J . Phys. Chem. 1990, 94, 61 17-6122
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Scanning Tunneling Microscopy Evidence of Semicrystalline and Helical Conducting Polymer Structures R. Yang, D. F. Evans,* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
L. Christensen, and W. A. Hendrickson 3M Company, Corporate Research’Laboratory, 3M Center, Bldg. 208-1 -01, S t . Paul, Minnesota 55144 (Received: October 6, 1989; In Final Form: January 16, 1990)
Scanning tunneling microscopy images of strands, microislands, and thin films formed by the conducting polymers polypyrrole tosylate, pol ypyrrole tetrafluoroborate, and polythiophene tetrafluoroborate are presented. The polymer strands and microislands contain two types of helical structures with diameters of 1.5-1.8 nm, a simple helix, and 5-6 nm, a superhelix. The polymer films gradually transform from an ordered crystalline array located at the anode surface to an amorphous material at the air-polymer interface.
Introduction
Conducting polymers have been extensively investigated since the discovery in 1977 that doped polyacetylene was electrically conducting.’ In recent years, heterocyclic polymers based on pyrroleZ and thiopene3 have been the focus of intense study due to their simple formation, relatively high conductivity, and good long-term stability under ambient conditions. Polypyrrole (PPY) and polythiophene (PTP) are most conveniently produced by the electrochemical polymerization of the heterocycle monomer directly onto an anode. The individual monomers maintain their integrity in the polymer and are linked primarily through the a-carbon sites4 as indicated in Figure 1 where the structures of the neutral and conducting polymers are shown. Oxidation of the polymer chain involves the removal of one electron from the *-electron system of the pyrrole ring and the formation of a positively charged radical defect called a polaron with a net spin of 1 / 2 . 5 A second oxidation either on the same chain or on an adjacent chain followed by the two polarons diffusing together results in the formation of a bipolaron with no net spin (Figure Ib). Within the region of the bipolaron, the heterocycles lose their aromatic nature and become equinomial. I t is not known whether the bipolarons form during polymerization or after the chain is completed. It is these positive charges created on the polymer backbone that are the charge carriers for the electrical conduction. Transport occurs via hopping of charges contained in the polymer; and their number and relative mobility determine the bulk electrical conductivity. The counterions (called dopant anions) stabilize the charge on the polymers, but they are not very mobile and appear to play no direct role in the electronic conduction. Thus, these heterocyclic polymers are truly electronic conductors of the p t y p e in which the mobile species are positive carriers.6 There has been a considerable effort to correlate electrical conductance with growth conditions, chemical composition, and ( I ) Shirakawa, H.; Louis, E. J.; MacDiarmid, A . G.; Chiang, C. K.; Heeger, A . J. J . Chem. SOC.,Chem. Commun. 1977, 578. (2) Bryan Street, G . In Handbook of Conducting Polymers; Skotheim. Terje A.. Ed.: Marcel Dekker: New York. 1986; Vol. I , Chapter 8. (3) Tourillon, Gerard. In Handbook of Conducting Polymers; Skotheim, Terje A.. Ed.; Marcel Dekker: New York, 1986, Vol. I . Chapter 9. (4) Clarke, T. C.; Scott, J . C.; Street, G . B. IBM J . Res. Deu. 1983, 27. 313. ( 5 ) Brtdas. J . L.; Scott, J . C.; Yahushi, K.; Street, G. B. Phys. Reo. B 1984, 30. 1023. ( 6 ) Keiji Kanazawa, K.; Diaz, A. F.; Gill, W. D.; Grant, P. M.; Street, G . B. Synth. Mer. 1980, 1 , 329.
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(b)
Figure 1. (a) Ideal structure of neutral (nonconducting) polypyrrole; (b) ideal structure of oxidized (conducting) polypyrrole showing proposed bipolaron defect structure.
structure of the For a variety of reasons this has proved difficult. ( I ) The bulk polymer is amorphous and virtually insoluble in almost all solvents. Thus many of the standard techniques for characterizing polymers cannot be used. There is almost no information on polymer molecular weights. (2) The chemical composition can show considerable diversity depending upon the electrodeposition conditions. There are indications that changes in the growth rate (or anode voltage) are accompanied by changes in the reaction kinetics.1° A variety of oxygen-containing species are sometimes observed in the polymer films.” (3) There is some evidence that during the growth of the polymer, chain linking defects such as a-(3 or (3-0bonding may occur.12 Such defects are expected to perturb chain flexibility and bipolaron distribution. As a consequence of the complexity of this polymer system, no clear picture has yet emerged. (7) Osaka, T.;Naoi, K.; Ogano, S. J . Electrochem. SOC.1988, 135, 1071. (8) Street, G. B.; Lindsey, S. E.; Nazzal, A. 1.; Wynne, K. J. Mol. Cryst. Liq. Cryst. 1985, 118, 137. (9) Bargan, J.; Mohmand, S.; Waltman, R. J. IBM J . Res. Deu. 1983, 27,
330.
(IO) Asdvapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A,; Pletcher, D. J . Elecrronal. Chem. 1984 177, 229. ( I I ) (a) Saleneck, W. R . ; Erlandsson, R.; Prejza, J.; Lundstrom, 1.; Ingarrao. 0 .Synth. Met. 1983.5, 125. (b) Diaz. A. F.; Lee, W.-Y.; Logan, A . J . Electroanal. Chem. 1980, 103, 311. (12) Street, G . B.; Clarke, T. C.; Krounbi, M.; Konazawa, K.; Lee, V.; Pfluger. P.; Scott, J. C.; Weiser. G. Mol. Cryst. Liq. Cryst. 1982, 83, 253.
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Figure 2. Microislands and helical strands of conducting hctcrocyclc polynicrs formcd during film nucleation and initial growth. (a) Two microislands of polypyrrolate tosylate (PPY-TOS) connncctcd by multiplc lincar hclicrs. ( b ) Small microislands of polythiophenc tctrafluoroborate (PTP-BF,) conncctcd by hclical strands. ( c ) Coiilcsccncc of t i v o polypyrrolc tctr;ifluorobor;\tc(PPY-TOS) to form a 1;irgcr island. (d) Scvcral isolated PPY-BF4 hclical strands.
All of the available evidence suggests that films of these conducting polymers exhibit a disordered noncrystalline molecular structure. However, the X-ray measurements of Mitchell and GeriI3 provide evidence for anisotropy which the! suggest arises from preferential alignment of the heterocycle rings to the clcctrode surface. In addition, Garner et aI.'* obtained evidence for microcrystalline regions from TEV and X-ray measurements. They proposed that these regions contained helical structures. However, these regions accounted for less than 5'7 of the total volume of the sample. In this paper we present scanning tunneling microscopy images of conducting polymer films. These images provide new insight into the structure of conducting polymers. I n particular the!. provide an explanation for the anisotropy and crystalline domains described above. Experimental Section
The samples were prepared by clcctrochcinical deposition onto 6 X 12 mm2 pieces of freshly cleaved. highly oriented pyrolytic graphite (HOPG) using ;i PAR Model I73 potcntiost;it/g~ilvnnostat equippcd with a PAR Model 170 coulomctcr. A onecompartment cell, with a platinum mesh counter electrode. \tiis ( I 3 ) Mitchcll. G. R.; Gcri. A. J . Phjas. I): Appl. /%I..$. 1987. 20. 1236. (14) Garnier. F.: Tourillon. G.:Rarraud. .I.Y.: Ihpcrt. ti. J . Marer Sci. 1985. 20. 2687.
filled with 0.1 M electrolyte solutions (tetraethylammonium ptolucncsulfonate (TOS-),or tetraethylammonium tetrafluoroborate (RF,-)) in either acetonitrile (polypyrrole) or propylene carbonate (polythiophcnc) and 0.1 'M pyrrole or 0.5 M thiophene. All reagents were used a s received. The solutions were deaerated with nitrogen and kept under an argon iitmosphcrc with no additional measures taken to exclude oxygen. The oxidation and polymerization reaction was carried out a i constant current densities of 1.2. 2.5. and 4-5 mA/cm2 for the TOS- and RFq- doped polypyrrole and RF4- dedoped polythiophene, respectively. The ;\mount of polymer deposited onto the tHOPG was controlled by measuring the charge that passed during the dcposition. In these experiments this varied from 30 to 200 niC. Polythiophene dcdopcd samples were produced by electrochemical reduction of the samples already prepared. After formation all samplcs were rinsed with acetonitrile and air dried. During the sample preparation the HOPG anode was partially submerged in the solution so that approximntcly one-half of the siiiiplc \\ils ii\ti\iliiblc for polymer deposition. Partially submerging thc electrode in the electrolyte solution results in a gradation of liliii thicknesses appropriate for studying the nucleation and growth of thc polymers. A commercially available Xanoscopc I I froin Digital Instrunicnts15 used to obtain all ST34 images in this paper. The ( I S ) Digital Instrumcnts. Inc.. Santa Barbara. C A 931 IO.
STM Evidence of Conducting Polymer Structures
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Figure 3. (a) An isolated polythiophene single chain with the helical formation on the clcan HOPG substrate. (A segment of a second chain
is visible at the top of the figure.) (b) A plot of height vs distance along the center of the polymer chain (Figure 3a) showing a regular periodic pattcrn. Thc distance between arrows is 0.9 nm. Fouricr transform analysis of such helices gives a pitch of 0.8 f 0.15 nm. images shown in this report were taken in air from samplcs prepared previously and stored in air. The STM was run in both thc constant height and constant current mode. Where height measurements are reported those images were run at constant current. While STM is uniquely suited to provide information on the height of surfaces features down to atomic levels, caution should bc used to evaluate height data due to the combination of work function and height changes that are not separable.I6 In our study on the helical structures of polythiophcne and polypyrrole, the height data we report are the same as the measured cross-scctional diameter, a measurement less affected by work function changes. The best images for each polymer were obtained by using thc following STM sctpoints: polypyrrole (TOS- and BF,- doped) 20-60 mV bias, 0.5-1.5 nA tunneling current, and 19-39 Hz scan frequencies: polythiophene ( BF,- doped) 60-1 20 mV bias, 0.4-0.7 nA tunneling current. and 4.3-8.7 Hz scan frequencies; and polythiophenc (dedoped BF,-) 0.4-3 V bias. 0.12-0.4 nA tunneling current, and 3-9 HI. scan frequencies. AI1 images obtained for this paper are real-time photographs taken directly from the computer screen. Results and Discussion
We have carried out two types of studies. The first focuses on nucleation and initial growth of conducting polymer structures. Hcrc there are discernible differences in structure that can be ascribed to the influence of the dopant counterion. The second involvcs experiments on electrodeposited films in which wc can trace the transformation from structured nuclei to disordered noncrystalline polymer structures. Initial Growth of Conducting Polymers Films. In Figure 2a-c we show microislands formed by PPY-TOS. PTP-RF,. and
Figure 4. (a) The cross-section profile of the polythiophene polymer chain along the chain dircction. The distance between the two arrows is 0.9 nm. (b) The cross-section profile of the dedoped polythiophene polymer chain along the chain direction. The distancc between the first two arrows is 0.9 nm and the distance between the second two arrows is 0.5 nm. (c) A proposcd modcl for the position of the dopant ions is brtwcen the helical coils ( 0 dopant ion).
PPY-BF,. respectively. These microislands are composed of many discernible polymer stands, appear semicrystalline,and are often intcrconncctcd by multiple polymer strands. In some instances we see individual polymer strands like those shown in Figure 2d for PPY-BF,. Examination of numerous STM images establishes that rill of the polymer strands have diameters of either 1.5-1.8 or 5-6 nm. Structures possessing the 1.5-1.8 nm diameter strands were observed 15 times on separate samples and multiple images of the larger strands were scen on 17 different samples. Wc consider first the structure of thc small polymer strands. They are helices. In a previous note,” we reported a helical structurc for thc PPY-TOS polymer with a pitch of 0.5 nm and a diamctcr of 1.8 nm. A detailed image of the PTP-BF, helix is shown in Figure 3a. Fourier transform analysis of all data gives ;i pitch of 0.8 f 0.15 nm and a diameter of 1.5 f 0.3 nm. These valucs agree quite well with the values of 0.7 and 2.0 nm predicted by Garnicr ct aI.l4 While the counterions appear to play no direct role in thc conducting properties of the polymer, there remains considerable intcrcst in how they are accommodated in the polymer structure. Comparison of the samples containing doped and dedoped polymer providcs a dircct answcr. Cross-sectional profiles of a PTP-BF, hclix with its uniform pitch of 0.8 nm and a partially dedoped PTP helix displaying two (17)
(16)
Baratoff. A. Physicu
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proposed helical model for polythiophene polymer chain. ( b ) The cross-section profile through the polythiophene polymer chain perpendicular to the chain direction. The distance between two arrows is 0.33 nm. Figure 5. (a) A
values of 0.8 and 0.5 nm arc shown in Figure 4, a and b. We attribute the decrease in pitch to the release of counterions bound between the helices upon the reduction and loss of positive charge along the polymer chain. These observations substantiate the helical structure proposed by Garnier et aI.l4 based on X-ray analysis of polymer microcrystalline domains. Additional structural information can be obtained from the cross-sectional profile across a single strand of the dedoped PTP shown in Figure 5. The hemispherical profile exhibits three major and two minor side peaks with a spacing of 0.33 nm that corresponds to five of the individual five-membered thiophene units. Since we can see only the top of the helix with STM, our results indicate that a full coil of the helix will have 10-1 I individual thiophene molecules per coil which is in agreement with theory and space-filling molecular models that we constructed. We might expect that counterion specificity should effect the structure of the helixes. Indeed inspection of interconnectingsingle helical strands for PPY-TOS (Figure 2a) and PTP-BF, (Figure 2b) reveals one significant difference. With TOS- counterions. the helices are linear over distances of 400 nm while those for RF,show a more random orientation. There are obvious differences in shape; BF4- is spherical while TOS- is ellipsoidal. In addition there is the possibility of cxcimcr-like interactions between the polypyrrole and the TOS- counterion. We now consider the structure of the microislands shown in Figure 2a-c. The polymer strands which comprise the structural building blocks of the microislands are linear and a preferential orientation between strands is evident. Fourier transform analysis reveals that these strands as well as the interconnecting strands arc helical with a height and diameter of approximately 5-6 nm (Figure 7a and 8b) and a pitch which is about 2.6 nm (Figure 6a.b). These dimensions arc clearly incornpatable with the formation of the simple helices described above. However. Fourier transform analysis along the individual pitches (perpendicular to the chain axis) shows a very weak, but reproducible. pitch of 0.9 nm (Figure 6c) which is consistent with the pitch for the PTP-RF, si nglc-st ra ndcd helix. We propose that these large structures are super helices formed by the coiling of single-stranded helix. Such structures arc well documented in thd DNA literature'* and result from either the ( 18)
Lilley. D.M.J. Siochenr. So(*.Trans. 1986. / 4 . 489.
Id
Figure 6. (a) The cross-section profile of a polymer strand, which shows regular peaks pattern. The distance between arrows is 2.9 nm. (b) Fourier transform spcctrum curve. where the vertical axis represents the amplitudc of spectral components and the horizontal axis represents the hpcctral pcriod. showing the major periodicity a t 2.58 nm. (c) The crohs-section profile pcrpcndicular to the strand which shows several small pcaks \\ith ;i hcmisphcrical shape. The distance between arrows is 0.9 niii. (d) A proposed model for polymcr supcrhcliccs structure.
coiling of closed DNA loops or the interaction with DNA organizing proteins. While we have no detailed structural information on the polymer super helices, we illustrate a possible structure in Figure 6d. Formarion o/'Corrdircring Polrnier Filnis. The preparation of thc polymer films involved using a partially immersed HOPG sheet a t the anode of the electrochemical cell. Examination of the rcwliing polymer film allowed us to identify four regions as shown in Figures 7 and 8 . W e discuss each of thcsc regions in the paragraphs bcloh . The topmost region in Figures 7a and 8 a is graphite. While this is not of direct interest. it docs play an important role in our STM mc;isurements. First. obtaining atomic resolution of graphite xcrvc\ to cstnblish that the apparatus is working properly and in particular that thc tip is well formed and imaging from only a ingle atorii. Sccond. the graphite surface serves a s a convenient rcfcrcncc plane for height measurements to estimate film thickness.
STM Evidence of Conducting Polymer Structures
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Figure 7. A series of STM images of polypyrrole polymer morphology which shows that the structure changes with the polymer film thickness. (a) The interfacial boundary between graphite and polymer. (b) A transition region from the thin structure film to the thicker amorphous region showing larger fibril structures. (c) A transition boundilr\ between the thin structure film and the thicker amorphous area. The height difference between the two regions is about I IO nm. (d) The thick polymer films showing a nodular structure.
Thc second region is shown in the bottom of Figures 7a and 8a. This thin film region ranges in height from 13 to SO nm and displays the ordered polymcr helical structures seen in the microislands. In some cases only parallel strands are seen as shown in Figure 7a while in other cases multiple layers with different orientations are observed as shown in Figure $a. We propose that this thin layer forms by growth and joining of the microislands as the polymerization proceeds. The third region is shown in Figures 7b.c and 8c. I t consists of a transition region between the thin structured films and the thicker amorphous rcgion shown in Figures 7d and 8d. This transition rcgion ranges in thickness from SO to 900 nm and displays an oriented but less distinct set of fibrils not unlike those reported by Walker and Bu~k1ey.l~These fibrils are on average considerably wider than the helical structures seen in the thin-film rcgion and no internal structure is disccrniblc with Fourier transform analysis. The fourth region is the typical amorphous polymer region possessing a nodular surface structure previously characterized by SEM measurement^.'.^^ The PTP-BF4 nodules shown in Figure 8d range in diameter from 200 to 1000 nm. Those for the
PPY-TOS are 100-200 nm in size as determined by both STM and SEM measurements on the same sample. These sizes arc smallcr than those of 1000-50000 nm reported by Salmon et aLZ1 on thicker polymcr films. From STM measurements we found that the size of the nodules increases in diameter as the film thickness increases. The change from the transition region to the nodular surface (Figure 7c) is rather abrupt. As the boundary between these two regions is approached, the fibril strands which range in width from SO to 100 nm become thicker and shorter. The height difference bctwccn the two regions is about 1 IO nm. We note that these individual helical structures are also observed on gold surfacesZ2 and thus are not a manifestation of the special properties of oriented graphite. The observation of distinct structural layers in conducting polymer films provides additional insight into several previous studies. Garnier et aI.l4 found evidence in their TEM and X-ray data for small microdomains of crystallized material. They obtaincd thcir samples by "scratching off thick deposits from the cl~ctrodc".'~ Our results suggest that their microdomains were portions of the polymer film located nearest the electrode. They
(19) Walkcr. J.; Buckley. L. J. 'Morphological Studies of Conductivc Polymers"; Report No. NADC-87092-60. 1987. (20) Diaz. A. F.; Hall. B. ISM J . Res. Der. 1983. 77. 342.
(21) Salmon. D.;Diaz. A. F.; Logan, A. J.; Krounbi. M.; Bargon, J. Mol. CrJ-.fr.Liq. Cr.v.fr. 1982. 83. 265. (22) Personal note from W. Smyrl.
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Figure 8. A series of STM images of polythiophene polymer morphology which shows that the structure changes with the polymer film thickness. (a) The interfacial boundary between graphite and polymer. (b) The polymer nascent growth showing polycrystal areas. (c) A transition region from thc thin structure film to the thicker amorphous region showing larger fibril structures. (d) The thick polymer films showing a nodular surface structure.
reported that the structured regions constituted less than 5% of the total sample, a result consistent with the thickness of the thin and transition regions. From X-ray measurements on oriented films, Mitchell and GeriI3 obtained evidence for anisotropy which they attributed to preferential alignment of the heterocycle rings with the electrode surface. Our results suggest that the anisotropy simply reflects the formation of helical and fibril structures adjacent to the electrode surface.
to the known repertory of structures formed by PPY and PTP poly”. 2. Conducting polymer films have a structure which gradually changes from an ordered crystalline array at the surface to an amorphous matcrials in the bulk. Whether this transformation rcflccts a change in polymerization mechanism or the loss of surface tcmplating is not known. But it does suggest a structural hicrarachy which adds a new dimension to our understanding of these systems.
Conclusions
Acknowledgment. Support by the Center for Interfacial Engineering (CI E), a National Science Foundation Engineering Kcsearch Center, the 3 M Co., a CIE Sponsor, and the NIH (GM 34341) is gratefully acknowledged.
STM On conducting heterocyclic polymers provide two conclusions concerning their structure. 1. Under some conditions PPY and PTP polymers form both simple and super helices which assemble to form crystalline arrays. The existence of such ordered structures does add a new dimension
Registry No. TOS-. 733-44-8; RF4-. 429-06-1 ; polypyrrole, 306048 1-0; pobthiophcnc. 25233-34-5.
