Figure 1 SEM planar view of an anodic alumina membrane anodized at 130 V. Effect of applied voltage To evaluate the effect of anodizing voltage, both the first and the second anodizing steps are carried out by applying similar DC voltages ranging from 100 to 130 V for fix anodizing time of 20 h. This range of voltages
is selected based on our previous observation on the optimized semiconductor activity of the PAAO membranes formed via aluminum anodizing at approximately 115 V for up to about 20 h . Different excitation wavelengths are tested in order to identify most of the details of the subband states. It is observed that under 265-nm excitation wavelength, DMXAA the PL emission includes most of the emission peaks which are observed by exciting the membranes under different excitation wavelengths solely. Hence, our interpretation of the defect-based subband states is MRT67307 based on the PL emissions measured under 265-nm excitation. All the measured PL emission spectra of the membranes produced at 100, 115, and 130 V, are presented in Figure 2. It is observed that all the membranes
show PL emission in the 300- to 550-nm wavelength range. Qualitatively, a redshift is observed within some of the measured PL spectra (see Figure 2). It is evident that an increase in anodizing voltage leads to a slight shift in the emission peaks toward the visible region. Thus, the subband gaps present in the electronic structure of the membranes are learn more narrowed slightly by an increase in anodizing voltage. It should be pointed out that the shift rate is much more below 115 V, and it decreases afterward. It could be deduced that in these membranes, an increase in anodizing voltage by approximately 115V enhances formation of optically active defects with subband gaps which lay in the visible range. Figure 2 PL emission spectra of PAAO membranes formed, using different anodizing voltages, in phosphoric acid. The PL emission of metal oxides usually has various origins like intrinsic electronic point defects. It is known that for isolated similar
point defects in an amorphous material, the PL emission has a normal (Gaussian) shaped distribution. In the case of different light-emitting point defects, the PL emission regarding each defect type will contribute Amino acid to the whole emission spectrum through a Gaussian-like peak. Gaussian fitting analyzes these contributions and assists us to identify different electronic point defects which arise in the PAAO membranes. The analyzed emission spectra of Figure 2 are shown in Figure 3a,b,c. Those figures show that PL emission of all the membranes are composed of five different Gaussian-shaped functions. The Gaussian functions in Figure 3a are fitted to peaks about 361, 381, 415, 453, and 486 nm which correspond to 3.43, 3.25, 2.99, 2.74, and 2.55 eV subband transitions, respectively.