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Friction Effects in Atomic Force Microscopy of Patterned Octadecyltriethoxysilane-on-Glass Self-Assembled Monolayers† Baek-woon Lee and Noel A. Clark* Condensed Matter Laboratory, Department of Physics, University of Colorado, Boulder, Colorado 80309 Received February 27, 1997. In Final Form: April 9, 1998 We present a quantitative study of the simultaneous effects of surface topography and friction in atomic force microscopic imaging of octadecyltriethoxysilane-on-glass self-assembled monolayers, patterned by illumination with ultraviolet radiation through a mask. The ultraviolet light leaves a ∼1 nm deep topographic depression in the exposed areas, and produces larger surface-tip friction which can be the dominant contribution to the apparent topography.
Introduction 1 has become a key research tool in the surface
The AFM characterization of materials such as polymers and organic thin films where low conductivity precludes the use of the scanning tunneling microscope. On surfaces with relief deeper than =100 nm, such as on compact disks or integrated circuits, the AFM accurately yields the surface topography. However, the interpretation of low relief AFM images is not as straightforward, especially for inhomogeneous surfaces where the tip-surface friction varies from place to place. The contribution of frictional forces to AFM images has been observed and analyzed theoretically,2,3 showing that the frictional force coupled with the spring constant of the cantilever contributes to the apparent topography as well as the normal force. The friction4,5 depends on the various tip-surface interactions, for example, van der Waals,6 hydrophobic and hydrophilic,7 and solvation.8 The tip-surface frictional force is most pronounced in the contact mode AFM, and indeed, its spatial variation may itself be used to obtain surface images.2,3 The apparent topography of such surfaces depends in a complex way both on the relief and on the spatial variation of frictional forces and is known to be affected by the shape and dimension of the tip,9 surface properties of the tip and the sample,4,5 and the cantilever spring constant.10 †
Supported by NSF MRG Grant DMR 92-24168.
(1) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (2) Radmacher, M.; Tillmann, R. W.; Fritz, M.; Gaub, H. E. Science 1992, 257, 1900. (3) Grafstrom, S.; Ackermann, J.; Hagen, T.; Neumann, R.; Probst, O. J. Vac. Sci. Technol. B 1994, 12, 1559. Ascoli, C.; Dinelli, F.; Frediani, C.; Petracchi, D.; Salerno, M.; Labardi, M.; Allegrini, M.; Fuso, F. Ibid. 1994, 12, 1642. (4) Burnham, N. A.; Dominguez, D. D.; Mowery, R. L.; Colton, R. J. Phys. Rev. Lett. 1990, 64, 1931. (5) Nakagawa, T.; Ogawa, K.; Kurumizawa, T. J. Vac. Sci. Technol. B 1994, 12, 2215. (6) Hutter, J. L.; Bechhoefer, J. J. Vac. Sci. Technol. B 1994, 12, 251. (7) Christenson, H. K.; Claesson, P. M. Science 1988, 239, 390. (8) O’Shea, S. J.; Welland, M. E.; Rayment, T. Appl. Phys. Lett. 1992, 60, 2356. (9) Montelius, L.; Tegenfeldt, J. O.; van Heeren, P. J. Vac. Sci. Technol. B 1994, 12, 2222. Westra, K. L.; Thomson, D. J. Ibid. 1994, 12, 3176. Keller, D. Surf. Sci. 1991, 253, 353. Akamine, S.; Barrett, R. C.; Quate, C. F. Appl. Phys. Lett. 1990, 57, 316. Doris, B. B.; Hedge, R. I. Appl. Phys. Lett. 1995, 67, 3816. (10) Weisenhorn, A. L.; Hansma, P. K.; Albrecht, T. R.; Quate, C. F. Appl. Phys. Lett. 1989, 54, 2651. Ohnesorge, F.; Binnig, G. Science 1993, 260, 1451.
Frictional effects are of particular importance in the study of photopatterned self-assembled monolayers (SAMs),11,12,13,14 because the topographic relief is very small and the tip-surface interactions and friction in the exposed regions can be made very different from those of the unexposed regions. Indeed, the UV photopatterning of hydrophilic/hydrophobic alkanethiol-gold SAM surfaces have been imaged via friction, in the presence of undetectable surface topography.11 UV-patterned SAMs are ideal for the study of friction because the topographic steps in the surface relief are small ( 1 Hz and that it varied linearly with loading force, FN, as shown in Figure 5, which presents δzfl vs setpoint voltage, VS ∝ FN, data for the SAM. Thus, the friction for both SAM and glass could be characterized by a coefficient of sliding friction, κ. A basic question to be addressed is whether κ depends on scan angle φ. The imaging process is characterized by two distinct orientations, that of the sample and of the scan direction, relative to the cantilever orientation. As mentioned above, the apparent height difference between regions A and B is independent of the sample orientation.
AFM of Octadecyltriethoxysilane-on-Glass SAMs
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Figure 6. (a) Image addition of the trace and retrace scans for the images of Figure 3. The edges in the composite are less well resolved because of imperfect superposition of the two images due to the hysteresis of the piezotube. (b) The cross-section of (a) showing that the exposed area is depressed. The vertical distance between the two triangular markers is 1.15 nm, and the topography in absence of friction is obtained by dividing the height scale by 2. (c) The frictional component can be isolated by subtracting trace from retrace. The resultant image shows that the exposed region (grids) is more frictional than the unexposed region (squares).
Since the photodetector is only sensitive to the cantilever deflection in the xz-plane, we need only consider the x components of the frictional force, FFxt ) FFt(φ) cos φ, and FFxr ) FFr(φ) cos φ where FFt(φ) and FFr(φ) are the trace and retrace mode frictions, assumed to possibly depend on scan angle φ. In this case the height difference obtained between the trace and retrace scans in the friction loop, δzfl(x), will be given by
δzfl(φ,x) ) γδθfl(φ,x) ) γ[δθFxt(φ,x) - δθFxr(φ,x)] ) γβ[Ft(φ,x) - Fr(φ,x)] cos φ (3) where the angular dependence of F(φ) ) Ft(φ) - Fr(φ) would arise from anisotropy in the tip shape. To probe the angular dependence of Ft(φ) - Fr(φ), δzfl(φ) was measured for the unpatterned OTE-SAM and for clean float glass, representing the limiting case of a hydrophilic, OTE-free surface. δzfl(φ) was obtained for a scan by subtracting the height during retrace from that during trace and averaging the middle 312 points out of a total of 512. This was repeated for five different scan lines, and these results were averaged. The first and the last 100 points of each scan were discarded because of the tip oscillation when the AFM scan direction reverses. Figure 4b shows the resulting angular dependence on cos φ of δzfl(φ) for the OTE-G SAM, 0, and for clean float glass, ∆. The error
bars indicate the standard deviations of the data. δzfl(φ) is always higher for glass than for OTE-SAM at every scan angle, and both curves depend nearly linearly on cos φ, crossing zero for φ ≈ 90°. This indicates that there is little effect of tip anisotropy and that simple sliding friction having a velocity independent coefficient of friction applies for the component of the motion along x and enables measurement of φ for the glass and SAM to be obtained from the linear fits to δzfl(φ) vs cos φ and the use of eq 2. The results are κOTE ) 0.067 for the OTE-SAM-Si3N4 tip and κG ) 0.20 for the glass-Si3N4 tip. Figure 6c shows a purely frictional image obtained by subtracting the retrace from the trace image and dividing the result by 2. This composite shows higher frictional bands around the edges of the squares, which is also evident in the bright bands in the trace scan and the dark bands in the retrace scan. Topography in Absence of Friction. There are several ways to obtain the topography of the patterned sample in AFM images free of friction effects. One method is to average the trace and the retrace scans, as was suggested by Radmacher4 and shown for our data for φ ) 0° in Figure 6. This method assumes that the tip is symmetric with respect to the trace and retrace scans, i.e., that the frictional force will change sign but not magnitude when the velocity of the tip moving along the
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Figure 7. OTE-G SAM UV exposed for 45 min on the grid pattern imaged in air by tapping mode at φ ) 30°. The left image is the trace scan and the right, the retrace. They are almost identical. The grids appear higher here because of a water layer on the surface. Attempts to penetrate the layer to image the UV-exposed SAM surface below by increasing the loading force resulted in positive feedback oscillation.
surface changes sign. Since the piezoscanners have a certain amount of hysteresis, the image pixels of each scan do not always represent the exact same sample points, resulting in some loss of resolution in this method. The cross-section in Figure 6b shows that, for the 45 min exposure, the exposed areas are depressed relative to the unexposed by about 0.6 nm, half of the height difference between the gray arrows. The depression of exposed relative to the unexposed increases with exposure to 1 nm for 90 min. True topography may also be obtained using the tapping mode, in which the tip oscillates far off the surface and it only makes contact with the sample for a fraction of time of the oscillation. Since it is not in contact with the sample, virtually all frictional effects should disappear. Figure 7 shows tapping mode images of UVexposed OTE-G SAM in air, illustrating that the trace and retrace images are almost identical and thus frictionfree, as expected. Similar friction-free scanning can be obtained under water (Figure 8). The grid appears higher in Figure 7 because the tip does not penetrate the water layer that forms on the hydrophilic grid surface at finite humidity. Under water, the surface topography in absence of friction has the grid appearing depressed by about 1 nm, but the image quality is poor, since, under a medium, it is difficult to increase the tapping mode feedback gain until the quality is acceptable without causing oscillation. Discussion Our AFM imaging experiments show that UV illumination produces a topographic depression, i.e., removes material from the surface. Measurements of water-SAM surface contact angle on UV-exposed unmasked slides show that UV exposure reduces the contact angle to zero, indicating that the UV-exposed surfaces become more hydrophilic. Thus it appears that it is aliphatic material that is being removed by UV illumination, exposing the more hydrophilic glass below. UV exposure alters the frictional characteristics of the surface, increasing friction with the Si3N4 tip. Our experiments suggest two possible mechanisms for this effect. The OTE-SAM is hydrophobic and of low surface energy, a consequence of the termination of the monolayer molecules with methyl groups, and their tight packing, the movement of molecules in the saturated SAM being very limited because of the steric interaction between
Figure 8. OTE-G SAM UV exposed for 45 min on the grid pattern imaged under water by tapping mode at φ ) 30°. Here, the grid is imaged free of friction effects and is lower, as expected. Analysis of the cross-sections and matching of the color and hue of the grid and square regions to the AFM color height bar indicates that the topographic depression of the grid is about 1.0 nm.
neighboring molecules. As a result, the surface is almost incompressible and of low friction. However, upon UV irradiation, this picture changes, the surface-tip interaction becoming more frictional (Figure 9). First, the surface becomes more hydrophilic, the UV breaking the bond between the silane and the alkyl tail, exposing the more hydrophilic bare glass (water wets the UV-exposed areas while forming isolated drops on the unexposed SAM), which has a greater interaction with the hydroxylterminated silicon tip and thus more friction. Second, the tails of the OTE molecules now have more room to move around because of the vacant spots created by UV irradiation. The surface is now more compressible, which increases the contact area with the tip and the friction increases. This picture is consistent with the observation of higher frictional bands around the edges of the squares, regions of intermediate UV exposure which have finite
AFM of Octadecyltriethoxysilane-on-Glass SAMs
Figure 9. Schematic of possible mechanisms giving the observed frictional effects on UV-patterned OTE-G SAMs. The UV exposure-related increase of friction with the Si3N4 tip can arise in two ways: (a) The glass surface is more exposed and terminated with hydroxyl groups. Under water, the tip, which is also terminated with hydroxyl groups, is more strongly attracted to the region B via hydrogen bonding, increasing the friction. (b) Upon UV removal of some of the OTE tails, the remaining have more room to move, producing a more compressible surface that increases the contact area with the tip and increases friction.
width because of diffraction and the imperfect collimation. The unexposed SAM starts with very low friction and as UV exposure increases, the friction increases by the mechanisms in Figure 9. However, as more and more
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molecules are removed, more of the hard surface of glass is revealed. This reduces the contact area with the tip, decreasing friction. This results in a maximum in the friction vs UV exposure curve. The intermediate regions will have higher friction than both of the 45-min exposed and the unexposed regions. With respect to determination of the topography in the absence of friction effects, the contact mode image subtraction (Figure 6) yielded a step of 0.6 nm from the exposed to unexposed regions, whereas the tapping mode under water (Figure 8) yielded a step of 1 nm. While the image quality was superior in the contact mode, this method is subject to the assumption that the tip frictional force is exactly antisymmetric with respect to a change in the sign of the velocity. The results indicate that while material is removed by the UV illumination, the observed step size is less than 2.5 nm, that expected for complete SAM removal and identical SAM-tip and glass-tip interactions, whereas the friction results indicate that there is nearly complete SAM removal. The origin of this difference may lie in that the SAM-tip and glass-tip interactions are not identical or in that the SAM and glass have different mechanical properties. Studies of the dependence of apparent step size on loading force and scan rate are currently being carried out in order to develop a better understanding of the observed friction-free step sizes. Acknowledgment. We acknowledge the assistance of Charles Liberko, Jon Moore, Andrew Winningham, and Patrick Sullivan. This work was supported by NSF MRG Grant DMR 92-24168. LA970217F
