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The Influence of Headgroup on the Structure of Self-Assembled Monolayers As Viewed by Scanning Tunneling Microscopy Christopher B. Gorman,* Yufan He, and Richard L. Carroll Department of Chemistry, North Carolina State University, Box 8204, Raleigh, North Carolina 27695-8204 Received October 3, 2000. In Final Form: April 24, 2001 Molecular resolution scanning tunneling microscopy images are shown of alkanethiol self-assembled monolayers (SAMs) containing alkene, cyano, and carboxylic acid headgroups. The alkene-terminated thiolate SAM displayed a lattice structure indistinguishable from those of methyl-terminated thiol. The cyano-terminated thiolate SAM showed hexagonal, square, and double-row lattice structures, indicative of polymorphism and/or multiple chemical species on the surface. The carboxylic acid-terminated thiolate SAM showed a double-row lattice structure very similar to that of the cyano-terminated thiolate SAM suggesting that perhaps the cyano groups had undergone hydrolysis to form amide and/or carboxylic acid groups. This hypothesis was supported by the results of a friction-force microscopy experiment on microcontact printed patterns of these molecules.
Introduction Since the advent of scanning probe microscopy techniques, a large number of investigations have been conducted to illustrate the structure of n-alkanethiolate (e.g., methyl-terminated) self-assembled monolayers (SAMs).1 Comparably little has been reported on the structure of similar SAMs that contain other functional headgroups. It is likely that many kinds of SAMs will have little or no periodic order and will thus not reveal an ordered lattice structure by scanned probe microscopy. This low order is anticipated if the headgroup is sufficiently different in shape (shape mismatched) with the rest of the chain connecting it to the surface. However, early work established that there was apparently a great deal of chain order in SAMs composed of various other small headgroups such as vinyl, cyano, and carboxylic acid.2-4 A SAM that presents functional headgroups in an ordered fashion at a surface can potentially form a basis for chemically well-defined molecular-scale surface science. This element could be of substantial utility in, for example, nanolithography via chemical (rather than physical) writing at potentially the molecular length scale. Any lithographic process that approaches the molecular length scale must necessarily be cognizant of chemical structure. The chemical basis for such lithographic processes has yet to be defined. In this paper, scanning tunneling microscopy images are presented of lattice and superlattice structures on SAMs containing functional headgroupssspecifically alkene, cyano, and carboxylic acid-terminated SAMs. Further, it is shown that, as might be expected, these functional groups are susceptible to chemical reaction and subsequent rearrangement. Specifically, it is suggested that a cyano-terminated SAM may be susceptible to * To whom correspondence should be addressed. Tel: (919)-5154252. Fax: (919)-515-8920. E-mail:
[email protected]. (1) Touzov, I.; Gorman, C. B. J. Phys. Chem. B 1997, 101, 5263-5276 and references therein. (2) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (3) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.
hydrolysis that converts it from a single-row, nonhydrogen-bonded structure to a double-row, hydrogenbonded lattice structure. There is some literature precedent for imaging SAMs with non-methyl headgroups. Several images of adlayers of short chain thiols including single-ring aromatic thiols,5-8 cysteine,9,10 cysteamine,11 and mercapto propionic acid12-14 have been obtained. SAMs composed of a few longer chain thiols including fluorinated chain thiols15,16 and hydroxy-12,17-19 and amine-terminated19,20 thiols have also been imaged. In a few cases, images of SAMs containing larger headgroups such as azobenzene17,21,22 and calixarenes23,24 have been obtained. Here, we focus (5) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101-111. (6) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, M. J. Phys. Chem. B 1998, 102, 5943-5946. (7) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565-3569. (8) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C.H. Langmuir 1997, 13, 4018-4023. (9) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir 1996, 12, 2849-2852. (10) Zhang, J. D.; Chi, Q. J.; Nielsen, J. U.; Friis, E. P.; Andersen, J. E. T.; Ulstrup, J. Langmuir 2000, 16, 7229-7237. (11) Kawasaki, M.; Sato, T.; Yoshimoto, T. Langmuir 2000, 16, 54095417. (12) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073-5078. (13) Giz, M. J.; Duong, B.; Tao, N. J. J. Electroanal. Chem. 1999, 465, 72-79. (14) Sawaguchi, T.; Sato, Y.; Mizutani, F. J. Electroanal. Chem. 2001, 496, 50-60. (15) Scho¨nherr, H.; Vancso, G. J. Langmuir 1997, 13, 3769-3774. (16) Liu, G.-y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301-4306. (17) Takami, T.; Delamarche, E.; Michael, B.; Gerber, C.; Wolf, H.; Ringsdorf, H. Langmuir 1995, 11, 3876-7107. (18) Poirier, G. E.; Pylant, E. D.; White, J. M. J. Chem. Phys. 1996, 105, 2089-2092. (19) Sprik, M.; Delamarche, E.; Michel, B.; Ro¨thlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116-4130. (20) Esplandiu´, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828-838. (21) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michael, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102-7107. (22) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264-3271. (23) Scho¨nherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567-1570.
10.1021/la0013998 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/20/2001
Influence of Headgroup on Self-Assembled Monolayers
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on organothiol SAMs containing methylene chains of 10 carbons or greater. In this case, the methylene chains in the SAM are anticipated to make a contribution to its packing and structure. Experimental Section 11-Mercaptoundecanoic acid, n-dodecanethiol, ethanol, and dodecane were purchased from Aldrich and used as received. 10-Cyanodecanethiol4 and undec-10-ene-1-thiol25 were synthesized according to literature procedures. For the preparation of the SAMs, two Au(111) substrates were employed. For the experiments shown in Figures 1-3, facets on a gold ball were employed and were prepared by a method similar to that reported by Schoer et al.26,27 except that annealing was performed in a hydrogen flame rather than the use of an electrochemical method. The experiment shown in Figure 4 was performed on templatestripped gold by a procedure of Wegner using physical stripping of the mica backing rather than solvent stripping.28 SAMs were prepared by vapor deposition or alternatively by deposition from ca. 5 mM ethanol solutions for 6-12 h. Scanning tunneling microscopy (STM) measurements were performed with mechanically cut Pt-Ir (90:10) tips using a Digital Instruments NanoScope III MultiMode microscope. All experiments were performed under a nitrogen atmosphere that was purged sufficiently to ensure that the humidity was
