33, 3.16, 2.90, 2.65, and 2.5, and of 3.32, 3.15, 2.91, 2.65, 2.49 eV, respectively (see Figure 3b,c). These results show that an increase in anodizing voltage from 100 to 115 V leads a rather equal amount of redshift in the position

of all the PL emissions, see for instance peaks 1 and 2 in Figure 3a,b. Figure 3 Fitted PL emission spectra of the aluminum oxide membranes of Figure 2 . The membranes are anodized at (a) 100, (b) 115, and (c) 130 V. In Figure 3a, the 415-nm peak reveals the maximum emission intensity. This emission wavelength is close to the beginning of the blue region. However, the maximum emission locates about 427 nm in Figure 3b,c, which is close to the middle of the blue region. This wavelength shift can slightly improve the PL activity of the membranes in the visible range. In Figure 3c, peak positions show negligible shift compared with Figure 3b. A tolerance error should be considered for selleck both PL measurement and graph fitting procedures because the fluorescence spectrophotometer precision lies at approximately 0.1 nm, and there exists a possibility of error in the fitting process. Consequently, it could be deduced that an increase in the anodizing voltage beyond 115 V has insignificant shifting

effect on the emission spectrum see more (see Figure 3b,c). These findings indicate that an increase in the anodizing voltage beyond 115 V cannot enhance the PL activity of the membranes

in the visible range. Most of the previous reports have related the PL properties of PAAO layers to the optical transitions within individual oxygen vacancies. However, there is a clear-cut distinction between their interpretations on the type of the oxygen vacancies. Some researches claim in their articles that the PL spectra are concerned to the singly ionized oxygen vacancies [12, 13, 15]. But others relate the spectra to both singly ionized and neural oxygen vacancies [11, 14]. Singly ionized oxygen vacancies are generally called F+ centers. These point defects form when an electron is trapped in a double ionized oxygen vacancy. Neutral oxygen vacancies are often called F centers. They can be formed if Lck two electrons are trapped in a double ionized oxygen vacancy. Our results could not confirm the interpretations of the first group; otherwise, our results would not agree with the results on crystalline Al2O3. According to Lee and Crawford studies on sapphire [19] and Evans and coworkers on crystalline α-Al2O3[20], if crystalline Al2O3 is excited under a 4.8 eV (260 nm) wavelength, it would emit UV PL radiation due to the F+ color centers at approximately 3.8 eV (326 nm). Only one PL emission about 3.8 eV is fitted out among our results (see the 323-nm peak in Figure 4c). But several visible emissions far greater than 323 nm are identified (Figure 3a).