1594 Langmuir 1993,9, 1594-1600

Atomic Force Microscopy of AgBr Crystals and Adsorbed Gelatin Films

Greg Haugstad and Wayne L. Gladfelter'

Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455

Michael P. Keyed and Elizabeth B. Weberg

Imaging Systems Department, E. I . du Pont de Nemours and Company, Inc., Brevard, North Carolina 28712

Received January 14,1993

Atomic force microscopy of the (111) surface of macroscopic AgBr crystals revealed steps ranging in height from two atomic layers up to 10 nm, lying predominantly along the (110) and (112) families of crystal directions. Rods of elemental Ag, formed via photoreduction, were observed along the (110) family of directions. Images of adsorbed gelatin films revealed circular pores with diameters of order 10-100 nm, extending to the AgBr surface. The length of deposition time, the pH and concentration of the gelatin solution, and the presence of steps on the AgBr surface were observed to affect the size, number, and location of pores in the gelatin films.

Introduction The principal format for rendering visual images remains

silver halide-based photographic films. The properties of these films are determined by the chemical and physical properties of the silver halide microcrystals, which in turn are determined largely by their surface structure. Surface- sensitive techniques such as photoelectron diffraction,' surface-extended X-ray absorption fine structure (SEX- AFS),2 and low energy electron diffraction (LEEDP recently have been employed in order to elucidate silver halide surface structures. An important goal of surface characterization is to correlate silver halide surface struo ture with conventional photographic responses.

Many surface-analytical techniques are limited by spatial resolution and/or necessary sample preparations which can alter the silver halide surface. Atomic force microscopy (AFM) has been demonstrated to be an effective tool for surface characterization of AgBr (refs 4-6) and requires no preliminary surface treatments, such as metal f i i deposition, which is essential in electronic imaging (e.g. scanning electron microscopy, scanning tunneling microscopy). The lack of conduction electrons in AgBr during AFM imaging also precludes the need for cryogenic sample temperatures to reduce Ag+ diffusion and the ensuing reduction to elemental silver by free

+ Current affiliation: Eastman Kodak Co., Research Laboratories, Rochester NY 14650.

(1) Yang, A.-B.; Brown, F. C.; Rehr, J. J.; Mason, M. G.; Tan, Y. T. Phys. Reu. E 1992,45,6188.

(2) Tan, Yen T.; Luehington, K. J.; Tangyunyong, P.; Rhodin, T. N. J. Imaging Sci. Technol. 1992,s. 118. (3) Wagner, M. K.; Ha", J. C.; desouza-Machado, R.; Liang, S.;

Tobin, J. G.; Maeon, M. G.; Brandt, 5.; Tan, Y. T.; Yang, A.-B.; Brown,

(4) (a) Haefke, H.; Meyer, E.; Guntherodt, H.-J.; Gerth, G.; Krohn, M. J. Imaging Sci. 1991,35,290. (b) Meyer, E.; Guntherodt, H.-J.; Haefke, H.;Gerth,G.;Krohn,M.Europhys.Lett. 1991,15,319. (c)Heinzelmann, H.; Meyer, E.; Guntherodt, H.-J.; Steiger, R. Surf. Sci. 1989,221,l. (d) Meyer, E.; Heinzelmann, H.; Grutter, P.; Hidber, H.-R.; Guntherodt, H.-J.;Steiger,R. J. AppLPhys. 1989,66,4243. (e) Hegenbart,G.;Mwig, Th. Surf. Sci. Lett. 1992, 275, L655.

(6) (a) Key-, M. P.; Phillips, E. C.; Gladfeltar, W. L. J. Imaging Sci. Technol. 1992,36,268. (b) Phillips, E. C.; Gladfelter, W. L.; Keyes, M. P. Proceedings of the IS&T Symposium on Electronic and Ionic hoperties of Silver Halides, 44th Annual Conference; St. Paul, MN, 1991; p 168.

(6) Schwan, U. D.; Haefke, H.;.Jung, T.; Meyer, E.; Guntherodt, H.-J.; Steiger, R.; Bohonek, J. J. Imaging Sci. Technol. 1992,36, 361.

F. c. PhYS. Reu. E 1991,43, 6405.

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electrons. In AFM studies of AgBr, steps from one to several atomic layers in height on thin films' and tabular microcrystals6 have been reported. In addition, individual ions on some AgBr surfaces were apparently resolved.' In the present work, AFM was explored as a technique for imaging adsorbed gelatin on AgBr. Gelatin is ubiquitous in photographic science, being present from the time silver halide microcrystals are grown, to film development where the photographic image is formed. In order to have the most atomically smooth and clean surfaces poseible with AgBr samples grown from solution, tabular-shaped crystals grown in the absence of gelatin were studied. Use of these samples precluded the need for removal of the gelatin from conventionally grown tabular microcrystals.

Experimental Details Crystals were grown from a solution made by dissolving 0.26

g of AgBr (All-Chemie, Ltd., 99.999%) in 12.6 mL of 48% HBr solution (Aldrich). A small puddle consisting of several drops of this solution was placed on a polyester substrate. Very slow evaporation over 1-2 weeks allowed octahedral crystals with reduced heighthidth aspect ratios to precipitate. Typical dimensions were 400 to 700 pm laterally and less than 300 pm vertically. A horizontal, triangular (111) face formed the top surface of each crystal. Though much larger, these crystals resemble tabular microcrystals grown in gelatin and frequently used in photographic-sensitive emulsions. Following precipita- tion, residual solution was carefully blotted with the tip of a paper towel, avoiding contact with the crystals themselves, and AFM imaging performed.

A Kodak safelight lamp (Model B with a no. 1A red filter) was used to illuminate the workbench and the instrument during sample preparation and positioning in the AFM. Some of the crystals were treated with solutions of limed-bone gelatin (Kind and Knox, type 2688). These solutions were prepared to the desired concentrations (1O-LlOd wt %) in deionized water and deposited on the crystals with an eye dropper. After 5-600 min of exposure time the crystals were dried with a paper towel as above, the process being monitored with optical microscopy. Surface tension allowed the abrupt removal of gelatin solution from the top face of the crystals. No additional rinsing of the crystals was performed. AFM imaging of the resulting surfaces was performed within 24 h of this treatment. Reproducible AFM images of the gelatin-treated crystals were obtained only with a fresh batch of solution, which was prepared weekly.

T h e stainless steel AFM lid, supplied with the instrument and designed to cover the sample, shielded the AgBr crystals from

0 1993 American Chemical Society

AFM of AgBr Crystals Langmuir, VoZ. 9, No. 6,1993 1595

ambient light during imaging. The Nanoscope AFM (Digital Instruments, versions I1 and 111) was used for all images, which were obtained in ambient air. The AFM was the optical beam- deflection type’ operated a t constant deflection, using micro- fabricated 200 pm cantilevers (spring constant = 0.12 N/m) with pyramidal SisN, tips. Typically, the contact force between the tip and sample was maintained a t =lV N. AgBr surfaces were damaged by the tip if the contact force was above le7 N. The 616D scanner, supplied by Digital Instruments, which could scan up to 15 pm in the x - and y-directions and 4.4 pm in the z-direction, was used for all images presented here; atomic resolution of highly oriented pyrolytic graphite (HOPG) was also obtained using this scanner, verifying instrument performance.

Since the maximum scanning area was considerably smaller than the size of the crystals, only a small part of one crystal could be imaged at any given time (as opposed to AFM imaging of tabular microcrystals6s). Typically a crystal was imaged a t several locations separated by more than 100 pm to obtain images representative of the entire crystal. Because the AgBr crystals were large enough to be visible with the naked eye, the orientation of image features with respect to the crystal axes was precisely known. All the observations discussed below were reproduced over at least several individually grown and prepared crystals.

Results and Discussion 1. AgBr(ll1) Surface. In Figure 1 representative

AFM images are presented of the AgBr crystals investi- gated. Figure l a shows a three-dimensional rendering (60° angle of inclination) of a surface region loo00 X loo00 nm in lateral dimension. Differences in height are also indicated by gray scale, the brightest regions being the highest; the total range in the z-direction is 50 nm. Rod- shaped features approximately 10 nm high by 100 nm wide by 2000-8000 nm long lie along the (110) family of directions, at 60’ and 120’ with respect to one another. Regions between rods typically contain atomically-flat plateaus, as large as several hundred nanometers laterally. This is seen in Figure lb, a representative smaller-scale image (3000 X 3000 nm, z-range = 5 nm). The smallest steps in Figure l b are 0.32 f 0.02 nm in height; all steps seen in the image are multiples of 1, 2, or 3 times this value. This was determined using the commercial AFM software to produce a histogram of pixel “elevations” by integrating over an approximately 200 X 200 nm area which contains a single step. A two-peak histogram was obtained corresponding to two plateau elevations; the distance between the peaks indicated the step height, and the peak widths provided a precise gauge of the uncertainty of the measurement. The lattice constant of rocksalt AgBr is 0.58 nm, giving an interplanar spacing of 0.33 nm for same- atom planes in the polar [ill] direction. Our results, repeated for several crystals, indicate that the smallest steps which are imaged on AgBr(ll1) surfaces are steps between same-atom planes. Steps and rods appear to form independently of each other.

The AFM image in Figure IC is typical of AgBr surface regions which contain both large steps and superimposed rods. The image (9000 X 9OOO nm, z-range = 150 nm) incorporates computer-simulated illumination of features. Steps are =l-10 nm in height and =lOOo-loo00 nm in length and lie predominantly along the (110) and ( 112) families of directions. These observations are consistent with other AFM studies of AgBr thin films.4 The AgBr(ll1) surface is thought to be reconstructed2va10from that expected on an ideally-truncated crystal. The fact that the small steps seen in Figure l b do not necessarily

(7) Meyer, G.; h e r , N. M. Appl. Phys. Lett. 1988,53,1045. (8) Haefke, H.; Krohn, M. Surf. Sei. Lett. 1992,261, L39. (9) Baetzold, R. C.; Tan, Yen T.; Tasker, P. W. Surf. Sci. 1988,195,

(10) Hamilton, J. F.; Brady, L. E. Surf. Sei. 1970,23,389. 579.

Figure 1. AFM images of the (111) face of a AgBr crystal, displayed at a 60° angle of inclination. (a) A region loo00 X loo00 nm in lateral dimension (2-range = 50 nm) displaying rod- shaped features lying along the ( 112) family of crystal directions. (b) A 3000 X 3000 nm image (z-range = 5 nm) of a surface region located between the rods. (c) A 9OOO X 9OOO nm image (2-range = 150 nm) of a region which contains both large steps and superimposed rods. The image incorporates computer-simulated illumination of features. The steps lie predominantly along the (110) and (112) families of directions.

1596 Langmuir, Vol. 9, No. 6, 1993

lie along (110) or (112) may reflect surface-energetic driving forces away from plateau truncation along a single row of atoms. It may also reflect the complex atomic structure of the AgBr(ll1) surface, where several surface- atom configurations may coexist, or even intercon~ert .~J~

Rod-shaped features seen in images like those in parts a and c of Figure 1 are often composed of segments and grow during approximately the first hour of imaging, after which the images remain essentially unchanged for several hours. Subsequent imaging at locations more than 100 pm from the initial location reveals distributions of rods of a similar number and size (also no longer changing with time) indicating that the effect is not localized to the initial scanning region and hence not induced by interactions with the scanning tip. Data from X-ray diffraction experiments performed on these crystals have shown the presence of crystal planes for both elemental silver and AgBr. We assign the rods to elemental Ag formed by a photoreduction process induced by residual light from the red laser of the AFM instrument. Optical microscopy confirmed the illumination of the entire crystal by (scattered) light originating from the laser.s

Generally, AgBr crystals which were grown more slowly, by controlling the rate of solvent evaporation, and which sat dry for at most several hours prior to AFM imaging displayed fewer rods. Occasionally rod formation was not observed at all. If the crystals containing rods were rewet by placing several drops of water on the them, and again dried, AFM images revealed that the rods were usually replaced by trenches of somewhat greater length and width than the original rods; trench depths (-10 nm) were comparable to rod heights. Apparently, the water loosened the rods and washed them away; no attempts were made to recover the elemental Ag for analysis. The presence of trenches following rod removal, and the lack of observa- tions of similar depressions prior to rod growth, suggests that the atoms comprising the rods originate from the immediate AgBr region.

Upon exposure to high intensity white light, we have observed with AFM the growth of micrometer-scale crystallites on the (111) faces of AgBr crystals. These crystallites, presumably elemental Ag, have the shape of square-based pyramids, indicating a AgBr(lll)/Ag(100) interface. They are oriented with base-edges parallel to the (110) and (112) directions. From these observations we propose an epitaxial relationship for AgO rod growth on AgBr(lll), displayed schematically in Figure 2. The directions of the crystal axes are shown in the lower left. The closed circles indicate the positions of Ag atoms in AgO, and the open circles indicate the positions of Ag or Br atoms (nearest-neighbor distance of 4.1 A) in the polar AgBr(ll1) plane. Because the AgBr(ll1) surface is thought to reconstruct,2Pal0 we do not intend in Figure 2 to indicate surface atom positions definitively, but rather the underlying (111) atomic positions in the lattice, for illustrating the relative orientation of the Ag( 100) lattice. Note that in Figure 2 a lattice match occurs along the lil01 direction, but a 15% mismatch exists along [ii21. To our knowledge the only similar reported results involve the formation of AgO particles along the (112) family of directions in the interior of tabular microcrystals.

2. AgBr( 111)/Gelatin. We have investigated the morphology of adsorbed gelatin films on AgBr( 11 l), while varying several film deposition parameters: deposition time, the pH of the gelatin solution, and gelatin concen-

Haugstad et al.

(11) Sprackling, M. T. J. Photogr. Sci. 1984, 32, 96.

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1 4 . 1 A k v i a ] 15 % mismatch - 0 % mismatch

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Figure 2. Schematic representation of the proposed relationship between the atomiclattice inA$rods, which form on AgBr(lll), and the bulk AgBr lattice. The cloeed circles indicate the positions of AgOatoma in the rode and the open circles indicate the positions of Ag or Br ions in the polar AgBr(ll1) plane. Crystal directions are shown in the lower left.

tration. In section 2.1 we present the dependence of gelatin morphology on deposition time at a single gelatin solution concentration and at the intrinsic pH. We briefly sum- marize the dependence of gelatin morphology on concen- tration. In section 2.2 we describe the effect of pH variation of the gelatin solution at a single gelatin concentration. In section 2.3 we present results which reveal the nonneg- ligible effect of AgBr surface features on gelatin film morphologies.

2.1. Variable Time of Gelatin Deposition. Figure 3 contains lo00 X lo00 nm AFM images (z-range = 20 nm) of the (111) surface of a representative AgBr crystal exposed to a 10-3 wt % aqueous gelatin solution at the intrinsic pH (typically 6.0-6.2) for increasing cumulative times of (a) 5 min, (b) 50 min, and (c) 500 min. The three images are representative of observed morphologies at numerous locations for each exposure, but were not taken from the same location on the crystal. For the 5-min exposure (Figure 3a) z-variations in topography are very small on the 20-nm scale; computer-enhanced contrast is employed to enhance the surface features. The image in Figure 3a reveals a homogeneous distribution of small circular holes approximately 20-40 nm in diameter and 0.5-1 nm in depth. In addition a small number of larger depressions, 100-150 nm in diameter and 1 nm deep, are observed (upper left and lower right of Figure 3a). For a 50-min exposure (Figure 3b), homogeneously-distributed holes (20-80 nm diameter and 5-8 nm in depth) are observed. For the 500-min exposure (Figure 3c), the AFM image again displays homogeneously-distributed holes of depths in the 6-12 nm range; the distribution of diameters is similar to the 50-min case but contains fewer in the lower end of the diameter range, the smallest being about 30 nm. In addition, the total number of holes is smaller by about a factor of 2.

Additional quantitative analysis of the images in Figure 3 yielded the root mean square (rms) roughness of the gelatin films. This was obtained using the commercial software provided with the AFM instrument by squaring

AFM of AgBr Crystals Langmuir, Vol. 9, No. 6,1993 1597

Figure 3. AFM images of the (111) face of a AgBr crystal following successive exposures to a 10-9 wt % aqueous gelatin solution a t the intrinsic pH. Cumulative exposure times are (a) 5 min, (b) 50 min, and (c) 500 min. The image size in each case is lo00 X lo00 nm, z-range = 20 nm. The three images were not taken from the same location on the crystal. In part a computer- enhanced contrast is employed to enhance the surface features.

the difference between elevation at each point on the surface and average surface elevation, summing over all points, and taking the square root. The rms roughness

thus determined for Figure 3 are (a) 0.5 nm, (b) 1.8 nm, and (c) 2.1 nm.

We interpret these images of gelatin-exposed AgBr crystals, as shown in Figure 3, as pores in an adsorbed gelatin film. Cross-sectional views of images of the largest- diameter pores (40 nm) display nearly flat bases on the atomic scale, while smaller pores (diameter = 10-30 nm) have rounded, approximately parabolic bases. The flat nature of the bases of the larger pores resembles clean AgBr surfaces discussed above and suggests that the pores extend down to the AgBr. In contrast, the gelatin film observed between the pores displays variations in surface elevation on the 2-10 nm scale. The curved nature of the bases of the smaller pores may indicate that the AFM tip cannot fit far enough into the pores to reach their bases. A simple calculation shows that the tip will reach the base of a pore 30 nm in diameter and 5 nm deep only if the tip radius of curvature is at most 25 nm, smaller than measured values (see section 3) for the commercial tips used in this study.

The increasing depth of the pores displayed in Figure 3 with increasing deposition time suggests a large increase in gelatin film thickness during approximately the first hour of deposition, with much smaller changes occurring with longer times. This is qualitatively consistent with the ellipsometry results of gelatin films on AgBr(lll).12 The reduction of the number of pores by about a factor of 2, from 50- to 500-min exposure, is due to the filling of some of the pores with gelatin, verified by carefully comparing AFM images of the same surface location at the two exposure times.

Images of gelatin films like that in Figure 3c were generally sharp and reproducible (at contact forces S5 X 10-8 N) over successively-collected images, indicating the lack of tip-induced effects in the measured f i i morphol- ogy. However, the images tended to be streakier and less sharp at lower film coverages. This generally is indicative of a film which is more susceptible to mechanical defor- mation. Apparently, when the gelatin films are thin and film cohesion is weak, the tip is more likely to perturb the surface. Increasing film thickness produces a more cohesive film, resulting in greater resistance to tipinduced mechanical deformation.

Gelatin films deposited on AgBr(ll1) during 21-h exposures to higher concentration (10-1 wt %) aqueous gelatin solutions displayed significantly fewer pores than at 103 wt % (Figure 3), but an otherwise similar mor- phology. At lower gelatin concentrations (106 wt %), gelatin films either contained pores of much larger diameter than at higher concentrations or were simply not continuous films, i.e. consisted of isolated islands of gelatin. Significant variations in morphology were ob- served between different crystals, possibly due to greater effects of contamination at this low concentration. The general trend, however, was clearly toward a decreasing degree of film continuity, i.e. an increasing presence of breaks or pores in the film, with decreasing gelatin solution concentration.

2.2. Variable pH of Gelatin Solution. Figure 4 shows representative lo00 X lo00 nm AFM images (z-range = 20 nm) of gelatin films deposited on AgBr(ll1) in 3-h exposures to 103 wt % aqueous gelatin solution at (a) pH = 4.0 and (b) pH = 8.9. The pH was lowered/raised from the intrinsic value (6.0-6.2) with HN03 or NaOH. The

(12) (a) Mabmaghan, T. J.; Bangham, 0. B.; OtbwiN, R H. J. Photogr. Sci. 1980,28,1. (b) Bangham, 0. B.; Mabrnaghan, T. J.; Otbwill, R. H. Photogr. Sci. Eng. 1979,23,4!5. (c) Wrighton, G. C.; Herglotz, H. K. J. Photogr. Sci. 1980,28,49.

1598 Langmuir, Vol. 9, No. 6, 1993 Haugstad et al.

(b) Figure 4. AFM images displaying the dependence of the morphology of gelatin films on the pH of the gelatin solution. AgBr(ll1) was exposed for 3 h to a 1o-S w t ?6 aqueous solution at (a) pH = 4.0 and (b) pH = 8.9, in pBr = 2.0 conditions. The image size is lo00 X lo00 nm, z-range = 20 nm.

gelatin solutions also contained M KBr to produce excess Br ion conditions (pBr = 2.0). In Figure 4a (pH = 4.0) we find a gelatin film morphology quite similar to that of Figure 3c in terms of pore diameter (=30-80 nm) and distribution (homogeneous), but with shallower pores. Control experiments performed at the intrinsic pH, but also at pBr = 2.0, produced results similar to those in Figure 3c. Pore depths in Figure 4a are 2-6 nm and rms film roughness is =l.l nm. In Figure 4b (pH = 8.9), we note a greater variation in pore diameters (10-120 nm) and depths (2-10 nm), along with gelatin agglomerates =30-100 nm in lateral dimension and -2-6 nm in height, and an rms film roughness of k2.2 nm.

The lower-pH gelatin solution leads to a more spatially- uniform film. The effect was found to be more repro- ducible when the deposition was performed at an elevated Br ion concentration, as was done for the films shown in Figure 4. We also find that pores in films deposited from low-pH solutions are shallower on average, implying a reduced film thickness. These results are qualitatively

Figure 5. AFM images displaying the differences in gelatin f i i morphology on a AgBr( 11 1) surface region previously containing a A$ rod (lower left). A 3-h exposure to 1o-S wt ?6 gelatin solution, at pH = 4.0, produced the results shown. The image size is 3000 X 3000 nm, z-range = 30 nm.

consistent with an ellipsometry study of gelatin films on AgBr(ll1) surfaces.12 At low pH, the gelatin molecules have a net positive charge and are expected to form a relatively strong bond to the bromide ions on the AgBr- (1 11) surface. Because positively-charged groups are distributed along the length of gelatin molecules, the molecules are expected to adsorb in flatter configurations,12 which in turn should contribute toward greater film uniformity.

2.3. Effect of AgBr Surface Features on Gelatin Morphology. Figure 5 is a representative 3000 X 3000 nm AFM image (z-range = 30 nm) of a gelatin-covered AgBr(ll1) surface region which contained a Ag rod removed by water prior to gelatin exposure. The gelatin solution used in this case was 103 wt % concentration, pH = 4.0, and the deposition time was 3 h. In Figure 5 the upper region (region 1) displays the usual porous film morphology, while the region at the bottom left (region 2) where the Ag rod was removed displays a film with no apparent pores. Examination of profile plots verifies that pores on region 1 extend well below the elevation of the surface in region 2, indicating that gelatin is in fact adsorbed in both regions, but completely wets the surface in region 2. Thus a distinct difference in gelatin adhesion results from chemical changes involved in the photore- duction of AgBr to AgO and subsequent removal of AgO with water.

In many images of adsorbed gelatin f i i , the underlying AgBr surface structure is evident. Figure 6a shows a 4000 X 4000 nm image (z-range = 30 nm) of a gelatin film deposited in a 3-h exposure to 1o-S wt % gelatin at pH = 4.0. A porous gelatin film qualitatively similar to that in Figure 4a is observed, but superimposed on a AgBr(ll1) surface containing steps from 5 nm in height down to the subnanometer regime. Demonstrated in the 1000 X 1000 nm image in Figure 6b (z-range = 10 nm), larger pores (>50 nm) tend to form along steps in the AgBr surface. Smaller pores (diameter = 10 nm) appear instead to be more homogeneously distributed.

3. Graphite/Gelatin. For comparison with results on AgBr( 111) we have performed a preliminary investigation

AFM of AgBr Crystals Langmuir, Vol. 9, No. 6, 1993 1599

Figure 6. AFM images of gelatin films on AgBr(111) surface regions containing steps, following a 3-h exposure to 1W wt % gelatin at pH = 4.0. (a) A 4000 X 4000 nm image (2-range = 30 nm) shown following the subtraction of a plane fit to the raw data, to render visible both the small-scale porous gelatin morphology and the large-scale steps in AgBr(ll1). (b) A lo00 X 1000 nm image (2-range = 10 nm) displaying the tendency of larger pores in gelatin to form along steps in AgBr(ll1).

of the morphology of gelatin films deposited on cleaved highly oriented pyrolytic graphite (HOPG). Figure 7 is a representative 10oO X loo0 nm AFM image (2-range = 5 nm) of the surface resulting from a 1-h exposure of HOPG to 103 wt % gelatin solution (pH = 6.0-6.2). A porous gelatin film is observed, similar to results on AgBr(ll1). Most of the pores are 10-30 nm in diameter, while a small number have diameters in the 100-150 nm range. The larger pores are typically surrounded by a lip of gelatin =l nm in height, are =2 nm deep relative to the mean surface elevation between the pores, and have atomically-flat bases, suggesting that these pores extend down to the HOPG surface.

Cross-sectional plots of the smaller pores in Figure 7 display variable depths of less than 2 nm and approxi- mately parabolic bases. We estimated an “operational” tip radius of curvature (a function of the mechanical

Figure 7. AFM image of a gelatin film deposited on highly oriented pyrolytic graphite, after a 1-h exposure to 10-9 w t !% gelatin solution (pH = 6.0-6.2). The image size is 1000 X 1000 nm, z-range = 5 nm.

compliance of gelatin) of 75 nm from the measured broadening of large steps in AFM images of the gelatin- covered graphite surface. The minimum pore diameter for which the tip will reach the base of a 2 nm deep pore is =35 nm, larger than the range of diameters of pores which do not display flat bases in Figure 7. It is therefore likely that the AFM tip does not reach the bases of these smaller pores.

The similarity of the qualitative morphology of gelatin films on both AgBr(ll1) and HOPG indicates a similar degree of wetting on these surfaces and suggests that the presence of pores in gelatin films on AgBr is not related to surface features or contaminants unique to AgBr( 11 1). Quantitative differences between the two cases are evident, however. On HOPG the gelatin film is apparently =2 nm thick and the vast majority of pores have diameters of 10-30 nm. By comparison, a similar time of exposure to 103 wt % gelatin at pH = 6.0-6.2 (Figure 3b) appears to yieldafilm5-8nm thickon AgBr(lll),withporediameters rather evenly distributed in the 20-80 nm range.

4. Technological Implications. Gelatin is always present during the lifetime of silver halide microcrystals, which are routinely used in photographic films. The purpose of the gelatin is to prevent the microcrystals from aggregating and to protect them from undesirable mor- phological changes while in the emulsion. At the same time, gelatin must allow the migration of a wide variety of reagents to the microcrystal surfaces for controlled growth of the microcrystals, for surface reactions to control their photosensitivity, and for nucleation and growth of elemental Ag during the development of the photographic film. Few other polymeric materials have been found which protect the microcrystals while allowing the nec- essary surface processes to occur.

At the gelatin coverages used in this study, pores were observed in gelatin films, whether adsorbed on AgBr or graphite, indicating similar wetting behavior on the two diverse surfaces. The presence of pores in photographic emulsions would be an important feature of adsorbed gelatin which would allow the migration of reagents to the silver halide microcrystal surfaces in photographic emul- sions. The results suggest in this case that the manipu-

1600 Langmuir, Vol. 9, No. 6, 1993

lation of pore size, by appropriate choice of parameters (concentration, pH, temperature, etc.) would possibly lead to more precise control of surface reactions on the microcrystals. Indeed, under conditions where pore size is at least partially dependent on the silver halide surface structure (e.g. Figure 6b), site-directed surface reactions might be possible.

Conclusions AFM was applied to flat triangular and hexagonal AgBr

crystals resembling conventional tabular microcrystals grown in the presence of gelatin and routinely used in the photographic film industry. Steps, which varied in height from 0.3 nm (two atomic layers, the distance between same- atom planes) to 10 nm were observed on AgBr(ll1) surfaces, the larger steps lying predominantly along the (110) and (112) families of directions. Rods of elemental Ag, formed by photoreduction, were also observed along the (110) family of directions.

Haugatad et a 1.

AFM images of adsorbed gelatin showed the presence of circular pores with diameters of order 10-100 nm. Similar pore structure was apparent on a graphite surface. The size and number of pores, and the film thickness, depended on the deposition time and the pH and concentration of the gelatin solution. The presence of AgBr surface features, such as steps, also influenced the size and location of the pores. Such information may ultimately lead to ways to more precisely control the architecture of silver halide microcrystals during their growth and to produce more photoefficient microcrystale.

Acknowledgment. Support by the Center for Inter- facial Engineering (CIE), a National Science Foundation Engineering Research Center, and a grant from E. I. du Pont de Nemours and Co., Inc., are gratefully acknowl- edged.