Fabrication and optical absorption of ordered indium oxide nanowire arrays embedded in anodic alumina membranes

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1 9 February 2001 Chemical Physics Letters 334 (2001) 298±302 Fabrication and optical absorption of ordered indium oxide nanowire arrays embedded in anodic alumina membranes Maojun Zheng a,b, *, Lide Zhang a, Xinyi Zhang a, Jun Zhang a, Guanghai Li a a Institute of Solid State Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei , People's Republic of China b Department of Applied Physics, University of Petroleum, Dongying , People's Republic of China Received 7 August 2000 Abstract Ordered semiconductor In 2 O 3 nanowire arrays embedded in anodic alumina membranes were fabricated by electrodeposition and oxidizing methods. The X-ray di raction and transmission electron microscopy indicate that the In 2 O 3 nanowires with polycrystalline structure are uniformly assembled into the hexagonally ordered nanochannels of the anodic alumina membranes. The optical absorption band edge of In 2 O 3 nanowires array exhibits a marked red shift with respect to that of the bulk In 2 O 3, and depends on the post-heat treatment temperature. This is attributed to the oxygen vacancy in In 2 O 3 nanowires and the interface interactions between the anodic alumina membranes and the In 2 O 3 nanowires. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction * Corresponding author. Fax: address: nanolab@mail.issp.ac.cn (M. Zheng). Indium oxide is a very important wide band gap transparent semiconductor and has been widely used in microelectronic applications including window heaters, solar cells and liquid crystal displays [1]. Many previous studies focus on the preparations and electrical properties of the indium oxide lms. Recently, the studies on the syntheses and optical properties of the indium oxide nanoparticles have been reported and exhibit some new properties di erent from bulk indium oxide [2]. In recent years, there has been increasing interest in quasi-one-dimensional nanostructure systems because of their numerous potential applications in various areas such as material sciences, electronics, optics, magnetism and energy storage. The anodic alumina nanoporous structures templates have received considerable attention in synthetic nanostructure materials due to their several unique structure properties such as controllable pore diameter, extremely narrow size distribution for pores diameter and their interval, ideally cylindrical shape of pores. They have been extensively used to fabricate nanometer-size brils, rods, wires and tubules of metals, semiconductors, carbons, and other solid materials [3±8]. Electrochemical deposition, sol±gel deposition and chemical vapour deposition have been employed as major template synthetic method. Until now, there have been no reports on the fabrication and optical properties of indium oxide nanowire arrays embedded in ordered porous solid. In this Letter, we rst report the syntheses and optical properties of indium oxide nanowire arrays embedded in ordered anodic alumina nanoholes. New optical /01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S (00)

2 properties that is di erent from those of indium oxide lms and nanoparticles dispersed in disorder mesoporous silica were observed. M. Zheng et al. / Chemical Physics Letters 334 (2001) 298± Experimental The alumina membrane was formed by a twostep anodization process as described previously [9]. Brie y, high purity (99.999%) aluminum plate was annealed in vacuum (673 K) and degreased in acetone, Al plate was anodized in 0.3 M oxalic acid solution under constant voltages 40 V for 4 h. After removal of the anodic oxide layer in a mixture of phosphoric acid (6 wt%) and chromic acid (1.5 wt%) solution at 60 C, for 6 h, the textured Al plate was anodized again for 8 h under the same conditions as for the rst anodizing. After coating a protecting layer on the surface of the porous alumina lm, the remaining Al layer was removed in a saturated HgCl 2 solution. A subsequent etching treatment was carried out in a 6 wt% phosphoric acid solution at 32 C for 1 h to remove the barrier layer on the bottom side of the anodic alumina membranes (AAM). A layer of Au was sputtered onto one side of the membrane used as the working electrode in a standard three-electrode electrochemical cell. The In nanowires were electrodeposited into the nanoholes by a three-probe dc method in 8.5 g/l InCl 3, and 25 g/l Na 3 C 6 H 5 O 7 2H 2 O solution at room temperature. The electrodeposition was performed at )1 V (versus Ag/ AgCl), with carbon serving as the counter electrode. After electrodepositing the In nanowires array embedded in AAM were annealed in air at di erent temperatures (973±1073 K) for 10 h. 3. Results and discussion Fig. 1 shows a atomic force microscope (AFM) micrographs of the AAM. One can see that the AAM has almost perfect hexagonally arranged nanochannel array with the channel diameter about 60 nm. The transmission electron microscopy (TEM) (JEM-200CX) micrograph of AAM with In 2 O 3 Fig. 1. AFM image of AAM. nanowires in its channels was shown in Fig. 2. The dark and bright areas are corresponding to In 2 O 3 nanowires and Al 2 O 3 supporting frame, respectively (Fig. 2a). The size of In 2 O 3 nanowire in diameter basically equals to those of the nanochannels, and they are periodically distributed in AAM and form a parallel aligned array. Fig. 2b shows In 2 O 3 nanowires micrograph after the AAM was removed from the In 2 O 3 /AAM samples by dissolving the AAM in 0.5 M NaOH solution and washing several times with distilled water. The di raction patterns of In 2 O 3 nanowires indicate that the nanowires in AAM pores were cubic polycrystalline structure. Fig. 3 shows the X-ray di raction (XRD) spectrum of the In 2 O 3 /AAM assembly system oxidized at 1073 K. X-ray di raction analysis was performed on a Philip's PW1700 X-ray di ractometer using Cu Ka X-ray source (k ˆ 0:15418 nm). The di raction peaks of (2 1 1), (2 2 2), (4 0 0), (4 4 0) and (6 2 2) correspond to the cubic In 2 O 3 phase. The other peaks are assigned to the AAM system phase. This indicates that In 2 O 3 nanowires array embedded in AAM are cubic polycrystalline, which is in agreement with the result of TEM. Sample color of the as-deposited samples is grey white, it may mainly consist of In as well as a low

3 300 M. Zheng et al. / Chemical Physics Letters 334 (2001) 298±302 TEM and XRD results all indicate that the postannealed samples are polycrystalline In 2 O 3, some chemical reactions may be taking place during the heat treatment. The possible reactions in the pore of AAM are listed below [10] 4In l 3O 2! 2In 2 O 3 s ; 4In l O 2! 2In 2 O l ; 4In l In 2 O 3 s!3in 2 O l ; In 2 O 3 s!in 2 O l O 2 ; Fig. 2. TEM micrograph of In 2 O 3 nanowire arrays embedded in AAM with channel diameters of 60 nm: (a) In 2 O 3 /AAM system; (b) In 2 O 3 nanowires. In 2 O l O 2! In 2 O 3 s : 5 In and In 2 O are in the liquid phase in the AAM due their low melting points. The primary reactions taking place in AAM is the conversion of In and In 2 O into In 2 O 3 according to above equations. These reactions may be responsible for the formation of the polycrystalline phase of In 2 O 3. Optical absorption spectra were obtained on a Cary-5E spectrophotometer at room temperature. Fig. 4 shows the optical absorption spectra of the blank AAM and the In 2 O 3 /AAM assembly system annealed at di erent temperatures. It can be seen Fig. 3. XRD spectrum of the In 2 O 3 /AAM system annealed at 1073 K. amount of In 2 O. When the samples were annealed at above 973 K, their colors changed into slight yellow. This indicates that In 2 O 3 has formed. Since Fig. 4. Optical absorption spectra of In 2 O 3 nanowire arrays in AAM annealed at di erent temperatures.

4 M. Zheng et al. / Chemical Physics Letters 334 (2001) 298± that the spectra of the annealed In 2 O 3 /AAM assembly system are quite di erent from that of the blank AAM (solid line). We did not observe the exciton absorption peaks observed in some semiconductor nanocrystals system [11]. The absorption spectra of the samples have di erent tails. The optical band gap of the In 2 O 3 nanowires embedded in AAM can been evaluated by following equation: ahm 2 / hm E g, where a is the absorption coe cient, hm is photon energy and E g is the band gap energy. Fig. 5 shows ahm 2 vs hm plots for the samples annealed at 973, 1023 and 1073 K, respectively. For all samples, the optical absorption in the edge region can be well t by the relation: ahm 2 / hm E g, which shows that In 2 O 3 nanowires embedded in AAM have the direct band gap structure, similar to the bulk case. The band gap of In 2 O 3 nanowires in AAM are about 3.4, 3.2 and 3.1 ev for annealed at 973, 1023 and 1073 K, respectively. This shows that the optical band edge of the In 2 O 3 nanowires embedded in AAM exhibits marked red shift respect to that of bulk In 2 O 3 (3.7 ev). It is well known that the semiconductor nanoparticle energy gap increases with decreasing the grain size, which leads to a blue shift of the optical ab- Fig. 5. ahm 2 vs hm plots for the samples annealed at di erent temperatures (data from Fig. 4). sorption edge and have been observed in many semiconductor nanoparticles system. Optical absorption spectra of the In 2 O 3 /AAM assembly system are di erent from those observed in In 2 O 3 lms and In 2 O 3 /SiO 2 assembly system, as previously reported [2]. The red shift and tails of the optical absorption spectra can be attributed to the variation of In 2 O 3 nanowires within channels in the structure, induced by annealing at di erent temperatures. In this work, In 2 O 3 nanowires were in situ formed within pores by thermal oxidizing In nanowires. There are oxygen vacancies at the interface of the In 2 O 3 nanowires. It has been evidenced that the concentration of oxygen vacancy dramatically depend on the heat treatment temperatures [12]. So the red shift may be from new energy levels in band gap formed by the oxygen vacancy in In 2 O 3 nanowires. With increasing the heat treatment temperature, the interface interaction between the In 2 O 3 nanowires and the wall of the AAM was enhanced, which may induce the formation of deep oxygen vacancy energy level in valance band and thus result in larger red shift of the optical band gap. In addition, surface con guration disorder may exist in the surface of the In 2 O 3 nanowires, which may also in uence the nanowires optical properties [13]. The variation of In 2 O 3 nanowires in size cannot in uence the optical band gap variations. According to quantum con nement theory [14], the exciton Bohr radius (a B ) plays an important rule in quantum con nement e ect. If a B is much larger than the radius of particle, the quantum con nement is obvious. It can be estimated by formulate: a B ˆ eh 2 =4ple 2, Where e is the static dielectric constant, h is Planck's constant, l is the reduced mass of an electron hole pair, 1=l ˆ 1=m e 1=m h, m e and m h are the e ective mass of electron and hole, respectively, and e is the electronic charge. For In 2 O 3 m e ˆ 0:3m 0, m h ˆ 0:6m 0, e ˆ 9, [1], we get it's a B is about 2 nm. The crystal size of the samples used are about 60 nm in diameter, R=a B is larger than 4. So In 2 O 3 nanowires embedded in AAM can be regarded as very weak quantum con nement system. It can be quantitatively discussed by the effective mass approximation model [14]. When R=a B P 4, the con nement of the electron and hole

5 302 M. Zheng et al. / Chemical Physics Letters 334 (2001) 298±302 wave functions is weak, and the shift (DE g ) of absorption band edges can be given as following: DE g ˆ E Ry h 2 =8MR 2 6 where M is the sum of the electron and hole effective masses, E Ry is the e ective Rydberg energy of the bulk exciton. The variation of the band gap of In 2 O 3 nanowires relative to the bulk band gap is estimated to be about )0.042 ev. This indicates that the variation of the band gap induced by the quantum con nement is very small and is negligible. Although we did not clearly know the details of the interface interactions, we believed that the annealing enhances the interaction between the In 2 O 3 nanowires and the AAM. We suggest that the red shift of optical absorption band edge arises from the formation of the new energy levels in the band gap due to the oxygen vacancy. Optical absorption band gap of the In 2 O 3 nanowires in the AAM was mainly controlled by the interface oxygen vacancy states. 4. Conclusion High order In 2 O 3 nanowires were uniformly assembled into the hexagonally ordered nanochannels of the AAM by oxidizing the In nanowire arrays electrodeposited in the nanochannels of the AAM. The method is applicable to other low melt point metal oxide nanowire arrays, which might be found technological applications in the future. Optical band gap red shift of the In 2 O 3 /AAM system was observed and is mainly attributed to the oxygen vacancy in In 2 O 3 nanowires. Previously experimental results have indicated that the stoichiometric form of In 2 O 3 is an insulator. While nonstoichiometric indium oxide has the properties of high conductivity, high transparency in the visible region and high re ectivity in the IR region, can change the conductivity by about three orders of magnitude [15]. It is possible for controlling the numbers of oxygen vacancy in the In 2 O 3 nanowire arrays embedded in the pores of anodic aluminum oxide lms by changing annealing temperature and time as well as the environmental atmosphere. This In 2 O 3 /AAM assembly system could be used in microelectronic applications, optical and gas sensitive devices. Acknowledgements This work was supported by the National Major Fundamental Research Project: Nanomaterials and nanostructures. References [1] I. Hamberg, C.G. Granqvist, J. Appl. Phys. 60 (1986) R123. [2] H. Zhou, W. Cai, L. Zhang, Appl. Phys. Lett. 75 (1999) 495. [3] T.M. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993) [4] G.S. Cheng, L.D. Zhang, Y. Zhu, G.T. Fei, L. Li, C.M. Mo, Y.Q. Mao, Appl. Phys. Lett. 75 (1999) [5] J.S. Suh, J.S. Lee, Appl. Phys. Lett. 75 (1999) [6] D. Xu, Y. Xu, D. Chen, G. Guo, L. Gui, Y. Tang, Adv. Mater. 12 (2000) 520. [7] C.R. Martin, Science 266 (1994) [8] P.R. Evans, G. Yi, W. Schwarzacher, Appl. Phys. Lett. 76 (2000) 481. [9] H. Masuda, K. Fukuda, Science 268 (1995) [10] C.A. Pan, T.P. Ma, J. Electrochem. Soc.: Solid-State Sci. Tech. 128 (1981) [11] N. Chestnoy, R. Hull, L.E. Brus, J. Chem. Phys. 85 (1986) [12] K.B. Sundaram, G.K. Bhagavat, Phys. Status Solidi 63 (1981) K15. [13] Y. Wang, N. Herron, J. Phys. Chem. 91 (1987) [14] Y. Kayanuma, Phys. Rev. B 38 (1988) [15] C. Xirouchaki, G. Kiriakidis, T.F. Pedersen, H. Fritzsche, J. Appl. Phys. 79 (1996) 9349.