2508
Langmuir 1998, 14, 2508-2514
Nanomodification of Polypyrrole and Polyaniline on Highly Oriented Pyrolytic Graphite Electrodes by Atomic Force Microscopy X. W. Cai, J. S. Gao, Z. X. Xie, Y. Xie, Z. Q. Tian, and B. W. Mao* State Key Laboratory for Physical Chemistry of Solid Surfaces, Institute of Physical Chemistry, Department of Chemistry, Xiamen University, Xiamen 361005, China Received June 24, 1997. In Final Form: November 3, 1997 This paper describes the first observation of localized electropolymerization of pyrrole and aniline on highly oriented pyrolytic graphite (HOPG) substrates under atomic force microscopy (AFM) tip-sample interactions. A scanning or oscillating AFM tip, providing the horizontal scratching force and the vertical tapping force, is essential as the driving force for the surface modification with the conducting polymer. The significant tip effect on the electropolymerization has been discussed on the basis of the electropolymerization mechanism. It has been shown that under the AFM tip interaction, the electropolymerization can be blocked on the bare HOPG substrate or enhanced on the as-polymerized film. The localized electropolymerization in selected surface areas enables the nanomodification of lines, square platforms, or hollows of polypyrrole (PPY) and polyaniline (PAN) on the substrates. The result indicates that AFM can be used as a unique tool for nanofabrication of conducting polymers.
Introduction The electrochemically synthesized conducting polymers have been extensively investigated because of their wide range of useful applications in electronic devices 1-3 as well as for other purposes.4 The most interesting and potentially useful aspects of the conducting polymers are that they can be reversibly “switched” between the insulating and the conducting states so that this feature has been directed toward applications in the fabrication of microelectronic devices.5,6 Some conducting polymers such as polypyrrole (PPY) and polyaniline (PAN) have received particular interest because of their ease of preparation as thin films with good thermal and chemical stability. With the development of the scanning tunneling microscopy (STM) and atomic force microscopy (AFM), there have been intensive studies in detailed structural or mophorlogical characterization of the conducting polymers.2,7-12 Reports on in-situ monitoring of the electropolymerization processes have also appeared in recent years.13-17 However, in comparison with the large * To whom correspondence should be addressed. (1) Naarmann, H., Bredas, J. L., Chance, R. R., Eds. Conjugated Polymeric Materials; Kluwer: Dordrecht, The Netherlands, 1990; p 11. (2) Bond, S. F.; Howie, A.; Friend, R. H. Surf. Sci. 1995, 333, 196. (3) Bloor, D. Phys. Scr. 1991, T39, 380. (4) Shiratori, S. S.; Nishikawa, T.; Yokoi, K. Jpn. J. Appl. Phys., Part 2 1996, 35, L1455. (5) Gardner, J. W.; Bartlett, J. W. Nanotechnology 1991, 2, 19. (6) Heinze, J. In Topics in Current Chemistry; Anonymous, Ed.; Springer-Verlag: Berlin, Heidelberg, 1990; p 1. (7) Yang, R.; Evans, D. F.; Christensen, L.; Hensdrickson, W. W. J. Phys. Chem. 1990, 94, 6177. (8) Madsen, L. L.; Carneiro, K.; Zaba, B. N.; Underhill, M. S. Synth. Met. 1991, 41-43, 2931. (9) Froeck, C.; Bartl, A.; Dunsch, L. Synth. Met. 1995, 40, 97. (10) Armes, S. P.; Aldissi, M.; Hawley, M.; Beery, J. G.; Gottesfeld, S. Langmuir 1991, 7, 1447. (11) Wan, M. X.; Zhu, C. F.; Yang, J.; Bai, C. L. Synth. Met. 1995, 69, 157. (12) Wessling, B.; Hiesgen, R.; Meissner, D. Acta Polym. 1993, 44, 132. (13) Lukkari, J.; Heikkila, L.; Alanko, M.; Kankare, J. Synth. Met. 1993, 55, 1311. (14) Li, J.; Wang, E.; Green, M.; West, P. E. Synth. Met. 1995, 74, 127.
number of reports that have appeared in the literature on the polymer structure, morphology, and properties using scanning probe microscopic (SPM) techniques, studies on such SPM tip induced polymerization or surface modification by tips are still limited.16,18-24 With the recent development of nanotechnology based on the SPM techniques, investigations have been directed toward the SPM tip induced polymerization16,18-22 or modification23,24 on surfaces and grafting of molecules onto the substrate25 based on various mechanisms. For example, Penner and co-workers18 have achieved nanometer-scale electropolymerization of aniline on platinum by applying a two-step bias voltage to the STM tip; Sasano et al.16 observed the localized polymerization of pyrrole with submicrometer-scale spatial selectivity on graphite substrates using STM. Control over the fabrication of the conducting polymer with a line width from 50 µm to 20 nm has been achieved. It should be noted that most of the previous work on SPM assisted polymerization at surfaces involve the use of either the STM tip26 or the microelectrode of a scanning electrochemical microscope (SECM)20,21 to provide driving forces either electrically or chemically for the localized polymerization at the selected (15) Nyffenegger, R.; Gerber, C.; Siegenthaler, H. Synth. Met. 1993, 55, 402. (16) Sasano, K.; Nakamura, K.; Kaneto, K. Jpn. J. Appl. Phys., Part 2 1993, 32, L863. (17) Everson, M. P.; Helms, J. H. Synth. Met. 1991, 40, 97. (18) Nyffenegger, R. M.; Penner, R. M. J. Phys. Chem. 1996, 100, 17041. (19) Yang, R.; Evans, D. F.; Hendricksons, W. A. Langmuir 1995, 11, 211. (20) Kranz, C.; Ludwig, M.; Gaub, H. E.; Schuhmann, W. Adv. Mater. 1995, 7, 38. (21) Borgwarth, K.; Ricken, C.; Ebling, D. G.; Heinze, J. J. Phys. Chem. 1995, 99, 1421. (22) Yaniv, D. R.; McCormick, L. D. Nanotechnology 1992, 3, 44. (23) Hua, Z. Y.; Xu, W.; Cai, L. Surf. Sci. 1996, 349, L111. (24) Tang, S. L.; Mcghie, A. J.; Suna, A. Jpn. J. Appl. Phys., Part 2 1994, 33, L466. (25) (a) Chen, J. S.; Cousty, J.; Charlier, J.; Lecayon, G. Langmuir 1996, 12, 3252. (b) Xu, S.; Liu, G. Langmuir 1997, 13, 127. (26) (a) Sheats, J. R. Langmuir 1994, 10, 2044. (b) Xhou, L.; Zhang, P. C.; Ho, P. K. H.; Xu, G. Q.; Li, S. F. Y.; Chan, L. J. Mater. Sci. Lett. 1996, 15, 2028.
S0743-7463(97)00675-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/04/1998
Nanomodification of Polypyrrole and Polyaniline
surface area. However, work on in-situ localized electropolymerization involving an AFM tip is lacking. As one of the SPM techniques, AFM is suitable for imaging nonconducting surfaces. The attractive and repulsive forces acting between a probe and a surface are also used for scanning force experiments. The AFM tipsample interaction may become another sort of driving force for some surface processes under the tip. Very recently, Bourgion and co-workers have shown that Langmuir-Blodgett (LB) films of phthalocyanine can be patterned using AFM by scanning the surface of the films at high speed, showing the strong effect of the force or force modulation applied to the tip.27 Koinuma and Uosaki have reported the use of a scanning AFM tip to enhance the electrochemical dissolution of p-GaAs(100) electrodes.28 In fact, AFM is becoming a technique in mechanically patterning various as-formed materials such as photoresist, metal, and semiconductors.29-33 The aim of the present study is to explore the potential of the localized electropolymerization with the assistance of the AFM. In this article, we report the first observation of localized electropolymerization of pyrrole and aniline on HOPG substrates under strong AFM tip-sample interactions. Results of the in-situ AFM investigation on electropolymerization behaviors and AFM tip enhancing or inducing electropolymerization of pyrrole and aniline will be discussed. Experimental Section Electrochemical AFM (ECAFM) of Nanoscope IIIa was employed to both monitor and assist the electropolymerization. A freshly cleaved highly orientated pyrolytic graphite (HOPG) and a Pt wire were used as the substrate and the counter electrode, respectively. The silver wire was used as a quasi-reference, but the potentials quoted in this paper are versus a saturated calomel electrode (SCE). The electrochemical control of the system was realized via the CHI660 potentiostat of CH Instruments. The electropolymerization was carried out in solutions of 0.1 M HNO3 + 0.02 M pyrrole and 0.1 M HNO3 + 0.02 M aniline, respectively. The pyrrole and aniline monomers were newly distilled before use. All other chemicals were in AR grade, and solutions were prepared with triply distilled water. The instructive cyclic volammograms was recorded in the ECAFM cell separately to determine the potentials suitable for AFM studies. A 200 µm wide leg Si3N4 AFM tip with a spring constant of 0.12 N/m was used. The forces applied to the AFM cantilever were typically 1 nN. The AFM was activated and the tip was scanned over the substrate at different stages of the electropolymerization for a period of 5 min with a scan rate of 8 Hz unless otherwise indicated. Images were taken during or after the polymerization with an expanded scanning area at a lower potential where the polymerization was not allowed to proceed.
Results and Discussion Figure 1 shows a typical cyclic voltammogram for the electropolymerization of pyrrole at a HOPG surface. Although the current rises dramatically at 0.63 V due to the polymerization of PPY, a small electrochemical oxidation current of pyrrole can be observed at 0.53 V (27) Bourgoin, J. P.; Sudiwala, R. V.; Palacin, S. J. Vac. Sci. Technol., B 1996, 14, 3381. (28) Koinuma, M.; Uosaki, K. Surf. Sci. 1996, 358, 565. (29) Yano, T.; Nagahara, L. A.; Hashimoto, K.; Fujishima, A. J. Vac. Sci. Technol., B 1994, 12, 1596. (30) Wendel, M.; Irmer, B.; Cortes, J.; Kaiser, R.; Lorenz, H.; et al. Superlattices Microstruct. 1996, 20, 349. (31) Roue, L.; Chen, L.; Guay, D. Langmuir 1996, 12, 5818. (32) Eliadis, E. D.; Nuzzo, R. G.; Gewirth, A. A.; Alkire, R. C. J. Electrochem. Soc. 1997, 144, 96. (33) Cai, X. W.; Xie, Z. X.; Mao, B. W. Fourth International Conference on Nanometer-Scale Science & Technology; Beijin University Press: Beijin, China, 1996; abstract, 11-Tup14.
Langmuir, Vol. 14, No. 9, 1998 2509
Figure 1. Typical cyclic voltammogram of the electropolymerization of pyrrole on a HOPG electrode in 0.1 M HNO3 + 0.02 M pyrrole. Sweep rate: 100 mV/s.
where the rate of polymerization is very low. This potential is, therefore, chosen for the polymerization and the AFM observation. Accordingly, a potentiostatic polymerization was initiated at 0.53 V, and the process was periodically monitored in situ with AFM in a time interval of 2 min. It has been found that pyrrole behaves essentially very differently when prepared under slightly different conditions34 such as the electrode pretreatment and electrochemical deposition conditions, including applied potential, supporting electrolyte, solvent, and substrate. The polymerization is also expected to be strongly affected by the AFM tip involvement. Therefore, two sets of experiments were performed for comparison. In the first set of experiments, the tip was brought to the region of interest only when an AFM image was intended to be taken. It is observed that there are two stages during the growth of the PPY. The morphologies of the PPY films formed in the two stages are considerably different. The PPY film in the initial stage is very thin, composed of some chains adhering to the HOPG surface, Figure 2a. The PPY film in the second stage is compact, composed of square grains, Figure 2b. The adhesion to the surface of the PPY in the intial stage is so weak that the scanning AFM tip can disturb the film structure, and the images taken in this period are blurred. This result is consistent with that of previously reported AFM investigation of the poly(N-methylpyrrol) on HOPG.29 However, after about 5 min, the loose PPY grows to be a compact film composed of ordered squares of grains suddenly; see Figure 2b. This process was too quick to be followed clearly even if continuous recording of images was activated. In the second set of experiments, the AFM tip was purposely involved during the polymerization to interfere with the process. Starting from the bare HOPG surface at 0 V, AFM was activated by scanning in an area of 1 µm × 1 µm followed by a potential step to 0.53 V, and the polymerization was allowed to proceed for 20 min with the tip scanning above it. After this process, the AFM scanning area was expanded to include a wider region of the surface for taking the image. As is shown in Figure 3, a compact PPY film of ca. 24 nm thick built up in the region outside the actual scanning area. In contrast, there is no observable PPY film in the region right under the tip scanning area, leaving a square, bare electrode surface. (34) Kupila, E. L.; Kankare, J. Synth. Met. 1995, 74, 241.
2510 Langmuir, Vol. 14, No. 9, 1998
Figure 2. ECAFM images of the electropolymerized pyrrole at two stages recorded at 400 mV: (a, top) the initial stage with loose PPY chains; (b, bottom) the second stage with a compact film composed of ordered squares of grains.
However, there is a clear enhancement at the scanning boundary, particularly at the corner of the scanning area in comparison with the normal polymerization outside this area. One of the mechanisms for explaining the square, bare area could be a tip-blocking effect resulting in a very large shielding of the diffusive flux of pyrrole to the area or some unfavorable tip-sample interactions which suppress the polymerization. However, for the former case, pyrrole can still be delivered to the area, although it may be at a very slow rate, while, for the latter case, the time that a specific region of the surface experiences such a tip interaction is very short when the tip scans over it (depending on the scan rate and size) in comparison with the time that it is left free of interaction during imaging. It is unlikely, therefore, that such a short interaction period from the tip would completely block the very initial stage of the polymerization, e.g., the oxidation of the monomer and dimer of pyrrole. The absence of PPY film may indicate that the loose oligomer/ polymer chains formed on the surface are probably swept away. The enhanced peripheries and corners seem to be in favor of the assumption of the tip-sweeping effect. A schematic diagram describing the initial stage of the electropolymerization and deposition process at the surface is given by Figure 4 for better understanding of the tipsweeping effect. The first step (step 1) is the oxidation of the monomer to give a radical cation that may couple with another radical cation to form a dimer species. The soluble dimers formed may undergo further oxidation at the surface and then couple with other dimers or monomers in the vicinity of the electrode. The soluble oligomers formed at this step can either diffuse away from the electrode or participate in the formation of nucleation sites on the electrode surface; see step 2. Fermin and Scharifker
Cai et al.
have identified oligomers of pyrrole accumulated in aqueous solution during the anodic oxidation of pyrrole at stationary and rotating electrodes and separated them using UV-visible spectroscopy and chromatographic techniques.35 They have observed the oligomers of up to nine pyrrole units. If the dimers/oligomers remain in the vicinity of the electrode, they may be oxidized and continue to couple, resulting in lower solubility. As shown in step 3, the establishment of nuclei for polymer growth occurs by precipitation of oligomeric species formed on the electrode surface and accumulated in its vicinity. It has also been determined that the oxidation potential shifts in the negative direction as the chain length increases, facilitating the progress of the reaction. The precipitation and deposition on top of polymeric nuclei, rather than at the bare substrate, will be favored since the monomers/ oligomers in solution are more readily oxidized at the polymer surface than at the substrate, in the same way as with metal deposition. Accordingly, it is reasonable to explain the apparent enhancement of the polymerization at peripheries and corners in terms of the tip-sweeping effect. The precipitated oligomers on the bare HOPG surface are easily swept away by the AFM tip and accumulated at the peripheries of the scanning area at the initial stage of the nucleation. In addition, since it was found that the continual electroprecipitation is the major route by which conducting polymers grew on electrode substrates especially on top of the nuclei already formed,36 the swept oligomers of the present case will further contribute significantly at the same time to the formation and subsequent growth of the polymeric nuclei due to fast continual electroprecipitation. By carefully analyzing the way of AFM tip scanning, it is not difficult to understand that the swept PPY species ought to accumulate substantially at the slow scanning direction due to the fast tip movement along the fast direction. This is consistent with the experimental results which show unequally enhanced polymerization with PPY accumulated more along the slow directions. Therefore, the tipsweeping effect on the oligomer/polymer chains could be the main factor in explaining the apparent enhancement of the polyermization. However, it is necessary to note that the additional tip interaction with the oligomers/ polymer chains, named the tip-enhancing effect, will become a dominant factor on the prepolymerized PPY thin film, which will be discussed in more detail in the following sections. When the tip scans over an as-polymerized PPY thin film, partially or totally enhanced polymerization can be achieved, depending on the stage at which the PPY film was formed. If the tip scans over a PPY film in the initial stage, the part of the PPY film within the tip-scanning area will be completely or partly swept away. If the tip scans over a PPY film in the second stage, the polymerization of PPY within the tip-scanning area will be enhanced. This was done as follows: after the polymerization at 0.53 V for various times, the AFM tip then scanned over a selected area of the as-polymerized PPY film for another 10 min. The polymerization was enhanced over all scanning areas resulting in a 10 nm height square platform; see areas “A” and “B” of Figure 5. Square PPY platforms of different sizes ranging from several mi(35) Fermin, D. J.; Scharifker, B. R. J. Electroanal. Chem. 1993, 357, 273. (36) John, R.; Wallace, G. G. J. Electroanal. Chem. 1991, 306, 157. (37) Genies, E. M.; Tsintavis, C. J. Electroanal. Chem. 1985, 195, 109. (38) Zotti, G.; Cattarin, S.; Comisso, N. J. Electroanal. Chem. 1988, 239, 387. (39) Duic, L.; Mandic, Z.; Kovac, S. Electrochim. Acta 1995, 40, 1681.
Nanomodification of Polypyrrole and Polyaniline
Langmuir, Vol. 14, No. 9, 1998 2511
Figure 3. (a, left) ECAFM image showing the absence of polymerization within the scanning area. (b, right) Section analysis of the image (see text).
Figure 4. Schematic diagram describing the initial stage of electropolymerization and deposition processes at the surface (see text).
Figure 5. ECAFM image illustrating various types of enhanced polymerization of PPY grains (A-D) (see text).
crometers down to several 10 nm can be obtained by scanning the AFM tip at different sizes. It is of special interest that there is a limitation in the smallest achievable size of the enhancement which is the grain size of the PPY. It was found that no matter how limited the AFM tip scanning area was, the growth rate over the entire grain area would be enhanced, though only part of it experienced the tip interaction. This phenomenon may be regarded as the AFM tip activated epitaxial type of growing of polymers; i.e., the size and shape of the enhanced polymer are dependent not only on the scanning area of the tip but also on the polymer grain arrangement underneath. Figure 5 also illustrates the above phenomenon by showing various types of enhanced polymerization of PPY grains. The two big bright platforms (A and B) were obtained by scanning on the preformed PPY with big area (250 × 250 nm2) and prolonged time (5 min).
When the tip was scanning within the size of one PPY grain but across the boundary of two PPY grains, see the square indicated in the figure, the growth of all of the two grains were enhanced (“C”). By further reducing the scanning size with the tip just scanning within a part of a single grain, the individually enhanced grain can be obtained (“D”). It is necessary to point out that by disabling the y scanning at a different position and by rotating the scanning by 90°, cross-lines with different heights can also be obtained after scanning of different times. Figure 6 shows the AFM image together with the profile of the cross-lines. Again, no matter how short the time of scanning on the preformed polymer thin film, the patterned line width cannot be thinner than about 60 nm, which is the typical size of a grain composing the PPY film. A conclusion may be drawn at this stage that the AFM tip-enhancing polymerization takes place only on a preformed PPY thin film of certain thickness which adheres to the surface firmly enough. The tip interaction with a certain part of a PPY grain is shared by the whole grain, leading to the spread over enhancement for the grain growth. More detailed inspection of the enhancement factor becomes possible by simple calculations. The apparent enhancement factor can be estimated as the ratio of the net film thickness grown with the tip assistance over the normal film thickness. The final film thickness involving the tip interaction is doubled in comparison with the normally polymerized film thickness of ca. 10 nm. Accordingly, the apparent enhancement factor is thus estimated as 10 nm/20 nm ) 2. However the enhancement factor was observed to be increasing with the decreasing
2512 Langmuir, Vol. 14, No. 9, 1998
Cai et al.
Figure 6. ECAFM image of cross-lines obtained by disabling the y scanning at different positions or by rotating the scanning by 90°. The cross-lines show different heights (20-70 nm) due to different scanning times (0.5-2 min).
of the scanning area. Under the present experimental condition, it takes 32 s to scan over a selected region of 1 µm2 with an 8 Hz scan rate and 256 sample numbers. The equivalent time to scan over a PPY grain of about 0.15 × 0.15 µm2 within the 1 µm2 scan area is 0.72 s. The accumulated time of tip interaction with a PPY grain equals ca. 27 s out of a total of 20 min (1200 s) of polymerization. Therefore, the real enhancement factor should be estimated on the basis of the tip interaction time; i.e., (10 nm/27 s)/(10 nm/1200 s) ) 44. In the above given results, the AFM tip interacts with the existing PPY film by exerting a force which contains both horizontal and vertical components. It has been found that an even more efficiently enhanced polymerization can be achieved with a scanning and oscillating tip by raising the integral gain of the feedback loop system for AFM measurements. An oscillating tip can exert a stronger force in the vertical direction on the PPY film than a nonoscillating tip. This vertical force can be responsible for the enhancement of the PPY. The vertical force may be beneficial to the polymerization of pyrrole through two effects. First the vertical force tends to press the PPY film to be more compact. As a result, it raises the density of the PPY film, which might raise its conductivity and thus raise the rate of localized polymerization. As a consequence, the thicker compacted film is formed. Second the vertical force offered by the AFM tip is that it presses the oligomer in the solution phase downward to the as-polymerized PPY film and thus assists the coupling of the oligomer in the solution phase and the surface polymer. Then the general result is that the vertical force increases the transport rate of the pyrrole in the solution phase to the electrode surface and thus enhances the polymerization within the tip-scanning area. Similar results have also been reported by Bourgion and co-workers in a very recent report27 which shows a strong effect of the force or force modulation applied to the tip that has been used to pattern LB films of phthalocyanine, although the mechanism of the patterning in their work is completely different. As has been described above, the tip-enhancing polymerization can only be achieved on the prepolymer thin film. However, it would be much more useful for practical applications if a PPY line or square could be created directly on the bare surface. For this purpose, an alternative way of using the tip-sweeping effect has been designed. First of all, AFM was activated by scanning in an area of 5 × 5 µm2 at 0.53 V, and it allowed slow polymerization for about 2 min. Then the x scanning was offset by 1.7 µm left, leaving the new scanning area (solid
Figure 7. ECAFM image of a PPY line about 150 nm wide and 5 µm long on the bare HOPG substrate.
frame) partially overlapping with the original scanning area (dashed frame). The potential was controlled at 0.0 V so that the polymerization of pyrrole was not allowed at this time. The AFM image was taken after the new area was scanned for another 2 min, as shown in Figure 7. It can be seen clearly that only a PPY line that was about 150 nm wide, 5 µ m long, and 100 nm high remained and the rest of the PPY chains have all been swept out of the observation area. The tip effect played different roles depending on the area underneath the scanning tip. In the first 2 min, the very initial stage of electropolymerization and deposition proceeded and a square, bare electrode surface and the enhanced polymer peripheries were formed, as was already described and shown in Figure 3a. During the second 2 min of scanning, the ultrathin PPY film that formed in the region outside the original scanning area was swept away from the new scanning area, while only the enhanced polymer peripheries remained at the surface and was even further enhanced to grow to a clear polymer line. This primary result demonstrates a possible way of fabricating PPY lines on the bare surrounding of the substrate. For comparison, aniline was also electropolymerized on HOPG under AFM tip interaction. The potential of 0.53 V is just across the threshold of the oxidation of aniline. It has been found that PAN is difficult to form on the surface when the potential is kept constant and it is normally prepared under the cyclic voltammetric condition.37-41 Therefore, the potential was cycled in the present study between 0.15 and 0.53 V at a scanning rate of 100 mV s-1 during the AFM tip scanning over the (40) Stilwell, E. M.; Park, S. M. J. Electrochem. Soc. 1990, 36, 182. (41) Yang, H. J.; Bard, A. J. J. Electroanal. Chem. 1992, 339, 423.
Nanomodification of Polypyrrole and Polyaniline
Langmuir, Vol. 14, No. 9, 1998 2513
Figure 8. ECAFM images of PAN films taken after scanning for 5 min (a, top) and after stopping the scanning over the same area for 20 min (b, bottom) with the sectional analysis for the images. The height differences of the PAN within and outside the enhanced areas are 1.5 and 6.5 nm for a and b, respectively.
surface. It has been found that the AFM tip can also enhance polymerization of PAN in the tip-scanning area on as-polymerized PAN film, just like the tip-enhancing polymerization of PPY. It is of special interest that the enhanced polymerized area which had been scanned by the tip seems to have higher reactivity for both PPY and PAN. Even when the tip stops scanning this area, it remains more reactive than the rest of the polymer. Parts a and b of Figure 8are two images taken over the same area of PAN in different time periods. The first one was taken after tip scanning for 5 min. The height of the tip-enhancing polymerized PAN is 1.5 nm, Figure 8a. Then the tip was moved to another area so the tip-enhancing effect on this particular area was stopped. After 20 min, the tip was moved back to the previous scanning area to take the second image. The height of that part of PAN is 6.5 nm, Figure 8b. By comparison between these two images, the PAN at the area which received the previous tip scanning grows faster than the rest of the area, even without the tip-enhancing effect. It can be seen clearly from the cross-sections that there is a 5 nm net increasement of the growth of the PAN in the enhanced area, even without further tip interaction. So one may conclude that the part of PAN having experienced the tip scanning has higher reactivity for the electropolymerization. It may infer that the applied potential is more effective on that special area, probably due to the considerable increase of the conductivity. At present we are not able to explain how the conductivity increases for the tip-scanned area. Future systematic investigation is required on the localized conductivity and
the correlated structure of the polymer affected by the tip interaction in order to draw a definite conclusion. The final factor that should be considered in the present study is the possible laser induced effect. This is because the laser illumination has been found to greatly enhance the pyrrole polymerization in previous reports.41 It is therefore important to rule out a contribution by laser activated polymerization in the region of AFM observation. The triangular AFM cantilever with the tip on the apex is ca. 200 µm in length for each side of it. The 75 µm spot of the laser beam for the AFM measurement falls partially onto the top of the triangular cantilever and partially onto the sample surface directly. The incident laser beam on the cantilever is effectively reflected, the influence of the laser in this part of the area is neglectable, and the area which does receive direct laser illumination is far away from the area of investigation by the AFM tip. Therefore, the laser enhancing polymerization is possible only in the region outside the cantilever projection area, which dose not influence the results discussed in this paper. Conclusions This paper presents a novel method for surface nanomodification with electroconductive polymers by AFM. A scanning or oscillating AFM tip, instead of a static tip, is essential for the enhanced electropolymerization in selected surface areas. The horizontal scratching force and the vertical tapping force are the other two driving forces for the polymerization in addition to the externally applied electrode potential. The increased speed of precipitation of oligomers under force is the main part that is responsible
2514 Langmuir, Vol. 14, No. 9, 1998
for such localized enhanced polymerization. Under different conditions, including the applied electrode potential and surface pretreatment, lines, square platforms, or hollows of PPY and PAN can be fabricated on aspolymerized surfaces or bare HOPG surfaces. The tipenhancing polymerized areas have higher reactivity for the further electropolymerization without the tip interaction. It may infer a considerable increase of the local conductivity of that special area. The present study reveals that AFM can be used as a unique tool for nanofabrication of conducting polymers. Moreover, sys-
Cai et al.
tematic and extensive work may be useful in extending the study to the fields of practical applications such as manufacturing microelectronic devices. Acknowledgment. Financial support from the Natural Science Foundation of China and the State Key Laboratory for Physical Chemistry of Solid Surfaces is gratefully acknowledged. LA970675O
