Langmuir 1997, 13, 4323-4332
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Modification of Silicon Nitride Tips with Trichlorosilane Self-Assembled Monolayers (SAMs) for Chemical Force Microscopy Takashi Ito,† Madoka Namba, Philippe Bu¨hlmann, and Yoshio Umezawa* Department of Chemistry, School of Science, The University of Tokyo, Bunkyo-Ku, Tokyo 113, Japan Received October 8, 1996. In Final Form: June 5, 1997X Chemical modification of Si3N4 atomic force microscopy tips was performed by a mild pretreatment using ozone and alkaline/acid solutions and subsequent formation of self-assembled monolayers (SAMs) with two kinds of ω-alkenyltrichlorosilanes (CH2dCH(CH2)nSiCl3, n ) 4, 9; HTS and UTS, respectively). Hydroboration of their vinyl groups gave hydroxyl-terminated SAMs. The extent of SAM formation was assessed by determining the adhesion between Si3N4 tips on one hand and bare mica or mica substrates covered with UTS SAMs on the other hand. All adhesion forces were determined with the samples immersed in water to eliminate effects of capillary forces. Contact angles on analogously treated planar Si3N4 substrates were measured for comparison. Proper conditions for SAM formation on Si3N4 tips were found to be similar to those for formation of SAMs on Si3N4 substrates, as shown by the good temporal correlation between the adhesion forces observed between UTS-modified tips and mica and the contact angles of water on planar Si3N4 substrates modified under identical conditions. Due to hydrophobic interactions, strong adhesion between tips modified with HTS or UTS SAMs on one hand and UTS SAMs on mica on the other hand was observed. The adhesion of tips modified with HTS to hydrophobic surfaces decreased only slightly upon hydroboration of the vinyl groups. Furthermore, hydroxyl-terminated tips derived from HTS adhered more strongly to vinyl-terminated monolayers on mica than to hydroxyl-terminated monolayers. This suggests substantial disorder in the HTS-type SAMs formed on Si3N4 tips, resulting in strong hydrophobic interactions between exposed alkyl chains on the tips and the hydrophobic surfaces. This seems to conceal the specific interactions between the terminal functional groups. Evidence for such disorder is also given by FTIR spectra and by the larger friction forces obtained for HTS monolayers on Si3N4 than for UTS monolayers on Si3N4. On the other hand, conversion of the terminal vinyl groups of the UTS SAMs on the Si3N4 tips to hydroxyl groups led to a large decrease in the adhesion to hydrophobic UTS monolayers and a small increase in the adhesion to hydroxyl-terminated monolayers derived from UTS. This shows that formation of SAMs of trichlorosilanes with long alkyl chains on Si3N4 tips and subsequent functional group conversion is a promising method for the preparation of modified tips for chemical force microscopy.
Introduction Atomic force microscopy (AFM) has the sensitivity to probe surfaces with molecular and/or atomic resolution.1-3 Recently it has been shown that imaging of specific chemical functional groups (chemical force microscopy) is possible with AFM tips modified with self-assembled monolayers (SAMs).4-6 For the modification of the AFM tip surface, gold deposition and subsequent SAM formation with mercaptans4-6 and formation of SAMs with alkyltrichlorosilane derivatives7-10 directly on Si3N4 tips have been demonstrated. Tips modified with trichlorosilane monolayers terminated by methyl and trifluoromethyl groups have been applied for the recognition of the chemical composition of surfaces,7,8 for the discrimination * Author to whom correspondence should be addressed: Telephone: +81-3-5802-2989. Fax +81-3-5802-2989. E-mail:
[email protected]. † Research Fellow of the Japan Society for the Promotion of Science (JSPS). X Abstract published in Advance ACS Abstracts, July 15, 1997. (1) McGuire, G. E.; Swanson, M. L.; Parikh, N. R.; Simko, S.; Weiss, P. S.; Ferris, J. H.; Nemanich, R. J.; Chopra, D. R.; Chourasia, A. R. Anal. Chem. 1995, 67, 199R-220R. (2) Bottomley, L. A.; Coury, J. E.; First, P. N. Anal. Chem. 1996, 68, 185R-230R. (3) Louder, D. R.; Parkinson, B. A. Anal. Chem. 1995, 67, 297A303A. (4) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071-2074. (5) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943-7951. (6) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960-10965.
S0743-7463(96)00978-X CCC: $14.00
of the molecular length of alkylsilanes adsorbed on silicon wafers,9 and for the reduction of capillary forces in in-air AFM measurements.10 While SAMs from trichloro- or trialkoxysilanes offer the advantage that they can be directly formed on Si3N4 (or Si) AFM tips without preceding deposition of gold onto the tip surface, formation of wellordered monolayers with silanes is well-known to be difficult because of possible formation of fairly thick, probably polymeric, layers.11 This problem is particularly pronounced for silanes with terminal functional groups (e.g., hydroxyl and amino groups) that can react with the trichlorosilyl and trialkoxysilyl groups. It restricts the variety of functional groups available as terminal groups of SAMs. In addition, trichlorosilanes may not only react with silanol groups on the Si3N4 surface12 but can alternatively react with water that is adsorbed on the Si3N4 surface. When trichlorosilanes react only with adsorbed water, no covalent bonds between the Si3N4 substrates and the monolayer are formed. While van der Waals forces between the alkyl chains, covalent ether linkages between the silicon atoms, and hydrogen bonds (7) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992, 20, 633-638. (8) Nakagawa, T.; Ogawa, K.; Kurumizawa, T. J. Vac. Sci. Technol. B. 1994, 12, 2215-2218. (9) Nakagawa, T.; Ogawa, K.; Kurumizawa, T.; Ozaki, S. Jpn. J. Appl. Phys. 1993, 32, L294-L296. (10) Alley, R. L.; Komvopoulos, K.; Howe, R. T. J. Appl. Phys. 1994, 76, 5731-5737. (11) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607-3614 and references cited therein. (12) Ogbuji, L. U. T.; Jayne, D. T. J. Electrochem. Soc. 1993, 140, 759-766.
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between the SAM and the Si3N4 surface stabilize a SAM formed in this way, SAMs without covalent bonds to the underlying substrates are prone to be removed mechanically. Application of an external force, as occurring in AFM measurements, is a typical condition under which mechanical removal may occur.7 Notwithstanding, the conditions necessary for formation of SAMs of silane derivatives on Si3N4 have so far not been studied in detail. In the present study, trichlorosilane SAMs have been formed on Si3N4 tips after pretreatment of the Si3N4 surfaces under mild conditions. To determine the optimum reaction time for the formation of SAMs on AFM tips, we measured forces of adhesion in water between freshly cleaved mica and SAM-modified Si3N4 tips and compared the changes in the measured adhesion forces with the water contact angles obtained for SAMs formed under identical conditions on planar Si3N4 substrates. To show that SAMs with terminal functional groups can be formed on AFM tips, vinyl-terminated SAMs formed in this way with two kinds of ω-alkenyltrichlorosilanes (CH2dCH(CH2)nSiCl3, n ) 4, 9) were converted to hydroxyl-terminated SAMs by hydroboration.13,14 This functional group conversion was confirmed by changes in the adhesion forces between tips and mica substrates covered with vinyl-terminated or hydroxyl-terminated SAMs (formed with UTS under the same conditions as those used for the tip modification) and bare mica in water. Since the arrangement of molecules in SAMs is known to be influenced by the length of the alkyl chain in the SAMs,15 we also determined the effect of the chain length on the adhesion force between ω-alkenyltrichlorosilane modified AFM tips and the three kinds of planar substrates. To assess the quality of the SAMs formed on Si3N4, contact angles for SAMs formed on planar Si3N4 substrates were measured, which made it possible to discuss the observed adhesion forces on the basis of the JKR theory. Some disorder of the molecules in these SAMs is suggested by FTIR spectra and friction force measurements. Experimental Section Materials and Reagents. Dodecyltrichlorosilane (Shin-Etsu Silicon Chemicals, Tokyo, Japan), octadecyltrichlorosilane (OTS; Tokyo Kasei Kogyo Co., Tokyo, Japan), HPLC grade toluene (Wako Pure Chemical Industries, Osaka, Japan), dehydrated toluene (water:
