ARTICLE pubs.acs.org/Langmuir
Chemical Vapor Deposition Synthesis of Tunable Unsubstituted Polythiophene Siamak Nejati and Kenneth K. S. Lau* Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States
bS Supporting Information ABSTRACT: Despite having exceptional electroactive properties, applications of unsubstituted polythiophene (PTh) have been limited due to its insolubility. To overcome this challenge, we have employed oxidative chemical vapor deposition (oCVD) as a unique liquid-free technique to enable the oxidative polymerization of PTh using thiophene as the starting monomer and vanadium oxytrichloride as an effective vaporizable oxidant initiator. Vibrational and phototelectron spectroscopy indicated the formation of unsubstituted polythiophene. Cyclic voltammetry revealed its electrochromic behavior in solution. Significantly, polymer conjugation length and electrical conductivity can be tuned by controlling oCVD process variables. Polymerization is found to be adsorption-limited, so by providing sufficient monomer and limiting the amount of initiator at the growth surface, PTh is believed to be formed through αα thiophene linkages.
1. INTRODUCTION Polythiophenes with their exceptional thermal and environmental stability and potential electrical conductivity have gained a lot of attention since their first synthesis through metalcatalyzed polycondensation.15 Importantly, the optoelectronic properties of polythiophenes have found use in fabricated devices, e.g., thin film transistors, hybrid solar cells, and organic light emitting diodes.610 So far, only substituted polythiophenes like poly(3-alkylthiophenes) have been applied extensively due in large part to their solubility and ease of solution processing. The parent unsubstituted polythiophene (PTh) is not soluble, and this has severely limited its study and hindered its incorporation into devices as an active material. However, the use of insulating alkyl side chains is known to introduce anisotropy in charge transport as well as negatively impact polymer chain orientation;1113 thus, the ability to provide a suitable and facile means for making PTh that would be amenable for device integration would be significant. Yet to date, there are only a very limited choice of synthesis routes because, in order to incorporate the intractable polymer into thin film device architectures, these methods would require tandem polymer synthesis and thin film formation. Electrochemical polymerization synthesis of PTh from the thiophene monomer is one common approach.14,15 Although this pathway has been used to incorporate PTh in electrochromic, solar cell, and light emitting devices, the area of deposition is limited and the method requires the electrode material to be electrically conductive.7,1618 An alternative synthesis pathway is to rely on vapor phase polymerization (VPP) that directly converts monomer vapor into solid polymer thin film. This relies on the use of a solid oxidant polymerization initiator that is first precast onto the substrate surface. Although VPP of PTh has been reported using r 2011 American Chemical Society
bithiophene and terthiophene as the starting monomers, the single unit thiophene monomer, which is more easily accessible and more economical, was not used due to its higher oxidation potential that was found to be unfavorable for polymerization.19 In addition, having a solvent-cast oxidant on the substrate to initiate and sustain the oxidative polymerization reaction might limit the extent of reaction after an initial growth of the polymer film over the cast oxidant due to diffusion constraints. Here, we present oxidative chemical vapor deposition (oCVD) as a unique liquid-free synthesis method to enable continuous polymerization and thin film deposition of PTh in a single step to address the problems associated with the lack of polymer processability in solution as well as the limitations of current vapor phase synthesis. Unlike VPP, oCVD relies on the transport of both monomer and initiator in the vapor phase to the substrate.20 Thus, it benefits from being able to control the reactant concentrations at the growth surface, thereby offering tunability and continuous growth conditions during synthesis. Furthermore, oCVD as a CVD technique is a viable method for growing polymers on very small features and within complex structural topologies.21 Recently, oCVD has been successfully utilized for the formation of poly(3,4-ethylenedioxythiophene) (PEDOT) and PEDOT copolymers.2224 These reports have focused on PEDOT-based polymers synthesized using iron(III) chloride or bromine as vaporizable oxidants. Here, we report on the novel oCVD synthesis of unsubstituted polythiophene PTh using a new oxidant, vanadium oxytrichloride, as an effective Received: August 23, 2011 Revised: October 29, 2011 Published: November 02, 2011 15223
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Langmuir vaporizable polymerization initiator having sufficient oxidation potential to enable PTh polymerization from the single unit thiophene monomer. To the best of our knowledge, there is no prior report on the synthesis of PTh with the ease and freedom offered by oCVD. Using this chemistry, we have deposited PTh with control over film structure and properties. Vibrational, UVvis, and X-ray photoelectron spectroscopy along with cyclic voltammetry were used to identify the deposited PTh films. By controlling oCVD synthesis parameters, we were able to tune the optical properties of PTh and relate them to the effective conjugation length of the polymer chain. By demonstrating a liquid-free synthesis approach, the potential for applying the intractable PTh in thin film devices particularly at the nanoscale can potentially lead to enhanced device performance.25
2. EXPERIMENTAL SECTION To enable oxidative chemical vapor deposition (oCVD), we have used a CVD reactor system described in detail elsewere.25 Briefly, the reactor chamber was evacuated to base pressure (ca. 5 mTorr) using a dry vacuum pump (Edwards Vacuum). Monomer, thiophene (97%, Sigma Aldrich), and oxidant initiator, vanadium oxytrichloride (99%, Strem Chemicals), were used as received and metered independently from glass source vessels into the chamber using precision metering valves (Swagelok). The initiator was heated up to 45 °C to achieve sufficient vapor pressure, and its temperature was kept constant using a temperature controller (Omega Engineering). The monomer had sufficient vapor pressure at room temperature and was not heated. The chamber pressure was measured with a pressure transducer (MKS Instruments) and automatically maintained by using a downstream throttle valve connected to a pressure controller (MKS Instruments). The substrate temperature was kept constant through backside cooling of the reactor stage by using a recirculating chiller (Thermo Scientific Neslab). In order to tune the oCVD polymerization reaction and synthesis chemistry at the surface, the ratio of the reactant (monomer, initiator) partial pressure to its saturated vapor pressure at the temperature of the substrate (i.e., Pr/Pr,sat) was carefully adjusted and controlled (see Discussion). Thus, pressures ranging from 12 to 22 Torr and monomer and initiator flow rates of 0.57.0 and 0.11.0 sccm (standard cm3/min), respectively, were studied. The substrate temperature was set at 5 °C. Unsubstituted polythiophene (PTh) films were deposited on various substrates, including fluorine-doped tin oxide glass (15 Ω/0, Hartford Glass), silicon wafers (WRS Materials), microscope glass slides (Fisher Scientific), and quartz glass (Chemglass). The silicon wafers were used as received, while all the other substrates were sonicated in dilute detergent solution (Citranox) and thoroughly rinsed in deionized water prior to use. After each oCVD deposition, the substrate temperature was raised to 80 °C for 4 h before the samples were extracted for analysis. Fourier transform infrared spectra (FTIR) were acquired on a Thermo Nicolet 6700 spectrometer in normal transmission mode using an MCT/A detector at a resolution of 4 cm1 averaged over 64 scans. UVvis spectra of deposited films on quartz glass were acquired between 280 and 800 nm with 1 nm resolution using a Shimadzu UV1700 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics PHI 5000 VersaProbe with a scanning monochromatic source from an Al anode and with dual beam charge neutralization. Survey XPS spectra were acquired at 100 W with pass energy of 117 eV over the range of 01100 eV with 0.5 eV resolution and 50 ms dwell time, and averaged over 5 scans. High resolution XPS spectra of C1s, O1s, V2p, Cl2p, and S2p core electrons were acquired in high power mode of 100 W with a pass energy of 23.5 eV using different acquisition times chosen based on the observed intensity of the elements from the surveys. For depth profiling, C60 was used as the ion source with a sputtering time of 30 s in between each
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depth acquisition and a sputtering rate of ∼4 nm/min. Raman spectra were collected on a Renishaw RM1000 microspectrometer using an Ar ion laser 488 nm with ∼1 μm lateral spot size and 11 mW total power. Cyclic voltammograms were recorded in a three electrode setup under a nitrogen blanket with a Gamry Reference 600 potentiostat. The PTh samples deposited on FTO glass served as the working electrode while a 2.5 2.5 cm2 platinum gauze (Princeton Applied Research) was used as the counter electrode. The silver reference electrode (Princeton Applied Research) was filled with 0.1 M silver nitrate (99.9999%, Sigma Aldrich) and 0.1 M tetraethylammonium perchlorate (electrochemical grade, Sigma Aldrich) in acetonitrile (ACS grade, Sigma Aldrich).26 A supporting electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate (electrochemical grade, Fluka) in acetonitrile was bubbled for 1 h with nitrogen prior to use. The potential was swept between 0.4 and 1.2 V vs Ag/AgNO3 with a sweep rate of 80 mV/s. Film conductivity was estimated through measuring sheet resistivity from an Alessi four-point probe connected to a Keithley 2400 source meter and film thickness from cross-sectional SEM averaged over 5 different locations along the same line as the four-point probe pins. All characterizations were performed on as-deposited PTh films. In addition, characterizations were done on PTh films washed by first soaking in 0.1 M hydrochloric acid (Sigma Aldrich) for 1 h to remove any vanadium compounds, then neutralized with 0.1 M sodium hydroxide (BDH Chemicals), and finally washed thoroughly with distilled water and dried in air. Besides oCVD films, analysis was also performed for comparison on oligothiophene standards, including 2,20 -bithiophene (97%, Acros Organics), terthiophene (99%, Alfa Aesar), α-quarterthiophene (TCI America), and α-sexithiophene (Sigma Aldrich).
3. RESULTS AND DISCUSSION In the oxidative chemical vapor deposition (oCVD) of unsubstituted polythiophene (PTh), thiophene monomer and vanadium oxytrichloride oxidant/initiator were continuously introduced as vapors into the chamber, where adsorption of the reactants on the cooled substrate resulted in polymer film formation at all the different oCVD conditions of pressure, flow rates, and substrate temperature explored. It was found that film deposition rate is a strong function of reactant surface concentration, particularly of the initiator, with increasing concentration leading to faster growth kinetics. For example, at a fixed substrate temperature (5 °C) and constant flow rates of monomer and initiator (2.0 and 0.5 sccm, respectively), increasing the total pressure resulted in a higher deposition rate. Similarly, with all other conditions remaining unchanged, an increase in substrate temperature by 2 °C significantly reduced the deposition rate. Importantly, these observations point to the fact that oCVD is an adsorption-limited process within the range of conditions studied here. Thus, a substrate temperature of 5 °C was chosen as a balance between the decrease in reactant reactivity at lower temperatures and reduced adsorption at higher temperatures. With the flow rates of monomer and initiator set at 2.5 and 0.6 sccm, respectively, and the total reactor pressure of the reactor at 18 Torr, the as-deposited PTh film in its doped state showed an electrical conductivity as high as 20 mS/cm, which is lower than the reported values for highly doped polythiophene (0.7200 S/cm) and higher than that of undoped polythiophene.2729 Dedoping the film by washing resulted in the loss of electrical conductivity and a color change of the film from brown to orange. Figure 1 shows the FTIR spectrum of the as-deposited PTh film. It has strong distinct features in between 1100 and 1500 cm 1 , which can be assigned to the doping induced vibrations of unsubstituted polythiophene.30,31 The peaks observable at 790 and 690 cm1 are characteristic of CH 15224
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Langmuir
Figure 1. FTIR spectra of as-deposited and washed PTh. The doping induced vibrational bands in the 11001500 cm1 region for the asdeposited film disappear after washing as a result of dedoping.
Figure 2. Raman spectra of as-deposited and washed PTh. The spectra can be divided into three main regions in the ranges 13501500, 10001250, and 650740 cm1. Insets magnify the latter two regions of lower intensity. Washing results in narrowing of the peaks in the highest intensity region as a result of a loss of the quinoid vibration.
out-of-plane bending of the thiophene ring.32 The CH stretches located above 3000 cm1 also indicate that the aromatic thiophene ring is preserved. The peak located at 1490 cm1 on the shoulder of the doping induced vibration is attributed to the antisymmetric CdC stretch. Also shown in Figure 1 is the FTIR spectrum of the washed PTh film. After washing with acid, the CH out-of-plane bending modes have become the most prominent features, while the doping induced peaks seem to have disappeared, indicating that the acid wash removes the dopant from the as-deposited film. There are small, broad features at 1140 and 1260 cm1 that remain, which are possibly from residual doping. Also noticeable besides the peak at 1491 cm1 of the antisymmetric CdC stretch is the appearance of a peak located at 1439 cm1 due to its symmetric vibration that was previously obscured in the as-deposited film by the doping induced bands. Additional peaks at 1042 and 1069 cm1 are assigned to CC stretches, and ring deformation can be
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observed at 740 cm1. The above assignments have been confirmed by literature30,32 and comparison with experimental spectra of undoped standard oligothiophenes, as well as computational modeling results of neutral α-oligothiophene molecules from density functional theory (DFT) calculations in Gaussian 0333 (see Supporting Information, Figures S1S4). The small broad peak at 950 cm1 in only the as-deposited PTh is possibly due to the presence of vanadium compounds formed during deposition that is removed with acid washing. As Figure 2 shows, the Raman spectra of the as-deposited and washed PTh share similar features. The features can be divided into three main regions in the ranges 13001510, 10001250, and 620740 cm1. The strongest peaks can be observed in the 13001500 cm1 region that can be deconvoluted into four peaks located at 1365, 1417, 1457, and 1504 cm1 (see Supporting Information, Figure S5).34,35 The vibration bands at around 1500 (ν1) and 1460 cm1 (ν2) are assigned to asymmetric and symmetric CdC ring stretching modes of unsubstituted polythiophene, respectively.32 A shift of the asymmetric CdC (ν1) band toward lower wavenumber indicates a higher conjugation length, e.g., sexithiophene in doped form is at 1507 cm1, while polythiophene is at around 1500 cm1.36 In our case, this peak is located at ∼1504 cm1, which could suggest a relatively short conjugation length and offer an explanation for the lower observed conductivity of doped PTh. The weak shoulder at 1365 cm1 is assigned to the CβCβ0 vibration in the thiophene ring and the peak located at 1417 cm1 can be attributed to the quinoid vibration in polythiophene. Lastly, we observe in this region that, although the as-deposited and washed films are comparable, the latter shows narrower peaks that can be attributed to the loss of the quinoid vibration of the doped state which consequently explains the loss of film conductivity as a result of dopant removal during washing. The second region in the 10001250 cm1 range (Figure 2 inset) can also be deconvoluted into four distinct peaks (see Supporting Information, Figure S6). The strongest peak located at 1045 cm1 is attributed to the CH bending mode, and the band at 1222 cm1 is assigned to the vibration of the CαCα0 linkage between adjacent thiophene rings. The remaining two bands in this region can be assigned to inter-ring CC stretches of the distorted part of the molecule.37 The third region within 620740 cm1 (Figure 2 inset) showing the weakest intensity of the three regions can also be resolved into four distinct peaks (see Supporting Information, Figure S6). The peak at 697 cm1 describes the CSC coplanar vibration of the thiophene ring, and distortional vibrations can be seen at 680 and 652 cm1. The band at 740 cm1 can also be assigned to ring deformation. The intensity ratio of the sharp peak at 697 cm1 in this region to the distortion peak at 680 cm1 has been used to correlate with electrical conductivity and defects in polythiophene.32 In our case, by deconvoluting this region into its constituent peaks, the intensity ratio of 680 (D) to 697 (ν7) is calculated to be 0.67, which is higher compared with the typical values (
