Langmuir 2001, 17, 7789-7797
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Formation of Nanometer Domains of One Chemical Functionality in a Continuous Matrix of a Second Chemical Functionality by Sequential Adsorption of Silane Self-Assembled Monolayers Nitin Kumar,† Charles Maldarelli,†,‡ Carol Steiner,† and Alexander Couzis*,† Department of Chemical Engineering and The Levich Institute of Physicochemical Hydrodynamics, City College of the City University of New York, 140th Street and Convent Avenue, New York, New York 10031 Received February 19, 2001. In Final Form: June 29, 2001 In this paper, we describe a procedure to prepare mixed self-assembled monolayers containing nanometer to micrometer domains of a chemical functionality surrounded by another chemical functionality, using sequential adsorption. Partial monolayers of octadecyltrichlorosilane (OTS) consisting of condensed islands with controlled size are prepared by varying the deposition conditions. The area surrounding the OTS islands is backfilled with 11-bromo undecyltrichlorosilane (BrUTS) or decyltrichlorosilane (DTS) to obtain nanometer to micrometer scale domains of OTS in a monolayer of DTS or BrUTS. First, we describe in detail the methodology to form partial OTS monolayers composed of domains of a desired size. Then, we discuss the procedure and optimum conditions for successful backfilling. These monolayers were analyzed by atomic force microscopy (AFM) to obtain height and friction images in contact and tapping modes. In addition, we have studied (1) the friction properties of various phases in OTS monolayers, (2) the morphology of monolayers on silicon substrates with various degrees of hydration, and (3) in situ adsorption of OTS monolayers using AFM.
1. Introduction In recent years, there has been a great deal of interest in the development of molecularly engineered surfaces that have regions of one chemical functionality with sizes ranging from the nanometer to the micrometer in a continuous matrix of a second functionality. Such surfaces can be used for molecular recognition applications, such as the arraying and immobilization of proteins,1 cells,2 and DNA,3 the localization of crystal growth,4,5 and phase separation of polymer mixtures.6,7 Methods for patterning micrometer features on surfaces are well developed and include microcontact printing8-10 and microlithography of self-assembled units.4,11 On the other hand, methods for developing nanometer scale features, such as nanowriting,12,13 phase-separated Langmuir-Blodgett (LB) films,14,15 and block copolymers,16 are time-consuming and * Corresponding author. E-mail:
[email protected]. cuny.edu. † Department of Chemical Engineering. ‡ Levich Institute of Physicochemical Hydrodynamics. (1) Wadu-Mesthrige, K.; Xu, S.; Amro, N. A.; Liu, G.-Y. Langmuir 1999, 15, 8580. (2) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877-5878. (3) Levicky, R.; Herne, T. M.; Tarlov, M. J.; Satija, S. K. J. Am. Chem. Soc 1998, 120, 9787. (4) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1998, 394, 868-871. (5) Fan, F.; Maldarelli, C.; Couzis, A. Cryst. Growth, submitted. (6) Seok, C.; Freed, K. F.; Szleifer, I. J. Chem. Phys. 2000, 112, 6443. (7) Seok, C.; Freed, K. F.; Szleifer, I. J. Chem. Phys. 2000, 112, 6452. (8) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (9) John, P. M. S.; Craighead, H. G. Appl. Phys. Lett. 1996, 68, 1022. (10) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576-9577. (11) Friebel, S.; Aizenberg, J.; Abad, S.; Wiltzius, P. Appl. Phys. Lett. 2000, 77, 2406. (12) Piner, R.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. Science 1999, 283, 661. (13) Liu, G.-Y.; Song Xu; Qian, Y. Acc. Chem. Res. 2000, 33, 457466.
cumbersome. Additionally, there are concerns that the LB film transfer process does not preserve the phaseseparated structure of the air-water interface. Methods involving sequential adsorption17,18 or competitive coadsorption17,19 of self-assembling monolayers (SAMs) have been suggested as an alternate and simpler route for making such surfaces. In this paper, we focus our study on the sequential adsorption approaches. SAMs consist of amphiphiles, which spontaneously adsorb onto a solid surface from solution to form a densely packed two-dimensional ordered monolayer. SAMs have been formed by the adsorption of alkylthiols on gold surfaces, long-chain carboxylate acids on alumina surfaces, and alkylsilanes (such as octadecyltrichlorosilane (OTS)) on silica surfaces (as described by Ulman20). In our study, we use the silane-based SAMs, which were first studied by Sagiv el al.21-24 Recent studies 25-28 suggest that the silane monolayers are formed by the adsorption of the hydrolyzed silanes25-27,29 onto a water layer, which typi(14) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K.-i. Langmuir 1998, 14, 1786-1798. (15) Fang, J.; Knobler, C. M. Langmuir 1996, 12, 1368-1374. (16) Thurn-Albrecht, T.; Steiner, R.; DeRouchev, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. Adv. Mater. 2000, 12, 787-791. (17) Fadeev, A. Y.; McCarthy, T. Langmuir 1999, 15, 7238. (18) Mauthauer, K.; Frank, C. W. Langmuir 1993, 12, 3446-3451. (19) Offord, D.; Griffin, J. H. Langmuir 1993, 9, 3015. (20) Ulman, A. An introduction to ultrathin organic films: from Langmuir-Blodgett to self-assembly; Academic Press: Boston, 1991. (21) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92-98. (22) Netzer, L.; Isovici, R.; Sagiv, J. Thin Solid Films 1983, 100, 67-76. (23) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465-494. (24) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112, 457-472. (25) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647-1651. (26) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215-1219. (27) Britt, D. W.; Hlady, V. J. Colloid Interface Sci. 1996, 178, 775784. (28) Zhao, X.; Kopelman, R. J. Phys. Chem. 1996, 100, 11014-11018.
10.1021/la010257q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/04/2001
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cally exists on a silica surface (see for example Muller,30 Hair and Hertl,31 and Gee32 and references within). In addition, there is no evidence of direct binding of the adsorbing silanes to the surface silanols of the substrate. Because of this water layer and the lack of direct bonding, the adsorbed molecules are mobile and can diffuse laterally on the surface.33 This mobility allows OTS molecules to aggregate into islands, which can be exploited to form nanometer size domains by sequential adsorption. These islands, which are formed by aggregation, have been imaged by atomic force microscopy (AFM)27,34-38 and have been found to be “fractal”-like in structure. This is the result of an adsorption-surface diffusion-aggregation mechanism,39 in which surface diffusion is rate determining. During studies of complete monolayers, Rondelez et al.33,40,41 using infrared spectroscopy, ellipsometry, and goniometry found that below a critical deposition temperature (28 ( 4 °C), the monolayers formed condensed into ordered phases resembling the liquid condensed (LC) states analogous to Langmuir monolayers.27,33-41 A complete monolayer prepared above this critical temperature showed regions of chain disorder, which can be interpreted as a “melting” to a liquid state resembling the liquid expanded (LE) state of the Langmuir monolayers. The authors drew an analogy to the LE-LC phase coexistence of Langmuir films and therefore interpreted this critical deposition temperature as the triple point temperature. Increasing the deposition temperature resulted in more disorder, until the monolayers were in a completely LE state. A similar study by Iimura et al.42 also identified such a critical temperature, but it was found to be significantly lower (∼10 °C). AFM measurements of partial monolayers at room temperature (∼22 °C) have shown a two-step growth mechanism.27,35,38 In the first step, fractal islands of constant height, ∼21 Å,27,35,38 relative to the background, appear and grow on the surface. This step is referred to as the primary growth step. The tilt angle of the OTS molecule in a complete monolayer was measured by infrared spectroscopy, and its value was about 11° from the normal to substrate.40,43 Given the fact that the fully extended OTS molecule is 26.4 Å long,33 a tilt angle of ∼11° leads to a monolayer height of ∼25 Å. This value agrees well with the thickness of about 25 ( 1 Å measured by ellipsometry.44-46 Comparison of these values with the (29) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120-1126. (30) Muller, H. J. Langmuir 1998, 14, 6789-6792. (31) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73, 4269-4276. (32) Gee, M. L.; Healy, T. W.; White, L. R. J. Colloid Interface Sci. 1990, 140, 450. (33) Parikh, A. N.; Allara, D.; Azouz, I. B.; Rondelz, F. J. Phys. Chem. 1994, 98, 7577. (34) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354-3357. (35) Carraro, C.; Yauw, O. W.; Sung, M. M.; Maboudian, R. J. Phys. Chem. B 1998, 102, 4441-4445. (36) Sung, M. M.; Carraro, C.; Yauw, O. W.; Kim, Y.; Maboudian, R. J. Phys. Chem. B 2000, 104, 1556-1559. (37) Davidovits, J. V.; Pho, V.; Silberzan, P.; Goldmann, M. Surf. Sci. 1996, 369-373. (38) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143-2150. (39) Doudevski, I.; Hayes, W. A.; Schwartz, D. K. Phys. Rev. Lett. 1998, 81, 4927-4930. (40) Brzoska, J. B.; Azouz, I. B.; Rondelez, R. Langmuir 1994, 10, 4367-4373. (41) Rye, R. R. Langmuir 1997, 13, 2588-2590. (42) Iimura, K.-i.; Nakajima, Y.; Kato, T. Thin Solid Films 2000, 379, 230-239. (43) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H.; Leitner, T.; Resch, R.; Friedbacher, G. J. Phys. Chem. B 1998, 102, 7190-7197. (44) 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.
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height of a primary island (∼21 Å in AFM images) implies that the primary fractal islands are in fact LC phases surrounded by a low-density liquid phase. In the second step, the growth of the primary fractal islands stops, and the difference of height between the islands and the background begins to decrease.35,37,38 Both these facts indicate that the surrounding is a liquid expanded phase whose density and therefore height are increasing. With increasing concentration of the LE phase, the surface diffusion to the primary islands is reduced, thereby arresting the primary growth. Finally, nucleation and growth of smaller islands of the same height (as the primary islands) occur in the regions surrounding the primary fractal islands. This second step is referred to as the secondary growth step, and it continues until a complete monolayer of uniform height is formed. Following Rondelez’s33,40 analogy, the secondary nucleation and growth occurs as the density of the surrounding LE state reaches the value for its condensation to the LC phase. However, the appearance of the LE phase in monolayers deposited at room temperature (∼22 °C)35,37 suggests that the critical temperature (analogous to the triple point temperature for Langmuir monolayers) as inferred by Rondelez el al.33,40 may be too high but more consistent with the value reported by Iimura et al.42 AFM imaging of partial monolayers deposited at temperatures well below room temperature (
