Photoluminescence and structural characterization of MgAl2O4 irradiated with swift Bi ions

Abstract

The optically stimulated luminescence in MgAl2O4 single crystals associated with defects produced by high energy (670 MeV) Bi ions with electronic stopping power Se = 36.6 keV/nm has been studied. Such energy loss is almost five times higher than the threshold for latent track formation in spinel, about 7.5 keV/nm. High resolution transmission electron microscopy (HRTEM) examination has revealed that ion tracks in the sub-surface layer of irradiated specimens retain a crystalline structure. Photoluminescence (PL) spectra from the near surface region were recorded during excitation at 355 nm, 473 nm and 532 nm using a laser confocal microscopy technique. It was found that PL spectra were always composed of broad overlapping bands positioned over the entire spectral range from the excitation wavelengths till 850 nm. It is suggested that such spectral composition might be due to a high concentration of antisite defects with large variation of the Mg/Al ratio in the disordered regions surrounding ion track cores. PL measurements on 23 MeV Ne (Se = 3.5 keV/nm) ion irradiated spinel, when no latent tracks are formed, have produced similar spectra as those for Bi ions. This indicates the same nature of defect-related luminescent centers formed at significantly different levels of electronic stopping power– below and above the threshold for latent track formation. The conclusion and findings may provide new insights for interpreting the optical spectra in this material.