Photoluminescence and optical absorption caused by the F+ centres in anodic alumina membranes

Anodic alumina membranes (AAMs) with highly ordered nanochannel arrays were prepared by anodizing aluminum in acid solutions. X-ray diffraction reveals the amorphous nature of AAMs. The photoluminescence (PL) excitation spectra of AAMs consist of a broad band centred at 235 nm and a narrow band peaking around 360 nm under a monitoring wavelength of 470 nm. There are five prominent absorption bands (or edges) at 370, 294, 254, 220 and 204 nm in the absorption spectra of AAMs prepared in oxalic solution. With the increase of the annealing temperature, the intensities of PL and absorption bands increase first, then decrease, and a blue-shift is found. The photoluminescence and optical absorption in the wavelength range of 200 nm to 500 nm are caused by the F+ centres in AAMs. The energy levels of F+ centres in AAMs are split and a new energy level of 3.35 eV appears due to the amorphous nature of AAMs.