Langmuir 2001, 17, 1549-1551
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Structural Changes in Self-Assembled Monolayers Induced by Photodimerization: A Scanning Force Microscopy Investigation Jiyu Fang,* Mu-San Chen, and Ranganthan Shashidhar Code 6900, Naval Research Laboratory, Washington, D.C. 20375-5438 Received August 10, 2000. In Final Form: December 21, 2000 Evidence of photodimerization-induced positional order in self-assembled monolayers (SAMs) is reported. Scanning force microscopy studies of SAMs of an alkylsilane with an olefinic group on a silicon surface show that the terminal alkyl chains are initially disordered but become highly ordered upon irradiation with UV light. Development of the crystalline order in the SAMs is accompanied by changes in frictional properties.
Introduction
Experimental Section
Self-assembled monolayers (SAMs) of alkanethiols and alkylsilanes provide a new route for modifying surface properties at the molecular level.1 A feature of alkanethiols is that they can form extremely well-packed, ordered monolayers on a gold surface. For example, dodecanethiol self-assembles into a hexagonal and commensurate (x3 × x3) R30° overlayer structure on Au(111).2 It has been shown that the self-assembly is a balance between the chemical interaction of the sulfur group-gold surface and the hydrophobic interaction of the hydrocarbon chains. Alkylsilanes have also attracted considerable scientific and technological interest because they can form SAMs on a wide range of substrates.3 It is generally believed that the self-assembly is through the formation of silanols as precursors, which are then covalently bonded to the hydroxyl groups of substrate surfaces. In addition, cross linking might take place for alkyltrichlorosilanes having more than one hydrolyzable group, which results in a twodimensional network of Si-O-Si.4 Ordering in the SAMs could be important for a variety of applications. Unlike alkanethiols on Au(111), however, alkysilanes on silicon do not have long-range positional order.5-8 In this paper, we describe a method of inducing longrange positional order in a siloxane SAM. This is accomplished by photodimerization of olefinic groups incorporated in the middle of alkylsilane molecules.
Silicon wafers and quartz were used as substrates. They were soaked in concentrated H2SO4 for 30 min, followed by boiling in distilled water at 100 °C for 15 min and drying with N2. The SAM of the alkylsilanes with olefinic groups, which was prepared on substrates according to the procedure reported in a previous paper,9 is illustrated in Figure 1. Briefly, cleaned substrates were submerged in a bath of 94% methanol, 5% distilled water, 1% 3-aminopropyltriethoxysilane (APS), and 10-3 M acetic acid for 45 min. Following the reaction, the substrates were washed with fresh portions of methanol. Upon removal from methanol, the substrates were immediately dried under N2 and baked at 120 °C for 4 min. The root-mean-square roughness of the APS layer is about 1.26 Å.10 The APS layer then was treated with a 2.4 mM solution of 4-hexoxy-cinnamoyl chloride (C6CC) in anhydrous acetonitrile for 60 min. Contact angles of water on SAMs were measured by the sessile drop method in the ambient environment at room temperature. The precision of the contact angle measurements is (2°. UV irradiation of SAMs was conducted with a 500 W high-pressure mercury lamp (260-320 nm) under an ambient atmosphere. Photodimerization of the SAMs was monitored by absorption spectra, which were recorded on a Cary 2400 spectrophotometer. A scanning force microscope (Nanoscope IIIa, Digital Instruments) was used to image the SAMs. All measurements were performed at room temperature in the contact mode with a triangle Si3N4 cantilever with a normal spring constant of 0.05 Nm-1. The vertical bending and lateral torsion of the triangle Si3N4 cantilever were monitored by reflecting a laser beam from the end of the cantilever onto a four-segment photodetector, so that the topographic and frictional force images of the samples could be obtained.
(1) Ulman, A. An Introduction to Ultrathin Organic Film: From Langmuir-Blodgett to Self-Assembly; Academic: Press: San Diego, CA, 1991. (2) Strong, L.; Whitesides, G. M. Langmuir 1989, 4, 546. Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. Chidsey, C. E. D.; Liu, G. Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421. (3) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 468. Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Stenger, D. A.; Tare, T. L.; Calvert, J. M. Science 1991, 252, 551. Kajiyama, T.; Kozuru, H.; Takashima, Y.; Oishi, Y.; Suehiro, K. Supramol. Sci. 1996, 3, 123. Sugimura, H.; Hozum, A.; Takai, O. IEICE Trans. Electron. 2000, E38C, 1099. (4) Angst, D. A.; Simmons, G. W. Langmuir 1991, 7, 2236. Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (5) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay D. Europhys. Lett. 1992, 20, 633. (6) Xiao, X. D.; Liu, G. Y.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (7) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wassermann, S. R.; Whitesides, G. M.; Axe, J. D. Phys. Rev. B 1990, 41, 111. (8) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mo¨hwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9.
Results and Discussion Figure 2 shows a topographic image of the monolayer of C6CC on a silicon wafer before irradiation. The film is quite smooth, except for the presence of few “holes”. These holes are 5-11 Å in depth, representing a loosely packed (9) Heiney, P. A.; Gruneberg, K.; Fang, J. Y.; Dulcey, C.; Shashidhar, R. Langmuir 2000, 16, 2651. (10) The structure of APS layers is complex. The molecule can form many possible conformations and orientations with respect to the surface. The surface coverage and structure depend sensitively on experimental conditions (Wandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Udval, K.; Erlandsson, R.; Elwing, H.; Lundstrom, I. J. Colloid Interface Sci. 1991, 147, 103.). In some cases, we did see polymerized mounds resting on the smooth self-assembled monolayers, whereas in other cases, under particularly rigorous deposition conditions, such mounds were less evident. The data we present in this paper were from the best batch, in which such polymerized mounds were almost completely absent.
10.1021/la001156f CCC: $20.00 © 2001 American Chemical Society Published on Web 01/31/2001
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Fang et al.
Figure 1. Schematic representation of the self-assembly processes of an alkylsilane with an olefinic group. After the irradiation with UV light, the double bonds of adjacent monomer units react to form a cyclobutane ring.
Figure 4. (A) Frictional force image of the olefin-incorporated monolayer. The left side is the irradiated region, and the right side is the unirradiated region. (B) The friction-load force plates on irradiated and unirradiated areas.
Figure 2. Topographic image of the olefin-incorporated monolayer on a silicon wafer.
Figure 3. Absorption spectra of the olefin-incorporated monolayer on quartz (A) before and (B) after UV irradiation.
C6CC layer or an exposed APS region. The coverage of the closely packed C6CC layer is over 93%, which is estimated from several images. The water contact angle on the film is 87 ( 2°. Absorption spectra of the C6CC SAM on quartz are shown in Figure 3. Before irradiation, the spectrum exhibits an absorption band at 280 nm (Figure 3a), assigned to the absorption of the conjugated chromophore. Upon irradiation, the absorption band decreases gradually with the increase of exposure time. After irradiation for 30 min, the absorption band drops more than 70% (Figure 3b). This decrease is attributed to the reduction in conjugation length because of the cycloaddition of olefinic
groups. Upon further irradiation, the intensity of the absorption band remains unchanged within experimental error, suggesting that no further conversion takes place in the film. Figure 4a shows a frictional force image of the C6CC SAM. The film was irradiated through a mask. In this picture, the left side constitutes the irradiated region and the right side corresponds to the unirradiated portion. A decrease in frictional force (dark) is observed on the irradiated region, compared to the unirradiated region (gray). Holes, which are observed in the topographic image (Figure 2), appear as bright spots (highest frictional force) in both regions. The contrast in frictional force, arising from chemically identical terminal groups, suggests that there is a difference in the packing density or crystalline order of alkyl chains before and after irradiation. The frictional force exhibits a linear increase with the load for both regions (Figure 4b), but the rate of increase in frictional force is faster for the unirradiated area than for the irradiated region. Wear appears first on the unirradiated region when the load is above 60 nN. To reduce tip damage, the loading force was adjusted to the smallest possible value at which the samples could be imaged, typically
