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Reversible Displacement of Chemisorbed n-Alkanethiol Molecules on Au( 111) Surface: An Atomic Force Microscopy Study Gang-yu Liut and Miquel B. Salmeron*J Material Science Division, Lawrence Berkeley Laboratory, University of California at Berkeley, Berkeley, California 94720, and Department of Chemistry, University of California at Berkeley, Berkeley, California 94720 Received October 7,1993. In Final Form: December 14,1999
Atomic force microscopy has been used to study the structure and the stability of CHs(CH2)9SH and CH3(CH2)1,SH molecules self-assembledon a Au(ll1) surface as a function of the load applied by the tip. Atomic resolution images taken during a loading and unloading cycle have revealed two sudden transitions corresponding to changes of the periodicity from a (d3Xd3)R30° (due to thiol layers) to a (1x1) (due to Au(lll)),and back to the (d3Xd3)R30° of thiol layers. These results represent the first observation that under high load, self-assembled n-alkanethiol molecules on Au(ll1) can be reversibly displaced on the substrate surface by a sharp tip during the scan.
It has been a common knowledge that thin film lubricants separate the contacting surfaces and prevent their strong physical and/or chemical interactions when under pressure. However, the mechanism of boundary lubrication is still poorly understood at the atomic level because very few techniques can provide the necessary information about the microscopic structures and mechanical properties of the thin films of lubricant molecules. A few recent investigations using interfacial force microscopy (IFM) have shed some light into the behavior of molecules under pressure.l-3 A time-dependent, elastic response is observed for n-alkanethiol monolayers on kl Au(111)under pressure.lv2 Another important study in this area involves the use of the surface force apparatus (SFA), pioneered by Israelachvili et al., which allows investigation of the behavior of thin liquid films (as thin as several monolayers) between two surfaces as they are moved normally and laterally to each other.4 Since presently existing work on the mechanical properties of these monolayers has not yet reached the atomic resolution, we have undertaken the study of the atomic structure of n-alkanethiol molecules, CH3(CHz)&H (Clo) and CH3(CH2)1,SH (CIS), self-assembled on a Au(ll1) surface as a function of the applied load using an atomic Figure 1. (a)Topographic and (b) simultaneous frictional force forcemicroscope (AFM).These thiol moleculesare known images, -(50 X 50) A2, of self-assembled monolayer of CHSfrom diffraction and microscopy techniques to form a (CH2)9S/Au(lll)mica. The load during imaging is 28 nN. (c) commensurate ( d 3 X d 3 ) R 3 0 ° structure on A ~ ( l l l ) . ~ l ~Topographic and (d) simultaneous frictional force images of a (50 X 50) A2monolayer of CH&H2)1,S/Au(lll)/mica. Theload Our results confirm these earlier findings about the during imaging is 100 nN. Both monolayers have a hexagonal lattice with a lattice constant of 5.0 i 0.2 A. All the images are t Department of Chemistry. lightly smoothed and corrected for background. Material Science Division, Lawrence Berkeley Laboratory. Abstract published in Advance ACS Abstracts, February 1,1994. periodicity of the thiol layers. In addition, we have found (1) Joyce,S.A.;Thomas,R.C.;Houston, J.E.;Michalske,T.A.;Crooks, RE. M. Phy. Reo. Lett. 1992,68,2790. that the monolayers are mechanically robust and maintain (2) Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. their ordered structure under applied load from 10 to 200 Science 1993,259, 1883. nN (Si3N4tip with tip radius of 700A). Most importantly, (3) Siepmann, J. I.; McDonald, I. R. Phvs. Rev. Lett. 1993. 70.453. (4) Israelachivili,J. N.; MeGuiggan, P. M.;Homola, A. M: Schnce however, we have observed that at a critical load, e.g. 280 1988,240,189. nN for c18, the monolayers are locally disrupted by ( 5 ) Strong, L.; Whitesides, G. M. Langmuir 1988,4,516. displacement of molecules under the tip. This phenom(6) Chidsey, C. E. D.; Liu, G.; bwntree, P. R.; Scoles, G. J. Chem. Phys. 1989,91,4421. enon of lateral displacement of strongly bound molecules (7) Samant,M. G.; Brown, C.A.; Gordon, J. G. Langmuir 1991,7,437. is reversible and is observed here for the first time. It (8) Fenter, P.;Eisenberger,P.; Liang, K. S. Phys. Rev. Lett. 1993,70, provides new insights to the mechanism of lubrication 2447. (9) Camillone, N.;et al. J. Chem. Phys. 1993,99,744. since it demonstrates that direct contact between two (10) Alves, C. A.; Smith,E. L.; Porter, M. D. J. Am. Chem. SOC.1992, objects can occur locally while the lubricant molecules 114,1222. (11) Widrig, C. A,; Chung, C.;Porter, M. D. J. Am. Chem. SOC.1991, remain intact, and such contact can heal after the local 113, 2805. high pressure is reduced. (12) Kim, Y.T.; Bard, A. J. Langmuir 1992,8,1096. The preparation of the samples, C ~and O CISmonolayers, (13) Pan, J.; Tao, N.; Lindsay, S . M. Langmuir 1993, 9, 1556.
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0743-7463/94/2410-0367$04.50/0
0 1994 American Chemical Society
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Figure 2. (a-f) (50 X 50) A2frictional force images of CHs(CHz)gS/Au(lll)/mica(top row) and CH~(CH2)1,S/Au(lll)/mica(second row). (g-1) The corresponding 2D Fourier transforms of images (a-f). Since low frequency noise (e.g. 120 Hz) is sometimes present along our slow scanning or Y direction, the corresponding Fourier spots along the x-direction are weakened. As a result, only four of the six reciprocal lattice points are clearly visible. The weak spots at (g-1) correspond to the 120-Hz noise which we did not filter out. The images shown in the first column represent the images taken when the normal force exerted by the tip (-700 A) is below a critical value, -280 nN. S ecifically, the loads in a and d are 74 and 31 nN, respectively. Both monolayers have a hexagonal lattice At a critical load, 270 and 280 nN for C ~and O Cra respectively, the thiol molecules are displaced and with the lattice of 5.0 0.2 a new periodicity, correspondingto the Au(ll1) lattice, is observed, which has also a hexagonal symmetry but with a lattice constant of 2.9 0.1 A. In addition, this new hexagonal lattice is rotated 30' with respect to the thiol lattice. Images in the second column are taken under high load. In b, the load is 358 nN and in e is 281 nN. Comparison of the images in the first column to that in the second column proves that the thiols form a (d3Xd3)R3Oostructure with respect to the Au(ll1) lattice. Decreasing the load, e.g. to 80 nN (c) and 26 nN (f), restores the original (d3Xd3)R30° periodicity.
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involves two steps, namely the preparation of the gold substrate and the formation of the monolayers. The substrates were prepared by evaporating a 1800A thick gold film onto freshly cleaved mica sheets in a vacuum of le7 Torr. The mica sheets were preheated to 250 "C
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and the gold evaporation rate was kept at -5 A/s. Under such conditions, the gold films have predominantly (111) orientation and relatively large flat surface domains.14 After evaporation, the mica-supported gold films were cooled to room temperature and immediately immersed
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Langmuir, Vol. 10, No. 2, 1994 369
into a -2 mM thiol solution, where they remained for about 36 h at room temperature to complete the formation of a monolayer. The home-built AFM instrument used in this study has been described elsewhere.16 All the measurements have been carried undar ambient laboratory conditions. The cantilevers are purchased from Digital Instruments and have a pyramidal Si3N4tip attached to a V-shaped beam with a force constant of 0.58 N/m. A quadrant photodiode detector allows measurement of the normal deflection and lateral torsion of the lever simultaneously. All the images shown in this report were taken in the contact mode under constant normal force. The value of the normal force or load includes the capillarity contribution and the force acting on the surface due to cantilever bending. The magnitude of the capillary force is determined by measuring the force versus distance dependence during an approach and retract cycle before each individual experiment. The x and y calibration was established by imagingC(0001), mica (OOOl), and Au(ll1) surfaces, whose lattice parameters are well known. Typical images of CISand Clo thiol monolayers taken under low load are shown in Figure 1. Both topographic and corresponding frictional force images show molecular resolution and were reproducible over a period of at least 3 weeks. The bright spots in the topographic and frictional force images have a hexagonal symmetry with a nearest neighbor distance of 5.0 f 0.2 A, consistent with the (d3Xd3)R30° structural model for thiol monolayers on Au(ll1) proposed from electron, atomic, and X-ray diffraction experiment^.^^ The lattice constant and the uncertainty are obtained from measuring a large number of unit cells (10-15) and from the Fourier transform of the images. The (d3Xd3)R3Ooperiodicity of the thiol monolayers remained unchanged with increasing load over a very large range, e.g., 20-200 nN, indicating the rigidity of these selfassembled monolayers. However, when the load was increased further to 270 nN for CIOand 280 nN for C18, a new periodicity was suddenly observed, a hexagonal unit cell rotated 29’ f 1”with respect to the thiol lattice with a lattice constant of 2.9 f 0.2 A (examples are shown in parts b and e of Figure 2). Such a periodicity corresponds to the bare Au(ll1) lattice (or 1x1 structure). The large static frictional force at the larger loads is responsible for the elongated features at the edges of images b and e in Figure 2. As a result, the elongated feature is on the left side of image 2b because the scanning direction is from left to right. If the scanning direction is reversed, i.e. from right to left, the elongated feature appears on the right side of the image (see Figure 2e). The Au(ll1) periodicity was preserved during unloading even below the critical value of 280 nN, the transition point found during loading. Only when the load was decreased below 100 nN, the (d3Xd3)R30° structure was observed again. This hysteresis of the loading and unloading cycle is depicted in Figure 3. To further investigate this hysteresis behavior, we moved our tip 5000 A away from the perturbed area during unloading before the second transition occurs, e.g., under a load of -150 nN. The periodicity of thiol was observed for the new scanning area, which confirms the existence of the hysteresis. Such a hysteresis behavior may be understood in terms of an energy barrier which must be overcome in order to initialize the lateral motion of the thiol molecules. The load dependence of the image periodicity, as shown in Figure 3, is very reproducible for
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(14) Chidaey, C. E. D.; Loiacono, D. N.; Sleator,T.;Nakahara,S. Surf. Sci. 1988, 200,-45. (15) Kolbe, W. F.; Ogletree, D. F.; Salmeron, M. B. Ultramicroscopy 1992,42-44,1113.
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Figure 3. Schematic diagram of the image periodicity change ofCHS(CHz)l,S/Au(lll)/mica ( C 3 duringaloading (solidcircles) and unloading (solid triangle) cycle. The (d3Xd3)R30° periodicity of Cle is preserved during loading until the load exceeds 280 nN. At this point, the Au(ll1) periodicity or 1x1 structure is suddenly observed. The thiol (d3Xd3)R30° structure reappears only after the load decreases below 100 nN. The tip used during this measurement has a force constant of 0.58 N/m and a tip radius of -700 A as deduced from measurements of gold step heights and widths.
each tip. However, the force threshold at which the image periodicity change occurs varies from tip to tip, in one extreme case, by 40%. We think that the variation in the force threshold is mainly due to the variation in tip radius. Sharp tips always result in smaller force threshold than dull tips. Using a simplifiedmodel,’e the Hertzian sphereflat configuration, we estimated the pressure threshold to be -0.8 f 0.1 GPa. To avoid making too muchassumption about the tip-surface contact situation, we chose to show the image vs force (instead of pressure) curve in Figure 3. Therefore, Figure 3 should be considered only as an example representing a general trend showing how the thiol monolayer image changes during a loading and unloading cycle. The reciprocal lattice vectors for the thiol monolayers and the underlying Au(ll1) lattice can be determined from the corresponding two-dimensional Fourier transforms shown in Figure 2g-1. The angle between the reciprocal space unit mesh vectors is 60 f 2’, indicating that both thiol layers and the bare Au(ll1) surfaces have a triangular unit cell. The ratio of the reciprocal space unit mesh vectors of thiol and gold is 0.58 f0.1 cv l d 3 . In addition, the Au(ll1) (see parts h and k of Figure 2) lattice rotates 29 f 1’ with respect to the thiol lattice (see parts g, i, j, and 1 of Figure 2). The above observations, therefore, provide further evidence in situ that the thiol monolayers have a (d3Xd3)R30° structure with respect to the Au(ll1) lattice.618 To further demonstrate the reversibility and to understand the nature of the transition during the loading and unloading cycle, we increased the scan area up to 500 X 500 A at a high load, 333 nN, under which Au(ll1) was observed. Then the normal force was decreased to 65 nN and images were taken of the central (50 X 50) A2 area. Again, the (d3Xd3)R3Oo structure was observed. This observation indicates that the local Au(ll1) structure Ycreatednby the AFM tip under high load is indeed unstable, and thiol molecules from nearby areas move back to the scanned area where the thiol molecules were removed. Under our imaging conditions, at room temperature and in air, this process occurs much faster than (16) Salmeron, M.; et al. Langmuir 1993,9, 3600.
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the time scale of our AFM measurements (about 20 s per image). The exact mechanism for these observed transitions is not completely clear to us. One possibility is that thiol molecules are attached to gold but laterally displaced by the tip under high load. When the load is decreased, or the tip is moved away, the packing and molecular conformation recover. Another possibility is that at high load, the thiol molecules under the tip bind to the tip, and this “new tip” images the periodicity of Au(ll1). The thiols are detached from the tip somehow when the load decreases. The third possibility is that the thiol molecules under the tip are desorbed and liquefied during loading, and the tip penetrates the “newly formed liquid” and images the gold substrate. During unloading, the thiol molecules reassembled into an ordered layer. We think that the first model is more likely because the S-Au chemical bond is much stronger than the thiol-SiSNd tip interaction. Therefore, it is less likely that thiol molecules desorb and climb up and down the tip during the force cycle. In addition, since lateral diffusion of adsorbates on metals require less energythan desorption, we believe that lateral motion is the process that occurs during the displacement and recovery of the thiol layer. Further
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experiments are in progress to understand the mechanism of the observed traneition and hysteresis. The observation presented in this report has demonstrated that chemisorbedthiol molecules can be displaced on the Au(ll1) surface by sharp AFM tips, e.g. -700 A, under sufficiently high loads. The displacement is found to be reversible and the molecules diffuse back to the displaced area when the load is decreased. This has important implications in adhesion and frictional studies where self-assembled monolayers are used as model lubricants.
Acknowledgment. G.Y.L. gratefully acknowledgesthe Miller Institute for a Miller ResearchFellowship sponsored by Professor Yuan T. Lee. We thank Drs. Carmen Morant and Bill Kolbe for their contributions in constructing the AFM. We also thank Dr. Vickie Hallmark for her help in preparation of the Au(lll)/mica film and Dr. Frank Ogletree for helpful discussions. This project is supported by the Director, Office of Energy Research, Office of Basic Energy Science, Materials Science Division, and the US. Department of Energy under the Contract Number DEAC03-76SD00098.
