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Nanoscale Patterning of Alkyl Monolayers on Silicon Using the Atomic Force Microscope Jill E. Headrick, Matt Armstrong, Justin Cratty, Stephanie Hammond, Bonnie A. Sheriff, and Cindy L. Berrie* Department of Chemistry, University of Kansas, Lawrence, Kansas 66045-7582 Received July 19, 2004. In Final Form: February 1, 2005 Self-assembled monolayers (SAMs) of 1-alkenes on hydrogen-passivated silicon substrates were successfully patterned on the nanometer scale using an atomic force microscope (AFM) probe tip. Nanoshaving experiments on alkyl monolayers formed on H-Si(111) not only demonstrate the flexibility of this technique but also show that patterning with an AFM probe is a viable method for creating welldefined, nanoscale features in a monolayer matrix in a reproducible and controlled manner. Features of varying depths (2-15 nm) were created in the alkyl monolayers by controlling the applied load and the number of etching scans made at high applied loads. The patterning on these SAM films is compared with the patterning of alkyl siloxane monolayers on silicon and mica.
Introduction In recent years, the drive for technological miniaturization has intensified and triggered significant interest in molecular computing, nanotechnology, and biomimetic engineering. During this time, the emerging field of molecular electronics1 has undergone significant advancement. The development of functioning molecular components, including wires, switches, and memory/storage devices, is a testament to technological growth and the scientific commitment to nanosystems and molecular manufacturing.2 The desire to integrate molecular electronics with traditional silicon-based technology has spurred the development of surface patterning techniques capable of creating nanometer-sized domains that can serve as active binding sites for the specific arrangement and immobilization3 of molecular-scale components and biomolecules.4,5 Nanofunctionalized surfaces and patterned ultrathin films have potential use in moleculebased devices, molecular computing and electronics, sensor technology, biomaterials research, and many other systems and disciplines. Many patterning techniques use self-assembled monolayer (SAM) films as resists for pattern creation.6 SAMs have numerous practical and fundamental applications because of their stability, flexibility, and highly ordered nature.7 In recent years, SAM systems have been used as passivating layers on electrodes,8,9 antistiction coatings for micromechanical devices,10 and active supports for fabricating chemical and biosensor arrays.11 In addition, (1) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 56, 43-49. (2) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804. (3) Williams, R. A.; Blanch, H. W. Biosens. Bioelectron. 1994, 9, 159167. (4) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848. (5) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189-201. (6) Ulman, A. An Introduction to Ultrathin Films From Langmuir Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (7) Poirier, G. E. Chem. Rev. 1997, 97, 1117-1127. (8) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147-7173. (9) Finklea, H. O. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; Wiley: New York, 2000; Vol. 11, pp 10090-10115. (10) Maboudian, R.; Howe, R. T. J. Vac. Sci. Technol., B 1997, 15, 1-20.
they serve as interfaces for studying boundary lubrication (tribology),12-14 biomolecule-surface interactions,15-18 monolayer adsorption kinetics,19,20 and the effect of spatial confinement on morphology, reactivity, and conductivity.6,19-27 Numerous monolayer-substrate combinations have been investigated and reported in the literature,6 including alkanethiols on gold, alkylsilanes on silicon, glass, and mica, and phosphonic28 and carboxylic acids29 on metal oxide substrates. Many different methods have been developed and used to pattern these SAM films, including electron beam lithography, photolithography, and microcontact printing. These and other techniques have been discussed in recent reviews.30,31 Several other (11) Metzger, S. W.; Natesan, M. J. Vac. Sci. Technol., A 1999, 17, 2623-2628. (12) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235-237. (13) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163-1194. (14) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880-2883. (15) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383-4385. (16) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. Langmuir 2001, 17, 5605-5620. (17) Chapman, R. G.; Ostuni, E.; Yan, L.; Whitesides, G. M. Langmuir 2000, 16, 6927-6936. (18) Sigal, G. B.; Mrksich, M.; Whitesides, G. M. Langmuir 1997, 13, 2749-2755. (19) Xu, S.; Laibinis, P. E.; Liu, G. Y. J. Am. Chem. Soc. 1998, 120, 9356-9361. (20) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457-466. (21) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (22) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122-8127. (23) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017-8021. (24) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721-2732. (25) Sek, S.; Misicka, A.; Bilewicz, R. J. Phys. Chem. B 2000, 104, 5399-5402. (26) Ishida, T.; Mizutani, W.; Aya, Y.; Ogiso, H.; Sasaki, S.; Tokumoto, H. J. Phys. Chem. B 2002, 106, 5886-5892. (27) Ishida, T.; Mizutani, W.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 2000, 104, 11680-11688. (28) Goetting, L. B.; Deng, T.; Whitesides, G. M. Langmuir 1999, 15, 1182-1191. (29) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813-824. (30) Gates, B. D.; Xu, Q.; Love, J. C.; Wolfe, D. B.; Whitesides, G. M. Annu. Rev. Mater. Res. 2004, 34, 339-372.
10.1021/la0481905 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/25/2005
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patterning techniques rely on scanning probe technology.32 Examples include dip-pen nanolithography,33,34 nanoshaving and nanografting,20,35 local anodic surface oxidation,36 STM-based lithography,37 and a variety of other nanoelectrochemical and chemomechanical methods.32,38-41 These scanning probe lithography techniques are similar in that they all take advantage of the nanometer-sized dimensions of the probe tip to fabricate nanoscale structures. Several recent reviews focus on the many different scanning probe lithography methods.20,32,42,43 Some of these methods are intrinsically sensitive to or dependent on the electrical and chemical properties of the underlying substrate. Unfortunately, this reliance restricts the substrates that can be successfully patterned. Patterning techniques that allow significant flexibility in the patterning conditions and the type of substrates that can be used are particularly desirable for applications such as nanoelectronics and biotechnology.44 This work investigates the flexibility of the recently developed nanoshaving method.45 In this technique, SAM molecules are mechanically displaced from the underlying substrate by scanning over the film with an AFM probe at an applied load greater than the displacement threshold. This technique has been used previously to measure the thickness of a DNA film under buffer on a gold substrate.46 This creates a negative pattern in the SAM matrix and exposes new reactive sites for subsequent functionalization. Very small features (18.2 MΩ‚cm and pH ) 6.0) was used during sample preparation and in the contact angle measurements. Type V-1 ruby muscovite mica was purchased from Lawrence and Company (New York). The single-sidepolished silicon samples (Virginia Semiconductor) used in this experiment were cut from phosphorus-doped Si(100) wafers (resistivity ) 10-18 Ω‚cm) and boron-doped Si(111) wafers (resistivity ) 3.0-6.0 Ω‚cm). Oxide-sharpened Si3N4 probes (NPS) from Veeco were used to pattern the OTS monolayers on mica. These probes have V-shaped cantilevers, relatively low force constants (k ) 0.6 N/m), and tips with a 5-40 nm radius of curvature. Stiff (k ) 4.5 and 14 N/m) NSC12 silicon probes from MikroMasch, which have rectangular beam cantilevers and tips with a radius of curvature
