Investigation of pure and Co2+-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional

The electronic structures of pure and Co-doped ZnO quantum dots (QDs) with sizes up to 300 atoms were investigated using three different density functional theory approximations: local spin density approximation (LSDA), gradient-corrected Perdew–Burke–Ernzerhof (PBE) and the hybrid PBE1 functionals with LANL2DZ pseudo-potential and associated basis set. Qualitative agreement among the three methods is found for the pure ZnO nanostructures, but only the hybrid functional reproduces the correct bandgap energies quantitatively. For Co-doped ZnO QDs, both LSDA and PBE incorrectly model interactions between Co d levels and the valence band of ZnO, which will strongly impair predictions of dopant–carrier magnetic exchange interactions based on such computational results. Experimental observations are reproduced well in calculations at the hybrid PBE1 level of theory, making this the method of choice for exploring the magnetism of transition metal ions in ZnO QDs computationally. The qualitative features of the Co 3d levels do not change appreciably with changes in cluster size over the range examined, leading to size-dependent dopant-band edge energy differences. The results presented here thus provide an experimentally calibrated framework for future ab initio descriptions of dopant–carrier and dopant–dopant magnetic exchange interactions in diluted magnetic semiconductors (DMS) nanocrystals. 1 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 055013 1367-2630/08/055013+12$30.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft