Manipulation of structural and optical properties in ZnO/ZnS type-II and ZnS/ZnO inverted type-II core/shell nanocrystals: tight-binding theory

Based on the atomistic tight-binding theory (TB), I report here the results of my theoretical investigation into the electronic structures and optical properties of ZnO/ZnS type-II and ZnS/ZnO inverted type-II core/shell nanocrystals, with special emphasis on the single-particle spectra, atomistic orbital characters, optical band gaps, overlaps of ground electron and hole wave functions, ground-state oscillation strengths, electron-hole interactions and Stokes shift under the variation of the coated shell thicknesses. The underlying mechanism from an analysis of the computations is sensitive to the band alignments and growth shell thickness. In the presence of the terminated shell, the optical band gaps of ZnO/ZnS core/shell nanocrystals are reduced, while those of ZnS/ZnO core/shell nanocrystals are increased. With the increasing monolayers of the growth shell, the optical band gaps of ZnO/ZnS and ZnS/ZnO core/shell nanocrystals are decreased because of quantum confinement. It is expected that selecting ZnS as a shell for ZnO nanocrystals, with its large core and growth shell sizes, leads to the formation of the lower band gaps into the visible light spectrum. High optical behaviour is observed in ZnO/ZnS core/shell nanocrystals. As described by electron-hole coulomb interaction, a strong confinement between electrons and holes is found in ZnS/ZnO core/shell nanocrystals. The energies of electron-hole coulomb interaction are reduced with the increasing thickness of the coated shell. The Stokes shift of ZnS/ZnO core/shell nanocrystals is greater than that of ZnO/ZnS core/shell nanocrystals as elucidated by the electron-hole exchange interaction. The reduction of Stokes shift is presented with the increasing passivated shell monolayers. Finally, the present systematic study on the structural and optical properties mainly exposes a new viewpoint to understand the shell-size- and band-profile-dependent behaviours of ZnO/ZnS type-II and ZnS/ZnO inverted type-II core/shell nanoparticles to provide detailed information for use in designing electronic nanodevices.