Jahn-Teller Defects in Solids: Integrated Infinite-mode Coupling Theory and Applications

Transition metals (TM), with their flexible chemistry and rich physics, are among the most versatile dopants for semiconductors to achieve exotic functionalities that would not be possible otherwise. Unfortunately, an accurate understanding of TM defects in semiconductors is often hampered by a multitude of challenges such as strong Coulomb correlations and the Jahn-Teller (JT) effect. Unlike in the case of molecular systems, JT defects in solids in principle involve an infinite number of phonon modes, and theory needs to go beyond the conventional few-mode JT models. Herein, we develop an integrated infinite-mode theory for treating JT defects in solids with quadratic coupling effects included and apply the theory to Ni doped CdS. Doping Ni in CdS has been shown to improve its photocatalytic activities, but the underlying mechanism is still not understood. We find that the interplay among crystal-field splitting, JT distortion and on-site Coulomb correlation results in the appearance of active defect states near the conduction band edge. Therefore, introducing Ni dopants in CdS may not only provide reaction sites but also introduce near-edge electronic states that can enhance photo-absorption or fine tune the band edge position for selective photocatalytic reactions.