Anisotropic highly-conductive films of poly(3-methylthiophene) from epitaxial electropolymerization on oriented poly(vinylidene fluoride)†

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Introduction
The strategies for improving the performance of organic thin lm electronic devices are diverse. Research is especially focused on synthesizing new active materials, device interface control, device structure modication and lm morphology regulation. [1][2][3] The control of morphology of the thin active layers in organic electronic devices by exploiting new deposition techniques is an essential and rational way towards optimization. 4,5 An effective direction of charge transport is along a p-p stacking axis. [6][7][8] This leads to anisotropic charge transport in crystals. 9,10 Therefore, large-area well-ordered organic semiconductor thin lms are expected to have excellent charge transport behavior in the specic p-p stacking direction.
Electropolymerization (EP) with concurrent polymer lm deposition has proved to be an especially useful method for the in situ preparation of electroactive and conducting polymer lms. [11][12][13] During the EP process, the precursor monomers are oxidized electrochemically; coupling reactions occur at the electrode surface, resulting in the formation of a polymer thin lm on the electrode. The growth rate and thickness of the polymer lms can be easily modulated by controlling the applied potential (or current density) and the total amount of charge passed through the cell, respectively. The morphology and properties of the EP lms can also be optimized through choice of the EP conditions including preparation techniques and experimental parameters. 14,15 For example, Ma et al. have recently developed a route utilizing electrochemical copolymerization and layer-by-layer polymerization to construct crosslinked networks, which are very promising candidates for applications in color-stable electroluminescent devices and solar cells. [16][17][18][19][20] Electrochemical polymerization of thiophene and its derivatives has been researched extensively because of the outstanding optoelectronic properties of polythiophenes. [21][22][23][24][25] However, the irregular molecular coupling and branching which usually occurs during traditional electrochemical deposition, is detrimental to the regularity of the conjugated polymer structure and prevents long-range p-p stacking. Thus the charge transport performance of the polymer is considerably reduced. Moreover, the crystalline domains in the electrochemically deposited polymer thin lms are randomly oriented. It is therefore difficult to obtain thin lms with optimal conductivity by electropolymerization.
Epitaxy is an efficient method for preparing well-dened thin lms with controlled crystal structure and molecular orientation. [26][27][28][29] Signicant progress has been made in the epitaxial growth of organic semiconductive molecules on highly oriented polymer substrates. [30][31][32] For example, in our previous work, vapour phase epitaxial growth of perylo [1,12-b,c,d]thiophene (PTH) on highly oriented polyethylene (PE) gave large-area wellarranged lms of PTH with a unique crystal structure. 33 From this background we considered that introducing epitaxy into an electropolymerization process could provide an unusual method for preparing highly ordered thin lms of semiconductive materials. Goto et al. demonstrated that electrochemical polymerization of 2,7-di(2-furyl)uorene in a macroscopically aligned nematic liquid crystal yields a uniaxially ordered polymer lm. 34 Electrochemical polymerization of bithiophene in a crystalline electrolyte has also been described. 35 Sakaguchi et al. reported electrochemical epitaxial polymerization of 3-butoxy-4-methylthiophene on an iodinecovered gold electrode by applying voltage pulses. A surfacepropagation mechanism gave single polythiophene wires which were observed by scanning tunnelling microscopy imaging. 36 These studies 34-36 did not report conductivity data for the aligned polymers. We are not aware of any previous studies on epitaxial electropolymerization induced by standard voltammetric cycling.
Herein, we report the preparation of highly oriented poly(3methylthiophene) (P3MT) through epitaxial electrochemical deposition. To achieve this goal, highly oriented poly(vinylidene uoride) (PVDF) ultrathin lms were deposited on indium tin oxide (ITO) electrodes followed by electropolymerization of 3-methylthiophene on the PVDF modied ITO. In this process, epitaxial growth of an oriented P3MT layer is achieved on the PVDF thin lm during electrochemical deposition; the lm displays high conductivity with anisotropic charge transport. This is a new strategy for preparing anisotropic semiconductive polymer lms.

Electrodeposition and characterization
Highly oriented PVDF lms were prepared according to a meltdraw technique introduced by Petermann et al. 37,38 Fig. 1a shows the polarized optical micrograph of a PVDF lm, which shows weak birefringence since it is thin (30-50 nm). The drawing direction during lm preparation is indicated by an arrow, i.e. the molecular chain direction of the PVDF is 45 relative to the polarization direction. When rotating the sample about the light beam axis by 45 , leading to a parallel alignment of the polarizer and PVDF drawing direction, extinction of the light takes place (see Fig. 1b) due to the high orientation of the PVDF lm.
P3MT was chosen as the semiconductive polymer as it is known to be readily obtained by standard electropolymerization of 3-methylthiophene. [39][40][41] Electrochemical coupling of 3-methylthiophene was performed using cyclic voltammetry (CV). 13 The PVDF layer is thin and combined with its dielectric properties electron transfer can occur readily to the electrode to enable the P3MT lm to grow on the PVDF. A scanning electron microscopy (SEM) image of the glass-ITO-PVDF-P3MT assembly is shown in the ESI. † For comparison, 3-methylthiophene was rst electropolymerized through potential sweeps between À0.2 and 1.0 V vs. Ag/Ag + at 40 mV s À1 on bare ITO glass in a 0.5 mM chloroform-acetonitrile (3/2 v/v) solution containing 0.1 M Bu 4 NPF 6 electrolyte. Then, the obtained P3MT lm was reduced (dedoped) at 0 V for 2 h following reported procedures. 42,43 The P3MT lm changed from greenish-black to red upon dedoping. As shown in Fig. 1c and d, the resultant dedoped P3MT lms exhibit an isotropic feature with birefringence that remains unchanged during the rotation of the sample about the light axis. Dedoping served to improve the crystallinity of the lm; doped P3MT is known to form a disordered semicrystalline structure on ITO. 42 The electrochemical polymerization potential of 3-methylthiophene on highly oriented PVDF covered ITO glass (1.22 V) is slightly higher than that on bare ITO glass (0.95 V) in the same electrolyte ( Fig. S1 †). Thus 3-methylthiophene was electropolymerized through potential sweeps between À0.2 and 1.2 V vs. Ag/Ag + at 40 mV s À1 on PVDF modied ITO glass in a 0.5 mM chloroform-acetonitrile (3/2 v/v) solution. As shown in Fig. 2, both the anodic and cathodic peak currents increase in the successive cycles, indicating the coupling reaction of 3-methylthiophene units and the growth of the polymer lm on the electrode. 13,25,44 In the inset of Fig. 2a, the evolution of the oxidation peak current at ca. 0.6 V versus cycle number illustrates a linear increase of the polymerization of 3-methylthiophene on the electrode. In addition, the X-ray photoelectron spectrum (XPS) of dedoped P3MT on the PVDF lm (Fig. 2b) shows the characteristic sulfur and uorine peaks, with no detectable phosphorus (from the PF 6 À counterion at ca. 130 eV) or nitrogen (from the NBu 4 + electrolyte at ca. 400 eV) indicating the successful deposition and dedoping of P3MT. Fig. 3a and b present the polarized optical micrographs of a P3MT lm electrochemically deposited onto oriented PVDF covered ITO glass and then dedoped. A regular structure with lathlike P3MT crystals of microns in length can be observed with strong birefringence. The crystals exhibit a well-ordered structure with their long axes aligned along the drawing direction of the PVDF lm, indicating the high orientation of the P3MT crystals, i.e. the occurrence of epitaxial crystallization of P3MT on the highly oriented PVDF during electropolymerization, and not simply the growth of nanobres. This is further conrmed by the occurrence of light extinction when the sample was rotated about the beam axis (see Fig. 3b). A possible driving force for epitaxial growth of the P3MT could be intermolecular hydrogen-bonding H(thiophene)/F(PVDF) interactions 45 which align the growing polymer chains on the PVDF surface. This mechanism is conceptually similar to the proposal of Sakaguchi et al. that the oriented substrate-lattice structure of iodine adsorbed onto Au(111) facilitates an iodinethiophene interaction leading to epitaxial polymerization. 36 Roncali et al. also showed by SEM the existence of "parallel grooves" in P3MT lms grown under standard voltammetric conditions on platinum electrodes. 40 However, unlike the present study, these authors did not report anisotropy in the optoelectronic properties of the lms.
The morphologies of the P3MT lm can be well tuned by changing the condition of electrochemical deposition. As shown in Fig. 3c, with a low scan rate and few scan cycles, lathlike crystals with a width of 10-20 mm are obtained. These crystals do not cover the whole PVDF substrate because of the short scan time. The thickness of the P3MT crystals is ca. 0.6 mm, as determined by the atomic force microscopy (AFM) image shown in Fig. 3d. With increasing numbers of cycles, a twodimensional close packed lm composed of lathlike P3MT crystals is obtained, similar to that shown in Fig. 3a. The thickness of the P3MT crystals in Fig. 3a is also ca. 0.6 mm. P3MT has very poor solubility in organic solvents, so the degree of polymerization of the bulk material cannot be accurately determined by GPC. The degree of polymerization of the soluble portions of the P3MT lm in tetrahydrofuran was found by GPC to be 16, with polydispersity ca. 1.3. The UV-Vis absorption spectrum of the dedoped lm (l max 510 nm) is consistent with previous data for electrochemically dedoped P3MT lms, 39 whereas the spectrum of the THF solution (l max 400 nm) is assigned to oligomer chains with a lower degree of polymerization, based on literature precedents 46,47 (see ESI †). Even thicker lms of P3MT can also be produced by further increasing the number of CV cycles.
To probe the chain arrangement of P3MT in the lathlike crystals, X-ray and electron diffraction data were obtained. In the X-ray diffraction prole, Fig. 3e, except for the weak reection peaks corresponding to the thin PVDF substrate and ITO glass (indicated by the asterisks), several peaks are observed for the P3MT with a main sharp peak located at 2q ¼ 11.5 . This reects the successful deposition of P3MT on the PVDF surface. By detaching the P3MT from the PVDF modied ITO glass (see ESI †) the electron diffraction prole of the P3MT was obtained. As inserted in Fig. 3e, sharp and well-dened diffraction spots can be observed. All of these spots can be accounted for by a hexagonal unit cell with a-axis parameter of 0.886 nm. The appearance of only (hk0) diffraction spots indicates a at-on orientation of P3MT crystals with molecular chains perpendicular to the substrate surface. Due to the difficulty in detaching the P3MT/PVDF double layers together from the ITO glass, no  superimposed electron diffraction has been obtained. In this case, the mutual orientation between the P3MT and PVDF crystals cannot be determined.
To further conrm the arrangement of molecular chains of P3MT in the crystals, Reection Absorption Infra-red Spectroscopy (RAIRS) was used. For RAIRS the resultant electric eld vector is perpendicular to the metal surface. Therefore, if molecules are adsorbed onto the substrate with a preferred orientation, vibrational modes having transition moments perpendicular to the surface will appear with greater intensity than modes having transition moments parallel to the surface. 48,49 Fig. 4 displays the characteristic bands of P3MT. Herein, we focus on the out-of-plane deformation modes of thiophene C b -H at 825 cm À1 for which the transition moments are perpendicular to the P3MT backbones, and the methyl deformation mode at 1378 cm À1 which is used sometimes as the internal standard because the frequency and intensity of this vibration mode is not sensitive to structural changes. 50,51 Comparing the FTIR (Fig. 4a) with RAIR spectra (Fig. 4b), the band of C b -H shis to the higher wavenumber in FTIR. It has been reported that reectance spectra clearly show large changes in both peak position and shape compared to transmission spectra. 52,53 To gain qualitative information about the molecular orientation, we avoid the disturbance of the band distortions by comparing the peak area of bands at 825 and 1378 cm À1 (A 825 /A 1378 ) in the same spectra. The value of A 825 /A 1378 is greater in the FTIR (Fig. 4a) than that in RAIR spectra (Fig. 4b). On the basis of the mutually perpendicular direction of the electric eld vector for transmission vs. p-polarized RAIR modes, we conclude that the main-chain of P3MT in the crystalline lm is aligned perpendicular to the substrate. These data corroborate the polarized optical microscopy data in Fig. 3 and further establish the epitaxial orientation of P3MT at the molecular level, rather than a macroscopic bre-like structure.

Conductivity measurements
The I-V characteristics of the dedoped P3MT lm in different directions were measured using a two-probe method. Au electrodes were placed onto the lms through mask deposition. As shown in Fig. 5A and B, two types of devices were fabricated in order to compare the conductivity of P3MT in the direction parallel and perpendicular to the drawing direction of the PVDF lms. Fig. 5C shows the I-V properties of the oriented P3MT lms in two measuring directions. It is clear that the slope of the I-V proles decreased dramatically when measured in the direction parallel to the drawing direction of PVDF. On the other hand, the electric currents in the direction perpendicular to the stretching direction of PVDF are much larger. This demonstrates unambiguously an anisotropic electrical conduction of the dedoped P3MT lms. The electrical conductivity of dedoped P3MT along the direction perpendicular to the drawing direction of PVDF is s rt 59 AE 3 S cm À1 , for several different samples. This value is comparable to that reported for P3MT doped with FeCl 3 . 54 By contrast, the electrical conductivity of P3MT along the stretching direction of PVDF is reproducibly 1.2 AE 0.4 S cm À1 . The anisotropy is ca. 50. For comparison, the electrical conductivity of the as-prepared (doped) P3MT on bare ITO displays isotropic conductivity values of 1840 AE 25 S cm À1 and 0.08 S cm À1 for doped and dedoped lms, respectively. This data is consistent with a previous value (s max 1975 S cm À1 ) for electrochemically doped P3MT lms supported on adhesive tape. 39 In the present study the comparative higher conductivity (1.2 AE 0.4 S cm À1 ) of the dedoped P3MT along the stretching direction of PVDF is explained by a regular arrangement of P3MT molecular chains on the PVDF. The high conductivity of our samples of dedoped P3MT can be attributed to the well-ordered structure of its single crystals related to stereoregular conjugated molecular chains, rather than branched chains. Moreover, the excellent conductivity of the P3MT lm suggests efficient p-p stacking of the P3MT molecules along the direction perpendicular to the drawing direction of PVDF. The proposed structural ordering is represented in Fig. 5D. We note that conductivity values as high as s rt 400 S cm À1 have been reported for crystals of single-  component molecular metals based on metal-dithiolenes which are non-doped (neutral) p-stacked species. 55,56 To establish that our strategy is versatile for obtaining anisotropic highly conductive polythiophene derivatives, the analogous polymerization of pure thiophene and 3-hexylthiophene on oriented PVDF was shown to give anisotropic lms. Polarized optical micrographs are shown in Fig. S5. † The resulting dedoped PT and P3HT lms display anisotropic conductivity, with values of 70 AE 4 S cm À1 and 50 AE 2 S cm À1 , respectively, along the direction perpendicular to the drawing direction of PVDF, and 1.6 AE 0.3 S cm À1 and 0.9 AE 0.1 S cm À1 along the stretching direction of PVDF.

Conclusions
In summary, highly oriented P3MT lms with lathlike crystals were successfully prepared via electropolymerization of 3-methylthiophene on a highly oriented PVDF surface by cyclic voltammetry. The molecular chains in the crystals are aligned perpendicular to the lm plane. These P3MT lms aer electrochemical dedoping exhibit anisotropic electrical conductivity with the anisotropy of conductivity of ca. 50. The value in the direction perpendicular to the long axis of the P3MT crystals is s rt 59 AE 3 S cm À1 , which is remarkably high for a dedoped polymer lm. The high conductivity should be attributed to the well-ordered structure of P3MT single crystals. Many chemical coupling methods can control the regioregularity of a single molecular chain of poly(3-alkylthiophene) derivatives, but cannot control the arrangement of all the polymer chains within the lm. 57 We have demonstrated that a main advantage of epitaxial electrochemical deposition is that structure can be ordered at both of these levels (intramolecular and intermolecular). This technique provides a new way to prepare high conductivity lms of semiconductive polymers with anisotropic charge transport and could open new applications in optoelectronic devices.

Materials
All reactants and solvents were purchased from commercial sources and used without further purication. Anhydrous and deoxygenated solvents were obtained by distillation over sodium benzophenone complex.

Electrochemical polymerization to yield P3MT
ITO-coated glass substrates were cleaned in an ultrasonic bath with toluene, acetone, ethanol and deionized water, respectively, and then dried with nitrogen. Highly oriented PVDF lm was then pasted onto the ITO by electrostatic forcing. Electrodeposition of P3MT was performed using a standard one-compartment, three-electrode electrochemical cell attached to a CHI 660E Electrochemical Workstation. The Ag/Ag + nonaqueous electrode was used as reference electrode. ITO (1 cm 2 ) was used as the working electrode and titanium metal was used as the counter electrode (area: 3 cm 2 ). A mixture of 3-methylthiophene (0.5 mM) and Bu 4 NPF 6 (0.1 M) in chloroform and acetonitrile (3/2 v/v) was the electrolyte solution. The electrodeposited lms were prepared by CV with these experimental parameters: scan range from À0.20 to +1.20 V, scan rate of 5-100 mV s À1 . Aer the electrodeposition process, the obtained P3MT lm was dedoped at 0 V for 2 h. Finally, the resulting lms were washed with a mixture of chloroform and acetonitrile (3/2 v/v) to remove unreacted precursors and supporting electrolytes, and then dried in a vacuum oven. All measurements were carried out under ambient conditions.

Conductivity measurements
The current-voltage (I-V) characteristics of the sandwich devices were recorded with a Keithley 4200 SCS semiconductor parameter analyzer (Keithley, Cleveland, OH) equipped with a Micromanipulator 6150 probe station in a clean and metallically shielded box in an ambient environment. The P3MT together with PVDF lm were detached from the ITO by HF solution (5% aqueous) and placed on a silicon wafer. Gold electrodes were then deposited onto the lms as shown in Fig. 5A. The conductivity (s) extracted from the linear region of the I-V proles was calculated using the equation s ¼ Id/(VS), where d is the distance between the electrodes, and S is the cross section of the samples.