Luminescent vacuum ultraviolet spectroscopy of Cr 3 ions in nanostructured aluminum oxide

Impurity Cr 3 centers in submicron and nanostructured Al 2O 3 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, 4T 2, and 4T 1 excited states of Cr 3 ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al 2O 3 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 O 2p→Al 3s electron transition in surroundings of an impurity Cr 3 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 δθ-Al 2O 3 crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al 2O 3 at an excitation energy above 19 eV (more than 2E g). © 2012 Elsevier B.V. All rights reserved