Adhesive and Friction Forces between Chemically Modified Silicon

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Adhesive and Friction Forces between Chemically Modified Silicon and Silicon Nitride Surfaces Vladimir V. Tsukruk* and Valery N. Bliznyuk† College of Engineering & Applied Sciences, Western Michigan University, Kalamazoo, Michigan 49008 Received April 9, 1997. In Final Form: October 30, 1997 We report the results of probing adhesion and friction forces between surfaces with functional terminal groups with chemically modified scanning probe microscopy (SPM) tips. Surfaces with terminal groups of CH3, NH2, and SO3H were obtained by direct chemisorption of silane-based compounds on silicon/silicon nitride surfaces. We studied surface properties of the resulting self-assembled monolayers (SAMs) in air and aqueous solutions with different pHs. Work of adhesion, “residual forces”, and friction coefficients was obtained for four different types of modified tips and surfaces. Absolute values of the work of adhesion between various surfaces, Wad, were in the range 0.5-8 mJ/m2. The work of adhesion for different modified surfaces correlated with changes of solid-liquid surface energy estimated from macroscopic contact-angle measurements. Friction properties varied with pH in a register with adhesive forces showing a broad maximum at intermediate pH values for a silicon nitride/silicon nitride mating pair. Similar broad maxima were observed in the acid range for a NH2-terminated SAM and in the basic range for a SO3H-terminated SAM. This behavior can be understood considering the changes of the surface charge state determined by the zwitterionic nature of silicon nitride surfaces with multiple isoelectric points.

Introduction Interactions between two surfaces at a nanometerseparation scale have been studied for a variety of interfaces with a surface force apparatus and other macroscopic techniques in relationship with chemical composition, environment, and external fields.1-8 Several years ago, scanning probe microscopy (SPM) techniques had emerged as a new tool for probing surface forces with a high lateral resolution.9-16 Very recently, “chemical force * To whom all correspondence should be addressed. Fax: 616387-6517. E-mail: [email protected]. † Present address: Physics Department, University of California, Santa Cruz, CA 95064. (1) Meyers, D. Surfaces, Interfaces, and Colloids; VCH, Weinheim, 1991. (2) Buckley, D. Surface Effects in Adhesion, Friction, Wear, and Lubrication; Elsevier P. C.: Amsterdam, 1981. (3) Yamaguchi, Y. Tribology of Plastic Materials; Elsevier P. C.: Amsterdam, 1990. (4) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley & Sons: New York, 1990. (5) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Claredon Press: Oxford, 1950. (6) Rabinowicz, E. Friction and Wear of Materials; Wiley & Sons: New York, 1965. (7) Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. Wear 1993, 168, 161. (8) Parker, J. L.; Claesson, P. M. Langmuir 1994, 10, 635. (9) Sarid, D. Scanning Force Microscopy; Oxford University Press: New York, 1991. (10) Magonov, S.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH: Weinheim, 1996. (11) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1. Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791. (12) Overney, R. M. Trends Polym. Sci. 1995, 3, 359. (13) Bhushan, B.; Israelachvili, J.; Landman, U. Nature 1995, 374, 607. Bhushan, B. Tribol. Int. 1995, 28, 85. Overney, R. M.; Takano, H.; Fujihira, M.; Meyer, E.; Guntherodt, H.-J. Thin Solid Films 1994, 240, 105. Mate, C. M. Phys. Rev. Lett. 1992, 68, 3323. Meyer, E.; Overney, R. M.; Brodbeck, D.; Luthi, R.; Frommer, J.; Gu¨ntherodt, H.-J. Phys. Rev. Lett. 1992, 69, 1777. Tsukruk, V. V.; Bliznyuk, V. N.; Visser, D.; Hazel, J. Tribol. Lett. 1996, 2, 71. Tsukruk, V. V.; Foster, M. D.; Reneker, D. H.; Schmidt, A.; Knoll, W. Langmuir 1993, 9, 3538. Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. Aim, J. P.; Elkaakour, Z.; Gauthier, S.; Mitchel, D.; Bouhacina, T.; Curely, J. Surf. Sci. 1995, 329, 149.

microscopy” (CFM) has been introduced as a new SPM mode.17-23 This technique allowed discrimination of the local surface forces related to intermolecular interactions of different chemical groups with a nanometer resolution. Nanoprobing chemical composition for organic surfaces with heterogeneous surface morphology, thus, is getting closer. CFM requires a chemical modification of the SPM tip by the fabrication of a molecular layer firmly tethered to the tip’s surface. The most widely implemented approach uses self-assembled monolayers (SAMs) from thiol molecules by chemisorption onto a gold surface and silane molecules to form SAMs on a silicon oxide surface according to well-established procedures developed in the 80s.24 Use of thiol molecules to modify surfaces requires (14) Tsukruk, V. V.; Reneker, D. H.; Bengs, H.; Ringsdorf, H. Langmuir 1993, 9, 2141. Josefowicz, J. Y.; Maliszewskyj, N. C.; Idziak, S. H.; Heiney, P. A.; McCauley, J. P.; Smith, A. B. Science 1993, 260, 323. Janietz, D.; Festag, R.; Schmidt, C.; Wendorff, J. H.; Tsukruk, V. V. Thin Solid Films 1996, 284/285, 289. Tsukruk, V. V.; Bengs, H.; Ringsdorf, H. Langmuir 1996, 12, 754. Tsukruk, V. V.; Einloth, T. L.; Van Esbroeck, H.; Frank, C. W. Supramol. Sci. 1995, 2, 219. (15) Tsukruk, V. V., Ratner, B., Eds. Scanning Probe Microscopy in Polymers; ACS Symposium Series; in press. (16) Tsukruk, V. V. Rubber Chem. Technol., accepted. (17) Frisbie, C. D.; Rozsnyai, L. W.; Noy, A.; Wrington, M. S.; Lieber, C. M. Science 1994, 265, 2071. Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. Akari, S.; Horn, D.; Keller, H.; Schrepp, W. Adv. Mater. 1995, 7, 549. (18) Nakagawa, T.; Ogawa, K.; Kurumizawa, T.; Ozaki, S. Jpn. J. Appl. Phys. 1993, 32, L294. Alley, R. L.; Komvopoulos, K.; Howe, R. T. J. Appl. Phys. 1994, 76, 5731. (19) Vezenov, D. V.; Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (20) Green, J.-B.; McDermott, M. T.; Porter, M. C.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (21) Berger, C. E.; van der Werf, K. O.; Kooyman, R. P.; de Grooth, B. G.; Greeve, J. Langmuir 1995, 11, 4188. Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925. (22) Frommer, J. E. Thin Solid Films 1996, 273, 112. (23) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (24) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164; Bain, C. D.; Troughton, E. B.; Tao, Yu.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.

S0743-7463(97)00367-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998

Silicon and Silicon Nitride Surfaces

coating of the silicon nitride tip by an ultrathin gold layer before attaching the monolayer. To date, several examples of this type of modified tips with CH3, COOH, CH2OH, CO2CH3, CH2Br, and NH2 surface groups have been demonstrated.17-23 “Nanotitration” data have been obtained by CFM for different functional surfaces.19,23 These results indicated that the adhesive behavior of the modified SPM tips was controlled by the nature of the surface terminal groups and was a subject of dramatic changes in the vicinity of isoelectric points. The results obtained stressed a role of electrostatic interactions between ionizable terminal groups; those are controlled by the ionic strength and pH of the solution. Friction behavior followed closely the variation of the adhesive properties and can be used for the identification of different microphases on multicomponent surfaces with a nanoscale resolution.20-23 However, a gold sublayer on the SPM tip is not desirable for some applications involving significant local stresses (nanotribology, nanomechanics, or nanoindentation). The major concern is the weak adhesion between the gold and silicon nitride. Therefore, another approach which relies on a direct surface modification of the silicon or silicon nitride tips by silane-based molecules is considered.18,25,26 This approach eliminates the intermediate stage of the gold coating and allows fabricating more robust modified tips. Initial results show that chemical modification of the tip surface can be, indeed, achieved by this method.18,25,26 In the present article, we report the fabrication of SPM nanoprobes with various surface groups using direct, silane-based modification of the silicon nitride and silicon tips. We discuss adhesive interactions and friction forces between chemically modified surfaces of silicon wafers and silicon nitride ceramics in various aqueous environments.

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Figure 1. General scheme of tip modification via silane-based self-assembled monolayers with different terminal groups (X, Y ) NH2, SO3H, and CH3) (a) and chemical formulas of silane compounds used for surface modification (b).

Silane-based molecules with various functional terminal groups have been used to modify surface properties of silicon wafers, silicon nitride chips, and the SPM tips (see Figure 1a for a general scheme). Silicon and silicon nitride tips (both manufactured by Digital Instruments, Inc.) are used for chemical modification. Silane compounds for the tip modification form robust self-assembled monolayers (SAMs) chemically tethered to silicon oxide surfaces as a result of hydrolysis of terminal Si(Cl)n or SiO-C2H5 groups.27 A silicon nitride ceramic in air has a several-nanometer-thick surface layer predominantly composed of silicon dioxide with surface silanol and silylamine groups.28-30 As surface modification reagents we use (3-aminopropyl)triethoxysilane (NH2-terminated SAM), (2-(4-(chlorosulfonyl)phenyl)ethyl)trimethoxysilane (SO3H-terminated SAM as a result of hydrolization of chlorosulfonate groups), and octadecyltrichlorosilane (OTS) (CH3 terminated SAM) (see Figure 1b for chemical formulas). To monitor the SAM quality, atomically smooth solid substrates such as silicon wafers were probed by the SPM technique. In the present work, we report mainly the results of investigations of symmetrical pairs of surfaces with identical terminal groups. Cleaning and hydrophilization of the surfaces were done in saturated chromic/sulfuric acid solution at room temperature according to the well established procedure.27 The silicon wafers and the SPM tips were then immersed in solutions of different concentrations for different periods of time (from 10 s to 1 h) to

monitor the self-assembling process and to ensure complete SAM formation. The procedures for tips and substrates were identical. After modification was completed, the substrates and tips were removed from the solution, rinsed several times, dried by a stream of dry nitrogen, and placed in a sealed glass storage container. Highly polished silicon single-crystal (100) silicon wafers (SAS Inc., Woodcliff Lake, NJ) (4-in. diameter and 100-µm thick) were cut in pieces of approximately 2 cm by 3 cm before modification. The silane compounds for surface modification were purchased from Gelest Inc. and Aldrich. All solvents used here were purchased from Aldrich and Sigma and were ACS grade. Solutions were used immediately after preparation. Water used for solution preparation was Milli-Q deionized water (R > 1018 Ω cm, pH ) 6.7 for fresh water and decreasing to 5.6 shortly). We varied the pH of the aqueous solution by adding a 0.1 N solution of HCl (for pH < 6) or NaOH (pH > 6). After each measurement at a specific pH, the SPM tip and substrate studied were rinsed with pure water and dried by dry nitrogen. All titration curves (adhesive forces versus pH)19,23,24 were obtained by the variation of pH from 2 to 10. A combination of contact topographical, friction, and tapping scanning modes in air and fluid was used to characterize the modified surfaces and probe their adhesive properties according to the well-established procedure described in detail elsewhere.11,16,31-33 The microscope used was the Dimension 3000 (Digital Instruments, Inc.) equipped with a special cantilever holder for scanning under fluid.34 Modified surfaces were probed

(25) Johnson, C. A.; Lenhoff, A. M. J. Colloid Interface Sci. 1996, 179, 587. (26) Tsukruk, V. V.; Bliznyuk, V. N.; Wu, J.; Visser, D. Polym. Prepr. 1996, 37 (2), 575. (27) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (28) Bergstrom, L.; Bostedt, E. Colloids Surf. 1990, 49, 183. (29) Senden, T. J.; Drummond, C. J. Colloids Surf. 1995, 94A, 29. (30) Parks, G. A. Chem. Rev. 1965, 65, 177.

(31) Tsukruk, V. V.; Bliznyuk, V. N.; Hazel, J.; Visser, D.; Everson, M. P. Langmuir 1996, 12, 4840. (32) Tsukruk, V. V.; Everson, M. P.; Lander, L. M.; Brittain, W. J. Langmuir 1996, 12, 3905. Tsukruk, V. V.; Lander, L. M.; Brittain, W. J. Langmuir 1994, 10, 996. (33) Bliznyuk, V. N.; Hazel, J. H.; Wu, J.; Tsukruk, V. V. In Scanning probe Microscopy in Polymers; Ratner,B., Tsukruk, V. V., Eds., in press (34) DimensionTM 3000 Scanning Probe Microscope Instruction Manual; Digital Instruments Inc.: Santa Barbara, CA 93117, 1995.

Experimental Section

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Results and Discussion

at several randomly selected locations with scan sizes varying in the range from 50 µm × 50 µm to 200 nm × 200 nm. To evaluate adhesive forces, we analyzed force-distance curves (number of measurements, n ≈ 10) at randomly selected locations (n ≈ 12). The final results were averaged over a total set of 50-100 curves. Pull-off forces were determined from the cantilever deflection (point B, Figure 2) in a retraction mode of a force-distance curve. The number of data points collected in one approaching-retracing cycle varied from 128 to 256 with increments in the range 2-4 nm. Elimination of the capillary forces was achieved by scanning in fluid. For these experiments we used a 1-mL drop of aqueous solution placed on the substrate surface. To study frictional properties of the surfaces, a cross-section of surface topography and variation of torsion deflections (a friction loop) were detected simultaneously according to the wellestablished protocol.12,31,35-38 Loading curves (friction forces versus normal load) were measured over selected surface areas with a smooth topography at a scanning rate of 10 µm/s. The fundamental resonant frequency was used for monitoring of the cantilever quality before and after scanning and for cantilever spring constant evaluation according to the procedure proposed earlier.37 Normal spring constant, kn, was determined to be 0.230.25 N/m, and the torsional (lateral) constant, kt, equals 110120 N/m for different short, narrow-leg V-shaped cantilevers. The tip-torsion system was calibrated using the friction loop for small lateral motions.36 To estimate the tip-end radius, we used a specially prepared sample of mixed gold nanoparticles (5 and 14 nm in diameter) tethered to a thiol-terminated SAM in accordance with the established procedure.39 Tip radii, Rc, before and after modification were fairly similar but varied widely in the range 40-500 nm from tip to tip. Modified surfaces were probed by dynamic and static contactangle measurements on a custom-designed optical-microscopic system.40 Water droplets (10 µL) were placed randomly over the surface studied for static-contact-angle measurements. For dynamic-contact-angle measurements, substrates were removed from MilliQ water with a constant velocity (100 µm/s). The shape of the water/substrate interface was observed with a microscope equipped with a CCD camera, and contact angle was measured at a monitor screen.

SAM Surface Morphology. As the first step of our investigations, we probed the quality of SAMs formed on the silicon wafer and silicon nitride surfaces. Preliminary results obtained for silicon wafers were used to modify silicon nitride chips. Probing SAMs fabricated at various conditions allowed establishment of time and concentration regimes required to form complete monolayers. The surfaces of all complete SAMs on silicon wafers were very smooth (rms 0.2-0.7 nm within 2 µm × 2 µm area) with occasional holes and bumps (Figure 3). The surface morphology observed was similar to the morphologies of silane SAMs reported earlier.11,32 A light grainy topography was observed for NH2 and SO3H films. Friction images showed homogeneous distributions of the friction response over surface areas several microns across. Surfaces of silicon nitride chips showed grainy topography with the average microroughness 1-3 nm. The SAMs formed are very stable and cannot be damaged under contact mode scanning with high normal forces. An example of the resulting surface variation of the NH2-terminated SAM after multiple scanning with high forces in air (>500 nN) is shown in Figure 3. The exceptions are SO3H-terminated monolayers which do not withstand extensive scanning at the highest load applied. SAM thickness determined from a “scratch test” (scratches were produced by a sharp steel needle) is in the range 0.5-0.9 nm for NH2 and SO3H films and 2 nm for OTS layers, which is close to expected thickness of monolayers estimated from molecular models.27 Thicknesses of surface layers were also probed by spectroscopic ellipsometry. Thicknesses of the silicon oxide layer and CH3-, NH2-, and SO3H-terminated SAMs were determined to be 2.0-2.2, 2.0-2.5, 0.4-0.5, and 0.5-0.6 nm, respectively.41 These values are close to the ones expected for completely formed monolayers and detemined from SPM data.27 Apparently, a thin silicon oxide surface layer on the silicon nitride provides the necessary surface conditions for hydrolization of ethoxysilane and chlorosilane terminal groups similar to a classical chemisorption of silanes on silicon surfaces.27 Contact-angle measurements revealed that modification of the silicon nitride surfaces resulted in a slight increase in surface hydrophobicity. The dynamic contact angle, Θ, increased from 32° for a silicon nitride surface to 42° and 36° for NH2- and SO3H-terminated surfaces, respectively (Table 1). The OTS-modified silicon surface was hydrophobic with a dynamic contact angle of 104°. Static contact angles were similar to those reported above except for highly hydrophilic silicon and silicon nitride surfaces where the static contact angle was too close to zero to be measured.4,40 Adhesive Forces in Aqueous Solution. Adhesive forces between modified tips and different surfaces were collected from SPM force-distance data.42-46 Initially, adhesive forces were measured for several combinations of modified surfaces and tips in neutral aqueous solutions.

(35) Ruan, J.; Bhushan, B. J. Tribol. 1994, 116, 378. (36) Liu, Y.; Wu, T.; Evans, D. E. Langmuir 1994, 10, 2241. (37) Hazel, J.; Bliznyuk, V. N.; Tsukruk, V. V. J. Tribol., accepted. (38) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1994, 64 (2), 403. Sader, J. E.; Larson, I.; Mulvaney, P.; White, L. R. Rev. Sci. Instrum. 1995, 66 (7), 3789. Albrecht, T. R.; Akamine, S.; Carver, T. E.; Quate, C. F. J. Vac. Sci. Technol. 1990, A8, 3386. Sader, J. E. Rev. Sci. Instrum. 1995, 66 (9), 4583. (39) Butt, H.-J.; Siedle, P.; Seifert, K.; Seeger, T.; Bamberg, E.; Weisenhorn, A. L.; Goldie, K.; Engel, A. J. Microsc. 1993, 169, 75. (40) Chan, C.-M. Polymer Surface Modification and Characterization, Hanser Publ.: Munich, 1994. Lander, L. M.; Siewierski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237.

(41) Tsukruk, V. V.; Nguen, T.; Hazel, J.; Lemieux, M.; Weber, W.; Sherchenko, V. MEMS Workshop, Ohio State University, November 1997, to be published by Kluwer Press. (42) Israelachvili, J. Intermolecular and Surface Forces, Academic Press, San Diego, CA, 1992. (43) Sviridenok, A. I.; Chizik, C. A.; Petrokovets, M. I. Mechanics of A Discreet Friction Contact; Nauka I Tekhnika: Minsk, 1990. (44) Micro/Nanotribology and Its Applications; Bhushan, B., Ed.; NATO ASI Series; Kluwer Academic Publ.: Dordrecht, 1997. (45) Burnham, N. A.; Colton, R. J.; Pollock, H. M. Nanotechnology 1993, 4, 64. (46) (a) Hues, S. M.; Colton, R. J.; Meyer, E.; Guntherodt, H.-J. MRS Bull. 1993, 1, 41. (b) Bliznyuk, V. N.; Tsukruk, V. V. In preparation.

Figure 2. Force-distance curve and different regimes of the tip-surface contact: (A) jump-in contact (designated as a point of physical contact in the SPM technique); (B) pull-off (adhesive force); (C) loading part (compliance). Inset shows the long-range repulsive forces. Separation distance is defined by using point A as a reference point with zero coordinate.

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Figure 3. Topographical (left) and frictional (right) images of NH2-terminated SAM with a track of a previous scan performed with high normal forces. Results of scannings are that as rubbing of the SAM surface caused lower friction forces rather than wear. Table 1. Parameters of Modified Surfaces contact angle surface

Θ,a deg

Θ,b deg

Wad, mJ/m2

pHmaxc

µH2Od

µaire

SiOH Si3N4 NH2 SO3H CH3

> ΣσSiOH) that compensates strong van der Waals forces within the contact area and results in repulsive behavior at small separation (Figure 4b). Gradual decrease of surface group ionization at intermediate pH increases the van der Waals attraction due to diminishing electrostatic contribution (ΣσSiNH+ ≈ ΣσSiO-). Within this pH range, surfaces are essentially neutral, which should result in maximum adhesion caused by noncompensated van der Waals forces. Further pH increase causes gradually decreasing adhesive forces because of an additional repulsive contribution between negatively charged surfaces (ΣσSiNH+ 9 and pH < 3. The behavior observed can be understood considering a balance of electrostatic and van der Waals interactions between composite surfaces with multiple isoelectric points. Maximum adhesive forces revealed at intermediate pHs are due to a predominant role of van der Waals interaction between essentially neutral surfaces and the balanced surface charges of acidic and basic terminal groups of bicomponent ceramic surfaces. The silicon nitride surfaces of SPM tips possess complex composition with an approximately 2:1 ratio of silanol (SiOH) and silylamine (SiNH and SiNH2) groups. Any variations of surface charge state are ultimately determined by the zwitterionic nature of the silicon nitride surfaces studied. Friction force-normal load behavior can be described by the generalized Amontons law for all mating surfaces studied. Absolute values of the friction coefficient in aqueous solution vary in a wide range from 0.2 for the silicon nitride/silicon nitride mating pair at low pH to 2.3 for SO3H surfaces at intermediate pH. Absolute values of the friction coefficient for NH2/NH2 and SO3H/SO3H mating SAMs in aqueous solutions are very high as compared to usual values reported for organic surfaces from macroscopic measurements. The friction properties vary with pH in a register, with adhesive behavior showing a broad maximum at intermediate pH values for the silicon nitride/silicon nitride pair. This maximum is shifted from the intermediate position for NH2- and SO3H-terminated surfaces. The observed differences in the pH behavior of the friction and adhesive properties of different chemical groups can be used for “chemical force mapping” and identification of different microphases and intermolecular interactions on composite surfaces. Acknowledgment. This work is supported by The Surface Engineering and Tribology Program, The National Science Foundation, CMS-94-09431 and CMS-9610408 Grants, and by U.S. Air Force Office for Scientific Research, Contract F49620-93-C-0063. The authors thank J. Wu and T. Nguyen for technical assistance and J. Hazel and V. Shevchenko for helpful discussion. LA970367Q