Photoluminescence in ZnO:Co²⁺ (0.01%–5%) Nanoparticles, Nanowires, Thin Films, and Single Crystals as a Function of Pressure and Temperature: Exploring Electron–Phonon Interactions

This work investigates the electronic structure and photoluminescence properties of Co 2+ -doped ZnO and their pressure and temperature dependences through high-resolution absorption and emission spectroscopy as a function of Co 2+ concentration and their structural conformations as a single crystal, thin film, nanowire, and nanoparticle. Absorption and emission spectra of diluted ZnO:Co 2+ (0.01 mol %) can be related to the 4 T 1 (P) → 4 A 2 (F) transition of CoO 4 (T d ), contrary to MgAl 2 O 4:Co 2+ and ZnAl 2 O 4:Co 2+ spinels in which the red emission is ascribed to the 2 E(G) → 4 A 2 (F) transition. We show that the low-temperature emission band consists of a 4 T 1 (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 4 T 1 (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 Co 2+ coordination from 4-fold T d to 6-fold O h . We also show that the optical properties of ZnO:Co 2+ (T d ) are similar, independent of the structural conformation of the host and the cobalt concentration. The Co 2+ enters into regular Zn 2+ 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.