Structure dependent photoluminescence of nanoporous amorphous anodic aluminium oxide membranes: Role of F+ center defects

The origin of photoluminescence (PL) of the anodic aluminium oxide (AAO) membranes synthesized by electrochemical anodization of aluminium by using oxalic, phosphoric and sulfuric acid electrolytes for different anodization time duration have been investigated. All the as prepared AAO membranes exhibit strong visible blue luminescence around 410–430 nm at room temperature. The mechanism of the visible light emission in AAOs has been explained by correlating the structural studies of the membranes with the PL and electron paramagnetic resonance (EPR) spectroscopy measurements. It is found that the AAO membranes are amorphous in nature (XRD and HRTEM study) and they contain large concentration of structural of defects (HRTEM study). The extensive PL and EPR investigations have indicated the presence of singly ionized oxygen vacancy related defects (F+ center defects) in AAO membranes. Based on the experimental evidences, the F+ center defects are attributed to the visible blue light emission from the AAOs.