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Sticky Molecular Surfaces: Epoxysilane Self-Assembled Monolayers Vladimir V. Tsukruk,* Igor Luzinov, and Daungrut Julthongpiput College of Engineering & Applied Sciences, Western Michigan University, Kalamazoo, Michigan 49008 Received November 23, 1998. In Final Form: March 3, 1999 Dense, homogeneous, and complete self-assembled monolayers with epoxy surface groups were fabricated from epoxysilanes to serve as a template for chemical anchoring of ultrathin polymer layers. We formed epoxysilane layers on silicon oxide surfaces of silicon wafers, and a combination of scanning probe microscopy, ellipsometry, XPS, and contact angle measurements was used to study their morphology and surface properties. A low concentration of epoxysilane (less than 0.5 vol %) led to significant aggregate formation caused by a prevailing hydrolization/polymerization of epoxysilane molecules in bulk solution. Epoxysilane SAMs prepared from 1% solution were truly monomolecular films with a virtually normal molecular orientation of densely packed molecules, which were firmly attached to the substrate. Self-assembly deposition of epoxysilanes at optimal conditions resulted in the formation of homogeneous SAMs 0.85 ( 0.2 nm thick with the surface roughness 0.22 ( 0.05 nm.
Introduction A variety of modern applications require highly ordered ultrathin coatings with molecularly controllable surface properties. Some recent examples are molecular lubrication for microelectromechanical systems (MEMS) and biocompatible interfaces for biofunctionalized surfaces.1,2 Mechanical stability always is a critical issue for tethered polymer layers, which affects their long-term durability under shear stresses. A concept of molecular lubrication through composite molecular layers has been recently proposed.3,4 Composite molecular films firmly tethered to solid surfaces possess superior tribological properties due to the combination of the low shear strength of a supporting compliant sublayer and the high hardness of the rigid polymer layer.3 As a first example, we used amine- and sulfonic-terminated self-assembled monolayers (SAMs) capped with various rigid polymers and monomers.4 Classical compounds used to enhance the stability and integrity of polymer-solid interfaces include epoxysilanes, which are applied widely to a variety of reinforced composite materials.5 These compounds also found applications in biomedical sciences to provide a strong binding of biological polymers to glass surfaces for * To whom correspondence should be addressed. Fax: 616-3876517. E-mail:
[email protected]. (1) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. Bhushan, B., Ed. Tribology Issues and Opportunities in MEMS; Kluwer Academic Publishers: Dordrecht, 1998. Komvopoulos, K.; Howe, R. T. J. Appl. Phys. 1994, 76, 5731. Komvopoulos, K. Wear 1996, 200, 305. Srinivasan, U.; Houston, M. R.; Howe, R.; Maboudian, R. J. Microelectromech. Syst. 1998, 7, 252. Muller, R. S. In Micro/Nanotribology and Its Applications; Bhushan, B., Ed.; Kluwer Press: Dordrecht, 1997; p 579. Maboudian, R., MRS Bull. 1998, 23 (6), 47. (2) Elam, J. H.; Nygren, H. J. Biomed. Mater. Res. 1984, 18, 953. Tsukruk, V. V. Prog. Polym. Sci. 1997, 22, 247. (3) Bliznyuk, V. N.; Everson, M. P.; Tsukruk, V. V. J. Tribol. 1998, 120, 489. (4) Tsukruk, V. V.; Nguyen, T.; Lemieux, M.; Hazel, J.; Weber, W. H.; Shevchenko, V. V.; Klimenko, N.; Sheludko, E. In Tribology Issues and Opportunities in MEMS; Bhushan, B., Ed.; Kluwer Academic Publishers: Dordrecht, 1998; p 608. (5) Pluddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1991. Lan, L.; Gnappi, G.; Montenero, A. J. Mater. Sci. 1993, 28, 2119. Xue, G.; Koening, J. L.; Wheeler, D. D.; Ishida, H. J. Appl. Polym. Sci. 1983, 28, 2633. Yamaguchi, M.; Nakamura, Y.; Iida, T. Polym. Polym. Compos. 1998, 6, 85. Hong, S. G.; Lin, J. J. J. Polym. Sci., Polym. Phys. 1997, 35, 2063.
biocompatibilization of inorganic surfaces.6,7 However, despite obvious prospectives for use of epoxysilanes as a binding interlayer for molecular interfaces, their ability to form stable, smooth, and homogeneous monolayers was not proven. In several recent studies, attempts to build epoxysilane films by either dip-coating or vapor deposition lead to the formation of molecularly thick films composed of at least 2-3 aggregated molecular layers with unknown surface morphology and microstructure.6-8 Obviously, such layers cannot serve as an ideal template for chemical tethering of molecular films. Therefore, in this study, we focus on the fabrication of truly monolayer epoxysilane films appropriate for chemical binding of composite molecular interfaces on silicon substrates. In the present communication, we report the results of the formation of such a smooth, complete, homogeneous, and dense epoxysilane monolayer. Experimental Section The epoxysilane compound (3-glycidoxypropyl)trimethoxysilane (see Chart 1) was purchased from Gelest Inc. Toluene and ethanol were obtained from Aldrich and were ACS grade. They were used as received. The epoxysilane solutions in toluene were prepared in oven-dried glassware in a nitrogen-purged glovebox. Highly polished single-crystal silicon wafers of {100} orientation (PureSilicon, Inc.) were cut in pieces of approximately 1.5 × 2 cm2 before modification. The substrates were first cleaned in an ultrasonic bath for 30 min, placed in a hot piranha solution (3:1 concentrated sulfuric acid/30% hydrogen peroxide) for 1 h, and then rinsed several times with high-purity water (18 MΩ‚cm, Nanopure). After the rinsing, the substrates were dried under a stream of dry nitrogen and immediately taken into the nitrogenfilled glovebox and immersed in epoxysilane solutions of different concentrations (from 0.1 to 1 vol %) for different periods of deposition time (from 10 s to 24 h). After the deposition was complete, the modified substrates were removed from solution and rinsed several times with toluene and ethanol. To remove unbound deposited materials, the substrates were additionally placed in ethanol in the ultrasonic bath for 20 min. The SAMs formed were dried overnight at ambient conditions before (6) Elender, G.; Kuhher, M.; Sackmann, E. Biosens. Bioelectron. 1996, 11, 565. (7) Salmon, L.; Thominette, F.; Pays, M. F.; Verdu, J. Compos. Sci. Technol. 1997, 57, 1119. (8) Petrunin, M. A.; Nazarov, A. P. Mater. Res. Soc. Symp. Proc. 1994, 351, 305.
10.1021/la981632q CCC: $18.00 © 1999 American Chemical Society Published on Web 04/01/1999
3030 Langmuir, Vol. 15, No. 9, 1999
Letters
Chart 1. Epoxysilane Compound Studied Here
measurements. All sample preparations were performed inside a Cleanroom 100 facility (Laminare Corporation). After preparation, samples were stored in a dry atmosphere in the clean room. The properties of the fabricated films (contact angle and thickness) were observed to be unchanged several months after preparation. Modified surfaces were examined by static contact angle (sessile droplet) measurements using a custom-designed optical microscopic system. Droplets (1.5-2 µL) of Nanopure water were placed randomly over the surface. Contact angles were determined within 1 min after droplet deposition. All reported values were an average of at least six measurements. The shape of the drop was observed with a microscope equipped with a CCD camera, and the contact angle was measured at a monitor screen. Ellipsometry was performed using a COMPEL automatic ellipsometer (InOmTech, Inc.) with a fixed angle of incidence of 70°. The silicon oxide thickness was measured for each silicon wafer after the piranha solution treatment and varied within 0.8-1.2 nm. The indexes of refraction of the epoxysilane monolayer and the silicon oxide were considered to be constant and equal to the “bulk” values 1.4299 and 1.46,10 respectively. All reported thickness values were averaged over six measurements from different parts of the substrate. Scanning probe microscopy (SPM) was used to obtain topographical, friction, and phase mode images in air. Studies were performed on a Dimension 3000 (Digital Instruments, Inc.) microscope according to the known procedure.11,12 Silicon nitride and silicon tips with spring constants from 0.2 N/m for contact mode to 50 N/m for tapping mode were used. Imaging was done at normal load, ranging from several tens of Newtons for contact mode to several Newtons for tapping mode. For thickness evaluation from SPM data, we used a “scratch” test. Scratches were produced with a sharp steel needle at different loads or by a multiple scanning with a stiff tip with a high normal load (several microNewtons). Molecular models of epoxysilane molecules were built, and geometrical dimensions were measured with the CERIUS2 package on a Silicon Graphics workstation.13
Results and Discussion Films were prepared by using toluene solutions with epoxysilane volume concentrations from 0.1% to 1%. In the present communication, we limit ourselves to discussion of only the 1% concentration. At other concentrations, significant aggregation and formation of inhomogeneous films occur due to prepolymerization of silane molecules in bulk solution, as will be discussed in detail elsewhere.14a Low-concentration epoxysilane solutions lead to the formation of tiny molecular aggregates composed of hundreds to thousands of molecules packed in bi- and trilayers instead of smooth monolayer films. These aggregates are loosely packed, which causes a high heterogeneity on a molecular scale. Aggregate formation arises from the presence of an excess amount of water per (9) Catalog no. 19007; Gelest, Inc.: Tullytown, PA, 1998; p 173. (10) Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996. (11) Tsukruk, V. V. Rubber Chem. Technol. 1997, 70 (3), 430. (12) (a) Ratner, B., Tsukruk, V. V., Eds. Scanning Probe Microscopy in Polymers; ACS Symposium Series 694; 1998. (b) Sheller, N. B.; Petrach, S.; Foster, M. D.; Tsukruk, V. V. Langmuir 1998, 14, 4535. (13) CERIUS2, Molecular Simulations, v. 3.8, 1998. (14) (a) Luzinov, I.; Julthongpiput, D.; Liebmann-Vinson, A.; Tsukruk, V. V. In preparation. (b) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (c) Trip, C. P.; Hair M. L. Langmuir 1995, 11, 1215. Trip, C. P.; Hair, M. L. Langmuir 1995, 11, 149. Trip, C. P.; Hair, M. L. Langmuir 1992, 8, 1120.
Figure 1. Variation of contact angle and calculated “apparent” surface coverage of the epoxysilane film versus deposition time.
epoxysilane molecule in the bulk and occurs at epoxysilane concentrations lower than 0.5%. This conclusion is supported by several recent studies showing that an optimal ratio of water in the bulk/surface is required to form a complete homogeneous monolayer.14b,c For the short-chain epoxysilanes studied here, we observed that hydrolization/ polymerization in bulk toluene solution is a major reason for the formation of inhomogeneous surface coverage (see for detail ref 14). Contact angle measurements show typical kinetics of molecular adsorption from solution (Figure 1). The contact angle rises very quickly within the first 10 min from close to zero (
