Abstract: This work investigates the electronic structure and photoluminescence properties of Co2+-doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co2+ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co2+ (0.01 mol %) can be related to the 4T1(P) ? 4A2(F) transition of CoO4 (Td), contrary to MgAl2O 4:Co2+ and ZnAl2O4:Co2+ spinels in which the red emission is ascribed to the 2E(G) ? 4A2(F) transition. We show that the low-temperature emission band consists of a 4T1(P) zero-phonon line and a phonon-sideband, which is described in terms of the phonon density of states within an intermediate coupling scheme (S = 1.35) involving all ZnO lattice phonons. Increasing pressure to the sample shifts the zero-phonon line to higher energy as expected for the 4T1(P) state upon compression. The low-temperature emission quenches above 5 GPa as a consequence of the pressure-induced wurtzite to rock-salt structural phase transition, yielding a change of Co2+ coordination from 4-fold Td to 6-fold Oh. We also show that the optical properties of ZnO:Co2+ (Td) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co2+ enters into regular Zn2+ sites in low concentration systems (less than 5% of Co 2+), although some slight shifts and peak broadening appear as the dimensionality of the sample decreases. These structural effects on the optical spectra are also supported by Raman spectroscopy. © 2013 American Chemical Society.

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