Effects of Surface Hydrophobization on the Growth of Self-Assembled

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Langmuir 2004, 20, 2701-2706

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Effects of Surface Hydrophobization on the Growth of Self-Assembled Monolayers on Silicon Johann Foisner,† Andreas Glaser,† Thomas Leitner,‡ Helmuth Hoffmann,‡ and Gernot Friedbacher*,† Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164-AC, A-1060 Wien, Austria, and Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163, A-1060 Wien, Austria Received December 2, 2003. In Final Form: January 20, 2004 The growth of self-assembled monolayers from octadecyltrichlorosilane (OTS) on modified silicon surfaces has been investigated. The influence of different immersion times in a deactivation reagent on the growth mechanism and the ordering of the films has been studied. Characterization of the films and the submonolayer coverage has been performed with tapping mode atomic force microscopy, ellipsometry, and infrared spectroscopy. We found that a deactivation of active sites led to a higher mobility of adsorbed molecules on the surface resulting in circular islands of highly ordered alkylsiloxane. However, upon prolonged immersion in OTS these ordered islands did not continue to grow and full monolayer coverage could not be obtained. Instead, an exchange reaction with the deactivation reagent leading to a disordered film between the ordered islands was observed. This was confirmed by external reflection infrared spectroscopy.

Introduction The study of self-assembled monolayers (SAMs) and their applications has largely expanded in recent years. Layers with very specific properties can be obtained through self-assembly. They are used in various applications such as protective coatings, modeling of biomembrane functions, biosensors, lubricant additives, and surface modifications [e.g., refs 1 and 2 and references therein]. For organosilicon compounds, the growth depends on various deposition parameters such as solvent,2,3 temperature,4,5 solution age,6,7 precursor concentration,8 water content of the solvent,6,8 deposition method,3 and the substrate type.9,10 Self-assembled monolayers grown by use of n-alkyltrichlorosilanes as precursors are robust, organized films, chemisorbed on solid substrate surfaces. Their stability and resistance against mechanical, thermal, and chemical attacks result from strong covalent bonds to the substrate surface and cross-polymerization between adjacent octadecyltrichlorosilane (OTS) molecules. The covalent bonds to the substrate are formed by a reaction of the hydrolyzed OTS with surface silanol groups. In the past, various groups have investigated the relationship * Corresponding author. Phone: +43 58801 15110. Fax: +43 1 58801 15199. E-mail: [email protected]. † Institute of Chemical Technologies and Analytics. ‡ Institute of Applied Synthetic Chemistry. (1) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (2) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Parikh, A. N.; Allara, D. L.; Azuz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (5) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441. (6) Leitner, T.; Friedbacher, G.; Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Mikrochim. Acta 2000, 133, 331. (7) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 790. (8) Foisner, J.; Glaser, A.; Kattner, J.; Hoffmann, H.; Friedbacher, G. Langmuir 2003, 19, 3741. (9) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (10) Brunner, H.; Vallant, T.; Mayer, U.; Hoffmann, H.; Basnar, B.; Vallant, M.; Friedbacher, G. Langmuir 1999, 15, 1899.

between the hydration state of silicon and the formation of alkylsiloxane films. Angst and Simmons11 found closely packed monolayers of ODS (octadecylsiloxane) on fully hydrated silicon surfaces, as well as lower coverage on dry silicon wafers. Similarly, Allara et al.12 found that proper substrate hydration is a fundamental prerequisite for the deposition of densely packed, highly organized alkylsiloxane monolayers of reproducible structure. The presence of a water layer acting as a lubricant layer between the monolayer and the substrate suggests that film organization is decoupled from the substrate chemistry. Thus, monolayers of similar structure can be obtained on substrates of widely different chemical characteristics. In contrast to Allara et al.,12 Le Grange et al.13 reported that a completely hydrated surface is not necessary for obtaining full monolayer coverage. In an early atomic force microscopy (AFM) study, Bierbaum et al.9 found that the characteristics of island growth are influenced by the chemical state of the silicon substrate. They suggested that changes in functional groups on the substrate affect surface diffusion of OTS molecules and therefore may also influence shape and size of the growing islands. In a comparative study of two widely different substrate materials, silicon and mica, Brunner et al.10 found two types of substrate effects influencing the growth of ODS, the number of active OHgroups for chemisorption and surface charges attracting organic molecules from solution. The total number of OHgroups on a fully activated silicon wafer, which is the most frequently used substrate for investigation of self-assembly processes of alkylsiloxanes, has been assumed to be 5 × 1014 per cm2.13,14 These silanol groups act as binding sites and therefore restrict the diffusion of hydrolyzed OTS molecules on the surface. This results in a large number of smaller islands, which show sharp and “spiky” edges. (11) Angst, D. L.; Simmons, G. W. J. Langmuir 1991, 7, 2236. (12) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (13) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (14) Wassermann, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852.

10.1021/la036261e CCC: $27.50 © 2004 American Chemical Society Published on Web 03/06/2004

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On mica, the number of OH-groups is negligible. Therefore, large ordered islands with circularly shaped, smooth edges have been observed. By deposition of full monolayers of ODS on mica and subsequent oxidation of the hydrocarbon chains, well-defined silicon oxide layers were formed. The deposition of only one layer of silicon oxide on mica led to an abrupt change of the morphology of subsequently deposited submonolayer islands of ODS. The authors concluded that island morphology and structures are only controlled by the chemical composition (number of reactive sites) of the topmost substrate layer and that there is no influence of the underlying bulk material. In contrast to that, a continuous decrease of adsorption rates has been observed for an increasing number of silicon oxide layers. The different growth rates on mica and silicon have been discussed to arise from the negative surface charge of freshly cleaved mica. Long-range interactions with the polar headgroup of the OTS molecules might enhance adsorption compared to an uncharged substrate like silicon. When the oxide layer thickness is increased, this enhancement of the adsorption is reduced due to increasing shielding of the surface charge. The findings described above lead to the conclusion that two parameters, on the one hand, the number of active sites on the surface, and on the other hand, the presence of a water film on the substrate surface, govern the selfassembly process. This water film, consisting of a few monolayers of water, preferentially develops on hydrophilic surfaces such as mica or silicon oxide. To study the influence of this parameter, we have considered hydrophobization of our silicon substrates. Moreover, chemical modification of our substrates was aimed at control of the number of reactive hydroxyl groups through blockage with a small molecule. Tripp and Hair15,16 have investigated the reaction of different chloromethylsilanes on silica by infrared spectroscopy. They proposed an order of reactivity with the surface silanol groups of (CH3)3SiOH > (CH3)2Si(OH)2 > CH3Si(OH)3 (after hydrolysis of the respective chlorosilanes). They observed that trimethylchlorosilane (TMCS) reacts with surface silanols blocking about 80% of the accessible OH-groups. A later study using XPS, DRIFT, and Si-MAS NMR confirms this reactivity scale.17 Concerning the hydrophobicity of the surface, Zhao and Lu18 have shown that the surface chemistry of MCM-41 molecular sieves can be effectively modified toward higher hydrophobicity by chemical attachment of trimethylchlorosilane. They demonstrated that a silylated surface is very hydrophobic and does not show significant adsorption of water. Fuji et al.19 investigated the relationship between coverage of hydrophobic groups on glass bead surfaces and macroscopic properties such as wettability. They observed a strong increase of the contact angle at surface coverages of about 50%, which means that the sample surface changes significantly from a hydrophilic to a hydrophobic surface. These results as well as the fact that TMCS is a monofunctional reagent with a simple reaction pathway, compared to multifunctional silanes,15,16 encouraged us to utilize TMCS for our hydrophobization experiments. In this paper, the effects of chemical modification of silicon with TMCS on subsequent growth of alkylsiloxane (15) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149. (16) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7, 923. (17) Garbassi, F.; Balducci, L.; Chiurlo, P.; Deiana, L. Appl. Surf. Sci. 1995, 84, 145. (18) Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. (19) Fuji, M.; Fujimori, H.; Takei, T.; Watanabe, T.; Chikazawa, M. J. Phys. Chem. B 1998, 102, 10498.

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Figure 1. Layer thickness of ODS layers versus deactivation time of the silicon substrate in TMCS. The adsorption time for OTS was 20 s in all cases. A strong decrease of the coverage can be observed even for short deactivation times.

monolayers have been studied with AFM, external reflection infrared spectroscopy, and ellipsometry. Experimental Section Film Preparation. Monolayers of alkylsiloxanes were prepared from solutions of octadecyltrichlorosilane (Aldrich, 90+%) in HPLC-grade toluene (Aldrich, 99.8%). All experiments have been performed at OTS concentrations of 1 mmol/L. The deactivation of the active sites has been performed with trimethylchlorosilane (Aldrich, >99%), using a concentration of 5 mmol/L. The water concentration was adjusted by doping commercial toluene (water concentration ∼ 5-6 mmol/L) with a calculated amount of distilled water. The residual water concentration of the solvent was measured with a commercial Karl Fischer setup (Metrohm Karl Fischer Automat E547). The water concentration was kept in a range of 15 ( 0.5 mmol/L for all experiments. Substrates. For all experiments, silicon wafers from Wacker Siltronics AG (prime grade single side polished CZ-Si, orientation 〈100〉, p-type (boron), diameter ) 100 mm, thickness ) 500-550 µm, resistivity ) 7-21 Ω/cm) were used. The wafers were cut into 1 × 1 cm2 pieces and cleaned in toluene in an ultrasonic bath for 5 min, followed by rinsing with toluene, acetone, and ethanol. Afterward, the pieces were put in a UV chamber for 15 min in order to activate the surface for the adsorption process (formation of silicon oxide and silanol groups). The wafers used for the infrared measurements were cut into 1.5 × 2.5 cm2 pieces in order to increase the analytical signal. Film Deposition. The deposition experiments were performed under a nitrogen atmosphere in a glovebox. Toluene with the adjusted water content was pipetted into a glass jar. Then the desired reagent volume was added using an Eppendorf pipet. Afterward, the wafers were immersed into the solutions, followed by rinsing with toluene. In the case of deactivated substrates, the wafers were first immersed into TMCS solutions, rinsed with toluene, and subsequently deposited into OTS solutions. Finally, they were deposited in a beaker with toluene. During these experiments, only the immersion times were varied. The deposition temperature was kept constant at 20 °C. AFM Measurements. The AFM measurements were accomplished with a Nanoscope III Multimode SPM from Digital Instruments, Veeco Metrology Group, Santa Barbara, CA, using an E-scanner (10 × 10 µm2 scan range). They were performed in tapping mode using silicon cantilevers with integrated silicon tips (spring constant ) 28-52 N/m, resonance frequency ∼ 330 kHz) under ambient conditions. The images were recorded in constant amplitude mode with a resolution of 512 × 512 pixels at scanning rates from 1.0 to 4.0 Hz. Ellipsometric Measurements. The ellipsometric measurements were performed with a PLASMOS SD 2300 instrument. Determination of the silicon oxide film thickness and the organic film thickness is based on a model of three layers (Si/SiOX/air) and a four-phase model (Si/SiOX/adsorbate/air), respectively. A commercial software based on the McCrackin20 algorithm was

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Figure 2. AFM images of ODS films on silicon modified through increasing deactivation times of (a) 0, (b) 30, (c) 60, (d) 120, and (e) 300 s in TMCS (c ) 5 mmol/L). The films were grown through immersion in OTS (c ) 1 mmol/L) for 20 s. Circular islands appear after about 60 s of deactivation in TMCS. used to calculate the film thickness from the measured ellipsometric angles. For each sample, five different spots were measured and the average value was determined. In this manner, typical standard deviations between 0.1 and 0.5 Å have been obtained. IR Measurements. External reflection infrared spectra were measured with p-polarized radiation and at a fixed incidence angle of 80°, using a custom-made external reflection optical system connected to a Mattson RS1 FT-IR spectrometer, as described in detail elsewhere.21 For each spectrum, 1024 scans at 4 cm-1 resolution were coadded from both the sample and the reference.

Results and Discussion Figure 1 shows the effect of deactivation in TMCS for immersion times from 0 (no deactivation, reference wafer for the OTS adsorption) up to 10 min. The immersion time in OTS was 20 s in all cases. The layer thickness decreases from about 2.2 ( 0.1 nm to around 0.9 ( 0.1 nm for a deactivation time of only 30 s. A sharp decrease in adsorption within only 1 min of deactivation can clearly be seen. Then, the layer thickness slowly decreases until a practically constant value is reached after 5 min. The layer thickness after 5 min of deactivation, followed by adsorption in a 1 mmol/L OTS solution for 20 s, is about 0.2 nm higher than that of a sole TMCS layer. Immersion of silicon wafers in a 5 mmol/L TMCS solution for 10 min resulted in a maximum layer thickness of only about 0.2 nm as determined by ellipsometry. These results clearly show that deactivation of the natural silicon oxide surface with TMCS leads to a dramatical decrease of the growth rate of subsequent OTS adsorption. Prolongation of the TMCS treatment up to 6 h did not change this behavior, which indicates that a full deactivation of the surface cannot be achieved. Figure 2 shows AFM images corresponding to the data shown in Figure 1. Figure 2a shows fractally shaped (20) McCrackin, F.; Passaglia, E.; Stromberg, R.; Steinberg, H. J. Res. Natl. Bur. Stand. Sect. A 1963, 67, 363. (21) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151.

Figure 3. Scheme of the exchange reaction between TMCS and OTS in the presence of surface water.

islands which are typically achieved for OTS adsorption onto a fully activated silicon substrate. Surface coverages of about 85% (∼2.2 nm) are usually obtained for an adsorption time of 20 s of OTS under the given conditions (toluene solution containing 15 mmol/L H2O). In Figure 2b, the situation after 30 s of immersion in TMCS and 20 s of deposition in OTS is illustrated. The island size decreases from over 2 microns to around 400 nm for the larger islands. These islands exhibit a spiky, circular shape. Many smaller islands with an average size of about 100 nm can be observed as well. The height of the islands corresponds to totally aligned OTS molecules perpendicularly resting on the surface. A further increase of the deactivation time leads to a decrease in size and number of the smaller islands until they almost completely disappear after 10 min of deactivation, whereas the larger islands seem to be constant in both size and number. The shape of the resulting islands more and more adopts a circular geometry upon increasing deactivation. This transition in shape from a fractal form for the fully activated wafer surface (Figure 2a) to an almost perfect circular shape for the deactivated surfaces (Figure 2c-e) can be explained by a decrease of surface hydroxyl groups. In the case of full activation, a maximum

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Figure 4. AFM images of ODS films on silicon obtained after deactivation with TMCS for 30 min. The wafers were deposited into a 1 mmol/L OTS solution for (a) 5 min, (b) 30 min, (c) 60 min, (d) 4 h, and (e) 20 h. Circular islands can be observed in all cases. (f) Section profile along the white lines marked in images a and e emphasizing the decreasing height contrast.

number of OH-groups is available on the silicon surface. Thus, there is a high probability for the approaching OTS molecules or aggregates to be trapped by formation of a covalent bond (through condensation) with the substrate. The fewer active sites that are accessible on the surface, the longer the adsorbed species are able to diffuse on the substrate. Thus, they are able to migrate on the surface and to rearrange themselves into energetically favorable circular islands. In the AFM images, spiky circular islands have been observed up to a deactivation time of 2 min. For longer immersion times in TMCS, visible fringes at the periphery of the islands have not been detected anymore. At this point, it should also be stressed that we believe that growth of the observed islands is initiated by adsorption of larger ordered aggregates from solution which can rearrange themselves on the surface depending on the surface properties of the substrate. This assumption is supported by the fact that fairly large islands are observed right from the beginning of the adsorption process and by dynamic light scattering experiments of the precursor solution described elsewhere.22 Besides the number of active silanol groups, however, a further parameter influencing the mobility of OTS molecules is the water film on the surface. With higher hydrophobicity of the surface, the contiguity of the water film decreases, restricting diffusion of OTS molecules to smaller areas. Thus, smaller islands are observed. Next, the persistence of the deactivation was investigated by increasing the adsorption times for OTS. For this purpose, the silicon wafers were immersed into a 5 mmol/L TMCS solution for 30 min. Afterward, they were transferred into a 1 mmol/L OTS solution for different periods of time up to 20 h. Ellipsometric data of the corresponding experiments have shown that the layer thickness doubles from 0.4 nm for 20 s of adsorption to 0.8 nm after 5 min of adsorption. Prolongation of the deposition time in the OTS solution leads to a further increase in (22) Glaser, A.; Foisner, J.; Hoffmann, H.; Friedbacher, G. Langmuir, submitted.

layer thickness up to 2 nm after 4 h and 2.3 nm after 20 h. Extended sonication in ethanol and intensive wiping of the wafers did not affect the layer thickness significantly. Therefore, a corruption of the data by physisorbed species can be excluded. It should also be stressed that even after a prolonged period of exposure for 20 h, the thickness of a highly ordered, densely packed film could not be obtained on such deactivated wafers. It cannot be excluded, however, that full monolayer coverage may be obtained after even longer adsorption times. However, such longterm experiments are not feasible due to degradation of the sensitive precursor solution during the experiment. The results described above indicate that the deactivation is either not complete or reversible. In the case of an incomplete deactivation, only a small percentage of the surface should be covered by TMCS. Since the achieved layer thicknesses were about 80% of full monolayer coverage,23 this would mean that only the remaining 20% should have been covered by TMCS. However, Tripp and Hair16 have reported that TMCS reacts with surface silanols blocking about 80% of the accessible OH-groups. Therefore, our results cannot be solely explained by incomplete deactivation of the silicon substrate. In the case of a reversible deactivation, an exchange reaction between TMCS and OTS would need to take place. This could explain the obviously decelerated growth of the monolayers after deactivation. Such exchange reactions have been reported for self-assembled monolayers of thiols on gold,24-26 but not yet for alkyltrichlorosilanes. However, Wang et al.27 found that chemisorbed siloxanes are able to migrate a few nanometers on silicon under certain conditions. They proposed a mechanism including (23) The value of 80% corresponds to a layer thickness of 2.1 nm which is obtained by subtracting 0.2 nm (maximum measured thickness for the sole TMCS layer) from the total measured thickness of 2.3 nm. (24) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (25) Chung, C.; Lee, M. J. Electroanal. Chem. 1999, 468, 91. (26) Kajikawa, K.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1997, 36, L1116.

Effects of Surface Hydrophobization on SAM Growth

Figure 5. Height of ODS islands versus adsorption time. Data correspond to the images shown in Figure 4. The wafers were immersed in OTS for 5, 30, 60, 240, and 1200 min after a deactivation period of 30 min in a 5 mmol/L TMCS solution.

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Figure 6. IR spectra of ODS films on silicon substrates: (A) closely packed ODS monolayer, (B) “hybrid” spectrum of a film showing more ordered than disordered portions, (C) “hybrid” spectrum of a film showing almost equal parts of ordered and disordered contributions, and (D) disordered ODS film.

slow hydrolysis of the siloxane bonds in the presence of water. This hydrolysis reaction leads to physisorbed species, which are able to migrate on the surface and to rebind to the surface at adjacent silanol sites. At the first glance, on our surfaces hydrophobized with TMCS, a surface water layer involved in such a reaction should not exist. However, Ye et al.28 have found interfacial water between a fused quartz surface and an ODS monolayer on top of it by in situ SFG (sum frequency generation spectroscopy). The existence of such a water film has also been proposed by other groups. It may be explained by the fact that not every single OTS molecule in the film is bonded to the surface. Thus, enough space between organic film and substrate surface is available for the water molecules. From these considerations, we conclude that water plays an important role in our observed exchange reaction. In fact, we expect a similar reaction mechanism as reported by Wang et al.27 (Figure 3). AFM images corresponding to this experiment are shown in Figure 4. The stripes visible in the images originate from cleaning the wafer with a tissue after the deposition experiment in order to remove physisorbed OTS molecules and aggregates which are always present after long time deposition experiments. Circular islands can be found in all images. When the immersion time in OTS is increased to 20 h, the number and size of the islands do not change significantly. The size of the islands varies in a range between 100 and 500 nm. Regarding the height of the islands (shown in Figure 5), a decreasing height contrast from 2.2 to 1.8, 1.3, 0.7, and 0.3 nm can be observed. This further supports an exchange reaction of the small TMCS molecule by OTS. The fact that further island growth is not observed could be explained by local replacement of individual TMCS molecules. Thus, the OTS molecules are not able to align to each other by van der Waals interactions leading to disordered films in that region. To obtain information on the state of ordering of the adsorbed OTS molecules, infrared spectra of the deposited monolayers were recorded. Figure 6 shows the CH stretch region of IR spectra for four different states of order. In the case of a densely packed and ordered ODS film (Figure 6A), obtained after an immersion time of 30 min in a 1 mmol/L OTS solution, two bands are visible, corresponding

to the symmetric methylene stretch νs(CH2), at about 2850 cm-1, and the antisymmetric methylene stretch νas(CH2), at about 2918 cm-1. At about 2967 cm-1, a small downward pointing band for the terminal methyl group can be seen. Figure 6D shows the IR spectrum of a disordered ODS film. The corresponding sample was prepared by OTS deposition above the critical temperature TC4,5,29 for ordered self-assembly. The film thickness obtained by ellipsometry was 1.3 ( 0.1 nm. Broad, downward pointing absorption bands at about 2856 cm-1 (νs(CH2)) and 2928 cm-1 (νas(CH2)) are visible. The inversion of the absorption bands and the shift toward slightly higher wavenumbers can be utilized for a very sensitive detection of structural order-disorder transitions, for example, caused by a change in the surface coverage. A detailed explanation of the reason for this band inversion can be found elsewhere.7,30 The spectra shown in Figure 6B,C correspond to wafers that were deactivated with TMCS for 30 min. After deactivation, they were immersed into an OTS solution for 4 and 20 h, respectively. The spectra represent a linear combination of the spectra shown in Figure 6A,D, indi-

(27) Wang, H.; Harris, J. M. J. Am. Chem. Soc. 1994, 116, 5754. (28) Ye, S.; Nihonyanagi, S.; Uosaki, K. Phys. Chem. Chem. Phys. 2001, 3, 3463.

(29) Rye, R. R. Langmuir 1997, 13, 2588. (30) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304.

Figure 7. Scheme for the island termination reaction.

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Figure 8. AFM images of ODS films on silicon modified through deactivation in a 1 mmol/L TMCS solution for 20 s. OTS adsorption times were (a) 0, (b) 20, (c) 60, (d) 300, (e) 600, and (f) 3600 s. Circular islands cannot be observed, and full monolayer coverage is achieved after 1 h.

cating that both densely packed, highly ordered regions and disordered areas are present on these samples. The spectra shown in Figure 6B,C differ only in the relative intensity ratios of the absorption bands indicating an increase of the portion of the disordered film upon prolonged adsorption, which is also confirmed by the ellipsometric data. The film thickness was measured to be about 1.9 ( 0.1 nm for the sample presented in Figure 6B and 2.3 ( 0.1 nm for the sample in Figure 6C. Since the absolute portion of the ordered areas is constant, this increase in layer thickness must be contributed to the disordered fraction of the film. Considering the described results, a few aspects require further discussion. Surprisingly, the initial number of islands does not increase on the deactivated silicon surfaces during adsorption for a prolonged period of time. From that, it could be concluded that adsorption of islands only takes place at preferred adsorption sites with a higher concentration of free silanol groups. Furthermore, the size of the islands does not change significantly with increasing adsorption time. This indicates that particularly reactive groups at the border of the ordered regions are blocked. An explanation for such a blocking of reactive groups at the border of the ordered islands is shown in Figure 7. The OTS molecules are adjacent to TMCS molecules for an extended period of time, and thus TMCS can react with an island-terminating OTS molecule blocking its last freely available OH-group. Therefore, such terminal OTS molecules are no longer preferred sites for adsorption and island growth is stopped as observed by AFM. To further confirm the exchange mechanism proposed in this paper, an additional experiment with a lower TMCS concentration and a shorter deactivation time has been performed. Under these conditions, the number of reactive sites should be higher and the ordering of the resulting film should be higher as well. Figure 8 shows the AFM images obtained in this experiment. The islands are still smaller than expected for an adsorption of OTS on a fully activated silicon wafer; however, they show a fractal, spiky shape. Increasing the adsorption time of OTS leads to a

decrease of the height contrast of the islands. After 10 min, only a very weak island contrast can be observed, and after 60 min the islands disappear completely indicating the formation of a closed monolayer film, which is also confirmed by the ellipsometric film thickness of 2.6 ( 0.1 nm. This behavior can be explained by a decreased number of blocked silanol groups. This in turn increases the probability for OTS molecules to directly interact with each other via van der Waals forces leading to a highly ordered film. Conclusion We have shown that a blockage of surface hydroxyl groups by a short-chain molecule like TMCS opens up the opportunity to contribute to the study of the mechanism of alkylsiloxane self-assembly processes on silicon surfaces. We found that the growth behavior of self-assembled monolayers of alkylsiloxanes changes dramatically by a stepwise deactivation of active sites (surface OH-groups). Decreasing the number of binding groups results in a decrease in size and number of the islands. The island shape changes from a fractal to a nearly circular geometry. Furthermore, it can be concluded that the size of the islands is influenced by the contiguity of the water film which in turn can be controlled through hydrophobization with TMCS. We found that the deactivation is reversible, which can be explained by an exchange reaction, where OTS molecules replace the covalently bonded TMCS. In the case of longer deactivation times, the monolayer consists of two different ODS regions with different states of order: ordered islands which are formed rather quickly in the beginning of the adsorption experiment and disordered regions formed during a comparatively slow exchange reaction. Acknowledgment. Financial support of this work by the Austrian Fonds zur Fo¨rderung der wissenschaftlichen Forschung (FWF, Project Number P14763) is gratefully acknowledged. LA036261E