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An Improved Method for the Assembly of Amino-Terminated Monolayers on SiO2 and the Vapor Deposition of Gold Layers Denise F. Siqueira Petri,*,†,‡ Gerhard Wenz,† Peter Schunk,§ and Thomas Schimmel§ Polymer Institut, Universita¨ t Karlsruhe, Hertzstrasse 16, D-76187 Karlsruhe, Germany, and Institut fu¨ r Angewandte Physik, Universita¨ t Karlsruhe, Engesserstrasse 7, D-76131 Karlsruhe, Germany Received October 5, 1998. In Final Form: March 15, 1999 Amino-terminated self-assembled monolayers on SiO2 surfaces were obtained by means of a simple silylation method. The thickness of the monolayers was measured with ellipsometry. Washing tests with strong solvents indicated that the amino-silane is covalently bonded to the silica surface. Amino-terminated monolayers on SiO2 surfaces are very flat and homogeneous, as shown by atomic force microscopy. Contact angle measurements showed that such monolayers interact very quickly with air impurities and should be used just after the silylation. The adhesion between the amino-terminated surface and gold was evidenced by ultrasonic and tape tests.
Introduction SiO2 surfaces modified by silylation reactions have been widely used as substrates for a variety of technological applications, as for instance, the development of stationary phase for chromatography. In particular amino-terminated self-assembled monolayers on SiO2 surfaces have been applied as an anchor surface for proteins1,2 and polymers.3,4 Although the aminosilane monolayers have the advantage of interacting with many different functional groups, they also frequently contain microstructural defects such as holes or clumps at the surface. This may cause problems, if the desired applications require a high surface homogeneity. The morphology of these assemblies depends strongly on the silylation conditions. Parameters such as concentration, solvent quality, temperature, and reaction time play an important role in the final morphology of aminopropylsilane monolayers onto the surface of Si wafers or glass slides.1-20 Haller20 published some * To whom correspondence should be addressed. † Polymer Institute. ‡ Present address: Instituto de Quı´mica, Universidade de Sa ˜o Paulo, Av. Lineu Prestes, 748, 05508-900 Sa˜o Paulo - SP, Brazil. Fax: 0055 11 815 55 79, Tel: 0055 11 818 21 68. E-mail:
[email protected]. § Institut fu ¨ r Angewandte Physik. (1) Kurth, D. G.; Bein, T. Langmuir 1993, 9, 2965-2974. (2) Sugimura, H.; Nakagiri, N. J. Am. Chem. Soc. 1997, 119, 92269229. (3) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 1995, 11, 2713-2718. (4) Chang, Y.-C.; Frank, C. W. Langmuir 1998, 14, 326-334. (5) Brzoska, J. B.; Ben Azouz, I.; Rondelez, F. Langmuir 1994, 10, 4367-4373. (6) Chang, Y.-C.; Frank, C. W. Langmuir 1996, 12, 5824-5829. (7) Heid, S.; Effenberger, F.; Bierbaum, K.; Grunze, M. Langmuir 1996, 12, 2118-2120. (8) Heise, A.; Menzel, H.; Yim, H.; Foster, M. D.; Wieringa, R. H.; Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723-728. (9) Wieringa, R. H.; Schouten, A. J. Macromolecules 1996, 29, 30323034. (10) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143-2150. (11) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215-1219. (12) Bierbaum, K.; Kinzler, M.; Wo¨ll, C.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512-518. (13) Culler, S. R.; Ishida, H.; Koenig, J. L. J. Colloid Interface Sci. 1985, 106, 334-346.
Figure 1. Contact angle measurements as a function of aging time in the laboratory atmosphere: advancing angle for freshly prepared APS monolayer (b), advancing (0), and receding (*) angles for samples kept in closed glasses in the laboratory atmosphere and advancing angles for samples kept in the desiccator (∆).
systematic work concerning the formation of aminoterminated self-assembled monolayers on SiO2 surfaces. The surface modification was performed in an apparatus where silicon wafers were in contact with the vapor phase of a 5 wt % solution of (3-aminopropyl)triethoxysilane in toluene refluxed for 16 h at a high temperature (∼120 °C). Wieringa and Schouten also used the same procedure.9 Although this silanization method leads to homogeneous monolayers,20 it appeared to be quite cumbersome to us. Cooper et al. dipped glass slides in a 2 wt % solution of (14) Tsukruk, V. V.; Bliznyuk, V. N.; Visser, D.; Campbell, A. L.; Bunning, T. J.; Adams, W. W. Macromolecules 1997, 30, 6615-6625. (15) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446-455. (16) Sano, K.; Machida, S.; Sasaki, H.; Yoshiki, M.; Mori, Y. Chem. Lett. 1992, 1477-1480. (17) Hild, R.; David, C.; Mu¨ller, H. U.; Vo¨lkel, B.; Kayser, D. R.; Grunze, M. Langmuir 1998, 14, 342-346. (18) Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791-1808. (19) Rodriguez-Fernandez, O. S.; Gilbert, M. J. Appl. Polym. Sci. 1997, 66, 2121-2128. (20) Haller, I. J. Am. Chem. Soc. 1978, 100, 8050-8055.
10.1021/la981379u CCC: $18.00 © 1999 American Chemical Society Published on Web 05/13/1999
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Figure 2. Typical AFM image of (a) a freshly prepared APS monolayer on silicon wafer after the first scan (image size: 1.0 µm × 1.0 µm) and (b) a freshly cleaned silicon wafer image size: 1.0 µm × 1.0 µm).
Figure 3. Frictional (LFM) image of freshly prepared APS monolayers on silicon wafer after five image scans.
N-[3-(trimethoxysilyl)propyl]ethylenediamine in 95% ethanol for 10 min. After this, the samples were annealed at 110 °C for 10 min.3 Tsukruk et al. used a similar silanization procedure of Si wafers with (3-aminopropyl)triethoxysilane and found stable monolayers with occasional holes and bumps by means of atomic force microscopy.14,15 The aim of this work was to obtain reproducibly aminoterminated self-assembled monolayers on SiO2 surfaces both flat and homogeneous by means of a simple silylation method. Method of Silanization. Silicon wafers (2 × 2 cm2, CrysTec Berlin, covered with a SiO2 layer approximately 1.5 nm thick) were cleaned according to the following procedure. The wafers were kept 15 min in dichloromethane, and then they were immersed in a mixture of NH3 (25% in volume), H2O2 (30% in volume), and distilled water in the volume ratio of 1:1:5 and at the temperature of 70 °C during 20 min. Afterward, the wafers were washed with distilled water and dried by a stream of N2. The amino-terminated monolayers were obtained by dipping freshly cleaned silicon wafers into a 1 wt % solution of (3-aminopropyl)trimethoxysilane, APS (Fluka, Germany),
in toluene for 4 min at 60 ( 1 °C. After 4 min the wafers were washed five times with toluene and dried by a stream of N2. Ellipsometry. A Rudolph Auto EL-II Null-Ellipsometer (NJ) equipped with a He-Ne laser (λ ) 632.8 nm) with an angle of incidence fixed at 70.0° was used to determine the thickness21 of the APS monolayer, assuming the following indices of refraction for Si, n ) 3.858 - 0.018i;22 for SiO2, n ) 1.462; for APS, n ) 1.424. The incident laser beam covered an area of approximately 3 mm2. The measurements were performed at three different spots of a given sample. The reported results are the average of 20 different samples. For the interpretation of the ellipsometric data, a multilayer model21 which applies Jones matrixes calculations was used. Contact angles of water drops (4 µL) were measured according to a standard method23 before and after the surface modification at room temperature at three different spots of a given sample. The reported results are the average of six different samples. Atomic force microscopy (AFM) investigations were carried out with an instrument from Park Scientific (Sunnyvale, CA) equipped with a home-built head with a laser deflection detection system in the contact mode in air at room temperature. V-Shaped silicon nitride cantilevers with sharpened pyramidal tips and force constants between 0.03 and 0.1 N/m were applied. All AFM images represent unfiltered original data and are displayed in a linear gray scale. Results and Discussion The mean thickness of the APS layers obtained by the described silanization process was determined by means of ellipsometry as 9 ( 1 Å. This value corresponds to the expected thickness of an APS monolayer, assuming an extended conformation perpendicular to the surface. For comparison, Tsukruk15 and Haller20 obtained aminoterminated layers 5 and 21 Å thick, respectively. To check the chemisorption of the amino-terminated layers, we performed “washing off” tests. Amino-silanized (21) Azzam, R. M.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland Publication: Amsterdam, 1979. (22) Edward, P., Ed.; Handbook of Optical Constants of Solids; Academic Press Inc.: London, 1985. (23) Adamson, A. Physical Chemistry of Surfaces; John Wiley & Sons: New York, 1982.
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Figure 4. AFM image of (a) gold sputtered on silicon wafers and (b) gold sputtered on APS monolayers.
samples freshly prepared were characterized by means of ellipsometry, as described above. After this, three samples were washed 10 times in dimethyl sulfoxide, and the other three samples were washed in tetrahydrofuran. These strong solvents usually destroy layers, which are just physically adsorbed, but they do not destroy those covalently bonded. After washing the samples, they were dried under a stream of N2 and again characterized with ellipsometry. The thickness measured for the layers before and after the washing process differed from each other in 1 Å in average. This discrepancy is within the experimental error and show that the layers are chemisorbed on the Si wafer. Contact angle measurements showed very unexpected results. The freshly silanized samples had a high hydrophilicity, as shown in Figure 1. The advancing angle was θa ) 23 ( 3° (solid circle), while the receding angle was θr ) 19 ( 2°. The low hysteresis ∆θ of the contact angle is indicative for a high flatness of the APS monolayers. These data are in contrast to advancing contact angles measured for water on amino-terminated layers reported in the literature, which were in the range of θa ) 42 to 68°.1,4,8,15,16,20 For the clarification of this discrepancy, aging experiments were performed under two different conditions. APS monolayers on Si wafers were kept either in a desiccator under the pressure of ∼1 mbar or in closed glass vials in the laboratory atmosphere for different periods of time. Already 3 h after the silanization a significant increase in the contact angles was observed for those APS monolayers kept in closed vials in a normal atmosphere. The longer the samples were kept under this atmosphere, the higher became both the advancing (open squares) and the receding (stars) contact angles, as depicted in Figure 1. After three weeks the advancing contact angle reached θa ) 70°. This change in the surface wettability is probably due to the adsorption of impurities from the atmosphere, which might be hydrophobic. These observations explain the discrepancies to the literature results and make clear that the APS monolayers should be used for the desired purpose directly after the silanization reaction. A much weaker aging effect was observed for those APS substrates, which were kept in the desiccator. However, after three weeks the average advancing angle had also increased to θa ) 41°. This could be
explained by the fact that at a pressure of 1 mbar there are still enough impurities available to adsorb onto the APS films. Another interesting point is the increase of the hysteresis of the contact angles ∆θ during the aging process. The mean values of ∆θ measured for the samples kept in a closed vial in a laboratory atmosphere or in a desiccator at 1 mbar for a period of three weeks amounted to 12° and 10°, respectively. This finding is indicative for some increase of surface roughness caused by the random adsorption of impurities from the air. AFM measurements showed that the APS monolayers (Figure 2a) freshly prepared by the present method are very homogeneous and smooth (roughness rms ) 2.2 ( 0.2 Å). The total force load, which includes capillary and adhesion forces, was considerably high, in the order of 70 nN. The average roughens (rms) value of 2.2 ( 0.2 Å was obtained for the first scan in a scan area of 1.0 µm2. After five scans, the APS surface turned rougher and friction effects could be observed in the lateral force microscopy (LFM) image as shown in Figure 3. The light area indicated clearly the frictional contrast obtained after scanning. Similar effects were reported by Tsukruk et al.15 They measured by AFM the adhesive forces and the work of adhesion between amino-terminated surfaces and functionalized tips. The highest values they found for aminocovered tips: the work of adhesion equaled to 4.5 mJ/m2 and adhesive forces to 8 nN. Tips covered with SO3H, SiN, and CH3 groups led to adhesive forces of 1.5, 2.5, and 5.0 nN, respectively. These values explain the high adhesive forces we observed, since our AFM measurements were done with Si3N4 tips. Moreover, they support our conclusion that amino-terminated surfaces are highly attractive for impurities, as discussed before. For comparison, we also investigated pure silicon wafers by AFM (Figure 2b). These showed an average roughness (rms) of 1.25 ( 0.07 Å in a scan area of 1.0 µm2 at a total force load, which includes capillary and adhesive forces in the order of 20 nN. The present silanization method yield flat and homogeneous amino-terminated surfaces. In this method the silylation reaction takes place at 60 °C for 4 min. These conditions provide the chemisorption of the (aminopropyl)trimethoxysilane on silica. In the traditional silylation
Assembly of Amino-Terminated Monolayers on SiO2
Figure 5. AFM image of the remaining surface after performing the “tape test” for gold sputtered on APS monolayers.
method the silylation reaction takes place at room temperature for a period of time, which depends on the solvent quality. Afterward the samples are cured at 60 °C for 2 h. With the method presented here we obtain a stable and flat amino-terminated in a short period of time, eliminating the traditional curing process. The traditional method generally leads to the formation of clumps on the surface. Actually the silane clumps are already present in the solution as a result of moisture absorbance at room temperature. At 60 °C the aggregation of silane coupling reagent in the solution is unfavored, avoiding the adsorption of clumps on the silica surface. APS monolayers obtained according to our procedure were tested as substrates for the vapor deposition of gold. The unsatisfactory interaction between gold layers and surfaces covered by SiO2 is known as an old problem. Therefore, when gold is thermally evaporated or sputtered onto glass slides or silicon wafers, commonly a thin intermediate layer of chrome is used to improve the adhesion between the metal and SiO2. However, this method has some disadvantages. At elevated temperature, chrome atoms may diffuse into the gold layer. Moreover, when gold substrates are used for adsorption experiments and the detection methods are based on optical methods such as ellipsometry, the chrome layer needs to be well characterized prior to the gold deposition. Freshly cleaned silicon wafers and freshly aminosilanized silicon wafers were coated with a 45 nm layer of gold by sputtering. These layers were investigated afterward by AFM at different spots for different samples
Langmuir, Vol. 15, No. 13, 1999 4523
as well using the same tip. A 45 nm gold layer sputtered on untreated silicon wafers (Figure 4a) showed a mean roughness (rms) of 12.6 ( 0.9 Å, while gold sputtered on APS monolayers (Figure 4b) showed a mean roughness (rms) of 9.5 ( 0.7 Å, both with a scan area of 1.0 µm2. The small decrease in the roughness due to the presence of the APS monolayer can be explained by a fast nucleation of Au from the gas phase due to a strong interaction between the amino groups at the surface and the precipitating Au clusters. Bubeck already demonstrated that the rate of the vapor deposition of metals is strongly influenced by the composition and the flatness of the substrate.24 Aminoterminated surfaces are known to possess a high affinity for Au clusters.25 To demonstrate the improvement of the adhesion between gold and SiO2 by an APS monolayer, samples of gold directly sputtered on silicon wafers (as that in Figure 4a) and samples of gold sputtered on amino-terminated layers (as that in Figure 4b) were immersed in water in an ultrasonic bath at room temperature. After 45 min, for the samples without APS no gold was left on the surface anymore, while those with APS layer at the interface remained perfectly covered with gold. Moreover, “tape tests” were performed for gold on SiO2 and gold on aminoterminated monolayers. After pulling the tape out from the gold surfaces, the gold layer was completely removed from SiO2 surfaces. In contrast, gold particles remained on amino-terminated monolayers. The upper gold layers were taken out, but small gold clusters dispersed on the surface were observed by means of AFM, as shown in Figure 5. These qualitative results prove that the adhesion of Au to glass is significantly improved by the APS monolayer. Conclusions We presented an improved silanization method to obtain flat, stable, and homogeneous amino-terminated surfaces. This method is highly reproducible and straightforward. Contact angle measurements showed that the aminoterminated layers attract impurities from ambient air and, therefore, they should be used directly after the preparation or stored in a vacuum. These APS monolayers significantly improve the adhesion between gold and SiO2, which will be very useful for further self-assembly experiments. Acknowledgment. The authors thank Prof. Dr. Jutzi and Dr. Scherer for the gold sputtering and for the access to their ellipsometer. LA981379U (24) Bubeck, C. Thin Solid Films 1989, 178, 483-491. Bubeck, C. Thin Solid Films 1992, 210/211, 674-677. (25) Doron, A.; Ratz, E.; Willner, I. Langmuir 1995, 11, 1313-1317.
