Langmuir 1992,8,2810-2817
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Preparation and Characterization of Laterally Heterogeneous Polymer Modified Electrodes Using in Situ Atomic Force Microscopy Jay C. Brumfield, Charles A. Goss, Eugene A. Irene, and Royce W. Murray' Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received June 12,1992. In Final Form: August 5, 1992 This paper describes the use of tip-perturbation effects in atomic force microscopy (AFM)to pattern thin f i i of in situ electrochemically-formed polymers. Applications of this technique to fabricate microelectrodes and junctions between the edges of very thin, dissimilar polymer films are presented. These are the f i t examplesof spatiallypatterned, laterallyheterogeneouspolymer f i b s having thickneseea in the 5-200-nm range.
Introduction This paper describes consequences and applications of atomicforce microscopy(AFM)' tip-sampleperturbations in thin, electrochemically-deposited polymer films, including the first spatially patterned junctions between the edges of dissimilar polymer films which are only a few nanometers thick. The AFM images presented simultaneously illustrate the capabilities and some of the complications associated with this application of AFM. A variety of thin film materials have been patterned with the probe tips of the scanning tunneling microscope2 (STM) and atomic force microscope. STM was employed by Eigler and co-workers3to order xenon atoms adsorbed on a Ni(ll0) surface. McCord and Pease' employed the STM tip as an electron field-emission source to expose conventional resists and produce a thin-film resistor, the first working microelectronic device fabricated with the STM. AFM tip-sample force alterations have been used to manipulate proteins6 and Langmuir-Blodgett (L-B) filmss on mica substrates and tert-butylammonium ions on zeolite surfaces.' Hoh et al.8used AFM to dissect and image isolated hepatic gap junctions adsorbed to glass in phosphate-buffered saline solutions. Pita and scratches have been constructed on polycarbonate surfaces,S and complex featureshave been formed on metal chalcogenides by layer-by-layer etching.1° Tip-induced ordering of polymer surfaces perpendicular to the AFM fast scan direction observed on polystyrene" is here also seen on poly(methy1methacrylate) and poly(pheny1ene oxide). (1) (a) Binnig, G.;Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986,56, 930-933. (b) Sarid, D.;Elings, V. J. Vac. Sci. Technol., B 1991,9,431437. (2) Cataldi, T.R.I.; Blackham, I. G.;Briggs, A. D.; Pethica, J. B.; Hill, H. A. 0. J. Electroanal. Chem. 1990,290,l-20. (3) Eigler, D. M.; Schweizer, E. K. Nature 1990,344,524. (4) McCord, M. A.; Pease, R. F. W. J. Vac. Sci. Technol., B 1988,6, 293-296. (5) Lea, A. S.;Pungor, A.; Hlady, V.; Andrade, J. D.; Herron, J. N.; VW, E. W., Jr. Langmuir 1992,8, 68-73. (6) Hausma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.;Longo, M. L.; Zasadzineki, J. A. N. Longmuir 1991, 7, 1051-1054. (7) Weisenhom, A. L.; Mac Dougall, J. E.; Gould, S. A. C.; Cox, 5.D.; D.;Ha"a, Wise, W.S.;Maeeie, J.;Maivald,P.;Elings,V.B.;Stucky,G. P. K. Science 1990,247, 1330-1333. (8) Hoh, J.H.;Lal,R.; John,S. A.;Revel, J.-P.;Arnsdorf,M. F. Science 1991,253,1405-1408. (9) Wiesendanger, R. Appl. Surf. Sci. 1992,54, 271-277. (IO)Delawski,E.;Parkinaon,B. A.J. Am. Chem. SOC.1992,114,16611667. (11) hung, 0.M.; Goh, M. C. Science 1992,255,6146.
We recently reported12atomic force microscopy images chronicling the in situ electrochemically-inducedformation13of a dielectric polymer film (poly(pheny1eneoxide), PPO)" on HOPG (basal plane highly oriented pyrolytic graphite)surfaces. These experimentsindicatedthat PPO films gradually roughen and thicken as a result of subsequent polymer depositions, and demonstrated that film topography is a function of depositionprotocol. Film thickness measurements could be conducted in situ through a process of tip-sample force adjustments (described as "nanodozing") where the tip is used to remove polymer from a selected region of the sample. Film thicknemes ranged from ca. 4.5 to 35 nm; thickneeeee up to ca. 200 nm have since been determined for other materials. Atom-resolved images of the HOPG electrode surface inside nanodozed holes and the ability to redeposit PPO into these holes have demonstrated12that thew regions are free of insulatingpolymer and are thus active electrode surfaces. We exploit this observation in the present paper by filling the nanodozed patterns with other polymers to make laterallyheterogeneousfilmsin a spatiallycontrolled manner. Scheme I outlines such experiments,which rely on PPO as a pattern-transfer material that can be removed from a submicrometerelectrode area by the AFM tip, and electrochemical formation of a secondary polymer which ideally occurs only on the exposed electrode area. Secondary polymers include the dielectric polymer poly(mfluorophenylene oxide) (P(m-FPO)) and the conductive polymers poly(tetrakis(0-aminopheny1)porphyrin) (P(H2(o-NH2)TPP))and poly(cobalt tetrakis(0-aminopheny1)porphyrin) (P(Co(o-NH2)TPP)). We are able to ascertain the degree and topography of hole filling, and produce a junction between the edges of two different electrochemically-formed polymer films, each of which is only a few nanometers thick. These resulte represent, to the best of our knowledge, the first examples of spatially patterned, laterally heterogeneous polymer films produced by a microprobe tip technique. The possible future utility of this technique in the fabricationof microelectronicdevices will be determined by the polymer materials available and ~
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(12) Goss,C. A.; Brumfield, J. C.;Irene, E. A.; Murray,R. W. Langmuir 1992,8,1459-1463. (13) (a) Abrunfla, H. D. Electrode Modification with Polymeric Reagents. In Electroresponsiue Molecular and Polymeric Systems; Skotheim, T.,Ed.;Marcel Dekker: New York, 1988. (b)Molecu&rDesign of Electrode Surfaces; Murray, R. W., Ed.; J. Wiley: New York, 1992. (14) (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.
0743-7463/92/2408-2810$03.00/0Q 1992 American Chemical Society
Langmuir, Vol. 8, No. 11,1992 2811
Laterally Heterogeneous Polymer Modified Electrodes Scheme I. General Protocol of a Nanodozing/ Hole-Filling Experiment.
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Polymer
Different Secondary Polymer Topologies
a Bare electrode is initially investigated with the AFM both in air and under phenolate monomer solution. PPO is then electrochemicallydeposited in situ, forming a film which completely covers the electrode surface. The PPO film surface is imaged at low forces. Polymer is then removed from a selected area of the electrode surface via the nanodozing process. The nanodozed pattern is then imaged a t low forces. Phenolate solution is rinsed from the AFM fluid cell, and the cell is filled with the monomer solution of the secondarypolymer. Secondary polymer is then electrochemicallydeposited, and the patterned area is reinvestigatedto determine the extent of the hole filling.
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Figure 1. In situ polymer depositionvoltammetry. Pt auxiliary electrode and Ag wire pseudoreference employed throughout. (A) PPO deposition at a bare HOPG electrode from 24 mM monomer solution. Potential scanned (0.1 V/s) from 0 to +1.6 V and held there for 15 min. (B) P(H~(o-NH~)TPP) deposition at a HOPG electrode coated with a PPO film deposited as in (A) and subsequentlypatterned through the nanodozingprocess (0.5 mM porphyrin monomer). Potential scanned (0.1 V/s) from 0 to +1.6 V and back. (C) Three (not continuous) P(m-FPO) depositioncycles at a HOPG electrodecoated with PPO as in (A) and subsequently nanodozed. Potential scanned (0.1 V/s) from 0 to +1.6 V and back in 24 mM monomer solution. Fresh monomer solutioninjectedbetween depositioncycles,which were ca. 20 min apart. Note decrease in peak current over subsequent cycles, indicating the deposition of a blocking (dielectric)film. S = 1000, 20, and 200 pA for (A), (B), and (C), respectively. AgQRE potential vs SSCE ca. -0.76,0, and -0.39 V in phenolate, H~(o-NH~)TPP and, m-fluorophenolate solutions, respectively.
PMMA Film Preparation. Poly(methy1 methacrylate) (PMMA) films were prepared by placing a drop of 1%(mass/ vol) PMMA (Aldrich, medium M W ) in chlorobenzene (Alfa) on a freshly-cleaved HOPG electrode and spin-coating (Headway Research Model EC101) at 2000 rpm until dry. Conventional Experimental Section surface profilometry was performed with a Tencor Alpha-Step 100 profilometer. Electrode Preparation. HOPG (supplied by Dr. Arthur W. AFM/Electrochemistry. Experimentswere conductedusing Moore, Union CarbideCo.) was employed as the working electrode a NanoscopeI1AFM, Nanoprobe cantileverswith integral SisN4 material in all experiments. Electrodes were fabricated by tips (100 pm, 0.58 N/m spring constant), and a glass cell a f f f i g HOPG to magneticsteeldiskswithdouble-sidedadhesive attachment (Digital Instruments, Santa Barbara, CA) containtape (Scotch). Electrical contact to the disk was made with a ing the AFM tip and HOPG working, Pt counter, and Ag thermally-cured Ag epoxy (Epoxy Technology, Inc., epo-Tek pseudoreference electrodes. The Ag and Pt electrodes were H20E). Fresh HOPG surfaceswere obtainedfor each experiment inserted in the cell inlet and outlet ports, respectively. Potentials by cleavingwith adhesivetape (Scotch). The geometricelectrode herein are cited vs AgQRE. area exposed in the AFM cell was limited to ca. 0.16 cm2 by AFM/electrochemicalexperimentsbegan by first imaging the masking the surface with a thin layer of thermally-cured epoxy HOPG working electrode in air. Typical AFM acquisition (Dexter Adhesives, Epoxi-Patch). parameters were A + B signal -4.5 V, A - B signal --1.7 V, Chemicals. Phenolate monomer (PhO-) (24 mM) or m-flusetpoint voltage -1.3 V, integral gain 0.8, proportional gain 1.5, orophenolate monomer (m-FPhO-) (24 mM) in 0.2 M tetrabutwo-dimensional gain 0, scan rate 4.34 Hz, 2 attenuation l X , tylammonium perchlorate (TBAP)/CH&N solutions were emfilters off. After obtaining atom-resolved images,lSthe tip was ployed during PPO and P(m-FPO) depositions, respectively. withdrawn 1+15 pm from the surface, the cell filled with Phenolate solutions were prepared" by first dissolving phenol phenolate monomer solution,and the bare HOPG surfaceimaged (EM Science) or m-fluorophenol (Aldrich) and TBAP (GFS again. HOPG image quality was generally identical in air and Chemicals) in CH3CN (EM Science, OmniSolv) followed by a under monomer solution. The dielectric polymer PPO was molar equivalentof tetramethylammonium hydroxide pentahydeposited by scanning the HOPG electrode potential (0.1 V/s) drate f Aldrich), dissolved via sonication. Since phenolate and from 0 to +1.8 V vs AgQRE and holding it at this value for 15 m-fluorophenolate solutions undergo a color change from clear min. The observed electropolymerizationvoltammetry (Figure to yellow/brown over a period of several days (photooxidation 1A) agrees with that previously observed on Au and Pt elecand air oxidation of phenol have been documented), fresh t r o d e ~ Potential .~~ control was provided by a Pine Model RDE4 monomer solutionswere used in each experiment. The solutions potentiostat. do not appear to change during the experiment. H~(o-NH~)TPP Successful imaging of these polymer films requires minimi(0.5 mM) or Co(o-NH2)TPP (0.5 mM) in 0.2 M tetraethylamzation of the tipsampleforce. Thiswas accomplished by engaging monium perchlorate (TEAP, Fluka)/CH&N solutions were the AFM tip while scanning over a relatively small area (ca. 100 employed in the poly(porphyrin)depositions. TBAP and TEAP X 100 nm) of the polymer surface, decreasing the tipsample were recrystallized 2 X from ethanol (EM Science) and H2O (Barnstead nanopure), respectively, and CH&N was dried over (15) (a) White, B. A.; Murray, R. W. J. Electtoanal. Chem. 1986,189, 4-A molecular sieves. Porphyrins were synthesizedaccordingto 345. (b) Bettelheim, A.; White, B. A.; Raybuck, S. A.; Murray, R. W. published procedure^.^^ Other chemicalswere used as received. Inorg. Chem. 1987,26,100~1017. All monomer solutions were filtered through 0.2-pm PTFE (16) We routinely obtain atom-resolvedimages of HOPG in air,under membranes (Acrodisc, Gelman Scientific). CHsCN, and at the bottom of holes nanodozed in PPO.
the ability to reproduciblypattern submicrometerfeatures on these surfaces with the AFM tip.
Brumfield et al.
2812 Langmuir, Vol. 8, No. 11, 1992
a nm 5000
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Figure 2. (a) Plane-fit filtered 14.7- X 14.7-pm image of hole nanodozed in PPO film and line scan traversing hole. (b) Same as (a) except after a single deposition of P(H~(o-NH~)TPP). Note image drift. Unfiltered image. Line scan traverses filled hole. (c) Unfiltered 4- X 4-pm image and line scan traversing a 1.2- X 1.2-pm hole nanodozed in PPO film after a single P(H~(o-NH~)TPP) deposition. (d) Unfiltered image; same as (c) except after two additional P(H~(o-NH~)TPP) depositions. Line scan again traverses filled hole. Line scan height scales as indicated. Arrows indicate approximate line scan positions. Image gray scales 100,400,60, and 500 nm, respectively. All images obtained with HOPG electrode at open circuit. Image a obtained under 24 mM Ph0-/0.2 TBAP/CHaCN Images b, c, and d obtained under 0.5 mM H2(o-NH2)TPP/0.2M TEAP/CHsCN. force by adjusting the set-point voltage to increasingly negative values until the image became unstable, and then backing off by +0.2-0.3 V. PPO films are occasionallydamaged during routine imaging if the force is not minimized sufficiently. 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 separatesfrom the sample,which is assigned a value of zero. In air, stable and reproducible images of PPO molecular film surfaceswere obtained using forces of ca. 100nN. Under CH3CN solution, the force used for stable imaging was lower,typically ca. 10nN. These values are approximatebecause differences between individual tips and instrument drift give rise to 100-200% variations in force. Removal of polymer with the tip was performed by nanodozing as previously described12 by decreasing the scan area and increasing the tip-sample force (to ca. 250-3000 nN in air or 25-250 nN under CH3CN) to produce an unstable image, indicating the onset of polymer removal." Once the image restabilizes, nanodozingis terminated by decreasing the force to the minimum stable value, and the scan size is then increased to image the patterned region. Secondarypolymers were deposited following patterning and imaging of PPO films. Withdrawingthe tip 10-15 Fm, phenolate monomer was flushedfrom the cell with CH3CNwhich was refilled (17) The tip scan rate is often increased from ca. 4 to 2&78 Hz to initiate and speed the nanodozing process.
with the new monomer solution. Poly(p0rphyrin)and P(m-FPO) depositions were performed by cycling the working electrode potential (at 0.1 V/s) from 0 to +1.6 V vs AgQRE and back (Figure 1B,C, respectively). Root mean square (RMS)surfaceroughnesswas calculated by the vendor's software in the top-view mode for representative 1-pm X 1-pm regions. Film thicknesses were determined by averaging the values obtainedfrom 20-30 differentlocationsalong line scans traversing empty or secondary polymer-filled nanodozed holes. Uncertainties are given as 1 standard deviation and represent errors in the precision of height measurements attributable to sample tilt and surface roughness. Error bars do not reflect possible inaccuracies caused by piezoscanner nonlinearity or elastic deformationof the polymer surfaceby the tip.
Results and Discussion Description of Tip-InducedFeature Formation. A variety of features can be induced in thin polymer films with the AFM tip through tip-sample force alterations. When relatively low forces (ca. 10 nN under CH3CN) are employed, the polymer surface can be imaged nondestructively to produce images such as previously presented.12 Our working definition of nondestructive imaging is that the sample surface can be scanned for long periods of time with no apparent changes in the image, and that enlarging the scan area shows no evidence for irreversible changes in the area previously scanned. Application of
Laterally Heterogeneous Polymer Modified Electrodes
Langmuir, Vol. 8, No. 11,1992 2813
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Figure 3. (a) Unfiltered 5- X 5-pm AFM image of nanodozed hole in a PPO film deposited on a HOPG electrode. Image obtained under 24 mM Ph0-/0.2 M TBAP/CH&N with the HOPG electrode at open circuit. Image gray scale 50 nm. (b) Same as (a) except after depositing P(m-FPO) for one oxidation cycle. Imaged under 24 mM m-FPhO-/0.2 M TBAP/CH&N. (c) Same as (b) except bottom half of image depicts preferential tip-induced damage of P(m-FPO) surface. Image gray scale 100 nm. (d) Unfiltered 14.7X 14.7-pm image of the previously depicted region after two additional oxidative depositions of P(m-FPO). Gray scale 800 nm.
high forces (up to ca. 250 nN under CH3CN) results in the removal of polymer material from the substrate surface via the nanodozing process, producing holes in the film such as those illustrated in Figures 2a,c, 3a, and 4C. Intermediate forces (insufficientto cause complete removal of polymer from the electrode surface) can cause polymer material to become oriented by the tip into ridges which generally lie perpendicular to the AFM fast scan direction, such as in Figure 4A,B. The different features in polymer films resulting from these AFM tip force changes will be discussed beginning with features formed at high (nanodozing) forces. Scheme I outlines the nanodozing and hole-filling experiment, which begins with in situ deposition of a poly(phenylene oxide) film which is then imaged at low forces and nanodozed over a smaller area to produce a hole in the film which extends down to the electrode surface. The nanodozed hole and the surrounding film are then imaged at low forces, and the PPO film thickness is determined from line scans traversing the hole as in Figure 2a,c. The polymer films do not appear to undergo any significant changes over time, neither when dry nor when immersed in monomer solution. AFM measurements of film thickness before and after exposure to CH3CN indicate that PPO films may exhibit a small degree of initial swelling (1-h equilibration times after assembling and filling the electrochemical cell can minimize such effects. Identification and elimination of thermal and/or mechanical drift would be crucial for use of the AF’M tip for precise patterning applications. However, the shape of the nanodozed features is not a central feature in the present experiments. The line scan shown below the image in Figure 2a gives a PPO thickness of 12 f 0.6 nm with an RMS roughness of 1.4 f 0.2 nm. The thickness measurement was read near the bare HOPG/PPO film interface since the line scan appears bowed. Two HOPG step defects can be seen in Figure 2a: one at the lower right and one in the upper center that is partially exposed by the nanodozed hole. The PPO film follows the contours of these defects (and
those in Figures 3 and 4) as previously reported.12 The PPO f i isurfacearound the nanodozed hole exhibits some long ridges of material of 30 f 8 nm average height which possibly may have resulted from tip-sample interactions (videinfra), may represent regions of the electrode surface over which the PPO film growth passifies less quickly (hillocks and apparently increased polymer growth at HOPG defects are occasionallyobserved12),or may simply be adventitious debris like that previously reported.la P(Hz(o-NH2)TPP) was next deposited as in the Experimental Section, with associated voltammetry as in Figure lB,and the nanodozed hole imaged (Figure 2b). The result is an irregular-shaped island of poly(p0rphyrin) roughly 2 pm X 7 pm, surrounded by the substantially unchanged PPO surface. We observe that the deposited poly(porphyrin)extendslaterally beyond the original hole, and its top surface lies 120 f 4 nm above the surrounding (18)(a) Clemmer, C. R.; Beebe, T. P.,Jr. Science 1991,261,640-642. (b) Chang, H.; Bard,A. J. Langmuir 1991,7,1143-1153.
Laterally Heterogeneous Polymer Modified Electrodes PPO film surface(ie., Scheme Id). By overlayingidentical scale images of the original (Figure 2a) and filled (Figure 2b) hole, it appears that the lateral spreadingis not uniform around the entire hole perimeter, varying from ca. 100 to 800 nm, with the largest amount of spreading occurring at the right side of the hole. Line scans as that below Figure 2b show that the top of the poly(porphyrin) island is quite flat, having a 2.1 i 0.7 nm RMS roughness which amounts to variations of only a few porphyrin monomer molecular dimensions over many lateral micrometersof poly(porphyrin)surface. The “shore”of the poly(porphyrin) island slopes1sup to its top at a ca. 14O angle, but this is approximate because the image is to a certain extent a convolution between actual surface geometry and the shape of the tip.2o A repetition of this experiment but with a smaller (1.2 X 1.2 pm) hole nanodozed in a PPO film following the fiist of severalP(H2(0-NHz)TPP)depositionsis shown in Figure 2c,d. Figure 2c shows the ca. 1.2- X 1.2-pm hole in the 15.2 f 0.4 nm thick film, which in this example is more square shaped due to better control of drift. As we commonly observe, the smaller nanodozed hole is surrounded by a lip of excavated polymer. The lip tends to mostly occur at the left edge of the nanodozed hole, and ita apparent dimension is exaggerated by what we suspect are stickslip effects there.l2T2lSubsequent fiing of this nanodozed feature by oxidative electropolymerizationof H2(o-NH2)TPP is shown in Figure 2d. The result is a roughly 1.6X 1.6-pmsquare of porphyrin polymer. Line scans (Figure 2d) across the poly(porphyrin) island show that its center is again nearly molecularly smooth as in Figure 2b. Its shore has a ca. 35O slope, and in this case, there has been an enhanced deposition of polymer around the rim of the island (i.e., SchemeIC).The borders of the poly(p0rphyrin) island extend ca. 400-700 nm over, and the central part and rim lie, respectively, ca. 100 and 200-300 nm above the surrounding PPO film surface. The experiments in Figure 2 serve to define many of the principal features of microscopic hole-filling experiments using the porphyrin monomer. One feature of importance is the delineation of the poly(porphyrin) island, or the degree to which the electropolymerizationreaction yields a polymer film of the same dimensions as the active electrode. Parts b and d of Figure 2 show that the extent of lateral spreading (100-800and 4W700 nm, respectively) is larger than is the vertical thickness (132 and 115 nm, respectively, accounting for the PPO thickness) of the electrodepositedporphyrin. The enhanced lateral growth is plausibly attributed to a degree of diffusive removal of activated porphyrin monomer from the active electrode area before monomer-monomer coupling reactions occur and lead to formationof insoluble,precipitating porphyrin oligomer. That is, polymer chain growth must be substantially initiated in solution as opposed to grafting of an activated porphyrin monomer onto a poly(porphyrin)film. The cross-sectional profile of the poly(porphyrin) island, (19) Note that the X-Y scale in Figures 1-4 is greatly compreased relative to the 2 scale. (20) We have performed calculationa which predict the image dependence on tip shape,the angle of the tip with respect to the sample surface, and the height of features on the sample surface. The tip shape is assumed to be pyrimidal with 2-cm sides and a 70° internal angle and is mounted at a ca. loo angle. (21) (a) 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. (b) Previous observations12 suggest the tip is a better bulldozer in one direction than in the other. (c) Most artifactual heights/streakii are observed when the tip traverses a very high feature or step in the low to high direction. Analogous artifacts occur in conventional surface profilometry when scanning over large features in the low to high direction.
Langmuir, Vol. 8, No.11,1992 2815 flat in Figure 2b for the larger and rimmed in Figure 2d for the smaller island, also attests to the importance of diffusive transport in the electropolymerization reaction. Enhanced diffusive flux at the edges of small electrodes (radial transport) is well kn0wn,2~and the rim of poly(porphyrin)in Figure 2d undoubtedlyarose in this manner. (Its formation may be influenced by the much smaller, preexisting excavatedPPO lip discussedabove.) Diffusive fluxes and transport lengths can be controlled to some extent by the time scale of the electropolymerization e~periment?~ which will be considered in subsequent studies as will the electroactive behavior of the poly(porphyrin) film. The Figure 2 results show clearly, however, that the AFM imaging and nanodozing experiments are useful tools in probing microscopic electrodeposition processes. These are the fiit results probing the microscopic delineation and topography of the edges of electrochemically-deposited polymer films. Another aspect of the hole-fillingexperimenta in Figure 2 is the extent to which electropolymerizationof porphyrin monomer is blocked elsewhere by the low permeability14 of PPO to this bulky monomer. In Figure 2b, following the porphyrin electropolymerization, the PPO surface roughened slightly (from RMS roughness 1.4 to 2.3 nm). Flushing the electrochemical cell, introducing fresh Hz(o-NHz)TPPmonomer solution,and carrying out a second stage of porphyrin electropolymerization produced additional roughening of the PPO surface (to RMS 2.7 nm). Nanodozing a fresh hole nearby in the PPO film revealed that it had been thickened by 7.9 nm to 20.2 i 1.5 nm by the two electropolymerization steps.24 The thickness of the poly(p0rphyrin)film at this point was ca. 144nm, and its roughness had increased from 2.1 f 0.7 to 3.8 f 0.9 nm. The extent to which the PPO film thickening and the rougheningof both films represent simply isolated patches of precipitated poly(porphyrin) as opposed to monomer permeation and formation of poly(porphyrin) within or on the PPO surface is not clear at this point. It is likely to some extent that slow permeation of porphyrin monomer into PPO does contribute to ita thickening. Nonetheless, the relative extent of poly(porphyrin) film growth at the nanodozed hole is quite high, as hoped at the inception of these experiments. Filling of Holes Nanodozed in PPO with Poly(mfluorophenylene oxide). A Higher Permeability Monomer. Figure 3a showsa PPO film that was depoeited in situ and nanodozed as in the Experimental Section. The hole shape again is distorted by tip drift. The PPO film is 8.2 f 0.6 nm thick with an RMS roughness of 4.7 f 0.9 nm. This roughness is greater than usual12 owing to many large raised features (15 f 3 nm average height, up to ca. 0.8 pm long1% These features arose from an initial imaging at forceshigher than those usually employed for stable images and to avoid tip-sample interactions; they and the ridges extending from the edges of the nanodozed hole will be discussed later. The result of flushing the cell with CH3CN and then 24 mM m-fluorophenolate in 0.2 M TBAP/CH&N followed by electropolymerization of P(m-FPO),Figure lC, scan 1, is shown in Figure 3b. Separate experiments on m-fluorophenolateelectropolymerizationshow that it undergoes dielectric film formation analogous to that exhibited by (22) Wightman, R. M.; Wipf, D. 0. Voltammetry at Ultramicroelectrod-. In Electroanalytical Chemistry: A Series of Advances; Bard, A. J., Ed.;Marcel Dekker: New York, 1989 pp 267-348. (23)Zhang, H.; Murray, R. W. J. Am. Chem. SOC. 1991, 113, 51835187. (24) Thickening after one poly(porphyrin) deposition has been demonatrated in analogous experimenta.
2816 Langmuir, Vol. 8, No.11,1992 phenolate. The new island of P(m-FPO) is readily distinguished in Figure 3b, having been imaged at low tip force and exhibiting a smoother topography than the surrounding, previously tip-perturbed PPO. The RMS roughness for the P(m-FPO) film inside the hole is 3.8 f 0.6 nm. Nanodozing a smaller hole, seen at the left of the filled hole in Figure 3b, and taking line scans across the new hole, the filled one, and the surrounding PPO show that the thickness of both the deposit and the neighboring PPO film is 16 f 1nm (i.e., Scheme Ib). The PPO film was noticeably thickened from ita previous value, by ca. 7.7 nm, as a result of P(m-FPO) deposition. The image in Figure 3b representa an edge-to-edge interface between two dissimilar, very thin polymeric materials, and line scans across it present a unique view of the topography of a polymer-polymer interface. The line scans reveal depressions at the PPO and P(m-FPO) interface between 1and 9 nm, an observation suggesting that the two films are quite immiscible, in spite of the secondarypolymerization of m-fluorophenolatethat occurs in or on the PPO film as discussed below. The thickening of the surrounding PPO film in Figure 3b is attributed to oxidative activation and polymerization of m-fluorophenolatemonomer that had permeated into the PPO film. That the degree of permeation of m-fluorophenolate into PPO is substantially more severe than that of the porphyrin monomer (vide supra) is fully anticipated from previous work14 on the permeability of PPO films to redox monomers of different molecular volumes. The difference in the porphyrin and m-fluorophenolate molecular volumes (ca. 1.2 vs 0.1 nm3)should, roughly, yield a 10-1O2-fold difference in permeability. Furthermore, electropolymerizationof m-fluorophenolate solutionsdoes not lead to electrode passivation as quickly as that of phenolate iteelf. Correspondingly,once initiated, additional P(m-FPO) film growth in (or on) the PPO film seems to occur more readily. This is seen from results (Figure 3d) of flushingfresh m-fluorophenolatemonomer solution into the cell followed by two additional electropolymerization scans. The original hole is almost obscured by the further thickening and a film topography now resembling that characteristic of P(m-FPO) films deposited independently on bare HOPG. The film has a RMS roughness of 88f 38 nm and a rather lumpy surface. Similar behavior was observedin depositionsof P(m-FPO) at PPO-coated electrodes that had not been nanodozed. The extensive,further growth of polymer observed in P(mFPO) depositions is in sharp contrast to that observed for poly(porphyrin) depositions (Figure 2). The different behavior of porphyrin and m-fluorophenolate deposition at nanodozed PPO films, particularly in regard to the concurrent thickening of the PPO, stresses the importance of PPO film permeability in preparing laterally heterogeneous polymer films on a nanoscopic scale. The larger, more slowly permeating porphyrin monomer was more effective in this respect. Deformation of Polymer Films at Intermediate TipSample Forces. Ridge Formation. Scanning of the AFM tip across the polymer sample at loading forces exceedingthose necessary for nondestructiveimaging (>10 nN in CH3CN) but less than that employed when nanodozing (ca. 250 nN) frequently produces ridgelike features. These ridges usually run perpendicular to the AFM fast scan direction. Such behavior has been observed for a variety of materials6J1including, in our laboratory, PMMA, PPO, and P(m-FPO). Figure 4A shows a 3- X 3-pm image of a PPO film at low (ca. 10 nN) forces that includes a 1-X 1-pm region which
Brumfield et al.
had undergone repeated scanning at an increased tipsample force (>lo nN). This 8.3 f 0.7 nm thick PPO film had been deposited in situ with a potential scan from 0 to +1.6 V vs AgQRE where it was held for 60 8. The width of the ridges seen in Figure 4A is at the base 121 f 12 nm, and the averageheight as determined from line scansabove the PPO film surface is 10 f 2 nm. The line scan over the film is depressed to the right of each ridge?la*bsuggesting a mechanism of polymer displacement and aggregationto form the ridge. The recorded height of the ridges may be exaggerated since their apparent volume exceeds that of the associated depressions. The height may be somewhat artifactual due to stick-slip motions of the tip when it encounters these features.12t21a*cThat the ridges are present and were formed due to a force-related tip disturbance seems, however, clearly established. Figure 4B shows ridges induced in a PMMA film in an analogous experiment. The poly(methy1 methacrylate) film was prepared on HOPG by spin coating as in the Experimental Section. The film thickness is ca. 20 nm as determined by conventionalsurface profdometryof similar films on glass microscope slides. The tip-disturbed 1pm2 region at the center of Figure 4B was generated by scanning the tip at 78 Hz for 40 min at high force (ca.25&3000 nN). The surrounding film exhibits a large number of features which seem to be pinhole defects ca. 20 nm deep. The observed ridges are 122 f 19 nm wide and 20 f 3 nm high as determined from line scans like that in Figure 4B.The ridges are similar in appearanceto those obaervedby hung and Gob" on a ca. 1-pm-thick film of poly(styrene) on mica. The exact force at which this kind of tip-induced disturbance is initiated has been difficult to determine, but must depend upon variables that include the mechanical rigidity of the polymeric material, ita adhesion to the substrate (in the case of a thin film), the tip geometry, and whether the sample is swollen by a bathing solvent. Lea et have suggested a mechanism of tip-induced orientation of proteins on mica surfaceswhich may explain the behavior observed on polymers. In their model, when imaging proteins at high force, the tip sweeps surface material as it moves in the fast scan direction. Swept protein moleculespile up until the verticalforcecomponent that the aggregate exerts on the tip is sufficient to cause a deflection of the cantilever, opposing ita vertical force component. At this point, the instrument feedbacksystem retracts the 2 piezoelectric and the tip traverses the ridge of protein and touches down on the other side to begin the sweeping process again. A possibile additional aspect of tip perturbations, in view of the behavior observed for PPO films, is the level of adhesion between the extremelythin polymer film and the HOPG substrate. Poor adhesion may initiate a physical motion under the lateral force of the tip that wrinkles the polymer film. This would be analogous to the wrinkling observed from a push on a rug that is tacked down only at one end. Poor adhesion between PPO and HOPG is consistent with the apparent tearing (vide infra) of films beyond nanodozing scan areas. This would also explainthe relative ease with which holes can be nanodozed in PPO films to the point that the bottom of the holes are completely free of PPO.16 The relative mechanical stabilityof the neighboringPPO and P(m-FPO) surfaces was in Figure 3c probed by momentarily increasing the force between the tip and sample. Preferential tip-induced deformation of the P(mFPO) surface was observed. The bottom half of Figure 3c, which was imaged under increased force, shows ridges in the P(m-FPO) material (35 f 8 nm high) oriented
Laterally Heterogeneous Polymer Modified Electrodes
perpendicularly to the fast scan direction. The force between the tip and sample was quickly reduced such that the image of the top half of the filled hole would be unaffected. This observation agrees with other data (frequent unstable images (streaking) and tip-induced perturbations), suggestingthat P(m-FPO)surfacesare less mechanically stable than those of similar PPO films. Ridges of raised material are also occasionallyobserved that extend from the edges of nanodozed holes into the surrounding PPO polymer surface, as exemplified by Figure 4c.% The ridges seen at the bottom of the nanodozed hole are 20 i 3 nm high (determined from line scans). The PPO film is 12 f 1nm thick. The hole-edge ridge features seem to orient perpendicular to the fast tip scan direction, although this is not always the case as seen from the analogous hole-edge ridges seen in Figures 2a and 3a. Tip-wrinkling like that in Figure 4A,B may thus (26) The nanodozed feature in Figure 4C was formed in situ on a PPO f i i following deposition of P(H~(o-NHz)TPP) for one oxidative scan.
Langmuir, Vol. 8, No. 11,1992 2817
not be the sole mechanism that produces hole-edgeridges. Significantly, while frequently observed at irregularly shaped holes, the hole-edge ridges have not been observed at the borders of square holes. The hole-edge ridges are observed on the initial low force (ca. 10nN under CHsCN) imaging scan following nanodozing and are unchanged thereafter. Nonetheless, it is unclear whether the holeedge ridges are formed during the nanodozing process or during the fmt imaging scan. In any event, it seems clear that polymer at lateral interfaces (where the material is not surrounded by a supporting matrix) is more prone to disturbance by the AFM tip, and accordingly even additional caution must be taken in the interpretation of images in near-edge regions.
Acknowledgment. This research was supported in part by a grant from the National Science Foundation, Materials Chemistry Initiative. -@try NO. PMMA, 9011-14-7; P(Ha(o-NHz)TPP),9776648-4;P(CO(O-NH#I'PP),107667-67-4.
