Characterization of Excited-State Magnetic Exchange in Mn2+-Doped ZnO Quantum Dots Using Time-Dependent Density Functional Theory

Absorption spectra of Mn2+-doped ZnO quantum dots have been studied with the linear-response time-dependent density functional theory. Spectral changes caused by excited state dopant-carrier and dopant–dopant magnetic exchange couplings are investigated. The excitonic transition maximum shifts to higher energy and decreases in intensity with increasing Mn2+ concentration. The lowest excitonic transitions split in the spin-up and spin-down manifolds due to sp–d magnetic exchange between the Mn2+ and ZnO conduction and valence band carriers. Increased Mn2+ concentration leads to a broadening and increase in the intensity of the midgap charge-transfer electronic absorption band. The charge-transfer band broadening results from excited-state splitting arising from double exchange magnetic interactions involving Mn2+ ions and the photogenerated hole. The excited-state double exchange leads to stabilization of the ferromagnetic configuration in the charge-transfer state. The strength of this ferromagnetic double exchange interaction depends on the inter-Mn2+ distance within the quantum dot.