Photoluminescent properties of anodic aluminum oxide films formed in a mixture of malonic and sulfuric acid

• Fabrication of highly ordered anodic aluminum oxide (AAO) membranes by electrochemical anodization in the mixture.
• Photoluminescence (PL) from the films in visible blue wavelength region.
• The effect of sulfuric ions on the PL properties of AAO films.
• IR transmission spectra study confirms that the origin of defects in AAO films.
• The malonic impurities and the defects center are responsible for the PL emission from the AAO films.

Anodic aluminum oxide (AAO) films with ordered nanopore arrays were fabricated electrochemically in the mixture electrolytes of malonic and sulfuric acids. Their photoluminescent (PL) properties have been investigated. There is no obvious PL band for the AAO films prepared in pure sulfuric acid, however, with the increase of the concentration of malonic acid in the mixture electrolytes, a PL band in the range from 300 to 500 nm appears, and its intensity is enhanced gradually. Obviously, the PL band is mainly attributed to the malonic impurities incorporated into AAO films, which corresponds to the excitation center at about 270 nm. Meantime, the increase of the sulfuric acid concentration results in a blueshift of the PL band. Further, the effect of annealing process on the PL properties is also investigated. As annealing temperature increases, the malonic impurities incorporated into AAO films are gradually decomposed, the PL intensity corresponding to excitation center at about 270 nm gradually becomes weak. But interestingly, with the decomposition of malonic impurities, the defects in AAO films mainly caused by a lattice mismatch and the difference in the coefficients of thermal expansion between alumina and incorporated malonic impurities are gradually produced, which form a new excitation center at about 250 nm, and its PL is firstly enhanced to maximum at 400 °C and then decreases with the further increase of annealing temperature.