Complete alignment of polyaniline monolayers on

Thin Solid Films 393 Ž2001. 186᎐192

Complete alignment of polyaniline monolayers on muscovite mica: epitaxial effects of a lattice-matched substrate Jack Y. Josefowicz a , Jamshid K. Avlyanov b,U , Alan G. MacDiarmid c a

Department of Materials Science and Engineering, Uni¨ ersity of Pennsyl¨ ania, Philadelphia, PA, and Hughes Research Laboratories, Malibu, CA 90265, USA b Eeonyx Corporation, Pinole, CA 94564, USA c Department of Chemistry, Uni¨ ersity of Pennsyl¨ ania, Philadelphia, PA 19104, USA

Abstract Totally aligned monolayers of the conducting polymer, polyaniline, have been deposited ‘epitaxially’ on a lattice-matched ‘Moscovite’ mica substrate using an in-situ deposition method in aqueous solution. The polyaniline molecules have a repeat unit that is an excellent match to the mica surface oxygen spacing. Images of the monolayers, recorded by atomic force microscopy, show that the polyaniline molecules align parallel to each other on the mica and that the interchain alignment appears to be strongly influenced by nitrogen atoms on adjacent polyaniline molecules that line up to form highly ordered rows of nitrogen atoms between chains. These results suggest the possibility of growing multilayer crystalline films which would be expected to have significantly higher conductivity compared with the more typical disordered polyaniline thin films. Ordered films could also promote and enhance the performance of a large variety of electronic devices where conducting polymers are the active material. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; In-situ deposition; Atomic force microscopy; Mica; Monolayers; Epitaxy

1. Introduction A conducting polymer, commonly referred to as a ‘synthetic metal’, is an organic polymer that possesses the electronic, magnetic and optical properties of a metal while retaining the mechanical properties and processibility of a conventional polymer. The polyanilines constitute a large class of conducting polymers which are formed by the chemical or electrochemical oxidative polymerization of aniline or its ring- or nitrogen substituted derivatives w1x. They can be prepared in three different oxidation states. Chemical oxidative polymerization of aniline first produces the pernigraniline oxidation state w2x; the three different oxidation states can be readily inter-converted w1x. They can be proto-

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Corresponding author. Tel.: q1-510-741-3632; fax: q1-741-3657. E-mail address: [email protected] ŽJ.K. Avlyanov..

nated Ž‘doped’., the ‘doped’ emeraldine oxidation state giving the greatest final electronic conductivity with a concomitant increase in conductivity of approximately eight orders of magnitude. Polyaniline can be produced in the form of powders, films or fibers having conductivity in the metallic-conducting regime. Scientific and technological interest in the polyanilines stems from the richness of their chemistry and their physics due to the many ways by which their chemistry, electrochemistry, electronic, magnetic and optical properties can be fine-tuned by the facile synthesis of a large variety of ring- and nitrogen-substituted derivatives. Their ready processibility adds yet another dimension to methods by which their intrinsic properties can be both explored and exploited. The conductive form of polyaniline is stable in air and has been used in applications such as rechargeable batteries w3,4x, electrochromic displays w4x, light emitting diodes w5x, photovoltaics w6x and chemical sensors

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J.Y. Josefowicz et al. r Thin Solid Films 393 (2001) 186᎐192

w7x. For many of these applications it is important that the polymer has high electronic conductivity, and that it can be produced by a simple deposition process which results in smooth continuous conductive films with reproducible thickness on a variety of different substrates. It has been demonstrated recently that the conductivity of free-standing films of polyaniline is greatly dependent on the molecular orientation of the polymer and on the degree of molecular alignment. Increases in conductivity of approximately 10 2 , parallel to the stretch direction, have been obtained by mechanically stretchorienting the films. This also leads to a concomitant increase in crystallinity w8x. The resulting decrease in resistance to electron flow between polymer chains resulting from this molecular alignment between the chains is largely responsible for this effect. Recently, a 1000-fold decrease in resistance has been reported for ‘doped’ polyaniline films deposited from solution having a ‘rod-like’ molecular conformation which permits increased crystallinity due to spontaneous alignment of the polymer chains w9x. However, there have been no reports of fully Ž100%. aligned polyaniline; only partially crystalline material has been previously obtained. Recent reports on X-ray diffraction ŽXRD. w10x structural studies of protonated andror non-protonated polyaniline in various oxidation states have shown that the polymer has a repeat unit, d, along the chain ˚ Both thin films screw axis ranging between 9 and 10 A. and bulk samples of polyaniline have been found to be substantially disordered and consequently a complete understanding of the molecular conformation and structure has been difficult to obtain. In this report, we show that monolayers of polyaniline can be formed on the surface of a lattice-matched surface where the polyaniline molecular chains are completely aligned relative to each other and to the substrate. This was accomplished using an in-situ deposition method w11x in which polyaniline was spontaneously deposited from dilute aqueous solutions of polymerizing aniline onto a lattice-matched substrate, ‘Moscovite’ mica. The polyaniline chains align along rows of oxygen atoms that are in a threefold symmetric hexagonal arrangement of ‘lattice-matched’ oxygen

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atoms on the mica surface. We have identified the structural features of the mica that induce the alignment of polyaniline, as well as the structure of the oriented polyaniline using atomic force microscopy ŽAFM. w12x. The orientation of the chains and the chain-to-chain spacing are strongly influenced by the mica. These results suggest that it should be possible to deposit crystalline multi-layer films of polyaniline having significantly higher conductivity along the oriented polymer direction than films consisting of randomly oriented molecules. The substrates used for these experiments were ‘Moscovite’ mica, one of a class of phyllosilicates, KAl 2 ŽAlSi 3 O 10 .ŽOH. 2 . Since the repeat unit of the ˚ Ždeoxygen atoms on the surface of mica is ; 4.6 A termined from the AFM results discussed below., it provides a good lattice match for the epitaxial growth of polyaniline which, from the aforementioned XRD ˚ ŽFig. 1.. Mica results, has a repeat unit, d, of ; 9.5 A offers several attractive materials properties as a substrate. Ži. It can be cleaved easily just prior to the deposition, providing an atomically fresh, clean and flat surface. Žii. The surface of freshly cleaved mica is composed of a hexagonal arrangement of oxygen atoms that present a hydrophilic surface when submerged in aqueous solution. Water droplets completely wet a freshly cleaved mica surface, i.e. there is no measurable contact angle at the water mica interface which suggests a highly polar hydrophilic surface. The surface of cleaved mica is normally occupied by potassium ions which balance the charge deficiency generated by the isomorphous Žaluminum. substitution in the dioctahedral sheets. In the presence of water, because of entropy effects, the potassium ions Žsurface density is ; 2 = 10 14 rcm2 . w13x tend to move away from the mica surface into the water leaving the surface negatively charged. The protonated nitrogen atoms on the aniline monomer units in solution are attracted to and adsorbed onto this negatively charged surface. In this position the monomers can be oxidized and undergo polymerization, where the alignment is influenced by the association of the protonated nitrogen atoms and the mica surface oxygens. Žiii. The oxygen atoms on the surface of mica are arranged in a hexagonal configura-

˚ from the Fig. 1. Partly protonated polyaniline Žpernigraniline oxidation state. 2 showing the repeat unit, d, which was determined to be ; 9.2 A AFM image in Fig. 3.

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Fig. 2. An AFM image of clean ‘Moscovite’ mica and a Fourier transform ŽFT, upper right hand corner. calculated from the image. The image and the FT show that the oxygen atoms are in a hexagonal ˚ The images of the arrangement with a repeat spacing of ; 4.6 A. mica and polymer surfaces were obtained with a Nanoscope III AFM ŽDigital Instruments Inc.. at 21⬚C, with a 1 g = 1 g piezoelectric substrate translator having a lateral and vertical resolution of less ˚ All AFM imaging experiments were carried out with a than 0.5 A. constant force applied between a micro-fabricated silicon nitride cantilever tip and the surface of the mica andror polymer film. Typical applied forces were on the order of 10᎐30 nN. Images were found to be stable over several hours of repeated scanning and did not change when scanning angles were varied over a range of angles between 0⬚ and 90⬚.

˚ determined from tion with a repeat unit of ; 4.6 A, the AFM image of mica ŽFig. 2.. This offers an excellent ‘epitaxial’ match between the nitrogen sites of the ‘zig-zag’ segment in polyaniline, where the distance between nitrogen atoms separated by two rings is ; 9.2 ˚ ŽFig. 1.. Živ. The surface structure, i.e. the arrangeA ment of oxygen atoms on mica, can be imaged using AFM, offering a good structural reference for comparison with the deposited polyaniline. For samples where the polymer monolayer does not completely cover the mica ŽFig. 3., the mica surface offers a ready reference which can be used to determine the thickness and relative configuration and orientation of the polymer molecules.

solution was made containing 1.2 g Ž0.010 mol. of ammonium metavanadate in 400 ml of 4 M orthophosphoric acid at room temperature. Both solutions were stirred for 10 min and then cooled to ; 2⬚C in an ice bath. Using plastic clamps, freshly cleaved mica substrates were suspended in 400 ml of magnetically stirred ammonium metavanadate in a beaker. The 400-ml solution containing aniline was then poured into the beaker to produce the dipping solution. Dipping times were recorded relative to the time of initial mixing of the aniline and ammonium metavanadate solutions. Substrates on which the polyaniline Žpernigraniline oxidation state. films were being deposited were withdrawn from the solution at different time intervals and were immediately placed in a beaker containing a 500ml solution of 1 M HCl, at 0⬚C. This step is necessary in order to exchange orthophosphate anions for chloride anions in the deposited polyaniline films. After soaking for 30 min, the substrates, which were covered with polyaniline film, were washed for 60 s in 1 M HCl and then dried using an air jet for 20 min. 3. Results and discussion An example of the surface of a freshly cleaved mica substrate is given by the AFM image, with accompanying Fourier transforms ŽFT., ŽFig. 2.. From the FT it was determined that the repeat unit for the oxygen ˚ All atoms, in a hexagonal arrangement, was ; 4.6 A. AFM images presented in. this report are shown ‘as

2. Experimental All in-situ polyaniline thin film growth took place in an acidic aqueous solutions containing 4 M orthophosphoric acid. Aniline ŽAldrich Co.. was freshly distilled under vacuum prior to use. All other chemicals were of the highest grade and used as received. The water which was used in all reactions was de-ionized and filtered ŽFischer Scientific Co... Solutions were made containing 2.0 g Ž0.021 mol. aniline in 400 ml of 4 M orthophosphoric acid at room temperature. Another

Fig. 3. An AFM image of the surface of mica partially covered by a monolayer of the pernigraniline form of polyaniline Žin the area at the top, labeled A.. A FT of the entire image is shown in the upper right hand corner. A hole in the polyaniline monolayer Žthe area at the bottom, labeled B. corresponds to the mica surface. The vertical bands associated with the polyaniline monolayer in area A, which ˚ above the mica have a spacing of 9.2, have a height of ; 2.5 A surface, area B, characterized by diagonal bands that have a spacing ˚ of ; 4.6 A.


recorded’; they have not been filtered or otherwise enhanced. The in-situ deposition of polyaniline on mica substrates was accomplished using different dipping times varying from seconds to several minutes, using intervals of 5 s. Mica which was submerged in solution for deposition times up to 35 s did not have any measurable polyaniline deposit. For these deposition times, there likely were only short chains Ždimers, trimers, etc.. of polyaniline deposited on the mica surface so that upon washing the mica substrates, after removal from the dipping solution, the polyaniline molecules could be dissolved during the washing steps which follow the deposition process. A top view AFM image of a mica substrate on which monolayers of in-situ deposited polyaniline monolayers, with a dipping time of 39 s, showed two different repeating band structures ŽFig. 3.. The FT of this image is shown as an insert in the upper right corner of the AFM image ŽFig. 3.. This image is of particular interest because it shows two distinctly different structures, the structure at the top in area A is associated with the polyaniline monolayer and area B corresponds to a hole in the polyaniline monolayer through which the mica surface was imaged. A cross-section analysis of this image determined that the thickness of area A, i.e. the surface of the vertically oriented bands, relative to ˚ i.e. approximately the surface in area B was ; 2.5 A, one molecule thick. Area B in the AFM image, corresponding to the mica surface, is characterized by a set ˚ of diagonally oriented bands which have a ; 4.6 A spacing that is approximately half that of the vertical ˚ .. The two different bands could be bands Ž; 9.2 A identified in the FT of the AFM image. Two bright spots located near the horizontal axis correspond to the vertical bands associated with the polyaniline; having a ˚ The diagonal bands in the repeat unit of ; 9.2 A. AFM image produced the large spots in the FT located at angles of 120⬚ and 240⬚, and were determined to ˚ Furthermore, the FT have a repeat unit of ; 4.6 A. also shows a series of six spots arranged in a hexagonal ˚ simipattern, also having a repeat spacing of ; 4.6 A, Ž . lar to that of the mica Fig. 2 . The bright spots in the FT associated with the diagonal bands on the mica, area B ŽFig. 3., are at approximately the same locations as two of the series of six hexagonally arranged spots at 120⬚ and 240⬚ angles. Using the repeat units and relative angles for the band structures in the AFM image of the polyaniline monolayer on mica ŽFig. 3., a model for the configuration of the polyaniline monolayer film on mica was formulated ŽFig. 4.. The red circles correspond to the oxygen atoms on the mica surface, which are arranged in a hexagonal structural configuration, as suggested by the AFM image and FT. In order to produce the vertical bands observed in the AFM image the polyani-

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Fig. 4. A schematic illustration corresponding to the AFM image of polyaniline on mica Žfollowing from Fig. 3.. Represented at the top are totally aligned polyaniline molecules where every other nitrogen atom Žgreen dots. on the chain associates with the negatively charged mica surface Žcovered by oxygen atoms, red dots. through electrostatic andror hydrogen bonding. With the polyaniline molecules standing on edge or at a tilt angle with respect to the surface, there is an alignment of the nitrogen atoms on the chains, ‘bridge nitrogens’, which line up giving rise to high amplitude bands, shown in blue, that are observed in area A of Fig. 3. The region at the bottom of the schematic Žcorresponding to area B in Fig. 3. shows an area of uncoated mica. The AFM tip seems to have delineated primarily one series of oxygen atoms on the mica surface ŽFig. 3., shown as red bands in this illustration, which have a spacing one half that of the ˚ corresponding to the distance between ‘bridge blue bands, ; 9.2 A, nitrogen’ atoms.

line molecules must be configured horizontally, as shown in the top portion of the schematic ŽFig. 4., where every other nitrogen is associated with every other oxygen on the mica; the attraction to the surface comes from associated protons Žfor simplicity, not shown in the illustration .. These polyaniline chains are assumed to be standing on edge or at a tilt angle on the ˚ as in mica surface. The bands are spaced at ; 9.2 A, the FT, and are a consequence of the alignment of the polyaniline chains so that the nitrogen atoms more remote from the surface form rows of ‘bridge’ structures that the AFM tip images as bands. This can be illustrated using a schematic representation of the ‘bridge’ structure ŽFig. 5.. This three-dimensional illustration shows an ‘edge on’ Žcross-section. perspective which features electrostatically attracted protonated nitrogen atoms and the alignment of the ‘bridge’ nitrogen atoms that are responsible for the bands associated with the polyaniline that were imaged by the AFM tip ŽFig. 3.. The AFM image ŽFig. 6., corresponds to an expanded top view of area B ŽFig. 3., and appears to be representative of the mica surface. As shown by the white arrows, the hexagonal configuration of the oxygen atoms which are associated with the six spots in the FT, can be clearly delineated. These spots are in line and are part of the series of oxygen atoms that form the diagonal bands. The reason that the AFM tip ‘high-lighted’ this diagonal series of oxygen atoms may

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Fig. 5. A schematic illustration showing a totally aligned pernigraniline polyaniline monolayer on the mica surface. This three-dimensional ‘edge’ view shows how an AFM tip would have recorded the bands corresponding to aligned ‘bridge nitrogen’ atoms for polyaniline molecules configured ‘edge on’ to the mica surface.

be due to some interference resulting from the simultaneous imaging of short sections of polyaniline molecules which may also be present in this area. Some evidence of faint vertical bands can also be seen in the AFM image ŽFig. 6.. It is interesting to note that aligned conducting polyaniline can be polymerized ‘endotaxially’ inside FeOCl by ‘in-situ’ oxidation involving an intercalation reaction within the lattice-matched interior framework of the FeOCl layered structure w14x. The ordering of polyaniline within the layered FeOCl galleries occurs along the Ž101. direction, i.e. diagonal to the a- or c-axes, where the Cl᎐Cl distance determined from XRD ˚ approximately one-half of the polyaniline is 5.2 A, repeat unit, d, along the chain. It was suggested that

the nitrogen atoms along the polyaniline chains were hydrogen-bonded to the negatively charged Cl atoms which are on the surfaces of the FeOCl in the galleries. Based on the AFM images presented above, depicting the mica and polyaniline monolayer, we propose a polymerization sequential mechanism w15x, shown by a series of schematics ŽFig. 7.. Starting with the acidic monomer solution, the sequence begins with the attraction and adsorption of protonated aniline monomers onto the mica surface which is covered by negatively charged oxygen atoms. After adding the oxidant to the solution, the monomers are oxidized, converting the monomers to cation radicals with either the quinoid or benzenoid form ŽFig. 7b.. The next step is the formation of a dimer from a quinoid and benzenoid pair, as shown in steps Žb. and Žc.. This is followed by the formation of a trimer, shown in steps Žd. and Že.; the process continuing in this fashion to form long aligned chains of polymer. The ‘bridge’ nitrogen atoms occur naturally due to the match in spacing between alternate nitrogen atoms on the chain which can be electrostatically bonded to every other oxygen. Once one chain begins to polymerize along a row of oxygen atoms on the mica, it influences the polymerization of chains on both sides. It has been reported w16x that oxidized polyaniline can oxidize monomer more rapidly than can oxidant molecules diffusing in solution. Steric hindrance would preclude the monomers from locating on mica oxygen groups adjacent to existing polymer chains already formed and aligned on the mica surface. Electron charge transfer from a monomer Žor dimer, trimer, etc.. to the polyaniline chain would occur when the nitrogen group of a monomer, either diffusing towards or adsorbed on the mica surface, aligns with the ‘bridge’ nitrogens on the polyaniline chains, thereby inducing the alignment of the nitrogen ‘bridges’ on adjacent chains. 4. Conclusions

Fig. 6. An expanded view of the AFM image in area B ŽFig. 3.. The associated FT is shown in the upper right comer. The hexagonal arrangement of oxygen atoms on the mica surface are shown by the bright spots in the image, highlighted by the white arrows, as well as the hexagonal pattern of bright spots in the FT which had a mea˚ The diagonal bands seen in the AFM sured repeat unit of ; 4.6 A. image result in the bright spots in the FT which seem to be associated with the mica related spots at the expected angle of 120⬚ and 240⬚.

Since the discovery of conducting polymers, it has been the goal of many research efforts to control the molecular orientation and alignment of these materials, especially because the molecular alignment appears to be important in increasing conductivity. We have shown that aligned monolayer films of polyaniline can be deposited on lattice-matched mica substrates using an in-situ deposition method. Our results also suggest that a similar approach could be used for other conductive polymers where an ‘epitaxial’ match can be made between the polymer repeat unit and that of a substrate having the appropriate surface chemistry. This approach has the potential for producing oriented multilayers of conductive polymer. Such crystalline thin material would be expected to have significantly higher conductivity which would improve the performance of


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electronic devices where such multilayers are the electronically active material. They should also provide crystalline material for a host of structural investigative techniques that will improve our understanding of this important class of organic conductors. Acknowledgements The authors gratefully acknowledge the support of this research by A.N. Chester and R.A. Reynolds of Hughes Research Laboratories, Malibu, CA; and the support by an ONR grant ŽNO0014-012q 1369. to J.K.A. and to A.G.M. References

Fig. 7. Schematic illustrations summarizing the proposed sequence of polymerization steps during the conversion of aniline to polyaniline Žpernigraniline oxidation state. 2. In Ža., the protonated aniline monomer, in the acid solution, is attracted to the negatively charged oxygen atoms on the mica surface. Radical cations are formed after reaction with the vanadate oxidant ion in solution. In order for the polymerization reaction to proceed, active benzenoid and quinoid cations must be configured as shown in Žb. so that there is enough room to allow a third cation to bridge the gap between the monomers attracted to the mica surface. Since there is not enough room to bring three cations into the proper configuration for a polymerization reaction if the monomers are located on adjacent oxygen atoms, stearic hindrance is avoided when the monomers are located on every other oxygen; this leads to the reaction that forms a dimer, in Žc., and the reaction Žd. leading to a trimer in Že,f.. Polymerization continues in a similar manner as the chain propagates along the surface as shown in Žf. and Žg.. The species depicted in Žd᎐g. will all be partly protonated in aqueous acid solution. For simplicity, only the species given in Žg. is shown in protonated form.

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