Optical properties of porous alumina membranes

Optical properties of aluminum oxide have been investigated since 1970s.

Ultraviolet absorption, emission and excitation spectra were studied for high-purity crystalline alumina by Evans [4]. Crystallized alumina [5], including α-, γ-, and η phases, or sapphire [6-7] doped with titanium and chromium ions had also reported the optical behaviors by using various methods from the 70’s to the 90’s. The photoluminescence properties of the electrochemically produced porous alumina membranes were increasingly investigated in the late 20^th century because of the potential use in optoelectronics and nanotechnology. Although many studies have been done on the optical properties of crystalline alumina, little information is available on the optical behaviors of porous alumina membranes. Due to the mechanisms and the characterization of the photoluminescence band are quite complex, the photoluminescence phenomenon has been studied by several researches until now. There are three major models to explain this phenomenon stated in following paragraphs.

Du and his co-works [8] reported that a very strong PL band of the porous alumina membrane anodized in oxalic acid solution was observed when excited by a Xe lamp. Figure 2-2 shows a broad PL band centered about 450 nm occurs in the wavelength of 400-600 nm. The process parameters of the tested samples are shown in table 2-1. The intensity of the PL band increases with the heat treatment temperature, Ta, and reaches a maximum when Ta is 773 K. Electron paramagnetic resonance (EPR) measurement are carried out to find the origin of the blue PL band, as shown in Fig. 2-3. It describes that an obvious EPR signal appears in the spectrum.

This means that there are many singly ionized oxygen vacancies (F⁺ centers) in the porous alumina membranes because other oxygen vacancies (F and F⁺⁺ centers) are not paramagnetic. Oxygen vacancies have several categories, such as F, F⁺, F⁺⁺, etc.

F means an oxygen vacancy with two electrons, F⁺ center is formed by an oxygen vacancy trapping an electron, and F⁺⁺ is an oxygen vacancy without electron.

Therefore, the authors thought that the PL band originated from single ionized oxygen vacancies (F⁺ centers) in the porous alumina membranes. The similar results were also described in other reports [9-10]. Li et al. [9] observed that the PL band peaking around 470 nm in the wavelength range of 200 nm to 500nm was caused by the F⁺ centers of the porous alumina membrane with excitation wavelength of 360 nm. The PL properties from silicon based porous alumina films were also investigated by Wu et al. [10]. The PL spectra of as anodized samples showed that there were three strong PL bands centered at 295, 340, and 395 nm with an excitation wavelength of 240 nm. They suggested that oxygen-related defects, F⁺ centers, were responsible for the observed PL peaks.

Fig. 2-2 PL spectra of porous alumina membranes and the porous silicon [8].

Table 2-1 Parameters of the heat treatment for the porous alumina membranes [8].

sample conditions

a as prepared

b 473 K for 4 hours c 573 K for 4 hours d 673 K for 4 hours e 773 K for 4 hours f 823 K for 4 hours

g aging in air for 15 days after heating at 823 K for 4 hours

h annealing at 823 K in H2 for 1 hour after aging in air for 15 days

i porous silicon

Fig. 2-3 EPR trace at room temperature for porous alumina prepared in oxalic acid [8].

A further investigation of PL properties from alumina membranes anodized in 0.5 M oxalic acid solution was reported by Huang and his co-works [11]. Figure 2-4 shows that a PL peak in the Blue can be divided into two bands around 405 nm and 455 nm. From the spectra, the intensity of the 455 nm band relative to the 405 nm band increases with the electrolyte concentration. When the electrolyte concentration increases, the current is larger and more charge carriers can move to the alumina.

Moreover, the oxygen vacancies in porous alumina membrane can trap two electrons easily, and the density of the F center becomes larger. Therefore, the writers suggested that the two luminescent bands arose from two kinds of different defects (F and F⁺ centers) rather than only one kind of defect center, F⁺, as reported previously. A defect distribution model in the alumina membrane also presents as shown in Fig. 2-5. The density of the F centers is the largest near the surface because the oxygen vacancies located on the surface can easily trap two electrons and become the F center. Then, the density of the F centers decreases gradually with an increase in the pore wall depth. The situation of the F⁺ centers is just reversed.

Fig. 2-4 The PL spectra of the alumina membranes obtained by anodization of Al foils in 0.5, 0.23, and 0.1 M oxalic acid solutions [11].

Fig. 2-5 A model for distribution of the F (dark square) and F⁺ (open square) centers in the alumina membranes [11].

The Second model concerning the origins of the blue PL band in porous alumina membranes was proposed first by Yamamoto et al. [12]. They reported that the oxalic impurity was the reason for the blue PL band. Later research by Gao et al.

[13] supported the viewpoint of Yamamoto et al. Figure 2-6 shows the PL excitation

and emission spectra of the alumina films anodized in 0.3 M oxalic acid solution in the ultraviolet-to-green region. In Fig. 2-6 (b), an intensive and broad PL emission band peaks around 470 nm. The corresponding excitation spectrum shown in Fig. 2-6 (a) describes that a major excitation band around 360 nm and a weaker side band located around 250 nm are observed. From the results of PL and PLE spectra, it can be suggested that the 470 nm emission band is related to the two excitation centers.

Figure 2-7 shows that the intensity of the 470 nm PL emission peak and the intensity of the EPR signal peak for the alumina films vary as a function of annealing temperature. The intensity of the 470 nm PL emission increases with the rise of the temperature, meanwhile, the intensity of the EPR signal decreases with the rise of the temperature. This reveals that the origin of the 470 nm emission band is different from that of the EPR signal. In addition, during the anodization process, the oxalic impurities can be incorporated into porous alumina films [12]. The dissociation of acids generates conjugate base anions (from reaction 1 and 2), and the conjugate base anions can partly replace the O^2- in the alumina film.

It is reasonable that the incorporated impurities existing in the alumina films have important influences on their optical properties. Therefore, Gao et al. concluded that the evidence for F⁺ centers in oxalic alumina membranes was slight. The PL centers produced from the incorporated oxalic impurities during the anodization process were responsible for the 470 nm blue luminescence.

However, the details of the oxalic impurities existing in the alumina films anodized in oxalic acid solution, such as their exciting and distributing forms, are not very clear at present.

Fig. 2-6 The PL excitation (a) and emission (b) spectra of the porous alumina film anodized in oxalic acid solution [13].

Fig. 2-7 The intensity of the 470 nm PL emission peak and the EPR signal of the oxalic alumina film as a function of annealing temperature [13].

The last model describes F⁺ centers and the oxalic impurities are both the origins of the PL band in porous alumina membranes. Li et al. [14] indicated that a PL band range from 300 to 600 nm was observed. The PL intensity and peak position depended strongly on the excitation wavelength. Figure 2-8 shows that there are two peaks in the PL band: one (P1) is at constant wavelength of 460 nm, and the other (P2) increases almost linearly from 420 to 465 nm with excitation wavelength. The authors concluded that there were two PL centers, one originating from the

oxygen-related defects in the barrier layer (the relatively pure alumina), contributing mainly to the second PL band (460 nm), and the other correlated with the aluminum incorporated into the anion-contaminated alumina layer, contributing mainly to the first PL band (420 ~ 465 nm). The similar phenomenon was also observed by Li and his co-works [15]. Indeed, they suggested that there were three optical centers in the annealed alumina membranes. Due to the F⁺ centers could convert to F centers at high temperature annealing, the first was originated from the F center, the second was correlated with F⁺ centers, and the third was associated with the oxalic impurities incorporated in the alumina membranes.

Fig. 2-8 (a) PL emission spectra for porous alumina membranes prepared in oxalic acid solution.(b) The Gaussian fitting of emission spectra with the changes of Gaussian fitting peak position and PL intensity with wavelength [14].

2.3 Reviews of the PbS nanocrystals