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Langmuir 2006, 22, 4170-4178
Spectroscopic Characterization of ω-Substituted Biphenylthiolates on Gold and Their Use as Substrates for “On-Top” Siloxane SAM Formation S. Stoycheva,† M. Himmelhaus,*,† J. Fick,† A. Korniakov,‡ M. Grunze,† and A. Ulman‡,§ Lehrstuhl fu¨r Angewandte Physikalische Chemie, INF 253, 69120 Heidelberg, Germany, and Department of Chemical and Biological Sciences and Engineering, Polytechnic UniVersity, 6 Metrotech Center, Brooklyn, New York 11201 ReceiVed NoVember 17, 2005. In Final Form: February 7, 2006 Self-assembled monolayers (SAMs) of ω-substituted biphenylthiolates (ω-MBP) on gold were characterized by spectral ellipsometry, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and vibrational sum frequency generation spectroscopy (VSFG). The vibrational studies of the SAMs were supported by an ab initio frequency analysis at HF/6-31G and BP86/6-31G levels, yielding an assignment of all relevant spectral features in the range from 3500 to 1200 cm-1. We were able to demonstrate that hydroxy-terminated MBP (HMBP) SAMs are basically featureless in the range of the CH stretching vibrations. Accordingly, the adsorption of a SAM of octadecyltrichlorosilane (OTS) on top of this model surface could be studied. A red shift of the C-O stretching vibration from 1281 to 1264 cm-1 was observed during the chemisorption of OTS, thus allowing for a quantification of the number of OTS molecules involved in surface binding of OTS, which was found to be about 26% on average.
1. Introduction Future progress in nanotechnology relies significantly on the molecular level control of surface properties. Self-assembled monolayers (SAMs) have proven to be an appropriate way for modifying the chemical and physical properties of surfaces with minute precision and in a highly reliable fashion. In particular, since SAMs form spontaneously from the gas or liquid phase on suitable substrates, they are easy to deal with and allow for processing of wide scale areas as required in many fields of state-of-the-art technology. So far, mainly SAMs of aliphatic moieties, such as alkyl siloxanes on metal/semiconductor oxide surfaces or alkanethiolates on the coinage metals have been adapted to applications in nanotechnology and life sciences. However, to control surface properties on molecular scale, these systems show insufficient precision due to their high degree of conformational freedom. In particular, it was demonstrated that ω-functionalized aliphatic moieties which are highly ordered in air may show solventinduced disordering when in contact with the liquid phase.1,2 This limits the applicability of aliphatic SAMs in cases where either the precise lateral spacing of the ω-functionalities or their direct exposure to the ambient is mandatory.3 Studies on the formation of structured water layers in the vicinity of surfaces with well-defined hydrophilicity,4,5 the molecular origin of friction forces,6 and investigations into the fundamentals of self-assembly * Corresponding author. E-mail:
[email protected]. Present address: Fujirebio, Inc., 51 Komiya-cho, Hachioji-shi, Tokyo 192-0031, Japan. † University of Heidelberg. ‡ Polytechnic University. § Present address: Department of Chemistry, Bar-Ilan University, RamatGan 52900, Israel. (1) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (2) Kacker, N.; Kuman, S. K.; Allara, D. L. Langmuir 1997, 13, 6366. (3) Ong, T. H.; Wang, R. N.; Davies, P. B.; Bain, C. D. J. Am. Chem. Soc. 1992, 114, 6243. (4) Israelachvili, J.; Wennerstro¨m, H. Nature 1996, 379, 219. (5) Lum, K.; Chandler, D.; Weeks, J. D. J. Phys. Chem. B 1999, 103, 4570. (6) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607.
on solid surfaces7 are examples for systems with such strict requirements. To overcome such problems, it was suggested to utilize ω-substituted mercapto biphenyls (ω-MBP), whichslike their aliphatic counterpartssform SAMs on the coinage metals via adsorption from solution, however, lack conformational freedom due to their rigid backbone. As has been demonstrated in a number of studies,8,9 those SAMs formed by ω-MBP adapt a regular superstructure on the surface, which is stable in contact with the fluid phase. Despite the thoroughness by which ω-MBP8 and related systems10 have been investigated in terms of formation and structure of the resulting SAM, no direct evidence has been given for the applicability of these films as model substrates for the study of interfacial phenomena. One prerequisite of such use is a detailed knowledge of the spectroscopic fingerprint of the ω-MBP SAMs and the identification of spectroscopic criteria, which allow for the study of interfacial processes. In this article, we will first give a detailed spectroscopic analysis of three different ω-MBP SAMs on gold by means of spectral ellipsometry, photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and vibrational sum frequency generation spectroscopy (VSFG). This experimental investigation is complemented by a frequency analysis based on ab initio calculations for proper assignment of the various features observed in the linear and nonlinear optical vibrational spectra. We will then demonstrate how certain spectral modes can be utilized to track the chemisorption of octadecyltrichlorosilanes (OTS) on hydroxyl-terminated MBP (HMBP). In particular, we will show that the CH stretching region of the HMBP SAM is basically free of spectral features, so that it can be utilized unambiguously for the analysis of the OTS film formed on top (7) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (8) Ulman, A. Acc. Chem. Res. 2001, 34, 855. (9) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662. (10) Azzam, W.; Cyganik, P. ; Witte, G.; Buck, M.; Wo¨ll, Ch. Langmuir 2003, 19, 8262.
10.1021/la0531188 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006
ω-Substituted Biphenylthiolates on Gold
of the HMBP model surface. Further, by transfer of the OTS film onto a metal substrate, the fingerprint region becomes accessible by means of infrared reflection absorption spectroscopy (IRRAS) with a quality, i.e., signal-to-noise ratio, not obtainable for OTS films directly adsorbed on semiconductor surfaces, thereby allowing the tracing of characteristic frequencies of the HMBP to monitor OTS chemisorption. Accordingly, our study exemplifies that the utilization of ω-MBP as model surfaces for surface adsorption provides new access to otherwise hidden interfacial phenomena. 2. Experimental Section Chemicals. Toluene and ethanol used for the preparation of ω-MBP films and for cleaning of the ω-MBP and OTS SAMs were ACS grade and were used without further purification. Anhydrous toluene (absolute, over molecular sieve, H2O < 0.005%) and octadecyltrichlorosilane (OTS, 90+%) were obtained from Aldrich. 4-Mercaptobiphenyl (MBP), 4′-methyl-4-mercaptobiphenyl (MMBP), and 4′-hydroxy-4-mercaptobiphenyl (HMBP) were prepared in the Ulman group. For the details of the synthesis, we refer to ref 11. Gold Substrates. Polycrystalline gold films were prepared by evaporation of 100 nm of gold (99.99%) at a base pressure of 5 × 10-7 mbar onto polished single-crystalline silicon (100) wafers (Silicon Sense). A titanium interlayer of 5 nm thickness was used as an adhesion promoter. Monolayer Preparation. Gold substrates were cleaned by ethanol and immediately immersed into the freshly prepared ethanolic thiol solutions (100 µM). The substrates were immersed for 3 days. After removal from solution, they were rinsed with ethanol and ultrasonicated in pure ethanol for 20 min to remove physisorbed moieties. Subsequently, samples were blown dry in a jet of nitrogen and stored under pure Ar. The resulting SAMs were investigated by ellipsometry, contact angle measurements, and IR spectroscopy within a few hours after preparation. Finally, samples were cut into halves, which were further investigated by VSFG and XPS, respectively. To chemically adsorb OTS on the HMBP SAM, the HMBPcoated substrates were transferred into a glass jar immediately after their preparation. The jar was evacuated for 1 h at a base pressure of 10-2 mbar, i.e., below the vapor pressure of water, to remove water, oxygen, and airborne contamination. Then, the jar was disconnected from the vacuum line and filled with pure nitrogen to reach atmospheric pressure. After opening of the jar, the OTS solution (25 µM OTS in anhydrous toluene, H2O < 0.005%, i.e.,
