Electronic structure and local vibrational modes of Ni doped CdS: Crystal field splitting, Jahn-Teller instability, and on-site Coulomb correlation effects

Doping transition metals (TMs) such as Ni in CdS quantum dots (QDs) have been shown to improve their photocatalytic activities[1-4], but the underlying mechanism is still not understood. TMs are well-known for their rich chemistry due to their variable oxidation states, high degree of electronic degeneracy, and ability and flexibility to form complexes with reagents. We have carried out density functional theory (DFT) plus U studies of Ni-doped CdS. A T₂ ⊗ (e+t₂) Jahn-Teller (JT) model is developed to understand the ground state adiabatic potential energy surface (APES). We find that the interplay among crystal-field splitting, JT distortion, on-site Coulomb correlation, and magnetism results in the appearance of active states near the conduction band minimum. Therefore, introducing TM dopants in CdS QDs can not only provide reaction sites but also introduce near-edge electronic states that can enhance photo-absorption or fine-tune the band alignment for selective photocatalytic reactions. Our work also contributes to a deeper understanding of the interplay among various degrees of freedom in defect physics.