Structure and Optical Properties of thin Porous Anodic ... - Science Direct

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ScienceDirect Physics Procedia 84 (2016) 415 – 420

International Conference "Synchrotron and Free electron laser Radiation: generation and application", SFR-2016, 4-8 July 2016, Novosibirsk, Russia

Structure and optical properties of thin porous anodic alumina films synthesized on a glass surface R.G. Valeev*,a, D.I. Petukhov a,b, V.V. Kriventsov a,c a Physical-Technical Institute of the Ural Branch of RAS, Kirova str. 132, Izhevsk, 426000 Russia b Department of Chemistry Lomonosov Moscow State University, Leninskie hills 1-3, Moscow, 119991 Russia c Boreskov’s Catalysis Institute of the Siberian Branch of RAS, Lavrientieva str. 5, Novosibirsk, 630090 Russia

Abstract The structure and luminescent properties of thin nanoporous aluminum oxide films obtained by anodization of aluminum films thermally deposited on glass have been investigated. The pore size and the interpore distance depend on the anodization voltage. For all studied samples the highest emission intensity obtained at the excitation wavelength equal to 330 nm. This behavior of luminescence curves caused by defect F+ luminescent centers (O- oxygen vacancies). The presence of porous alumina films on the glass surface increases the optical absorption in the visible light region. The oscillations on the spectra are caused by FabryPerot interference on the anodic alumina oxide film/glass interface. The suggested technique can be used for obtaining porous aluminum oxide films on other substrates, including Indium-Tin-Oxide, and can be applied in the technology of light-emitting devices and infrared-visible-ultraviolet detectors. © Published by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SFR-2016. Peer-review under responsibility of the organizing committee of SFR-2016.

Keywords: microporous materials; chemical synthesis; luminescence; microstructure; optical properties

1. Introduction Porous anodic aluminum oxide (AAO) has become widespread in nanotechnology applications [1] due possibility of its utilization as a template for preparation different nanostructured materials and as a dielectric matrix with pores filled with metals or semiconductors [1-5]. Furthermore, anodic alumina have been utilized in different optical

* Corresponding author. Tel.: +7 3412 430163; fax: +7 3412 722529. E-mail address: [email protected]

1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SFR-2016. doi:10.1016/j.phpro.2016.11.070

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applications, such as photonic crystals [6,7], optical sensors [8], carrier for surface-enhanced Raman scattering active particles [9,10] and different luminescent materials [11,12]. Therefore, studying optical properties of anodic alumina is important for practical applications. In recent years, most of investigators have focused their attention on the optical properties of free-standing anodic alumina films obtained from the thick aluminum foil [13-18]. Nevertheless, some applications require nanostructure arrays to be obtained on different surfaces including glass, silicon wafers, and ITO films on glass [19-22]. AAO synthesis on different smooth surfaces is basically similar to obtaining them on aluminum plates or foils. The main difference is the request of deposition aluminum film on the substrate prior to anodization. Traditional methods of aluminum deposition are radio-frequency (RF) or DC magnetron sputtering [23], electron-beam evaporation [24] and thermal evaporation [25]. All of these techniques have their advantages and drawbacks. Aluminum films obtained by RF-sputtering have high purity, but low adhesion to the substrate due to a low-temperature of deposition process, which leads to exfoliation of the deposited film from the substrate. The most reliable and adhesive Al films are obtained by the electron-beam evaporation technique, but this method requires expensive equipment. The method of aluminum thermal evaporation is technologically the most efficient method. Films deposited using this technique are chemically clear and have good adhesion to substrates. In this work aluminum films with thickness of 500 nm were deposited on the glass substrate via thermal evaporation technique. Anodic alumina films were fabricated by anodization of sputtered layer in 0.3 M solution of H2C2O4 and anodization voltages 30, 40 and 50V. The influence of anodization voltage on the structural and optical properties of synthesized microporous material was studied. 2. Materials and methods High pure aluminum films (99.99%, 0.5 μm) were deposited on soda lime glass substrates by a thermal evaporation method. Before the deposition process, the substrates were thoroughly cleaned by the standard procedure: washing in alcohol and acetone, ultrasonic alcohol rinsing for 10 minutes and drying with compressed air. The substrates temperature during deposition process was 25 ºC. Anodization was carried out in a two-electrode cell using a stainless steel wire as a cathode. During the anodization the electrolyte temperature was kept at about 2-3 °C. Chemical synthesis was performed in 0.3M aqueous solution of oxalic acid H2C2O4 at anodization voltages 30, 40, and 50 V. XRD investigations were carried out using Rugaku Miniflex 600 difractometer with Co-Kα wavelength. SEM micrographs were obtained using scanning electron microscope (Leo Supra 50VP). Optical transmission and absorption spectra were recorded by a UV/VIS spectrometer PerkinElmer Lambda 950 in the full reflection mode, and calculations were performed using the Kubelka-Munk equation [26]. Luminescence analysis was carried out at room temperature using a PerkinElmer LS 55 spectrometer. 3. Results and discussion Micrograph of sputtered aluminium layer is shown on the Fig.1. The obtained aluminum films are quite smooth and formed from small grains. Thickness measured by interferometry method was 500 nm. The anodization of thermally deposited aluminum films was carried out in a solution of oxalic acid (COOH)2 with a molar concentration of 0.3 M and at anodic voltages of 30, 40 and 50 V. As a result, samples whose typical form is shown in Fig.2 were synthesized. During the anodization process the current density vs. time graphs were recorded for all samples. It had been shown earlier in [27-29] that, regardless of electrolyte concentration and anodization voltage, the passing charge density about 2 Q/cm2 leads to the formation of a anodic alumina film with a thickness of 1 μm. The area of our samples was 3.14 cm2, and passing 3 Coulomb charges led to the formation of 0.57 μm thick films. Figure 3 shows chronoamperometric curves recorded during oxidation at different voltages. On the first stage we observe the nucleation of porous structure, the uniform growth of oxide film on the second stage with following drop of current density to zero due to dissolving all aluminum. Increasing of anodization voltage leads to rising current density which indicates to an accelerated oxidation rate. This leads to decreasing anodization duration.


Fig.1. Micrograph of sputtered aluminum film.

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Fig.2. Typical photo of a PAA film on a glass substrate.

Fig.3. Chronoamperometric curves obtained at anodization voltages of 30V, 40V and 50V.

A SEM study of the surface morphology of synthesized porous films (Fig.4) shows that the increase anodization voltage leads to increasing pore diameter. X-ray diffraction investigations (Fig.5.) show an amorphous state of porous alumina films. It should be noted that the earlier XRD studies by Zaghdoudi et al [30] show the presence of aluminum peaks, which implies an incomplete aluminum anodization. This is probably due to the large thickness (up to 5.5 μm) of aluminum films used by them, which leads to non-uniform oxidation of aluminum.

Fig.4. SEM images of porous alumina films, obtained at different anodic voltages: a) 30V; b) 40V; c) 50V.

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PL studies of samples have shown the presence of a luminescence peak with the maxima of 400 nm (Fig.6). The dependence between luminescence intensity and excitation wavelength has nonmonotonic behavior: as the wavelength decreases from 350 nm to 330 nm, the luminescence intensity increases. Further decrease in the wavelength leads to a decrease in luminescence intensity. Moreover, with increasing excitation wavelength the maximum of luminescence shifts towards larger wavelengths. This shift is due to the presence of F+ centers, i.e., oxygen vacancies with a single valence electron (O-) [31]. The concentration of such defects has a maximum in deeper layers of the boundaries between the pores. This explains the fact that the wavelength of the luminescence maximum for the sample synthesized at a voltage of 30 V is smaller (blue curve in Fig.6b), since, as Fig.5. XRD patterns of porous alumina fims on glass. seen from the SEM micrograph of the surface of the sample (Fig.4a), the distance between the pores’ boundaries is maximal. In general, the results obtained are in good agreement with the data presented in [31] for porous alumina films synthesized on aluminum plates.

Fig.6. PL spectra of 30V sample recorded at different excitation wavelengths (a) and samples obtained at anodic voltages of 30V, 40V and 50V under excitation at a wavelength of 330 nm (b).

Figure 7 represents optical absorption spectra of samples obtained at anodic voltages 30, 40 and 50 volts. For sample 30V spectra were obtained at incidence of light to the substrate and the AAO film. The wide optical transparence in the UV to IR range is seen to be limited by the range of optical transparence of the glass substrate. The absorption by porous alumina films is higher than by glass. Oscillations observed on the spectra for samples of AAO films on glass are due to Fabry-Perot interference on the AAO film/glass interface. Fabry-Perot interference was used for calculation of film thickness h using following equation [32]:

h=

λi ⎛ λ ⎞ 2n cosθ ⎜⎜1 − i ⎟⎟ ⎝ λi +1 ⎠

(1)

where λi and λi+1 are positions of neighboring reflection maxima (λi < λi+1), θ is the incident angle of light with respect to the normal to the sample, and n is refractive index of the alumina. The calculated thickness of oxide films


is equal to 580 nm that thicker then aluminum film on 14%, which can be explained due to expansion of aluminum after anodization [28].

Fig.7. Optical absorption spectra of 30V sample obtained at incidence of light to the substrate (red line) and to the AAO film (green line) (a) and samples 30V, 40V and 50V at incidence of light to the AAO film (b).

4. Conclusion The structure, luminescent and optical properties of thin nanoporous aluminum oxide films obtained by anodization of aluminum films thermally deposited on glass have been studied. Pore diameter and interpore distance depend on the anodic voltage. For all studied samples the highest emission intensity obtained at the excitation wavelength equal to 330 nm. This behavior of luminescence curves is due to the presence of defect F+ centers (oxygen vacancies with a single electron). The presence of porous alumina films on the glass surface increases the optical absorption in the visible light region. The oscillations on the spectra are caused by Fabry-Perot interference on the AAO film/glass interface. The methods presented can be used for the synthesis of porous aluminum oxide films on other substrates, including Indium-Tin-Oxide (ITO), and can be applied in the technology of light-emitting devices, and IR-visible-UV detectors. Acknowledgements This work was carried out within the framework of the subject of the State assignment for the Department of Surface Physics and Chemistry of the Physical-Technical Institute of the Ural Branch of the Russian Academy of Sciences (registration No. 01201157502) and was supported by the Russian Foundation for Basic Research (grant No. 16-48-180303). References [1] G.E.J. Poinern, N. Ali, D. Fawcett. Materials 4 (2011) 487-526. [2] R.G. Valeev, D.V. Surnin, A.N. Beltyukov, V.M. Vetoshkin, V.V. Kriventsov, Ya.V. Zubavichus, N.A. Mezentsev, A.A. Eliseev. J. Struct. Chem. 51 (2011) 132-136. [3] R. Valeev, E. Romanov, A. Deev, A. Beltukov, K. Napolski, A, Eliseev, P. Krylov, N. Mezentsev, V. Kriventsov. Phys. Stat. Sol. C 7 (2010) 1539-1541. [4] R. Valeev, E. Romanov, A. Beltukov, V. Mukhgalin, I. Roslyakov, A. Eliseev. Phys. Stat. Sol. C 9 (2012) 1462-1465. [5] K.S. Napolskii, I.V. Roslyakov, A.A. Eliseev, D.I. Petukhov, A.V. Lukashin, S.-F. Chen, C.-P. Liu, G.A. Tsirlina. Electrochimica Acta 56 (2011) 2378-2384. [6] G. Wang, J. Wang, S.-Y. Li, J.-W. Zhang, C.-W. Wang. Superlattices and Microstructures 86 (2015) 546–551. [7] S.-Y. Li, J. Wang, G. Wang, J.-Z. Wang, C.-W. Wang. Mater. Res. Bul. 68 (2015) 42–48. [8] T. Kumeria, A. Santos, D. Losic. Sensors 14 (2014) 11878-11918. [9] S.N. Terekhov, S.M. Kachan, A.Y. Panarin, P. Mojzes. Phys. Chem. Chem. Phys. 17 (2015) 31780(9).

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