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Patterning Surfaces Using Tip-Directed Displacement and Self-Assembly Nabil A. Amro, Song Xu, and Gang-yu Liu* Department of Chemistry, Wayne State University, Detroit, Michigan 48202 Received January 19, 2000 An atomic force microscopy (AFM)-based procedure, a nanopen reader and writer (NPRW), is developed to produce nanometer scale patterns on surfaces. In each NPRW experiment, a self-assembled monolayer serves as the resist, while an AFM tip displaces the resist molecules from desired locations by using a high shear force. The AFM tip is precoated with adsorbate molecules which can adsorb to the newly exposed substrate areas. This procedure combines the advantages of two recently developed AFM lithography methods: nanografting (Xu, S.; Liu, G. Y. Langmuir 1997, 13, 127) and dip-pen nanolithography (Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661). NPRW is easy to complete and able to produce desired patterns with multiple components. In addition, NPRW can reach high spatial resolution, independent of the texture of the paper and the humidity of the environment. The size and geometry of these nanopatterns are well preserved because the resist molecules efficiently prevent diffusion and smearing.
Patterning of self-assembled monolayers (SAMs) with nanometer precision has attracted tremendous attention.1,2 The resulting nanostructures exhibit promising applications in the development of nanometer-scale electronic devices, biochips, and sensors.1,3 While microfabrication techniques such as photolithography,4 microcontact printing,2,5-7 micromachining,5 and microwriting5,6 can produce patterns as small as 0.1 µm, production of sub-100-nm structures still poses a significant challenge. At present, such high-resolution fabrication can be achieved using scanning probe lithography (SPL).8 The key to reach high spatial precision is to selectively break chemical bonds or direct chemical reactions using local tip-surface interactions during fabrication.8 Various successful approaches in controlling the tip-surface interactions have been reported. These methods include atomic force microscopy (AFM)-based lithography such as tip-catalyzed surface reactions,9 tip-induced displacement (or shaving),10 and scanning tunneling microscopy (1) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (b) Bishop, A.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127. (2) Jeon, N. L.; Lin, N.; Erhardt, M. K.; Girolami, G. S.; Nuzzo, R. G. Langmuir 1997, 13, 3833. (3) (a) Jones, V. W.; Kenseth, J. R.; Porter, M. D.; Mosher, C. L, Henderson, E. Anal. Chem. 1998, 70, 1233. (b) Tender, L. M.; Worley. R. L.; Fan, H.; Lopez, G. P. Langmuir 1996, 12, 5515. (c) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580. (d) Bamdad, C. Biophys. J. 1998, 75, 1989. (e) Proudnikov, D.; Timofeev, E.; Mirzabelov, A. Anal. Biochem. 1998, 259, 34-41. (4) (a) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395. (b) Huang, J. Y.; Dahlgren, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626. (5) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 8, 219. (6) (a) Abbott, N. L.; Kumar, A.; Whitesides, G. M. Chem. Mater. 1994, 6, 596. (b) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380. (7) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274. (8) (a) Nyffenegger, R. M.; Penner, R. M. Chem. Rev. 1997, 4, 11195. (b) Ho, W. Acc. Chem. Res. 1998, 31, 567. (c) Liu, G. Y.; Xu, S.; Qian, Y. Submitted for publication in Acc. Chem. Res. (9) Muller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; McEuen, P. L.; Schultz, P. G. Science 1995, 268, 272. (10) (a) Liu, G.-Y.; Xu, S. In New Directions in Materials Synthesis; Winter, C. H., Hoffman, D., Eds.; ACS Symposium Series 727; American Chemical Society: Washington, DC, 1998; p 199. (b) Xiao, X.-D.; Liu, G.-Y.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (c) Jung, T. A.; Moser, A.; Hug, H. J.; Brodbeck, D.; Hofer, R.; Hidber, H. R.; Schwartzm U. D. Ultramicroscopy 1992, 42-44, 1446.
(STM)-based lithography such as tip-assisted electrochemical etching, oxidation, and field-induced desorption.11,12 In this Letter, we report a new AFM-based lithographic method, which is built on two complementary techniques: nanografting13 and dip-pen nanolithography (DPN).14 The principal procedure of our approach is illustrated in Figure 1. A thiol SAM on gold is used as the resist, while an AFM tip is used as a shaver to displace thiols from desired locations under a high force (e.g., 5-10 nN). The tip is precoated with a different adsorbate, normally another thiol. As the tip displaces the matrix molecules, new thiols on the tip adsorb onto the freshly exposed gold substrate following the shaving track of the tip. The resulting patterns can then be characterized under a reduced load (0.05-5 nN). From its principle, this new approach can be considered as nanografting with a coated tip, or DPN using SAMs instead of polycrystalline gold surfaces as the paper. Hereafter, we refer this method as “nanopen reader and writer (NPRW)” as it can accomplish nanofabrication (writing) as well as characterization (reading) in situ and with the same AFM tip (pen). NPRW is developed to combine the advantages of both nanografting13 and DPN.14 Similar to nanografting, the spatial precision of NPRW is determined by the intrinsic stability of the AFM and the tip-substrate contact area during fabrication. The tip-substrate contact depends on the force exerted during fabrication and the sharpness of the tip. Therefore, unlike DPN, the resolution of NPRW is independent of the texture of the paper and the humidity of the environment. In addition, NPRW is easier to perform and expands the medium of nanografting from solution phase to both solution and ambient environments. In nanografting, fresh (11) (a) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (b) Schoer, J. K.; Zamborini, F. P.; Crooks, R. M. J. Phys. Chem. 1996, 100, 11086. (c) Chen, J.; Reed, M. A.; Asplund, C. L.; Cassell, A. M.; Myrich, M. L.; Rawlett, A. M.; Tour, J. M.; Van Patten, P. G. Appl. Phys. Lett. 1999, 75, 624. (d) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073. (12) Bard, A. J.; Denuault, G.; Lee, C. M.; Mandler, D.; Wipf, D. O. Acc. Chem. Res. 1990, 23, 357. (13) (a) Xu, S.; Liu, G. Y. Langmuir 1997, 13, 127. (b) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G. Y. Langmuir 1999, 15, 7244. (14) (a) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (b) Hong, S.; Zhu, J,; Mirkin, C. A. Science 1999, 286, 523.
10.1021/la000079l CCC: $19.00 © 2000 American Chemical Society Published on Web 03/08/2000
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Figure 1. Schematic diagram of NPRW illustrating the three basic steps to produce and characterize a pattern under ambient laboratory conditions.
solvent must be injected after fabrication to prevent exchange reactions,13 whereas with NPRW, the exchange reaction is effectively prevented because the imaging medium does not contain adsorbate molecules. As a proof-of-concept experiment, nanostructures of various thiols were produced under ambient laboratory conditions. In the examples shown in Figure 2, the resist was a CH3(CH2)9S/Au(111) SAM (abbreviated as C10S/ Au). The tip, made of Si3N4 (ThermoMicroscope, sharpened microlever, force constant 0.1 N/m), was first coated with CH3(CH2)17SH(C18SH) by soaking in a saturated C18SH (2-butanol) solution for 15 min and then allowed to dry under a gentle flux of N2 for 30 min. The gold film was prepared at high temperature (350 °C) and thus had a relatively flat morphology containing plateau areas of Au(111) separated by single atomic steps. Within the 200 × 200 nm2 pattern of C18S shown in Figure 2A, three single atomic Au(111) steps are clearly visible. The C18S pattern is 8.3 Å higher than the surrounding C10S monolayer and thus appears brighter in the topographic image shown in Figure 2A. Zooming into any areas within the pattern or matrix, the (x3×x3)R30° periodicity15 can be resolved (see Figure 2B). Other adsorbates, such as a fluorinated thiol, CF3(CF2)11(CH2)2SH(CF12C2SH), were used to test the general applicability of NPRW. In Figure 2C, a 100 × 120 nm2 rectangular pattern of CF12C2S was produced within the C10S/Au matrix. This pattern is 7.7 ( 0.9 Å (15) (a) Fenter, P.; Eberhardt, A.; Eisenberg, P. Science 1994, 266, 1216. (b) Poirier, G. E.; Taylor, M. J. Langmuir 1994, 10, 2853. (c) Camillone, N., III; Chidsey, C. E. D.; Liu, G.-Y.; Putvinski, T. M.; Scoles, G. J. Chem. Phys. 1991, 94 (12), 8493.
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taller than the surrounding layer. A high-resolution image of the CF12C2S pattern is shown in Figure 2D, revealing a hexagonal lattice with a lattice constant of 5.6 ( 0.2 Å, larger than that of corresponding n-alkanethiols. Together the height and periodicity measurements compare well with the known structure of fluorinated alkanethiol SAMs.16 The coated tip lasted throughout the experiment (more than 5 h) without becoming dull or running out of ink. Figure 2E shows a line of 100 nm in length produced by a single scan at the end of the experiment. The width of the line is 8.9 nm measured from the full width at halfmaximum (fwhm) of the corresponding cursor profile in Figure 2F.17 In addition to methyl- and fluoromethylterminated patterns, NPRW has been used to produce nanostructures with various functionalities including -CHO, -COOH, -SH, -OH, etc., which demonstrates the generality of this approach and provides opportunities to build complex architectures using these patterns. The thiols within the patterns are ordered and closely packed as shown in the high-resolution images in Figure 2. These nanostructures have a very low density of defects and no observable impurities. In addition, the selfassembly occurs following a very fast kinetics (faster than the scanning speed of 40 ms per line). Following the example of our systematic study in nanografting experiments,18 these observations are attributed to the change in reaction pathway due to a spatially confined microenvironment, i.e., the newly exposed gold surface is confined by the AFM tip and the surrounding thiols.18 Since thiols are located on the tip, the adsorption is further accelerated because the reactant is delivered to the surface by the tip. As a result of the spatial constraint and the high local density of thiols, the self-assembly process in NPRW follows a pathway similar to nanografting,18 which differs from the unconstrained self-assembly. The fast kinetics in NPRW allows fast fabrication and the use of a wide range of scanning speeds because the adsorption process is faster than the scan. Multiple-component fabrications can be completed using NPRW. As successfully demonstrated in DPN, surface registry could be maintained by using landmarks fabricated in the first step.14 Since NPRW may be considered as DPN with a SAM as the paper, it can maintain the surface registry and produce multiple ink patterns following the same approach as DPN.14 Alternatively, multicomponent patterns may be produced by combining NPRW and nanografting. In the example shown in Figure 3, a rectangular C18S pattern (200 × 200 nm2) was first produced in air using NPRW. A 2-butanol solution containing 0.1 mM HOOCC15SH was then injected into the sample container. A new pattern of HOOCC15S, 150 × 130 nm2, was produced next to the C18S pattern using nanografting. Both patterns can be visualized clearly from the topographic as well as frictional force images in panels A and B of Figure 3, respectively. The image contrast in NPRW experiments is sharper than similar images acquired without coating the tips. This observation is contrary to the speculation that a coated tip may compromise the AFM resolution because the tip becomes duller and more hairy than the corre(16) (a) Liu, G. Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberg, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301. (b) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (17) Because of tip convolution, the actual line width is narrower than the measured value from the fwhm in Figure 2F. For more detailed discussions, see: (a) Ramirez-Agilar, K. A.; Rowlen, K. L. Langmuir 1998, 14, 2562. (b) Markiewiz, P.; Goh, C. Rev. Sci. Instrum. 1995, 66, 3186. (18) Xu, S.; Laibinis, P. E.; Liu, G. Y. J. Am. Chem. Soc. 1998, 120, 9356.
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Figure 2. Three nanopatterns produced and imaged in air within a C10S/Au SAM using NPRW: (A) a 270 × 270 nm2 topographic image of C10S/Au SAM containing a 200 × 200 nm2 square of a C18S pattern; (B) a high-resolution 22 × 22 nm2 image of the matrix C10S as indicated in (A); (C) a 170 × 160 nm2 topographic image of a C10S/Au SAM containing a 100 × 120 nm2 rectangular island of CF12C2S; (D) a high-resolution 10 × 10 nm2 image of the fluorinated thiol area as indicated in (C); (E) a 10 × 100 nm2 line of CF12C2S produced within the C10S/Au; (F) the corresponding cursor profile as indicated in (E).
Figure 3. An example of two-color fabrication. 200 × 200 nm2 C18S and 150 × 130 nm2 HOOCC15S patterns were produced within a C10S/Au matrix via NPRW and nanografting, respectively. (A) and (B) are a 670 × 690 nm2 topographic and the corresponding frictional force image acquired in 2-butanol.
sponding uncoated tip. Etch pits, single atomic Au(111) steps, and the periodicity of the thiol lattices can be observed routinely in air (e.g., parts A and B of Figure 2). In Figure 2B, the periodicity and domain boundaries were both visible in a relatively large scan, 220 × 220 Å2. The improved resolution may be attributed to a strong tipsurface interaction as a result of coating tips with a
functionality similar to the surface. Such strong interactions enhance the surface corrugation of the SAMs detected by the AFM tip and thus sharpens the image contrast. As demonstrated in this and our previous studies,13 AFM can reach true molecular resolution. High-resolution AFM images of these nanopatterns show no presence of impurities. We attribute this observation to the high local
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concentration of thiols on the tip and the fact that matrix thiols were displaced and removed from the adsorption sites efficiently during the fabrication process. We have presented a preliminary study of a new nanofabrication method, NPRW. The principle of NPRW enables us to combine the advantages of DPN and nanografting. Similar to DPN, NPRW is easy to use and able to produce patterns with multiple components under ambient laboratory conditions. Similar to nanografting, NPRW does not require changing tips, and the fabrication can reach high spatial resolution regardless of the texture of the substrate and the imaging medium. The size and geometry of these nanopatterns are well preserved because the surrounding matrix molecules efficiently prevent diffusion and smearing. The spatial resolution of AFM images is improved because the tips are functionalized with molecules similar to the surface molecules. We hope this initial study will attract more researchers to explore (19) (a) Jordan, J. S.; Cruchon-Dupeyrat, S. J.; Huang, Y.; Kuo, P. K.; Liu, G. Y. Langmuir 1999, 15, 6495. (b) Overney, R. M.; Meyer, E.; Frommer, J.; Guntherodt, H. J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 10, 1281.
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the capability of NPRW to produce more complicated nanoarchitectures. In addition, the potential applications in prototypical nanoelectronic devices and in the development of biosensors and biochips need to be explored. The NPRW procedure itself provides an opportunity to study fabrication mechanisms and the fundamental aspects of tip-surface interactions. In addition, the resulting nanostructures allow researchers to investigate the physical properties of the materials19 and chemical reations13 at the nanometer level. Acknowledgment. We thank Professor Paul Laibinis at MIT and Jayne Garno and Sylvain Cruchon-Dupeyrat at WSU for many helpful discussions. G.Y.L. gratefully acknowledges the Camille and Henry Dreyfus Foundation for a New Faculty Award and the Arnold and Mabel Beckman Foundation for a Young Investigator Award. This work was also supported by National Science Foundation (CHE-9733400 and IGERT-970952), the ACSPetroleum Research Fund (PRF-AC), and the Whitaker Foundation (Biomedical Engineering Grant). LA000079L
