Preparation and Tribological Study of a Peptide-Containing

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Preparation and Tribological Study of a Peptide-Containing Alkylsiloxane Monolayer on Silicon Shiyong Song,†,‡ Sili Ren,† Jinqing Wang,† Shengrong Yang,*,† and Junyan Zhang*,† State Key Labaratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, China, and Graduate School of Chinese Academy of Science, Beijing 100080, China ReceiVed October 25, 2005. In Final Form: May 9, 2006 A peptide-containing alkylsilane self-assembled monolayer on silicon surface has been prepared successfully by a simple one-step strategy. The formation and structure of the peptide-containing SAMs were characterized by means of contact angle measurement, ellipsometry, FT-IR, and AFM morphology observation. It was found that the water content in the hydrolysis solution plays a key role in determining the quality of the monolayers. The micro-tribological properties of various films were evaluated by using AFM, while the macro-tribological study was performed on a ball-on-plate tribometer. It was found that the peptide-containing monolayers possess excellent friction-reduction and antiwear ability, which was attributed to its amide-containing structure. In other words, the interchain hydrogen bonds among the molecules enhance the stability of the monolayers against rubbing the counterpart ball and thus endow it an outstanding antiwear ability.

Introduction Self-assembled monolayers (SAMs) have been widely investigated in the past 20 years because of their potential applications in the field of surface modification, boundary lubricant, sensor, photoelectronics, and functional bio-membrane modeling, etc.1-6 On the basis of the surface chemical reaction and synthetic approaches, the chemical structures of SAMs can be manipulated easily both at the individual molecular and at the material levels.7-15 As a potential lubricant for controlling stiction and friction in micro-electromechanical systems (MEMS), the nano-tribological properties of SAMs are closely related to their intrinsic chemical composition and structures. For example, the frictional behaviors of SAMs are chain length and terminal group dependent.16-20 SAMs with longer chains are generally densely * Corresponding author. Phone: 86-931-4968088. Fax: 86-931-8277088. E-mail: [email protected] (S.Y.); [email protected] (J.Z.). † Chinese Academy of Science. ‡ Graduate School of Chinese Academy of Science. (1) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (2) Hsu, S. M. Tribol. Int. 2004, 37, 537-545. (3) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 3-30. (4) Gulino, A.; Mineo, P.; Scamporrino, E.; Vitalini, D.; Fragala, I. Chem. Mater. 2004, 16, 1838-1840. (5) Foisner, J.; Glaser, A.; Leitner, T.; Hoffmann, H.; Friedbacher, G. Langmuir 2004, 20, 2701-2706. (6) Wang, M.; Liechti, K. M.; Wang, Q.; White, J. M. Langmuir 2005, 21, 1848-1857. (7) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (8) Sabapathy, R. C.; Shattacharyya, S.; Leavy, M. C.; Cleland, W. E., Jr.; Hussey, C. L. Langmuir 1998, 14, 124-136. (9) Gershwitz, O.; Grinstein, M.; Sukenik, C. N. J. Phys. Chem. B 2004, 108, 664-672. (10) Monsathaporn, S.; Effenberger, F. Langmuir 2004, 20, 10375-10378. (11) Mowery, M. D.; Kopta, S.; Ogletree, D. F.; Salmeron, M.; Evans, C. E. Langmuir 1999, 15, 5118-5122. (12) Ren, S.; Yang, S.; Zhao, Y. Langmuir 2003, 19, 2763-2767. (13) Choi, J.; Morishita, H.; Kato, T. Appl. Surf. Sci. 2004, 228, 191-200. (14) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee S. J. Am. Chem. Soc. 2003, 126, 480-481. (15) Chi, Y. S.; Lee, J. K.; Lee, S.; Choi, I. S. Langmuir 2004, 20, 3024-3027. (16) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235-237. (17) Lio, A.; Carych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 38003805. (18) Nakano, M.; Ishida, T.; Numata, T.; Ando, Y.; Sasaki, S. Jpn. J. Appl. Phys., Part 1 2003, 42, 4734-4738.

packed, while the shorter-chain ones are not. With the same terminal groups, loosely packed SAMs generally possess higher friction force due to the larger energy dissipation during the sliding, and higher adhesive force as well due to the liquidlike disordered structure. On the other hand, altering the terminal groups, for instance, from apolar (-CF319 and -CH3) to polar (-COOH,20-23 -OH,19,24 and NH212), could result in the increase of adhesion and friction. This is because SAMs with more polarized groups generally possess higher surface energy and a relatively strong interaction during the sliding, and therefore higher adhesion and more energy loss are expected, which leads to a higher friction force. Co-deposition of molecules with different terminal groups25-28 or alkyl chain lengths29,30 to form mixed SAMs is also extensively studied, which allows an in-depth understanding of the relationship between structure and performance of SAMs. Several reports have revealed the frictional behaviors of the mixed SAMs derived from akanethiols or alkylsilanes. For instance, mixed monolayers of mercaptoundecanoic acid (MUA) and dodecanethiol (DDT) provided a chemically heterogeneous surface,27 on which friction force increases with increasing the relative amounts of MUA in the mixed SAMs. Another study shows that mixed SAMs composed of two alkylsilanes with different alkyl chain lengths (19) (a) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaduddin, S.; Lee, T. R.; Perry, S. S. Langmuir 1999, 15, 3179-3185. (b) Houston, J. E.; Doelling, C. M.; Vanderlick, T. K.; Hu, Y.; Scoles, G.; Wenzl, I.; Lee, T. R. Langmuir 2005, 21, 3926-3932. (20) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 17, 19701974. (21) Brewer, N. J.; Foster, T. T.; Leggett, G. J.; Alexander, M. R.; McAlpine, E. J. Phys. Chem. B 2004, 108, 4723-4728. (22) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, S. M. J. Am. Chem. Soc. 1995, 117, 7943-7951. (23) Kim, H. I.; Houston, J. E. J. Am. Chem. Soc. 2000, 122, 12045-12046. (24) Leng, Y.; Jiang, S. J. Am. Chem. Soc. 2002, 124, 11764-11770. (25) Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109-4115. (26) Li, L.; Chen, S.; Jiang, S. Langmuir 2003, 19, 666-671. (27) Beake, B. D.; Leggett, G. J. Langmuir 2000, 16, 735. (28) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345. (29) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (30) (a) Zhang, Q.; Archer, L. A. J. Phys. Chem. B 2003, 107, 13123-13132. (b) Zhang, Q.; Archer, L. A. Langmuir 2005, 21, 5405-5413.

10.1021/la052868e CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006

Tribological Study of SAMs on Silicon

exhibit lower friction coefficients than one-component SAMs, which is opposite from previous findings for mixed SAMs formed from alkanethiols on gold.30 Introducing a functional group, such as diacetylene,11 peptide,7,8,12 and sulfone,31 into straight hydrocarbon chains to construct robust SAMs is another alternative way to improve their tribological properties. It is hypothesized that, within the SAMs, the functional groups interact laterally taking the form of hydrogen bonding, dipole interaction, π-stacking, or covalent attachment, which may enhance the mechanical integrity and stability. To the best of our knowledge, only a few of studies11,12 have been engaged in the investigation on the frictional behaviors of the functional-group-embedded SAMs. The focused targets are the alkylthiols derivatives due to their convenient molecular synthesis and easy formation of the monolayer on gold surface, while the use of alkylsilanes is seldom reported. Because SAMs based on alkylsilanes are more suitable than alkanethiols SAMs as the potential lubricant layer for MEMS,12 more attention should be paid to SAMs derived from alkylsilanes. In a previous study,12 a peptide-containing self-assembled duallayer film was constructed by a two-step strategy, and the tribological properties of the film were investigated. The film exhibits excellent friction reduction and wear-resistant performance. It was supposed that the interchain hydrogen bonds enable the adjacent molecules to interlock and enhance the stability of the SAMs against rubbing with the counterpart Si3N4 ball. However, by the two-step strategy, the real structure of the “duallayer” film might be more complex and uncontrollable. In this work, a peptide-containing adsorbate of alkylsilane was synthesized, and then the corresponding SAMs were fabricated on Si(111) wafer by using a simple one-step strategy. The formation mechanism of SAMs was discussed, and the nano- and macrotribological properties were studied as well. Experimental Section Materials. 3-Aminopropyltriethoxylsilane (APS) and octadecyltrimethoxylsilane (OTMS) were purchased from Aldrich. Stearic acid (STA) and thionyl chloride, analytical purity, were from Shanghai Reagent Co. Acetone and toluene were of analytical purity. All reagents were used as received. P-type polished single-crystal Si(111) wafer used as substrates was obtained from GRINM Semiconductor Materials Co., Ltd., Beijing. Ultrapure water (>18 MΩ) was used in this work. Synthesis of the Monolayer Precursor. The monolayer precursor N-((3-triethoxylsilyl)propyl)octadecanoylamide (coded as TPOA) was synthesized according to the literature.32 First, stearic acid (42.67 g, 0.15 mol) was dissolved in 106 mL of dry benzene. Thionyl chloride was then added (22 mL, 0.30 mol) over a period of 2.5 h at 50 °C. The mixture temperature was then increased to 90 °C, and the heating was continued for an additional 3 h. The excess thionyl chloride was then removed under a reduced pressure, and the product was fractionally distilled. The intermediate, stearic chloride, was obtained at 200-205 °C at 0.015 MPa (equal to 12-13 mmHg). Second, the mixture of stearic chloride (30.30 g, 0.1 mol) and toluene (50 mL) was added dropwise to a solution of 3-aminopropyltriethoxylsilane (APS) (22.10 g, 0.1 mol) and triethylamine (14 mL, 0.1 mol) in toluene (50 mL) at 0 °C. The reaction mixture was then warmed to 40 °C and stirred for another 3 h. Finally, TPOA was obtained by removal of the solvent under reduced pressure.

Preparation of the Monolayers. The mixture of acetone and toluene was used as the hydrolysis solvent, which was saturated with 1 mol/L hydrochloride acid for 24 h. The amount of water in

Langmuir, Vol. 22, No. 14, 2006 6011 the hydrolysis solvent was controlled by varying the ratio of acetone to toluene, and three different ratios of Vacetone to Vtoluene at 1:4, 1:2, and 1:1 were used. (A mixture with higher ratios of acetone to toluene was also used in this work. However, the poor solubility of TPOA in the mixture solvent and excess water caused a very turbid TPOA solution. Therefore, further work using the mixture solvent with a higher acetone concentration was not performed.) The hydrolysis solution of TPOA in a concentration of 20 × 10-3 mol/L was prepared in the mixed solvent, and then sealed and stirred at room temperature for 48 h. After that, 1 mL of hydrolysis solution was diluted by adding 20 mL of cyclohexane. Silicon wafers were cleaned and hydroxylated in Piranha solution (mixture of 7:3 (v/v) 98% H2SO4 and 30% H2O2) at 90 °C for 30 min. After being rinsed by ultrapure water and blown dry in ultrapure nitrogen, they were placed in the diluted TPOA solutions for 24 h at room temperature. The functionalized substrates then were washed sequentially with toluene, ethanol, and ultrapure water to eliminate any possible physically absorbed impurities. Finally, the SAMs of TPOA (TPOA-SAMs) were baked at a constant temperature of 110 °C for 2 h. For comparison, SAMs of OTMS were also prepared on silicon wafers, and the preparation procedure was the same as that of TPOA. The hydrolysis solvent with a ratio of Vacetone to Vtoluene at 1:4 was used. Characterization of the SAMs. A CA-A type contact angle meter (Kyowa Scientific Co. Ltd.) was used to measure the static water contact angle on the films. At least five points were measured for each specimen, and the measurement error is (1°. The ellipsometric thickness measurements were performed with a L116-E ellipsometer (Gaertner), which was equipped with a He-Ne laser (632.8 nm) set at an incident angle of 50°. A real refractive index of 1.46 was set for the silica layer and 1.45 for organic monolayers. The measurements were carried out seven times for each specimen to obtain average data, and the thickness was recorded to an accuracy of 0.3 nm. Transmission mode FTIR spectra were recorded on a Bruker IFS 66V Fourier transformation infrared spectrometer. The spectrum was collected for 1024 scans with a resolution of 1 cm-1 based on air background. To eliminate the effect of H2O and CO2, the pressure in the sample chamber and optical chamber was kept below 3.0 × 10-4 Pa. A digital Instruments Nanoscope IIIa Multimode atomic force microscope (AFM, Digital Instruments, Cambridge, UK) was employed to observe the film morphology using tapping mode. All of the characterizations were performed on at least three samples for each kind of film. Tribological Study of the SAMs. The micro-tribological study of the SAMs was performed using the SPA400 AFM. Commercially available Al-coated V-shape Si3N4 cantilevers were used with an announced torsional force constant, 0.1 N/m, and a normal force constant, 2 N/m. A tip (uncoated) with a radius of less than 10 nm was used. No attempt was made to calibrate the torsional force constant. The output voltages were directly used as relative frictional force. A series of measurements of friction-load behavior was made from friction loops obtained from at least five separate locations on each sample surface. For all measurements, the same cantilever was used. The friction coefficients were determined from the gradients of the detector response-load plots by using linear regression. Experiments were carried out under ambient conditions of 20 °C and 40-50% relative humidity. Macro-tribological properties were studied on a UMT-2MT tribometer (CETR.USA) using ball-on-plate mode. The upper ball counterpart was fixed, while the lower sample plate adhered on the flat base kept reciprocating at a distance of 0.7 cm. The balls used here were commercially available ruby balls (Al2O3, φ 4 mm). The load of 10 g (0.1 N) was applied for all measurements, and the corresponding initial Hertzian contact stresses were estimated to be about 0.1 GPa. The friction coefficient-time plots were recorded (31) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (32) (a) Fang, K.; Ming, Q.; Luo, W. Chem. Reagents (Beijing) 1989, 11, 121-122. (b) Ralston, A. W.; Selby, W. M. J. Am. Chem. Soc. 1939, 61, 10191020.

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automatically, and more than five repeated measurements were performed. To investigate the relationship between friction coefficient and velocity, the average friction coefficients within the first 5 min were measured at different reciprocating rates from 50 to 700 rev/ min, and measurements were performed at least three times for each velocity.

Results and Discussion The Formation of the TPOA-SAMs. The trialkoxysilane derivative, but not trichlorosilane, was chosen as the target molecule to form SAMs because it was less sensitive to water and thus relatively easy to synthesize and purify. According to the previous reports,33,34 the precursor solution should be hydrolyzed first. A certain amount of water should be added for the creation of silanol according to the proposed mechanism. We deduced that the control of the proper amount of water in the hydrolysis solution was very important, because insufficient water may result in an uncompleted film and excess water could result in the polymerization of the precursor and then the deposition of polysiloxane on the surface. Therefore, we first investigated the effect of water in the hydrolysis solution on the formation of TPOA monolayers. Because of the poor solubility of the synthesized precursor TPOA in tetrahydrofuran (THF), the monolayer fabrication procedures used in the previous literature33 were not suitable. Toluene is a good solvent for TPOA, but direct addition of water into toluene will result in a hetero-phase. So, we developed a method to control the amount of water in toluene by adding a certain amount of acetone to toluene and then saturating the mixture with 1 mol/L hydrochloride acid for 24 h. It was supposed that the amount of water in the mixture increased with increasing acetone ratio. In this work, three mixtures in ratios of Vacetone to Vtoluene at 1:4, 1:2, and 1:1 were used, and the corresponding prepared films were marked as films A, B, and C, respectively. The water contact angle and thickness of various films were given in Figure 1. It can be seen that the water contact angle on silicon wafer cleaned by piranha solution is less than 5°. After the deposition of the TPOA monolayer, the contact angle increased to more than 90°, which reflects that the TPOA-SAMs have been formed on the silicon surface. Furthermore, we can see that the water contact angle increases from 91° to 99°, and to 100° with increasing acetone ratio in the hydrolysis solution from 1:4 to 1:2, and to 1:1. Increase of the acetone ratio means an increasing water amount in the precursor’s solution, indicating that enough water will help produce better quality TPOA-SAMs. Besides, pure toluene saturated by the acid solution was also tested as the hydrolysis solvent. A film with a contact angle of about 60° and thickness around 1.2 nm was obtained, suggesting the film was a disordered film and probably the hydrolysis was not processed completely due to the lack of enough water in the hydrolysis solution. However, it also can be seen that there is not much difference in the water contact angles for films B and C (99° and 100°, respectively), which indicates that the mixture solvent in a ratio of 1:1 will be able to offer sufficient and proper water to produce the most densely packed TPOA-SAMs. The measured ellipsometric thickness also confirmed the formation of TPOASAMs on silicon wafers, and the films thickness varied with the water content in the hydrolysis solutions, which is in good agreement with the results of contact angle measurements (Figure 1). A schematic view of a peptide-containing self-assembled monolayer on silicon surface is shown in Figure 2. (33) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532-538. (34) Tian, F.; Xiao, X.; Loy, M. M. T.; Wang, C.; Bai, C. Langmuir 1999, 15, 244-249.

Figure 1. Water contact angle and ellipsometric thickness of various films.

Figure 2. A schematic view of the peptide-containing self-assembled monolayers on silicon.

It has been reported that the ideal -CH3 terminated SAMs possessed a water contact angle of about 110°.35,36 Also, the thickness of the ideal TPOA-SAMs should be about 3.1 nm according to 0.14 nm per CH2 unit.35 Yet the best TPOA-SAMs we obtained here only had a water contact angle of 100° and a thickness of 2.5 nm. As compared to the hydro-carbon chain molecules such as OTMS (octadecyltrimethoxylsilane), the big amide group (OdC-NH) in the chain of TPOA may result in a steric hindering effect and complex interchain interaction, which prevent the formation of more ordered and condensed SAMs. To obtain more insight into the structure of the TPOA-SAMs, the transmission mode FTIR spectra were collected as shown in Figure 3. Because of the weak signal, only the C-H stretching mode could be seen. For a comparison, the spectra of films A and C were placed together. For film C, the asymmetric and symmetric methylene vibration peaks appeared at 2922 and 2853 cm-1, while they appeared at 2925 and 2854 cm-1 for film A. Previous studies indicate that the asymmetric and symmetric methylene vibrations are typically in the range of 2915-2918 and 2846-2850 cm-1 for the all-trans extended ordered chain, and ∼2928 and ∼2856 cm-1 for liquidlike disordered chains.30 Therefore, TPOA-SAMs obtained here are less ideally ordered but much better than its liquid precursor. Comparing the two (35) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (36) Ren, S.; Yang, S.; Zhao, Y.; Zhou, J.; Xu, T.; Liu, W. Tribol. Lett. 2002, 13, 233-239.

Tribological Study of SAMs on Silicon

Figure 3. The transmission mode FTIR spectra of the TPOA-SAMs (films A and C) in the high-frequency C-H region.

Figure 4. The AFM morphology of TPOA-SAMs (film C) over a scan area of 1 µm × 1 µm.

films, film C is more ordered than film A, which is consistent with the water contact angle and thickness measurement results. To obtain information on the surface characteristics, such as the uniformity, roughness, grain distributions, and defect formation, we observed the surface morphology of the prepared films by using AFM.37 The morphology of film C is shown here as in Figure 4. It can be seen that the TPOA-SAMs surface is characterized by regular grains distributed on the surface, which might be caused by the deposition of polysiloxane during the fabrication. Nevertheless, the surface of the TPOA-SAMs film is still rather smooth on the micrometer scale, with a microroughness of root-mean-square (rms) of about 0.5 ( 0.1 nm over a scanning range of 1 µm × 1 µm. Micro-tribological Study of the TPOA-SAMs. The microfriction behavior of silicon substrate and the TPOA-SAMs (film C) was investigated by using AFM (Figure 5). The friction force was given here in the form of voltage signal, which should be proportional to the real friction force.20,30 Therefore, the results from various film surfaces could be compared to each other. It is evident from Figure 5 that the formation of TPOA-SAMs on the silicon wafer greatly reduces the friction force. Also, the friction force has an approximately linear relationship with applied load, which can be described by a modified form of Amonton’s (37) Rosidian, A.; Liu, Y. J.; Claus, R. O. AdV. Mater. 1998, 10, 1087-1090.

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Figure 5. Friction versus load curve for the bare silicon (SiO2/Si) and TPOA-SAMs (film C).

Figure 6. The relative friction coefficients of the bare silicon (SiO2/ Si) and the three different packing density TPOA-SAMs.

Law, in which the lateral force (FL) is given by

FL ) µFN + F0 where µ is the coefficient of friction, FN is the normal load, and F0 is the friction force when the external load is zero.19 The slope of the friction-versus-load curve can be used as the relative friction coefficient because it is supposed to be proportional to the real friction coefficient.20,25 The relative friction coefficients of various films were summarized in Figure 6. It can be seen that the uncoated bare silicon wafer has the highest friction coefficient value of 1.29, and the friction coefficient for the three different packing density films of A, B, and C are 0.36, 0.32, and 0.29, respectively. Apparently, the relative frictional coefficient of various TPOASAMs could well mirror the trends in packing density and crystalline order of the films mentioned above. In other words, the higher are the packing density and crystalline order, the more lubricity properties there would be. Perry et al.38 have reported that, with the same chemical structure and composition, the densely packed crystalline monolayer films of hydrocarbon chain exhibit lower friction as compared to loosely packed or liquidlike ones. This might be attributed to the relatively strong adhesion resulted from the increased contact area between the AFM tip and the terminal methyl groups for a more liquidlike film, which (38) Lee, S.; Seok, Y.-S.; Colorado, R., Jr.; Guenard, R. L.; Lee, T. R.; Perry, S. S. Langmuir 2000, 16, 2220-2224.

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Figure 7. Variation in friction coefficient with time for TPOA-SAMs at different sliding frequencies at an applied load of 0.1 N: (a) film A, at 1 Hz; (b) film C, at 1 Hz; (c) film A, at 10 Hz; (d) film C, at 10 Hz.

give rise to the observed higher friction. Another interpretation is that less densely packed or disordered films possess more channels of energy dissipation, and thus result in a higher fiction force.16,17 Macro-tribological Study of the SAMs. The ball-on-plate test was used to investigate the macro-tribological properties of the films, and the results are shown in Figure 7. It can be seen that films A and C have similar friction coefficients at a sliding frequency of 1 Hz (equal to 1.4 cm/s, in Figure 7a,b). At the early stage, the friction coefficients for both films are below 0.1; after more than 10 000 s (equal to 10 000 sliding cycles), the coefficient gradually increased to 0.15, which is also much lower than that of the SiO2/Si film. (The friction coefficient of SiO2/Si is about 0.5, which was obtained by using the same counterpart and test conditions.) So, we can conclude that both films, even for the less packed and ordered film A, exhibit good friction reduction and wear-resistant properties. Because the operation of the micromechanical systems is often in a high-speed state, it is necessary to evaluate the tribological properties of the TPOA-SAMs film in high sliding frequency. The frictional behavior of the TPOASAMs film in a sliding frequency of 10 Hz (equal to 14 cm/s) is shown in Figure 7c,d. It can be seen that both films A and C still possess good friction reduction and wear-resistant ability. For film C, the coefficients remain below 0.15 after 90 000 sliding cycles, while it rises from 0.12 to 0.16 after more than 70 000 sliding cycles for film A. We proposed that the excellent antiwear performance of the peptide-containing TPOA-SAMs is attributed to its special film structure. As shown in Figure 2, the presence of an amide group in the methylene chains results in interchain hydrogen bonds, which interlock the molecular chains and therefore enhance the stability of the SAMs against rubbing the counterpart ball.12,39 Furthermore, even if the hydrogen bonds

Figure 8. Variation in friction coefficient with time for film OTMSSAMs at a normal load of 0.1 N and a sliding frequency of 10 Hz.

were broken in the presence of the mechanical interaction, they could be recovered and remain to stabilize the film after sliding.39 For a comparison, SAMs of octadecyltrimehoxylsilane (SAMsOTMS) were prepared, which have a water contact angle of 104° and thickness of 1.9 nm similar to those of the TPOA-SAMs. The macro-tribological properties were also evaluated at a normal load of 0.1 N and sliding frequency of 10 HZ. The result is shown in Figure 8. We can see that the initial friction coefficient of the SAMs-OTMS is almost the same as that of TPOA-SAMs. However, it registers a sharp increase in the friction coefficient after less than 1500 sliding cycles, indicating that the OTMSSAMs have been destroyed in this case. Noting that the OTMS and TPOA monolayers have a molecular order and bond mode (39) Ren, S.; Yang, S.; Wang, J.; Liu, W.; Zhao, Y. Chem. Mater. 2004, 16, 428-434.

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friction coefficient-increasing trend with the increasing sliding velocity can be seen. We contribute this phenomenon to the higher degree of the oscillation and distortion of the molecules at higher shear velocity, which accelerate the dissipation of the accumulated energy.39

Conclusions

Figure 9. Variation in friction coefficient with sliding velocity for TPOA-SAMs (film C) at a normal load of 0.1 N.

similar to that of the substrate, it was supposed that the different antiwear abilities of them might have resulted from their different chemical structures. In other words, the hydrogen bonds in the TPOA-SAMs cannot be available in the OTMS monolayer. Comparing the frictional behaviors of TPOA-SAMs at different sliding frequencies (Figure 7), it was found that the friction coefficient could be affected by the sliding speed. Thus, the relationship between friction coefficient and sliding speed was investigated, and the result was shown in Figure 9. The average coefficients for film C during the first 5 min for each sliding frequency were collected. Despite the scatter of the data, the

We have prepared a peptide-containing alkylsilane monolayer on a silicon wafer. It was found that the water content in the hydrolysis solution plays an important role in determining the quality of the formed TPOA-SAMs. A series of films with different packing density were obtained by adjusting the water content in hydrolysis solutions. Micro-tribology studies on AFM have revealed that the more ordered film exhibited a smaller frictional coefficient. Because of the amide group contained in the hydrocarbon chains, which leads to the formation of interchain hydrogen bonds, the peptide-containing SAMs possessed outstanding antiwear performance. To gain a further understand about the effect of chemical structure on tribological properties of SAMs, it is suggested that more effort should be focused on this subject in the future. Acknowledgment. We would like to thank the National Natural Science Foundation of China (Grant Nos. 50375151, 50323007, 50271080, and 50572107), 863 Plan (Grant No. 2002AA302609), and “Top Hundred Talents Program” of the Chinese Academy of Sciences for financial support. We also thank the Lubricant and Functional Material Laboratory of Henan University for use of the AFM facility. LA052868E