Suppression of ferromagnetism due to N co-doping in Cr(II)-doped ZnS nanowires and their optical properties: Insights from density-functional calculations

Abstract

Spin-related optical properties will be the main basis for future information technology, but they require microscopic ferromagnetic and anti-ferromagnetic couplings between the d-states of the transition metal ions or magnetic interaction between excitons and magnetic ions, all of which remain unclear in these diluted magnetic semiconductors.In this work, we studied the influence of Cr doping and co-doping of Cr with N on the electronic, optical, and magnetic properties of ZnS nanowires using spin-polarized density functional theory. According to our findings, Cr(II) doping induces a half-metallic state with a magnetic moment of 4 μB per Cr(II) ion. The ferromagnetic behaviors and high Curie temperature in undoped Cr(II)-doped ZnS nanowires were suppressed due to the hole-carriers introduced by N co-doping. The band-coupling model was used to explain ferromagnetic (FM) stability in Cr(II)-doped nanowires with and without N co-doping. The optical properties of undoped ZnS nanowire, doped, and co-doped wires have been computed, and it is found that the bandgap (3.49 eV) exhibits a red shift upon Cr(II) doping, with the energy bandgap falling from 3.49 to 3.20 eV, and the bandgap being further decreased to 3.05 eV upon N co-doping. Compared with pure ZnS nanowires, mono- and co-doped nanowires have a higher electron-hole pair formation rate and better optical properties. The d-state to d-state transitions of the Cr(II) ion in the wire occur in the mid-infrared and near-infrared red regions and are shifted to the high energy side by N co-doping. Furthermore, we correlated the optical band gap of ZnS nanowire and the d-d transition peak of Cr ions under different modes of magnetic coupling, and we found that the optical band gap and d-state to d-state transition peaks of Cr(II) ions shift to lower and higher energies in FM and AFM configurations, respectively. Such a shift in bandgap and d-d transition peaks of Cr(II) ions in the FM and AFM states of the far configuration was not observed.