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Preparation, Structure, and Mechanical Stability of Alkylsilane Monolayers on Mica Xu-Dong Xiao, Gang-yu Liu, Deborah H. Charych, and Miquel Salmeron" Materials Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 Received July 6, 1994@ The preparation, structure, and mechanical stability of self-assembled monolayers formed by octadecyltriethoxysilane (OTE) on mica have been studied by atomic force microscopy. The nanometer scale morphology of the films (3-D clusters, pinholes, etc.) is compared for various preparation methods and corrqlated with their macroscopicwettability. High-resolutionimages of the atomically smooth monolayer (1.5A root mean square roughness) reveal the existence of only short range order. By applyinga load above 10 nN with sharp Si3N4 tips (radius < 300 A), the film could be removed leaving 25 A deep holes. Using the same tip, thiol monolayers on gold could be displaced at loads ~5 nN, although, in this case, the displacementwas reversible. In contrastto the case of OTE, films formed by octadecyldimethylmethoxysilane (ODMS) showed the presence of only 3-D clusters with poor adhesion to the mica substrate. On the basis of these results, we conclude that the mechanical strength of the films formed by OTE is due to siloxane cross-linkingbetween molecules rather than to chemical bonding to the mica substrate. 1. Introduction
Self-assembled monolayers (SAM) provide excellent models for studies of adhesion and lubrication by organic films. In a recent study in our laboratory, we found that the SAMs formed by alkylthiols on Au(111)could support the pressure exerted by the AFM tip while preserving their (& x &)R30 structure up to critical 1oad.l Above this critical load, the thiol molecules in the layer were displaced by the AFM tip such that the gold substrate was imaged. This displacement was found to be reversible, i.e., by reducing the load below a critical value (smaller than the one needed for the initial displacement), the (& x &)R30 structure of the thiol molecules is recovered without any visible damage to the film or substratp. The radius of the tips used in this work was ~ 7 0 A. 0 Smaller or higher tip radii shifted the critical load for thiol displacement (and recovery)to substantially smaller or higher values, respectively.2 To understand the relative contributions of molecule-molecule and moleculesubstrate interactions to film stability against external forces, we directed our next studies to films formed by molecules that form intermolecular chemical bonds. SAMs formed by alkylsiloxanes on mica were chosen for that purpose. Mica is an attractive substrate for such mechanical strength studies because molecule-molecule bonding can, in principle, be isolated from moleculesubstrate bonding. This is because the mica substrate is anticipated to be free of hydroxyl functionalities which are known to form siloxane bonds with trichloro- or trialkoxyalkylsilanes. However, the disadvantage ofmica is that unlike thiols on gold, the preparation of alkylsilane films on this atomically flat, inert substrate is not straightforward. It is a well-established fact that alkylsilanes can easily self-assemble onto glass, silicon oxide, and metal oxide surfaces by reaction of the chlorosilyl groups with the hydroxylated surface. As a result, siloxane bonds are formed with both the surface and neighboring ~ i l a n e s . ~ , ~ Because few hydroxyl groups exist on the mica surface, @Abstractpublished in Advance A C S Abstracts, May 1, 1995. (1)Liu, G.-y., Salmeron, M. Langmuir 1994, 10,367. (2) Salmeron, M.; Liu, G.-y.; Ogletree, D. F. In Forces in Scanning Probe Methods; eds. Gunterodt, H.-J., Anselmetti, D., Meyer, E., Eds.; NATO AS1 Series E.; Kluwer Academic Publishers: Dordrecht, The Netherlands. In press. ( 3 ) Sagiv, J. J . Am. Chem. SOC.1980,92, 102.
methods have been developed with the intent to modify its chemical nature. One involves pretreating mica by exposure to water vapor or sodium ethoxide to help introduce silanol groups (SiOH).5,6 Another involves prehydrolysis of OTE followed by self-assembly and crosslinking onto the mica ~ u r f a c e . ~ In either case, the formation of covalent bonds between the silane molecules and the mica surface has not been established. Although IR spectroscopy can provide information about chemical bonding, it cannot easily distinguish the Si-0 bonds formed between the silane molecules and mica from those formed by cross-linkingamong the silanes. Other methods of characterization in addition to IR, such as contact angle measurement and ellipsometry, provide information on wettability, packing density, average molecular orientation, and film thickness8 Although the formation of a single monolayer is assumed, silane molecules may polymerize in the solution and form large clusters on top of the monomolecular film. In addition, uncovered areas (holes)can be present. In light of this, results of packing density measurements have to be taken with caution. Atomic force microscopy (AFM)is a recent technique that is suitable for solving some of the above problems because of its high spatial resolution in both surface normal and lateral directions, providing information on multilayer or cluster formation and molecular packing density. The purpose of this paper is 2-fold: one is to describe our preparation method to produce SAM films from OTE on mica and to correlate the nanometer scale morphology of these films with their macroscopic property of water contact angle that is often used to characterize their quality. The second is to present our findings on the mechanical stability ofthe OTE films and compare it with that of thiol films of the same alkyl chain length on Au previously studied using the same AFM tips. These findings, in addition to the results found using octadecyldimethylmethoxysilane (ODMS),a molecule with only one siloxane bond, indicate that it is the molecular cross(4) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991; Chapter 3, pp 237 and references therein. ( 5 ) Nakagawa, T.; Ogawa, K. Langmuir 1994, 10,525. (6)Schwartz, D. K.; Steinberg, S.; Israelachvilli, J.;Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (7) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (8)Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074.
0743-746319512411-1600$09.00/0 0 1995 American Chemical Society
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Alkylsilane Monolayers on Mica linking through the siloxane bonds, rather than their binding to the mica substrate, that is responsible for the mechanical stability of the OTE films.
2. Experimental Section 2.1. Materials. N-Octadecyltriethoxysilane (OTE) and noctadecyldimethylmethoxysilane(ODMS)were purchased from Huls America, Piscataway, NJ. OTE was vacuum-distilled upon receipt. OTE and ODMS were filtered through 0.2 pM PTFE membranes immediately prior t o preparation of prehydrolysis solutions. All alkylsilane materials were stored in a desiccator. House-distilled water was passed through a four-cartridge Millipore pQF purification train producing a resistivity of 18.2 MQ cm. Tetrahydrofuran (THF) and cyclohexanewere spectral quality. Glassware for preparation of prehydrolysis solutions and for self-assembly of OTE and ODMS was cleaned with Nochromix reagent or chromic acid immediately prior t o use. 2.2. Film Preparation Methods. The OTE and ODMS films were prepared essentially following the procedure of Kessel and Granick;' however, several process variables were introduced. Prehydrolysis solutions were prepared by dissolving 0.2 gof OTE or 0.16 g of ODMS (19 mM) in 24 mL of THF containing 0.2 g of 1N HC1. The solution was stirred at room temperature for 2-3 days. Before further dilution of the solution for depositing onto mica, the prehydrolysis solution was subjected to the following treatments: series I, no treatment; series 11, centrifugation at 3000 rpm for 1 h (sample temperature was -35 "Cafter centrifugation); and series 111, filtration through 0.2 pm nylon or PTFE membranes. Following the treatments, the prehydrolysis solution (1.5-2.0 mL) was diluted with 25 mL of cyclohexane. The cloudy solution was added to cleaned Petri dishes containing freshly cleaved mica samples. For each series, the freshly cleaved mica is immersed into the diluted solution for a variable period of time. Following reaction, the samples were briefly rinsed with fresh cyclohexane and dried under a stream of argon or nitrogen. In some cases, the samples were baked at 115 "C for 2 h. No difference in results were observed, however, if this step was left out. Samples were immediately characterized by AFM and water contact angle measurements. The resulting OTE/mica films were stable under cyclohexane,as judged by their hydrophobicity and molecular scale morphology. 2.3. Contact Angle Measurement. Advancing contact angles were measured with a Rame-Hart contact angle goniometer (Mountain Lakes, NJ). Typically, a droplet of 1-3 pL of water was applied t o the sample. Time-dependent water contact angles were measured in a humid local environment maintained by water-saturated filter papers in the enclosed sample chamber. 2.4. AFM Measurement. Two different atomic force microscopes have been used in our study. In both cases, the AFM images were taken in the contact mode at constant force (load). Large area imaging ('1 pm x 1 pm) was carried out with an Autoprobe AFM Park Scientific Instrument. In this case, only topographic images were obtained. Small scale images were obtained with a home-built AFM9 using commercial Si3N4 cantilevers, Two types of tips were used that have very different apex radii. The first type of tip was purchased from Digital, Inc. They have a nominal force constant of 0.58 N/m. Their radius, as estimated from step images (height and width) is typically between 700 and 1000 A. The second type was the sharpened Park Scientific tips with a nominal force constant of 0.1 N/m. We found their radius to be between 100 and 300 A. Since pressure, rather than force or load, is the important parameter in the mechanical test experiments, these two types of tip can produce similar effects at loads that might differ by 1order of magnitude or more. Where appropriate, then, we specify the type of tip used in the experiment. Atomic resolution images were routinely obtained on mica substrates in both cases. A quadrant rather than a bisector photodiode enabled acquisition offrictional images in addition to topographic images. The mechanical stability of the films is evaluated by measuring the force required to move the adsorbed molecules from their sites (scratch removal). The AFM piezoelectric scanners were calibrated using the mica periodicity in thex-y direction and atomic Au(ll1) steps in the z direction. The load applied to measure the mechanical stability (9) Kolbe, W. F.;Ogletree, D. F.;Salmeron, M. Ultramicroscopy 1992, 42-44, 1113.
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Time (min) Figure 1. Time evolution of the water contact angle on a selfassembled film formed by OTE on mica. Three preparation conditions from a prehydrolysis solution were used: (a) no treatment, (b) after centrifuging, and (c) after filtering the solution. For each of the three series, four different immersion times were evaluated.
of the films is calculated from the nominal force constant and the amount of cantilever deflection. Increasing load is achieved by advancing the sample toward the cantilever. All measurements were carried out under ambient laboratory conditions.
3. Results and Discussion 3.1. Correlation between Water Contact Angle Stability and Surface Morphology. All the OTE film samples prepared have an initial water contact angle of 110". However, the time dependence or stability of the contact angle vanes dramatically among the samples as shown in Figure 1, for series, I, 11, and 111. For series I and I1 only, the stability of the water contact angle
1602 Langmuir, Vol. 11,No.5, 1995
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Figure 2. 10 pm x 10pm AFM images of the OTE films on mica corresponding to the three series of Figure 1. Immersion time is increasing from left to right. First row: no treatment of the prehydrolysis solution (unfiltered). Soecondrow: after centrifuging qf solution. Third row: after filtering the solution. The clusters range in size from 100 to 10 000 A with heights from 10 to 500 A.
improves significantly with increasing immersion time in the OTE solution. At 30 s immersion, the contact angle drops from 110" to -40' in 30 min. At 60 min immersion time, the contact angle is unchanged in 30 min. The stability for series I improves with increasing immersion time while that of series I1 improves rapidly a t first and then stabilizes;otherwise, the trends are similar for both series. On the other hand, the contact'angle for samples in series I11 does not stabilize. It drops from an initial value of 110" to -40" in 30 min, even for a 60 min immersion time. Following previous criteria' that stable water contact angles are a measure of the tenacity and quality of a monolayer, we would conclude that the first two methods provide well-organized OTE films, while the third method results in poorly formed or poorly anchored OTE layers. However, the AFM images shown in Figure 2 do not support the above conclusion. For series I and 11, where the highest contact angle stabilityis observed, many threedimensionalclusters are observed (firsttwo rows of images in Figure 2). The clusters arise from 3-D polymerization in the prehydrolysis solution, followed by deposition onto the surface. Their diameter range? from 100 to 10 000 A and their height from 10 to 500 A. The density of the clusters also increases with the immersion time. The binding of these aggregates to the underlying monolayer is weak because the clusters can be removed by ultrasonic cleaning or by the AFM tip under low load (40 nN with
a tip radius of ca. 1000 A). In series 111, filtering of the prehydrolyzed solution removes most of the large OTE clusters and provides flatter surfaces, as shown in Figure 2 (bottom row). Although a few clusters are still present from time to time, their density is significantly less compared to series I and I1 and is relatively the same for all immersion times. Therefore,all immersion times show contact angle instability. The results indicate that the presence of three-dimensionalclusters may help stabilize the water-film interaction, perhaps by providing microscopic hydrophobic boundaries to spreading. In an effort to obtain atomicallyflat samples with stable water Contact angles, we have also tried to ultrasonically clean the samples from series I and 11. After 30 min sonication, the samples indeed become cleaner, however, three-dimensional aggregatesare still present a t a density which is higher than that of series 111. In agreement with our previous conclusion, the water contact angle stability is lower than that of the original unsonicated samples, but is higher than that of series 111. From the above observations, we conclude that the contact angle stability is neither a sufficient nor a necessary condition for high-quality,aggregate-free monolayer formation with angstrom-level surface flatness. 3.2. Structure of OTEMica Films. Images of OTE films from series I11 corresponding to less than 1s and to 30 s immersion times are shown in parts a and b and part c, respectively, of Figure 3. For short immersion times,
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pinholes a few hundred angstroms wide and 25 A deep are observed, in agreement with previously published results in which similar pinholes were observed in OTS films prepared by pretreatment of mica with sodium e t h ~ x i d e .Unlike ~ the latter preparation, however, we believe that the pinholes observed here are due to incomplete surface coverage rather than an intrinsic surface defect. At a different spot, we can also image areas of partiFlly covered mica (Figure 3b). The OTE islands are 25 A in height, in agreement with the thickness obtained by ellipsometrylo and X-ray reflectivity measurements. l1 With longer immersion times, typically >30 s, the surface appears to be fully covered and no pinholes are visible under our imaging conditions (figure 3c). This result is very different than both that of the ethoxy-treated mica5 and also from that of the steam-treated mica.6 “Scars”were observed in the former films even a t complete coverage. The difference in quality may arise from inhomogeneities in the pretreated mica surface or to impurities adsorbed on the mica during the following pretreatments. In these pretreatments, the mica surface is exposed to a nitrogen atmosphere overnight prior to OTS self-assembly,possibly producing adsorbed contaminants. The use of prehydrolyzed OTE allows immediate immersion of freshly cleaved mica into the solution. Also, since no prior treatment of the mica surface is required, the mica is in the most pristine state possible. All these factors result in the formation of high-quality OTE films. High-resolution AFM images were taken, as shown in Figure 4. The topographicimages reveal a smooth surface with a root mean square corrugation of 1.5 A. Unlike the alkylthiols/Au(11l), however, topographic and frictional force images indicate that there is no long-range molecular order. This result is in agreement with previous measurements using grazing-angle FTIR. l2 The trisiloxyl group requires a tetrahedral bonding environment and may prevent formation of a two-dimensional crystalline array of hydrocarbons. Some molecular size features are identificable in both the topographic and frictional force images. They form lines that are aligned more or less along the diagonql direction in the images. Their average separation is 8 A, and is similar for all three series of samples. This distance is larger than the close-packing distance of alkane chains (-5 A) or than the Si-0-Si bond length (-3 A). They might correspond to loosely packed chains bound through Si-0-Si bonds. The disorder may reflect a decreased mobility of adsorbed dimers, trimers, and oligomers,as well as a lack of registry with the underlying mica substrate. Because of the lack of crystalline molecular order in the OTE layer, it is (10) Maoz, R.;Sagiv, J. J. Colloid Inte$. Sci. 1984, 100, 465. (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) Tilman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J.Am. Chem. SOC.1988,111,6136.
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II Figure 4. Small area (50 A
x 50 A) topographic (a) and frictional force (b) images of OTE films on mica. Right-to-left and left-to-right scan directions are shown. Well-ordered lattices are not observed although some short range residual ordering is visible in the form of lines or bands running approximately along the diagonal of the images.
difficult to measure molecular density using the AFM technique. 3.3. Substrate Binding vs Intralayer Binding: Adsorption of ODMS. As discussed above, it is not clear whether chemical bonds are formed between the alkylsilane molecules and the mica surface since mica has no hydroxyl groups and is chemically inert. On the other hand, no barriers exist for the formation of siloxane bonds between OTE molecules, as shown by the easy formation of 3-D clusters on the surface from polymerization in the solution. To further clarify this point, we have attempted to prepare ODMS films on mica. In this case, the dimethyl groups prevent formation of cross-linked siloxane bridges. Thus, chemical bonding can only occur between the silane and the mica surface, or between pairs of molecules with no bonds left for other attachments. Following the same preparation procedures as for OTE, we found that complete ODMS monolayers do not form on mica. The mica surface is only coveredby dilute patches of OMDS, often higher than a monolayer. The resulting
Xiao et al.
1604 Langmuir, Vol. 11, No. 5, 1995 surface is still wettable by water. Within a patch, the monolayer is easily removed by a load of -1 nN using Park-sharpened tips. The fact that ODMS does not form stable monolayers on mica indicates that no chemical covalent bond can be formed between ODMS and mica. In contrast, stable monolakoxysilane monolayers are readily formed on hydroxylated surfaces such as ~ i l i c a . ~ J ~ It appears then that the primary molecular bonding of the S A M monolayers formed by OTE on mica is between molecules within the layer through bridging siloxane networks with a much weaker binding to the mica substrate. This is in contrast to preparations involving mica steam treatment.6 In that case, it is postulated that the primary mode of monolayer stabilization is via binding to formed silanol groups or the mica. 3.4. Mechanical Stability of OTE/MicaFilms. We have studied the mechanical strength of the OTE layers by attempting to scratch them off with the AFM tip. Since the tip size varies from case to case, comparative studies were performed using the same tie for different samples. Using Si3N4Digital tips of ca. 1000A radius, no mechanical wear of the OTE films could be observed for loads up to 300 nN. Under similar conditions, however, n-alkanethi01son gold (n = 10to 18)could be displaced.' Using sharper tips (Park Instruments) with
