Exchange of Fluorinated Cyanine Dyes between Different Types of

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to study dye adsorption on silver halide (AgBr or Ag(Br,I)) microcrystals. The dyes ...
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Exchange of Fluorinated Cyanine Dyes between Different Types of Silver Halide Microcrystals Studied by Imaging Time-of-Flight Secondary Ion Mass Spectrometry Jens Lenaerts,* Geert Verlinden, Luc Van Vaeck, and Renaat Gijbels Department of Chemistry (MiTAC), University of Antwerp (UIA), B-2610 Wilrijk, Belgium

Ingrid Geuens and Paul Callant Agfa-Gevaert N. V., B-2640 Mortsel, Belgium Received June 8, 2001. In Final Form: August 9, 2001 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to study dye adsorption on silver halide (AgBr or Ag(Br,I)) microcrystals. The dyes are labeled with fluorine to allow their detection at the surface of the microcrystals by means of atomic ions. In this study, particular attention is paid to dye adsorption selectivity as a function of microcrystal morphology and to the possibility of dye exchange between different types of microcrystals. Using a gallium (Ga+) liquid metal ion gun operating at 25 keV as the primary ion source, secondary ion images with a lateral resolution of 65 nm have been collected. This high lateral resolution makes it possible to distinguish between octahedral and cubic AgBr crystals even with an edge length of only 0.4 µm, based on their bromide images. It also enables us to detect and localize the fluorine-labeled dyes at the surface of these microcrystals. When larger cubic crystals with an edge length of 0.8 µm are used, the site selectivity of the fluorine-labeled dye for the edges of these crystals can be studied. Apart from cubic and octahedral crystals, dye adsorption on tabular crystals is also studied.

I. Introduction Cyanine dyes are used in the photographic industry as spectral sensitizers for silver halide materials. The intrinsic sensitivity of silver halide microcrystals is limited to wavelengths shorter than 410 nm for AgCl and shorter than 490 nm for AgBr crystals. To extend the spectral region of the photographic response, it is necessary to use sensitizing dyes, which are adsorbed as thin monomolecular films on the silver halide microcrystals. The sensitizers most widely used are cyanine dyes. More extensive information about sensitization of silver halide microcrystals can be found in the work of Maskasky1,2 and Tani.3 The properties and dynamic behavior of the adsorbed sensitizers are of utmost importance for the proper functioning of the coated silver halide microcrystals in photographic applications. Tani has investigated selective dye adsorption and dye exchange between cubic and octahedral crystals.4 The experiments were conducted on macroscopic populations of silver halide microcrystals. However, full characterization of the sensitizer coatings requires individual microcrystals to be analyzed. The detection of monolayers of an organic dye on silver halide crystals of only 0.4-1 µm in size still represents a major challenge to analytical chemistry. One of the few candidates that is able to achieve this analytical task is time-of-flight secondary ion mass spectrometry (ToF-SIMS). It is a surface sensitive analytical technique that has been widely used for the characterization of inorganic as well as organic surface layers.5-9 Three different types of information can be (1) Maskasky, J. E. Langmuir 1991, 7, 407-421. (2) Maskasky, J. E. J. Imaging Sci. 1991, 35, 29-38. (3) Tani, T. Photographic Sensitivity; Oxford University Press: New York, 1995; Chapter 5. (4) Tani, T. J. Imaging Sci. 1985, 29, 165-171. (5) Benninghoven, A. Angew. Chem., Int. Ed. Engl. 1994, 33, 10231043.

obtained by ToF-SIMS analysis corresponding to three modes of operation. First, ToF-SIMS allows mass spectra to be recorded for the identification of elements and molecules that are present at the surface of solid materials (mass spectrometric mode). In the second place, the technique can visualize their lateral distribution, in the case of elements even with high lateral resolution (imaging mode). In the third place, ToF-SIMS is also capable of monitoring the in-depth distributions of elements in a solid sample (depth profiling mode). Previous ToF-SIMS work in this laboratory has shown the feasibility of studying the in-depth and lateral halide distributions in individual silver halide microcrystals.10,11 As to organic applications, the secondary ion formation of cyanine dyes adsorbed on a silver surface under Ga+ primary ion bombardment12 has been studied. The next step and the purpose of the present paper is the direct microscopical analysis, that is, characterization and localization, of these organic cyanine dye layers adsorbed at the surface of silver halide microcrystals. This will enable us to study, in a direct manner, the adsorption behavior of these dyes at different types of silver halide microcrystals. To do so, imaging experiments at high lateral resolution are needed. The number of dye molecules (6) Van Vaeck, L.; Adriaens, A.; Gijbels, R. Mass Spectrom. Rev. 1999, 18, 1-47. (7) Adriaens, A.; Van Vaeck, L.; Adams, F. Mass Spectrom. Rev. 1999, 18, 48-81. (8) Pacholski, M.; Winograd, N. Chem. Rev. 1999, 99, 2977-3005. (9) Benninghoven, A.; Hagenhoff, B.; Niehuis, E. Anal. Chem. 1993, 65, 630A-640A. (10) Verlinden, G.; Gijbels, R.; Geuens, I.; De Keyzer, R. Secondary Ion Mass Spectrometry, SIMS XII; Benninghoven, A. et al., Eds.; Elsevier: Amsterdam, 1999; pp 213-216. (11) Verlinden, G.; Gijbels, R.; Geuens, I. J. Anal. At. Spectrom. 1999, 14, 429-434. (12) Lenaerts, J.; Verlinden, G.; Van Vaeck, L.; Gijbels, R.; Geuens, I. Secondary Ion Mass Spectrometry, SIMS XII; Benninghoven, A. et al., Eds.; Elsevier: Amsterdam, 1999; pp 115-118.

10.1021/la010862t CCC: $20.00 © 2001 American Chemical Society Published on Web 10/12/2001

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Figure 1. Structure of the cationic fluorinated dye (dye 1).

Figure 2. Structure of the zwitterionic fluorinated dye (dye 2). Table 1. List of Silver Halide Suspensions Used in This Study

suspension

type of microcrystal

1 2 3 4 5 6 7 8 9 10 11

cubic (0.8 µm) cubic (0.4 µm) octahedral (0.4 µm) tabular (1 µm) cubic (0.8 µm) cubic (0.8 µm) cubic (0.4 µm) cubic (0.4 µm) tabular (1 µm) tabular (1 µm) tabular (1 µm)

cationic dye (dye 1)

zwitterionic dye (dye 2)

x x x x x x x

surface coverage (%) 0 0 0 0 80 40 80 80 5 50 80

available within a spot of about 60 nm diameter is limited so that the ion yield will become an important factor that needs to exceed the detection limit. Molecular ions from organic molecules exhibit low ionization yields, practically preventing the direct imaging of these substances in the applications aimed at. Therefore, the elaborated methodology used fluorine-labeled dyes to exploit the high ion yield of the elemental ions. II. Experimental Section Products. Four different types of silver halide suspensions (dispersions of microcrystals in gelatin) were used: two suspensions of cubic AgBr crystals with edge lengths (EL) of 0.4 and 0.8 µm, a suspension of octahedral AgBr crystals with an EL of 0.4 µm, and a suspension of tabular Ag(Br,I) crystals with an average equivalent circular diameter (ECD) of 1 µm and a thickness of 150 nm. Looking at the crystal plane orientation of these crystals, some differences can be found: the cubic crystals are characterized by planes with a {100} silver bromide surface and slightly rounded edges with a {111} and {110} silver bromide surface; the octahedral crystals typically have a {111} silver bromide surface just like the major faces of the tabular crystals. The dye adsorption of two fluorinated dyes was studied: a cationic dye (dye 1) and a zwitterionic dye (dye 2). The structures of these dyes are represented in Figures 1 and 2. The suspensions as well as the dyes were obtained from Agfa-Gevaert N.V. Mortsel. The combinations of the silver halide suspensions and dye solutions used in this study are summarized in Table 1. The percentages of dye coverage that are given are calculated on the basis of experimental data specifying that a dye molecule typically covers 1 nm2.13 Hence, about 1 mg/m2 of dye corresponds to a 100% monolayer coverage. Sample Preparation. The dyes were dissolved in methanol (dye 1) and in a mixture of methanol and phenoxy-ethanol (dye 2) to a concentration of 4 g/L. The solutions were added to the different gelatinous silver halide crystal suspensions at 40 °C. (13) Deprez, L. Personal communication.

Figure 3. Schematic representation of the primary ion transport in the crossover mode (left) and in the collimated mode (right). The mixture was stirred for some time depending on the type of experiment. The suspensions were diluted with distilled water and degelled by centrifugal treatment. A drop of the dispersion was placed on a piece of silicon wafer to transfer the crystals into the ToF-SIMS instrument. All manipulations and experiments were performed under appropriate darkroom conditions. Instrumentation. The measurements were performed on a ToF-SIMS IV instrument (Cameca/Ion ToF) equipped with a Ga+ liquid metal ion gun operating at 25 keV. More details about the instrument and the principles of the ToF-SIMS technique used can be found in the work of Briggs et al.14 and Niehuis et al.15 Depending on the type of analysis, different pulse lengths and different direct primary ion currents were applied. For the registration of secondary ion mass spectra, short primary ion pulses of 1 ns were used. This resulted in secondary ion mass spectra with high mass resolution (typical value of 7000 on C2H3). In the mass spectrometric mode, the primary ion beam was directed through the primary ion gun and on the surface by crossover lenses. In this way, a direct current of approximately 2.5 nA could be reached. Secondary ion images on the other hand were obtained by bombarding the sample by primary ion pulses with a pulse length of 200 ns. The result was a deterioration of the mass resolution to unit mass resolution. The primary ion beam was no longer directed toward the sample by crossover lenses; instead, the primary ion beam transport occurred in the collimated mode. Part of the primary ion beam was blocked at the first aperture; the remaining part yielded a direct primary ion current of approximately 100 pA at the sample surface. A schematic representation of the two modes of primary ion beam transport is shown in Figure 3. In the imaging mode, a lateral resolution of 65 nm could be obtained for elemental species (cf. Figure 4), according to the 16%, 84% criterion. Methodology. The analysis of the samples was done in three different steps. First, secondary ion mass spectra were taken at high mass resolution, to verify the presence of isobaric interferences. In a second step, imaging was performed in a different field of view at low magnification (and low mass resolution). The beam was rastered over fields of 15 × 15 µm2. The purpose was to visualize the homogeneity of the dye distribution over the total crystal population. In the final step, images with high lateral resolution were collected, again in a different field of view to be sure to obtain surface information. The primary ion beam was then rastered over fields of maximum 5.5 × 5.5 µm2. These highresolution images made it possible to distinguish between cubic and octahedral silver halide crystals with an edge length of only 0.4 µm.

III. Results and Discussion Use of Fluorine as a Label. As is described in the previous part, the first step in the analysis of our samples (14) Briggs, D.; Seah, M. Practical Surface Analysis: Ion and Neutral Spectroscopy; Wiley: New York, 1990; Vol. 2. (15) Niehuis, E.; Heller, T.; Ju¨rgens, U.; Benninghoven, A. J. Vac. Sci. Technol., A 1989, 7, 1823-1828.

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Figure 4. Left: Line scan across the surface of a single tabular Ag(Br,I) microcrystal. Right: Plot of the Br- ion intensity versus the distance.

Figure 5. Positive secondary ion mass spectrum of a carbocyanine dye adsorbed on cubic silver bromide crystals (80% theoretical dye coverage).

was a mass spectrometric study. Figure 5 shows a positive secondary ion mass spectrum of a nonfluorinated carbocyanine dye (molecular weight 617) adsorbed on AgBr tabular crystals. For this sample, a surface coverage of 80% was estimated from the known surface area of the silver halide crystals per gram of gelatine suspension and the amount of dye added. Only element ions and small organic fragment ions (e.g., C2H5+ and C3H6+) without any structural relevance were detected. There were no intense secondary ion signals present above m/z ) 200. As a result, the positive secondary ion mass spectra did not contain relevant molecular information. In the negative secondary ion mass spectrum, no molecular information was detected either (cf. Figure 6). Only some element

Figure 6. Negative secondary ion mass spectrum of a carbocyanine dye adsorbed on cubic silver bromide crystals (80% theoretical dye coverage).

ions and small organic fragments were observed. At higher masses, the spectra were dominated by silver halide cluster ions. Because of the absence of characteristic high m/z secondary ions, in both the positive and the negative mode, it was necessary to label the dye molecules, to permit their identification and localization at the surface of the silver halide microcrystals (by means of elemental ions). In these experiments, fluorine was used as a label. The lateral resolution of secondary ion images in ToF-SIMS is inherently linked to the ion yield.16,17 Therefore, the (16) Rulle, H.; Rading, D.; Benninghoven, A. Secondary Ion Mass Spectrometry SIMS X; Benninghoven, A. et al., Eds.; Wiley & Sons: Mu¨nster, Germany, 1995; pp 153-156. (17) Hagenhoff, B. Mikrochim. Acta 1999, 132, 259-272.

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Figure 7. Secondary ion images (F- and Br-) of cubic crystals (edge length 0.8 µm), covered with 80% zwitterionic dye (suspension 5).

Figure 8. Secondary ion images (F- and Br-) of cubic crystals (edge length 0.8 µm), covered with 40% zwitterionic dye (suspension 6).

incorporation of fluorine in the organic dyes allows the Fions to be generated with high yield, and this in turn offers the perspectives to retrieve the dyes at the surface of silver halide microcrystals. Moreover, there were no alternative sources of fluorine in the samples (apart from the fluorinated counterion in the samples where dye 1 was used as a sensitizing agent), so that the fluorine signal could be directly assigned to the dyes. Other research groups have also used fluorine-labeled organic molecules for submicrometer imaging ToF-SIMS experiments. Steiger has shown some images of fluorinelabeled dyes adsorbed on cubic AgBr crystals with an EL of 1.25 µm.18 Frisbie et al. on the other hand used fluorinelabeled molecules to image self-assembled monolayers (SAM layers) on a gold surface.19 In the subsequent part, four different cases are presented where ToF-SIMS imaging of fluorine-labeled dyes has been used to study the behavior of those dyes at the surface of silver halide microcrystals. Carbocyanine Dye Adsorption on Cubic Crystals. The first aim of this part of the study was to investigate the homogeneity of dye adsorption over a crystal population. A homogeneous dye distribution is desired for optimal photographic response. The second aim was to study the selectivity of dye adsorption at edges and corners of cubic crystals. The study of the homogeneity of dye distribution was performed on suspension 5 (dye 2). The secondary ion images of this sample (Figure 7) indicate that the dye is equally distributed over the crystal population. There is a perfect match between the F- image (representative for the dye) and the Br- image (representative for the AgBr crystals). There are no differences in F- intensities when different microcrystals in one analysis field are compared. Moreover, the dye covers the entire crystal surfaces; that is, there is no indication of preferential adsorption at the edges of the cubic crystals. In a second sample, the amount of dye 2 added to the suspension was decreased to an amount equivalent to a 40% surface coverage (suspension 6). The secondary ion images of this sample again indicate that the dye remains homogeneously spread over the total crystal population (data not shown). In the high-resolution secondary ion images where individual crystals are studied, it can be observed that the dye molecules are no longer adsorbed over the entire microcrystal surface. As seen in Figure 8, they tend to become adsorbed preferentially at the edges of the cubic microcrystals, while the {100} planes remain uncovered. There are two possible explanations for this region-selective dye adsorption. On one hand, it can be explained by differences in crystal plane orientation

between the edges and the major planes of cubic silver halide crystals. As was mentioned before, the slightly rounded edges of the cubic crystal form a {111} or a {110} silver bromide surface, while the planes form a {100} silver bromide surface. This difference in crystal surface type can influence the interaction with the dye molecules. On the other hand, the number of defects in the silver bromide lattice can play a significant role. There are more defects at edges and corners of cubic crystals than at the cubic planes. Possibly, the dye aggregates are preferentially nucleated at these sites.20 Adsorption Selectivity of Carbocyanine Dyes for Cubic versus Octahedral Microcrystals. The study of differences in adsorption selectivity as a function of crystal morphology is important for applications where different types of microcrystals are present in the suspension or when selective adsorption on a certain side is required. In these experiments, cubic crystals and octahedral crystals with a mean edge length of 0.4 µm were used. ToF-SIMS imaging was able to distinguish between these two types of crystals based on the secondary Brimages. The nonidentical angle of incidence of the primary beam on the different faces of the crystals results in different secondary ion yields (contrast) and creates a three-dimensional effect that makes the morphology of the crystals clearly visible in the image.20 While the cubic crystals have an intense square top face and two less intense side faces, the octahedral crystals are characterized by two triangular faces, an intense one and a less intense one (cf. Figure 9, parts A and B). In a mixture of cubic and octahedral crystals, the latter has typically lower secondary Br- ion intensities. For these experiments, a mixture of suspensions 2 and 3 was prepared; the number of cubic and octahedral crystals was adjusted so that both crystal surfaces were available for dye adsorption in a 1/1 surface proportion. The dye solution was added to this mixture in an amount equivalent to a 40% surface coverage. The mixture was then stirred for 30 min at 40 °C. The adsorption selectivity was studied for both dye 1 and dye 2. If the dye solution is added to a suspension containing only one type of microcrystal, it results in a homogeneous coverage of the dye over the entire crystal population. This is the case for both dye 1 (cationic dye) and dye 2 (zwitterionic dye). The low-resolution images (15 × 15 µm2) of the mixed suspension make it clear (data not shown) that both dyes are no longer homogeneously spread over the entire population. One can already assume that there has been preferential dye adsorption on one of the two crystal morphologies. The high-resolution images

(18) Steiger, R. Chimia 1994, 48, 444-446. (19) Frisbie, D.; Wollman, E.; Wrighton, M. Langmuir 1995, 11, 25632571.

(20) Verlinden, G. Three-dimensional chemical characterization of complex silver halide microcrystals using secondary ion mass spectrometry. Ph.D. Thesis, University of Antwerp, Antwerpen, 1999.

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Figure 9. Secondary ion images (F- and Br-) of a mixture of suspension 2 and suspension 3 covered with cationic dye (A) and with zwitterionic dye (B). The estimated dye coverage in both cases is 40%.

Figure 10. High-resolution secondary ion image of a mixture of suspension 1 and suspension 3 covered with 40% zwitterionic dye.

shown in Figure 9A clearly indicate that there is a correlation between the F- image of dye 1 and the Brimage of the octahedral crystals. The same correlation is retrieved for dye 2 (cf. Figure 9B). The cubic crystals in Figure 9A,B remain blank; no dye molecules are adsorbed on their surface. It can be concluded that the zwitterionic as well as the cationic dye molecules adsorb preferentially at the {111} silver bromide surface of the octahedral crystals, while the {100} surfaces of the cubic crystals remain uncovered. Because of the small dimension of the microcrystals (EL of 0.4 µm), it was not feasible to observe any preferential dye adsorption at the edges of the cubic crystals. To overcome this problem, a mixture of suspension 1 and suspension 3 was prepared. In this case, only the zwitterionic dye adsorption selectivity was studied (cf. Figure 10). The results confirm those of the previous experiment (cf. Figure 9B). It can be seen that three octahedral crystals at the top of the image are homogeneously covered by the zwitterionic dye molecules. The larger cubic crystal at the bottom of the image shows some increase in the F- intensity at the edges, while the planes remain blank. Both experiments (cf. Figure 9B and Figure 10) point out that the zwitterionic dye preferentially adsorbs at the {111} surface of silver bromide crystals. The strong interaction of these sites with the dye molecules allows selective dye sensitization in a suspension that contains a mixture of different crystal morphologies. Exchange of Carbocyanine Dyes between Cubic and Octahedral Crystals in a Gelatine Matrix. To test the exchange of dyes between cubic and octahedral microcrystals, a suspension of cubic AgBr microcrystals covered with dye (80% theoretical surface coverage) was mixed with a blank suspension of octahedral AgBr microcrystals. To permit the dye molecules to exchange between the crystal surfaces of different morphologies, the mixture was stirred for 4 h at 50 °C. Since only cubic and octahedral crystals with an EL of 0.4 µm were used

Figure 11. Secondary ion images (F- and Br-) after thermal treatment (4 h at 50 °C) of a mixture of suspension 8 and suspension 3.

Figure 12. Secondary ion images (F- and Br-) after thermal treatment (4 h at 50 °C) of a mixture of suspension 7 and suspension 3.

in these experiments (suspensions 3, 7, and 8), the behavior of dye molecules adsorbed at the edges of the cubic crystals could not be monitored. The secondary ion images in Figures 11 and 12 display a different exchange behavior for the zwitterionic and the cationic dye; therefore, both experiments will be discussed separately (vide infra). In the process of dye exchange, three steps can be distinguished: desorption of the dye from the microcrystal surface into the gelatine matrix, transport of the dye molecules through the gelatine medium, and readsorption at the surface of the silver bromide microcrystals. Important parameters4 influencing the rate of these three steps are temperature, solubility, aggregation state, and cosolvent. At the starting point of these experiments, the dye molecules are adsorbed at the surface of the cubic crystals as large aggregates.3,21 To transport the dye through the gelatine medium, monomers, dimers, or small dye aggregates have to be released from the microcrystal surface. The dye molecules then diffuse through the gelatine (21) Ceulemans, T. An electron resonance study of the charge transfer kinetics at the surface of spectrally sensitized silver halide microcrystals in photographic emulsions. Ph.D. Thesis, University of Antwerp, Antwerpen, 1997.

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Figure 13. Left: Secondary ion images (F- and Br-) of suspension 9 (5% dye coverage). Right: Secondary ion images (F- and Br-) of suspension 10 (50% dye coverage).

medium and renucleate on another silver halide crystal surface to form a new supramolecular aggregate. This implies that not all the dye molecules are adsorbed on a crystal surface when the samples are prepared for ToFSIMS analysis; some of them remain in the gelatine matrix, but this matrix is removed during the sample preparation. The background fluorine signal, which is detected in the secondary ion images, can result from rests of the gelatine matrix or from small amounts of dye molecules that desorb from the microcrystal surface after the dilution process. When the exchange capacity (at 50 °C) for the cationic dye (dye 1) is examined for suspensions 3 and 8, the secondary ion images show that the dye is homogeneously distributed over the total crystal population (cf. Figure 11). This indicates that during this thermal treatment of 4 h, the dye molecules desorb from the surface of the cubic crystals, but they do not selectively readsorb at the surface of the octahedral crystals. The secondary ion images recorded from this sample show a rather diffuse fluorine image; however, the individual microcrystals can still be seen separately. To have an indication of the stability of the dye during the thermal treatment, UV-vis diffuse reflectance spectroscopy was used. This technique has often been used to study dye aggregation at silver halide surfaces.22-24 The measurements on this sample indicate that the dye molecules are unaffected by the treatment; the absorption maximum of the aggregates remains unchanged. Diffusion of the fluorinated counterion in the gelatine matrix may be a possible explanation for the diffuse fluorine image that is recorded; this diffusion does not affect the light absorption characteristics of the dye molecules. It must be taken into account that these measurements do not prove that the aggregation of the dyes remains unchanged. In the second experiment (cf. Figure 12), the exchange of the zwitterionic dye (dye 2) between cubic and octahedral crystals was studied under the same conditions. The lowresolution images of this sample indicated that the dye molecules were heterogeneously distributed over the crystal population after the 4 h thermal treatment. The high-resolution images revealed that the octahedral crystals were covered with zwitterionic dye molecules, while the cubic crystals had lost their coverage. This result pointed out that the zwitterionic dye molecules desorbed from the {100} surface of the cubic crystals, were transported through the gelatine medium, and selectively adsorbed at the {111} surface of the octahedral crystals. (22) Kawasaki, M.; Ishii, H. J. Imaging Sci. Technol. 1995, 39, 210221. (23) Janssens, G.; Gerritsen, J.; Van Kempen, H.; Deroover, G.; Callant, P.; Vandenbroucke, D.; De Keyzer, R. Proceedings Int. Congress on Imaging Science track 1, Antwerp, 1998. (24) Watanabe, S.; Tani, T. J. Imaging Sci. Technol. 1995, 39, 8185.

Figure 14. Secondary ion images of a mixture of tabular crystals covered with zwitterionic dye (80%) and blank tabular crystals, after a thermal treatment at 60 °C for 4 h.

It can be concluded that dye adsorption of the zwitterionic dye on an octahedral AgBr crystal surface is favored, even when the dyes are originally adsorbed on the surface of a cubic crystal. For the cationic dye, the interaction with the octahedral crystal surface seems to be less favored. Dye Adsorption on Tabular Ag(Br,I) Crystals. Tabular Ag(Br,I) crystals used in this study are characterized by large {111} planes and therefore have the same surface orientation as octahedral microcrystals. The major difference is the presence of iodide in the top layer; this iodide may influence the interaction with the adsorbed dye molecules. To examine this interaction, two types of experiments were carried out. First, the homogeneity of dye depositions was checked for various amounts of dye solution added to the suspension. In a second experiment, the possibility of dye exchange between covered and blank tabular crystals in a gelatine medium was studied. Because of the problems with the cationic dye during thermal treatment (discussed in the previous section), only the zwitterionic dye was used in this study. In the first experiment, three different suspensions were studied: suspensions 9, 10, and 11. Figure 13 presents the secondary ion images of the first two suspensions. As can be seen in the image on the left, the dye is heterogeneously distributed over the total crystal population when an amount of dye equivalent to a 5% surface coverage is added. The tabular crystal in the center of the image shows a clear fluorine image, while the surrounding crystals remain blank. This result points to an experimental problem: the dye molecules are adsorbed very fast on the crystal surface, so that they do not have the opportunity to dissolve in the suspension. This can be due to local supersaturation, resulting in very fast nucleation of the dye, when a drop of the dye solution is added to the suspension. As can be seen below (cf. Figure 14), once the dye molecules are adsorbed, they are immobilized by the strong interaction with the crystal surface and no longer have the opportunity to redistribute themselves over the crystal population.

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The experimental problem was less pronounced when larger amounts of dye were added. In the second sample where enough dye solution was added to cover 50% of the crystal surface, homogeneous dye coverage over the population was observed. This result points out that when higher dye concentrations are added to the suspension, not all the dye molecules are adsorbed at the surface at once. The results of the third sample (80% dye coverage) are comparable with those of suspension 10 and will not be shown in detail. To visualize whether the dye molecules are irreversibly bound to the tabular crystal surface, a mixture of suspension 11 and suspension 4 was thermally treated. Several conditions were tested ranging from 1 h stirring at 40 °C to 4 h at 60 °C. As can be seen in Figure 14, even the most severe conditions do not cause a redistribution of the dye molecules. Only four crystals in the imaged field are covered with dye; the others remain blank. These results show that dye adsorption is irreversible, which is comparable with our observations for the octahedral crystal surface. IV. Conclusion It has been demonstrated that ToF-SIMS is a suitable technique for studying the adsorption behavior of fluorinated cyanine dyes at the surface of silver halide microcrystals. The ion images allowed direct localization of these dyes at the surface of microcrystals in the size range of 0.4-1 µm. A lateral resolution of 65 nm could be attained

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with the Ga+ liquid metal ion gun, because of the sufficient ion yield of the fluorine label. Note that only 6000 dye molecules are available in a monolayer within an area of 6000 nm2 (corresponding to 1 pixel). As a consequence, recording high lateral resolution images of the dyes by monitoring molecular (fragment) ions has proven to be unfeasible at the present state of the art. Therefore, fluorine-labeled dyes have been studied to exploit the high yield of the F- atomic ion. The imaging capabilities have made it possible to distinguish between octahedral and cubic crystals with an edge length of 0.4 µm. They even allowed observing preferential adsorption of the fluorine label at the edges of cubic crystals. In this way, ToF-SIMS has proven to be a valuable tool for studying adsorption selectivity of fluorinated dyes as a function of crystal morphology; it has also been shown that this technique can be used to image dye exchange experiments in a direct way on octahedral, cubic, and tabular microcrystals. Hence, the method is foreseen to be of great use for the further refinement of photographic materials. Acknowledgment. This work is supported by the Federal Services of Scientific, Technical and Cultural affairs (DWTC/SSTC) of the Prime Minister’s Office (IUAP IV. Conv. P4/10) and by the Flemish Fund for Scientific Research (FWO-Vl). LA010862T