Materials Science and Engineering B 112 (2004) 171–174 www.elsevier.com/locate/mseb
Spectroscopic characteristics of doped nanoporous aluminum oxide W.M. de Azevedoa,*, D.D. de Carvalhoa, H.J. Khouryb, E.A. de Vasconcelosc, E.F. da Silva Jr.c a
Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, Cidade Universita´ria, Recife-PE 50670-901, Brazil b Departamento de Energia Nuclear, Universidade Federal de Pernambuco, Cidade Universita´ria, Recife-PE 50740-540, Brasil c Departamento de Fı´sica, Universidade Federal de Pernambuco, Cidade Universita´ria, Recife-PE 50670-901, Brazil
Abstract In this work, we present photoluminescence characterization of rare earth ion doped nanoporous aluminum oxide synthesized by anodization process in different aqueous electrolyte solutions. We found that the luminescence of doped aluminum oxide strongly depends on the synthesis medium. When synthesized in inorganic acid only rare earth ions fluorescence is present, whereas nanoporous aluminum oxide synthesized in organic solvent presents two strong unexpected luminescence emission lines, one at 429 nm and the other at 491 nm, with quite long decay time when excited with long wavelength ultraviolet light. Theses results suggest that light simulation of primary colors and chromaticity control of the emitted light can be achieved by carefully doping aluminum oxide nanoporous with a combination of different rare earth ions. # 2004 Elsevier B.V. All rights reserved. Keywords: Rare earth; Nanoporous; Luminescence; Aluminum oxide
1. Introduction Nanotechnology research, to a great extend is based on fabricating functional nanoscale structures and devices in a well-controlled way, which represents one of the most difficult challenges facing today’s researchers and engineers. The means to organize nanoelements into device structures in order to realize their desired functionalities, using inexpensive fabrication techniques, is essential from a technological point of view. Due to the small dimensions of these nanoelements, a bottom-up self-assembly process often provides a viable approach to overcome such technological challenges [1,2]. One of the important aspects of selfassembly lies in its capability of forming a large area of uniform structures through inexpensive chemical or biological processes. A major concern of using self-assembly processes to fabricate nanoscale devices for electronic or optoelectronic applications is their compatibility with * Corresponding author. Tel.: +55 81 32718440; fax: +55 81 32718442. E-mail address:
[email protected] (W.M. de Azevedo). 0921-5107/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.05.039
high-vacuum technologies. Most electronic and optoelectronic devices are based on high-quality semiconductors, and their production involves complicated micro or nanofabrication processes. Aluminum anodization [3] is one of the most controllable self-assembly processes, and nanoporous anodic aluminum oxide has been employed to synthesize a variety of nanoparticles and nanowires through a template-mediated approach [4]. The electrochemical self-assembly consist of basically three steps: (a) electro polishing an aluminum foil in a suitable electrolyte to clean and prepare the surface, (b) anodizing the electropolished foil in a desired solvent with a dc current to form a porous alumina film on the surface, and finally (c) electrodepositing or chemically synthesizing the material of interest within the pores. Recently, aluminum anodization has been combined with traditional silicon processing to fabricate uniform anodic aluminium oxide thin films directly onto a silicon substrate [5], and it was also used to produce planarized microelectronic components from aluminum and anodic alumina layers on large scale of integration and hybrid circuits
172
W.M. de Azevedo et al. / Materials Science and Engineering B 112 (2004) 171–174
such as interconnections, inductors, capacitors and others [6-9]. The increasing interest for the development of luminescent materials for flat panel displays and optoelectronic devices has stimulated the development of rare earth activated luminescent materials in the form of thin films [10]. The use of rare earth ions in wide band gap materials such as ZnO or Al2O3 as luminescence sources offers the advantage of light emission with spectral characteristics fairly stable under different operation conditions, since the luminescence is mainly dependent on the electronic energy levels of the rare earth ions. Another interesting feature is the chromaticity control of the devices that can be achieved through the generation and control of the relative intensity of primary additive colors generated by the rare earth ions [11–15]. Aluminum and doped aluminum oxide has been also used as a host materials in the form of thin films for rare earth ions, however the techniques used to fabricate these films such as laser ablation, reactive sputtering, CVD and spray pyrolysis require in general expensive vacuum setups and complicated systems for handling the source reactants. In this work we present a simple and straightforward method to prepare highly luminescent aluminum oxide and rare earth doped aluminum oxide using the aluminum anodization process followed by reflux reaction. Also, we discuss the luminescent characteristics of theses materials as a function of the synthesis medium and the chromaticity control throughout generation of primary light colors.
2. Experimental details The anodizing aluminum oxide (AAO) template was generated with high purity aluminum foil (99.99%) in acid solution using a two step process [16]; the process was carried out either at a constant voltage of 40 V in 0.3 M oxalic acid solution at 10 8C or at 18 V for 0.5 M of sulfuric acid. After the first anodization, the oxide film was removed and the newly patterned aluminum substrate was anodized again. The anodization time was determined by the required thickness of the AAO film. After that, the AAO film was treated in reflux with a rare earth chloride solution 0,1 M, for four hours; finally the AAO doped films were thoroughly washed with deionized water and putted into a furnace for annealing treatment. The photoluminescence and infrared characterization were performed at room temperature using a Jobin Yvon Ramanor model U-1000 spectrometer using either a 150 W Xe Lamp or a argon laser as a excitation source. The emission light was detected by a water cooled photomultiplier RCA C31034-02 and processed by the Jobin Yvon spectralink data acquisition system, and a FTIR Bruker IF 66 spectrophotometer, respectively. The chromatic coordinate has been plotted using the Spectra LuxTM Software v.1.0. [17] A Siemens D-5000 X ray diffractometer with a Cu target was used to obtain the X-ray diffraction patterns of the samples, and the surface microstructure of
anodic aluminum oxide films were observed using scanning electron microscope (SEM) JEOL model JSM 5600 LV.
3. Results and discussion The structural characteristics of nanoporous aluminum oxide films synthesized by the anodization process in oxalic acid is illustrated in Fig. 1, where X-ray diffraction patterns for the undoped film without annealing treatment are shown. The patterns shows several peaks associated with Al and Al2O3 crystalline phases on top of a broad peak centered at 2u 258. The broad peak is indicative of a highly disordered and/or amorphous aluminum oxide compound. Fig. 2 shows the top view SEM surface micrographs of the alumina film prepared in oxalic acid as described in the experimental setup. The figure shows a typical view of the alumina surface, where the dark areas are the pores with a quite regular hexagonal characteristics pattern with average diameter of 70 nm, consistent with reports in the literature, and the surrounding light areas are alumina. The IR spectra of aluminum oxide film prepared in oxalic acid (full curve) and sulphuric acid (broken curve) are compared in Fig. 3. Both spectra have absorption bands at about 3400 cm1 and in the region 1250–400 cm1. The former is due to the O–H stretching vibration of the bound water within the film. The latter is due to the intrinsic vibrations of the alumina constituting the bulk of the film the band with a peak at 1246 cm1 observed with the sulphuric acid film, is assigned to the vibration arising from sulphate species incorporated into the film during anodization, and the band observed at 1107 and 1032 cm1 in oxalic acid can be assigned to the coupling of the C–C stretching vibration and the O–C=O bending vibration. The double absorption band with peaks at 1634 and 1485 cm1, observed with the oxalic acid film and absent for films formed in sulphuric acid electrolytes, can be reasonable assign to the antisymmetric O–C–O stretching vibration; and to the coupling band arising from the symmetric O–C–O stretching vibration and the C–C stretching vibration, respectively. Accordingly, it seems quite reasonable to assume that the double absorption band is inherent to the films formed in aliphatic carboxylic acid solutions and is
Fig. 1. X-ray diffractogram of anodized aluminum oxide synthesized in oxalic acid 0.3 M.
W.M. de Azevedo et al. / Materials Science and Engineering B 112 (2004) 171–174
173
Fig. 2. Scanning electron micrographs of fragments of a surface side of the alumina film prepared in oxalic acid.
due to carboxylate species incorporated into the films during anodization. The photoluminescence emission spectra for the nanoporous aluminum oxide prepared in oxalic acid and doped with Tb+3 and Eu+3 are shown in Fig. 4. We can see two unexpected broad peaks, one at 429 nm and the other at 491 nm when the sample is excited by UV at 320 nm. Theses peaks completely overcome the rare earth emission that appears as a slight shoulder in the spectra. It is interesting to mention here that these unexpected peaks do not appear in the emission spectra of the sample synthesized in sulphuric acid suggesting that probably these peaks are related with the presence of F centers and/or free radicals of carbon related centers, as shown in the infrared spectra. Fig. 5 shows the effect of the temperature annealing treatment on the sample doped with Tb+3 (a) and Eu+3 (b) rare earth ions and synthesized in oxalic acid. We can see that at the annealing temperature of 600 8C the broad peak intensity decreases, remaining only the rare earth ions emission lines at 490, 542 585 e 621 nm corresponding
Fig. 3. Comparison between the IR reflection spectra of the oxalic acid (solid line) and sulphuric acid films (dot line).
Fig. 4. Room temperature photoluminescence emission from nanoporous aluminum oxide doped with (a) Tb+3 and (b) Eu+3 rare earth ions.
to 5 D4 ! 7 F6 ; 5 D4 ! 7 F5 ; 5 D4 ! 7 F4 , and 5 D4 ! 7 F3 transitions for the Tb+3 ion and 580, 590, 614, 653 and 702 nm corresponding to the 5 D0 ! 7 F0 ; 5 D0 ! 7 F1 ; 5 D0 ! 7 F2 ; 5 D ! 7 F and 5 D ! 7 F transitions for Eu+3 ion. The 0 3 0 4 intensity, line width and the relative intensity of the spectral lines is a strong indication that the luminescence are due to the rare earth ions doping amorphous aluminum oxide, instead of rare earth chloride adsorbed in the nanoporous matrix. Again it is interesting to mention that at intermediated annealing temperatures the samples synthesized in oxalic acid present strong phosphorescence that last on several seconds.
Fig. 5. Room temperature photoluminescence emission from nanoporous aluminum oxide doped with Eu+3 (a) and Tb+3 (b) rare earth ions annealed at 600 8C.
174
W.M. de Azevedo et al. / Materials Science and Engineering B 112 (2004) 171–174
luminescence and the intensity relation between the electric dipole and magnetic dipole transitions of the rare earth ions Eu+3 and Tb+3 in the annealed films been bigger than 1 indicates that the ions are incorporated into the host material having low symmetry. The unusual luminescence presented by the sample synthesized in oxalic acid is probably originated from carboxylate species incorporated into the films during anodization or by polymeric condensation of aluminum ions creating pentahedrally coordinated aluminum sites. The light primary colors simulation has been achieved and chromaticity control of the emitted light has been proposed by the combination of the rare earth ions during sample preparation.
Fig. 6. CIE (1931) color coordinates of fluorescent doped aluminium oxide samples irradiated in the UV region.
The explanation for that unexpected luminescent behavior may be related with the incorporation of the oxalate anion or some complex, or carbonyl groups generated during the synthesis, another possibility could be the formation of some polymeric form of Al2O3 oxide as it has been noted in Al2O3–SiO2 porous glasses [18], and in sol gel derived oxide networks [19]. In both cases the luminescence decreases when the samples is annealed at high temperature. Another interesting result presented by this system is the chromaticity control of emitted light, as can been seem in Fig. 6, the primary colors, red (A) and green (B) are easily generated by the spectra of Eu+3 and Tb+3 ions, respectively, as the simulation of the secondary color inside the figure by the sample prepared in oxalic acid (C). Through careful control of the concentration and the number of rare earth ions on the aluminum oxide film, it is possible to generate white light. Efforts are underway in our lab to generate white light through the use of a combination of terbium, europium and thulium ions in the preparation of aluminum oxide under sulfuric acid process or by the combination of europium and terbium in samples prepared in oxalic acid.
Acknowledgements This work was supported in part by CNPq/NanoSemiMat and FINEP/CTPETRO under grants 550.015/01-9 and 65.00.0280.00, respectively.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
4. Conclusions Highly luminescent nanoporous materials at room temperature, have been synthesized by the anodization process followed by chemical reactions under reflux. The strong
[16] [17] [18] [19]
R. Bashir, Superlatt. Microstruct. 29 (2001) 1. J.H. Fendler, Chem. Mater. 13 (2001) 3196. H. Masuda, K. Fukuda, Science 268 (1995) 1466. C.R. Martin, Science 266 (1994) 1961. W.C. Hu, et al. Appl. Phys. Lett. 79 (2001) 3083. G.C. Schwartz, V. Platter, J. Electrochem. Soc. 123 (1976) 34. V. Surganov, A. Mozalev, G. Gorokh, Proceedings of the Academy of Science of Belarus, Physics, Mathematics Series, vol. 6, 1991, p. 92. V. Surganov, A. Mozalev, Foreign Electr. Tech. (Russian J.) 1–2 (1993) 40. V. Surganov, IEEE Trans. Comp. Packag. Manuf. Technol. B 17 (1994) 197. S.W. Depp, W.E. Howard, Sci. Am. 267 (1993) 90. L.E. Tannas Jr., Phys. Today 45 12 (1992) 52. S.C. Goh, J. Non-Cryst. Solid 161 (1993) 227. M. Dejneka, et al. J. Lumin. 65 (1995) 227. J.E.C. Silva, O.L. Malta, G.F. de Sa´ , P.A. Santa Cruz, Quimica Nova 21 (1998) 372. J.M. Souza, S. Alves Jr., G.F. de Sa´ , W.M. de Azevedo, J. Alloys Compd. 344 (2002) 320. H. Masuda, K. Fukuda, Science 268 (1995) 1466. P.A. Santa-Cruz, F.S. Teles, Spectra Lux Software Ponto Quaˆ ntico Nanodispositivos/RENAMI, vol. 1, 2003. T. Hayakava, A. Hiramitsu, M. Nogami, Appl. Phys. Lett. 82 (2003) 2975. B.E. Yoldas, J. Non-Cryst. Solids 147 (1992) 614.
