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Langmuir 2000, 16, 662-670
Tip in Situ Chemical Modification and Its Effects on Tribological Measurements Linmao Qian and Xudong Xiao* Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, China
Shizhu Wen The State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China Received May 10, 1999. In Final Form: September 8, 1999
In this paper, we present our experimental results on the process of the in situ chemical modification of the silicon nitride atomic force microscopy/frictional force microscopy tip by n-octadecyltrimethoxysilane (OTE)/mica, OTE/SiO2, and SiO2. The modified tips have different friction and adhesion properties against mica reference samples compared to those before modification. The resultant tip modification depends not only on OTE self-assembled monolayer (SAM) but also on the substrates the OTE SAM is prepared on. In the case of OTE/mica, the friction of the modified tip against mica reference is greatly reduced; in the case of OTE/SiO2, the friction of the modified tip against mica reference is greatly increased. It is surprising that bare SiO2 can also chemically modify the Si3N4 tip to increase the friction against mica reference. In the case of OTE modification, it was found that the tips could be cleaned by repetitive friction scans on mica. However, a tip modified by SiO2 cannot be mechanically cleaned. Moreover, it was found that humidity and load could also affect the tip chemical modification. Our results are important for interpreting tribological data since the actual contact chemistry was often overlooked in the atomic force microscopy experiments in the past.
1. Introduction During the past years, atomic force microscopy (AFM) and its offspring frictional force microscopy (FFM) have become common tools for studying the tribological properties of materials at the atomic scale.1-6 Often, a commercial silicon nitride (Si3N4) tip, due to its fine mechanical properties, is used for the measurements. The contact between the tip and the sample is assumed to be a clean tip/sample interface. This assumption, however, is questionable since the materials on the sample surface might be transferred to the tip as a result of tribochemistry, which may in turn affect the reproducibility and the understanding of the tribological measurements. Ex situ chemical modification of tips, or “controlled” tip chemistry, has been employed to control the contact chemistry. For example, in the so-called chemical force microscopy (CFM),7-11 the tips were modified by a layer of self-assembled molecules, typically alkanethiols on a gold-coated tip. Controlling the terminal groups of the * Corresponding author. E-mail:
[email protected]. Fax: (852)2358-1652. Tel: (852)2358-7494. (1) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (2) Overney, R.; Meyer, E. MRS Bull. 1993, May, 26-30. (3) Hu, H.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358. (4) Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H.-J. Phys. Rev. Lett. 1992, 69, 1777. (5) Frommer, J. E. Thin Solid Films 1996, 273, 112. (6) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (7) Frisbie, C. D.; Rozsnyai, L. W.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (8) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (9) Noy, A.; Vezenov, D. V.; Lieber, C. M. Annu. Rev. Mater. Sci. 1997, 27, 381. (10) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446. (11) Ito, T.; Namba, M.; Buhlmann, P.; Umezawa, Y. Langmuir 1997, 113, 44323.
molecules in the layer allows distinction of chemical identities of the molecules on a sample surface through measurement of adhesive or frictional properties.7 While modification of tips with alkanethiols requires that the silicon nitride tip be coated by a gold film, which is not suitable for tribological study under high load due to the softness of the gold film, direct modification of Si3N4 tips with alkanesilane monolayers is possible through reaction with the oxidation outlayer of the tip. With the silicon nitride tips modified by silane-based molecules, Tsukruk and Bliznyuk have studied the pH dependence of the adhesive and friction forces of the self-assembled monolayers (SAMs) with CH3, NH2, and SO3H terminal groups in aqueous solutions.10 Even in these cases, the inertness of the chemically modified tips is only an assumption rather than an experimental fact. There is relatively little research on the tip chemical modification during the contact with sample surfaces. However, knowledge of such in situ modification is important particularly for tribological experiments. When an AFM/FFM tip scans on a sample surface, the pressure in the contact areas is on the gigapascals scale and the temperature there may be much higher than that in the environment.12 Thus, complex tribochemical reactions can take place in the contact areas, which result in tip chemical modification. The resultant modification of the tip may depend on the environment conditions (e.g., temperature and humidity), the materials at the interface, and the load and the scan speed. A study of in situ tip chemical modification is important for the understanding of the tribological experimental results. In this paper, we present our experimental results of the in situ chemical modification of Si3N4 tips during friction scans on n-octadecyltrimethoxysilane (OTE), (12) Fischer, T. E.; Mullins, W. M. J. Phys. Chem. 1992, 96, 5690.
10.1021/la9905618 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/28/1999
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CH3(CH2)17Si(OCH3)3, monolayers self-assembled on mica (OTE/mica) and silicon oxide (OTE/SiO2), and friction scans on a clean silicon oxide surface. Other than serving the purpose for an in situ tip chemical modification study, the OTE/SiO2 system is also interesting because OTE is a good lubricant to reduce the stiction between two moving parts in micromachine systems (MEMS) which are made of Si-based materials.13,14 The stiction often causes MEMS operation failure. The behavior of OTE layers under high pressure is important for MEMS applications. In the experiment, we used clean mica as a reference to test the friction of the tip against it before and after tip modification. It was found that after friction scans to high enough load on the above samples, the friction of the modified tip against the reference is altered. Sometimes, the change in friction is as large as a factor of ∼3. On a mica reference, while the friction by the OTE/mica modified tip decreases compared to the tip before modification, the friction of the OTE/SiO2 modified tip has an opposite behavior. It is surprising that even silicon oxide can also chemically modify the tip. After friction scans on “clean” SiO2, the friction of the thus-modified tip against mica increases as compared to that with a tip before modification. We have also attempted to clean all these in situ chemically modified tips by having them repetitively scannned on the reference mica surface. For the OTE/mica and OTE/ SiO2 modified tips, the cleaning was successful. For SiO2 modified tips, the mechanical cleaning was not effective. Since humidity and load may also play a role in the tribochemical reactions, we also studied the dependence of the tip modification on humidity and load for each sample by carrying out the modification process at two relative humidity values, 5% and 90%, and different loads. 2. Materials and Experimental Methods Four kinds of samples, mica, OTE/mica, SiO2 (oxidation layer on clean silicon), and OTE/SiO2, were used. While the mica substrates were freshly cleaved before being immersed into OTE solution, the SiO2 substrates were prepared according to wellestablished procedures.15 The n-octadecyltriethoxysilane (OTE) was purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI), and was filtered through a 0.2 µm TPFE membrane prior to use. The solvents of tetrahydrofuran (THF) and cyclohexane were of spectral quality. The glassware for preparation of the prehydrolysis solution and for self-assembly was cleaned with a chromic acid cleaning solution. The preparation of OTE SAM on freshly cleaved mica and hydrophilic silicon oxide was carried out as described previously.16-19 Briefly, 0.2 g of the OTE and 5 mL of 1 N hydrochloric acid were added to 20 mL of THF to prepare the prehydrolysis solution. The solution was stirred at room temperature for 4 days. Prior to self-assembly, the hydrolysis solution was filtered through a 0.2 µm TPFE membrane. Following this treatment, the solution was diluted to 1:20 in cyclohexane and then added to a clean Petrie dish container. The freshly cleaved mica or clean silicon wafers were immersed into the diluted solution for 10 min at room temperature. Samples were then rinsed with fresh cyclohexane for many cycles. At last, they were baked at 120 °C for 2 h. According to refs 16, 17, and 19, this procedure could produce high-quality OTE monolayers on both mica and SiO2 substrates. As shown in ref 19 in detail, a smooth OTE monolayer was formed with a low density (∼5 (13) Srinivasan, U.; Houston, M. R.; Howe, R. T.; Maboudian, R. J. Microelectromech. Syst. 1998, 7, 252. (14) Maboudian, R.; Howe, R. T. Tribol. Lett. 1997, 3, 215. (15) Sunada, T.; Yasaka, T.; Takakura, M.; Sugiyawa, T.; Miyazaki, S.; Hirose, M. Jpn. J. Appl. Phys. 1990, 29, L2408. (16) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532. (17) Schwartz, D. K.; Steinberg, S.; Israelachvilli, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (18) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (19) Xiao, X.-D.; Liu, G.-y.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600.
Langmuir, Vol. 16, No. 2, 2000 663 clusters/µm2) of polymerized clusters of sizes ∼100-10000 Å and heights ∼10-500 Å on top. Imaging both the OTE/mica and OTE/ SiO2 samples made for the present study by AFM topographic mode showed consistent results with the cluster dimensions at the lower limits. For the friction experiment, we tried to avoid these weakly adhered clusters in the scan areas. Before the AFM/FFM measurements were taken, the water contact angles of the samples were checked using an NRL contact angle goniometer (Rame-Hart, Inc., Mountain Lakes, NJ). OTE/ mica showed a water contact angle of 100°, whereas OTE/SiO2 showed a water contact angle of 108°. The water contact angles of mica and SiO2 were less than 4°. The friction measurements were taken by a home-built Beetle type atomic force microscope/frictional force microscope (AFM/ FFM) with an RHK electronic controller (RHK Technology, Rochester Hills, MI). Briefly, a quadrant photodiode was used to measure both the friction force (cantilever torsion) and the normal force (cantilever deflection) simultaneously. Since the torsional force constant was unknown, we left the friction force as an arbitrary unit. The external load was computed as the product of the displacement of the cantilever and the nominal force constant. For all the measurements, commercially available Si3N4 triangular cantilevers/tips with a nominal force constant of 0.50 N/m (Park Instruments, USA) were employed. The nominal radius of curvature of the pyramidal tips was ∼500 Å. After mica treatment (see below), the radius increased to ∼1000 Å as checked by the step images of Au(111). All our experiments were carried out at room temperature in a glovebox in which the humidity can be controlled by the flow of dry air and evaporation of water. For the friction measurement, the feedback loop was disabled to allow changes of load via the applied voltages on the piezotubes. The load was increased (or decreased) linearly in each successive scan line. At a given load, the tip was scanned back and forth in the x direction while the lateral deflection of the lever was measured. The typical scan size was ∼400 Å. The difference in the lateral deflection between back and forth motions was taken as twice of the friction.3 A friction signal versus load curve could then be generated by averaging the friction force signal over x positions for a range of loads. Since the friction was measured in both approach and retract cycles, the simultaneously measured normal force allowed us to deduce the adhesive force through the pull-off force in the force-distance curve. Because the friction in the retract half cycle is more or less the same (a hysteresis of ∼10%) as that in the approach half cycle in the positive load region, and continuously decreases toward zero as the tip is pulled out in the negative load region, it does not add any extra information for the chemical modification effect. Thus, we will present data in the approach half cycle only. For brevity, we will call the above procedure of friction measurement a friction scan. To obtain reproducible results in the AFM/FFM experiments, we first wore a new silicon nitride tip in friction scans up to an external load of 150 nN on a cleaved mica surface until the frictionversus-load curves reached a steady state. Mica is chosen as a reference because of its flatness and easy cleavage. However, it is well-known that a newly cleaved mica surface is unstable due to charge neutralization and contaminant (water and organic molecules) adsorption.20 In our case, the friction of newly cleaved mica was observed to vary considerably in the first few hours. With the mica sitting in a relatively low humidity environment (∼10%) overnight, the friction could reach a stable value and was reproducible at different areas on the sample. A typical wear process of a new silicon nitride tip on mica is shown in Figure 1. While the magnitude of friction in the first few scans behaved somewhat randomly due to the instability of the tip geometry, in the latter scans the friction force increased slowly as the tip wore more and more. After 25 scans, the friction became stable, indicating that a stable “dull” tip was formed. As mentioned before, the tip radius after this process was on the order of 1000 Å, judging from the images of the width of Au(111) steps. For convenience, the tips at this condition are called mica-treated tips. The friction-versus-load curve of the mica-treated tip against mica serves as a reference for comparisons. To investigate the possible material transfer to the tips or the tip chemical (20) Christenson, H. K. J. Phys. Chem. 1993, 97, 12034.
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Qian et al. samples. The tip chemical modification process via possible material transfer was carried out by scanning the tip on the sample at different areas until no further change in the frictionversus-load curves could be observed. After this, the tip was brought back to the mica surface to perform friction measurements again. The difference of friction on mica measured before and after the tip chemical modification process on the above samples was used to decide whether there was material transferred to the tip or, in other words, whether the tip was chemically modified as a result of friction scans on the samples.
3. Results and Discussion
Figure 1. A typical blunting process of an Si3N4 tip via friction scans on mica as shown by the friction-versus-load curves for a series of scans. The measurements were taken at 5% relative humidity and room temperature. modification, a newly prepared mica-treated tip was used for each sample to avoid any possible interference between the
A. Material Transfer from OTE/Mica to Tip. (i) At 5% Relative Humidity. To study the material transfer from OTE/mica to the silicon nitride tip, or the in situ chemical modification of the tip, we made 20 friction scans on the OTE/mica sample at different areas at 5% relative humidity. The friction-versus-load curves for some of the friction scans are shown in Figure 2a. It appears that the friction curves have a relatively big change only in the first five friction scans, especially in the load regime between 60 and 100 nN. From the sixth friction scan on, the friction curves coincide with each other reasonably well. The behavior of the friction curves here are consistent
Figure 2. (a) In situ tip chemical modification process shown by a series of friction-versus-load curves on OTE/mica using a newly blunted clean tip (mica-treated tip). (b) The friction-versus-load curves on mica before and after the tip modification on OTE/mica, together with a representative friction curve on OTE/mica. (c) The tip cleaning process shown by friction-versus-load curves in successive friction scans. A reference friction curve on mica with the mica-treated tip is also plotted. (d) The adhesive force simultaneously taken during each friction scan performed from (a) through (c). All the above measurements were taken at 5% relative humidity and room temperature.
Tip Passive Chemical Modification
with those observed in previous studies,18,21 namely, a superlinear behavior in the low-load regime (110 nN). The latter has been identified as a result of direct contact with mica with the OTE monolayer being worn out.21 In Figure 2b, the friction-versus-load curves on mica before and after the tip modification on OTE/mica, together with a representative friction curve on OTE/mica are shown. What is surprising is that the friction of mica against the tip after the modification process on OTE/ mica is significantly smaller than that measured with the mica-treated tip. The decrease is about a factor of ∼2.5. This is an indication of tip chemical modification by OTE. While the lubrication effect of OTE/mica, that is, a reduction of friction by ∼10 times from mica to OTE/mica against the tip, is obvious as has been reported before by a number of papers,18,21 this is the first time that an in situ tip chemical modification by friction scans on OTE/ mica is reported. As is clear in Figure 2b, even against the OTE-modified tip, the OTE monolayer on mica still functions as a good lubricant (45 nN). In the low-load regime (
