While Co2+-doped ZnO shows high potentialfor spin-based electronic applications, the characteristics of dopant-centered mid-gaptransitions are not fully understood; also the interplay between these bandshas led to contradicting conclusions. In this work, mid-gap excited states ofCo2+-doped ZnO quantum dots were analyzed using linear response time-dependentdensity functional theory and the effective-mass theory with a focus on the geometryrelaxation in the donor photoionization excited state. Upon investigatingquantum dots with sizes 0.8 nm, 1.2 nm and 1.8 nm in diameter, one can see thatthe d-d (ligand field) bands reside below the donor photo-ionization (chargetransfer). Although d-d transition energy is independent on the quantum dot size, the charge transfer bands red-shift as the dot sizes increase. In addition, the -transfer state geometry relaxation leads to enourmous energy stabilization by up to 1.5 eV. Extrapolation of the charge-transfer state energies to the bulk limit  allows a prediction of the possibility of tunneling between the ligand-field and charge-transfer states, which has been observed experimentally (Johnson, C. A et al., Phys. Rev. B, 2010, 82, 224401). The interplay between the d-d band and charge-transfertransitios can be tuned by thermal energy, resulting in controllable luminescenceand photoconductivity in doped ZnO quantum dots of different sizes.

Tunable mid-gap excitations in Co<sup>2+</sup>-doped ZnO quantum dots

242nd National Meeting (2011)

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Tunable mid-gap excitations in Co2+-doped ZnO quantum dots

Tunable mid-gap excitations in Co²⁺-doped ZnO quantum dots