Adsorption Profiles of Chelating Aromatic Dithiols and Disulfides: Comparison to Those of Normal Alkanethiols and Disulfides Nupur Garg, Jonathan M. Friedman, and T. Randall Lee* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received August 16, 1999. In Final Form: January 24, 2000 This study provides a comparison of the rates of adsorption of the following thiols onto the surface of gold: 1,2-bis(mercaptomethyl)-4,5-dihexadecylbenzene (1), 1-mercaptomethyl-3,4-dihexadecyl-benzene (2), hexadecanethiol (4a), and and eicosanethiol (4b). This study also compares the rates of adsorption of these adsorbates to those obtained for the aromatic disulfide analogue of 1 (2,3-dithia-6,7-dihexadecyltetralin, 3) and the normal dialkyl disulfide analogues of 4a and 4b (hexadecyl disulfide, 5a, and eicosyl disulfide, 5b, respectively). The adsorption behavior was monitored using ex situ ellipsometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The adsorption profiles suggest that monolayer formation proceeds via two distinct kinetic regimes: (1) a fast initial adsorption, where ca. 80-90% of the monolayer forms during the first few minutes of immersion, followed by (2) a slower orientational ordering lasting several hours. Comparison of the rates of adsorption of the aromatic dithiols 1 to those of aromatic monothiol 2 and normal alkanethiols 4a and 4b reveals that the structure of the adsorbate plays a substantial role during the initial stages of thiol adsorption. The impact of structural and/or chemical variations is further illustrated by comparing the poor quality of the monolayer generated from the aromatic disulfide 3 to those of monolayers generated from 1, 2, 4, and 5. Introduction Self-assembled monolayers (SAMs) of alkanethiolates are formed by the spontaneous adsorption of organosulfur compounds onto the surface of metals such as Au, Ag, Pt, and Cu. 1-3 Gold is typically the metal of choice because it forms no oxide coating and is inert to most common contaminants. The structural features of alkanethiolate SAMs on gold have been characterized extensively using a wide variety of techniques. 2 The intrinsic mechanisms of film formation, however, remain poorly understood. It is commonly believed that SAMs derived from either thiols or disulfides adsorb onto the surface of gold as alkanethiol- ates. 4-6 Other studies have found, however, that the adsorbates exist as dimers (i.e., as disulfides) on the surface. 7,8 To establish the optimum conditions for SAM formation, our research seeks to probe the nature of the adsorption process by varying the structures and binding properties of the adsorbates. Previous studies of the adsorption process have utilized a variety of tools including spectral ellipsometry, 9 contact angle goniometry, 9 reflection/absorption infrared spec- troscopy (RAIRS), 10 scanning tunneling microscopy (STM), 11-13 surface plasmon resonance spectroscopy (SPRS), 14,15 quartz crystal microbalance (QCM), 16,17 second harmonic generation (SHG), 18 and near-edge X-ray ab- sorption fine structure (NEXAFS). 19 The consensus opinion favors a two-regime kinetic model for film formation: a fast initial adsorption regime, where 80-90% of the monolayer is formed, followed by a slow adsorption regime, where the monolayer undergoes orientational ordering to achieve complete film formation. While most studies favor the two-regime model, the relative duration of the adsorption regimes remains controversial. Other studies, such as those by DeBono et al., 15 suggest three distinct kinetic adsorption regimes, while the studies by Bucher et al. 11 and Sondag-Huethorst et al. 12 favor a single adsorption regime. These differences can plausibly arise from several factors including differences in (1) the purities, concentrations, and/or chain lengths of the adsorbates, (2) the nature of the adsorption medium, and (3) the quality of the gold substrate. While examining the kinetics of adsorption of normal alkanethiols on gold, previous studies have also examined the corresponding adsorption of dialkyl disulfides. 9,20 While the quality of the films generated from normal alkanethiols and their corresponding disulfides are largely indistin- guishable, competitive adsorption studies have revealed that the alkanethiols adsorb more rapidly from solution than their corresponding dialkyl disulfides. This difference * To whom correspondence should be addressed. E-mail: trlee@uh.edu. (1) Whitesides, G. M. Sci. Am. 1995, 9, 146. (2) Ulman, A. An Introduction to Ultrathin Organic Films; Aca- demic: Boston, 1991. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1993, 105, 4481. (5) Jung, Ch.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103. (6) Caster, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (7) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1995, 117, 12528. (8) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 412/413, 213. (9) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (10) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. Langmuir 1998, 14, 2361. (11) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1997, 13, 5335. (12) Sondag-Huethorst, J. A. M.; Schonenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826. (13) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 1941. (14) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (15) DeBono, R. F.; Loucks, G. D.; Manna, D. D.; Krull, U. J. Can. J. Chem. 1996, 74, 677. (16) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 979. (17) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469. (18) Buck, M.; Grunze, M.; Eisert, F.; Fisher, J.; Trager, F. J. Vac. Sci. Technol. A 1992, 10, 926. (19) Ha ¨ nher, G.; Wo ¨ll, C.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955. (20) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. 4266 Langmuir 2000, 16, 4266-4271 10.1021/la991100p CCC: $19.00 © 2000 American Chemical Society Published on Web 04/07/2000