Langmuir 1993,9, 3717-3721
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Friction Force Microscopy of AgBr Crystals: Ago Rods and Adsorbed Gelatin Films Greg Haugstad and Wayne L. Gladfelter’ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
Elizabeth B. Weberg Imaging Systems Department, E. I . du Pont de Nemours and Company, Inc., Brevard, North Carolina 28712 Received July 22, 1993. In Final Form: September 24, 199P Friction force microscopy was performed on the (111)surface of solution-growh AgBr crystals before and after deposition of gelatin films. Images of the initial surface identify the dissimilar chemical nature of the AgBr and rod-shaped crystallites which grow along the (110) family of crystal directions during imaging and are attributed to A$. Pores in gelatin films on AgBr and on highly-oriented pyrolytic graphite are seen to extend to the substrate, based on a reduced frictional force sensed at the bottom of the pores. The images also distinguish frictional contributions of “physical”versus chemical origin. The former are manifest as variations in image contrast seen at subnanometer-scalesteps in AgBr(ll1) and at the edges of the AgO crystallites and the gelatin pores and reflect additional cantilever torsion at these locations. The quantitative dependence of this effect on the size and shape of surface asDerities is discussed. and an expression is dedved which relates cantilever torsion to boih chemical and physical Components of friction.
Introduction The quest to resolve chemical variations spatially on an increasinglysmaller scale has been a powerful technological driving force in the field of surface analysis. .Methods developed range from workhorse techniques like scanning Auger microscopy1to more exotic ones, e.g. synchrotron radiation photoemission spectromicroscopy.2 The ongoing development of scanning probe techniques initiated by the invention of scanning tunneling microscopy3 (STM) and the related atomic force microscopy (AFM) includes friction force microscopy (FFM),S recently used for the first time to resolve spatial chemical variations on a submicrometer scale.6 Applications in tribology are also apparent.BJ Particularly intriguing is the potential of FFM to identify and separate chemicalfrom “physical” (asperityrelated) contributions to the overall frictional force between surfaces in contact in relative In this study we employed FFM to obtain complementary information on a material system we previously investigated topographically with AFM, the (111)surface of solution-grown AgBr crystals and adsorbed gelatin filmse8Silver halide-based photographic film remains the principal format for rendering visual images. Gelatin comprises the ubiquitous binding matrix used in photographic emulsions. The performance of photographic products depends on both the individual and interactive properties of the silver halide microcrystals, the gelatin, 0
Abstract published in Advance ACS Abstracts, November 15,
1993. (1) Hosler, W. Surf. Interface Anal. 1991,19, 543. (2) Margaritondo, G.;Cerrina, F. Nucl. Instrum. Methods Res., Sect. A 1990,291, 26. (3) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Phys. Rev. Lett. 1982,49, 57. (4) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986,56,930. (5) Mate, C. M.; McClelland, G. M.; Erlandsson, R;S., C. Phys. Rev. Lett. 1987, 59, 1942. (6)Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H. Phys. Rev. Lett. 1992,69,1777. (7) Overney, R.; Meyer, E. MRS Bull. 1993, 18, 26. (8) Haugetad, G.; Gladfelter, W.; Keyes, M.; Weberg, E. Langmuir 1993, 9, 1594.
0743-7463/93/2409-3717$04.00/0
and other sensitizers, e t ~ . ~ JAFM 0 has been used to probe the atomic-scale surface structure of AgBrl1J2 and to monitor in situ surface reactions occurring on AgBr microcrystals in chemically-active fluids.13J4 Our topographic study of macroscopic AgBr crystals grown from solution revealed the step structure of the (111)surface and the presence of rod-shaped objects which grew during AFM imaging.8 These were identified as AgO crystallites produced by photoreduction via residual light from the AFM laser, which is used to monitor cantilever deflection. Gelatin filmsdeposited on the AgBr crystals and on highlyoriented pyrolytic graphite (HOPG) were observed to contain a large number of pores 10-100 nm in diameter and several nanometers deep. Analysis suggested that these pores extend to the AgBr and HOPG substrates. The present findings clearly display the capability of FFM to probe chemical variations across the sample surface. In particular our results indicate the dissimilar frictional nature of (a) the rod-shaped A$ objects compared to the surrounding AgBr surface and (b) the bases of the larger pores in gelatin compared to the gelatin film surface, whether on a AgBr or HOPG substrate. We also analyze the nature of asperity-related aberrations in the frictional signal localized at the edge of the A$ objects and at steps in AgBr(ll1). We discuss the dependence of the measured frictional force on the size of the asperity relative to the radius of curvature of the imaging tip. In addition we derive an expression for the torque exerted about the cantilever axis as a result of both chemical and physical components of friction, in terms of the material(9) Hamilton, J. F. Adu. Phys. 1988,37, 359. (10) The Theory of the Photographic Process, 4th ed.; Mees, C. E. K., James, T. H., E&.; MacMillan Publishing Co.: New York, 1966. (11) Haefke,H.;Meyer,E.;Howald,L.;Schwarz,U.;Gerth,G.;Krohn, M. Ultramicroscopy 1992,42-4#, 290. (12) Hegenbart, G.;Mussig, T. Surf. Sci. Lett. 1992,275, L655. (13) Keyes, M. P.; PhilliDs, E. C.: Gladfelter, W. L. J . Imaging _ _ Sci. Technol. issz, 36, 268. (14) Phfips,E.C.;Gladfelter, W.L.;Keyes,M.P.InIS&TSympoaium on Electronic and Ionic ProDerties of Silver Halides. 44th Annual Conference; St. Paul, MN, 1961.
0 1993 American Chemical Society
3718 Langmuir, Vol. 9, No. 12,1993
Figure 1. Topographic (left) and friction force (right) images of the (111)surface of a AgBr crystal which contains a large number of rod-shaped AgO crystallites lying with their major axes along the (110) family of crystal directions. The scanning region is 12000 X 12000 nm and the AgO rods are about 10 nm in height. The friction force image (right) displays a lower friction (dark in contrast) between tip and sample on the A$ rods compared to the AgBr(ll1) surface.
specific frictional coefficient and the slope of the surface topography.
Experimental Details The details of our experimental methods were described previously.8 AgBr crystals were grown from AgBr/HBr solution and gelatin films deposited onto the AgBr crystals from aqueous solution, both in safelight conditions. The gelatin solution concentration was 10-3 w t % and the exposure time 3 h. The AgBr crystals were shielded from ambient light during imaging. The Nanoscope I11 (Digital Instruments) was used for all images, which were obtained in ambient air. The AFM/FFM was the optical beam-deflection type16 operated a t constant vertical deflection using triangular microfabricated 200-pm cantilevers (spring constant = 0.12 N/m) with pyramidal SiSN4 tips. The operating force regime allowableto avoid damaging the surfaces investigated was previously discussed.8 The 616D and 126F scanners, which could scan up to 15 and 23.5 pm laterally, respectively,and 4.4 pm in the z-direction,were used for all images presented here. Images were normally collected with the tip "scanning" left-bright relative to sample,unless otherwisenoted. Note that in the laboratory frame of reference the tip remains stationary while the sample is actually scanned from right-toleft. Multiple imaging locationswere systematically investigated to ensure reproducible and representative results.
Results and Discussion 1. AgBr(ll1) + A@ Rods. Figure 1 contains topographic (left) and friction force (right) images of the (111) surface of a AgBr crystal which contains a large number of rod-shaped objects previously attributed to elemental Ag, resulting from the photoreduction of AgBr by residual light from the AFM laser.8 The scanning region is 12000 X 12000 nm in dimension. Higher surface regions are rendered as brighter features in the topographic image (left). The height and width of the AgO rods are of order 10 nm and 100 nm, respectively, representative of the average size observed on the AgBr crystals investigated. These rods lie with their major axes along the (110) family of crystal directions.8 The friction force image (right) qualitatively displays a lower friction (dark in contrast) between tip and sample on the Ago rods compared to the AgBr(ll1) surface. Figure 2 contains topographic (left) and friction force (right) images of the (111)surface of another AgBr crystal. The scanningregion is 7000 X 7000 nm in dimension. The roughly rectangular AgO crystallites (20-50 nm in height, (15) Meyer, G.;h e r , N. M. Appl. Phys. Lett. 1988,53,1045. (16) Israelachvili,J. N.Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1991.
Haugstad et al.
Figure2. Enhanced-contrast topographic (left) and friction force (right) images of a AgBr(ll1) surface region containing subnanometer-scale steps and large A$ crystallites. The scanning region is 7000 X 7000 nm and the A$ objects are 20-50 nm in height. The friction force image (right) displays a lower friction on the A@ compared to the AgBr(ll1) surface. Localized high (low) friction a t sudden increases (decreases)in surface elevation as the tip scans from left to right, of greater magnitude for larger changes in elevation, are seen at the edges of A$ or at steps on the AgBr surface.
200-500 nm in width) in Figure 2 are representative of the upper end of the cross-sectional size range commonly observed. As in Figure 1we note a lower friction on these objects relative to the AgBr(111) surface. Preliminary friction-loopdata7(not shown) revealed an approximately 1:3 ratio of frictional forces; these forces increased as a function of load while maintaining a constant ratio. Tipsample adhesive loads of =30 nN were measured on both Agoand AgBr, via the negative applied load yieldingjumpfrom-contact in force versus distance measurements.16 However, the largest negative cantilever deflection obtainable during imaging with our instrument was not great enough to reach zero total load. Linear extrapolation indicated zero frictional force at an applied load of -30 nN, i.e. zero total load. Together these observations suggest that frictional forces sensed on the two surface regions in Figures 1 and 2 are linearly proportional to total load in the range examined. In Figure 2 we also observe localized high (low) friction a t sudden increases (decreases) in surface elevation as the tip scans from left to right, a t steps on the AgBr surface or a t the edge of the AgO objects. Moreover the magnitude of localized frictional aberrations a t such surface "asperities'' apparently increases with a greater change in elevation. A qualitative explanation of this is presented in Figure 3, an illustration of the variation of cantilever torsion a t six locations on a hypothetical heterogeneous surface, each site corresponding qualitatively to features in Figure 2. At each location we indicate with vectors the nature of the normal (solid vector) and tangential (dashed vector) forces extend on the tip by the sample surface. The sign and magnitude of the associated torque about the principal cantilever axis are indicated above each respective location number. In our commercial instrument a clockwise torque of greater magnitude corresponds to brighter features in the FFM images. A stronger chemical interaction between tip and surface on material I (light), relative to material I1(dark),producesa greater tangential force Ft via a larger coefficient of friction p, assumingthat the classical Amonton's law holds, Ft = pFn, where F n is the total (applied + adhesive) normal loading force. Thus a larger torque is sensed at location 2 ( p = 1)compared to4 ( p =0.5). The additional torque produced by a sudden change in surface elevation (1,3, 5, and 6) can also vary depending on the magnitude of the change relative to the radius Of curvature Of the AFM tip* At an edge which is comparable in height to the tip radius of curvature (3 and
Langmuir, Vol. 9, No. 12,1993 3719
Friction Force Microscopy of AgBr Crystals P
scan direction
1
3
2
I
4
‘I
5
6
1
Material I
-
normal force
(5)
---t tangential force
(5)
Figure 3. Schematic description of cantilever torsion on a hypotheticaltwo-componentsurfaceduringleft-bright scanning. At each of six locations the normal (solid vector) and tangential (dashed vector) forces exerted on the tip are indicated with vectors. The sign and magnitude of the associated cantilever torsion is indicated above each respective location. A stronger chemical interaction between tip and surfaceon material I (light), relative to material I1 (dark), produces a greater tangential force Ft. Forces exerted by surface asperities also affect cantilever torsion.
5 ) , the normal force has a large horizontal component and hence contributes substantially to cantilever torsion (positively or negatively);at small steps (1and 6) the torque contributed by the normalforce is correspondingly smaller. Note that Figure 3 depicts the idealization that instantaneous feedback maintains a constant vertical cantilever deflection, and thus a constant vertical component of F n (assuminguniform adhesive load). In reality, the feedback circuit responds in a finite time to sudden changes in surface elevation. In this case we nevertheless expect the “collision”of the tip with a taller asperity to produce greater cantilever torsion, for the same reasons as discussed above. The finite width of frictional aberrations at AgBr step sites in Figure 2 presumably reflects the finite radius of curvature of the probing tip. Additional broadening measured along the edges of Ago rods indicates a finite slope at these locations. This is seen in Figure 4a, a topographic (bottom) and corresponding frictional (top) trace along a segment of a particular scan in Figure 2 containing the Ago rod in the lower right portion of that figure. Zero height corresponds to the AgBr surface elevation. Significantly,the frictional trace resembles the first derivative of topography. To understand this result we illustrate in Figure 4b the contact forces exerted on a tip with radius of curvature R and total height L + R as it scans in the x-direction across a one-dimensionalsurface of variable height z ( x ) . By maintaining a constant vertical cantilever deflection and treating the adhesive load as constant, we ensure a constant total loading force 8’1 = F n cos 8, where F n is the force exerted on the spherical tip normal to its surface at the point of contact and 8 is the angle between the surface normal and the z-direction. The torque exerted by F n about the principal cantilever axis (labeled A) a distance L from the vertex of the spherical tip is given by T,
= FnL sin 8 = FILtan 8 dz =FLdx
where dz/dx is evaluated a t the point of contact. An
-4 t
Scan direction
A
t.
J Ft
Figure 4. (a) Cross-sectional traces (1500 nm) of topography (bottom) and frictional force (top) along a segment of a particular scan in the lower right portion of Figure 2, centered on the AgO rod visible in that figure. (b) Illustration of the torques exerted about the principal cantilever axis due to normal (F,) and tangential (Ft) forces exerted on a tip with radius of curvature R and height L + R.
additional torque is associated with the tangential force Ft between tip and surface. This is given by 7t
= Ft(L +
R =) COS e
Assuming Ft = ClF, = pFdcos 8 and noting that cos 8 = dx/(dx2 + dz2)1/2= [l + ( d z / d ~ ) ~ ] -we l / ~obtain
3720 Langmuir, Vol. 9, No. 12,1993
Haugstad et al. --
ypx
The total torque is thus given by
For the tips used here the ratio R / L is on the order of 0.01. The associated term in eq 1will thus be significant (210% of signal) only if the slope is roughly 10 or greater, which would exceed the aspect ratio of the tips used in this study. Thus our model only applies for lesser slopes, which is certainly the case in the data of Figure 4a. We then have
This is consistent with our observation in Figure 4a that as the tip scans across the edge of the AgO rod, the frictional force varies at the first derivative of topography.17 Note that eq 2 also implies that for the condition T = 0 we have p = -dz/dx, i.e. the (negative) slope of the topography which produces a vanishing torque provides a measure of the frictional coefficient. The slope correspondingto zero cantilever torsion in the data of Figure 4b is about -0.1, giving p~go= 0.1, and hence p ~ g3 ~p =~~0.3 (for the air-exposed surfaces). Importantly, we were able to bypass the complexities of solving the material problem of triangular cantilever torsion. To our knowledge this is the first published report of frictional coefficients determined by FFM employing triangular cantilevers.l8 2. GelatinFilmson AgBr( 111) and HOPG. In Figure 5we present representative AFM (left)and corresponding FFM (right) images or gelatin films deposited on the (111) face of two different AgBr crystals (a and b), at locations outside of regions containing Ago rods, to investigate the nature of gelatin adsorption to “unperturbed” AgBr. In Figure 5 the lateral size, z-range, and scanning direction are (a) 600 X 600 nm, 20 nm, left-to-right and (b) 700 X 700 nm, 10 nm, right-to-left. The FFM image contrast scale was inverted in Figure 5b to preserve the correspondenceof bright features with high friction (see Figure 3). In both cases the topographic images display pores several nanometers deep in the gelatin film, as reported in our earlier publication.8 The FFM images reveal a reduced and more uniform friction within the larger pores (>50 nm in diameter). The enhancedor reduced cantilever torsion occurring at the edges of the pores is consistent with the analysis of Figure 3 and the selected scan direction. Inspection of the topographic data in cross sectional form (not shown)indicates that nearly atomically-flatpore bases correspond to the lower frictional regions seen with FFM. We interpret the lower, more uniform friction as characteristic of the AgBr surface compared to gelatin. This is consistent with our previous interpretation of pores extending to the substrate surface.8 In the previous article we also quantitatively analyzed the importance of finite tip width, and concluded that geometric constraints
a
1
-
(17)For the purposes of illustration we presented analysis of a AgO rod whose surface normal lay almost solely in the xz plane, so that &lay = 0 and the derived expression for cantilever torsion could be related solely to the profile z(x). In general the orientation of the surface normal is specified in three dimensions by both polar and azimuthal angles 6 and 9, and the torque about the primary cantilever axis depends on &/ax = f(fl,d)>.
(18)The measurement of frictional forces via cantilever torsion is performed in commercial instruments like ours by monitoring (with a position-sensitivephotodetector) the lateral displacement of a laser beam reflected from the cantilever. Detailed analysisof instrumental calibration (Haugstad, G.; Gladfelter, W. L. Submitted for publication in Ultramicroscopy) reveals a nonlinearity in the laser displacement measurement which may be significant at frictional forces near the high end of the measurable range.
Figure 5. Topographic (left) and friction force (right) images of gelatin films deposited on the (111)face of two different AgBr crystals (a and b), at locations outside of regions containing A$ rods: (a) Region 600 X 600 nm, z contrast range 20 nm, scanning direction left-to-right; (b) region 700 X 700 nm, z contrast range 10 nm, scanning direction right-to-left, friction contrast was inverted to preserve the correspondence of bright features with high friction. The films contain pores =10-100 nm in diameter; a reduced and more uniform friction is observed within the larger pores observed.
prevented the tip from reaching the base of the smaller pores (
