Template-Assisted Patterning of Nanoscale Self-assembled

We report a general, simple, and inexpensive approach to pattern features of self-assembled monolayers (SAMs) on silicon and gold surfaces using porou...
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Langmuir 2006, 22, 8078-8082

Template-Assisted Patterning of Nanoscale Self-assembled Monolayer Arrays on Surfaces Han Gao,* Nitya N. Gosvami, Jie Deng, Le-Shon Tan, and Melissa S. Sander† Institute of Materials Research and Engineering, Singapore 117602 ReceiVed March 10, 2006. In Final Form: July 2, 2006 We report a general, simple, and inexpensive approach to pattern features of self-assembled monolayers (SAMs) on silicon and gold surfaces using porous anodic alumina films as templates. The SAM patterns, with feature sizes down to 30 nm and densities higher than 1010/cm2, can be prepared over large areas (>5 cm2). The feature dimensions can be tuned by controlling the alumina template structure. These SAM patterns have been successfully used as resists for fabricating gold and silicon nanoparticle arrays on substrates by wet-chemical etching. In addition, we show that arrays of gold features can be patterned with 10-nm gaps between the dots.

1. Introduction Surfaces patterned with regular arrays of chemically distinct nanoscale features have a wide range of applications.1,2 In particular, self-assembled monolayers (SAMs) are useful for modifying surface chemistry because they have tunable end groups and can be deposited on many substrates.3,4 Therefore, patterned SAMs have been widely used as resists,5-8 templates,9,10 and model surfaces11,12 for controlling the formation and assembly of inorganic materials, macromolecules, and biomolecules. A number of techniques have previously been developed to create SAM patterns on surfaces. For example, films of SAMs can be directly patterned with good precision and controlled geometries using photolithography13,14 or particle beam lithography.15-17 However, these techniques require sophisticated equipment and have high operation costs, especially for creating sub-100 nm features. Patterned SAMs can also be written using scanning probe-based lithography approaches,18-21 which enable * To whom correspondence should be addressed. E-mail: h-gao@ imre.a-star.edu.sg. Phone: +65 6872 7526. Fax: +65 6774 1042. † Current address: GE Global Research, Niskayuna, NY 12309. (1) Gates, B. D.; Xu, Q. B.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1171. (2) Geissler, M.; Xia, Y. N. AdV. Mater. 2004, 16, 1249. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (4) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (5) Wang, D. W.; Thomas, S. G.; Wang, K. L.; Xia, Y. N.; Whitesides, G. M. Appl. Phys. Lett. 1997, 70, 1593. (6) Finnie, K. R.; Nuzzo, R. G. Langmuir 2001, 17, 1250. (7) Weinberger, D. A.; Hong, S. G.; Mirkin, C. A.; Wessels, B. W.; Higgins, T. B. AdV. Mater. 2000, 12, 1600. (8) McLellan, J. M.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 10830. (9) Aizenberg, J.; Muller, D. A.; Grazul, J. L.; Hamann, D. R. Science 2003, 299, 1205. (10) Porter, L. A.; Choi, H. C.; Schmeltzer, J. M.; Ribbe, A. E.; Elliott, L. C. C.; Buriak, J. M. Nano Lett. 2002, 2, 1369. (11) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (12) Lee, K. B.; Park, S. J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295, 1702. (13) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (14) Sun, S. Q.; Leggett, G. J. Nano Lett. 2002, 2, 1223. (15) Golzhauser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. AdV. Mater. 2001, 13, 806. (16) Pallandre, A.; Glinel, K.; Jonas, A. M.; Nysten, B. Nano Lett. 2004, 4, 365. (17) Stamou, D.; Musil, C.; Ulrich, W. P.; Leufgen, K.; Padeste, C.; David, C.; Gobrecht, J.; Duschl, C.; Vogel, H. Langmuir 2004, 20, 3495. (18) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661. (19) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457.

excellent control over the size, composition, and spacing of features. Although these are typically serial processes, recently it has been shown that by using many tips in parallel it is possible to create patterns over large areas on surfaces simultaneously.22 Another patterning approach, contact printing, is a highthroughput and cost-effective technique for fabricating SAM patterns over large areas.1,3 When utilized for sub-100 nm fabrication, contact printing is somewhat limited by the distortion of the stamps and the lateral diffusion of the neighboring SAM features, although ∼40 nm features have been reported by using specially designed stamps to pattern high molecular weight inks.23,24 This approach also requires sophisticated lithography facilities to produce masters, which is particularly time-consuming for very small features. It is therefore of interest to develop new, general approaches that can be used to generate sub-100 nm SAM patterns over large areas at low cost and with high throughput. In this work, we describe a general and inexpensive approach to create dense, uniform sub-100 nm SAM features by transferring nanoscale patterns from porous anodic alumina templates to gold and silicon surfaces. The pores in the templates spatially confine the self-assembly of molecules on the exposed surfaces. With this approach, we have demonstrated the fabrication of SAM patterns over large areas (>5 cm2) with tunable feature sizes down to 30 nm and densities higher than 1010/cm2. In addition, by using closely spaced SAM patterns as resists, arrays of gold features with 10 nm gaps have been patterned by wet-chemical etching. In contrast to previous lithographic techniques used to deposit SAMs on surfaces, the patterns created using this template-based approach are the result of the self-organization of the pores in the alumina during anodization. The porous alumina templates are simple and inexpensive to fabricate. Dense features of SAMs with diameters smaller than 100 nm can be easily fabricated due to the ease of preparing sub-100 nm pores in the nanoporous templates. In addition, the diameter, shape, and spacing of the features can be tuned by controlling the template structure, as (20) Maoz, R.; Frydman, E.; Cohen, S. R.; Sagiv, J. AdV. Mater. 2000, 12, 424. (21) Liu, J. F.; Cruchon-Dupeyrat, S.; Garno, J. C.; Frommer, J.; Liu, G. Y. Nano Lett. 2002, 2, 937. (22) Salaita, K.; Lee, S. W.; Wang, X. F.; Huang, L.; Dellinger, T. M.; Liu, C.; Mirkin, C. A. Small 2005, 1, 940. (23) Odom, T. W.; Love, J. C.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Langmuir 2002, 18, 5314. (24) Schmid, H.; Michel, B. Macromolecules 2000, 33, 3042.

10.1021/la060658b CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006

Patterning of Nanoscale SAM Arrays on Surfaces

previously reported.25-30 The pore diameters can be controlled by adjusting the applied voltage during anodization and additionally by varying the time of chemical etching of the templates after anodization. In addition, template films can be removed either by a mechanical peel or in a selective chemical etch, while the alumina remains chemically and mechanically stable during processing and SAM deposition. Free-standing porous anodic alumina films have been previously used as templates and masks for fabricating a wide range of nanostructure materials.31-34 Recently, we and others have demonstrated the fabrication of nanoporous templates directly on various substrates.35-40 In this work, we take advantage of the intimate contact between the template and the substrate in these structures to deposit molecular features directly on the underlying substrate. Deposition is limited to the exposed surface regions defined by the template. By using nanoporous templates directly on substrates to create SAM features, we can avoid the problems of SAM diffusion that are inherent in methods that involve molecular transfer from a stamp or SPM tip.16,41,42 Because the size and density of the SAM features are determined by the template pore structure, very small and dense SAM features can be created. In addition, by using solution-based deposition for our template-based deposition, in contrast to the ambient environment that is used in many writing and stamping approaches, it may be possible to deposit molecular systems that are difficult to be deposited using other approaches, such as moisture-sensitive silanes or multilayer structures. The major limitation of this approach is that the organization of SAM features is restricted to the patterns that can be fabricated in the templates. In general, self-organizing template-based approaches are most useful for applications requiring high densities of uniform-size features on surfaces. These patterning approaches are therefore complementary to lithography-based techniques, which enable more precise control of the size and shape of features, but also require more sophisticated equipment and are typically limited to fabricating features over smaller areas on surfaces. 2. Experimental Section Experimental Methods. Figure 1 schematically outlines the two procedures we have used to create nanopatterned SAMs on surfaces. Both of the approaches are based on transferring patterns from porous anodic alumina templates to create SAM features on substrates. In (25) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (26) Li, A. P.; Muller, F.; Birner, A.; Nielsch, K.; Gosele, U. J. Appl. Phys. 1998, 84, 6023. (27) Li, F. Y.; Zhang, L.; Metzger, R. M. Chem. Mater. 1998, 10, 2470. (28) Peng, C. Y.; Liu, C. Y.; Liu, N. W.; Wang, H. H.; Datta, A.; Wang, Y. L. J. Vac. Sci. Technol. B 2005, 23, 559. (29) 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. (30) Cojocaru, C. S.; Padovani, J. M.; Wade, T.; Mandoli, C.; G. Jaskierowicz; Wegrowe, J. E.; Morral, A. F. I.; Pribat, D. Nano Lett. 2005, 5, 675. (31) Cheng, G. S.; Moskovits, M. AdV. Mater. 2002, 14, 1567. (32) Martin, C. R. Science 1994, 266, 1961. (33) Kovtyukhova, N. I.; Mallouk, T. E.; Mayer, T. S. AdV. Mater. 2003, 15, 780. (34) Masuda, H.; Yanagishita, T.; Yasui, K.; Nishio, K.; Yagi, I.; Rao, T. N.; Fujishima, A. AdV. Mater. 2001, 13, 247. (35) Cai, A. L.; Zhang, H. Y.; Hua, H.; Zhang, Z. B. Nanotechnology 2002, 13, 627. (36) Chu, S. Z.; Wada, K.; Inoue, S. AdV. Mater. 2002, 14, 1752. (37) Rabin, O.; Herz, P. R.; Lin, Y. M.; Akinwande, A. I.; Cronin, S. B.; Dresselhaus, M. S. AdV. Funct. Mater. 2003, 13, 631. (38) Sander, M. S.; Tan, L. S. AdV. Funct. Mater. 2003, 13, 393. (39) Sander, M. S.; Cote, M. J.; Gu, W.; Kile, B. M.; Tripp, C. P. AdV. Mater. 2004, 16, 2052. (40) Sander, M. S.; Gao, H. J. Am. Chem. Soc. 2005, 127, 12158. (41) Maury, P.; Peter, M.; Mahalingam, V.; Reinhoudt, D. N.; Huskens, J. AdV. Funct. Mater. 2005, 15, 451. (42) Lu, N.; Gleiche, M.; Zheng, J. W.; Lenhert, S.; Xu, B.; Chi, L. F.; Fuchs, H. AdV. Mater. 2002, 14, 1812.

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Figure 1. Schematics of two approaches employed to create patterned SAMs on silicon (a) and gold (b) substrates. PAA: porous anodic alumina template; BL: barrier layer; RIE: reactive ion etching; SAM: self-assembled monolayer. the first process shown in Figure 1a, a nanoporous template film is created directly on a substrate by evaporation of an Al film, imprinting and anodization, and this film is then directly used as a template for SAM deposition. In the second approach, shown in Figure 1b, an alumina template fabricated from Al foil is used as an etch mask to transfer the pore pattern to an SiO2 transfer layer by reactive ion etching (RIE), and the porous SiO2 layer is then used as a template for SAM deposition. Fabrication of Silane-Based Patterns and Si Nanoparticle Arrays on Silicon. The fabrication of alumina templates on silicon has been described in detail previously.38 Briefly, Al films ∼300 nm thick were deposited by e-beam evaporation onto clean silicon. To produce highly ordered nanopores in the alumina, an imprinting step was performed on Al/Si as previously reported.39 The mold was a piece of free-standing anodic alumina ∼5-10 µm thick. The pretextured Al film was then anodized in 0.3 M oxalic acid at 40 V and 2-5 °C to obtain pores with center-to-center spacings of ∼100 nm. After anodization, the thin barrier layer at the bottom of the pores was removed by immersion in 5 wt % H3PO4 for 40 min at room temperature. Octadecyltrimethoxysilane (OTMS, SigmaAldrich) was deposited from a 10 mM solution in toluene for 1 h, then rinsed in toluene and ethanol, and dried under N2. The template was removed by a mechanical peel using adhesive tape and then the sample was heated at 150 °C for 1 h, or alternately, the sample with template was first heated and then the alumina template was removed by immersion in 1 M KOH or 5 wt % phosphoric acid. (An OTMS layer is incidentally also formed on the alumina template, but this has no effect on the silicon patterning process.) To create a nanostructured silicon surface, the substrate was etched in 4 M KOH with 15% IPA (isopropyl alcohol) at 40 °C for approximately 2 min until a color change was observed.6 Fabrication of Thiol-Based Patterns on Gold and Gold Nanoparticle Arrays on Silicon. Alumina templates were fabricated from annealed and electropolished Al foil by anodization in 0.3 M oxalic acid using a two-step process, as previously reported.43 The first anodization was carried out at room temperature (∼23 °C) while the second was done at ∼2 °C. Low-temperature anodization during the second step allows for controlled fabrication of very thin alumina films. The aluminum anodization rate is ∼40 nm/min in 0.3 M oxalic acid at 40 V and ∼2 °C. Using a 5-10 min anodization (43) Gao, H.; Mu, C.; Wang, F.; Xu, D. S.; Wu, K.; Xie, Y. C.; Liu, S.; Wang, E. G.; Xu, J.; Yu, D. P. J. Appl. Phys. 2003, 93, 5602.

8080 Langmuir, Vol. 22, No. 19, 2006 for the second step leads to nanoporous templates with highly ordered pores and a thickness less than 400 nm. A thin PMMA layer was then formed by coating 5 wt % PMMA (in anisole) on the asprepared top alumina template surface and heating at ∼120 °C for 1 h. A thick nail polish layer was subsequently deposited on the PMMA layer. The remaining Al foil was removed in saturated HgCl2. The free-standing alumina film plus protective layers was rinsed in deionized water and then immersed in 5 wt % H3PO4 for 90-150 min to remove the barrier layer at the bottom of pores and to tune the pore diameters. The alumina film produced in 0.3 M oxalic acid at 40 V and 2 °C has pore diameters less than 30 nm. When just released from the Al foil, the pores in the alumina templates are closed, with the top end covered by the protective PMMA and nail polish layers while the other end is sealed with the barrier layer (BL). The BL can be selectively removed by immersing the sample in phosphoric acid. The etching rate of alumina in 5 wt % H3PO4 at 23 °C is ∼0.5 nm/min. Therefore, it takes about 80 min to dissolve the 40 nm thick BL formed by anodization at 40 V. Extending the immersion time after the removal of the barrier layer results in continued etching of the pore walls at the same rate. By controlling the etching time, it is possible to tune the pore diameter. For example, immersion of templates in 5 wt % H3PO4 at 23 °C for 90, 120, and 150 min produces templates with nominal pore diameters of 30, 60, and 90 nm, respectively. With variation of the pore diameter in the starting porous anodic alumina films, the size of the SAM features can be controlled. After pore widening, the free-standing template supported with protective layers was mounted onto a SiO2/Au/Si substrate that was made by thermal evaporation of ∼5 nm Cr (as an adhesion layer) and ∼15 nm Au on a Si wafer (Si(100), n-type) and followed by deposition of ∼50 nm SiO2 via plasma-enhanced chemical vapor deposition. The protective film was subsequently removed by repeated rinsing with acetone. Next the porous alumina film was used as a mask for reactive ion etching (RIE). RIE was performed using an Oxford RIE II Etcher, with 55 sccm CHF3, 5 sccm O2, at 35 mTorr and 160 W, for 5-10 min. The sample was then immersed in 1 mM hexadecanethiol (HDT, Sigma-Aldrich) in ethanol for >24 h. The alumina and SiO2 templates were removed to reveal the patterned HDT by dipping the sample into a 1% HF solution for a few seconds. To create gold nanoparticle arrays using the nanopatterned SAMs as a resist layer, gold etching was performed in a solution of 0.1 M Na2S2O3, 1.0 M KOH, 0.01 M K3Fe(CN)6, and 0.001 M K4Fe(CN)6 in deionized water, with gentle stirring for 5-10 min.7 Characterization. Field emission scanning electron microscopy (FESEM, JSM-6700F) and atomic force microscopy (AFM) were used for characterization of the nanopatterned surfaces. A commercial AFM (Molecular Imaging) with rectangular Si3N4 cantilevers (Olympus, ORC8-PS-W) with spring constants of 0.05 N/m were used to image the samples under ambient conditions. Lateral force microscopy (LFM) in the AFM was used to image the SAM features and contact mode AFM was used to image the nanoparticle arrays.

3. Results and Discussion As shown in Figure 1, we have used two approaches to create nanopatterned SAMs on surfaces. Each approach has advantages for particular substrates. For our template-based patterning process, it is critical that the template layer is in good contact with the underlying substrate in order to ensure that the SAM layer deposits only in the regions defined by the pores of the template. If the template is not in good contact with the underlying substrate, then the SAMs can spread underneath the template to create features that are larger than the pore size. In the development of this patterning approach, we first used the approach to pattern silianes on silicon, as shown in Figure 1a. When we attempted to use the same approach to pattern thiols on gold, we found that the contact between the anodized alumina template and gold surface was not good enough to enable direct transfer of the pore structure to the nanopatterned SAMs. We observed spreading of

Gao et al.

Figure 2. (a) SEM image (oblique view) of an alumina template on a silicon substrate with micrometer-sized domains of hexagonally ordered pores. (b) LFM image of OTMS features (bright) on a silicon substrate created by deposition into the template in (a). (c) Topography image of nanostructured silicon surface created by etching the silicon using the nanopatterned SAM in (b) as a resist; the inset shows a line scan across the ∼4-5 nm tall features.

the thiols under the template, resulting in larger, and in some cases, interconnected SAM features. To avoid this problem, we developed the approach shown in Figure 1b, which allows us to use a SiO2 layer as a transfer layer between the alumina template and the substrate. The sputtered SiO2 layer remains in good contact with the Au film after etching and during SAM deposition. These two approaches may be used to create a variety of silanebased SAMs on silicon, as well as thiol-based SAMs on gold, and it should be possible to adapt these general approaches to a variety of other surfaces as well. The first approach, as outlined in Figure 1a, has been used to create patterned silanes on silicon substrates. A scanning electron microscopy (SEM) image of an alumina template on a silicon substrate is shown in Figure 2a. The fabrication of nanoporous alumina films on substrates has been described previously.38 In these films, the pores are arrayed in micrometer-sized domains with hexagonal ordering due to an imprinting step prior to anodization.39 From the oblique view of Figure 2a, it is possible to see that the pores are open to the substrate. This is achieved by a short chemical etch to remove the thin alumina barrier layer (BL) that remains at the base of the pores after anodization. We employed these templates to pattern alkylsilanes on the underlying silicon. OTMS was deposited on the Si substrates from solution. The adhesion between the silicon substrate and alumina template is strong enough to enable SAM deposition only in the pores of the template, but weak enough to allow the template to be removed by a simple mechanical tape peel after SAM formation. The template could also be removed using a chemical etch. The lateral force microscopy (LFM) image in Figure 2b shows the chemical contrast between the hydrophobic silane features and the hydrophilic surrounding silicon substrate. The complementary topography image of this sample (not shown) revealed a surface roughness of 1010/cm2. The feature size can be easily controlled by varying the fabrication parameters of the starting alumina template. To generate the nanopatterned SAMs, we have used two complementary approaches for pattern transfer. In the first approach, we use a thin nanoporous alumina film fabricated directly on a silicon substrate as a template to pattern silane-based SAMs. In the second approach, a thin alumina film is used as an etch mask to create a nanoporous silica layer on a gold substrate for nanopatterned thiol deposition. We have also shown that the SAM nanopatterns can be used as resists for fabricating gold and silicon nanoparticle arrays through wet chemical etching of the unpatterned regions. Using tightly spaced SAM features on gold, we have also created gold nanoparticle arrays with gap widths down to 10 nm. This self-organizing template-based SAM patterning method possesses a number of advantages compared with other methods for patterning dense arrays of uniform size features on surfaces. The process is simple and allows for patterning large areas on surfaces simultaneously. In addition, because the template defines the SAM feature size, it is possible to pattern very closely spaced and small features using this approach. Here we have demonstrated the fabrication of features with diameters as small as 30 and 100 nm center-to-center spacing, and by using alumina templates with a higher density of smaller diameter pores,28-30 it may be possible to pattern sub-10 nm features with even higher densities. LA060658B (45) Zhang, H.; Chung, S. W.; Mirkin, C. A. Nano Lett. 2003, 3, 43. (46) He, L.; Musick, M. D.; Nicewarner, S. R.; Salinas, F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem. Soc. 2000, 122, 9071. (47) Felidj, N.; Aubard, J.; Levi, G.; Krenn, J. R.; Salerno, M.; Schider, G.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R. Phys. ReV. B 2002, 65, (48) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (49) Wang, H.; Levin, C. S.; Halas, N. J. J. Am. Chem. Soc. 2005, 127, 14992.