Photoluminescent properties of barrier anodic oxide films on aluminum

We have investigated the photoluminescence (PL) spectra of barrier anodic oxide films on aluminum, formed in citric acid and in boric acid + borax. Strong PL bands are present in spectra of both electrolytes, featuring two distinct peaks in the 300 nm to 600 nm range. The first one is centered at about 460 nm, while the other peak shifts with the excitation wavelength. The PL intensity increases upon annealing, reaching the maximum value at about 500 °C. Experimental results indicate that the PL originates from optical transitions in two kinds of centers which are related to oxygen vacancies, F+ and F. In addition, the PL of oxide coatings formed under anodic spark deposition was performed and a third peak at around 327 nm was observed.