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Boundary Friction of Aromatic Self-Assembled Monolayers: Comparison of Systems with One or Both Sliding Surfaces Covered with a Thiol Monolayer M. Ruths* Department of Physical Chemistry, A° bo Akademi University, Porthansgatan 3-5, FIN-20500 A° bo, Finland Received January 2, 2003. In Final Form: April 29, 2003 The boundary friction of three aromatic thiol monolayers with different packing densities on gold was studied with friction force microscopy. The friction of each monolayer was measured against unfunctionalized silicon tips and thiol-functionalized gold-covered tips. The experiments were done in ethanol to significantly reduce the tip-substrate adhesion and therefore the dependence of the friction force on contact area. This allowed a direct, quantitative comparison of load- and velocity-dependent friction with tips of different radii. In close-packed systems, the friction force was lower when both surfaces were covered with a monolayer, and plateaus in the friction force vs velocity curves appeared at higher velocity, suggesting a more fluidlike sliding. A transition in the monolayers at high loads was found to be dependent on the tip radius and appeared at higher average pressure for more close-packed systems but did not depend on whether the system contained one or two monolayers.
Introduction Aromatic sulfur-containing molecules such as thiophenes and thiols are naturally present as impurities in diesel fuel and in mineral-oil-based lubricants, where some of them are known to have favorable effects as antioxidants and antiwear compounds.1,2 Similar molecules can also be used as friction modifiers in aluminum-on-steel sliding.1 Their function at the molecular level has not been extensively investigated. This is in contrast to alkanethiols, for which a large amount of information is available both on the formation of chemisorbed layers on metal surfaces3-6 and on their frictional properties at different conditions.7-17 Aromatic molecules are less flexible than alkanes and are expected to have more complex intermolecular interactions, which makes their friction response interesting also from a fundamental point of view. (1) Heenan, D. F.; Januszkiewicz, K. R.; Sulek, H. H. Wear 1988, 123, 257. (2) Oumar-Mahamat, H.; Horodysky, A. G.; Jeng, A. U.S. Patent 5514289, 1996. (3) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (5) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (6) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (7) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (8) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (9) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (10) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (11) Lio, A.; Morant, C.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. B 1997, 101, 4767. (12) Kiely, J. D.; Houston, J. E. Langmuir 1999, 15, 4513. (13) Clear, S. C.; Nealey, P. F. J. Colloid Interface Sci. 1999, 213, 238. (14) Li, L.; Yu, Q.; Jiang, S. J. Phys. Chem. B 1999, 103, 8290. (15) van der Vegte, E. W.; Subbotin, A.; Hadziioannou, G.; Ashton, P. R.; Preece, J. A. Langmuir 2000, 16, 3249. (16) Salmeron, M. Trib. Lett. 2001, 10, 69. (17) Kim, H. I.; Boiadjiev, V.; Houston, J. E.; Zhu, X.-Y.; Kiely, J. D. Trib. Lett. 2001, 10, 97.
Information on the structure of chemisorbed aromatic layers has come mainly from molecular optics and electronics,18-36 where aromatic molecules are used as molecular wires or as anchoring layers for conducting polymers on metal substrates. Some questions related to the orientation of the molecules on different metals have been resolved and a consistent picture of the effect of molecular structure is emerging.26,28,30,31,35 Friction mechanisms at the molecular level are typically investigated in a single asperity contact in the surface forces apparatus37 (SFA) or with the technique used in (18) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955. (19) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (20) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992, 96, 7416. (21) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (22) Mohri, N.; Inoue, M.; Arai, Y.; Yoshikawa, K. Langmuir 1995, 11, 1612. (23) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (24) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (25) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511. (26) Tao, Y.-T.; Wu, C.-C., Eu, J.-Y., Lin, W.-L.; Wu, K.-C.; Chen, C.-h. Langmuir 1997, 13, 4018. (27) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705. (28) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570. (29) Abduaini, A.; Kera, S.; Aoki, M.; Okudaira, K. K.; Ueno, N.; Harada, Y. J. Electron Spectrosc. Relat. Phenom. 1998, 88-91, 849. (30) Jung, H. H.; Won, Y. D.; Shin, S.; Kim, K. Langmuir 1999, 15, 1147. (31) Whelan, C. M.; Barnes, C. J.; Walker, C. G. H.; Brown, N. M. D. Surf. Sci. 1999, 425, 195. (32) Sawaguchi, T.; Mizutani, F.; Yoshimoto, S.; Taniguchi, I. Electrochim. Acta 2000, 45, 2861. (33) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (34) Wan, L.-J.; Terashima, M.; Noda, H.; Osawa, M. J. Phys. Chem. B 2000, 104, 3563. (35) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstro¨m, H.; Kolb, D. M.; Mandler, D. J. Electroanal. Chem. 2000, 491, 55. (36) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2000, 17, 2408. (37) Israelachvili, J. N.; Berman, A. D. In Handbook of Micro/ Nanotribology, 2nd ed.; Bhushan, B., Ed.; CRC Press: Boca Raton, FL, 1999; pp 371-432.
10.1021/la034003b CCC: $25.00 © 2003 American Chemical Society Published on Web 07/03/2003
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this study: friction or lateral force microscopy38-40 (FFM or LFM), a development of atomic force microscopy (AFM). In FFM, the radius of curvature of the tip is commonly not measured and one may observe different magnitudes of the friction forces depending on the tip used. Furthermore, the conversion of the torsional deflection of the cantilever to friction force is not trivial in a microscopic system, especially since very high pressures occur in the small contact area. The conflicting data found in the literature on, e.g., the friction of CH3-terminated alkanethiol monolayers7-14,17 can to some extent be due to different calibration procedures. Calibration methods for AFM and FFM are currently developing very rapidly and a summary of the ones used in this work is given under Materials and Methods. Variations in friction data obtained with different FFM tips are common, especially in experiments on adhesive systems in a vacuum (already at an interfacial energy of γ ≈ 10 mN/m)9,41 and in air of very low humidity,42 or in ambient air where a microscopic water meniscus gathers around the contact and causes a strong capillary force.10,43 At low applied normal forces or loads, L, the friction in these systems is typically not a linear function of the load but follows the contact area, A, predicted by theories of contact mechanics of adhesive systems (adhesion-controlled friction);9,41,42 for example, there is a finite friction force even at zero applied external load (L ) 0). Similar results have been obtained with the SFA for strongly adhesive contacts.37 On the other hand, both FFM7,13,14 and SFA37,44 experiments in systems with low adhesion show a linear increase in friction force with load (loaddependent friction) already at low loads, with the friction force being zero at L ) 0. Also, in systems that initially show adhesion-dependent friction, the friction becomes a linear function of load at very high loads or friction coefficients. These experimental observations are generally expressed as37
Ff ) ScA + µL
(1)
where Sc is the critical shear stress and µ is the friction coefficient. Alternatively, the friction can be expressed as shear strength45 by dividing eq 1 by the contact area to obtain S ) Sc + µp, where p is the pressure L/A. A description of this type of friction is provided by the so-called “cobblestone model”, which proposes that work needs to be done normal to the surfaces in order to enable them to slide past each other.37,44 The work consists of two separate components, the adhesion and the load contributions, which are assumed to be additive. The cobblestone model is in fact an extension to the molecular scale of models for friction of “interlocking asperities” developed for nonadhesive systems,37 and the adhesion enters it as an additional contribution that one must also overcome to cause separation of the surfaces. It can be used to calculate values for the critical shear stress Sc and the (38) Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. (39) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1990, 57, 2089. (40) Meyer, E.; Overney, R. M.; Dransfeldt, K.; Gyalog, T. Nanoscience: Friction and Rheology on the Nanometer Scale; World Scientific: Singapore, 1998. (41) Enachescu, M.; van den Oetelaar, R. J. A.; Carpick, R. W.; Ogletree, D. F.; Flipse, C. F. J.; Salmeron, M. Trib. Lett. 1999, 7, 73. (42) Kim, H. I.; Houston, J. E. J. Am. Chem. Soc. 2000, 112, 12045. (43) Schwarz, U. D.; Allers, W.; Gensterblum, G.; Wiesendanger, R. Phys. Rev. B 1995, 52, 14976. (44) Berman, A.; Drummond, C.; Israelachvili, J. Trib. Lett. 1998, 4, 95. (45) Briscoe, B. J.; Evans, D. C. B. Proc. R. Soc. London A 1982, 380, 389.
friction coefficient µ that agree rather well with experimental values, thus giving the phenomenological expression in eq 1 a physical basis.37,44 Since in this model only Sc depends on the interfacial energy (the adhesion), it implies that a reduction in γ would reduce the area dependence of the friction force, even to the point of removing the area dependence so that only the term µL remains, which also been observed experimentally between surfaces that repel each other.37,44 The cobblestone model thus implies that in systems where there is no adhesion, the friction force is increasing linearly with the load and goes to zero as the load goes to zero. The different contributions from the adhesion and external load are of interest for understanding friction at the fundamental, molecular level. It is also of great practical importance to know under what circumstances friction measured with tips of different (and perhaps unknown) radii can be directly compared. This situation is commonly encountered in experiments, since it is often not possible to use the same tip for a large number of measurements, especially if its shape or surface properties have been irreversibly modified due to wear or intentional treatments. In this work it was found that under specific conditions of low adhesion (interfacial energy γ ≈ 1-4 mN/m), the friction of thiol self-assembled monolayers (SAMs) was not dependent on the tip radius, and showed a linear dependence on load already at low loads. This made it possible to separate the contribution to friction of dissipation within one monolayer from interactions between two monolayers by comparing experiments made with unfunctionalized Si tips and with thiol-covered tips. A parallel study on silane monolayers indicates that this approach can be extended to the direct comparison of FFM and SFA data obtained under conditions of low adhesion (γ < 1-3 mN/m).46 The prediction of a direct proportionality of the friction force to load in such systems was confirmed in the SFA experiments, where the real (molecular) contact area can be directly measured optically: At a given load, there was no detectable dependence of the friction force on the different contact areas obtained for surfaces with different radii of curvature.46 In the present work, the load- and velocity-dependent friction of two close-packed aromatic thiol monolayers, aminothiophenol and benzyl mercaptan, and one very loose-packed monolayer, thiophenol, was studied with FFM and compared to that of an alkanethiol monolayer. In a study of different aminothiophenols and thiophenol, Batz et al.35 concluded that the intermolecular interactions of amino groups lead to denser packing and better organization of the monolayer. The methylene group in benzyl mercaptan gives the molecule more rotational freedom than thiophenol, which leads to a more upright conformation with higher packing density but does not significantly influence the strength of the intermolecular interactions.26,30 It will be shown here that the difference in rigidity between aminothiophenol and benzyl mercaptan monolayers is reflected in their velocity-dependent friction, which is strongly different even though the molecular area is the same in both systems. Materials and Methods Thiol Monolayers. 4-Aminothiophenol (ATP, Fluka, 96.2%), benzyl mercaptan (BM, Aldrich, 99.7%), thiophenol (TP, Aldrich, 99.8%), and 1-octadecanethiol (C18, Fluka, 99.4%) were used as received. Self-assembled monolayers (SAMs) were formed onto template-stripped gold and gold-covered FFM tips by adsorption (46) Ruths, M.; Alcantar, N. A.; Israelachvili, J. N. Unpublished data.
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Ruths
Table 1. Properties of Thiol Monolayers Formed by Self-Assembly on Gold from Ethanol Solution thiol
mol area (nm2)
aminothiophenol (ATP)
0.1920,33 0.22 ( 0.0222 0.2335
benzyl mercaptan (BM)
0.2026
thiophenol (TP)
a
θH2O,adv (deg)
θH2O,rec (deg)
near-parallel24,35
80 ( 2a
34 ( 4a
9-1430
8326 83 ( 1a
66 ( 2a
7026 83 ( 1a
55 ( 2a
115 ( 35 115 ( 2a
1055 101 ( 2a
0.3826
0.3031 1-octadecanethiol (C18)
tilt from surface normal (deg)
0.224 0.2233
4230 4936 76 ( 528 80 ( 1031 275
mp (°C) 37.6b 37-42b 40-4667 -3068
-1569
30.7b
This work. b Manufacturer’s data.
from 1 mM solution in ethanol (99.5%, Primalco, Finland) for 24-40 h. The samples were rinsed in ethanol and blown dry with N2 gas after removal from the adsorption solution. The template-stripped gold was made by evaporating 100 nm gold (99.9%, Kultakeskus, Finland) onto freshly cleaved mica (S & J Trading, Glen Oaks, NY) at a rate of 0.2 nm/s. Pieces of polystyrene (ca. 2 cm × 2 cm, from a plastic Petri dish) were heated on a hot plate until slightly softened. The mica-supported gold was also cut in ca. 2 cm × 2 cm pieces and its exposed gold side (rms roughness 2.6 nm) was pressed against the soft polystyrene.47 The hot plate was switched off and the pieces were left on it to cool overnight. The mica was removed with tweezers or with adhesive tape to expose the smoother side of the gold (rms roughness 0.2 nm) right before immersing the substrates in thiol solution. The gold adheres well to the polystyrene during immersion for several days in ethanol.47 When a faster evaporation rate (0.5 nm/s) was used, the rms roughness of the exposed side of the evaporated gold was reduced from 2.6 to 1.9 nm, but this gold surface did not adhere well to the polystyrene. The packing density and tilt angle of thiol molecules in a monolayer depend on the metal substrate6,18,36 and on the solvent. Some literature data on the systems used in this study (SAMs formed from ethanol onto gold) are summarized in Table 1 together with data from a molecular dynamics simulation.30 When these molecular areas are compared to that expected on the basis of covalent and van der Waals radii for a vertically oriented benzene molecule, 0.21 nm2 (0.64 nm × 0.33 nm),26,30,34 or vertically oriented TP molecule,18 0.2479 nm2, it is apparent that ATP and BM monolayers reach a high degree of packing, while the TP monolayer does not. Ellipsometry measurements give monolayer thicknesses of 0.13 and 0.24 nm for TP and BM, respectively26 (quite low values, considering the expected orientation and coverage) and 2.2 nm for C18.5 XPS experiments suggest a thickness of 0.6 ( 0.1 nm for TP.36 The aromatic monolayers in this study are disordered,36 in contrast to monolayers of polyaromatic molecules, where the molecular stiffness and the intermolecular interaction are stronger.23,36 Higher packing densities and better order have also been reported for other adsorption conditions, as for example a tilt angle for TP on gold from an acetone solution19 of only 14.0° ( 1.5° (in which case multilayer formation has been suggested28), from aqueous solution 30° (molecular area 0.31 nm2)34 or nearperpendicular to the surface.29 The reported advancing contact angle of water on TP monolayers formed from aqueous solution was 80° ( 2° and the ellipsometric thickness again low: 0.1 ( 0.1 nm.21 In scanning tunneling microscopy (STM) measurements, no ordering of TP could be observed when the monolayer was formed from ethanol solution23,26 or from aqueous solution with molecular area 0.38 nm2,32 but when adsorbed from aqueous solution with a reported area of 0.31 nm2, a regular structure was observed.34 One observation on a regular structure in STM (47) Valtakari, D. Master’s thesis, Åbo Akademi University, Åbo, Finland, 2002.
images of ATP adsorbed from ethanol solution has been reported.20 An ATP layer from aqueous solution (molecular area 0.15 nm2, believed to be more than a monolayer) could be imaged with STM with difficulty; only a few rows could be seen within disordered areas.35 TP adsorbed at the same conditions had a molecular area of 0.44 nm2 and no structure was seen.35 A hexagonal pattern with a unit cell dimension of 0.49 ( 0.01 nm has been reported for BM from 1 mM solution in ethanol.26 In the present experiments, AFM images obtained in ambient air and in ethanol of TP and BM where featureless. Occasionally, a few rows with hexagonal structure could be observed on the ATP monolayers. The regular structures of the C18 monolayers and of bare gold substrates could be imaged reproducibly, with results in good agreement with the literature.9,11,16 Friction Force Microscopy. The friction of the monolayers was measured with an atomic force microscope (Nanoscope III, Digital Instruments). The deflection (bending) of the cantilever in the direction normal to the substrate is used to determine adhesion forces and height differences on the surface and provides control of the applied normal force or load. The torsion of the cantilever when the sample is scanned in the direction perpendicular to its long axis is decoupled from the normal deflection. It is normally not influenced by lateral bending due to the differences in stiffness and can be converted to lateral (friction) force.38-40 Sensitive measurements of friction forces require low lateral (torsional) spring constants, kl, to ensure that the cantilever is the weakest part of the system. Otherwise sample deformation or bending of the outermost portion of the tip itself will affect the measurement.9,48-50 However, to avoid coupling with other deflection modes, cantilevers with different spring constants may be needed when friction forces of very large magnitude or at high loads are measured. The normal spring constant, kn, can be measured from the resonance frequency of the cantilever51,52 and the lateral one from scanning on samples with known friction coefficients.9,53 The practical difficulty with the latter is that, for monolayer-covered tips, the necessary values are not always known. Furthermore, friction is strongly influenced by the environment because of capillary condensation or even contamination of the calibration sample or of the tip. The outermost layer on the tip (e.g., an organic monolayer) is typically of low (48) Lantz, M. A.; O’Shea, S. J.; Hoole, A. C. F.; Welland, M. E. Appl. Phys. Lett. 1997, 70, 970. (49) Carpick, R. W.; Ogletree, D. F.; Salmeron, M. Appl. Phys. Lett. 1997, 70, 1548. (50) Pie´trement, O.; Beaudoin, J. L.; Troyon, M. Trib. Lett. 1999, 7, 213. (51) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403. (52) Sader, J. E.; Chon, J. W. M.; Mulvaney, P. Rev. Sci. Instrum. 1999, 70, 3967. (53) Buenviaje, C. K.; Ge, S.-R.; Rafailovich, M. H.; Overney, R. M. Mater. Res. Soc. Symp. Proc. 1998, 522, 187.
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Langmuir, Vol. 19, No. 17, 2003 6791 calibration (taking into account the adhesion force) the lateral stiffness of the contact given by48-50
Table 2. Properties of Unfunctionalized Si Tips and Thiol-Functionalized Gold-Covered Tips Used for Friction Force Microscopy
kcont ) 8G*a
tip
chemical functionality
kn (N m-2)
kl (N m-2)
R (nm)
1 2 3 4 5 6
unfunctionalized Si unfunctionalized Si ATP on gold BM on gold TP on gold C18 on gold
0.04 0.34 0.04 0.25 0.11 0.08
4.6 37.8 4.6 16.0 9.1 9.0
11 33 11 19 13 31
stiffness, which can strongly influence such a calibration. It is often sufficient to determine the spring constants after the experiment, to avoid damage of the tip. Scanning electron microscopy (SEM) can be used to determine the dimensions of Si cantilevers also without covering them with a conductive layer. The FFM tips used in this study are listed in Table 2. Tips 1 and 2 were unfunctionalized Si tips, assumed to have a native oxide layer (CSC12 and NSC12, MikroMasch, Tallinn, Estonia). Tips 3-6 had an evaporated layer of Cr and Au (CSC12, BioForce Laboratory, Ames, IA) and were functionalized by adsorption of thiol as described above. The normal and lateral (torsional) spring constants of our rectangular cantilevers, given in Table 2, were calculated according to40,54
kn ) (Ewh3)/(4L3)
(2)
kl ) (GK)/(t2L)
(3)
where E is Young’s modulus, G is the shear modulus of the cantilever material (Si),40,54 and K ) (wh3/16)(16/3 - 3.36h/w). The manufacturer’s specifications for the width, w ) 35 µm, and length, L ) 300 µm (tip 4) and 350 µm (tips 1-3, 5, and 6), were assumed to be sufficiently accurate and were therefore not measured separately. The beam thickness h and tip height t (height of the tip cone + 0.5h), which were expected to have a larger relative variation, were measured to an accuracy of 0.02 µm with SEM. For tips 1 and 3-6, h was found to be in the range 0.89-1.66 µm (manufacturer’s specified range for CSC12 cantilevers is 0.7-1.3 µm). For tip 2, h was 2.14 µm (manufacturer’s specified range for NSC12 cantilevers is 1.7-2.3 µm). The effect on the stiffness of the reflective aluminum layer on top of the beam9,52,55 or the chromium-and-gold coating on the underside of some tips was assumed to be small and was not taken into account. The radius of each tip, R, was measured in two perpendicular directions by reverse imaging of a characterization grating consisting of sharp cones with an apex radius of curvature of less than 10 nm (model TGT01, MikroMasch, Estonia), and the geometric mean is given in Table 2. The raw data in FFM measurements consist of deflection voltage (torsion of the cantilever) vs sample position. In a simple conversion from voltage to friction force, this signal is assumed to be only from torsion of the cantilever.13,49,50,54,56 It is, however, influenced by deformation of the contact between the tip and sample (lateral contact stiffness)9,48-50 and by bending of the outermost part of the tip itself.48 Calibration measurements were done for tips 1 and 2 in strongly adhesive tip-mica contact in ambient air, to obtain a conversion of deflection voltage to torsion from the “stick-slope” part of the raw data in a system without a compliant thiol layer.50,54,56 This conversion was then compared to the slopes of tip-mica contact in ethanol and of the actual monolayer data, which were found to be very similar. The lateral stiffness of a contact between Si and mica is quite low (because of the low shear modulus of mica57). For the loads used in the (54) (a) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241. (b) Liu, Y.; Evans, D. F.; Song, Q.; Grainger, D. W. Langmuir 1996, 12, 1235. (55) Sader, J. E.; Larson, I.; Mulvaney, P.; White, L. R. Rev. Sci. Instrum. 1995, 66, 3789. (56) Cain, R. G.; Reitsma, M. G.; Biggs, S.; Page, N. W. Rev. Sci. Instrum. 2001, 72, 3304. (57) McNeil, L. E.; Grimsditch, M. J. Phys. Condens. Matter 1993, 5, 1681.
(4)
is ca. 70 and 150 N/m for tip 1 and 2, respectively. The radius of the contact area is a ) [(3RL)/(4E*)]1/3, with E* ) [(1 - ν12)/E1 + (1 - ν22)/E2]-1 where Ei is Young’s modulus and νi is Poisson’s ration for the materials. G* ) [(2 - ν1)/G1 + (2 - ν2)/G2]-1, where Gi is the shear modulus. The spring constants for bending the very end of tips 1 and 2 can be estimated as ca. 50 and 150 N/m, respectively.48 For five of the cantilevers used in this study, the lateral (torsion) spring constant (Table 2) was an order of magnitude less than in refs 48-50. In these cases, it is unlikely that tip bending or sample deformation contribute significantly to the “stick-slope” of the raw data, which instead shows the torsion of the soft cantilever. However, for the stiffer tip 2, these effects become important and are likely the reason for the differences between the measured friction coefficients with tip 1 and 2 (Figure 2a,b, Table 3). This will be discussed below in connection with the results. Slope calibration and conversion of this type cannot be done reliably for the monolayer-covered tips, because of the low stiffness of the monolayer, but the conversion for tips 3-6 was assumed to be the same as for tip 1, which had similar dimensions and spring constants. The experiments presented here have been done in ethanol to reduce the interfacial energy between the thiol monolayers and the tips to γ e 4 mN/m.7,13,58 Low adhesion leads to relatively small deformations and small contact area at low loads, as predicted from the DMT or Hertz theories of contact mechanics. For simplicity, the contact pressures have been calculated with the assumption of Hertzian (nonadhesive) contact and an average elastic modulus Keff ) (4/3)E* for Si-gold or gold-gold. The maximum Hertzian pressure in the contact area is
p0 ) 3/(2π) (Keff/R)2/3L1/3
(5)
Some literature data on the average pressure in the contact area of alkanethiol monolayers10,11,16,63 have been converted for comparison by use of p0 ) (3/2)pavg. To obtain the large range of sliding velocities shown in Figures 1b and 3, friction data were taken over three different scan sizes (20, 300, and 750 nm), at frequencies of 0.02-61 Hz. The agreement between measurements at the same velocity but different scan size was very good. The friction force vs load data in Figures 1a and 2 were measured at 1 Hz over a scan size of 150 nm.
Results and Discussion Strength of Adhesion in Ethanol. All adhesion and friction experiments were done in ethanol. No oscillatory force or layering in the ethanol was detected with any of the FFM tips as they were brought in contact with the monolayers or with bare gold substrates. Layering can be difficult to detect with a sharp tip, but in SFA experiments46 (R ) 0.2-2 cm) on monolayers of small aromatic silanes with a structure comparable to the thiols in this study, the layering of ethanol was also absent. It is also known that oscillatory forces do not appear in systems where the monolayers consist of branched molecules.59 The reason is that these monolayers are expected to be rough on a molecular scale, which perturbs the layering of the liquid. Similarly, the 0.2 nm rms roughness of the template-stripped gold surface might disturb the layering of small molecules.60 In the systems considered here, only a weak minimum (attraction) was seen instead of an oscillatory force below (58) Weisenhorn, A. L., Maivald, P.; Butt, H.-J.; Hansma, P. K. Phys. Rev. B 1992, 45, 11226. (59) Gee, M. L.; Israelachvili, J. N. J. Chem. Soc., Faraday Trans. 1990, 86, 4049. (60) Frink, L. J. D.; van Swol, F. J. Chem. Phys. 1998, 108, 5588.
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a distance of ca. 2 nm from the onset of strong repulsion. The average adhesion force on separation from this minimum, measured with Si tips 1 and 2 and normalized with the measured tip radius, was F/R ) -22 ( 5 mN/m for ATP and BM, -37 ( 5 mN/m for TP, and -36 ( 3 mN/m for C18. When also the tip was covered with a thiol monolayer (tips 3-6), the adhesion force was -20 ( 4 mN/m for ATP and BM, -26 ( 6 mN/m for TP, and -4 ( 1 mN/m for C18. The interfacial energy in these systems is thus reduced by the presence of ethanol to γ ≈ 1-4 mN/m (the range obtained from the F/R values by use of a JKR or DMT model), as has been observed previously for CH3-terminated alkanesilane and alkanethiol monolayers.7,13 The measured values for the adhesion force can be compared to the calculated van der Waals force in ethanol (n ) 1.36, ) 25) of monolayers (n ) 1.58, estimated thickness 0.8 nm for ATP and BM and 0.3 nm for TP; n ) 1.45 and thickness4 2.2 nm for C18) on gold. The expected interaction forces between alkanethiol monolayers on gold have been calculated in detail,61 and some parameters from that study were used to obtain estimates for the Hamaker constants in the aromatic systems. In all systems, including C18, one finds that the calculated ethanol film thickness corresponding to the measured adhesion is less than 0.2 nm between the monolayers and Si tips 1 and 2 and less than 0.3 nm in contacts with tips 3-6. Since the average diameter of an ethanol molecule is 0.44 nm,62 it is unlikely that a complete monolayer of ethanol is present between the monolayer and tip in contact already at the lowest loads. This is also expected since one can image C18 monolayers without significant disturbances from the ethanol. In SFA experiments on aromatic silanes,46 it was found that the monolayer thickness in contact in dry N2 and in ethanol was the same, within the experimental error of 0.1-0.2 nm. One can therefore expect the friction response of these systems to arise mainly from the monolayers, not from confined ethanol. However, ethanol molecules may penetrate into the monolayer structure as will be discussed later. Load Dependence of Friction. In Figure 1a it is shown that, despite a difference in tip radius of a factor of 3 (cf. Table 2), a similar friction coefficient was obtained on a template-stripped gold substrate in ethanol with both Si tips (µ ) 0.49 ( 0.02 with tip 1 and µ ) 0.42 ( 0.01 with tip 2). The sliding velocity was 0.15 µm s-1. Assuming a Hertzian contact, the tip-substrate contact areas of the two tips at a given load would differ by a factor of 2. If the friction were linearly proportional to contact area, as has been demonstrated for the very common situation of adhesive contacts between FFM tips and substrates in a vacuum or in ambient air9,41 and also between monolayers at a controlled, low humidity (RH ca. 4%),42 then the difference in area would be reflected in the friction force, but that was not observed here. Furthermore, the friction force increases linearly with load (whereas the Hertzian contact area increases as L2/3) and the linear fits go through the origin, within experimental error. Apparently the chosen condition of low adhesion reduces the area dependence of the friction, at least for this range of tip radii and loads, and we measure a load-dependent friction, which we will study in more detail below for self-assembled monolayers. The friction forces of the thiol monolayers against Si at a sliding velocity of 0.15 µm s-1 are shown as a function (61) Ederth, T. Langmuir 2001, 17, 3329. (62) Lo¨ning, S.; Horst, C.; Hoffmann, U. Chem. Eng. Technol. 2001, 24, 242.
Ruths
Figure 1. Friction force between bare template-stripped gold and two unfunctionalized Si tips of different radii and spring constant (cf. Table 2), measured in ethanol. (a) Friction force vs load. Main figure: Tip 2, R ) 33 nm, µ ) 0.42 ( 0.01 (cf. Table 3). Inset: Tip 1, R ) 11 nm, µ ) 0.49 ( 0.02. The sliding velocity was 0.15 µm/s. (b) Friction force vs sliding velocity at selected loads. The data at L ) 0.8 and 2 nN were measured with tip 1, and the higher loads were measured with tip 2.
of load in Figure 2. The data obtained with Si tips 1 and 2 are shown in panels a and b, respectively. Figure 2c shows the friction of the same monolayers against goldcovered tips functionalized with the corresponding monolayer (tips 3-6 in Table 2). For the TP and C18, two sets of data are shown in Figure 2a (open and solid symbols) for monolayers prepared from different solutions of similar concentration. One readily distinguishes two linear regions in each friction vs load curve, one at low and one at high loads, separated by a transition region or plateau (see, for example, the C18 data in the inset of Figure 2a). The maximum pressure in the contact region (p0, eq 5) at the onset of the plateau is given for each system in Table 3. Such transitions have been observed earlier in alkanethiol monolayers: C10, C16, and C18 thiols at a maximum pressure p0 of ca. 1.5 GPa,16,63 and also in C18 at 3.5 GPa.11 These transitions were ascribed to a reversible displacement of the monolayer so that the tip comes in direct contact with the gold substrate.11,16,63 When comparing the observations in these systems to the literature,11,16,63 one needs to take into account that, in contrast to the C18 monolayer at low load, one cannot easily obtain an image of any regular structure in the aromatic monolayers. However, the structure of the gold (63) Liu, G.-y.; Salmeron, M. B. Langmuir 1994, 10, 367.
Boundary Friction of Aromatic SAMs
Figure 2. Load dependence of the friction force of thiol monolayers in ethanol at a sliding speed of 0.15 µm/s: aminothiophenol (ATP, O), benzyl mercaptan (BM, b), thiophenol (TP, 0 and 9), and octadecanethiol (C18, 4 and 2). Panels a and b show the friction of single monolayers against unfunctionalized Si tips 1 and 2, respectively (cf. Table 2). Panel c shows the friction of each monolayer against a gold-covered tip functionalized with the same monolayer (tips 3-6 in Table 2). The insets in panels a and b show the data for C18 on different scales. The friction coefficients before and after the plateaus in the curves and the maximum pressure at the onset of each plateau are given in Table 3.
substrate appears above the critical pressure also in the aromatic thiol systems and disappears again when going to loads below the plateau, suggesting that the monolayer has reappeared also in this case. The friction at high loads (above the plateaus) is not the result of sliding against only bare gold in ethanol, i.e., the monolayer is not completely removed, since the friction force and friction coefficient in this regime varied between the systems and was not the same as on bare gold in ethanol (cf. Figure 1a and Table 3). For the aromatic monolayers, the friction coefficient decreased after the transition, whereas it increased for the C18 monolayer. A larger friction after the transition has also been seen for C16.16 Similar
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transitions appear also when monolayer-covered tips are used at approximately the same pressure as for a single monolayer of each thiol (Figure 2c and Table 3). The discussion in the following will mainly be concerned with the linear friction regime at low loads, i.e., the monolayers before the transition. The friction coefficients obtained for the different substrates against the Si tips and thiol-functionalized goldcovered tips are listed in Table 3. When comparing the results from Figure 2, panels a and b, given in the first columns in Table 3, one finds good agreement between the friction coefficients µ measured at low load (before the plateau in the Ff vs L curves), although the tip radii R differ by a factor of 3. Throughout, a slightly smaller value (not larger, as would be expected for area-dependent friction) is obtained for tip 2 also when sliding on the bare gold in Figure 1a. The difference likely arises from discrepancies in the conversion of voltage to nanometer torsion according to the simple procedure described under Materials and Methods. Because of deformation of the tip/substrate contact, whose lateral stiffness is only 4-5 times higher than the torsional spring constant for tip 2, the deflection (voltage) is lowered in the experiments done with this tip. Since this deflection was multiplied with kl (determined from the dimensions of the cantilever and tip) without correction for the contact deformation, the values of Ff for tip 2 become too low. The estimated difference in effective lateral spring constant at the conditions in these experiments is ca. 10-30%, which agrees well with the difference in the values measured with tip 1 and 2 at “low pressure” in Table 3. In terms of absolute values of the friction coefficient, some data for C18 can be found in the literature. The values in ambient air range from 0.0210,11,14 to 0.078,12 and 0.14,11 and in ethanol two studies indicate a large µ ) 0.8.7,13 The differences are partly due to the different adhesion strengths (making the measurements in air depend on the tip radius) and to calibration procedures. One study indicates that the friction force of ATP at low loads in air would be 9-10 times larger than that of C18,25 which is in good agreement with the observation in ethanol in Figure 2 and Table 3. When both sliding surfaces were covered with a monolayer, the friction coefficient at low loads decreased for the close-packed monolayers (ATP, BM, and C18), signifying better lubrication. The friction coefficient of TP at low loads increased, and further study is needed to clarify this mechanism. After the transition (plateau in Ff vs L curves), the friction coefficient of all aromatic monolayers was similar, when measured with either unfunctionalized or thiol-covered tips. This suggests that the original structure of these monolayers before the transition is not very important for the friction at very high pressures. Velocity Dependence of Friction. The friction of the substrate, bare gold, against Si tips in ethanol (Figure 1b) did not show a significant dependence on sliding velocity (scan speed). A slight gradual increase was seen toward the highest velocities at the loads investigated. The friction force in each thiol system as a function of sliding velocity is shown in Figure 3. In each case, the left panel of a pair shows the friction of bare Si tips 1 and 2 against a single monolayer, whereas the right panel shows the monolayer-monolayer sliding (tips 3-6, cf. caption for Figure 3). These data sets further confirm the observations in Figure 2 on the decrease in friction as a function of load for the close-packed monolayers in contact (both at the velocity 0.15 µm s-1 as in Figure 2 and at other velocities), whereas the two thiophenol monolayers in contact (Figure 3f) show increased friction at low loads.
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Table 3. Friction Coefficients µ of One Monolayer against Si Tips 1 and 2 (Figure 2a,b) Compared to Thiol-Covered Tips 3-6 (Figure 2c)a system bare gold ATP BM TP TP C18 C18
µ, tip 1 (unfunct Si) low pr high pr 0.49 ( 0.02 1.1 ( 0.1 1.1 ( 0.2 0.7 ( 0.2 0.62 ( 0.03 0.11 ( 0.02 0.12 ( 0.01
N/A 0.7 ( 0.2 0.6 ( 0.2 0.59 ( 0.03 0.23 ( 0.03
µ, tip 2 (unfunct Si) low pr high pr 0.42 ( 0.01 0.88 ( 0.03 0.8 ( 0.1 0.71 ( 0.05
N/A 0.60 ( 0.03 0.51 ( 0.02 0.37 ( 0.02
0.072 ( 0.002
0.3 ( 0.1
µ, thiol-covered tip low pr high pr
p0,tip1 (GPa)
p0,tip2 (GPa)
N/A 2.8 g2.7 2.5 2.3 2.9 3.2
N/A 3.2 3.2 2.6
N/A 0.69 ( 0.01 0.51 ( 0.04 1.0 ( 0.1
3.0
0.065 ( 0.002
N/A 0.61 ( 0.02 0.50 ( 0.02
p0,thiol-cov (GPa) N/A 3.2 2.6 2.4 >2.4b
a All experiments were done in ethanol. Low and high pressure refers to the linear regimes below and above the plateau in the F vs f L curves. p0 (error ca. 0.1 GPa) is the maximum Hertzian pressure at the onset of this plateau. b All experimental points were below a possible transition.
Figure 3. Velocity dependence of the friction of thiol monolayer systems in ethanol at selected loads: aminothiophenol (ATP) (a) against unfunctionalized Si tips 1 (two lowest loads) and 2 (three higher loads) and (b) against aminothiophenol-functionalized tip 3; benzyl mercaptan (BM) (c) against Si tips 1 and 2 and (d) against benzyl mercaptan-functionalized tip 4; thiophenol (TP) (e) against Si tips 1 and 2 and (f) against thiophenol-functionalized tip 5; octadecanethiol (C18) (g) against Si tips 1 and 2 and (h) against octadecanethiol-functionalized tip 6. Note that the friction force in panels g and h is shown on a different scale than in the other panels.
The velocity dependencies of friction in the close-packed aromatic systems (ATP and BM) are distinctly different. For both asymmetric systems, Si/ATP (Figure 3a) and Si/BM (Figure 3c), a plateau or maximum in the friction is observed as a function of sliding velocity. The onset of these plateaus in Si/ATP and Si/BM is moved toward lower speeds with increasing load, as is expected if a system becomes more solidlike.37,54,64 However, at comparable loads, the onset of the plateau occurs at a significantly lower speed for the ATP, suggesting that this system is
inherently more solidlike than BM, despite the similarity in packing density and orientation. This reflects the difference in rotational freedom between the two systems. There is possibly also an effect of interactions of the amino end group with the polar solvent, which may act to stabilize the ATP monolayer.65 The plateau was not observed for TP, which is expected to be more fluidlike than ATP and BM. In symmetrical systems of ATP and BM-covered surfaces (Figure 3b,d), a plateau was not observed at the
(64) Yoshizawa, H.; Chen, Y. L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128.
(65) Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 9662.
Boundary Friction of Aromatic SAMs
lowest load. At higher loads the onset of plateaus can be identified and appear at higher velocities compared to the asymmetrical cases (Figure 3a,c), but a shift to slower velocities with increasing load can still be seen. This difference between the asymmetric and symmetric cases is indicative of a more fluidlike sliding in the case of two close-packed monolayers in contact. Literature data on the velocity dependence of the friction between a CH3-terminated tip and a CH3-terminated alkanethiol monolayer66 at a load of 80 nN showed that the friction force (expressed in arbitrary units) increased continuously in the velocity region 0.1-ca. 10 µm s-1, with a leveling off in the region 10-60 µm s-1. In the regime 1-10 µm s-1, the friction increased by a factor of 3. Our investigation for the symmetrical C18 system was not done at such high loads, but it appears that our velocity dependence has the same general shape, and compared to the asymmetrical case, an increase in this regime by a factor of 2-3 is possible. Conclusions It has been shown here that, under conditions of low adhesion, it is possible to directly compare load-dependent (66) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 17, 1970. (67) Beilstein, 4th Suppl. Ser., 13, 1289. (68) Beilstein, 4th Suppl. Ser., 6, 2632. (69) Beilstein, 4th Suppl. Ser., 6, 1463.
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friction measured with FFM tips of different radii. The aminothiophenol (ATP) monolayer, which has a higher packing density than thiophenol (TP) due to intermolecular interactions between amino groups, shows a higher friction (due to higher rigidity to deformation) with a pronounced velocity dependence. Benzyl mercaptan (BM), which has more rotational freedom than TP, can attain a packing density comparable to that of ATP, but its intermolecular interactions are not as strong and this flexibility of the BM molecule is reflected in the friction of BM monolayers. Plateaus in the friction as a function of velocity appear at 10 times higher velocity for BM than for ATP, suggesting that the sliding of BM is much more fluidlike (although not as fluidlike as that of TP). Comparison of systems with one and two monolayers suggest that for ATP and BM (close-packed monolayers) the sliding is more fluidlike in the case of two monolayers in contact. Acknowledgment. I thank R. O ¨ sterbacka, C. Ekholm, J. Gustafsson, and B. Westman for technical assistance. Discussions with J. Israelachvili are gratefully acknowledged. This work was supported by the Academy of Finland (Grant 48879), the Ella and Georg Ehrnrooth foundation, the Research Institute at Åbo Akademi, and the Magnus Ehrnrooth foundation. LA034003B
