J. Phys. Chem. 1995, 99, 17198-17206
17198
Morphological Evolution of Chloroaluminum Phthalocyanine Thin Films Followed in Situ by Atomic Force Microscopy F. Santerre, R. Cbte, G. Lalande, L. Gastonguay,+ D. Guay,* and J. P. Dodelet" INRS-Energie et Mattriaux, 1650 MontCe Ste-Julie, C. P. 1020, Varennes, QuCbec, Canada J3X I S 2
L. T. Weng and P. Bertrand PCPM, UniversitC Catholique de Louvain, Place Croix du Sud, I B-1348 Louvain-la-Neuve, Belgium Received: April 17, 1995; In Final Form: September 9, 1995@
Chloroaluminum phthalocyanine (ClAlPc) thin films, vacuum sublimed on Sn02-covered glass substrates, were exposed to KC1 solutions of various pH values (from 2.0 to 8.0) in the fluid cell of an atomic force microscope (AFM).The evolution of the surface morphology of the film was followed in situ. Complementary experiments on the film morphology were also performed by scanning electron microscopy. The chemical evolution of the film surface was obtained by time-of-flight secondary ion mass spectrometry. On the one hand, when ClAlPc is immersed in KCl solutions at pH = 2.0 or 3.0, a slow dissolution of the outermost surface of the film occurs. This is followed by the hydrolysis of ClAlPc in solution to form first the hydroxyaluminum phthalocyanine (HOAlPc) and then the p-oxo-dimer, PcAlOAlPc. The latter recrystallizes as long needles on top of the initial film, thereby changing drastically its surface morphology. On the other hand, when ClAlPc is immersed in KC1 solutions at pH = 5.7 or 8.0, the hydrolysis of ClAlPc occurs at the surface of the film without noticeable morphological modifications. A subsequent immersion of the film in KC1 at pH = 2.0 may have the same effect on the film morphology as the one previously described for pH = 2.0 or 3.0, as long as mechanical energy is provided to the film, either by ultrasound or by the cantilever tip of the AFM. When the latter is involved, the source of mechanical energy is localized in space and patterning of the surface morphology becomes possible.
Introduction Phthalocyanines are molecular pigments which attract a lot of attention owing to their possible applications in extremely diversified areas of interest.'.2 Over the past few years, we have studied the electrical properties of these pigments and their use as semiconductors in photoactive devices3 Recently, we concentrated our attention on A1 phthalocyanine derivatives$-'5 paying special attention to the relation between the structure of the chloroaluminum phthalocyanine (ClAlPc) film, which is a p-type semiconductor, and its photoelectrochemical properties. Along the course of these studies, we have demonstrated that it is possible to improve drastically the photoactivity of ClAlPc films by immersing them into acidic solutions (pH = 2.0 or 3.0) of salts like KCl, KBr, or KI. Upon immersion, the partial hydrolysis of bulk ClAlPc occurs, transforming it into hydroxyaluminum Phthalocyanine (HOAlPc). Anions (Cl-, Br-, I-, 13-) are also incorporated in large number in the film, up to a proportion of about one anion for five macrocycles. The films thus transformed are characterized by an important red shift of the Q absorption band, with a maximum at wavelengths above 800 nm. This is the region of interest in electrophotography. Also, the photoactivity of the films under white light illumination improves by a factor of up to four and photocurrents above 1 mA cm-2are reached in simulated solar conditions. The modification of the ClAlPc film properties by their immersion in acidic solutions of salts was always accompanied by drastic changes in their morphology. The reasons of the morphological changes were never understood despite electron
* To whom correspondence should be addressed.
' Present address: Institut de Recherche d'Hydro-QuCbec, DCpartement de chimie des materiaux. C. P. 1000. Varennes, Quebec, Canada J3X 1S1. @Abstract published in Admnnce ACS Absrructs, November 1. 1995.
microscopy studies. An in situ technique was required to unravel the complexity of this morphological transformation. Over the past few years, the advent of atomic force microscopy (AFM)has revolutionized the field of surface science. This is so because AFM allows almost routinely the determination of the surface structure of nonconducting surfaces at the nanometer scale. Moreover, this determination can be pursued with the sample immersed in a liquid solution. This peculiarity, coupled to the fast imaging capability of most commercial apparatus, has yielded to direct in situ observation of crystal growth from s ~ l u t i o n . ' ~ - ' ~ This paper reports an in situ and real time AFM study of the evolution of the surface morphology of chloroaluminum phthalocyanine thin films immersed in KCl solutions of various pH values. Scanning electron microscopy (SEM) was used as a complementary technique. Films have also been studied by time-of-flight secondary ion mass spectrometry (ToF SIMS) to determine the nature of the different chemical species found at their outermost surface. Evidence that the change in the surface morphology of the film is due to a solvent-mediated recrystallization process is presented and a detailed mechanism is proposed. The possibility of creating microscopic patterns of various morphologies on the surface of the ClAlPc film is also highlighted.
Experimental Section Film Preparation. The synthesis of chloroaluminum phthalocyanine (ClAlPc) has been reported elsewhere.4 The crude phthalocyanine was purified twice by vacuum sublimation before being used for the preparation of thin films. Thin ClAlPc films with thickness of about 200 nm were obtained by vacuum sublimation of the purified phthalocyanine (base pressure < 1
0022-3654/95/2099-17198$09.00/0 0 1995 American Chemical Society
Chloroaluminum Phthalocyanine Thin Films x Torr). The sublimation temperature of ClAlPc was set at 350 “C in order to provide a growth rate of about 200 nm min-I.l2 The films were deposited onto conducting SnO2 substrates (ps = 25-30 Q sq-’). Prior to the sublimation, the substrates were cleaned with methanol and soaked 30 min in sulfochromic acid cleaning solution. They were then thoroughly rinsed with distilled water and dried under a N2 stream. Film Transformation. Demineralized water was used throughout this study, with a specific resistivity larger than 18.0 MQ cm. Potassium chloride (KCl, A.C.S. reagent from Aldrich) was used. The pH of the acidic solutions (pH = 2.0 and 3.0) was adjusted with HCl (A.C.S. reagent from Fisher). The pH of the basic solution (pH = 8.0) was adjusted with NaOH (A.C.S. reagent from Aldrich). Five different types of film were studied: as-sublimed ClAlPc and ClAlPc films immersed for 60 min in a 0.1 M KCl solution at pH = 2.0, 3.0, 5.7 (demineralized water and KCl only), or 8.0. In each case, and unless otherwise specified, a freshly sublimed film was immersed in the solution of interest. Some specific experiments were conducted by immersing successively a given film in more than one solution. Atomic Force Microscopy. The contact-mode AFM was a Nanoscope I11 (Digital Instruments, Inc., Santa Barbara, CA). All the experiments were done in the constant-force mode with a scanning head having a maximum scanning range of 15 pm. Most of the experiments were performed with a scanning rate of 2 Hz. Typically, one image (one frame) was acquired every 5 min or so. The fluid cell supplied by the manufacturer was used to perform the imaging with the sample immersed in the electrolytic solution. The volume of the fluid cell is less than 1 cm3. The solution in the fluid cell could be changed without drying out the sample through tubes connected to the cell and a reservoir containing the solution. Commercially available Si3N4 tips (nanoprobes supplied by Digital Instruments) mounted on a cantilever were used throughout this study. Unless otherwise specified, the cantilever with the largest spring constant was used (0.58 N/m). In most of the experiments, prolonged scanning of the cantilever tip on the film surface was performed. We found that there is considerable and unavoidable drift of the applied force during that period. Owing to that fact, and even though the authors are aware that the force constants of the tips bear sometimes little relation to the manufacturer’s nominal rating, no attempt was made to precisely determine the spring constant of the cantilever. Instead, the values quoted by the manufacturer were used. The force exerted by the cantilever tip on the surface was obtained by averaging the force measured before and after the scanning experiment. In most of the experiments the force of the cantilever tip on the surface of the film was always larger than 100 x N, which is a quite high imaging force. The images shown herein are really 3-D pictures of the sample surface, with the left to right (fast scanning direction) and upper to lower (slow scanning direction) image directions corresponding to the X and Y axis of the sample, respectively. The gray scale encoding displayed on the right-hand side of each figure corresponds to the Z axis. The images have not been filtered or processed, except for the subtraction of a “polynomial plane” which removes the image bow and tilt. ToF SIMS. The surface of the various films was probed by ToF SIMS. These measurements were performed with a ToF SIMS spectrometer from Charles Evans and Associates.20*21 In the ToF SIMS experiments, the surface of the sample was bombarded with 15 keV Ga+ ions. The secondary ions were first accelerated to f 1 0 keV by applying a bias on the sample
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Figure 1. Scanning electron micrographs of as-sublimed Cl AlPc film (A) and ClAlPc film chemically transformed by immersion for 60 min in a 0.1 M KCl solution at pH = 2.0 (B), 3.0 (C), 5.7 (D), or 8.0 (E). In this figure, a length of 4 mm corresponds to 100 nm.
and on the extraction lens. The secondary ions were then deflected through three electrostatic analysers. A 5 keV postacceleration step was then applied in order to increase the sensitivity at high masses (>200 amu). The analyzed area corresponds to a square of 100 by 100 pm2. With a data acquisition time of 10 min on the 0-5000 amu mass range, the total ion dose on the sample is less than 10I2ions cm-2 (static conditions).22 No charge compensation was needed. Data acquisition and analysis were performed using the software available from Charles Evans Associates.20 The best mass resolution obtained with this equipment is M/AM = 11 OOO at mass 28 amu on a Si wafer. Two measurements were recorded on the same film and the data shown herein are the mean average of these two trials. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was performed on a JEOL JSM 6300F apparatus, operated at 15 kV. All the samples were coated with a thin
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17200 J. Phys. Chem., Vol. 99, No. 47, 1995
film of gold (thickness of -50 A). The magnification of all micrographs shown herein is 40 OOOx .
Results Scanning Electron Microscopy. Figure 1 displays the scanning electron micrographs of an as-sublimed ClAlPc (A) and ClAlPc chemically transformed by immersion for 60 min in a 0.1 M KCl solution at pH = 2.0 (B), 3.0 (C), 5.7 (D), and 8.0 (E). Figure 1A shows that the as-sublimed ClAlPc has a clustered and grainy appearance. The average diameter of the structures observed in Figure 1A is roughly 50 nm. The lateral dimension of the grains observed in Figure 1A varies with the growth rate of the film, being larger for slower sublimation rate.'* From previous scanning and transmission electron microscopy studies6*11J4 it is known that the film is made of densely packed columnar grains with both crystalline and amorphous regions. As the film grows, the grains extend perpendicularly to the substrate surface, so that the surface morphology is practically independent of the film thickness.'* The pH of the solution in which ClAlPc is immersed has a profound influence on the surface morphology of the film. In acidic solutions (pH = 2.0 and 3.0, Figure 1, B and C, respectively), long needles are seen at the surface of the film. They are randomly oriented, with their long axis lying parallel to the substrate surface. These crystallites are about 50-70 nm wide and more than 1pm long. There seem to be no marked differences between the morphology of these two films. However, when ClAlPc films are immersed in solutions at pH = 5.7 (Figure 1D) or 8.0 (Figure lE), the initial morphology of the film is only slightly modified. The pH of the immersing solutions is therefore one of the parameters which determines the resulting morphology of the film. As will be shown later on, it is also of interest to consider what happens to the film morphology when ClAlPC is successively immersed in solutions of different pH. Figure 2, A and B, depicts the morphology of ClAlPc films immersed successively in two 0.1 M KCl solutions, the first one at pH = 8.0 for 1 h and the second one at pH = 2.0 for 5 min. Even so, there is a large difference in the morphology of the films shown in Figure 2A,B. This is the result of performing the transformation step of ClAlPc at pH = 2.0 under sonication (Figure 2A), while the solution containing the film was allowed to stand still for Figure 2B. The characteristic needles of Figure 2A (with sonication) are the same as those seen in Figure 1, B and C. On the other hand, Figure 2B (without sonication) depicts a morphology similar to that of Figure 1, A or E. A film immersed only in a 0.1 M KCl solution at pH = 2.0 for 5 min (Figure 2C) has the same morphology as the one presented in Figure 2A and resembles most closely that of the film which has been immersed in that same solution a for longer period of time (Figure 1B). It is clear from these observations that the history of the film has a marked influence on its resulting morphology. Atomic Force Microscopy. Figure 3 shows the surface topography of a ClAlPc thin film immersed in a 0.1M KCl solution at pH = 5.7. The film has a grainy appearance. The crystallites in Figure 3 show a tip shape artifact. The average diameter of the grains is about 75 nm. This image of the surface morphology is similar to the micrograph obtained by SEM (Figure 1A). The area in Figure 3 was scanned continuously during 20 min without showing any apparent modification. No effect of the prolonged scanning period was noticed when the size of the scanned region was increased. Figure 4 shows a sequence of images which were recorded continuously after changing the pH of the solution from 5.7 to
Figure 2. Scanning electron micrographs of ClAlPc films which have been sequentially immersed in a 0.1 M KCl solutions of various pH: 60 min at pH = 8.0 followed by 5 min at pH = 2.0 and sonication (A), 60 min at pH = 8.0 followed by 5 min at pH = 2.0 but without any ultrasonic agitation (B), 5 min at pH = 2.0 without previous prolonged immersion in a basic solution (C). In this figure, a length of 4 mm corresponds to 100 nm.
2.0. The change of the pH was done by allowing 20 cm3 of a 0.1 M KCl solution at pH = 2.0 to flow through the small volume of the cell still in place in the microscope. For the sake of conciseness, only a few images of the series have been reproduced. Figure 4B is the first image of the ClAlPc film recorded immediately after changing the solution. It is denoted t = 5 min €?om the time at the end of the scanning period. This image shows a number of streaks oriented in the fast scanning direction. These streaks are not observed in either the subsequent images, or in Figures 3 and 4A, which were taken in 0.1 M KCI at pH = 5.7 (t = 0 min). In Figure 4B also a few new elongated crystallites are observed lying on top of the film. Their long axis is more or less parallel to the surface of the film. In the
J. Phys. Chem., Vol. 99, No. 47, 1995 17201
Chloroaluminum Phthalocyanine Thin Films
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Figure 3. AFM image of a 6 by 6 pm area of a ClAlPc film immersed in a 0.1 M KCI solution at pH = 5.7. This area of the film was scanned continuously for 20 min without showing any apparent modification.
subsequent image (Figure 4C, t = 22 min), the surface of the film is covered with more crystallites and most of the previously observed ones in Figure 4B have increased in size. With further scanning (Figure 4D, t = 31 min), the number of crystallites does not increase any more and their size remains fairly constant. At the end of their growing cycle, some of the crystallites have a length of more than 1 pm and a lateral dimension up to about 200 nm. Following the sequence of images depicted in Figure 4, the 6 pm by 6 pm scan size was increased to 12 pm by 12 pm (Figure 5). The image thus obtained shows the previously scanned area, with its characteristic surface morphology, surrounded by a film whose morphology is identical to that of the as-sublimed CIA1Pc su$ace. This observation is quite unexpected. It suggests that the morphological modification of the ClAlPc film in the middle of Figure 5 is the result of the interaction of the AFM tip with the surface of the ClAlPc film. In order to observe this peculiar behavior, it is necessary first to fill the fluid cell containing the film with a neutral or slightly basic solution (pH = 8.0 or 5.7, respectively) and then to replace it with a more acidic one (pH = 2.0). If the first step of this double-dipping procedure is ignored, i.e. if the film is directly immersed in the KCl solution at pH = 2.0, then the AFM image anywhere on the surface of the film looks like Figure 4C or 4D. In this respect, the AFM results are totally consistent with those obtained by SEM. The AFM result of Figure 5 bears some resemblance to the consecutive dipping experiments described previously. The double dipping experiment consisting of an immersion in a solution at pH = 8.0 and then in a solution at pH = 2.0 without sonication (Figure 2A) induces a film morphology like that observed in the outermost portion of Figure 5. On the other hand, the double dipping experiment of ClAlPc with sonication (Figure 2A) induces a film morphology like that of the center part of Figure 5. Therefore, the interaction of the cantilever tip with the surface of the film during the double dipping experiment has the same effect on the surface morphology as does the ultrasonic agitation of the substrate in a similar
experiment. In the former case, however, the new phase characteristic of the morphological modification appears only in the region of the surface that is scanned by the tip and the effect is spatially localized. ToF SIMS. Figures 6 and 7 show the positive ion ToF SIMS spectra of ClAlPc thin films after the various chemical treatments: as-sublimed ClAlPC film (A) and ClAlPc after immersion in 0.1 M KCl solution at pH = 2.0 (B), 3.0 (C) and 5.7 (D). Figures 6 and 7 display the 500-600 amu and the 10801120 amu regions, respectively. All molecular ions containing at least one complete phthalocyanine macrocycle are above 500 amu. It is the region of interest for chemical identification of the molecular species found at the surface of the film. All the peaks are more or less arranged in clusters around a major component. In a given cluster, all these peaks differ by one or more H atoms. The peaks at 525.57, 539.54, 552.56, 556.52, 574.96, and 1095.02 amu are assigned to C32H&J,Al+, AlPc+, CHAlPc+, HOAlPc+, ClAlPc+, and PcAlOAlPc+ molecular ions, respectively. HOAlPc, ClAlPc, and PcAlOAlPc (the p-oxo-dimer) are the only molecular species found at the surface of the film. The intensities of the HOAlPc+, ClAlPc+, and PcAlOAlPc+ signals vary with the experimental procedure used to modify the films. In Figures 6 and 7, the peak assigned to AlPc+ (located at 539 amu) is the most intense, being at least 1 order of magnitude higher for any particular conditions than the peaks assigned to the other molecular ions. We used that peak as an internal reference to calibrate the intensity of the molecular ions. Figure 8 shows the histogram obtained when the intensity of each molecular ions is normalized with respect to the intensity of the AlPc+ peak for a given chemical treatment. Within this representation, the normalized intensity of the HOAlPc+ ion is fairly constant and independent of the parameters of the chemical treatment procedure. The normalized intensity of the ClAlPc+ molecular ion decreases steadily as the pH of the aqueous solution is increased. In the case of the PcAlOAlPc+ dimer, the normalized intensity goes through a maximum at pH = 3
Santerre et al.
17202 J. Phys. Chem., Vol. 99, No. 47, 1995
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Figure 4. A F M images of the surface of a ClAlPc thin film immersed in a 0.1 M KCI solution at pH = 2.0. The images were obtained by scanning continuously the film. From B to D, the time elapsed since the beginning of the experiment is 5.22, and 31 min, respectively. Figure 4A was taken prior to the introduction of the pH = 2.0 solution, while the film was immersed in 0.1 M KCI at pH = 5.7, in order to perform the necessary alignment.
and decreases by more than 1 order of magnitude when the pH of the solution is 5.7. The fact that the normalized intensity of the HOAIPc+ ion is nearly invariant with the chemical treatment needs some more explanations. The HOAIPc+ signal has two origins: the HOAlPc molecule and the p-oxo-dimer which fragments to yield HOAIPc+. At pH = 5.7, the HOAlPc+ signal arises mainly from HOAlPc, with only a small contribution from PcAIOAlPc. However, in more acidic solutions, the contribution of the dimer to HOAlPc+ becomes more important while that of HOAlPc decreases. We believe it is these two opposite trends that yield the observed behavior for the normalized intensity of the HOAIPc+ ion. Therefore, it can be concluded that the surface of films treated in acidic solution is mainly composed of dimeric phthalocyanine (PcAIOAlPc), while HOAlPc is the major component of the surface of the films treated in near neutral solution. HOAlPc is also the major component of the as-sublimed ClAlPc film, which are always in contact with the water vapor in ambient air. This variation of the surface chemical composition with the conditions of the chemical treatment will prove to be extremely useful in understanding the mechanism by which the treatment affects the morphology of the surface of the film.
Discussion The SEM micrographs of the chemically transformed ClAlPc have shown that the evolution of the surface morphology varies with the pH of the aqueous solution in which the films are immersed. For acidic solutions (pH = 2.0 or 3.0), the morphology of the film changes drastically. The columnar growth appearance of the as-sublimed ClAlPc is replaced by needles lying randomly on the substrate surface. This change in the morphology does not occur for more basic solutions (pH = 5.7 or 8.0). In order to gain some insight in the mechanisms governing the evolution of the morphology, an atomic force microscope equipped with a fluid cell was used to perform in situ imaging in real time of the surface of the ClAlPc film during the transformation occurring when the film is immersed in an acidic solution. A number of observations made from Figures 3, 4, and 5 are worth discussing. Firstly, it is clear that the new phase, which is composed of elongated crystallites, is formed on top of the initial one which still appears underneath, especially at the beginning of the transformation. The morphological transformation is therefore restricted to the outermost layer of the surface, the region probed by ToF SIMS.
J. Phys. Chem., Vol. 99, No. 47, 1995 17203
Chloroaluminum Phthalocyanine Thin Films
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Figure 6. Positive ion time-of-flight SIMS spectra between 500 and 600 amu of the ClAlPc film after various chemical treatments: as-sublimed (A), after immersion for 60 min in a 0.1 M KCl solution at pH = 2.0 (B), 3.0 (C), or 5.7 (D). Secondly, as far as we can tell, the crystalline needles at the surface of the transformed film cannot be columnar grown crystallites of the as-sublimed ClAlPc film detached from the
substrate and lying on top of it without any change in their nature or their morphology. The length of the columnar grown crystallites of the as-sublimed films is 200 nm, 1 order of
17204 J. Phys. Chem., Vol. 99, No. 47, 1995
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magnitude shorter than the long crystallites observed on top of the film. More probably, the newly formed crystallites, which are seen to nucleate and grow in Figure 4, arise as a consequence of a slow dissolution and reprecipitation process of the film material. This process is often called solvent-mediated recrystallization, since the solvent aids the modification process by allowing the dissolution of the molecules and their subsequent recrystallization.*3 The solvation step allows for an entirely new structure to grow on top of the film surface. In the present case, the recrystallized species is not of the same chemical nature as the starting one. As a result, two questions must be answered: what is the chemical identity of the species which dissolves in the acidic
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solution and what is the driving force to make the dissolved species precipitate as a new phase on top of the initial one. ToF SIMS results help in answering these two questions. It is clear from Figure 8 that the surface composition of the as-sublimed ClAlPc film is already altered by its contact with ambient air. Indeed, ToF SIMS reveals that besides CIAIPc, there is also HOAlPc and some PcAlOAlPc in the outermost layer of the film. More alterations yet result from the chemical treatment of the films. In particular, there is a large decrease in the surface concentration of ClAlPc when the film is immersed in a near-neutral solution. We believe that this change in the surface composition is at the origin of the different behavior observed in Figure 2, B and C. In Figure 2B (first dipped in a solution at pH = 8.0 followed by a 5 min dip in a solution at pH = 2.0), the surface of the film is made primarily of HOAIPc. The rate at which the solvent-mediated recrystallization proceeds for that film is much lower than for the film modified by the experimental conditions used in Figure 2C (5 min immersion in a solution at pH = 2.0). According to ToF SIMS, the surface of the latter films contains a much higher fraction of CIAIPc. Keeping in mind that the needles appear as the result of a slow dissolution and recrystallization mechanisms, this difference can be explained by assuming that the solubility of ClAlPc in 0.1 M KCI at pH = 2.0 is higher than that of HOAlPc in the same solution. There are two other pieces of experimental evidence to support the fact that ClAlPc is slighty soluble in an acidic solution. Firstly, the UV-visible absorption spectrum of the solution used to initiate the morphological transformation displays an absorption peak at 677nm, whose full width at halfmaximum is 23 nm, typical of a dissolved phthalocyanine.
J. Phys. Chem., Vol. 99, No. 47, I995 17205
Chloroaluminum Phthalocyanine Thin Films
at pH 2.0 or 3.0
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Figure 9. Schematic description of the sequence of events leading to the nucleation and growth of PcAlOAlPc needlelike crystalline phase on top of the initial ClAlPc film.
Secondly, there is an overall decrease of the film absorbancy when the solution used to initiate the morphological transformation is stirred. This means that part of the material is lost in the solution. On the other hand, one would expect the very slow dissolution of HOAlPc to proceed at a higher rate if some kind of mechanical energy is provided to the film surface, helping the dissolution step. In that case, one would expect a larger number of elongated crystallites to appear on the surface. This is done by sonicating the substrate and Figure 2A shows that this procedure yields a surface morphology similar to that of a film directly immersed in the acidic solution (Figure 2C). The same result is also obtained by the localized interaction of the cantilever.tip with the surface of the film, as seen in the center portion of Figure 5. Regarding the nature of the driving force responsible for the recrystallization, it is worth noticing that films treated in acidic solutions have a very high proportion of dimeric phthalocyanine on their surface. It is therefore postulated that the dimer is associated with the newly formed needlelike crystalline phase which is observed solely for films having been immersed in an acidic solution. PcAlOAlPc would result from the reaction in acidic solution of the solubilized phthalocyanines, followed by its precipitation as needles on the film owing to its lack of solubility in these conditions. The mechanisms responsible for the apparition of the elongated crystallites at the surface of the film can thus be summarized by the sequence of reactions depicted in Figure 9. With this summary at hand, it is easier to explain what causes the morphological variety observed in Figure 5. The ClAlPc film in the AFM fluid cell is first contacted with a KCl solution at pH 5.7 (Figure 3). From ToF SIMS experiments, it is known that ClAlPc at the surface of the film disappears and is replaced
by HOAlPc. Then a KCl solution at pH = 2.0 replaces the previous one (Figure 4). We believe that the repeated interaction of the cantilever tip with the surface of the film acts like a localized input of mechanical energy that improves the dissolution of HOAlPc which otherwise would be very slow in these conditions. The dissolved HOAlPc reacts near the surface of the film to produce PcAlOAlPc which is insoluble in the prevailing conditions and crystallizes as needles on top of it. It is not clear at this point what is the exact nature of the interaction which is responsible for the increase in solubility but various hypotheses can be put forward: mechanical agitation of the solution in front of the film, dissipation of energy from the frictional forces between the tip and the film, and physical removal (scraping) of a layer of PcAlOH molecules helping their dissolution. When the scanning region is enlarged (Figure 5), the region that has not been continuously scanned to produce the sequence of images depicted in Figure 4 remains morphologically unchanged. It was scanned only once to generate an image like that shown in Figure 5 and little HOAlPc dissolution occurred during this single event. Repeated interaction between the cantilever tip and the film is required to generate enough HOAlPc in solution to result in the nucleation and growth of PcAlOAlPc needles.
Conclusion A combination of SEM, in situ AFM, and ToF SIMS has been used to unravel the complexity of the mechanisms responsible for the appearance of a new phase at the surface of a ClAlPc thin film when it is immersed in an acidic solution. The AFM results have clearly demonstrated that the new phase appears as a result of a dissolution and recrystallization process. The ToF SIMS results on the other hand have allowed the
17206 J. Phys. Chem., Vol. 99, No. 47, 1995 identification of the molecules found at the surface of the film and an understanding of why the morphological evolution of the film is hampered in a given set of experimental conditions. By doing so, we believe we have shed some light on the nature of the interaction between the cantilever tip and the surface of the film which is sometimes responsible for the formation of very unique features.
References and Notes (1) Thomas, A. L. Phthalocyanine Research and Applications; CRC Press: Boca Raton, FL, 1990. (2) Leznoff, C. C., Lever, A. B. P., Eds. Phthalocyanines, Properties and Applications; VCH Publishers: New York, 1989. (3) Simon, J.; Andre, J. J.; Molecular Semiconductors; SpringerVerlag: Berlin, 1985. (4) Guay, D.; CBtC, R.; Marques, R.; Dodelet, J. P.; Lawrence, M.; Gravel, D.; Langford, C. H. J. Electrochem. SOC.1987, 134, 2942. (5) Guay, D.; Dodelet, J. P.; CBtC, R.; Langford, C. H.; Gravel, D. J. Electrochem. SOC. 1989, 136, 2272. (6) Guay, D.; Veilleux, G.; Saint-Jacques, R. G.; CBtC, R.; Dodelet, J. P. J. Mater. Res. 1989, 4, 651. (7) Tourillon, G.; CBtC, R.; Guay, D.; Dodelet, J. P. J. Electrochem. SOC.1989, 136, 2931. (8) Guay, D.; Tourillon, G.; Gastonguay, L.; Dodelet, J. P.; Nebesny, K. W.; Amstrong, N. R.; Garrett, R. J. Phys. Chem. 1991, 95, 251. (9) Dodelet, J. P.; Gastonguay, L.; Veilleux, G.; Saint-Jacques, R. G.; CBtC, R.; Guay, D.; Tourillon, G. Proc. SPIE-Int. SOC. Opt. Eng. (Photochem. Photoelectrochem. Org. Inorg. Mol. Films) 1991, 1436, 38.
Santeme et al. (IO) Dodelet, J. P.; Tourillon, G.; Gastonguay, L.; CBte, R.; Ladouceur, M.; Flank, A. M.; Lagarde, P. J. Phys. Chem. 1992, 96, 7202. (11) Gastonguay, L.; Veilleux, G.; CBtC, R.; Saint-Jacques, R. G.; Dodelet, J. P. J. Electrochem. Soc. 1992, 139, 337. (12) Gastonguay, L.; Veilleux, G.; CBte, G.; Saint-Jacques, R. G.; Dodelet, J. P. Proc. SPIE-Opt. Mater. Technol. Energy Eflciency Solar Energy Conuersion XI: Photovoltaics, Photochem., Photoelectrochem. 1992, 1729, 13. (13) CBt6, R.; DCnes, G.; Gastonguay, L.; Dodelet, J. P. Proc. SPIE-Opt. Mater. Technol. Energy Eflciency Solar Energy Conversion XI: Photovoltaics, Photochem., Photoelectrochem. 1992, 1729, 222. (14) Gastonguay, L.; Veilleux, G.; Cbtt. R.; Saint-Jacques. R. G.; Dodelet, J. P.; Chem. Mater. 1993, 5, 381. (15) Guay, D.; CBt6, R.; Marques, R.; Dodelet, J. P.; Lawrence, M.; Gravel, D.; Langford, C. H. Proc. Electrochem. SOC. (Photoelectrochem. Electrosynth. Semicond. Mater.) 1988, 88-14, 287. (16) Hillner, P. E.; Manne, S.; Gratz, A. J.; Hansma, P. K. Ultramicroscopy 1990, 42, 1387. (17) Hillner, P. E.; Gratz, A. J.; Manne, S.; Hansma, P. K. Geology 1992, 20, 359. (18) Durbin, S. D.; Carlson, W. E. J. Cryst. Growth 1992, 122, 71. (19) Hillner, P. E.; Manne, S.; Hansma, P. K.; Gratz, A. J. Faraday Discuss. 1993, 95, 191. (20) Schueler, B.; Sander, P.; Reed, D. A. Vacuum 1990, 41, 1661. (21) Schueler, B. Microsc. Microanal. Microstruct. 1992, 3, 119. (22) Briggs, D.; Heam, M. J. Vacuum 1986, 36, 1005. (23) Davey, R. J.; Cardew, P. T.; McEwan, D.; Sadler, D. E. J. Cryst. Growth 1986, 9, 648.
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