4964
J. Phys. Chem. C 2007, 111, 4964-4968
Combining Atomic Layer Deposition with a Template-Assisted Approach To Fabricate Size-Reduced Nanowire Arrays on Substrates and Their Electrochemical Characterization Lee Kheng Tan, A. S. Maria Chong, X. S. Eric Tang, and Han Gao* Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore ReceiVed: October 18, 2006; In Final Form: December 4, 2006
We report a combinational method to fabricate well-aligned size-reduced gold nanowire (Au NW) arrays on a Si substrate over large areas (>1.5 cm2). In this method, we employ a SiO2 atomic layer deposition (ALD) technique to reduce the pores of a porous anodic alumina (PAA) template which serves as a template for the electrodeposition of Au NW arrays and is directly integrated on the Si substrate. Unlike conventional method that simultaneously reduces the pore size and the interpore spacing by decreasing the applied potential during PAA anodization, our method allows for independently tuning these parameters. By using the ALD-reduced PAA template, we can fabricate the size-reduced Au NW arrays directly on the Si substrate with the average diameter reduced from ∼79 to ∼33 nm while their nanowire density and interwire spacing remain constant. The ALD technique enables the fine-tuning of the pore size or the nanowire diameter at an angstrom scale. Electrochemical characterization of the size-reduced Au NW arrays as nanoelectrodes is performed. The doublelayer charging current (noise of the nanoelectrodes) decreases with the reduction of the nanowire diameter. Our method could be used to fabricate nanowire arrays with large spacing and small diameters via high voltage anodization and subsequent ALD reduction on the PAA template. This type of nanowire arrays might have potential applications in electrochemical and field-emission devices.
Introduction Dense, aligned arrays of nanostructures created on various substrates have a wide range of applications in electronics, optoelectronics, information storage, sensing, and catalysis. Porous anodic alumina (PAA) templates have been widely used to fabricate one-dimensional nanostructure arrays, including metals, semiconductors, carbons, and polymers.1-5 The PAA template has several favorable characteristics in nanofabrication due to the ease of preparing the template, tuning pore structures, and removing the template after fabrication.6-8 For practical applications, direct integration of nanostructure arrays created by the PAA-assisted approach onto various substrates is highly desired. Recently, several techniques have been developed to transfer the PAA patterns onto substrates by using a free-standing PAA,9 ultrathin templates with protective layers,10,11 metal membranes duplicated from the PAA,12 and templates directly bonded onto substrates.5 In particular, we and others have demonstrated direct growth of the PAA template on various substrates (PAA/substrate) by anodizing an evaporated Al film on the substrate.13-22 The PAA/substrate shows several advantages over the free-standing PAA template, especially when used in device-oriented fabrication. First, the PAA/substrate allows for simultaneously growing and assembling nanostructures on the substrate. Next, the PAA template with a thickness ranging from a few hundred nanometers to several micrometers can be fabricated over large areas because of the support of the substrate, and even ultrathin templates (∼200 nm) can be easily created and handled during fabrication. Third, the fabrication of the PAA/substrate is relatively simple and easy15 in comparison with that of the freestanding PAA mask in which many steps are involved, including * To whom correspondence should be addressed. Tel.: (65) 6872 7526. Fax: (65) 6774 1042. E-mail:
[email protected].
PAA anodization, removal of the PAA from Al foils, chemical etching, as well as handling and attaching the PAA onto the substrate.11 Finally, the intimate contact between the PAA template and the underlying substrate makes it possible to prepare nanostructures via both solution-21,18 and vapor-based34 fabrication. To further exploit the potential applications of the PAA/ substrate, its inherent limitation must be addressed. During the fabrication of PAA/substrate, alumina barrier layer (BL) is formed at the base of the template, which will block the direct contact between the prepared nanostructure arrays and the substrate. To remove the BL, several approaches have been employed, including progressive reduction of the applied potential at the end of anodization,17,23 in situ penetration by electrochemically changing the local pH value,20,22 and isotropic chemical etching.15 Among these approaches, isotropic chemical etching is the simplest, most generally applicable, and wellcontrolled method. By simply immersing the template in a gentle acidic or basic solution for a period of time, the BL with even nonuniform thickness can be thoroughly removed without attacking the substrate and requiring “valve metal” substrates.20,22 However, this method is limited by the simultaneous pore enlargement, which is not favorable in fabricating eversmaller features on surfaces. For example, when using PAA/ substrate to prepare nanoelectrode arrays24,25 or field emitters,26 very large spacing between two neighboring nanostructures is highly desired to eliminate shielding effects or prevent diffusion layer overlap, but the nanostructures are required to be as small as possible. Large-spacing nanostructure arrays can be achieved using corresponding PAA templates, which can be prepared via a high-voltage anodization, for example, anodization of Al at 120 V will produce a PAA template with a center-to-center distance of ∼300 nm.6 Unfortunately, this conventional method simultaneously enlarges the pores and thickens the BL because
10.1021/jp066841v CCC: $37.00 © 2007 American Chemical Society Published on Web 03/10/2007
Template-Assisted ALD To Fabricate NW Arrays
Figure 1. Schematic of method used to fabricate size-reduced Au NW arrays on substrates. PAA template was first fabricated on the substrate. SiO2 ALD was employed to reduce the pores of the template, and the SiO2 layer at bottom of the template was removed by dry etching. Finally, Au NW arrays were electrodeposited into the template and released from the template by wet chemical etching.
the interpore spacing, the pore size, and the BL thickness are proportional to the applied voltage. Therefore, a prolonged period of chemical etching is required to remove the thick BL, during which the pore size is further enlarged. As such, techniques regarding pore reduction are of great importance in achieving the template with large spacing and small pore size. In this paper, we employed atomic layer deposition (ALD) of SiO2 to finely reduce the pores of the PAA/substrate while keeping the interpore spacing and pore density constant. The ALD-reduced PAA/substrate can then be used to fabricate Au NW arrays using electrodeposition. The Au NW arrays directly grown on the Si substrate can serve as nanoelectrode arrays. The double-layer charging current (DCC), noise of the nanoelectrode, was lowered as reducing the diameter of the nanowires with constant spacing. Although the pore reduction of the freestanding PAA template has been previously demonstrated using carbon films27 or alumina films,28 few reports have focused on the pore reduction of PAA/substrate and employed this template to fabricate the size tunable nanowires which are directly grown on the substrate. Our method offers the ability to fabricate the size-reduced nanowire arrays on the substrate. By using ALDreduced large-spacing PAA, the nanowire arrays with large spacing and small diameters could be fabricated for nanoelectrode or field-emission applications. In addition, ALD is a superb film deposition technique, allowing for conformal and layerby-layer coating of metal oxide films on nanoscale-sized highaspect-ratio features.16,28,29 The ability to finely reduce the size of the nanostructures is very useful in many applications but has seldom been demonstrated. For example, transport and thermoelectric properties of individual Bi nanowires will be affected by reducing the size of the nanowire.30 Magnetic behavior of Ni nanowire arrays is enhanced,31 and band gap shift of the two-dimensional photonic crystal could be realized by reducing the nanowire size.32 Experimental Section Experimental Methods. The process for reducing PAA pores is schematically shown in Figure 1. Initially, the template was created on a gold/silicon substrate by a two-step anodization of an evaporated Al film. The BL at the bottom of the template was chemically etched to obtain a through-pore template on the substrate. Concurrently, the pore size of the template was also enlarged. The through-pore template with a Au layer on the substrate then underwent a SiO2 ALD. Thin films of SiO2 with angstrom-controlled thickness were deposited on both the
J. Phys. Chem. C, Vol. 111, No. 13, 2007 4965 template and the Au surface. To expose the conductive Au layer for subsequent electrodeposition, dry etching techniques were applied to selectively etch the SiO2 overlayer on the template and the Au surface. After etching, Au was electrodeposited into the size-reduced template, and Au NW arrays on the substrate were released from the template by short dipping in dilute HF solution. PAA/Substrate Preparation. PAA/substrate was prepared by a two-step anodization as previously reported.15 In brief, an ∼1 µm thick Al film was deposited onto a Si substrate (p-type Si(100)) by electron beam evaporation (Edwards Auto 360). Prior to Al deposition, the Si substrate was first coated with a ∼5 nm Ti (as an adhesion layer) and a ∼50 nm Au that served as a working electrode for the Au NW deposition. The first anodization was carried out for 15-25 min in 0.3 M oxalic acid at 2-5 °C, 40 V. The first template was subsequently removed in a mixture of 3.5 vol % H3PO4 and 45 g/L CrO3 acid at ∼55 °C for 15-30 min. The second anodization was performed at the same conditions. Although the Al thickness for the first anodization is not thick enough to prepare highly ordered templates, the two-step anodization resulted in relatively uniform pore sizes for assessing the pore reduction. The end of anodization was indicated by a distinct color change. The alumina BL at the bottom of the template was removed by etching in 5 wt % H3PO4 for 30-70 min, depending on the desired pore diameter (during this process, pores are simultaneously widened). Size-Reduction of PAA/Substrate by SiO2 ALD. The asprepared through-pore PAA/substrate was rinsed repeatedly in deionized water (DI water). SiO2 ALD was carried out in a home-built setup at room temperature using sequential SiCl4 (Sigma-Aldrich, 99.998%) and DI water exposures. Anhydrous pyridine (Sigma-Aldrich, C5H5N, 99.8%) was co-dosed with each reactant exposure.29 This setup consists of a diffusionpumped stainless steel chamber equipped with three computercontrolled solenoid valves. The chamber was evacuated to 1.5 cm2) have been fabricated by combining an ALD technique and a PAA template-assisted approach. The diameter of the nanowires was reduced from ∼79 to ∼33 nm with a magnitude of one monolayer reduction while the interpore spacing and the pore density remain constant. The DCC (noise) of Au nanoelectrode arrays was found to decrease with reducing diameters. This technique holds potential for making nanoarrays with large spacing and small diameters which cannot be currently realized by conventional PAA template-assisted approaches. It is worth noting that, while this work used Au to demonstrate the method, arrays of various nanostructures with tunable size could also be fabricated on a range of substrates. Acknowledgment. We thank Dr. Jie Deng, Dr. Erik Johansson Cox, and Dr. Emma Philpott for their helpful discussions. References and Notes (1) Wang, Y.; Liao, Q.; Lei, H.; Zhang, X. P.; Ai, X. C.; Zhang, J. P.; Wu, K. AdV. Mater. 2006, 18, 943. (2) Wang, J. G.; Tian, M. L.; Kumar, N.; Mallouk, T. E. Nano Lett. 2005, 5, 1247. (3) Miao, Z.; Xu, D. S.; Ouyang, J. H.; Guo, G. L.; Zhao, X. S.; Tang, Y. Q. Nano Lett. 2002, 2, 717. (4) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Go¨sele, U. Science 2002, 296, 1997.
Tan et al. (5) Jung, H. Y.; Jung, S. M.; Gu, G. H.; Suh, J. S. Appl. Phys. Lett. 2006, 89, 013121. (6) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Go¨sele, U. J. Appl. Phys. 1998, 84, 6023. (7) Lee, W.; Ji, R.; Go¨sele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741. (8) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (9) Masuda, H.; Satoh, M. Jpn. J. Appl. Phys 1996, 35, L126. (10) Lei, Y.; Chim, W. K. J. Am. Chem. Soc. 2005, 127, 1487. (11) Lei, Y.; Yeong, K. S.; Thong, J. T. L.; Chim, W. K. Chem. Mater. 2004, 16, 2757. (12) Lee, W.; Alexe, M.; Nielsch, K.; Go¨sele, U. Chem. Mater. 2005, 17, 3325. (13) Chu, S. Z.; Wada, K.; Inoue, S. AdV. Mater. 2002, 14, 1752. (14) Cai, A. L.; Zhang, H. Y.; Hua, H.; Zhang, Z. B. Nanotechnology 2002, 13, 627. (15) Sander, M. S.; Tan, L. S. AdV. Funct. Mater. 2003, 13, 393. (16) Sander, M. S.; Coˆte´, M. J.; Gu, W.; Kile, B. M.; Tripp, C. P. AdV. Mater. 2004, 16, 2052. (17) Choi, J. S.; Sauer, G.; Goring, P.; Nielsch, K.; Wehrspohn, R. B.; Go¨sele, U. J. Mater. Chem. 2003, 13, 1100. (18) Gao, H.; Gosvami, N. N.; Ding, J.; Tan, L. S.; Sander, M. S. Langmuir 2006, 22, 8087. (19) Masuda, H.; Yasui, K.; Sakamoto, Y.; Nakao, M.; Tamamura, T.; Nishio, K. Jpn. J. Appl. Phys. 2001, 40, L1267. (20) Rabin, O.; Herz, P. R.; Lin, Y. M.; Akinwande, A. I.; Cronin, S. B.; Dresselhaus, M. S. AdV. Funct. Mater. 2003, 13, 631. (21) Sander, M. S.; Gao, H. J. Am. Chem. Soc. 2005, 127, 12158. (22) Tian, M. L.; Xu, S. Y.; Wang, J. G.; Kumar, N.; Wertz, E.; Li, Q.; Campbell, P. M.; Chan, M. H. W.; Mallouk, T. E. Nano Lett. 2005, 5, 697. (23) Furneaux, R. C.; Rigby, W. R.; Davidson, A. P. Nature 1989, 337, 147. (24) Menon, V. P.; Martin, C. R. Anal. Chem. 1995, 67, 1920. (25) Penner, R. M.; Martin, C. R. Anal. Chem. 1987, 59, 2625. (26) Tu, Y.; Huang, Z. P.; Wang, D. Z.; Wen, J. G.; Ren, Z. F. Appl. Phys. Lett. 2002, 80, 4018. (27) Mu, C.; Yn, Y. X.; Wang, R. M.; Wu, K.; Xu, D. S.; Guo, G. L. AdV. Mater. 2004, 16, 1550. (28) Elam, J. W.; Routkevitch, D.; Mardilovich, P. P.; George, S. M. Chem. Mater. 2003, 15, 3507. (29) Klaus, J. W.; Sneh, O.; George, S. M. Science 1997, 278, 1934. (30) Boukai, A.; Xu, K.; Heath, J. R. AdV. Mater. 2006, 18, 864. (31) Nielsch, K.; Wehrspohn, R. B.; Barthel, J.; Kirschner, J.; Go¨sele, U.; Fischer, S. F.; Kronmuller, H. Appl. Phys. Lett. 2001, 79, 1360. (32) Matsuu, M.; Shimada, S.; Masuya, K.; Hirano, S.; Kuwabara, M. AdV. Mater. 2006, 18, 1617. (33) Tian, M. L.; Wang, J. U.; Kurtz, J.; Mallouk, T. E.; Chan, M. H. W. Nano Lett. 2003, 3, 919. (34) Sander, M. S.; Prieto, A. L.; Gronsky, R.; Sands, T.; Stacy, A. M. AdV. Mater. 2002, 14, 665. (35) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (36) Hu, M. S.; Chen, H. L.; Shen, C. H.; Hong, L. S.; Huang, B. R.; Chen, K. H.; Chen, L. C. Nat. Mater. 2006, 5, 102. (37) Kovtyukhova, N. I.; Mallouk, T. E.; Mayer, T. S. AdV. Mater. 2003, 15, 780.
