Ultraviolet emission and Fano resonance in doped nano-alumina

Richard Laine


J. Appl. Physics, 101:053534.

Emission properties of Al2O3 nanopowders, synthesized by flame spray pyrolysis with Mg, Cr, and Sc dopants, are investigated, principally in the protein lysing range of 250–290nm (UV-C band). As expected, point defect densities depend on crystal phase and irradiation history and strongly influence emission properties at short wavelengths. Ultraviolet and visible emission intensities of aggregated point defect centers change upon electron beam exposure at high current densities, but ultraviolet emission from point defects is persistently enhanced over a narrow range of Mg-doped Al2O3 compositions slightly off spinel stoichiometry. At 40% Mg concentration, emission intensities at 320nm rise by over an order of magnitude after beam exposure. Quantum efficiency for cathodoluminescence in the 250–300nm range nevertheless remains low. Point defect ionization at high currents shifts the emission of Al2O3 nanopowders to the infrared and is shown to be correlated with a ubiquitous Fano resonance in ionized Cr-vacancy complexes. This base line spectroscopy permits us to attribute the very high quantum efficiency of UV-C luminescence in annealed Sc3+:Al2O3 primarily to the formation of α-phase Al2O3.