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Alternative Method for Fabricating Chemically Functionalized AFM Tips: Silane Modification of HF-Treated Si3N4 Probes Jill E. Headrick and Cindy L. Berrie* Department of Chemistry, University of Kansas, Lawrence, Kansas 66045-7582 Received December 20, 2003. In Final Form: February 25, 2004 An alternative method for fabricating functionalized, atomic force microscopy (AFM) tips is presented. This technique is simple and requires only minimal preparation and tip modification to generate chemically sensitive probes that have a robust organic monolayer of flexible terminal chemistry attached to the surface. Specifically, commercially microfabricated Si3N4 AFM tips were modified with self-assembled monolayers (SAMs) of octadecyltrichlorosilane and (11-bromoundecyl)trichlorosilane after removing the native silicon oxide surface layer with concentrated hydrofluoric acid. The structure of these SAM films on solid silicon nitride surfaces was studied using contact angle goniometry and Fourier transform infrared spectroscopy. Pull-off force measurements on various bare (mica, graphite, and silicon) and SAMfunctionalized substrates confirm that mechanically robust, long-chain organic silane SAMs can be formed on HF-treated Si3N4 tips without the presence of an intervening oxide layer. Adhesion experiments show that the integrity of the organic film on the chemically modified tips is maintained over repeated measurements and that the functionalized tips can be used for chemical sensing experiments since strong discrimination between different surface chemistries is possible.
Introduction The advent of chemical force microscopy (CFM) has made atomic force microscopy (AFM) measurements sensitive to the spatial arrangement of surface functional groups and facilitated the identification and discrimination of surfaces with different chemistries. It allows for the chemical characterization of a system by measuring interfacial forces with an AFM tip that has been intentionally modified to probe specific intermolecular interactions. The incorporation of chemical sensitivity1 in a method already having nanometer-scale resolution and piconewton sensitivity2 has made this application of AFM an ideal method not only for investigating and understanding molecular-scale interactions but also for probing both adhesion and frictional forces,1,3-38 monitoring surface * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (2) Hoh, J. H.; Cleveland, J. P.; Prater, C. B.; Revel, J. P.; Hansma, P. K. J. Am. Chem. Soc. 1992, 114, 4917. (3) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (4) Green, J. B. D.; Lee, G. U. Langmuir 2000, 16, 4009. (5) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345. (6) Vezenov, D. V.; Zhuk, A. V.; Whitesides, G. M.; Lieber, C. M. J. Am. Chem. Soc. 2002, 124, 10578. (7) Skulason, H.; Frisbie, C. D. Langmuir 2000, 16, 6294. (8) Wallwork, M. L.; Smith, D. A.; Zhang, J.; Kirkham, J.; Robinson, C. Langmuir 2001, 17, 1126. (9) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (10) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (11) Clear, S. C.; Nealey, P. F. J. Colloid Interface Sci. 1999, 213, 238. (12) Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925. (13) Green, J. B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (14) Werts, M. P. L.; van der Vegte, E. W.; Grayer, V.; Esselink, E.; Tsitsilianis, C.; Hadziioannou, G. Adv. Mater. 1998, 10, 452. (15) Beake, B. D.; Ling, S. G.; Leggett, G. J. J. Mater. Chem. 1998, 8, 2845. (16) Han, T.; Williams, J. M.; Beebe, T. P. Anal. Chim. Acta 1995, 307, 365.
reactions,23,24,39,40 mapping the spatial distribution of specific functional groups,9,10,41 measuring bond rupture forces,34,42-47 determining the acid-base properties and (17) Akari, S.; Horn, D.; Keller, H.; Schrepp, W. Adv. Mater. 1995, 7, 549. (18) Fiorini, M.; McKendry, R.; Cooper, M. A.; Rayment, T.; Abell, C. Biophys. J. 2001, 80, 2471. (19) McKendry, R.; Theoclitou, M. E.; Abell, C.; Rayment, T. Langmuir 1998, 14, 2846. (20) Zhang, H.; He, H. X.; Wang, J.; Mu, T.; Liu, Z. F. Appl. Phys. A: Mater. Sci. Process. 1998, 66, S269. (21) Duwez, A. S.; Nysten, B. Langmuir 2001, 17, 8287. (22) Dufreˆne, Y. F. Biophys. J. 2000, 78, 3286. (23) Werts, M. P. L.; van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4939. (24) Grinevich, O.; Mejiritski, A.; Neckers, D. C. Langmuir 1999, 15, 2077. (25) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446. (26) Vezenov, D. V.; Noy, A.; Lieber, C. M. Chemical force microscopy: Probing and imaging interactions between functional groups. In Scanning probe microscopy of polymers; Ratner, B. D., Tsukruk, V. V., Eds.; American Chemical Society: Washington, D.C., 1998; Vol. 694, p 312. (27) Ton-That, C.; Campbell, P. A.; Bradley, R. H. Langmuir 2000, 16, 5054. (28) Tsukruk, V. V.; Bliznyuk, V. N.; Wu, J.; Visser, D. W. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1996, 37, 575. (29) Ito, T.; Madoka, N.; Buhlmann, P.; Umezawa, Y. Langmuir 1997, 13, 4323. (30) Nakagawa, T.; Ogawa, K.; Kurumizawa, T. J. Vac. Sci. Technol., B 1994, 12, 2215. (31) Nakagawa, T.; Ogawa, K.; Kurumizawa, T.; Ozaki, S. Jpn. J. Appl. Phys. 1993, 32, L294. (32) Nakagawa, T. Jpn. J. Appl. Phys. 1997, 36, L162. (33) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992, 20, 633. (34) Scho¨nherr, H.; Beulen, M. W. J.; Bu¨gler, J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 4963. (35) Alley, R. L.; Komvopoulos, K.; Howe, R. T. J. Appl. Phys. 1994, 76, 5731. (36) Florin, E. L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415. (37) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (38) Kidoaki, S.; Matsuda, T. Langmuir 1999, 15, 7639. (39) Linford, M. R.; Chidsey, C. E. D. Langmuir 2002, 18, 6217. (40) Scho¨nherr, H.; Chechik, V.; Stirling, C. J. M.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 3679. (41) Papastavrou, G.; Akari, S. Nanotechnology 1999, 10, 453. (42) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 266, 771.
10.1021/la036425r CCC: $27.50 © 2004 American Chemical Society Published on Web 04/15/2004
Fabricating Chemically Functionalized AFM Tips
ionization state of surface-bound moieties,1,8,20,25,26,48,49 and studying molecular interactions important in biological systems18,42,46,50 and protein folding dynamics.51 Detailed information about CFM applications and advances can be found in recent review articles dedicated to this topic.52,53 It has been shown that chemical specificity can be introduced into AFM by modifying and controlling the surface chemistry of the probe tip.52,53 The tips most commonly used in AFM studies are composed of silicon nitride.16,54 While the favorable chemical and mechanical properties of silicon nitride54 (hardness, thermal resistance, wear resistance, and chemical stability, etc.) have made this a popular material with which to fabricate AFM probes, the surface chemistry of this ceramic is complex and can be difficult to control.25,26,44,54-58 Tips composed of Si3N4 are highly susceptible to hydrocarbon contamination under ambient conditions,1,17,32,59,60 and their usefulness in CFM studies is limited since they lack the chemical diversity that is needed to probe a variety of interactions.16 Tip-sample adhesion forces depend strongly on the nature of the functional groups exposed at the surface of both the substrate and the probe tip, as well as the nature of the surrounding medium.12 Consequently, chemical sensitivity and discrimination can be achieved by chemically derivatizing AFM probe tips with molecules having specific terminal functional groups. The primary method for preparing chemically functionalized (CF) tips involves adsorbing a covalently bound, self-assembled monolayer (SAM) onto the surface of the probe.52,53 One recently developed approach for chemically derivatizing AFM probes is to form self-assembled monolayers of 1-alkenes on the surface of hydrogen-passivated silicon tips.61,62 These monolayers covalently attach to the silicon surface via Si-C bonds and have been shown to be as robust and stable as thiol and silane SAMs.63 Experiments involving silicon probes coated with densely (43) Wenzler, L. A.; Moyes, G. L.; Raikar, G. N.; Hansen, R. L.; Harris, J. M.; Beebe, T. P.; Wood, L. L.; Saavedra, S. S. Langmuir 1997, 13, 3761. (44) Wenzler, L. A.; Moyes, G. L.; Olson, L. G.; Harris, J. M.; Beebe, T. P. Anal. Chem. 1997, 69, 2855. (45) Wei, Z. Q.; Wang, C.; Zhu, C. F.; Zhou, C. Q.; Xu, B.; Bai, C. L. Surf. Sci. 2000, 459, 401. (46) Mazzola, L. T.; Frank, C. W.; Fodor, S. P. A.; Mosher, C.; Lartius, R.; Henderson, E. Biophys. J. 1999, 76, 2922. (47) Skulason, H.; Frisbie, C. D. J. Am. Chem. Soc. 2002, 124, 15125. (48) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997, 101, 9563. (49) He, H. X.; Huang, W.; Zhang, H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 517. (50) Kidoaki, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17, 1080. (51) Carrion-Vazquez, M.; Oberhauser, A. F.; Fisher, T. E.; Marszalek, P. E.; Li, H.; Fernandez, J. M. Prog. Biophys. Mol. Biol. 2000, 74, 63. (52) Takano, H.; Kenseth, J. R.; Wong, S. S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (53) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (54) Senden, T. J.; Drummond, C. J. Colloid Surf., A 1995, 94, 29. (55) Ogbuji, L. U. T.; Jayne, D. T. J. Electrochem. Soc. 1993, 140, 759. (56) Bergstro¨m, L.; Bostedt, E. Colloids Surf. 1990, 49, 183. (57) Hoh, J. H.; Revel, J. P.; Hansma, P. K. Nanotechnology 1991, 2, 119. (58) Lee, G. U.; Chrisey, L. A.; Oferrall, C. E.; Pilloff, D. E.; Turner, N. H.; Colton, R. J. Isr. J. Chem. 1996, 36, 81. (59) Thundat, T.; Zheng, X. Y.; Chen, G. Y.; Sharp, S. L.; Warmack, R. J.; Schowalter, L. J. Appl. Phys. Lett. 1993, 63, 2150. (60) Lo, Y. S.; Huefner, N. D.; Chan, W. S.; Dryden, P.; Hagenhoff, B.; Beebe, T. P. Langmuir 1999, 15, 6522. (61) Ara, M.; Tada, H. Appl. Phys. Lett. 2003, 83, 578. (62) Yam, C. M.; Xiao, Z. D.; Gu, J. H.; Boutet, S.; Cai, C. Z. J. Am. Chem. Soc. 2003, 125, 7498. (63) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145.
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packed SAMs of long-chain 1-alkene compounds61 and alkene derivatives62 have been reported in the literature. The most widely adopted tip-functionalization method involves metallization prior to chemisorption of a selfassembling thiol monolayer.1,3-24,40,41,48-50 In this approach, AFM tips are coated with an ultrathin (20-100 nm) layer of gold before attaching an alkanethiol (HS(CH2)n-X) SAM to the surface.17,19 Alkanethiols are known to form highly stable, densely packed SAMs on gold surfaces.64,65 Due to weak adhesion between the gold and silicon nitride surfaces,25,28,45 a 2-5 nm film of an adhesion promoter, such as chromium3-15,41,48,50 or titanium,1,20-22,40,49 is usually deposited onto the Si3N4 tip before Au deposition. An unfortunate consequence of this type of tip modification is the possibility of substantial tip growth due to metallization.25,28 The attachment of alkylsilane SAMs on oxidized silicon35,66 and Si3N41,24-33,43-45,66,67 tips via silanization68,69 was one of the first tip-functionalization methods reported in the literature.31,33 Trichloro- and trialkoxysilane SAMs form directly on the native oxide that resides on silicon and silicon nitride surfaces, enabling one to control the surface chemistry of the tip without the need for intervening layers of metal.29 Alkylsilane monolayers are robust, stable (chemically and thermally), and resistant to degradation.43,64,70 The strength of these chemisorbed films is attributed to their structure, which involves interchain cross-linking via Si-O-Si bonds and direct bonding between the alkylsilane precursor and the silanol groups exposed at the surface of the oxidized silicon or silicon nitride substrate. Silanization of Si and Si3N4 probes is used less frequently than thiol modification of Au-coated tips because alkyltrichlorosilanes are sensitive to moisture and polymerization often inhibits uniform film formation.29,33,52,58,65,66,69,71,72 The density of exposed silanol groups on an oxidized silicon nitride surface is variable.25 In most studies, the adsorption of alkylsilane SAMs on Si3N4 probe tips is preceded by pretreatment processes that involve a combination of various cleaning and oxidation steps.25-33,44,45,59,64,66 Chemical pretreatment removes surface contaminants and enhances the hydrophilicity of the silicon nitride by increasing the density of surface Si-OH groups. Recently, it has been shown73 that the intervening SiOx layer and a high density of silanol groups are, in fact, not needed on the nitride surface to form monolayers similar to those formed on oxidized silicon. Sung et al.73 demonstrated that robust alkylsiloxane SAMs could be formed on planar Si3N4 substrates even after removing the native surface oxide with concentrated hydrofluoric acid. It has been suggested that trichlorosilanes can react with water adsorbed on the Si3N4 surface (as well as with (64) Ulman, A. An introduction to ultrathin organic films: From Langmuir-Blodgett to self-assembly; Academic Press: Boston, MA, 1991. (65) Ulman, A. Chem. Rev. 1996, 96, 1533. (66) Johnson, C. A.; Lenhoff, A. M. J. Colloid Interface Sci. 1996, 179, 587. (67) Nakagawa, T.; Ogawa, K.; Kurumizawa, T. Langmuir 1994, 10, 525. (68) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775. (69) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (70) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (71) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (72) Silberzan, P.; Le´ger, L.; Ausserre´, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (73) Sung, M. M.; Kluth, G. J.; Maboudian, R. J. Vac. Sci. Technol., A 1999, 17, 540.
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surface silanol groups) and that the presence of this water layer is critical for monolayer formation.29,73 In this paper, an alternative fabrication method is explored for producing chemically sensitive CFM tips. This technique exploits the fact that trichlorosilanes form stable SAMs on oxide-stripped Si3N4 surfaces and circumvents the need for metallization or complicated cleaning/ oxidation procedures that require superfluous tip-handling and exposure to elevated temperatures and multiple solvent systems. Furthermore, the potential for undesirable tip growth is minimized since thick metal layers and oxide films are not needed to promote monolayer formation on the surface of the tip. Commercially microfabricated Si3N4 tips are etched with concentrated hydrofluoric acid to remove the native oxide layer and then exposed to alkyltrichlorosilane compounds to form well-ordered, selfassembled siloxane monolayers that are covalently bound to the silicon nitride surface. Si3N4 tips were functionalized with octadecyltrichlorosilane (OTS) and (11-bromoundecyl)trichlorosilane (BrTS) to generate SAMs having -CH3 and -CH2Br terminal functionalities, respectively. Adhesion force measurements demonstrate that mechanically robust silane SAMs can be formed on oxygen-depleted Si3N4 tips to generate chemically sensitive AFM probes. Furthermore, the integrity of the organic film on the modified tips is maintained over repeated measurements despite the possibility of limited covalent bonding between the cross-linked silane monolayer film and the tip surface.29,73 Experimental Section Chemicals. Tetrahydrofuran (99%), chloroform (99.9%), sulfuric acid (95.5%), hydrogen peroxide (30%), and hydrofluoric acid (49%) were obtained from Fisher Scientific and used without further purification. Ethanol (absolute, 200 proof) purchased from AAPER Alcohol and Chemical Co. was also used as received. Hexadecane (99%), octadecyltrichlorosilane [OTS, CH3(CH2)17SiCl3, 90+%], dodecyltrichlorosilane [DTS, CH3(CH2)11SiCl3, 98%], and 11-mercaptoundecanoic acid [HOOC(CH2)10SH, 95%] were purchased from Aldrich. The (11-bromoundecyl)trichlorosilane [BrTS, BrCH2(CH2)10SiCl3] was obtained from Gelest. Milli-Q water (18.2 MΩ‚cm, pH ∼ 6.0) was used in the force measurements. Tips and Materials. The probes (Veeco Instruments) used throughout this study had a pyramidal, oxide-sharpened Si3N4 tip attached to a V-shaped beam, a nominal tip radius of 5-40 nm (as stated in the manufacturer specifications), and cantilever spring constants of ∼0.06 and ∼0.32 N/m. Bare probes were commonly cleaned with a chloroform rinse or by immersion in a heated (110 °C) piranha solution (a 7:3 (v/v) mixture of 95.5% H2SO4 and 30% H2O2) for 15-20 min. Caution: Piranha solution reacts violently with organic compounds and should not be stored in closed containers. The silicon samples (Virginia Semiconductor) used in this experiment were cut from boron-doped (p-type), single-sidepolished Si(111) wafers having a resistivity of 3.0-6.0 Ω‚cm. Monolayer formation was characterized on Si3N4-coated Si(100) wafers (University Wafer) that were commercially fabricated using low-pressure chemical vapor deposition. These planar silicon nitride samples consisted of a 1500 Å Si3N4 film deposited on a 1500 Å thermal oxide layer grown on a phosphorus-doped (n-type) Si(100) substrate. Highly ordered pyrolytic graphite (HOPG) and clear ruby muscovite mica (type V-1) were obtained from Veeco Instruments and Lawrence & Co., respectively. Both of these substrates were cleaved immediately before use. Gold surfaces were prepared using the well-documented template-stripping procedure.74-76 An Edwards Auto 306 thermal (74) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39. (75) Wagner, P.; Hegner, M.; Guntherodt, H. J.; Semenza, G. Langmuir 1995, 11, 3867. (76) Xu, S.; Miller, S.; Laibinis, P. E.; Liu, G. Y. Langmuir 1999, 15, 7244.
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Figure 1. Schematic representation of an HF-treated silicon nitride tip coated with a methyl-terminated SAM film. evaporator was used to deposit ∼200 nm of Au (Alfa Aesar, 99.999%) onto a heated (250 °C) mica surface at a typical rate of 0.2 nm/s. During gold evaporation, the chamber pressure was maintained below 5 × 10-6 Torr and a quartz crystal microbalance was used to monitor film thickness and deposition rate. The density of the deposited gold film was determined from the product of the bulk density of gold (19.3 g/cm3) and an experimentally calculated compensation factor (or “tooling factor”) that accounts for variability in the thickness of the Au films deposited on the surfaces of the crystal and the mica substrates. After deposition, the Au/mica substrates were annealed under vacuum for an additional 20 min at 250-300 °C to promote the formation of larger gold grains and then bonded to Si(111) samples with Epo-Tek 377 from Epoxy Technology. Prior to use, the Si/ Au/mica precursors were immersed in tetrahydrofuran (THF) for 5-10 min to remove the mica and expose the gold surface. The resulting template-stripped Au/Si samples were rinsed with ethanol, dried with flowing nitrogen, and placed immediately in the thiol solution. Preparation of SAMs on Tips and Surfaces. The OTS and BrTS SAMs were fabricated on the Si3N4 tips and planar substrates using a procedure very similar to that outlined in Sung et al.73 The tips and surfaces were cleaned and degreased in chloroform, immersed in 49% HF solution for 1 min (to remove the native oxide layer and hydrogen-passivate the surface), and then rinsed several times with Milli-Q water. After rinsing, the tips were allowed to air-dry for 10-15 min and the silicon nitride samples were dried with flowing nitrogen. The HF-treated Si3N4 probes and surfaces were immersed overnight in solutions of OTS (2.0 mM) or BrTS (5.0 mM). All of the alkylsilane solutions were prepared in a 4:1 (v/v) hexadecane/chloroform solvent mixture and stored at room temperature. The tips and surfaces were rinsed with chloroform after removal from the alkylsilane solutions and then dried. A schematic of a functionalized, methylterminated tip is given in Figure 1. Monolayers (OTS and DTS) were also fabricated on bare Si(111) surfaces. The silicon samples were cleaned and oxidized in a heated (110 °C) piranha solution for 15-20 min prior to monolayer formation to generate a SiO2/Si surface rich in hydrophilic silanol groups. After piranha-cleaning, the samples were rinsed with Milli-Q water, dried with nitrogen, and placed into dilute solutions (∼2.0 mM) of the corresponding alkylsilane precursor (OTS or DTS) for approximately 24 h. Once again, the alkylsilane compounds were prepared in a 4:1 (v/v) solvent mixture of hexadecane and chloroform. Self-assembled monolayers of 11-mercaptoundecanoic acid were formed on the template-stripped gold samples immediately after the Au surface was separated from the mica substrate. After rinsing away residual THF with ethanol, the freshly stripped Au surfaces were immersed in 2.5 mM ethanol solutions of the functionalized mercaptan for at least a day before removal. Prior to use, the acid thiol SAMs were rinsed with ethanol and dried with flowing nitrogen. Characterization of SAMs and Tips. A Rudolph AutoELIII (Rudolph Research) ellipsometer (λ ) 632.8 nm, angle of incidence ) 70°) was used to characterize the thickness of the various SAM films on both the gold and silicon substrates. Fixed values for the index of refraction (n) of the Si substrates (nSi ) 3.858, nSiO2 ) 1.456), and the organic films (nmonolayer ) 1.45)77-79 (77) Clear, S. C.; Nealey, P. F. Langmuir 2001, 17, 720. (78) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749.
Fabricating Chemically Functionalized AFM Tips were used as inputs for the computational analysis. The optical properties, including the index of refraction and the extinction coefficient (k), of the freshly stripped gold substrates were determined prior to immersion in the alkanethiol SAM solutions. Average values of nAu ) 0.16 and kAu ) 3.56 were measured for the bare gold surfaces. The reported monolayer thickness data were determined from the average of at least five separate measurements on different areas of the individual SAM samples. Ellipsometric measurements on the silicon nitride substrates were complicated by the multilayer configuration. Consequently, characterization of the SAMs on these particular substrates was limited to contact angle measurements and FTIR studies. A Model 100-00 NRL contact angle goniometer from Rame´Hart was used to study the wettability of the bare substrates and the alkanethiol/Au, alkylsilane/Si, and alkylsilane/Si3N4 systems. Static contact angles formed by sessile drops of deionized water were measured on three to five randomly distributed sites on each sample. A Thermo Nicolet Nexus 670 spectrometer equipped with an MCT detector and a KBr beam splitter was used to characterize the structure and order of silane SAMs on Si3N4 and Si(111) surfaces and thiol SAMs on gold. The position, relative intensity, and bandwidth of the CH-stretching modes of the methyl (-CH3) and methylene (-CH2) groups in the alkyl chains can be used to confirm the presence of an alkyl SAM and evaluate the molecular packing and orientation within the monolayer.64 Transmission spectra of SAMs on silicon and silicon nitride surfaces were generated by co-adding 2000 scans at 1 cm-1 resolution. To characterize the acid-terminated mercaptan SAMs on gold, the interferometer was fitted with a VeeMax II variableangle specular reflectance accessory (Pike Technologies). Infrared reflection-absorption spectroscopy (IRAS) measurements were performed by focusing p-polarized light onto the surface of the SAM/Au sample at a grazing angle of 80°. While the reflectance spectra are not included here, it is important to note that IRAS analysis of the 11-mercaptoundecanoic acid SAM/Au sample that was used in the adhesion studies gave spectral results consistent with literature reports for ordered monolayers of this type.80-82 A scanning electron microscope (SEM) was used to monitor changes in the surface chemistry of the Si3N4 tips after silanization. High-resolution SEM images of the uncoated and organically coated AFM tips were obtained with a LEO 1550 scanning electron microscope. It has been shown that SEM analysis of AFM tips can provide self-consistent and reproducible estimates of the probe radius6,7,83 and verify the condition of the tip coating after CFM experiments.6 Probe tips are typically sputter-coated with a metal layer prior to SEM imaging to enhance surface conductivity. To preserve the size and geometry of our AFM tips and avoid undue alteration, our measurements were taken without depositing a metallic coating on the surface of the probes. While the images of the uncoated Si3N4 tips showed no discernible artifacts due to surface charging, the SEM pictures that were captured for the OTS- and BrTS-coated tips were all obscured by charge buildup on the surface. The introduction of significant charging effects in the SEM images suggests that the surfaces of the HF-treated Si3N4 tips were successfully modified with the insulating, organic films. This distortion due to charging meant that we were unable to extract a reliable radius for the modified probe tips using this approach, but it did confirm that the gross structure of the tip was not significantly changed. Force Measurements. The force measurements reported in this study were performed with two commercial atomic force microscopes, the Multimode Nanoscope IIIa and Nanoscope E systems, from Veeco Instruments. A fluid imaging cell was used to conduct the adhesion force experiments in an aqueous environment. The average pull-off forces (Fpull-off) reported for each tip/sample combination were computed from more than 2000 individual force-distance (FD) curves captured with the Nanoscope software and analyzed with the Scanning Probe Image (79) 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. (80) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 2101. (81) Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1993, 9, 1775. (82) Arnold, R.; Azzam, W.; Terfort, A.; Woll, C. Langmuir 2002, 18, 3980. (83) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001, 123, 5549.
Langmuir, Vol. 20, No. 10, 2004 4127 Processing (SPIP) program from Image Metrology. The FD curves52,53,84 were obtained from at least five contact points randomly distributed on the surfaces of the different samples. For this work, cantilever spring constants were taken from the manufacturer specifications and not measured directly. The reported pull-off forces were calculated from the peak value of the adhesion force in the unloading curve.
Results and Discussion Characterization of SAMs and Tips. Contact angle and ellipsometric measurements were used to confirm the formation of the alkylsilane monolayers by studying the wetting properties and the thickness of OTS and BrTS SAMs on HF-treated Si3N4 surfaces and piranha-cleaned Si(111) substrates. The average static water contact angle measured for the OTS monolayer on Si3N4 was ∼103.5°. This is slightly lower than the 110° that Sung et al.73 measured for the same OTS/Si3N4 system. OTS SAM films on SiO2/Si(111) substrates gave contact angles on the order of 108°, which agrees well with previous reports.64,65,72 These measurements indicate that our OTS/Si3N4 monolayers are slightly less ordered than the analogous monolayers on oxidized silicon. This assessment is supported by FTIR measurements on the same surfaces. Since 110° contact angles have been measured for OTS SAMs on both SiO2/Si68,72,85,86 and HF-treated Si3N4,73 the observed discrepancies in surface wettability are not necessarily the result of the different substrate compositions. The average static contact angles measured for the BrCH2(CH2)10SiCl3 monolayers on Si3N4 and SiO2/Si(111) substrates were 85.5 and 80°, respectively. These values, which are consistent with the literature,44,85,87 are lower than those measured for OTS due to the addition of the bromine atom to the terminal group. Ellipsometric measurements of the OTS and BrTS monolayers on SiO2/ Si(111) gave estimated thickness values of 3.0 ( 0.3 nm and 2.9 ( 0.4 nm, respectively. While the data for the OTS SAM agree reasonably well with previous studies,78,85 the measured BrTS thickness is almost twice the expected value of 1.6 nm.88 It is likely that surface aggregation is responsible for the higher readings. Due to the complex substrate composition, ellipsometry measurements of OTS and BrTS films on Si3N4 proved inconclusive. Infrared spectroscopy in the C-H stretching region was used to probe the packing and orientation of the adsorbed monolayers on the planar substrates. Transmission FTIR spectra of self-assembled monolayers of OTS and BrTS on HF-treated Si3N4 substrates and an OTS SAM on a SiO2/Si(111) surface are displayed in Figure 2. Assignments of the C-H vibrational modes are listed in Table 1. As expected,89 three peaks were resolved in the spectra corresponding to the OTS monolayers and two were present in the spectra of BrCH2(CH2)10SiCl3 on Si3N4. The absence of the methyl stretching vibration in the BrTS monolayer is responsible for the loss of the feature at 2958 cm-1. The measured methyl and methylene stretching frequencies listed in Table 1 are relatively similar for all three SAM films, and the observed band positions are consistent with literature reports of OTS SAMs on oxidized silicon surfaces.89 The spectral data provide strong (84) Cappella, B.; Dietler, G. Surf. Sci. Rep. 1999, 34, 1. (85) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (86) Kallury, K. M. R.; Thompson, M.; Tripp, C. P.; Hair, M. L. Langmuir 1992. (87) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (88) Heise, A.; Menzel, H.; Yim, H.; Foster, M. D.; Wieringa, R. H.; Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723. (89) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357.
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Table 1. Transmission FTIR Data Collected for OTS and BrTS Monolayers on SiO2/Si(111) and HF-treated Si3N4 OTS Si3N4 substrate
a
BrTS SiO2/Si(111) substrate
Si3N4 substrate
mode
measured freq (cm-1)
measured freq (cm-1)
lit. freqa (cm-1)
measured freq (cm-1)
νs(CH2) νa(CH2) ν(CH3)
2850 2921 2958
2849 2917 2958
2850 2917 2958
2854 2926
Reference 90. Table 2. Pull-Off Force Measurements Made in Water with OTS-Functionalized Tips tip
sample
1a
mica OTS/Si(111) mica DTS/Si(111) HOOC(CH2)10SH/Au graphite SiO2/Si(111) OTS/Si(111)
2b
3a a
Figure 2. Transmission FTIR spectra of an OTS SAM on SiO2/ Si(111) and monolayers of OTS and BrTS on HF-treated Si3N4.
evidence that the OTS alkyl chains are tightly packed and form an all-trans configuration89,90 on SiO2/Si(111). Alkyl chains that are disordered with gauche defects (liquidlike) typically exhibit methylene stretching vibrations that are ∼5-10 cm-1 higher in frequency.78,81,89,91,92 The small (4 cm-1) blue shift observed in the peak position of the asymmetric (νa) methylene vibration for the OTS film on HF-treated Si3N4 suggests that this SAM is slightly less organized than the analogous film on SiO2/Si(111). The transmission spectrum of BrTS on HF-treated Si3N4 shows a blue shift in both the asymmetric and symmetric methylene vibrations. The νs(CH2) peak shifts up in energy by 4 cm-1, while the νa(CH2) band shows up at 2926 cm-1, 9 cm-1 higher in energy than that observed for the OTS monolayer on SiO2/Si. Steric constraints caused by the terminal bromine atom could contribute to the reduction in order. Interestingly, the recorded spectra suggest that OTS films on HF-treated Si3N4 and SiO2/Si(111) are very similar despite the fact that the density of surface silanol groups on silicon nitride and silicon dioxide is quite different. The oxygen content on Si3N4 surfaces following treatment with HF is extremely low (only ∼0.2 monolayer).73 This is in direct contrast to SiO2 substrates, which are known to have an estimated 5 × 1014 silanol groups/cm2 at the surface.89,93 One would imagine that the deficiency in surface silanol groups should severely limit the number of covalent bonds that are formed between the Si3N4 substrate and the monolayer. If direct surface bonding were critical in forming robust alkylsiloxane SAMs, then the packing of the OTS SAM would be very different on the oxygen-rich and oxygen-depleted surfaces. While peak shifts in the spectral data indicate that the packing of the (90) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (91) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D. Langmuir 1992, 8, 2707. (92) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (93) Zhuravlev, L. T. Langmuir 1987, 3, 316.
static contact angle (deg) 100). Results from the adhesion measurements were consistent with expectation (based on the chemistry exposed at the surface of the tip and the chemistry of the various surfaces) and with the literature. The force measurements indicate that the monolayers formed on the probe tips are stable since the functionalized tips maintain their chemical sensitivity, even after significant use. This method has been shown to be a simple, fast, robust approach for the chemical modification of probe tips. Acknowledgment. This material is based upon work supported by Kansas Technology Enterprise Corp. through a KTEC/NSF First Award, a 3M Nontenured Faculty Award, the Petroleum Research Fund (40142-G5S), and the University of Kansas General Research Fund. LA036425R
