Effect of Excited-State Structural Relaxation on Midgap Excitations in Co2+-Doped ZnO Quantum Dots

Abstract

The unique midgap excited states of Co2+-doped ZnO quantum dots have given rise to new applications in photocatalysis, sensing, magneto-optics, and magnetoelectronics. However, the electronic characteristics of these midgap transitions are not fully understood, and the uncertain interplay between these transitions has led to disagreement in the literature. In this work, midgap excited states of Co2+-doped ZnO quantum dots are analyzed using linear response time-dependent density functional theory and the effective mass theory with a focus on the geometry relaxation in the donor-type photoionization excited state. Relaxation of the excited-state geometry lowers the charge-transfer transition energy to be in the vicinity of the prominent spin-allowed 4A2 → 4T1 Co2+ d–d transition that gives this material its characteristic color. For large quantum dots, the excited-state population distribution between this Co2+ d–d excited state and the charge-transfer excited state can be tuned by thermal energy, resultin...