Electronic structures of ZnX (X = O and S) nanosheets from first-principles energy loss near edge structure studies

Abstract

This paper tries to study the core energy loss spectra of zinc based nanosheets (ZnO and ZnS) in density functional theory using the FPLAPW method. We have calculated the energy loss near edge structure (ELNES) spectra of zinc K- and L2,3-edges, and oxygen and sulfur K-edges in ZnO and ZnS nanosheet at magic angle conditions and compare to those of ZnO and ZnS wurtzite bulk structures. As a result of the smaller bond lengths in the nanosheet structures, all the ELNES spectra in nanosheets, including Zn K- and L2,3-edges, and O and S K-edges show a shift to the higher energies. The calculations reveal that in comparison to Zn edges in ZnO structures, all the ELNES spectra of ZnS structures including the bulk and sheet show a shift to lower energy region. This is a result of larger bond lengths in ZnS structures, and that it can be used to fingerprint each structure. The comparison of ELNES spectra and unoccupied symmetry-projected density of states (local DOS) confirms that Zn K-edges of both ZnO and ZnS nanosheets have main features including the electron transition to π* and σ* states which this is a sign of the strong sp2 hybridization and it is absent in Zn K-edge of ZnO and ZnS bulks. Moreover, in comparison to bulks, the energy differences between some main features in Zn based nanosheets decreases. The accordance of ELNES spectra and unoccupied local DOS shows that the main contributions in Zn L-edges ELNES of ZnO and ZnS nanosheets refer to transitions to mostly d-symmetry states and p-d hybridization. Orientation dependencies of the spectral features in nanosheet structures were also consideable. The overall dispersions of oxygen and sulfur K-edges in bulks and nanosheets are different. Since the inclusion of core-holes and super-cells is essential for accurate reproduction of features in ZnO and ZnS bulks and due to the rather wide band gaps of Zn based nanosheets, it may be essential as well for all the ELNES spectra of nanosheet structures.