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Langmuir 1992,8, 1459-1463
In Situ Atomic Force Microscopic Imaging of Electrochemical Formation of a Thin Dielectric Film. Poly(phenylene oxide) Charles A. GOSS, Jay C. Brumfield, Eugene A. Irene, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received March 2, 1992 Thispaper presents the fmt in situ atomic forcemicroscopy (AFM)images recordingthe electrochemicallyinduced formation of a dielectric polymer film (poly(pheny1eneoxide), PPO). The experiments describe electrochemically-inducedpolymerizationand changesin surfacemorphologythat accompany film growth, observations of nearly molecularly smooth dielectricsurfaces,use in an organic solvent,tip-driven removal of PPO f i to form submicrometer features, and in situ ultrathin film thickness measurements.
Introduction We present the first in situ atomic force microscopy (AFM)' images chronicling the electrochemically-induced formation of an ultrathin dielectric polymer film (poly(phenylene oxide), PPO). Electrochemical processes that have been studied in situ with scanning tunneling microscopy (STM) include surface oxidation,2 adsorption,3 and electrodeposition of metal^^^^ and conducting polymers.4d However, STM experiments are limited to sufficiently conducting surfaces, and interpretation of the images obtained can be clouded by the convolution of surface topology with the local density of states. On the other hand, AFM has been shown to provide nondestructive imaging of surface topology for both conducting and insulating samples with atomic resolution.' Gewirth and co-workers recently introduced the first in situ AFM experiments, in order to examine Cu electrodepositionSa and oxide formationsb a t Au(ll1) electrodes. One significant finding was that in perchloric acid electrolyte the underpotential deposited monolayer of Cu adopts a closepacked lattice (0.29 nm spacing), while in sulfuric acid electrolyte it forms a more open lattice (0.49 nm spacing), which the authors postulated to be caused by coadsorption of sulfate anions with the C U . ~ ~ Presently, the nanostructure of molecular films at electrodes remains poorly understood, yet the ability to control and monitor their molecular architecture will clearly be important to their ultimate utility in areas such (1) (a) Binnig, G.;Quate, C. F.; Gerber, Ch. Phys. Reu. Lett. 1986,56, 930-933. (b) Sarid, D.; Eliigs, V. J. Vac. Sci. Technol., B 1991,9,431437. (2) (a) Wiechem, J.; Twomey, T.;Kolb, D. M.; Behm, R. J. J. Electroanal. Chem. 1988,248,451-460. (b) Trevor, D. J.; Chidsey, C. E. D.; Loiicono, D. N. Phys. Rev. Lett. 1989, 62, 929-932. (c) Honbo, H.; Sugawara, S.; Itaya, K. AM^. Chem. 1990,62,2424-2429. (d) Chang, H.; Bard,A. J. J. Am. Chem. SOC. 1991,113,5588. (3) (a) Yau, S.-L.;Vitus, C. M.; Schardt, B. C. J.Am. Chem. SOC. 1990, 112, 3677-3679. (b) Yau, S.-L.; Gao, X.;Chang, 5.4.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. SOC. 1991,113,6049-6056. (4) (a)Green, M.P.; Hanson, K. J.; Scherson, D. J.; Xing, X.; Richter, M.;Row, P. N.;Cam, R.; Lindau, I. J.Phys. Chem. 1989,93,2181-2184. (b)Sonnenfeld, R.; Schardt, B. C. Appl. Phys. Lett. 1986,49,1172-1174.
(c) Magnuesen, 0. M.;Hotlce, J.; Nichols, R. J.; Kolb, D. M.; Behm, R. J. Phys. Reo. Lett. 1990,64,2929. (d) Fan, F.-R. F.; Bard, A. J. J. Electrochem. SOC.1989,136,3216-3222. (5) (a) Manne, S.; Hansma, P. K.; Maeeie, J.; Elings, V. B.; Gewirth, A. A. Science 1991,251, 183-186. (b) Manne, 5.;Massie, J.; Eliigs, V. B.; H m , P. K.; Gewirth, A. A. J. Vac. Sci. Technol., B 1991,9,950954.
0743-746319212408-1459$03.00/ 0
as microelectronics and biosen~ors.~Electrochemical deposition of molecular films718often permits nanometerscale control of film thickness and gives access to observation of film birth and early growth. Such films are appealing for microscopic study, and both in situ and ex situ STM imaging of electrochemically prepared conducting polymer films have been reported.4dp6The present work is to our knowledge the first application of AFM to the in situ study of an electrochemically polymerized dielectric molecular film,7J3which exhibits a nearly molecularly smooth polymer surface, and is also the first use of AFM in an organic electrolyte. The AFM images record changes in polymer surface morphology that accompany growth of a polymer film only a few nanometers thick, demonstrate tip-driven nanoremoval of PPO film to form submicrometer lateral dimension features, and illustrate in situ ultrathin film thickness measurements. These observations have provided new insight into the mechanism of PPO film growth, and demonstrate the ability to control film nanostructure. Experimental Section The experimentswere conducted using a Nanoscope I1 AFM, Nanoprobe cantileverswith integral SiaNd tips (100pm, 0.58 nN/ m spring constant),and a glass cell attachment (all from Digital Instruments,Santa Barbara, CA). The cell contained the AFM tip, HOPG (highly oriented pyrolytic graphite, supplied by Dr. Arthur W. Moore, Union Carbide Co.) working, Pt counter, and Ag pseudo-reference electrodes, and was filled with 24 mM phenolate monomer (MerN+PhO-)in 0.2 M BuSJClOdCHaCN solution. The Ag and Pt electrodeswere inserted in the cell inlet and outlet, respectively. A Pine Model RDE4 potentiostat was employed. In phenolate monomer solution, the silver wire potential is -0.76V vs SSCE potentialsherein are cited va SSCE. Fresh HOPG surfaces were obtained by cleaving with adhesive tape. The geometric area of the HOPG working electrode was ca. 0.4 cm2. Phenolate solutions prepared as described before* were filtered through a 0.2-pm PTFE membrane (Acrodisc, Gelman Scientific). Typical AFM acquisition parameters were as ~~
(6)(a) Jeon, D.; Kim, J.; Gallagher, M. C.; Willis, R. F.; Kim. Y.-T. J. Vac. Sci. Technol., B 1991,9,1154-1158. (b) Yang,R.; Naoi, K.; Evans, D. F.; Smyrl, W. H.; Hendrickson, W. A. Langmuir 1991,7,568-658. (c) Snyder, S. R.; White, H. S.; Mpez, S.; AbruAa, H. D. J. Am. Chem. SOC. 1990,112,1333-1337. (d) Yang, R.; Dalein, K. M.; Evans, D. F.; Christensen, L.; Hendrickson, W. A. J. Phys. Chem. 1989,93,511-512. (7) (a) Ab-, H. D. Electrode Modificationwith Polymeric Reagents. In Electroresponsiue Molecular and Polymeric Systems; Skotheim, T., Ed.; Marcel Dekker: New York 1988. (b)Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; J. Wiley: New York, in prees. (8) (a) McCarley, R. L.; Thomas, R. E.; Irene, E. A.; Murray, R. W. J. Electroanal. Chem. 1990,290,79. (b) McCarley, R. L.; Irene, E. A.; Murray, R. W. J. Phys. Chem. 1991,95, 2492-2498.
0 1992 American Chemical Society
1460 Langmuir, Vol. 8,No. 5,1992 follows: (A + B) signal = 5 V, (A - B) signal = -1.75 V; setpoint voltage = -1.2 V; integral gain = 1;proportional gain = 2; twodimensional gain = 0; scan rate = 4.34 Hz; filters off. AFM/electrochemistry was conducted by first assemblingthe electrochemical AFM cell and imaging the HOPG working electrode in air. If atom-resolved images were obtained: the tip was withdrawn 10-15 pm from surface, the cell filled with phenolate monomer solution, and the bare HOPG surface imaged again. HOPG image quality was generally identical in air and under monomer solution. The dielectric polymer PPO was deposited by cycling the HOPG electrode potential (0.1 V/s) to potentials where phenolate is oxidized to phenoxy radical (from -0.76 V to +0.44 V vs SSCE) and back. The observed electropolymerization voltammetry was similar to that seen earlie$ at Au and Pt electrodes. During voltammetric scans the AFM tip was withdrawn 10-15 pm from the HOPG surface. Successful imagingof the PPO film required minimization of the tip-sample force. This was accomplished by engaging the AFM tip while scanning over a relatively small area (ca. 100 X 100 pm) of the polymer surface and decreasingthe tip-sample force by adjusting the setpoint voltage to increasinglynegative values until the image became unstable, then backing off by +0.2 to 0.3 V. After the scan area was increased and images of the freshly formed PPO film were recorded, the cell was flushed with fresh phenolate monomer solution, and the above deposition/imaging procedure repeated to yield a record of progressive film growth. The force between the tip and the sample was estimated from calibration plots of tip deflection versus tip-sample separation, using the 0.58 N/m cantilever spring constant supplied by the manufacturer. The reported forces are measured relative to the value at which the tip separates from the sample,which is assigned a value of zero. In air, stable and reproducible images of PPO molecular film surfaces were obtained using forces ca. 100 nN. Under CH3CN solution, the force used for stable imaging was lower, typically ca. 10nN. These values are approximate because differences between individual tips and instrument drift give rise to 100-200% variations in force. The peak-to-valley heights were measured by recording height differences between adjacent high and low features in line scans of surface height. The reported peak-to-valley heights for each image are the average of 70-80 measurements from three line scans of surface height. Root mean square surface roughness values were calculated by the vendor’s software in the topview mode for the entire film area shown in the figure. Film thicknesses were determined by averaging the values obtained from 20-30 different locations along line scans like those shown in Figure 2 (vide infra). The uncertainties given are la.
Results and Discussion Oxidation of Me4N+PhO-/CH&N solutions by continuous cyclical scanning of electrode potential to positive values is known to coat the electrode surface with a relatively pinhole-free, electronically passivating PPO dielectric ca. 5-7 nm thick, and with a probably crosslinked structure.8 Typical AFM microscopy before and after PPO film deposition is shown in Figure 1. Panel lA, taken under phenolate monomer solution, but before electropolymerization, shows two atomically flat terraces9 separated by a ca. 0.75 nm step defect slanting across the initial HOPG surface. Panels 1B and 1C show the surface of the PPO polymer formed after a single, and after 32, oxidative potential scans. Images like those in panels 1B are 1C are stable and reproducible as long as the force between the tip and the sample is kept low (ca. N, see ExperimentalSection). Four featuresare noteworthyand are typical. First, the PPO film both covers the electrode completely and reproducesloaits contours, including the step defect in panel A, whose location drifted to the right (9) We routinely obtain atom-resolved images of HOPG in air, under CH&N, and at the bottom of holes nano-dozed in PPO. (10) (a)Occasionally,we observe apparentlyenhancedpolymer growth at step defects in the HOPG. (b) Note that the X-Y scale in Figures 1-3
is greatly compressed relative to the Z-scale.
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1000 nm Figure I. Unfiltered 1 X 1pm AFM images: (A) bare HOPG electrode; (B) PPO film deposited after one oxidative potential scan, anodic charge density ca. 6.2 mC/cm2;(C) PPO film after 32 oxidationscans with monomer solutionreplacement after scans 1, 2, 3, 4, 5, 7, 12, and 22, total anodic charge density ca. 35 mC/cm2. All were obtained under 24 mM Me4N+PhO-/O.l M BwNClOdCH3CN with the HOPG electrode at open circuit.
between panels 1A and 1B. Second, the surface of the initially formed PPO film is almost molecularly smooth.lob The panel 1B root mean square surface roughness is 0.55 nm and the average peak-to-valley difference is 0.8 (f0.5) nm. Since a phenoxy monomer unit is ca. 0.5 X 0.25 nm, these surface roughness values correspond to PPO chain protrusions above the average polymer surface of only one to two and two to four monomer units, respectively. No significant surface roughness differences have been observed between PPO films imaged in situ immediately after electropolymerization and those which are subsequently rinsed and dried and are imaged in air. Third, occasional large hillocks appear; panel 1B shows two that are ca. 100 nm wide a t base and 2-5 nm high. Five others appeared in a larger 25 X 25 pm image of this film. The hillocks seem to represent defect regions that somehow do not passify completely and allow continued, localized PPO film growth. Fourth, repetitive cell solution replacement and oxidative potential scanning (panel IC) enhances the PPO surface texture (2.1 nm root mean square roughness, 3.7 (f2.1) nm peak-to-valley) and thickens the film. The film thickening, which exceeds that in previous experiments: seems to be invoked by our procedure of introducing fresh monomer solution into the cell between successive oxidative potential scans (vide infra). When large tip-sample forces are applied, the AFM tip seems to function as a molecular bulldozer (a nano-dozer), pushing polymer out of the way in the scan area, and ultimately producing a film-free region whose shape and dimension roughly correspond to the scan area. Similar behavior was observed by Hansma et al.lla during AFM imaging in air of Langmuir-Blodgett monolayers of
Images of Formation of a Dielectric Polymer Film
Figure2. Patterns formed by tip at high force settings;see text. Top (unfiltered) images: (A) 2 X 2 pm scan of a (one oxidative scan) PPO film with a nano-dozed square hole, (B) 5 X 5 pm scan of the panel 1B film after forming a 1 X 1 pm hole, then a second oxidativescan,and forming a ca. 150 X 150 nm hole. Both images were obtained under monomer/electrolytesolution as in Figure 1. Bottom: Line scans along the direction of the arrow in the image; height scale corresponds to image gray scale.
cadmium arachidate on mica substrates. The authors reported that by applyinghigher tip-sample force,llbthey were able to form a 36-nm diameter hole in the film, whose =3 nm depth was close to that expected for a monolayer of cadmium arachidate. Hoh et al.12 recently used the same idea to dissect isolated hepatic gap junctions adsorbed to glass in phosphate-buffered saline solutions; increasing the tip-sample force from =l nN to =lo nN selectively removed the top membrane of the junction, thus exposing the extracellular surface of the bottom membrane. The present work demonstrates that AFM can be used in situ to pattern molecular films formed at electrodes and to follow changes in film thickness, which has provided insight into the mechanism of film growth. Figure 2A shows a ca. 0.7 X 0.7 pm film-free region in a PPO film (the film was prepared as in panel 1B) that was formed and imaged in situ under BwNC104/CH&N solution. The film-free region was formed by increasing the force between tip and polymer surface until the image became unstable, indicating the initiation of the nanodozing process.13 High force scanningwas continued until the PPO was completely removed from the scan area, whereupon the image restabilized. The images in Figure 2 were obtained, after nano-dozing, by returning the force to a low value, which gives stable images, and increasing the scan area. Depending on how the PPO film is prepared, nano-dozing can be accomplished either in air (with forces ca. 250-3000 nN) or under CH&N solution (forces ranged from ca. 25-250 nN). Patterns can be formed in thin PPO films (Figures 1B and 2A) using forces at the low end of the range. Indeed, PPO films formed during only a single electropolymerization scan are occasionally damaged during routine imaging if the force is not minimized sufficiently. Thicker (and possibly more cross-linked) films (panel 1C) are more robust and can require much larger forces. Observations of the hole formation process indicate that the rate of PPO film removal is not uniform across the entire area being scanned at high force. Typically film removal is initiated within one section of the scan area, where presumably some weakness in the film exists, and often the film is completely removed from ~~
(11) (a) Hansma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; Longo,M. L.;Zasadzinski,J. A. N. Langmuir 1991,7,1051-1054. (b) The authors do not report values for tip-sample force, so comparing the sensitivityof the Langmuir-Blodgett monolayer and the PPO film to the applied force is not possible. (12) Hoh, J. H.;Lal,R.;John,S.A.;Revel,J.-P.;Arnsdorf,M.F. Science 1991,253,1405-1408. (13) During nano-dozing the tip scan rate is often increased from ca. 4 to 20-78 Hz to help initiate and speed film removal.
Langmuir, Vol. 8, No. 5, 1992 1461 this section, while the rest of the scan area remains substantiallycovered with polymer. However, continued high force scanningresults in complete film removal within the scan area. We conclude that the nano-dozing halts when the tip encounters the HOPG surface. This is supported by atom-resolved images of HOPG obtained in the film-free regionsg and the observation that no patterning occurs on bare HOPG control surfaces scanned at the same applied force. Although transfer of PPO from the sample to the AFM tip may occur during film removal, there is no change in the quality of polymer surface and atom-resolved HOPG images, indicating that the tip is not significantly altered by the nano-dozing process. The thickness of the PPO film can be obtained from a surface height line scan that traverses the nano-dozed hole. Placing markers as shown in Figure 2A, bottom, gives a 7.1 (&0.9)-nmfilm thickness, which is in good agreement with our previous observations on PPO films by other methods.8 Interestingly, the tip’s nano-dozing appears to leave a lip of PPO around the hole’s perimeter, much as would a macroscopic bulldozer in digging a shallow pit. The lip in the images (Figure 2) seems most prominent to the left of the hole and has an apparent volume larger than that of the film that has been removed. The exact shape of the lip is not constant during normal imaging and is sensitive to the scan rate and the tip-sample force. These observations suggest that the cantilever tip geometry makes it a better nano-dozer of PPO chains during rightto-left scans than vice versa13and that stick/slip motions as the tip moves right-to-left over the lip14 caused an artifactual image of that region. Analogous artifacts occur in conventional surface profilometry when scanning over large features in the low to high direction. In addition, the lip polymer at the edge of the film-free region is not very adherent, as judged by occasional permanent changes in the shape of the lip during imaging. These difficulties complicate a detailed analysis of the lip shape. The smallest feature that can be accurately nano-dozed in PPO films also remains to be studied in detail; control of the lip, tip size, and f i i thickness are expected to be influential factors. An appealing aspect of the in situ AFM measurement is the ability to investigate nanometer scale increments of film thickening caused by further electropolymerization and to carry out microscopic deposition experiments into previously nano-dozed holes, as illustrated in Figure 2B. The image shows a ca. 1X 1pm film-free region (upper) nano-dozed in a PPO film formed by a single oxidative potential scan, which was subsequently refilled with PPO by flushing the cell with fresh phenolate monomer solution and performing a second oxidative potential scan. The smaller ca. 150 X 150 nm film-free region (lower) was generated by high force scanning following the second potential scan. The line scan shown traverses both features; the middle marker represents the HOPG bottom of the smaller hole. The left-hand marker, in the position of the original 1X 1pm film-free region, shows that this area was partly refilled with 6.2 (fl.O) nm of fresh PPO film during the second oxidation scan. This is the same film thickness typically produced by a single potential scan (see Figure 2A). The similarity of film deposition in the 1 X 1pm hole (effectively a microsquare electrode) and on a bare electrode suggest that it may be possible to prepare thin, patterned molecular films in which holes created in one film are filled, just to the brim, with a film of a different comr>osition. (14) The AFM tip is continuously rastered relative to the sample between the right and left boundaries of the scanned area, but the image displayed is composed of only right to left scans.
1462 Langmuir,
VoZ. 8,No. 5, 1992
A 400 nm
B 400 nm
Figure 3. Unfiltered 1 X 1 pm AFM images of PPO films on HOPG electrodes formed by (A) 22 continuous oxidative potential scans without solution replacement,total anodic charge density = 30.9 mC/cm2,and (B) oxidative potential scanning (22 total) with cell solution replacement after scans 1,2,3,4,5,7,12,and 17, total anodic charge density = 38.5 mC/cm2. The films in parts A and B were grown in a conventional two-compartment cell, rinsed with CH&N, dried, and imaged in air. The image gray scale is 15 nm.
The right-hand marker in the Figure 2B linescan shows that the PPO film formed by two oxidative potential scans is 9.7 (f0.5) nm thick, and thus grew by ca. 3 nm during the second scan. Continuing the sequence of phenolate monomer solution replacement and repeated oxidative scans (32 total) produced the film whose image appears in panel C of Figure 1. In addition to the previously mentioned roughening, film thickness measurements revealed that the film in panel C had grown from its original ca. 7 nm thickness to ca. 18 nm. This shows that further film thickening does occur, but by relatively small increments per oxidative scan. The film thickening upon continued oxidative potential scanning described above was not observed in our previous investigations of PPO films.8 The ability to grow significant amounts of additional polymer was surprising, given the insulating character of the films, and their established ability to block access to the electrode by redox probe molecules such as ferrocene.8 We hypothesize that the thickening is caused by slow permeation of phenolate monomer into the already-formed PPO film during the time spent imaging the film with the AFM (and in the replenished monomer solution) between individual potential cycles. Because of their proximity to the electrode surface,these intrufilm monomers can be oxidized to form new polymer during the next oxidative potential scan. Figure 3 presents results which support this hypothesis. PPO films were grown at HOPG electrodes in a conventional two-compartment electrochemical cell, using two different protocols. After being rinsed with CH&N and dried in a stream of Ar, the two films were imaged with the AFM in air. The first film (panel 3A) was prepared as reported previously: by continuously cycling the electrode potential, 22 times, through the phenolate oxidation wave (see Experimental Section). The second film (panel 3B) was prepared as in the in situ AFMI electrochemistry experiments: oxidatively scanning the
Goss et aZ.
electrode potential for a single cycle, flushing the cell with fresh monomer solution, waiting several minutes, then scanningthe potential again, flushing the cell periodically, etc., up a total of 22 potential scans. AFM analysis of these two films reveals striking differences in their morphology and thickness. Deposition by continuous potential cycling (panel 3A) producesa film with an appearance (0.72 nm root mean square roughness, 1.2 (*0.6) nm peak-to-valley) similar to that observed for a film formed during only a single potential cycle (panel 1B). The thickness was found to be 6.7 (f0.9) nm, again close to that for the film in panel lB, and near that reported previously.8 In contrast, for the PPO film deposited by the cycle/flush procedure (panel 3B), both the enhanced roughness (2.0 nm root mean square roughness, 3.7 (f2.1) nm peak-to-valley) and film thickness of 15 (f3.1) nm are very similar to that observed for the film prepared by the analogous procedure in the in situ AFMIelectrochemistry experiment (panel 1C). These results are significant because they demonstrate for the first time the ability to both control and monitor the nanostructure of an ultrathin molecular film. The differences in film deposition revealed in Figure 3 may arise from differences in the flux of phenolate monomer to the active HOPG electrode surface in the two deposition protocols. In the case of continuous potential cycling, the flux of phenolate monomer to the HOPG surface is quickly depressed after the first scan by the formation of a poorly permeable layer of PPO and by depletion of phenolate from the diffusion layer due to the irreversible nature of the oxidation reaction. In addition, many of the phenoxy radicals produced may go on to form soluble PPO oligomers that are too large to permeate the nascent PPO film and do not contribute to additional film growth. This is supported by the low current efficiency15 of the reaction, ca. 5% ,which indicates that only a fraction of the oligomers produced actually coat the electrode surface. Thus, film morphology and thickness resulting from many continuous scans differs little from that resulting from a single oxidative cycle (panels 1B and 3A). Replenishing the monomer solution between oxidative cycles, on the other hand, both refreshes the concentration of phenolate in the diffusion layer, and allows time for additional increments of phenolate to permeate into the nascent PPO film to be oxidized at the HOPG surface during the next potential scan. Consistent with this view is the enhanced current efficiency, ca. lo%, observed for this method. One possibility is that confinement of the phenoxy radicals produced within the PPO film increases the probability, relative to that in solution, that the PPO oligomers formed will add to the existing film. PPO films as thick as 35 nm have been grown by the solution replacement protocol; thicker films have not been attempted. Note that in both protocols we assume, given that PPO is an electronic dielectric, that phenolate monomer oxidation occurs at the electrode/PPO interface, i.e., that phenolate must permeate through the PPO film to be oxidized and activated for polymerization. Whether polymer actually forms there, or at the PPO/monomer solution interface, is a separate and as yet unresolved issue. The results in this paper that show complete electrode surface coverage by PPO with a molecularly smooth ca. 7 nm film after a single oxidative electropolymerization (15) We define current efficiencyas the ratio (nFN/Q)X 100%, where n = 2, F = Faraday’s constant, N is the number of moles of phenoxy monomer in a typical PPO film, and Q is the anodic charged passed. N
was estimatedfrom the film volume, assuminga density of 1.25 /cm3,and the molecular weight of phenol. Thus, for example, the current efficiency is 5.6% for the film in panel 3A and 10.0% for the film in panel 3B.
Images of Formation of a Dielectric Polymer Film scan differ from the behavior reported for conducting polymers. Fan and Barda observed, by STM, nonuniform electrodeposition of poly(pyrro1e) onto Pt at ca. 4 nm thickness; full coverage required growth to about 15 nm. Jeon et al. observedea similar behavior for poly(aniline) films on Au electrodes. Possible reasons for the more uniform coverage and molecular-level smoothness of the PPO films are (i) smoothness and low defect density of the electrode material, (ii) the dielectric nature of PPO films, which leads to passivation of the electropolymerization rather than extension of an active (conducting
Langmuir, Vol. 8, No. 5, 1992 1463 polymer) electrode surface, and (iii) mechanistic differences in the electropolymerization reactions (e.g., oligomer growth in solution vs at the electrode/polymer surface). Understanding these topics will require further studies but they appear to be central to the area of electrochemically-induced ultrathin polymeric film growth.
Acknowledgment. This research was supported in part by a grant from the National Science Foundation Materials Chemistry Initiative.
