In Situ, Vibrationally Resonant Sum Frequency Spectroscopy Study of the Self-Assembly of Dioctadecyl Disulfide on Gold Clayton S.-C. Yang, Lee J. Richter,* John C. Stephenson, and Kimberly A. Briggman National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899-8372 Received March 26, 2002. In Final Form: May 24, 2002 We report the results of an in situ, vibrationally resonant sum frequency generation (SFG) spectroscopy study of the assembly of perdeuterated dioctadecyl disulfide on gold substrates from ethanol solutions under laminar flow conditions. The coverage evolution of the SFG spectra can be well described by the coexistence of two distinct phases: a low-coverage, disordered phase and the full-coverage crystalline phase. The structure of the adsorbed thiolate fragments in the low-coverage phase is disordered but upright (as opposed to lying completely in the surface plane), characterized by significant gauche defects in the backbone but a near-normal orientation for the terminal methyl group. The crystalline phase is marked by an erect all-trans configuration of the alkane chain. The kinetics of the evolution of these two phases can be quantitatively described by a simple model, consistent with phase coexistence above a critical density of the disordered phase. I. Introduction Self-assembled monolayers (SAMs) are ordered molec- ular assemblies formed spontaneously by the adsorption of a molecule with a certain affinity of its headgroup to the substrate. Among SAMs, the assembly of organosulfur compounds, especially thiols (R-S-H) and disulfides (R-S-S-R), on coinage metals has received considerable attention. 1-4 These SAMs are excellent model systems for studies of monolayer structure, 5-7 adhesion, 8 and inter- facial reactions 9-11 and have novel applications as bio- sensors 12,13 and corrosion inhibitors. 14 The adsorption of alkanethiol molecules on Au from both solution and the gas phase has been extensively studied and recently reviewed. 15,16 There has been much less attention paid to the adsorption of dialkyl disulfides. 17-20 It is well estab- lished that the disulfides and thiols result in indistin- guishable monolayers, composed of erect gold thiolate (RS - ) species. 17,18 However, there have been very few stu- dies of the evolution of the surface species with coverage. Surprisingly, even for the extensively studied al- kanethiolates, there have been few vibrational spectros- copy studies of the assembly that probe the full coverage range and no in situ kinetic vibrational spectroscopy studies. Early ex situ infrared reflection-absorption spectroscopy (IRAS) and vibrationally resonant sum frequency generation (SFG) studies focused on the struc- ture of the completed films. 21-23 More recently, ex situ studies have been performed on partial monolayers removed from solution and rinsed to remove loosely attached adsorbate molecules. 24-26 However, changes of the film coverage and structure can occur in an ex situ measurement. 27 Therefore, in situ studies under actual deposition conditions in real time are preferable. Second-order nonlinear optical techniques are particu- larly well suited for probing chemical reactions at buried interfaces due to their high surface selectivity, submono- layer sensitivity, and excellent spatial, spectral, and temporal resolution. 28 Vibrationally resonant SFG, in which a vibrationally resonant IR photon is mixed with a visible photon in a second-order process, has the additional advantage of being able to probe the vibrational spectra of buried interfaces and thus provide insight into alkane chain order and orientation. 29 In this paper, we report an in situ SFG study of the assembly of SAMs from * Corresponding author. E-mail: lee.richter@nist.gov. (1) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (3) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (4) Ulman, A. 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Soc., Faraday Trans. 1995, 91, 1281. 10.1021/la0257790 This article not subject to U.S. Copyright. Published xxxx by the American Chemical Society PAGE EST: 7.2 Published on Web 00/00/0000