Surface Vibrational Spectroscopy of Alkylsilane Layers Covalently

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Surface Vibrational Spectroscopy of Alkylsilane Layers Covalently Bonded to Monolayers of (3-Mercaptopropyl)trimethoxysilane on Ag Substrates Mei Cai, Mankit Ho, and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received August 9, 1999. In Final Form: January 5, 2000 Fourier transform IR and Raman spectroscopy have been used to characterize the structure and conformational order of octadecyltrichlorosilane (OTS) and dimethylchlorooctadecylsilane (DOS) covalently bonded to Ag surfaces modified with a hydrolyzed and condensed self-assembled monolayer of (3mercaptopropyl)trimethoxysilane (3MPT). This 3MPT layer is used as a model silica surface with a very low surface silanol density and, hence, much less surface-confined water than normal silica surfaces. OTS bonds to this surface at the silanol sites in heterogeneous polymeric islands possessing both highly crystalline and disordered domains. DOS bonds in submonolayer coverages with its alkyl chains in a disordered state. The results from both FTIR and Raman spectroscopies confirm the critical role of surface water in forming highly crystalline assemblies of cross-linked alkylsilanes.

Introduction Alkylsilane-modified silica surfaces have received intense scrutiny in recent years, due to their importance in diverse applications including microelectronics,1,2 chromatographic separations,3-5 and adhesion.6-8 In many instances, these applications rely on the structure and conformational order of these bonded alkylsilane phases. Densely packed, highly organized monolayer films prepared from self-assembly of long-chain alkyltrichlorosilanes have been studied extensively for these applications;9-16 however, a complete understanding of the variables controlling the assembly process is still lacking. Two important variables known to play critical roles in the silanization process are the number of available surface silanols and the amount of surface-confined water. In this paper, a unique layered structure terminating in bonded alkylsilanes is investigated; this structure is based on a substrate with a significantly smaller number of surface silanols and a smaller amount of surface-adsorbed water than those of normal silica surfaces. * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: (520) 621-8245. Fax: (520) 621-8248. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Ulman, A. Adv. Mater. 1990, 2, 573. (3) Sander, L. C.; Wise, S. A. CRC Crit. Rev. Anal. Chem. 1987, 18, 299. (4) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, 857A. (5) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068. (6) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018. (7) Porro, T. J.; Pattacini, S. C. Appl. Spectrosc. 1990, 44, 1170. (8) Pantano, C. G.; Wittberg, T. N. Surf. Interface Anal. 1990, 15, 498. (9) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (10) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (11) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (12) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (13) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (14) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (15) Allara, D. L.; Parikh, A.N.; Rondelez, F. Langmuir 1995, 11, 2357. (16) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151.

Although bonded alkylsilane phases have been studied by a variety of surface analysis techniques, the most popular tool has been FTIR spectroscopy.13-16 Despite the use of FTIR for estimating the conformational order of bonded alkylsilane structures, strong interferences from bulk silica and water preclude a detailed analysis of the low-frequency region (i.e., 1000-1600 cm-1) where direct vibrational information about alkyl chain conformation is contained. In contrast, Raman spectroscopy is quite valuable for accessing conformational information in this region without interference from water or silica. Particularly prominent in this region are the ν(C-C) and δ(CH2) vibrations which are sensitive to alkyl chain conformation.17 Unfortunately, Raman scattering is an inherently weak phenomenon, especially for monolayers of alkylcontaining species, making spectral acquisition of these systems challenging. The layered structure reported here overcomes this sensitivity problem, allowing signals from alkylsilane coverages as small as submonolayer to be detected. This layered structure is also readily amenable to FTIR spectroscopic analysis in the low-frequency region with minimal background interference from water or silica. Thus, both Raman and FTIR spectroscopy were employed in this study providing complementary information about the bonded alkylsilanes. A schematic of the layered structure used for these studies is shown in Figure 1.18,19 A smooth, polycrystalline Ag substrate is used to provide a small enhancement (ca. 2 × 103)20 to the Raman scattering intensities at the interface. Onto this surface, a self-assembled monolayer of hydrolyzed and condensed (3-mercaptopropyl)trimethoxysilane (3MPT) is anchored. 3MPT is a bifunctional molecule that contains both thiol and silane functional groups. The thiol groups serve as binding sites for the covalent attachment of 3MPT to Ag surfaces.18,19 This hydrolyzed 3MPT monolayer is then used for the subsequent attachment of alkylsilanes. In this way, surface enhancement of the Raman scattering by the underlying Ag surface can be exploited to allow the weak signals of the alkylsilanes to be observed. Furthermore, background (17) Pemberton, J. E. In Characterization of Organic Thin Films; Ulman, A., Ed.; Butterworth-Heinemann: Boston, MA, 1995; p 87. (18) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1993, 5, 241. (19) Thompson, W. R.; Cai, M.; Ho, M. K.; Pemberton, J. E. Langmuir 1997, 13, 2291. (20) Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999, 53, 1212.

10.1021/la991075n CCC: $19.00 © 2000 American Chemical Society Published on Web 02/25/2000

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Experimental Section

Figure 1. Schematic showing 3MPT-modified Ag surface as a model silica surface.

interferences from silica absorption are greatly reduced for FTIR spectroscopy on this layered assembly; thus, the sensitivity of FTIR in the low-frequency region is also greatly enhanced. The chemistry associated with the self-assembly process of 3MPT and the number of surface silanols and amount of surface-confined water on the hydrolyzed and condensed 3MPT-modified Ag surfaces have been studied previously with FTIR, X-ray photoelectron spectroscopy (XPS), and ellipsometry.18,19 These previous studies have shown that careful control of the chemistry during self-assembly is critical in order for uniform, well-defined monolayers of 3MPT to be formed. Furthermore, results from these previous studies indicate that less than 5% of the initial surface Si-O-CH3 groups in a monolayer of 3MPT exist in the form of free silanols after hydrolysis and condensation. This number of surface silanol groups (ca. 0.2 per 100 Å2) is significantly smaller than on normal silica surfaces on which they occur at a density of ca. 4.5 per 100 Å2.21 Furthermore, the hydrolyzed 3MPT layer supports less than a monolayer of surface-bound water based on FTIR spectroscopy.19 Fewer silanols and considerably less water on the 3MPT-modified Ag surfaces make them distinct from normal silica surfaces; thus, the use of these as platforms for alkylsilane bonding may provide insight into the role of surface silanol number density and amount of surface water in the silanization process. In this sense, the studies reported here extend those of Parikh and coworkers in which mixed monolayers formed from ω-hydroxyhexadecanethiol and hexadecanethiol of different mole ratios on Au were used as substrates for octadecylsiloxane bonding.22 In this study, the smallest number of surface hydroxyls investigated was 12.5% of the total surface alkanethiols, more than twice as many surface silanols as found on our hydrolyzed 3MPT surfaces. Octadecyltrichlorosilane (OTS) and dimethylchlorooctadecylsilane (DOS) are used as representative alkylsilanes for this study of silanization. OTS has three chlorine atoms per molecule that can be hydrolyzed and condensed with either surface hydroxyls, to form siloxane bonds, or with each other to form a cross-linked polysiloxane backbone structure. In contrast, DOS has only one chlorine atom per molecule such that a single siloxane bond can be formed. The chemistry associated with covalent bonding of these moieties to self-assembled monolayers of 3MPT on Ag surfaces is characterized. (21) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979; Chapter 6. (22) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996.

Instrumentation. Raman spectra were obtained using 100 mW of 514.5 nm radiation from a Coherent Innova 90-5 Ar+ laser on a Spex 1877 “Triplemate” spectrometer as described previously.23-25 The detector in these experiments was one of two Princeton Instruments charge-coupled device (CCD) systems based on either a Tektronix TK-512T CCD of pixel format 512 × 512 or a Princeton Instruments RTE-1100-PB CCD of pixel format 1100 × 330. Both CCDs were thinned, back-illuminated, antireflection-coated chips and were cooled with liquid N2 to -100 °C. External reflection-absorption FTIR spectra were acquired with a Nicolet Magna 550 FTIR spectrometer with a liquid N2cooled narrow band MCT-A detector. 3MPT-modified Ag surfaces were used as the spectral reference system. Spectra of bonded alkylsilanes were obtained using p-polarized light incident at 80° with respect to the surface normal using an FT-80 grazing angle accessory (Spectra-Tech, Inc.) All spectra are the average of 600 scans of both the sample and the reference and were collected at 2 cm-1 resolution with Happ-Genzel apodization. Spectra are reported as -log(R/Ro), where R is the reflectivity of the sample and Ro is the reflectivity of the reference. Procedures. Polycrystalline Ag surfaces were mechanically polished to a mirror finish with successively finer grades of alumina down to 0.05 µm and sonicated in water for 2 min to remove residual alumina. 3MPT films were formed on these Ag surfaces by immersion into a solution of ca. 20 mM 3MPT in ethanol for 1-2 h. After film formation, the surfaces were rinsed with ethanol and allowed to air-dry. Subsequent hydrolysis and condensation of the 3MPT films were accomplished by immersion in aqueous acid (0.1 M HCl) for 1 h. The 3MPT-modified surfaces were then rinsed with copious amounts of water and allowed to air-dry. Gas-phase reaction was used for covalent attachment of OTS or DOS to the 3MPT-modified Ag surfaces. This strategy minimizes participation of adventitious water from solution in the silanization processes. A closed glass cell was rinsed with toluene, dried in an oven at 120 °C overnight, and cooled in a desiccator. The 3MPT-modified Ag surfaces were then placed in the closed glass cell purged with N2. OTS or DOS was introduced to the bottom of the glass cell, and the closed cell was warmed to 30-40 °C in an oil bath for 2 h to establish the appropriate vapor environment in the cell and allow the reaction to go to completion. After reaction, the surfaces were rinsed with copious amounts of toluene and methanol to remove any unbonded alkylsilane. Materials. (3-Mercaptopropyl)trimethoxysilane (3MPT, >97%), DOS and OTS were purchased from either United Chemical Technologies or Aldrich. 3MPT was vacuum-distilled prior to use. Anhydrous-grade methanol was purchased from Mallinckrodt. Anhydrous-grade ethanol was purchased from Quantum Chemical Company. Toluene (reagent-grade) was obtained from EM Science and distilled prior to use. Water was obtained from a Milli-Q UV Plus ultrapure water system. All reagents were used as received unless specified. Polishing supplies including alumina and polishing pads were purchased from Buehler. Polycrystalline Ag disks (99.999%) were obtained from Johnson Matthey.

Results and Discussion OTS/3MPT/Ag. FTIR Spectroscopy. Results from FTIR external reflection-absorption spectroscopy (FT-IRRAS) of OTS monolayers on 3MPT-modified Ag surfaces are presented first. Parts a and b of Figure 2 show FTIR spectra in the 4000-2600 and 2000-700 cm-1 regions, respectively. Using 3MPT-modified Ag as the spectral reference produces no interference from the substrate; therefore, all bands appearing in these spectra are from (23) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (24) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (25) Pemberton, J. E.; Bryant, M. A.; Sobocinski, R. L.; Joa, S. L. J. Phys. Chem. 1992, 96, 3776.

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Table 1. FTIR Peak Frequencies and Assignments for Bonded OTS and DOS surface DOSa

surface OTSa

assignmentsa,b

∼810 NOc 937 ∼1110 NO 1265 NO NO NO NO 1468 2856 NO 2928 2963 NO 3721

∼800 908 NO ∼1140 and ∼1025 1178-1274 NO 1310 1341 1378 1455 1468 2851 2874 2921 2961 ∼3380 3721

νs(Si-O-Si) ν(Si-OH) ν(Si-OH) νa(Si-O-Si) δ(CH2)wag δ(CH3) δ(CH2)kink δ(CH2)end-gauche δs(CH3) δa(CH3) δ(CH2) νs(CH2) νs(CH3) νa(CH2) νa(CH3)ip ν(SiO-H) ν(SiO-H)isolated

a In cm-1. b ν ) stretch, δ ) bend, a ) antisymmetric, s ) symmetric, and ip ) in-plane. c NO ) not observed.

OTS. Vibrational assignments and their corresponding frequencies are listed in Table 1.26-33 Four main bands at 2851, 2874, 2921, and 2961 cm-1 are clearly observed in the high-frequency region (Figure 2a). These are assigned to νs(CH2), νs(CH3)FR, νa(CH2), and νa(CH3) modes of the OTS alkyl chains, respectively. The peak frequencies for the νs(CH2) and νa(CH2) modes provide insight into alkyl chain packing in terms of crystallinelike or liquidlike structure. The peak frequency for the νa(CH2) mode of an all-trans extended alkyl chain in a crystalline environment is reported to be ca. 2915-2920 cm-1, and that for the νs(CH2) mode is reported to be ca. 2846-2850 cm-1.27,28 The frequencies of these peaks increase to ca. 2928 and 2856 cm-1 for the νa(CH2) and νs(CH2) modes, respectively, for liquidlike disordered chains.26 The peak frequencies observed for OTS bonded to 3MPT-modified Ag are intermediate between perfectly crystalline and liquidlike, suggesting either a heterogeneous mixture of discrete regions of conformationally ordered and disordered alkyl chains or a homogeneous array of alkyl chains that individually possess intermediate conformational order. Some level of alkyl chain disorder is further supported by the relatively large bandwidths of the ν(CH2) modes. For crystalline alkyl chains, the νa(CH2) and νs(CH2) modes are typically 7-10 and 10-13 cm-1 in width, respectively.34 In spectra of OTS bonded to 3MPT-modified Ag, widths of ∼25 and 15 cm-1 are observed for these two modes, respectively, indicating the presence of a disordered component in the alkyl chain assembly. A partially disordered alkyl chain structure for selfassembled monolayers of bonded OTS is significant and has not been reported previously for fully hydrated metal or oxide surfaces.15,16 Consequently, the presence of a (26) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (27) Snyder, R. G.; Schachtsneider, J. H. Spectrochim. Acta 1963, 19, 85. (28) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1982, 88, 334. (29) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (30) Snyder, R. G.; Maroncelli, M.; Qi, S. P.; Strauss, H. L. Science 1981, 214, 188. (31) Maroncelli, M.; Snyder, R. G.; Qi, S. P.; Strauss, H. L. J. Am. Chem. Soc. 1982, 104, 6237. (32) Snyder, R. G. J. Mol. Spectrosc. 1960, 4, 411. (33) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316. (34) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.

disordered component in bonded OTS makes it difficult and ambiguous to accurately determine the predicted average alkyl chain tilt from the surface spectrum using previously described approaches.34-36 The absorbances of the ν(C-H) modes are larger than expected for single monolayers of bonded OTS based on previous studies on fully hydrated metal and oxide surfaces under similar (although not identical) experimental conditions.15,16 For example, the absorbance of the νs(CH2) mode in Figure 2 is about twice as large as that reported in the literature for a monolayer of OTS.15,16 This discrepancy is even greater when the difference in angle of incidence of our experiment with that of Allara et al.15 is considered. Furthermore, on our 3MPT-modified Ag surfaces, these intensities increase with reaction time. Both of these observations are consistent with polymerization of the OTS on the 3MPT-modified Ag surface. One other attribute of the OTS spectrum in Figure 2a is noteworthy. For all-trans alkyl chains that are perfectly parallel to the surface normal, the νs(CH3) mode would dominate the intensity in this region and the intensities of the methylene modes would be zero on the basis of surface selection rules.37 Thus, the relative intensities of the νs(CH3) and ν(CH2) modes are indicators of alkyl chain tilt relative to the surface normal, with the ν(CH2) modes becoming relatively more intense as the tilt angle increases. For OTS on 3MPT-modified Ag, the ν(CH2) modes are considerably more intense than the methyl modes, suggesting a large average tilt angle with respect to the surface normal. Although these results cannot be interpreted in terms of an exact tilt angle, they can be used qualitatively for this purpose. Previous FTIR studies on self-assembled monolayers of octadecanethiol on Au (tilt angle of ca. 30°)34 and Ag (tilt angle of ca. 13°)38 report values for the intensity ratio of the νa(CH3)ip to νs(CH2) modes of ca. 0.839 and 1.8,38 respectively. The analogous value for alkanoic acids on Ag (tilt angle of ca. 20°) is ca. 0.9.40 Thus, the trend is established in this ratio that the smaller the value, the larger the average tilt angle. For OTS on 3MPT-modified Ag, this ratio is ca. 0.6, suggesting an average tilt angle larger than 30° on the basis of this trend. Although complexities associated with the existence of both ordered and disordered alkyl regions preclude a more detailed interpretation of this value, it supports the notion of an unusual bonding arrangement for OTS on these surfaces. Additional information about the conformational order of the bonded OTS alkyl chains can be obtained from the FTIR spectrum in the low-frequency region shown in Figure 2b. A series of reproducible, well-resolved peaks is observed between 1150 and 1300 cm-1 that is attributed to the progression of coupled ω(CH2) modes for all-trans alkyl chains.32,33 In contrast, a broad feature in this region would be observed for alkyl chains in a disordered, liquidlike state. The observation of these progression bands for OTS on 3MPT-modified Ag indicates the presence of at least a portion of conformationally ordered alkyl chains in this bonded OTS. Thus, coupled with the spectral results in the ν(CH) region, one can conclude that the alkyl portion of this bonded OTS phase is a hetero(35) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (36) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135. (37) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (38) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (39) Stole, S. M.; Porter, M. D. Langmuir 1990, 6, 1199. (40) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132, 93.

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Figure 2. FTIR spectrum of OTS attached to 3MPT-modified Ag surface: (a) ν(C-H) region and (b) low-frequency region.

geneous mixture of highly ordered and disordered regions and not a homogeneous film of intermediate order. Within the highly ordered OTS component, the average percentage of gauche conformers within an alkyl chain can be quantitatively estimated from the average spacings of these ω(CH2) progression bands. These bands are predicted to exhibit an average regular spacing of 18 cm-1 for crystalline octadecyl groups in an all-trans sequence.27 The spacing of these peaks (∆v) is related to the number of methylene units (n) in an all-trans conformation in an alkyl chain according to the following equation:27

∆v ) 326/(n + 1)

(1)

Thus, eq 1 correctly identifies n ) 17 for an all-trans octadecyl group with a spacing of 18 cm-1. For OTS on 3MPT-modified Ag, the average spacing of ca. 19.3 ( 1.2 cm-1 of the six progression bands clearly evident in the spectrum gives a value of n ) 16. Although the presence of the broad νa(Si-O-Si) feature at ca. 1100 cm-1 prevents the observation of the complete set of progression bands, the spacing calculated above is a useful estimate. Thus, an average of 1 out of 17 methylene units is in a gauche conformation, most likely the one adjacent to the methyl group. This value corresponds to ca. 6% average gauche population in the highly ordered region of the bonded OTS assembly. In fact, this result is similar to that obtained by Allara et al. for highly ordered hydrolyzed and condensed bulk OTS and self-assembled OTS monolayers on silica surfaces and on Au surfaces activated by UVozone exposure.15,36 The presence of small numbers of gauche conformations in the otherwise highly ordered region is further indicated by weak broad bands at 1341

and 1310 cm-1 (see inset in Figure 2b) from ω(CH2) modes associated with end-gauche and kink conformational defects, respectively.30,31 Although the relative intensities of these modes can be slightly variable, these modes are reproducibly observed for this system, confirming the small amount of alkyl chain disorder suggested by the vibrational behavior in other frequency regions. Collectively, the results from both spectral regions indicate heterogeneous OTS layers on 3MPT-modified Ag surfaces. Both ordered and disordered components coexist within what is most likely a polymeric OTS-bonded phase with a significant component in the ordered state, as suggested by the observation of the progression bands in the low-frequency region. On the basis of these results, it is reasonable to propose a polymeric island structure for bonded OTS. These islands are envisioned to initiate at the 3MPT surface silanol sites and consist of highly ordered inner cores surrounded by disordered regions The assembly mechanism suggested by this picture is discussed in more detail below. Covalent bonding of OTS to the 3MPT-modified Ag surface is confirmed by two small, negative bands at ca. 3720 and 937 cm-1, which have been attributed to the ν(O-H) and ν(Si-OH) modes, respectively, from free and hydrogen-bonded surface silanols on the 3MPT-modified Ag surface.19 The band at 908 cm-1 is assigned to the ν(Si-OH) mode from unreacted silanols attached to OTS headgroups.15,41-43 The relatively large intensity of this mode indicates a significant number of unreacted silanols, a consequence of incomplete cross-linking in the polymeric OTS islands proposed above. These conclusions are also consistent with the presence of a pronounced band due to the ν(SiO-H) mode at 3380 cm-1 from unreacted silanols on the OTS islands. Interestingly, negative bands due to the consumption of surface-bound water are not observed at ca. 3400 or 1620 cm-1 for the hydrogen bonded ν(O-H) or water deformation modes, respectively. Since the amount of surface-bound water on 3MPT-modified Ag surfaces has been determined to be a submonolayer from FTIR spectroscopy,19 the 3380 cm-1 band must prevent observation of the loss of 3MPT surface-bound water. Although water from solution is precluded with the gasphase reaction procedure used here, other sources of water (e.g., trace water adhering to the walls of the glass reaction cell, trace water from air that leaks into the cell, etc.) must exist that initiate the silanization reaction. Increasing the reaction time from 2 to 4 h almost doubles the peak absorbance of the νa(CH2) mode for OTS alkyl chains, indicating a greater surface coverage and implying a greater consumption of water. Once initiated, the formation of siloxane bonds must regenerate water that allows the reaction to propagate. The formation of a cross-linked siloxane structure is proven by the presence of broad, complex bands at ca. 1000-1140 cm-1 and a band at ca. 800 cm -1. These bands are assigned to νa(Si-O-Si), and νs(Si-O-Si) modes, respectively.44-47 The high intensities of these bands indicate that siloxane bond formation has occurred not only between the OTS and 3MPT surface silanols but also between at least a fraction of the headgroups of the OTS molecules. (41) Ishida, H.; Koenig, J. L. Appl. Spectrosc. 1978, 32, 462. (42) Ishida, H.; Koenig, J. L. Appl. Spectrosc. 1978, 32, 469. (43) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (44) Galeener, F. L.; Lucovsky, G. Phys. Rev. Lett. 1976, 37, 1474. (45) Duran, A.; Serna, C.; Fornes, V.; Fernandez Navarro, J. M. J. Non-Cryst. Solids 1986, 82, 69. (46) Wong, J. S.; Yen, Y. S. Appl. Spectrosc. 1988, 42, 598. (47) Yen, Y. S.; Wong, J. S. J. Phys. Chem. 1989, 93, 7208.

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Cai et al. Table 2. Raman Peak Frequencies and Assignments for OTS and DOS in the ν(C-C) and ν(C-H) Regions liquid crystal. surface OTS OTS OTS

1065 1080

1062

1124

1100 1128 1174

1303 2852 2890 2930 2965

1293 2849 2881 2930 2962

998 1036 1040 1061 1078 1096 1127 1174 1242 1295 2849 2881 2929 sh

liquid DOS

crystal. surface DOS DOS

1065 1080

1062

1124

1100 1128

1303 2852 2886 2931shb 2965 2978

1293 2849 2881 2929sh 2962 2979

998 1036 1040 1061 1078 1124 1174 1242 1300 2852 2929 sh

assign.a ν(C-C)T 3MPT rk(CH2)T 3MPT ν(C-C)G 3MPT νa(C-C)T ν(C-C)G τ(CH2)T νs(C-C)T τ(CH2) 3MPT τ(CH2) 3MPT τ(CH2)T,G νs(CH2) νa(CH2) νs(CH2)FR νa(CH3) νa(CH3

a ν ) stretch, δ ) bend, τ ) twist, T ) trans, G ) gauche, s ) symmetric, a ) antisymmetric, and FR ) Fermi resonance. b sh ) shoulder.

Figure 3. Raman spectra in ν(C-C) region of (a) neat liquid OTS, (b) crystalline OTS, (c) OTS attached to 3MPT-modified Ag surface, and (d) hydrolyzed 3MPT on Ag surface as a background spectrum. Integration times: (a) 60 s, (b) 5 min, (c) 60 min, and (d) 30 min.

The other weak peaks observed in the surface spectra are readily identified. The small peak observed at 1378 cm-1 is assigned to a δs(CH3) or methyl umbrella mode.33 The shoulder at ca. 1455 cm-1 is assigned to the δa(CH3), and the band at 1468 cm-1 is assigned to the CH2 scissor mode. Raman Spectroscopy. Results from Raman spectroscopy of OTS layers on 3MPT-modified Ag surfaces further confirm the conformational structure concluded from the FTIR studies described above. Figure 3 shows Raman spectra between ca. 970 and 1350 cm-1 for neat OTS (a), crystalline OTS (b), OTS bonded to a 3MPT-modified Ag surface (c), and a hydrolyzed and condensed 3MPTmodified Ag surface (d). Peak frequencies and their assignments for OTS and 3MPT are given in Table 2.18,19,48,49 The Raman spectra are fundamentally different from the FTIR spectra in that the Raman spectra contain contributions from both 3MPT and OTS. The surface Raman behavior of 3MPT-modified Ag has been wellcharacterized in this laboratory18,19 and, thus, poses no particular interpretational problem. However, the intensities of the 3MPT bands are greater than those from OTS, a result of the distance dependence of surface-enhanced Raman scattering (SERS),50,51 which makes molecules further away from the metal surface less enhanced. The OTS bands of interest in this region are the ν(CC)T, ν(C-C)G, and τ(CH2)T,G modes at 1065, 1124, and 1080 and 1303 cm-1, respectively. For OTS bonded to a 3MPTmodified Ag surface, 3MPT bands for the ν(C-C)T, r(CH2)T, and two τ(CH2) modes appear at 998, 1036, and 1174 and 1242 cm-1, respectively. These bands do not overlap those (48) Thompson, W. R.; Pemberton, J. E. Anal. Chem. 1994, 66, 3362. (49) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, CA, 1991. (50) Murray, C. A. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; p 203. (51) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143.

of OTS. More importantly, the band intensities and positions do not change upon attachment of OTS, suggesting that the structure and conformational state of the underlying 3MPT layer remain intact upon modification. The conformational order of the OTS alkyl chains can be estimated from the relative intensity ratio of the ν(C-C)T and ν(C-C)G bands and from the peak frequency, breadth, and degree of asymmetry of the τ(CH2)T,G band. For neat liquid OTS (Figure 3a), the large intensity of the ν(C-C)G band at 1080 cm-1 relative to the two ν(C-C)T bands at 1065 and 1124 cm-1, and the broad asymmetric τ(CH2)T,G at 1303 cm-1 indicate that the OTS alkyl chains are disordered. In contrast, the spectrum of crystalline OTS (Figure 3b) shows no evidence of the ν(C-C)G band, and the two ν(C-C)T bands shift ca. 3 cm-1 to lower frequencies. The τ(CH2) band is at ca. 1293 cm-1, 10 cm-1 lower in frequency than that in the liquid, and becomes much narrower and more symmetric, indicative of the crystalline state. When OTS is deposited on 3MPT-modified Ag surfaces, the Raman spectrum (Figure 3c) is complicated by the presence of 3MPT bands and an intense background. However, as the assignments in Table 1 indicate, the peaks for 3MPT are reasonably well-resolved from the bands of OTS. The νa(C-C)T, νs(C-C)T, and τ(CH2)T,G bands from OTS are at 1061, 1127, and 1295 cm-1, respectively. These frequencies are close to those for crystalline OTS, suggesting that a significant portion of the bonded alkyl chains are ordered. Further evidence for ordered alkyl chains is the relatively narrow and symmetric shape of the τ(CH2)T,G mode at 1295 cm-1. A weak ν(C-C)G band might be inferred from the unresolved shoulder at ca. 1080 cm-1. Although the exact intensity of this band is hindered by the signal from 3MPT, it is less intense than the ν(C-C)T modes, indicative of relatively few gauche conformers. These conclusions are further supported by the Raman spectral data in the ν(C-H) region. Figure 4 shows Raman spectra between ca. 2700 and 3100 cm-1 for neat OTS (a), crystalline OTS (b), OTS bonded to a 3MPT-modified Ag surface (c), and a hydrolyzed and condensed 3MPT-modified Ag surface (d). The prominent bands of interest in this region are the νs(CH2), νa(CH2), νs(CH2)FR, and νa(CH3) modes at ca. 2852, 2890, 2930, and 2965 cm-1, respectively. The peak frequencies and assignments are given in Table 2. This region is also complicated by contributions from both 3MPT and OTS. However, judicious choice of spectral parameters used for assessment of conformational order allows considerable insight into the chemistry of this system. In particular,

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Figure 5. FTIR spectrum of DOS attached to 3MPT-modified Ag surface: (a) ν(C-H) region and (b) low-frequency region. Figure 4. Raman spectra in ν(C-H) region of (a) neat liquid OTS, (b) crystalline OTS, (c) OTS attached to 3MPT-modified Ag surface, and (d) hydrolyzed 3MPT on Ag surface as a background spectrum. Integration times: (a) 15 s, (b) 2 min, (c) 20 min, and (d) 20 min.

since OTS has 17 methylene units and 3MPT has only 3, the ν(CH2) modes can be used for this purpose. The peak frequencies of the two ν(CH2) modes are useful indicators of the alkyl chain conformational order.48,52,53 The ν(CH2) modes shift to slightly lower frequencies in going from the liquid to the crystalline state. In the spectrum of crystalline OTS in Figure 4b, the νs(CH2) and νa(CH2) modes have decreased by 2 and 5 cm-1, respectively, relative to the spectrum of liquid OTS in Figure 4a. When OTS is tethered to a 3MPT-modified Ag surface (Figure 4c), two bands attributed to the OTS νs(CH2) and νa(CH2) modes at 2849 and 2881 cm-1, respectively, are clearly observed. The frequencies of these bands are identical to those in crystalline OTS, confirming the high degree of alkyl chain order inherent in this bound OTS system. The other prominent band at ca. 2930 cm-1 is assigned to the νs(CH2)FR from both OTS and 3MPT. Collectively, the results from Raman spectroscopy are consistent with those obtained from surface FTIR studies. DOS/3MPT/Ag. FTIR Spectroscopy. Parts a and b of Figure 5 show FTIR spectra of DOS bonded to 3MPTmodified Ag surfaces in the regions from 4000 to 2500 and 2000 to 700 cm-1, respectively. Peak frequencies and their corresponding assignments are given in Table 1. The spectrum in Figure 5a exhibits peaks at 2856, 2928, and 2963 cm-1 for the νs(CH2), νa(CH2), and νa(CH3) modes of DOS, respectively. The peak frequencies of the νs(CH2) and νa(CH2) bands indicate that the bonded DOS alkyl chains are in a disordered, liquidlike state based on the discussion above. The absorbance of the νa(CH2) mode is ca. 0.0004, almost an order of magnitude smaller than that for OTS bonded on 3MPT-modified Ag surfaces as shown in Figure 2. This absorbance clearly indicates submonolayer coverage by DOS, consistent with the small number of surface silanols on the 3MPT-modified Ag surface. (52) Gaber, B.; Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260. (53) Wallach, D. F. H.; Verma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153.

Additional information about DOS conformation can be obtained from the low-frequency region shown in Figure 5b. The ω(CH2) mode progression bands between 1150 and 1130 cm-1 are completely absent, further consistent with disordered alkyl chains. Covalent attachment of DOS to the 3MPT layer is indicated by negative bands at 3721 and 937 cm-1 for the ν(O-H) and ν(Si-OH) modes of free and hydrogen-bonded 3MPT surface silanols, respectively.19 The broad bands near 1110 and 810 cm-1 are assigned to ν(Si-O-Si) modes as described above. The sharp peak at 1265 cm-1 is due to the δ(CH3) mode from the two Si-CH3 groups.49,54 The weak band at 1468 cm-1 is from a CH2 scissor mode. Raman Spectroscopy. Figure 6 shows Raman spectra between ca. 970 and 1350 cm-1 for neat DOS (a), crystalline DOS (b), DOS on a 3MPT-modified Ag surface (c), and hydrolyzed 3MPT on a Ag surface (d). The bands of interest in this region are the ν(C-C)T, ν(C-C)G, and τ(CH2)T,G modes. Changes of the relative intensities and peak frequencies for the ν(C-C)T and ν(C-C)G modes in going from neat liquid DOS (Figure 6a) to crystalline DOS (Figure 6b) are similar to those discussed above for OTS. Changes in band shape and peak frequency of the τ(CH2)T,G mode are similar to those of OTS as well. For DOS on a 3MPT-modified Ag surface (Figure 6c), intensities of the alkyl chain bands are weaker than those from OTS, consistent with a smaller surface coverage. The ν(C-C)T band at 1065 cm-1 is not resolved from the 3MPT band, but it can be seen as a shoulder. In contrast to the spectral response of OTS, the ν(C-C)G mode at ca. 1078 cm-1 is distinct and is intense relative to the ν(CC)T band at ca. 1124 cm-1 consistent with a more liquidlike, disordered state. The peak frequency of the τ(CH2)T,G band is 1300 cm-1, a further indication of a disordered alkylsilane structure. Figure 7 shows Raman spectra in the ν(C-H) region between ca. 2700 and 3100 cm-1 for neat DOS (a), crystalline DOS (b), DOS on a 3MPT-modified Ag surface (c), and a hydrolyzed and condensed 3MPT layer on a Ag surface (d). Four main Raman bands are observed at ca. 2852, 2886, 2931, and 2965 cm-1 assigned to the νs(CH2), νa(CH2), νs(CH2)FR, and νa(CH3) modes, respectively. Changes in the relative intensities and peak frequencies (54) Nakanishi, K.; Solomon, P. H. Infrared Absorption Spectroscopy; Holden-Day, Inc.: Okland, 1977; p 55.

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Figure 6. Raman spectra in ν(C-C) region of (a) neat liquid DOS, (b) crystalline DOS, (c) DOS attached to 3MPT-modified Ag surface, and (d) hydrolyzed 3MPT on Ag surface as a background spectrum. Integration times: (a) 120 s, (b) 10 min, (c) 60 min, and (d) 30 min.

Figure 7. Raman spectra in ν(C-H) region of (a) neat liquid DOS, (b) crystalline DOS, (c) DOS attached to 3MPT-modified Ag surface, and (d) hydrolyzed 3MPT on Ag surface as a background spectrum. Integration times: (a) 30 s, (b) 4 min, (c) 40 min, and (d) 20 min.

for the νs(CH2) and νa(CH2) modes in going from neat liquid DOS (Figure 7a) to crystalline DOS (Figure 7b) are similar to those for OTS. When DOS is deposited on 3MPT-modified Ag surfaces, the intensities of bands from the alkyl chains are much weaker than those from OTS, consistent with the much smaller surface coverage. The νs(CH2) band of DOS is clearly observed at 2852 cm-1, but the νa(CH2) band is hidden under the broad ν(C-H) envelope of 3MPT and

Cai et al.

DOS. The low intensity of the νa(CH2) band is further evidence that the DOS alkyl chains are in a disordered state. Collectively, both FTIR and Raman spectra of submonolayers of DOS on 3MPT-modified Ag surfaces suggest that the bonded DOS exists in a disordered, liquidlike state. Interfacial Structure of OTS/3MPT/Ag and DOS/ 3MPT/Ag. FTIR and Raman spectroscopies provide a consistent picture of the structure and conformational order of OTS and DOS on 3MPT-modified Ag surfaces. As shown in Figure 8, both OTS and DOS covalently bond to the 3MPT-modified Ag surfaces during the silanization reaction. For DOS on 3MPT-modified Ag surfaces, submonolayer coverages are achieved with the alkyl chains in a disordered state. In contrast, OTS bonds in heterogeneous islands containing both highly crystalline and disordered segments. This picture for the OTS-bonded structure is distinctly different from that previously proposed for OTS bonded to fully hydrated silica surfaces.13,15,55-59 However, as noted below, the mechanism behind these assembly processes is similar. It has been demonstrated in multiple studies that an ultrathin, physisorbed layer of water at the film-substrate interface is important for hydrolyzed OTS molecules to self-assemble into a cross-linked polysiloxane backbone structure in which the alkyl portions are highly crystalline.13,15,36 For 3MPT-modified Ag surfaces in which less than 5% of the surface is covered by silanols, the small amount of surface water hydrating these moieties is likely to be heterogeneously distributed across the surface. As a result, the hydrolyzed OTS molecules easily selfassemble around these surface silanol sites where they subsequently cross-link into a polymeric, partially ordered structure. The relative intensity behavior of the ν(CH) modes is consistent with a large average alkyl chain tilt angle within these domains. The remainder of the siloxanelinked surface is relatively hydrophobic and at such sites with an insufficient amount of water, the hydrolyzed OTS is prevented from moving laterally at the interface to crosslink. This behavior is entirely consistent with the previously proposed mechanism for silanization reactions involving the participation of surface-confined water. Further understanding of the silanization process comes from consideration of the behavior of DOS on 3MPTmodified Ag surfaces. Monochloroalkylsilanes such as DOS have a single reactive chlorine per molecule that results in the formation of a single siloxane bond with surface hydroxyls. Polymerization or cross-linking cannot occur with such monohydrolyzable silanes, and the alkylsilane surface coverages are smaller than those from di- or trichloroalkylsilanes.3 Bonded alkyl chains from monochlorosilanes are expected to have a less organized structure due to a lack of chainchain interactions at these lower surface coverages. With much smaller surface coverage and no cross-linking, alkyl chains of bonded DOS are in a disordered state, suggesting that both van der Waals interactions between alkyl chains and the formation of a cross-linked polysiloxane backbone are important in forming ordered, well-organized films. This result also indicates the insufficient role of covalent bonding between DOS and surface silanols in promoting (55) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (56) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 111, 6146. (57) Carson, G. A.; Granick, S. J. Mater. Res. 1990, 5, 1745. (58) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D. L.; Bright, T. Langmuir 1986, 2, 239. (59) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607.

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Figure 8. Schematic showing the proposed monolayer structure for DOS (top right) and OTS (bottom right) on 3MPT-modified Ag surfaces.

a well-organized alkyl chain structure. Indeed, Allara et al.15 concluded recently from their work on OTS monolayers on SiO2 and on Au activated by UV-ozone exposure that increasing surface attachment of the cross-linked alkyl chains through surface silanol groups introduces disorder and film defects. Conclusions The results presented here demonstrate the utility of 3MPT-modified Ag as a model silica surface with very low surface silanol density for studying the structure and conformational order of bonded OTS and DOS alkyl chains. This layered structure approach provides a unique opportunity for acquiring both FTIR and Raman spectra on the same sample with good signal-to-noise ratio. Moreover, this model silica surface provides spectral access to the methylene wagging progression modes between ca. 1100 and 1400 cm-1. In all previous vibrational spectroscopic studies of alkylsilane monolayers on silica, observation of

these modes has failed as a result of the strong spectral interference from silica modes. The hydrolyzed 3MPT-modified Ag surface contains many fewer surface silanol groups and significantly less surface water compared to traditional silica surfaces. For OTS, an island structure with partially ordered domains is obtained, suggesting the critical role of surface water in promoting well-organized alkylsilane assemblies. In contrast, DOS bonds to the 3MPT-modified surface in a disordered structure, indicating that covalent bonding between alkylsilanes and surface silanols is insufficient to form a crystalline alkyl arrangement. Acknowledgment. The authors are grateful for support of this research by the Department of Energy (Grant DE-FG03-95ER14546) and for support of the instrumentation by the National Science Foundation (Grant CHE-9504345). LA991075N