Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr3+ centers in submicron and nanostructured Al2O3 crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of 2E, 4T2, and 4T1 excited states of Cr3+ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al2O3 nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O2p→Al3s electron transition in surroundings of an impurity Cr3+ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al2O3 crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al2O3 at an excitation energy above 19 eV (more than 2Eg).