Langmuir 2000, 16, 3249-3256
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Nanotribological Properties of Unsymmetrical n-Dialkyl Sulfide Monolayers on Gold: Effect of Chain Length on Adhesion, Friction, and Imaging Eric W. van der Vegte,† Andrei Subbotin,‡ and Georges Hadziioannou* Department of Polymer Chemistry and Materials Science Center, University of Groningen, Nijenborgh 4, NL9747 AG, Groningen, The Netherlands
Peter R. Ashton and Jon A. Preece* School of Chemistry, University of Birmingham, Edgbaston, B15 2TT, Birmingham, United Kingdom Received July 12, 1999. In Final Form: November 24, 1999
A nanotribological study of self-assembled monolayers on gold of unsymmetrical dialkyl sulfides with varying chain length was carried out using a scanning force microscope. Adhesion forces measured between dialkyl sulfide-coated probes and dialkyl sulfide-coated substrates show a direct correlation with variations in chain length as a result of chain interdigitation. The dependence of the friction force upon the sliding velocity and the applied load was investigated. The friction-velocity relationship shows a distinct behavior, i.e., the friction force initially increases with velocity, reaches a maximum, and then decreases. The absolute friction forces increase with increasing chain length and external load. The maxima of the friction forces shift to lower velocities (critical velocity) with increasing applied load, but shift to higher velocities with increasing chain length. This behavior is explained using the concept of friction anisotropy. The friction forces increase linearly with the applied load at a fixed sliding velocity and the calculated friction coefficients show an increase with increasing chain length of the molecules. The chain length dependence of adhesion and friction forces is used to differentiate between chemically similar dialkyl sulfides in compositional imaging. Chain length specific adhesion force mapping and friction force imaging is demonstrated on patterned surfaces bearing up to three different dialkyl sulfides created by the micro-contact printing technique. The influence of the medium on the adhesion and friction is addressed by performing measurements in different solvents.
Introduction 1
Ultrathin self-assembled layers of alkyltrichlorosilanes, n-alkanethiols,2 dialkylammonium surfactants,3 and Langmuir-Blodgett4 layers have gained much interest as boundary lubricants in various technologies.5 These compounds meet the requirements for effective lubricants, such as strong adhesion to the substrate, significant reduction of the friction force, and wear resistance. Moreover, the possibility to vary the chain length, chemical * Corresponding authors. Tel: +44 121 414 3528. Fax: +44 121 414 4403. Email:
[email protected]. Tel: +31 50364300. Fax: +31 50363400. Email:
[email protected]. † Present address: DSM Research Competence Centre, Material Properties, PO Box 18, 6160 MD Geleen, The Netherlands. ‡ Present address: Institute of Petrochemical Synthesis, Russian Academy of Science, Moscow 117912, Russia.
(1) (a) Xiao, X.-D.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (b) Gauthier, S.; Aime´, J. P.; Bouhacina, T.; Attias, A. J.; Desbat, B. Langmuir 1996, 12, 5126. (c) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (2) (a) McDermott, M. T.; Green, J.-B., D.; Porter, M. D. Langmuir 1997, 13, 2504. (b) Lio, A.; Morant, C.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. B 1997, 101, 4767. (3) 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. (4) Briscoe, B. J.; Evans, D. C. B. Proc. R. Soc. London A 1982, 380, 389. (5) (a) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Clarendon Press: Oxford, 1986. (b) Suzuki, M.; Saotome, Y.; Yanagisawa, M. Thin Solid Films 1988, 160, 453. (b) Miyamoto, T.; Sato, I.; Ando, Y.; Tribology and Mechanics of Magnetic Storage Devices, 2nd ed.; Bushan, B., Eiss, N. S., Eds.; Springer-Verlag: New York, 1990. (c) Bushan, B.; Gupta, B. K. Handbook of Tribology: Materials, Coatings and Surface Treatments; McGraw-Hill: New York, 1001.
functionality, degree of cross-linking, etc., makes them good model systems for fundamental studies aimed at understanding friction at the molecular level.6 In earlier studies7 we explored the influence of the chemical nature of the exposed surface on the friction properties. Others have demonstrated1c,2a,3b that the molecular chain length, which determines the structural order in the films, can also have a significant influence on adhesion and friction properties. In this study we investigate the structural aspect of chain interdigitation between two monolayers in relation to adhesion and friction. For this purpose we use another class of self-assembling organic sulfur compounds: unsymmetrical dialkyl sulfides (CH3-(CH2)n-S-(CH2)9CH3), to modify the SFM probe and substrate (Figure 1).8 The structural study by Troughton et al.8a reveals that dialkyl sulfides spontaneously adsorb onto crystalline gold surfaces, thereupon forming reasonably stable monolayers (6) For an overview see: Carpick, R. W.; Salmeron M. Chem. Rev. 1997, 97, 1163. (7) (a) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (b) van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. 1997, 46, 9563. (8) See for example: (a) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (b) Steinberg, S.; Rubinstein, I. Langmuir 1992, 8, 1183. (c) Hagenhoff, B.; Benninghoven, A.; Spinke, J.; Liley, M.; Knoll, W. Langmuir 1993, 9, 1622. (d) Zhang, M.; Anderson, M. R. Langmuir 1994, 10, 2807. (e) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (f) Beulen, M. W. J., Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M.G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170.
10.1021/la990911q CCC: $19.00 © 2000 American Chemical Society Published on Web 02/08/2000
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Figure 1. Schematic representation of the unsymmetrical dialkyl sulfide-coated probe and substrate.
in which the molecules exhibit a relatively disordered organic liquid, liquid-crystalline, or glassy structure. Dialkyl sulfides are very attractive for structure-property relationship studies since the structure of the two alkyl chains can be varied independently by straightforward chemical methods. In this way a higher degree of variation in the local monolayer structure is obtained, often difficult to attain with other organic sulfur compounds. Here, we report a systematic nanotribological SFM study of unsymmetrical n-dialkyl sulfide SAMs, by varying the length of one alkyl chain from 10 to 18 carbon atoms, in steps of 2 carbon atoms, while keeping the other alkyl chain length constant at 10 carbon atoms. The adhesion force between the dialkyl sulfide-coated SFM probes and substrates was found to be dependent on the degree of asymmetry of the two chain lengths of the dialkylsulfide compounds. The asymmetry can be correlated to the degree of chain interdigitation, as illustrated in our model (Figure 1). Friction measurements show a velocity dependence, similar to that observed for amorphous surfactant monolayers. The dependence of the friction and adhesion forces was used as the basis for structural or chain length dependent imaging of the different dialkyl sulfides. The specific imaging was demonstrated on chemically patterned surfaces using the micro-contact printing technique.9 The influence of the medium on friction and adhesion was studied by performing similar measurements in ethanol, water, and n-octane. Experimental Section Materials. Dodecanethiol, n-heptane p.a., THF, (Janssen Chimica), and ethanol p.a., n-octane p.a. (Merck) were used as received. Water used in the experiments was deionized (18 MΩ cm resistivity) with a Millipore Milli-Q filtration system. Gold wire (Scho¨ne Edelmetalen) was 99.99% pure. Mica (clear ruby grade 2) was obtained from Mica New York Corporation, Sylgard silicone elastomer 184 was a gift from Mavom b.v. (Alphen a.d. Rijn, The Netherlands). Synthesis of the Unsymmetrical Dialkylsulfides. The synthesis followed was similar to a procedure reported by Reinhoudt.8f THF (20 mL) was distilled from Na under an atomsphere of N2 directly into a round-bottom flask (50 mL) fitted with a septum inlet. Thiodecanol (2 mL, 10 mmol) was injected into the flask, followed by the alkene (10 mmol). The solution was cooled in an ice bath. A 0.5 M solution of 9-BBN in THF (390 µL, 0.2 mmol) was injected over a 15 min period. The solution was stirred overnight and the temperature allowed to rise to room temperature. The solvent was concentrated in vacuo, (9) (a) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (b) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513. (c) Lo´pez, G.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1513. (d) Wilbur, J. L.; Kim, E.; Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 649. (e) Xia, Y.; Whitesides, G. M. Adv. Mater. 1995, 7, 471.
van der Vegte et al. and the residue subjected to silica gel column chromatography using petroleum ether as eluent. The fractions containing the dialkylsulfide were combined and concentrated in vacuo. Analysis. It is has been reported8a that solutions of dialkylsulfides doped with 1% of thiols result in self-assembled monolayers on gold which are disproportionately rich in the thiol. Thus, it was crucial for the AFM experiments reported in this paper to ensure that there is no contamination with thiodecanol in any of the dialkysulfides used in this study. Thus, to ensure there was no contamination of the dialkylsulfides they were analyzed by GC-EIMS. All mass spectra were recorded on a VG ProSpec mass spectrometer at 70 eV in electron impact mode. GC/MS utilized a DB5 capillary column (25 m) operating at 150 °C to 280 °C at 10 °C per min, with helium as the carrier gas. The high-resolution mass spectra were recorded on a solids probe at a resolution of 7500. The thiodecanol eluted with a retention time much less than all the dialkylsulfides, and no thiodecanol was observed in the dialkylsulfide samples. Additionally, the EI mass spectra of thiodecanol affords a peak of 174 attributable to C10H21SH+, while all the dialkylsulfides show fragmentation peaks at 173 and 174 which are attributable to the fragments 12C H S+ and 13C H S+, in the correct isotopic ratio, indicating 10 21 10 21 that there is no C10H21SH contamination. It should be noted that a slight trace impurity (
