In-situ FTIR Studies on Self-Assembled Monolayers of Surfactant

In-situ FTIR Studies on Self-Assembled Monolayers of Surfactant Molecules Adsorbed on H-Terminated Si(111) Surfaces in Aqueous Solutions. Akihito Iman...
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Langmuir 2006, 22, 1706-1710

In-situ FTIR Studies on Self-Assembled Monolayers of Surfactant Molecules Adsorbed on H-Terminated Si(111) Surfaces in Aqueous Solutions Akihito Imanishi, Ryo Omoda, and Yoshihiro Nakato* DiVision of Chemistry, Graduate School of Engineering Science, Osaka UniVersity, Toyonaka, Osaka 560-8531, Japan, and CREST, JST ReceiVed September 13, 2005. In Final Form: December 4, 2005 The adsorption of a surfactant, sodium di-2-ethylhexyl sulfosuccinate (SDES), [C4H9CH(C2H5)CH2OCO][C4H9CH(C2H5)CH2OCOCH2]CHSO3-Na+, in an aqueous solution on an atomically flat H-terminated Si(111) [abbreviated as H-Si(111)] surface with a hydrophobic property was investigated by in-situ FTIR measurements. Immersion of the H-Si(111) surface in a solution of 1.0 × 10-2 M SDES for more than 2 h led to formation of a self-assembled monolayer (SAM) with the alkyl chains having a tendency to be assembled perpendicular to the Si surface. The in-situ FTIR observation revealed that the adsorption was nearly complete about 60 min after the start of the immersion, and after that the adsorbed molecules changed their arrangement into an ordered mode. The Si-H peak in the FTIR spectrum remained unchanged with time in aqueous surfactant solution, in contrast to the case of immersion in pure water, indicating that the adsorbed surfactant protects the H-Si(111) surface from oxidation. No structural change in the SAM was observed when a negative potential of -700 mV vs Ag/AgCl was applied to the Si, whereas the adsorbed molecules changed their arrangement, accompanied by their substantial desorption and oxidation of the Si surface, when a positive potential of +700 mV was applied.

Introduction The adsorption of surfactant molecules on solid surfaces in aqueous solutions has attracted much attention in the field of colloid and surface chemistry because of its interesting physical and chemical properties. In particular, the formation of selfassembled monolayers (SAMs) of adsorbed molecules is of much interest because they have a dense and stable molecular structure and potential applications to corrosion prevention, wear protection, etc. In addition, a biomimetic or biocompatible nature of the SAMs will allow us to apply them to chemical and biochemical sensors. Adsorbed surfactants at solid/liquid interfaces have been widely studied. In most studies cleaved surfaces of layered materials such as mica1-8 and graphite2,9-13 were used as substrates, in addition to silica surfaces.1,2,5,13 Recently, it has been established that successive etching with 5% hydrogen fluoride (HF) and 40% ammonium fluoride (NH4F) produces a nearly atomically flat and H-terminated Si(111) [hereafter abbreviated as H-Si* To whom correspondence should be addressed. Fax: +81-6-68506236. E-mail: [email protected]. (1) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (2) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892-898. (3) Li, B.; Fujii, M.; Fukada, K.; Kato, T.; Seimiya, T. Thin Solid Films 1998, 312, 20-23. (4) Ducker, W. A.; Wanless, E. J. Langmuir 1999, 15, 160-168. (5) Zou, B.; Qiu, D.; Hou, X.; Wu, L.; Zhang, X.; Chi, L.; Fuchs, H. Langmuir 2002, 18, 8006-8009. (6) Wall, J. F.; Zukoski, C. F. Langmuir 1999, 15, 7432-7437. (7) Sharma, B. G.; Basu, S.; Sharma, M. M. Langmuir 1996, 12, 6506-6512. (8) Fujii, M.; Li, B.; Fukada, K.; Kato, T.; Seimiya, T. Langmuir 2001, 17, 1138-1142. (9) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 42234228. (10) Fleming, B. D.; Wanless, E. J. Microsc. Microanal. 2000, 6, 104-112. (11) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207-3214. (12) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (13) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294.

(111)] surface with a well-defined step and terrace structure.14-22 The H-Si(111) surface is stable in aqueous solutions and has a hydrophobic property. Thus, this surface is quite appropriate as a substrate for the study of adsorption behavior and structures. In addition, this surface has merit in that it can be modified with chemically attached alkyl chains with or without functional groups to control the morphological and hydrophobic/hydrophilic properties. In previous work23 we investigated the adsorption of cationic surfactants in aqueous solutions on atomically flat H-Si(111) surfaces using in-situ atomic force microscopic (AFM) inspection. All surfactants used, i.e., dodecyltrimethylammonium chloride [CH3(CH2)11N(CH3)3Cl] (C12TAC), octadecyltrimethylammonium chloride [CH3(CH2)17N(CH3)3Cl] (C18TAC), and dioctadecyldimethylammonium chloride [{CH3(CH2)17}2N(CH3)2Cl] (2-C18DAC), formed high-density adsorption monolayers on the H-Si(111) surface, most probably with the alkyl chains assembled nearly normal to the Si surface. The surfactant, C12TAC, formed the most smooth and flat SAM. Troughs, arising from steps at the Si surface, were observed in the case of C18TAC and 2-C18DAC, indicating that the morphology of the adsorbed monolayer was strongly affected by the surface step of Si. (14) Takahagi, T.; Ishitani, A.; Kuroda, H.; Nagasawa, Y. J. Appl. Phys. 1991, 69, 803-807. (15) Yablonovitch, E.; Allara, D. L.; Chang, C. C.; Gimitter, T.; Bright, T. B. Phys. ReV. Lett. 1986, 57, 249-252. (16) Grundner, M.; Schulz, R. AIP Conf. Proc. 1988, 167, 329. (17) Graef, D.; Grundner, M.; Schulz, R. J. Vac. Sci. Technol. 1989, A7, 808813. (18) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656-658. (19) Jakob, P.; Chabal, Y. J. J. Chem. Phys. 1991, 95, 2897-2909. (20) Kim, Y.; Lieber, C. M. J. Am. Chem. Soc. 1991, 113, 2333-2335. (21) Hessel, H. E.; Feltz, A.; Reiter, M.; Memmert, U.; Behm, R. J. Chem. Phys. Lett. 1991, 186, 275-280. (22) Itaya, K.; Sugawara, R.; Morita, Y.; Tokumoto, H. Appl. Phys. Lett. 1992, 60, 2534-2536. (23) Imanishi, A.; Suzuki, M.; Nakato, Y. Trans. Mater. Res. Soc. Jpn. 2004, 29, 3223-3225.

10.1021/la052495h CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006

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Figure 2. Structure of the surfactant, sodium di-2-ethylhexyl sulfosuccinate.

Figure 1. Schematic illustration of a Teflon cell used for in-situ FTIR measurements in a multiple internal reflection (MIR) mode under a regulated potential of the Si.

The above results indicate that in-situ AFM inspection is really a powerful method to investigate the morphological structure of adsorbed layers. However, this method gives little information on the arrangement or orientation of individual surfactant molecules. In the present paper we thus adopted in-situ FTIR measurements to investigate the orientation or arrangement of adsorbed molecules in the adsorption layer. This technique has enabled us to investigate time-dependent and potential-dependent changes of the molecular arrangement in the adsorbed surfactant layers. Experimental Section Single-crystal n-type Si(111) wafers of a resistivity of 10-15 Ω cm with vicinal surfaces were obtained from Osaka Tokushu Gokin Co. Ltd., Japan. For in-situ FTIR measurements, the Si wafers were polished on both the front and rear sides and cut into the shape of a prism with dimensions of 50 × 17 × 0.5 mm. They were then cleaned by the conventional RCA cleaning method,24 which consisted of immersion in 98% H2SO4 + 30% H2O2 (4:1 in volume) for 10 min, 5% HF for 5 min, 25% NH4OH + 30% H2O2 + H2O (1:1:5 in volume) for 10 min, and 36% HCl + 30% H2O2 + H2O (1:1:6 in volume) for 10 min. Figure 1 schematically illustrates a Teflon cell for the in-situ FTIR measurements under a regulated electrode potential of n-Si. The above-mentioned cleaned Si prism was reoxidized again in all surfaces by immersion in 98% H2SO4 + 30% H2O2 (4:1 in volume) for 10 min to obtain FTIR signals only from the Si/solution contact, at which the Si surface is H-terminated, as explained later. Ohmic contact with n-Si was obtained by pasting indium-gallium alloy at edges of the rear Si surface after removal of the surface oxide. Much care was taken so that the alloy would not disturb the multireflection of the IR beam. At the front Si surface a rubber O-ring was placed for sealing, and then a Teflon block having a compartment, which allowed the solution to flow into the cell, was placed and fixed to the rear Teflon block with screws (Figure 1). After the surface-oxidized Si prism was mounted in the Teflon cell, 5% HF was flowed into the cell for 5 min, and then 40% NH4F was flowed for 1 min by using a diaphragm pump to get an atomically (24) Kern, W.; Puotinen, D. A. RCA ReV. 1970, 31, 187-206.

flat H-Si(111) surface at the region of the Si/solution contact.25 The FTIR spectra were then measured by flowing continuously an aqueous solution containing a surfactant into the cell with the same pump as above. As the surfactant, sodium di-2-ethylhexyl sulfosuccinate (C20H37O7SNa, Figure 2), SDES, was used in the present work. A potentiostat (Hokuto-Denko HA501) was used for regulating the electrode potential of n-Si together with an Ag/AgCl/satd KCl electrode as the reference electrode and a Pt wire as the counter electrode. The FTIR spectra were obtained with a Bio-Rad FTS 575C spectrometer, with a spectral resolution of 1 cm-1. The multiple internal reflection (MIR) method was adopted to get high sensitivity26,27 (Figure 1). The IR beam was focused at normal incidence on an edge (45-bevel) of the Si prism. A simple calculation showed that the IR beam was internally reflected about 100 times in the Si prism. Thus, one-hundredth of the observed absorbance, ln(I0/I), is indicated in the spectra of the next section, where I0 and I are the intensities of IR light passing through the reference and sample compartments of the spectrometer. For weak absorption (I0 = I), ln(I0/I) = (I0 - I)/I0 ) ∆I/I0. A chemically surface-oxidized Si prism, just after preparation, was used as the spectral reference sample. The compartments in the spectrometer were purged with nitrogen gas or dry air during the measurement. Special-grade chemicals were used without further purification. Pure water was obtained by purifying deionized water with a Milli-Q water purification system.

Results Formation of atomically flat, H-Si(111) surfaces by HF and NH4F etching was confirmed by observation of a clear step and terrace structure in air with an atomic force microscope (AFM)28,29 and observation of a sharp strong FTIR peak at 2083.6 cm-1, assigned to the stretching mode of Si-H bonds at the (111) terrace, in air, together with some weak peaks assigned to vibration modes of Si-H and SiH2 bonds at steps.26,29 Figure 3 shows in-situ FTIR spectra in the regions of SiHx (x ) 1, 2)26,27,29 and CHx (x ) 2, 3)30 vibrations for an H-Si(111) surface immersed in an aqueous solution of 1.0 × 10-2 M SDES. The spectrum denoted as “0 min” was measured just after immersion (or just after the surfactant solution was flowed), whereas those denoted as “60 min” and “240 min” were obtained 60 and 240 min after the start of the immersion, respectively. The IR band at 2078.0 cm-1, mainly assigned to terrace Si-H bonds, is fairly broad when the H-Si(111) surface is in contact with an aqueous solution, though the band is very sharp when the surface is in air. Similar broad peaks were reported by other workers.31 The broadening is nearly independent of the kind of solutions (or solvents)31 and thus most probably attributed to van der Waals interactions of the Si-H bonds with solvent molecules. (25) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656-658. (26) Zhou, X. W.; Ishida, M.; Imanishi A.; Nakato, Y. Electrochim. Acta 2000, 45, 4655-4662. (27) Zhou, X. W.; Ishida, M.; Imanishi, A.; Nakato, Y. J. Phys. Chem. B 2001, 105, 156-163. (28) Imanishi, A.; Ishida, M.; Zhou, X.; Nakato, Y. Jpn. J. Appl. Phys. 2000, 39, 4355-4358. (29) Imanishi, A.; Hayashi, T.; Nakato, Y. Langmuir 2004, 20, 4604-4608. (30) Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. Spectrometric Identification of Organic Compounds, 7th ed.; John Wiley & Sons Inc.: New York, 2004. (31) Ye, S.; Ichihara, T.; Uosaki, K. Appl. Phys. Lett. 1999, 75, 1562-1564.

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Figure 3. In-situ MIR-FTIR spectra in the regions of Si-Hx and CHx vibrations for an H-Si(111) surface immersed in an aqueous solution of 1.0 × 10-2 M SDES for 0, 60, and 240 min.

Figure 4. In-situ MIR-FTIR spectra for an H-Si(111) surface immersed in pure water for 0, 60, and 240 min.

The IR bands at 2961.8, 2932.7, 2874.1, and 2861.8 cm-1, observed in the spectra denoted as 60 and 240 min in Figure 3, can be assigned to asymmetric CH3, asymmetric CH2, symmetric CH3, and symmetric CH2 vibrations, respectively, according to the literature.30 The intensities of the bands are nearly zero just after immersion, whereas they increase largely after the 60-min immersion, clearly indicating that these bands arise from adsorbed surfactant molecules not from the molecules in solution detected by evanescent waves of the totally reflected IR beam. It is interesting to note that during immersion from 60 to 240 min the intensity of the CH3 and CH2 bands hardly changes but the intensities of the CH3 bands increase whereas those of the CH2 bands decrease. This result indicates that adsorption of the surfactant is nearly complete during the initial 60-min immersion and after then the adsorbed molecules change their arrangement or orientation within the adsorption layer. (The latter point will be discussed in more detail later.) The bands at 2852.2, 2926.1, and 2960.9 cm-1, observed in the spectrum just after immersion (denoted as 0 min), may be attributed to contaminating hydrocarbons in solution. It is to be noted also in Figure 3 that the intensity of the Si-H peak at 2078.0 cm-1 remains unchanged even after the 240-min immersion. This result is in sharp contrast to the result for an H-Si(111) surface immersed in pure water with no surfactant (shown in Figure 4). In the latter case the Si-H peak at 2078.0 cm-1 decreases in intensity and a new peak appears at 2236.5 cm-1, which can be assigned to Si-H bonds for a back-bond oxidized H-Si(O-Si)3 group.26,27 The results indicate that the H-Si(111) surface in pure water is oxidized during the 60-min

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Figure 5. In-situ MIR-FTIR spectra for an H-terminated Si(111) surface immersed for 240 min in aqueous solutions of (a) 1.0 × 10-2 and (b) 1.0 × 10-3 M SDES together with a spectrum (c) for the aqueous solution of 1.0 × 10-2 M SDES, measured by a transmission mode.

immersion, whereas that in an aqueous solution containing the surfactant is not oxidized. The difference will be attributed to the fact that the adsorbed surfactant molecules existing at a high density at the Si surface protect the H-Si(111) surface from oxidation by H2O or dissolved oxygen. Figure 5 shows the influence of the surfactant concentration in solution on the FTIR spectra of the adsorbed layer. Spectra a and b were measured for the H-Si(111) surface immersed for 240 min in aqueous solutions of 1.0 × 10-2 and 1.0 × 10-3 M SDES, respectively. Spectrum c is for the aqueous solution of 1.0 × 10-2 M SDES, measured by a transmission mode, for comparison. The ratio of the intensities of the CH3 bands to those of the CH2 bands in spectrum b for the low surfactant concentration is nearly the same as that in spectrum c for the bulk solution, indicating that the adsorbed surfactant molecules in spectrum b exist randomly and are not oriented on the H-Si(111) surface. On the other hand, the above-mentioned ratio in spectrum a is much higher than that in spectrum c, strongly suggesting that the adsorbed molecules in spectrum a have an arranged structure. We note here that the transition moment of the CH3 vibration is in parallel to the alkyl chain of the surfactant, whereas that of the CH2 vibration is normal to the alkyl chain. We note also that by the “surface selection rules”32,33 the vibrations in the adsorption phase with the transition moment normal to the surface are strongly enhanced whereas those with the transition moment parallel to the surface are quenched. These considerations lead to the conclusion that the surfactant molecules in spectrum a for high surfactant concentration are arranged with the alkyl chains having a tendency to be assembled perpendicular to the Si surface. In relation to the above arguments, it is important to note that the spectrum denoted as 60 min in Figure 3 (60-min immersion in 1.0 × 10-2 M SDES) resembles spectrum b of Figure 5 (240min immersion in 1.0 × 10-3 M SDES). This implies that the oriented adsorption of the surfactant molecules occurs only for immersion in an aqueous solution with a high surfactant concentration. We also investigated the effect of application of a potential to the Si on the structure of the adsorption layer. First, an adsorbed surfactant layer with an arranged structure was formed by immersion of an H-Si(111) surface in an aqueous solution of 1.0× 10-2 M SDES for 240 min. Then, a potential was applied (32) Kobayashi, Y.; Ogino, T. Surf. Sci. 1996, 368, 102-107. (33) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315.

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and disappeared completely after the 240-min application. This behavior is the same as that observed in Figure 4, suggesting that the H-Si(111) surface was oxidized. This conclusion is also in good harmony with the aforementioned fact that the adsorbed surfactant was desorbed at +700 mV vs Ag/AgCl (Figure 6) because the oxidized Si surface no longer has a hydrophobic property and loses power for adsorption of a surfactant.

Discussion

Figure 6. (a) In-situ MIR-FTIR spectrum in the region of CHX vibrations for an H-Si(111) surface immersed in an aqueous solution of 1.0 × 10-2 M SDES for 240 min, (b) that for the H-Si(111) surface in which a potential of -700 mV vs Ag/AgCl was applied for 240 min after measurement of spectrum a, and (c) that for the H-Si(111) surface in which a potential of +700 mV was applied for 240 min after measurement of spectrum a.

Figure 7. In-situ MIR-FTIR spectra in the region of the Si-H vibration for an H-Si(111) surface immersed in an aqueous solution of 1.0 × 10-2 M SDES. Spectra a-c were measured in the same way as in Figure 6.

to the n-Si prism and maintained for 240 min. Figure 6a and b shows in-situ FTIR spectra before and after a potential of -700 mV vs Ag/AgCl was applied, respectively. No difference is seen between the spectra, indicating that application of a negative potential does not affect the structure of the adsorbed surfactant layer. On the other hand, Figure 6c shows the FTIR spectrum after a potential of +700 mV vs Ag/AgCl was applied and maintained for 240 min. We can see that the intensity for the CHx bands decreases and also that the ratio of the intensities of the CH3 vibrations to those of the CH2 vibrations decreases. These results indicate that a part of the adsorbed surfactant molecules is desorbed from the Si surface and the remaining adsorbed molecules change their orientation into a random mode. Figure 7a shows an in-situ FTIR spectrum for an H-Si(111) surface in the region of the Si-H vibrations, obtained after immersion in an aqueous solution of 1.0 × 10-2 M SDES for 240 min. The terrace Si-H band is observed clearly, as mentioned earlier. After observation of spectrum a, a potential of + 700 mV vs Ag/AgCl was applied to the Si and in-situ FTIR spectra were observed at 5 and 240 min. The results are shown in Figure 7b and c, respectively. The Si-H band considerably decreased in intensity after the 5-min application of +700 mV vs Ag/AgCl

The experimental results described in the preceding section have shown that the self-assembled monolayer (SAM) of an adsorbed surfactant, which is in equilibrium with an aqueous solution with a high concentration (1.0 × 10-2 M) of surfactant, forms an arranged structure of the adsorbed molecules with the alkyl chains aligned nearly perpendicular to the Si surface. On the other hand, the SAM in equilibrium with an aqueous solution with a low concentration (1.0 × 10-3 M) of surfactant has a randomly arranged structure. Essentially the same behavior is reported for the SAMs of alkanethiols chemisorbed on Au-metal substrates.34-42 Many researchers reported that the S-S distance, in other words, the density of alkanethiol molecules, had a close relation with the tilt angle of the alkyl chains. The alkyl chains of alkanethiols were nearly parallel to the substrate when the coverage was low, whereas the chains became normal to the substrate surface owing to the mutual interaction among the alkyl chains when the coverage got high (or when the adsorbed molecules became densely packed).43-45 The similarity in the behavior gives strong support to the above-mentioned structure for the adsorbed surfactant layers in the present work. We also mentioned in the preceding section that adsorption was nearly complete about 60 min after the start of the immersion, and after that the adsorbed molecules changed their arrangement into an ordered mode. Essentially the same behavior is reported at this point for the SAMs of alkanethiols on Au substrates. Uosaki et al. reported that alkanethiol molecules are initially adsorbed with a random arrangement on an Au substrate with almost full coverage for a short time, and after that the order of molecular arrangement gradually increased.36 This similarity gives further support to the aforementioned structure for the adsorbed surfactant layers. An important difference between the SAMs in the present work and those of alkanethiols on Au substrates is that in the latter case adsorption of alkanethiols (R-S-H) occurs by formation of covalent bonds of R-S-Au, whereas in the former case adsorption of a surfactant occurs by a hydrophobic interaction through a hydrophobic property of the H-Si(111) surface. This interpretation is in harmony with the fact that almost no change (34) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426-429. (35) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4305. (36) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-1514. (37) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 10321037. (38) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 13851387. (39) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128-1132. (40) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307-2312. (41) Sato, Y.; Frey, B. L.; Corn, R. M.; Uosaki, K. Bull. Chem. Soc. Jpn. 1994, 67, 21-25. (42) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (43) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989, 5, 1147-1152. (44) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. F. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (45) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultans Bureau: New York, 1959; pp 177-217.

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in the IR band at 2078.0 cm-1, mainly assigned to terrace Si-H bonds, occurred by adsorption of the surfactant (Figure 3). As mentioned in the Results section, the terrace Si-H band becomes broad when the H-Si(111) surface comes into contact with solvents and the extent of broadening is nearly independent of the kind of solvents,31 probably because the broadening is caused by weak van der Waals interactions of the Si-H bonds with solvent molecules. Accordingly, adsorption of the surfactant (i.e., replacement of weakly adsorbed water molecules with weakly adsorbed surfactant molecules) should cause almost no change in the IR band at 2078.0 cm-1, as observed in Figure 3. It is known that the arrangement of alkanethiol molecules chemisorbed on Au substrates is changed by the applied potential.46-49 Sato et al. investigated the potential-dependent structural change of the SAMs of HS(CH2)2CN and HS(CH2)7CN on Au substrates.49 They reported that the alkyl chains became more perpendicular to the surface with the C-N bonds arranged nearly in parallel to the surface as the potential got more positive. A driving force for such a change may be given by the interaction between the dipole moment of a CN group and the potentialinduced charges at the gold surface. In the present work no structural change could be observed for the SAMs of the adsorbed surfactant when a negative potential of -700 mV vs Ag/AgCl was applied to the Si surface. This is quite reasonable if we assume the aforementioned structural model for the SAM in which the alkyl chains are assembled perpendicular to the Si surface, with the SO3- group located at the outermost region of the layer. Application of a negative potential will make the Si surface negatively charged, which will exert a repulsive force on the negatively charged SO3- group of the surfactant. However, the SO3- group is originally located at the outermost region of the layer, and therefore, no change in the arrangement of the surfactant molecules is induced (as long as the repulsive energy is less than the adsorption energy and the SAM is not broken). In other words, no structural change of the SAM by application of a negative potential strongly supports the aforementioned structure for the adsorbed surfactant molecules are arranged perpendicular to the Si surface. When a positive potential of +700 mV vs Ag/AgCl was applied, on the other hand, a serious structural change was caused in the SAMs together with oxidation of the Si surface. In this case the negatively charged SO3- group of the surfactant will be attracted by the potential-induced positive charges at the Si surface and

approach to it, thus leading to a large change in the molecular arrangement in the SAMs. It is to be noted also that the approach of the SO3- group to the Si surface will induce the approach of water molecules to it, thus inducing oxidation of the Si surface. This argument implies that protection of the Si surface from oxidation by the adsorbed surfactant is due to formation of a hydrophobic layer, which retards the approach of water molecules to the Si surface. The fact that the adsorbed surfactant protects the Si surface in aqueous solutions from the surface oxidation is very interesting. To protect the Si surface effectively is an important subject in the field of electrochemical semiconductor devices. Many researchers reported that surface alkylation through formation of Si-C covalent bonds50,51 is effective to protect the Si surface from surface oxidation. However, in this case the Si-H bonds are changed into Si-C bonds. Adsorption of surfactants gives an effective way to protect the H-Si surface from surface oxidation without changing chemical bonds in the H-Si(111) surface.

(46) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521-2530. (47) Shimazu, K.; Ye, S.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 375, 409-413. (48) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (49) Sato, Y.; Ye, S.; Haba, T.; Uosaki, K. Langmuir 1996, 12, 2726-2736.

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Conclusion The formation of SAMs of adsorbed surfactant (C20H37O7SNa, SDES) molecules on atomically flat H-Si(111) surfaces with a hydrophobic property was investigated by in-situ FTIR measurements. When H-Si(111) was immersed in an aqueous solution with a high concentration (1.0 × 10-2 M) of surfactant, a SAM with an arranged structure was formed, in contrast to the case with a low concentration (1.0 × 10-3 M) of surfactant, probably owing to interactions between the alkyl chains of the adsorbed surfactant. In the high surfactant concentration adsorption of surfactant was nearly complete 60 min after immersion, and then the order of molecular arrangement of the surfactant increased. The terrace Si-H bonds remained unchanged even 240 min after immersion, indicating that the adsorbed surfactant molecules protect the Si surface from oxidation by H2O or dissolved oxygen. No structural change was observed when a negative potential of -700 mV vs Ag/AgCl was applied to the Si substrate, supporting the structural model for the SAMs. Application of a positive potential of +700 mV vs Ag/AgCl, on the other hand, caused a serious structural change in the SAMs, accompanied by desorption of the adsorbed molecules as well as oxidation of the Si surface.

(50) Webb, L. J.; Lewis, N. S. J. Phys. Chem. B 2003, 107, 5404-5412. (51) Nakato, K.; Takabayashi, S.; Imanishi, A.; Murakoshi, K.; Nakato Y. Sol. Energy Mater. Sol. Cells 2004, 83, 323-330.