Accelerating the Kinetics of Thiol Self-Assembly on GoldsA Spatial Confinement Effect Song Xu, † Paul E. Laibinis, ‡ and Gang-yu Liu* ,† Contribution from the Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 ReceiVed June 3, 1998 Abstract: The adsorption of alkanethiols onto gold surfaces to form self-assembled monolayers (SAMs) occurs more than 10 times faster in a spatially confined environment than on unconfined bare substrates, and the adsorbed layers exhibit higher coverage and two-dimensional crystallinity. The spatially constrained reaction environment is prepared with use of an atomic force microscope tip to displace thiols within a previously formed SAM. During the displacement, the thiol molecules present in the solution above the SAM rapidly assemble onto the exposed nanometer-size gold area that is confined by the scanning tip and surrounding SAM. The accelerated rate is attributed to a change in the pathway for the self-assembly process as the spatial confinement makes it geometrically more probable and energetically more favorable for the initially adsorbed thiols to adopt a standing-up configuration directly in this microenvironment. In contrast, thiols that self-assemble onto gold surfaces in an unconstrained environment initially form a lying-down phase, which subsequently degrades and forms a standing-up phase. Our observations suggest that spatial confinement can provide an effective means to change the mechanism and kinetics of certain surface reactions by sterically preventing alternative reaction pathways and stabilizing particular transition states or reaction intermediates. In addition, the results underlie the development of a new method (“nanografting”) for patterning SAMs laterally with nanometer-level precision. Introduction Self-assembled monolayers (SAMs) offer many promising applications in the developments of boundary lubricants, anticorrosion coatings, and recently the microfabrication process because they can be laterally patterned and then used as resists for pattern transfer. 1-3 To be effective as a resist, the SAM should be stable and contain few defects. A popular system for these applications has been SAMs derived from the adsorption of alkanethiols onto metals such as gold, silver, and copper. 4 These thiol-derived SAMs contain ordered domains that are separated by boundaries (areas of lower surface coverage) and various defects, 1,5,6 whose size and distribution depend on the interplay of kinetic and thermodynamic factors during the growth of the SAM. Empirically, a thiol-derived SAM with a large domain size and low defect density can be prepared by contacting a gold surface with a dilute solution of the thiol for at least 24 h. 1,4,7 A molecular level understanding of the self-assembly process and kinetics is of fundamental importance for improving the quality and usefulness of SAMs. As shown in recent studies using scanning tunneling micros- copy (STM) 8 and helium and X-ray diffraction, 9 the self- assembly of these molecules from the vapor phase follows two major steps. First, the thiol molecules adsorb on gold and form a lattice-gas or mobile phase that gradually evolves into crystalline islands with the molecules oriented parallel to the gold surface. 8,9 At the saturation coverage of this lying-down phase, a solid-to-solid-phase transition occurs to produce islands of molecules in a standing-up configuration. 8 Using low-energy helium diffraction, Schwartz et al. revealed that the low-density lying-down monolayer degrades into a disordered state, from which the standing-up phase is formed. 9 Using an ultrahigh vacuum STM, Poirier and Pylant provided a molecular-level in situ picture at each step of the self-assembly process for thiols onto gold from the vapor phase. 8 In practice, most SAMs are prepared from a solution phase. 1,4 Under these conditions, * To whom correspondence should be addressed. † Wayne State University. ‡ Massachusetts Institute of Technology. (1) Ulman, A. An Introduction to Ultrathin Organic FilmssFrom Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (2) (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. (c) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (3) (a) Kumar, A.; Whitesides, G. M. Science 1994, 263, 60. (b) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274. (c) Jeon, N. L.; Nuzzo, R. G.; Xia, Y.; Mrksich, M.; Whitesides, G. M. Langmuir 1995, 11, 3024. (4) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 448. (b) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (5) (a) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (b) Poirier, G. E.; Tarlov, M. J.; Rushmeierf, H. E.; Langmuir 1994, 10, 3383. (c) Sondag-Huthorst, J. A. M.; Scho ¨nenberger, C.; Fokkink, L. G. J. Phys. Chem. 1994, 98, 6826. (d) McDermott, C. A.; McDermott, M. T.; Green, J. B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (e) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (6) (a) Camillone, N. C.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (b) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 4234. (c) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys. 1997, 106, 1600. (7) (a) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315. (b) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (8) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (9) (a) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (b) Schwartz, P.; Schreiber, F.; Eisenberger, P.; Scoles, G. Phys. ReV.B 1998, preprint. 9356 J. Am. Chem. Soc. 1998, 120, 9356-9361 S0002-7863(98)01938-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/20/1998