Electronic, magnetic and optical properties of Co(II) doped and (Al, Co) co-doped CdS nanowires: An ab initio study

In this work, we have carried out first-principles calculations for understanding the optoelectronic and magnetic properties of Co(II)-doped and (Al, Co) co-doped CdS nanowires. In the free carrier Co(II)-doped CdS NW, the spin-down Co(II)-t_a state is completely empty, leading to a super-exchange process. The exchange coupling of the electron carrier (provided by Al co-doping) with the spin-down Co(II)-t_a state promote a ferromagnetic ordering via double exchange interactions with a Curie temperature above room temperature. From the optical analysis, we found that Co(II) doping causes a blue-shift of the fundamental energy gap of CdS nanowire. The observed d-d spin-allowed transition peak of the Co(II) ion at 1.97 eV in the visible region is consistent with the experiments. The n-type Al co-doping not only generates an exciton-magnetic polaron (EMP) peak next to the fundamental bandgap but also exhibits the red-shift of the fundamental energy gap and Co(II)'s d-level-to-d-level transition bands. The correlation between spin-spin interaction and optical absorption shows that the d-d spin-allowed transition and optical energy gap both exhibit a blue-shift (red-shift) in the AFM (FM), supporting the experimental observation. Due to the paramagnetic behavior of the Co ions in the far configuration, there is no shift in the d-d transition peak or optical bandgap in the AFM and FM.