct band gap transition in ZnS/Si and Si/ZnS core/shell nanowires

An investigation on the electronic structure and band gap nature of hydrogen passivated wurtzite ZnS/Si and Si/ZnS core/shell nanowires (CSNWs) in the [0001] direction has been performed by means of the density functional theory calculations. We have studied the effects of size and core-to-shell chemical composition ratio on lattice parameter and band gap. Almost all of these CSNWs have the indirect band gap nature for different sizes and different chemical composition ratios; however, in the case of ZnS/Si CSNWs for x = 0.4 with size of 2.7 nm and 3 nm, they show a direct band behavior. This indirect to direct band gap transition is attributed to the direct band gap nature of the core region in their bulk counterpart and strain effect on the core/shell interface. The band gap also demonstrates an increase when the size of both CSNWs is reduced from 3 nm to 2.5 nm, as a consequence of quantum confinement. The band gap modulation is in perfect agreement with the experimental results. The cohesive energy indicates that CSNWs with larger diameters are energetically more stable, and Si/ZnS CSNWs are more stable than ZnS/Si CSNWs.