© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 48, NOVEMBER 28, 1996
LETTERS Atomic Force Microscopy and Kelvin Probe Force Microscopy Evidence of Local Structural Inhomogeneity and Nonuniform Dopant Distribution in Conducting Polybithiophene Oleg A. Semenikhin,†,‡ Lei Jiang,§ Tomokazu Iyoda,† Kazuhito Hashimoto,†,§ and Akira Fujishima*,§ Kanagawa Academy of Science and Technology, KAST Laboratory in Tokyo Institute of Polytechnics, 1583 Iiyama, Atsugi-shi, Kanagawa 243-02, Japan, A. N. Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninskii prosp. 31, 117071 Moscow, Russia, and Department of Applied Chemistry, Faculty of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: March 19, 1996; In Final Form: June 27, 1996X
Direct evidence of local structural inhomogeneity and nonuniform doping-level distribution in conducting polymer film has been obtained using Kelvin probe force microscopy (KFM) and atomic force microscopy (AFM). The KFM data suggests that the polymer consists of grains that constantly differ in work function and thus in the dopant concentration from the grain peripheral regions. For the as-grown polymer, most of the doping charge is located at the grain tops, whereas the electrochemically doped polymer features relatively higher doped grain periphery and less doped grain tops. The AFM study reveals two different kinds of the polymer molecular structure dependent on whether the image was taken at the top of a grain or the grain periphery. This result confirms the inherent inhomogeneity of conducting polymers demonstrated with KFM.
Introduction Electron-conducting polymers are promising materials for numerous applications from batteries to molecular electronic devices. The properties of such polymers can be easily and reversibly varied in a wide range by electrochemical or chemical oxidation/reduction. The involved process is called doping since the oxidation of the polymer backbone requires uptake of some anions from the solution or gas phase to preserve electroneutrality (for an anion-doped polymer; the picture for cationic doping is opposite). However, the actual values of the doping level imply that only a fraction of the polymer chains is oxidized. Therefore, the question occurs, what is the distribution of the doping ions or the oxidized and the reduced portions of the polymer chains (doping charge) within the polymer? * Corresponding author: phone +81-03-3812-9276; fax +81-03-38126227; e-mail
[email protected]. † Kanagawa Academy of Science and Technology. ‡ Russian Academy of Sciences. §The University of Tokyo. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
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First, since doping-undoping is a heterogeneous reaction, the dopant concentration varies along the thickness of the polymer film.1-3 Another aspect is the polymer domain structure. Electrochemical data on the kinetics of the doping/ undoping process4-7 indicates that some of the doping charge is localized in doped metal-like domains separated by a nonconductive polymer phase. This conclusion is also supported by the data on dielectric response spectra and temperature dependences of the magnetic susceptibility and dc conductivity.8 However, up to now, no direct observation of such domains has been made. Scanning probe techniques have been proved to be a powerful tool for the study of the surface morphology as well as for the local determination of various surface parameters. Among these techniques is the so-called Kelvin probe force microscopy (KFM),9-11 which provides information on the lateral distribution of the surface potential over the sample. The term of surface potential in this context means the potential difference between the sample and a conducting probe that is positioned © 1996 American Chemical Society
18604 J. Phys. Chem., Vol. 100, No. 48, 1996
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Figure 1. Energy digram for the tip-surface system: VFB, applied bias between the tip and the sample; EFermi, Fermi level; Evac, vacuum level; Ev, valence band; Ec, conduction band; F, work function of the tip and polymer. (a) VFB ) 0; the electrostatic force between the tip and surface is induced by the work function difference. (b) VFB has been applied to cancel the electrostatic force and corresponds to the work function difference.
close to the sample. Since this value correlates with the difference in the work functions of the probe and the sample, this technique offers a possibility of distinguishing between the regions with different chemical nature or composition. For instance, the method has been successfully applied to twodimensional dopant profiling of semiconductor surfaces.12,13 Correspondingly, this technique would provide an opportunity for studying local doping-level distribution in conducting polymer films. In this work, we present the first direct KFM and atomic force microscopy (AFM) evidence that conducting polymers feature nonuniform dopant distribution and local structural heterogeneity, both of which are consistently related to the polymer surface morphology. Experimental Section The KFM measurements were performed with an SPA 3700 STM/AFM system (Seiko Instruments) equipped with a KFM controller (Seiko Instruments) using commercial 200-µm rectangular gold-coated cantilevers (characteristic frequency f ) 27 kHz, spring constant C ) 1.6 N/m). An energy digram for the tip-sample system is shown in Figure 1. The surface potential images were taken in a potential feedback (FB) mode, that is, when maintaining a zero electrostatic force between the sample and the tip by applying external bias. These bias values (VFB) varied from point to point depending on the local work function and were acquired simultaneously with the topography information in the AFM tapping mode. More positive values of the FB surface potential corresponded to higher values of the work function, and vice versa (the tip was grounded). More detailed description of the KFM technique can be found elsewhere.10,13 The KFM images were taken in ambient conditions. The contact-mode AFM images were obtained in situ with the as-grown film immediately after the synthesis. The same system with commercially available triangular 200-µm Si3N4 sharpened cantilevers (spring constant Kz ) 0.021 N/m) was used. Polybithiophene (PBT) films were electrochemically deposited onto highly oriented pyrolytic graphite from propylene carbonate solution containing 0.05 M 2,2′-bithiophene and 0.1 M LiClO4. 2,2′-Bithiophene (Tokyo Kasei) was fractionally distilled under reduced pressure. Propylene carbonate (Wako Chemicals) and anhydrous lithium perchlorate (Wako Chemicals) were used as received. The polymer deposition was carried
Figure 2. Simultaneous 250 × 250 nm images of (a, top) topography and (b, bottom) surface potential acquired with an as-grown PBT film in the KFM feedback mode. The gray-scale range is (a) 12.9 nm and (b) 21 mV. The images were low-pass filtered at 1 kHz.
out in galvanostatic conditions under a stepwise current variation from 0 to 1 mA/cm2 and then back to 0. In some experiments, the film was further electrochemically doped in 0.1 M LiClO4 solution without the monomer at a potential of approximately +1.05 V vs normal hydrogen electrode, that is, in the vicinity of the PBT doping peak.14 The film thickness was approximately 10 nm according to our previous AFM data.15 After the synthesis, the films were rinsed with pure propylene carbonate and dried under vacuum for 2 h at room temperature. Results and Discussion Figure 2 presents the images of (a) topography and (b) surface potential acquired simultaneously at the same portion of the surface of an as-grown PBT film. The topography image shows polymer grain structure typical for these conditions.15 One can see that the surface potential distribution is not uniform and clearly correlates to the polymer surface morphology. A grain in the topography image always corresponds to a bright region in the image of the surface potential. That is, the values of the surface potential are constantly more positive at the top of grains than at the grain periphery. This implies that the polymer work
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Figure 3. Simultaneous 250 × 250 nm images of (a, top) topography and (b, bottom) surface potential acquired in the KFM feedback mode. The PBT film was subjected to further electrochemical doping in the supporting electrolyte solution. The gray-scale range is (a) 14.6 nm and (b) 45 mV. The images were low-pass filtered at 1 kHz.
function at the grains is higher; therefore, the grains are relatively oxidized and relatively doped as compared with the grain peripheral regions. In other words, the KFM data suggests that most of the doping charge for the as-grown film is located at the grains rather than at the grain periphery. Since the dopant concentration can be varied by electrochemical treatment, it was of interest to estimate the effect of the electrochemical doping of as-grown films on the dopant distribution. Figure 3 presents the images of the topography and the surface potential acquired on a film that was further electrochemically doped in the supporting electrolyte solution without the monomer. One can see that the dopant distribution in this case is different. Unlike the as-grown film, the very top of the grains is relatively less doped. These less doped regions are surrounded by heavily doped areas that form circular structures. These structures are in turn surrounded by a relatively undoped phase. Remarkably, the dopant distribution retains correlation with the surface morphology that was the case for as-grown films. However, the most heavily doped regions are located now along the grain boundaries, whereas the very top of the grains is
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Figure 4. Contact-mode AFM molecular-resolution images typical to (a, top) the grain periphery and (b, bottom) the grain tops taken with an as-grown PBT film in situ immediately after the synthesis. The images represent deflection and were low-pass filtered at 1 kHz and linearized. Parameters of the images were found to be helix pitch p ) 0.43-0.45 nm; chain separation s ) 0.55-0.65 nm; a ) 0.27-0.29 nm; b ) 0.37-0.40 nm.
relatively less doped. This fact suggests that the penetration of the dopant ions in the electrochemical doping of as-grown films occurs predominantly at grain peripheral areas. The grain tops turn to be more inert, although apparently still more highly doped as compared with the as-grown film. The fundamental difference between polymer grains and grain peripheral areas demonstrated by KFM was also supported by in situ contact-mode AFM data taken with the as-grown polymer film. Parts a and b of Figure 4 show molecular-resolution AFM images typical to grain periphery and grain tops, respectively. One can see that these images correspond to different structures. The image of Figure 4a is proposed to consist of separate helical chains. Similar structures are typical for polythiophenes as was shown by STM16-18 and molecular simulation.19-21 The image of Figure 4b is likely to represent a crystalline region. It probably consists of linear polymer chains stacked together. The partial crystallinity of the as-grown polymer film was also demonstrated by X-ray diffraction data,15 which agreed well with the parameters of the AFM molecular-resolution images. This observation suggests that the inhomogeneous doping density probably results from the local structural inhomogeneity in the PBT film.
18606 J. Phys. Chem., Vol. 100, No. 48, 1996 In conclusion, we may summarize that both AFM and KFM data show consistent differences in structure and properties between polymer grains and grain peripheral regions. According to the AFM molecular-resolution data, the grains comprise a highly ordered crystalline structure while the grain periphery features separate polymer chains with a some degree of disorder. At the same time, the KFM data clearly suggests inhomogeneous doping-level distribution between the grain tops and peripheral areas. Furthermore, comparison of the character of the dopant distribution for as-grown and electrochemically doped films shows that the grain peripheral regions are more active in the doping process than are the grain tops. The results suggest that KFM is a powerful tool for studying dopant distribution in conducting polymers. References and Notes (1) Pekker, S.; Janossy, A. In Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1; p 45. (2) Aoki, K.; Aramoto, T.; Hoshino, Y. J. Electroanal. Chem. 1992, 340, 127. (3) Tezuka, Y.; Kimura, T.; Ishii, T.; Aoki, K. J. Electroanal. Chem. 1995, 395, 51. (4) Kalaji, M.; Nyholm, L.; Peter, L. M. J. Electroanal. Chem. 1992, 325, 269. (5) Genz, O.; Lorengel, M. M.; Schultze, J. W. Electrochim. Acta 1994, 39, 179. (6) Vuki, M.; Kalaji, M.; Nyholm, L.; Peter, L. M. Synth. Met. 1993, 55-57, 1515.
Letters (7) Ovsyannikova, E. V.; Alpatova, N. M.; Kazarinov, V. E. Electrochim. Acta, in press. (8) Epstein, A. J.; Joo, J.; Kohlman, R. S.; Du, G.; MacDiarmid, A. G.; Oh, E. J.; Min, Y.; Tsukamoto, J.; Kaneko, H.; Pouget, J. P. Synth. Met. 1994, 65, 149. (9) Martin, Y.; Abraham, D. W.; Wickramasinghe, H. K. Appl. Phys. Lett. 1988, 52, 1103. (10) Fujihira, M.; Kawate, H.; Yasutake, M. Chem. Lett. 1992, 2223. (11) Yokoyama, H.; Saito, K.; Inoue, T. Mol. Electron. Bioelectron. 1992, 3, 79. (12) Henning, A. K.; Hochwitz, T.; Slinkman, J.; Never, J.; Hoffmann, S.; Kaszuba, P.; Daghlian, C. J. Appl. Phys. 1995, 77, 1888. (13) Kikukawa, A.; Hosaka, S.; Imura, R. Appl. Phys. Lett. 1995, 66, 3510. (14) Since the synthesis and treatment were performed in the AFM cell, the actual reference electrode was a Pt wire whose potential was periodically controlled against an aqueous Ag/AgCl electrode. (15) Semenikhin, O. A.; Jiang, L.; Iyoda, T.; Hashimoto, K.; Fujishima, A. To be published. (16) Caple, G.; Wheeler, B. L.; Swift, R.; Porter, T. L.; Jeffers, S. J. Phys. Chem. 1990, 94, 5639. (17) Yang, R.; Dalsin, K. M.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J. Phys. Chem. 1989, 93, 511. (18) Yang, R.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J. Phys. Chem. 1990, 94, 6117. (19) Cui, C. X.; Kertesz, M. Phys. ReV. B 1989, 40, 9661. (20) Corish, J.; Morton-Blake, D. A.; Veluri, K. Mol. Simul. 1995, 14, 381. (21) Corish, J.; Morton-Blake, D. A.; Veluri, K.; Beniere, F. J. Mol. Struct.: THEOCHEM 1993, 283, 121.
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