Langmuir 1993,9, 3310-3312
3310
Friction Effects on Force Measurements with an Atomic Force Microscope Jan H.Hoh' and Andreas Engel Maurice E . Miiller Institute for High Resolution Electron Microscopy at the Biocenter of the University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland Received June 14,1993. In Final Form: August 3,1993@ The deflection of an atomic force microscope (AFM) cantilever by a sample that is approached or withdrawn can be used to produce a plot of force versus sample position. These force curves are becoming important for investigatingintermolecular forces and surface properties. However, many components of the force curves are not yet understood. Here we show that friction as the tip slides on the surface contribute to the hysteresis in the contact part of the force curve. This sliding arises when the cantilever is mounted at an angle (about 15" in our microscope) to the surface, which forces the tip to move forward after contact is made during the approach. While the cantilever bends upward, it is bowed forward by the friction. This creates an offset in the contact line which is important to interpreting the curves, in particular when force curves are converted into curves of force versus separation distance, since the contact line is then taken as zero separation. In addition, the sudden onset of the friction at contact can cause a discontinuity in the force curve which sometimes appears e a short range attractive interaction. The friction effect on the contact line is uncoupled from the hysteresis in the noncontact part of the curve, which arises from the viscosity of the medium in which the measurement is made.
Introduction The atomic force microscope (AFM)I is becoming an important tool for investigating the intermolecular forces between surfaces with very small contact areas. The actual contact area between the two surfaces depends on their size and geometry, but the ability of the AFM to image structureson an angstrom scale suggests that near atomic size contacts are possible. The force measurements are carried out by monitoring the deflection of a cantilever as a sample is approached toward or withdrawn from the tip. These so-called "force curves" are sensitive to properties such as adhesion, elastic properties, viscous properties, and electrostatic and van der Waals forces.24 Despite their increasing use, a number of features regarding force curves are not well understood. Here we describe a friction induced effect that can cause a misinterpretation of true separation distance between tip and sample and can give rise to apparent attractivetadhesive interactions that do not exist. An AFM force curve can be divided into three general parts (Figure 1). When the tip and sample are well separated there is no interaction force that deflects the cantilever. This noncontact line is generally straight with the exception of optical effects that can produce a sinusoidal oscillation or slight slope. The second part of the curve is a t or near contact. Here there are a variety
* To whom correspondence should be addressed at Department of Physics, University of California, Santa Barbara, CA 93106. Phone: 805-893-3999.FAX: 805-893-8315. * Abstract published in Advance ACS Abstracts, October 1,1993. (1)Binnig, G.; Quate,C. F.; Gerber, C. Phys. Reu. Lett. 1986,56,930933. (2)Weisenhorn, A. L.;Hansma, P. K.; Albrecht, T. R.; Quate, C. F. Appl. Phys. Lett. 1989,54, 2651-2653. Ducker, W. A.;Senden, T. J.; Pashley, R. M. Nature 1991,353,239-241.Weisenhorn, A. L.;Maivald, P.; Butt, H.-J.; Hansma, P. K. Phys. Reu. Lett. B 1992,45,11226-11232. Hoh, J. H.;Cleveland,J. P.; Prater, C. B.; Revel, J.-P.; Hansma, P. K. J. Am. Chem. SOC.1992,114,4917-4918. Butt, H. J. Biophys, J. 1991, 60, 777-785. Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992,8,1831-1836. (3) Burnham, N. A.; Dominguez, D. D.; Mowery, R. L.; Colton, R. J. Phys. Reu. Lett. 1990,64,1931-1943. Bumham, N.A.;Colton, R. J. J. Vac. Sci. Techno 1989,A7,2906. (4)Meyer, E.;!einzelmann, I H.; Grtitter, P.; Jung, T.; Hibder, H.; Rudin, H.; Gihterodt, H.-J. Thin Solid F i l m 1989,181,527-544.
Contact Line
Non-contact Line
Sample Position
Figure 1. Three regions of a typicalforce versus sample position curve. When the cantilever and sample are well separated (right side of curve), the cantilever is not deflected. Near or at contact there can be a variety of attractive and repulsive interactions causingthe cantilever to deflect. In this case there is an attraction on the approach (solid lines) and adhesion on the withdrawal (dashedlines). After contact the cantilever and samplemovement are coupled, in the contact line. Hysteresis in the contact line is evident.
of complex attractive and repulsive interactions that contain the majority of information about the interaction between tip and sample. After contact the tip and sample movements are linearly coupled (if there are no elastic effects) in what is here called the contact line. This line is generally considered to represent zero separation between the tip and sample. However, the approaching and withdrawing portions of the contact line oftan exhibit a substantial hysteresis, manifest as a lateral separation of the two traces.
Experimental Methods A Nanwope I11 atomic force microscope (DigitalInstruments, Santa Barbara CA) with a J type scanner (164 Mm xy and 5 M m z range) and a standard fluid cell waa used, although the O-ring waa omitted from the fluid cell. Water (18 MQ)was from a
0743-7463/93/2409-3310~04.~IQ0 1993 American Chemical Society
Langmuir, Vol. 9, No. 11,1993 3311
Friction Effects on Force Measurements 40
I"
2 pmls water
-201
0
,
,
200
400
IC
L-. . . ,......... .-. . .. .........1-
L
a
0" .-2
0.6 umls
O.
1
40 water
1
.
,
0
200
400
b. D
I j
4.0
"0
.
C
-50 0
80
0
400
200
0
200
400
5 pmls ;,F air
E
500
1000
1500
0
10
20
30
40
50
Sample Approach Speed ( p V s )
195 pmls air
500
1000
1500
Sample Postion (nm) Figure 2. Dependence of the noncontact line on the viscosity
of the medium (for a silicon nitride tip on a carbon surface). (A-B) Force curves acquired in an aqueous solution of 1 mM NaCl show a typical double layer repulsion. As the scan rate is increased,a hysteresis in both the contactline and the noncontact line develops. (C-D) When force curvesare acquiredin glycerol, the noncontact line hysteresisis significanteven at very low scan rates. As the scan rate is increased, the separation of traces becomes extremely large, in this case 50 nm for a scan rate of 4.8 pm/s. ( D F ) In the converse case, when the viscosity of the medium is low (air), the noncontact line hysteresis is near zero. However, the contact l i e hysteresisis still scan rate dependent. At high scan rates the cantileversometimes oscillatessignificantly after it is pulled off the surface. Solid l i e s are approaching curves and dashed lines are retracting curves. Milliporepurificationsystem. Mica (NewYork Mica Corp., New York) was punched into 12-mmdisks,glued onto 11-mmmagnetic steel disks and cleaved immediatelybefore use. Glass cover slips (12 mm) were glued onto 11-mmmagnetic steel disks and coated with carbon by evaporation. Both mica and carbon surfaceswere used with no qualitative differences in the results. Cantilevers were standard 200 pm long and 40 pm wide silicon nitride probes (Digital Instruments)? Cantilevers were mounted at 1&15O relative to the sample. Data were collected by ramping the z piezo and monitoring the cantileverdeflection. All scan rates are reported in terms of z piezo movement. Force curves were captured with the Nanoscope software and converted into ASCII. The data were 128, 256, or 512 points along the scan direction and 16 bits for each cantilever deflection value.
Results and Discussion In aqueous solution both the contact line and noncontact line exhibit a scan rate dependent hysteresis. At scan rates of a few pmis, i.e. the velocity at which the sample (5) Albrecht, T. R.; Akamine, S.;Carver, T. E.;Quate, C. F. J. Vac. Sci. Technol. A 1990,8,3386-3396.
Figure 3. Friction-dependent behavior of the contact line hysteresis (i.e. the separation of the approaching and retracting contact lines). Different symbols are for different experiments. Open symbols are data collected in air and closed symbols are data collected in water. The curves collected in air show a behavior typical of stick slip friction. The hysteresis is high at very low scan rates and decreases as the rate is increased. After reaching a minimum, the hysteresis increases with higher scan rates when it is shear rate limited. The exact behavior of these curves is very complex and depends on many parameters such as tip structure and surface chemistry, though in air different surfaces studied consistently exhibited friction-dependent behavior. When water is present, the stick-slip effect is weaker and the measurements at very low scan rates are highly variable. The approaching and retracting contact lines are generally parallel, though occasionally they will converge toward the turn around point. In such cases the hysteresiswas measured at zero deflection.
is approached to the tip, the approaching and retracting curves lie close to each other (Figure 2A). However, as the scan rate is increased, there is a hysteresis between the traces that appears as a separation of both the noncontact line and contact line. In all cases we have examined (all media and tip-sample combinations), the approaching curve overlaps or runs above the retracting curve when far away from the surface. However, as the tip and sample approach, the two traces cross and the approaching curve runs below the retracting curve for the entire contact line (clearly seen in Figure 2B). A significant effect on the hysteresis in the noncontact line can be seen by altering the viscosity of the medium (Figure 2C-D). In a high viscosity medium such as glycerol (15 P versus 1 X 1k2 P for water), separation of the traces becomes extremely large even at very low scan rates. This is consistent with the common understanding that the trace separation of the noncontact line is primarily due to fluid dynamic effects. In a low viscosity medium, air (ca. 2 X 10-4 P), the hysteresis in the noncontact line is very small and appears scan-rate independent within the range examined (Figure 2E-F). However, the hysteresis in the contact line shows a clear scan-rate dependence. A closer examination of this scan-rate dependence reveals that at high scan rates the separation is large. As the scan rate is decreased, the hysteresis reaches a minimum after which i t increasea again (Figure 3). This rate-dependent increase below a certain minimum is typical of stick slip frictione and not consistent with an electronic effect. Friction above the minimum is typically dominanted by shear forces: as is seen here in (6) This type of friction behavior is well documented. For a recent example see Gee, M. L.; McGuiggan, P. M.; Israelachvili, J. N.; Homola, A. M. J. Chem. Phys. 1990,93, 1895-1906.
3312 Langmuir, Vol. 9, No. 11,1993
Figure 4. Schematic model of friction-inducedforward bowing of the centilever as the sample is approached. With the optical lever detection system the forward bow causes the beam to be offset downward, making both the tip and sample appear lower down than they are. The converse happens as the sample is withdrawn.
the linear dependence of the hysteresis on the scan rate, at high rates. We therefore propose that friction between the tip and sample causes the cantilever to bow forward after the tip makes contact, resulting in an offset in the contact line (Figure 4). Since the contact line is taken as zero separation, this results in a misplacement of the sample position that can be very large in some cases, up to 100nm in Figure 2F, for example. As the tip is retracted, the cantilever then bends upward, causing an opposite offset in the line. This interpretation is further supported by the behavior of the curves a t the turn-around points, where the deflectionsignaljumps or drops nearly vertically (most clearly seen in parts B and F in Figure 2) as would be expected when the cantilever turns up from a forward bow (or vice versa). Such friction-induced effects on the cantilever have been well documented in AFM imaging.**' It should be noted that additional contributions to the hysteresis in the force curves may result from the nonlinear behavior of the piezo scanner (R. Colton, personal communication). Since the deflection signal for the approaching curve is too low, and that for the retracting curve too high, the correct value must lie somewhere in between. Using a series of different scan rates, it might be possible to extrapolate scan-rate dependence to zero trace separation. However, this not possible with our microscope in which (7) Mate, C. M.; McClelland, G. M.; Erlandson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. Marti, 0.; Colchero, J.; Mlynek, J. Nunotechnology 1990, I, 141. Radmacher, M.; Tillmann,R. W.; Fritz,M.; Gaub,H. E. Science 1993,257,1900-1905. Erlandseon,R.;Hadziioaunou, G.; Mate, C. M.; McClelland, G. M.; Chiang, S. J. Chem. Phys. 1988,89, 51W5193. Baeelt, D.R.;Baldeschweiler, J. D. J. Vac. Sci. Technol. B 1992,10,2316-2322.
Hoh and Engel there is a small scan rate dependent offset. This causes both the approaching and retracting contact lines to shift, thereby also moving the true zero separation point. Therefore, to determine the real position of the contact line would require an independent method for determining the position of the surface, which we do not have. In lieu of direct surface position measurement or other solutions described below, the most prudent approach is to use the average of the approachingand retractingtraces and report the maximal error in sample position as halfthe separation distance of the traces. A further effect of friction in the contact line of the force curve is that the bending forward of the cantilever does not have to be smooth but can appear as a discontinuity in the curve. This would appear as a jump toward the surface, very similar to events often attributed to van der Waals attractions. Therefore caution is in order when interpreting these types of interactions. The friction effect described here is most significant when the cantilever is mounted a t an angle, as in the beam deflection detection system. Instruments in which the cantilever is parallel to the surface show little if any of this effect.3 There are many possible ways to address the sliding problem. The two most general approaches are compensated sample movement and nondeflecting cantilevers. In the first, the sample could be ramped a t an angle perpendicular to the cantilever, or the sample position could be adjusted in x and y using feedback from the deflection signal (for a given cantilever geometry).This would minimize the lateral movement between tip and sample. Nondeflectingcantileversshould not have lateral motion problems. One such system uses a tip on a very stiff beam and a capacitor-basedforce balance to keep the deflection zero.8 It is also possible to make the cantilever in a conventional beam deflection AFM non-deflecting, by making the cantilever magnetic and using an external magnetic field to maintain zero deflection (J.Cleveland, personal communication). In addition to reducing the frictional effect on the force curve, these solutions ensure that the interactions between tip and sample are localized to a small area. Acknowledgment. The authors thank Jason Cleveland for helpful discussions. This work was supported by a fellowshipfrom the Human Frontier Science Program (LT438/92 to J.H.H.). (8)Joyce, S. A.; Houston, J. E. Rev. Sci. Zmtrum. 1991,62,710-715.
