Article pubs.acs.org/JPCC
Effect of Regioregularity on Charge Transport and Structural and Excitonic Coherence in Poly(3-hexylthiophene) Nanowires Katherine A. Mazzio,‡ Andrew H. Rice,‡ Mathew M. Durban, and Christine K. Luscombe* Department of Materials Science and Engineering and Molecular Engineering and Sciences Institute, University of Washington, Seattle, Washington 98195, United States S Supporting Information *
ABSTRACT: This study explores the role of very small changes in poly(3-hexylthiophene-2,5-diyl) (P3HT) regioregularity on the physical and electronic properties of P3HT nanowires. Due to a high level of synthetic control, we are able to isolate the effects of regioregularity from those of polymer molecular weight and dispersity for the first time. A series of P3HTs with regioregularities from 96 to 99%, similar molecular weights, and low dispersities are synthesized. The charge transport properties of these polymers, along with a Soxhlet extracted 93% regioregular P3HT purchased from Rieke metals, are investigated in both thin film and nanowire transistors. The resulting structural characteristics are examined by atomic force microscopy and X-ray diffraction, and the optical characteristics are explored by UV−vis absorption. It is found that increasing the P3HT regioregularity results in improved charge transport characteristics, with an increase in mobility by a factor of 4 for the regioregularities examined. The increased mobility is shown to reflect increasing structural coherence lengths in the (010) direction, as well as improved J-aggregate characteristics due to greater planarity and reduced numbers of defect sites along the polymer nanowires. Overall, this study serves to emphasize the importance of determining and reporting even small changes in polymer regioregularity.
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polymer backbone and breaks up conjugation.14,15 The greater planarity in the conjugated backbone of highly RR polymers allows for both efficient π−π stacking and charge transport through both interchain and intrachain interactions. In practice, thin films of P3HT tend to be comprised of ordered crystallites surrounded by amorphous P3HT.16 This mixed domain system can impede charge hopping between the crystalline charge transport domains due to the disorder between the crystallites.8,17 As a result, much effort has been put forth to suppress the role of disorder in these systems and to promote the greatest degree of structural order. One method of interest is the development of self-assembled polymer nanowires that enable long-range charge transport without postprocessing treatments. P3HT nanowires exhibit supramolecular self-assembly with polymer chains perpendicular to the nanowire long axis, where growth occurs along this long axis via strong π−π interactions.18,19 These nanowires have been produced by various methods, including electrospinning, templated synthesis, and via the whisker method in solution, among others.19−21 The whisker method is one technique that provides a low-cost, solution processable route toward the mass production of these nanowires. The effects of MW, RR, and processing conditions on the structural and charge transport properties of nanowires have been previously investigated, but a lack of synthetic control
INTRODUCTION Conjugated polymers have seen significant academic and industrial interest for applications in flexible, low-cost, and solution processable electronic devices.1−4 Poly(3-hexylthiophene-2,5-diyl) (P3HT) is one of the most widely studied polymers in this class, owing to its good optoelectronic properties, along with its relative synthetic ease and solution processability.5−7 Many of the advantageous properties of P3HT arise from favorable π−π interactions that allow the formation of well-ordered, semicrystalline lamellar microsctructures, the domain sizes of which have been shown to have a strong influence on the resulting electronic properties.1,8 The molecular weight (MW), dispersity (Đ), regioregularity (RR), and processing conditions have all been shown to have profound effects on the charge transport properties of P3HT.9−12 The RR of P3HT is defined as the ratio of monomers adopting the head to tail (H−T) conformation to those adopting either tail to tail (T−T) or head to head (H− H) couplings, as outlined in Figure 1.13 The introduction of head to head coupling products in the chain results in torsion caused by steric effects that reduces the planarity of the
Received: March 26, 2015 Revised: June 16, 2015
Figure 1. Possible coupling products of P3HT. © XXXX American Chemical Society
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ization, however, even small changes in the isopropylmagnesium chloride (iPrMgCl) concentration during Grignard formation can affect active monomer formation, with increased i PrMgCl concentrations leading to Grignard formation on both sides of the monomer. This can ultimately influence the resulting RR of the polymer. We hypothesize that in our case this led to the formation of polymers with RR differing by as little as 1%. This resulted in the synthesis of a series of P3HT with RR ranging from 96 to 99% with similar molecular weights and low dispersities. This series of polymers was also compared to 4002-E P3HT purchased from Rieke Metals. All polymers were Soxhlet extracted in methanol and acetone and isolated from chloroform; in the case of the 4002-E P3HT, this resulted in a reduction of its dispersity from 1.8 for the commercial polymer to 1.5 for the Soxhlet extracted polymer. For each polymer, the molecular weights and dispersities were determined by GPC in THF with polystyrene as an external standard, while the RR was determined by NMR. Representative integrated NMR spectra can be found in the Supporting Information in Figures S1−S4 for each polymer. A summary of the physical properties for all polymers investigated can be found in Table 1. These polymers were later used to make a
over the base polymer has made it difficult to isolate the effects of each property. Baghgar et al. recently explored the role of MW on the exciton coupling in P3HT nanowires.22 They found that the effective exciton dimensionality changes with increasing polymer molecular weight from quasi 2D (exhibiting both inter- and intrachain coupling) to almost exclusively 1D (exhibiting primarily intrachain coupling) and attributed this to chain folding of individual polymers, which improved the planarity and reduced their torsional disorder. The RR and Đ values for each set of polymers were reported to be similar, but the RR values in particular were only described as being ∼98%. Snyder et al. have shown that even very small changes in P3HT RR (on the order of 0.5%) can influence the crystal lamellar thickness and emphasized the importance of reporting polymer RR; they have identified it to be one of the most critical properties dictating differences in morphology and device performance.23 Aiyar et al. examined the role of RR on the charge transport properties of P3HT, finding that differences in RR of as little as 4% are sufficient to drastically effect the electronic and morphological properties.24 They attributed the observed improvements to an enhancement in the main chain planarity and a reduced number of grain boundaries for the higher RR variant. Shimomura et al. looked at the role of solvent processing on the structural coherence and transport properties of isolated P3HT nanowires.25 They found an increase in mobility due to greater structural coherence for polymers processed from solvent mixtures with higher concentrations of good solvent. In this case, the RR and dispersity of the polymer were not identified. Similarly, Roehling et al. investigated the role of processing conditions from different solvents and solvent combinations on P3HTs of two different molecular weights.18 They found a positive correlation between the microstructural order and the solvent quality. However, the RR of the polymers under investigation were reported only as being >95%, and no dispersities were presented. The isolation of polymer properties represents a significant challenge due to synthetic limitations, and a rigorous description of the impact of polymer RR on charge transport phenomena has yet to be described. Bronstein and Luscombe previously developed a method for the externally initiated synthesis of P3HT that allows for excellent control over the molecular weight with very low dispersity and extremely high RR.26 Through this synthetic method, we have been able to produce a series of polymers with RR from 96 to 99%, differing by 1%, with similar molecular weights and low dispersities. This provides us with the unique ability to isolate RR effects from the other polymer properties. In this work, we demonstrate that with increasing, RR polymer nanowires exhibit better charge transport properties, as expected. We find that these improved electronic characteristics are a result of increased crystallinity with increasing RR, as determined by XRD, which corresponds to an elongation of the structural coherence length and is supported by enhanced intrachain order observed in the UV−vis spectra as analyzed by Spano’s model.27
Table 1. Number Average and Weight Average Molecular Weights, Dispersities, and RR for the P3HT Polymer Series Examined RR (%)
Mn (kg/mol)
Mw (kg/mol)
Đ
93 96 97 98 99
19 18 21 16 20
28 20 23 19 25
1.5 1.1 1.1 1.2 1.2
series of nanowires according to the whisker method19 by dissolving 2 mg/mL of each polymer in an 80:20 anisole:chloroform solvent mixture, heating at 70 °C overnight, and then allowing the polymers to self-assemble at room temperature for at least 3 days. When heated, each of these mixtures was a bright orange color that changed to a deep purple during the self-assembly process. It has been reported that small changes in RR can significantly influence the physical and electronic properties of P3HT.11,23,24,28,29 However, these results have yet to be fully decoupled from molecular weight and dispersity effects due to poor synthetic controls. In addition, differences in RR on the order of 1% have yet to be investigated. As mentioned above, Snyder et al. have reported that a change in RR of as little as 0.5% may significantly change the crystallization properties of P3HT.23 Therefore, a change in RR of 1% may actually represent a substantial difference. To investigate the effect of polymer RR on the charge transport properties, we made both thin film and nanowire transistors. Probing the charge transport characteristics of self-assembled nanowires allows us to predominately isolate the role of microstructure on the resulting properties. All transistors investigated were fabricated in a top-contact bottom-gate geometry with 2 wt % P3HT spin coated from chloroform for the non-nanowire samples and 2 wt % P3HT self-assembled in 80% anisole/20% chloroform for the nanowire samples. At least four substrates and 12 devices were examined for each processing condition. Representative transfer curves can be found in Figures S5 and S6 of the Supporting
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RESULTS AND DISCUSSION The externally initiated synthesis of P3HT was carried out using previously reported procedures.26 This method allows the production of highly regioregular P3HT with controlled molecular weights and very low dispersities through the nickel-catalyzed polymerization of a Grignard functionalized monomer from an external initiator. During each polymerB
DOI: 10.1021/acs.jpcc.5b02914 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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nanowires were fabricated under the same conditions and from polymers with similar molecular weights. Each polymer appears to produce nanowires with similar dimensions and morphology, indicating that these physical features are not dependent upon RR. Therefore, we expect that any variation in properties is a result of differing RR of the polymers and not simply changes in the film morphology. Intra- and Interchain Order. To understand the role of RR on the structural coherence of the resulting nanowires, each set of nanowires was examined by XRD. Figure 4 provides representative diffractograms, ordered from lowest RR on the bottom to highest RR on the top. While there are no changes in peak position for the (100) peaks at 5.8° (2θ) or (010) peaks at 23.8° (2θ), indicating that there is no fundamental change in the crystal structure due to changes in RR, other significant differences in the XRD spectra do exist. The main differences are primarily reflected in the narrowing of the (010) peak with increasing RR. The (010) peak arises from features in the π−π stacking direction, which correspond to the long axis of the nanofibers. The narrowing of the (010) peak with increasing RR indicates an increase in structural coherence, which can be thought of as increasing the average crystallite size or decreasing the number of grain boundaries within the nanofibers. The Scherrer equation can be used to quantify the structural coherence, ξ, of P3HT according to eq 130
Information. Table 2 provides a summary of the resulting charge transport properties. Table 2. Summary of FET Characteristics for Nanowire and Non-Nanowire Films Fabricated from P3HT with Varying RR RR (%) 93 96 97 98 99 93 96 97 98 99
film type nanowire nanowire nanowire nanowire nanowire non-nanowire non-nanowire non-nanowire non-nanowire non-nanowire
Vt (V) 18 13 11 9 12 6 8 6 9 7
μh (cm2 V−1 s−1)
Ion:Ioff 2
10 103 103 103 103 103 103 104 103 103
4.5 7.8 1.2 1.6 1.5 4.0 5.7 6.6 8.0 8.5
× × × × × × × × × ×
10−4 10−4 10−3 10−3 10−3 10−4 10−4 10−4 10−4 10−4
± ± ± ± ± ± ± ± ± ±
5.0 1.0 1.5 1.1 2.3 5.0 7.0 6.0 1.2 1.0
× × × × × × × × × ×
10−5 10−4 10−3 10−3 10−3 10−5 10−5 10−5 10−4 10−4
We found that the field-effect mobility increases with RR for both the nanowire and non-nanowire samples in a similar manner. There is an initial increase in mobility followed by a statistical leveling out for 98% and 99% RR. A visual representation of this trend can be found in Figure 2. With
ξ=
Kλ (Δθ )cos θB
(1)
where K is a dimensionless shape factor; λ is the X-ray wavelength; θB is the Bragg angle; and Δθ is the line broadening at half the maximum intensity. Calculated unit cell dimensions and structural coherence are reported in Table 4. The (100) interlayer distance, d(100), is located at 5.8° (2θ) and is similar for each set of nanowires. A distance of 0.17 nm for d(100) corresponds well to the sum of two rows of P3HT, with the hexyl chain tilted in an all trans conformation, and is consistent with dimensions for the (100) interlayer distance reported by other groups for P3HT.25,31 The (100) domain size, ξ(100), does not show any trend with changing RR. The (100) domain size is greater than the nanowire height as determined by AFM, which has been suggested by others18,19 to arise from nanowires stacking perpendicular to their long axis, which may occur during drying. The (010) interlayer distance, d(010), remains consistent at 0.38 nm for each set of polymer nanowires and corresponds to the π−π stacking of the thiophene units. The structural coherence in the longitudinal direction, ξ(010), shows significant elongation with increasing RR. Because the structural coherence is related to the average size of crystallites within the nanofiber, we expect that longer coherence lengths will result in fewer grain boundary scattering sites for charge transport, thereby improving the charge transport properties in nanowire transistors made with polymers of increasing RR, as observed in our transistor studies reported in Figure 1. This increase in long-range order is also supported by an increase in intrachain order with increasing RR, as determined by analysis of the UV−vis spectra according to Spano’s weakly coupled HJ-aggregate model.27 The weakly coupled HJaggregate model takes advantage of the analysis of vibronic modes in the absorbance spectra to describe the interplay
Figure 2. Relationship between RR and hole mobility for P3HT nanowire and non-nanowire films. Error bars represent one standard deviation from the mean.
the nanowire transistors, the initial improvements represent a 4-fold average increase in mobility. These improvements could be a result of better crystallinity and/or morphology. In order to determine the root cause of the observed improvement in electronic properties with increasing RR, we first examined whether there were any changes in film quality or morphology of the transistors by AFM. Representative AFM height images can be found in Figure 3. Non-nanowire films spin coated from chloroform are featureless, as expected.12,24 The morphologies of the nanowire films appear to be very similar, regardless of polymer RR, and are comprised of a tangle of nanowires with dimensions of about 24 nm in width and 5 nm in height. Average nanowire dimensions are reported in Table 3, as determined by examining a population of at least 100 nanowires from each polymer. There is no clear trend between the RR and the height, width, or surface coverage of the nanowires in the nanowire films. These properties have been previously shown to be dependent on the physical properties of the base polymers, as well as on the processing conditions employed. In our case, all C
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Figure 3. AFM height images for nanowire thin films comprised of 93%, 96%, 97%, 98%, and 99% regioregular P3HT. A smooth, featureless, nonnanowire P3HT film (99% RR) is included for comparison.
are few reports of J-aggregate behavior in supramolecular assemblies of P3HT, and they have generally been reported for P3HT nanowires self-assembled in toluene.22,33,34 It has been hypothesized that J-aggregate formation will only occur upon fractionation of the MW during self-assembly, which leads to fewer stacking faults.34 In our case, the formation of Jaggregates occurred as the RR of the polymer used for nanofiber formation increased while using a mixed solvent system of 80% anisole and 20% chloroform. By using P3HT with low dispersities, we limit the requirement for fractionation and promote J-aggregate behavior in highly regioregular polymers. It is also interesting that in previous reports Jaggregate behavior in nanowires made from lower RR P3HT and produced via self-assembly in toluene resulted in nanowires that were significantly wider and were 30−40 nm in width.33 By using our low Đ, high RR P3HT, and performing the selfassembly process in a mixture of good and poor solvents, we are able to produce J-aggregates in nanowires with much smaller widths. We believe that the narrower nanowires are a result of using a poorer solvent, which results in the kinetic trapping of polymer conformations. In the 93% RR P3HT, kinetic trapping causes the formation of H-aggregates, but using a low Đ, high RR P3HT allows for J-aggregate formation.33 Representative UV−vis spectra for 93% RR, 96% RR, and 99% RR nanowire samples can be found in Figure 5. It can be seen that with increasing RR there is an increase in the lowest energy absorption feature (A0−0). 93% RR P3HT nanowires exhibit Haggregate behavior, and 99% RR P3HT nanowires show Jaggregate behavior. Under our processing conditions, we found that our 96% RR P3HT nanowires were located approximately at a crossover point, with A0−0/A0−1 ≈ 1. This shows that the HJ-aggregate behavior in these supramolecular self-assemblies of P3HT can be tuned with small changes in RR. It should be noted that the 93% RR P3HT also has a greater dispersity, which might be expected to effect the HJ-aggregate characteristics by potentially introducing greater amounts of stacking faults with the inclusion of shorter polymer chains. However, Roehling et al. described the role of fractionation during nanowire formation, where one would expect short chains to be
Table 3. Nanowire Dimensions for P3HTs with Different RR RR (%) 93 96 97 98 99
height (nm) 4.9 4.8 5.2 4.1 5.1
± ± ± ± ±
1.1 1.3 1.1 1.1 1.0
width (nm) 24 24 24 24 24
± ± ± ± ±
2.5 4.4 3.4 3.3 2.5
between structural order and excitonic coupling, i.e., the role of intrachain excitonic coupling (through bond, or along the polymer backbone) relative to the amount of interchain excitonic coupling (through space, or between polymer chains in the π−π stacking direction). The relative H-like or J-like aggregate behaviors can be determined by comparing the relative absorption strength of the lowest energy 0−0 transition (A0−0) to that of the 0−1 first vibronic absorption transition (A0−1). Their ratio is related to the cofacial intrachain Coulombic coupling, J0, by the free exciton bandwidth, W, where W = 4J0. Assuming a Huang−Rhys factor of 1, this ratio can be expressed according to eq 2 2 A 0 − 0 ⎛ 1 − 0.24W /ℏω0 ⎞ ≅⎜ ⎟ A 0 − 1 ⎝ 1 + 0.073W /ℏω0 ⎠
(2)
where ℏω0 is 180 meV and corresponds to the energy of the primary vibronic modes (symmetric ring stretching) associated with the electronic transition. W will decrease with greater Hlike character in the system, corresponding to an increase in intrachain coupling as a result of fewer conformational changes and greater long-range order. An increase in the ratio of A0−0 to A0−1 indicates decreased excitonic coupling or greater intrachain order. In general, H-aggregate behavior is found for A0−0/ A0−1 values 1, where interchain excitonic coupling is limited due to extended intrachain exciton delocalization. There D
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Figure 4. XRD spectra for nanowire films made from P3HT with different RR.
Table 4. Summary of Unit Cell Values and Structural Coherence Determined from XRD Spectra RR (%)
d(100) (nm)
ξ(100) (nm)
d(010) (nm)
ξ(010) (nm)
93 96 97 98 99
0.17 0.17 0.17 0.17 0.17
8.1 8.7 8.5 8.7 8.4
0.38 0.38 0.38 0.38 0.38
6.2 11 14 21 27
excluded from the nanowires.18 Indeed, we found that there was primarily a reduction in the amorphous absorption feature, i.e., a reduction in the absorption feature of P3HT in chloroform solution with a maximum absorbance at 450 nm, between polymers made with neat 4002-E P3HT and Soxhlet extracted 4002-E P3HT. This supports the findings of Roehling et al., and we believe that that the observed changes in A0−0/ A0−1 are therefore primarily a result of changes in RR. The degree of J-aggregate behavior follows what we would expect from the XRD structural coherence measurements above, where greater structural coherence in the (010) direction promotes increased J-aggregate behavior, both of which are indicative of increased planarity in the polymer backbone and the existence of limited stacking faults. The continued increase in structural coherence with increasing RR does not account for the leveling off of mobility that we observed for 98% and 99% RR; we would expect to observe similar structural coherence based on the transport
Figure 5. UV−vis absorption spectra for 93% RR (long dashed line), 96% RR (short dashed line), and 99% RR (solid line) P3HT nanowires, highlighting an increase in the lowest energy vibronic shoulder with increasing RR.
characteristics. We hypothesize that areas of nanowire overlap in the thin films may be acting as defect sites for charge transport, where charge carrier hopping to neighboring wires through incoherent amorphous regions could result in a reduction in mobility. To support this hypothesis, we made transistors from relatively dilute solutions of P3HT nanowires, having ∼15% surface coverage, relative to the ∼45% surface E
DOI: 10.1021/acs.jpcc.5b02914 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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methanol, acetone, and a final extraction with chloroform. The solution was concentrated and precipitated in methanol, followed by filtrating to yield the purified P3HT polymer product. Representative 1H NMRs can be found in Figures S1− S5 of the Supporting Information (96% RR P3HT: Mn 18 kDa, Mw 20 kDa, Đ 1.1; 97% RR P3HT: Mn 21 kDa, Mw 23 kDa, Đ 1.1; 98% RR P3HT: Mn 16 kDa, Mw 19 kDa, Đ 1.2; 99% RR P3HT: Mn 20 kDa, Mw 25 kDa, Đ 1.2). Nanowire Synthesis. P3HT nanowire solutions were prepared according to the whisker method,19 using a solvent blend of 80 vol % anisole and 20 vol % chloroform. P3HT was added to this solvent at a concentration of 2 mg/mL, and the solution was stirred overnight inside a N2 glovebox at 70 °C. The solution was then removed from heat and stirring and left untouched for at least 3 days to allow the nanowires to selfassemble in solution. OFETs. Organic field-effect transistors were fabricated in a top-contact bottom-gate device geometry on heavily doped ptype silicon ⟨100⟩ wafers with a 300 nm thermal oxide layer, purchased from Montco Silicon Technologies. Substrates were cleaned by sequential ultrasonication with acetone, methanol, and isopropyl alcohol for 15 min each. They were then dried under a stream of nitrogen and treated by air plasma for 5 min before passivating the oxide layer with a self-assembled monolayer of octadecyltrichlorosilane (OTS) via vapor deposition. The substrates were then washed with chloroform and isopropyl alcohol to remove any physisorbed silanes. Solutions of each polymer were used as prepared for the nanowire experiments and spin coated on the OTS passivated substrates at 1000 rpm for 60 s. The substrates were then left in a nitrogen glovebox overnight without annealing to allow any remaining solvent to evaporate. Gold source and drain electrodes were then thermally evaporated through a shadow mask at a base pressure of 5 × 10−7 Torr at a rate of 1 Å/s to a thickness of 50 nm. The output and transfer characteristics of all transistors were measured in a nitrogen atmosphere using a standard four-probe setup with an Agilent 4155B semiconductor parameter analyzer. All devices had channel lengths of 50 μm and channel widths of 1000 μm. At least four substrates and 12 devices were tested for each processing condition. The saturated charge carrier mobility for each polymer film was calculated using the saturation current equation (eq 3)
coverage of the nanowire films discussed above. Figure S7 of the Supporting Information provides a visual representation of how dilution of nanowires effects the charge transport properties. We found that there is a significant increase in mobility with dilution, which we can attribute to a reduction in the number of charge carrier hopping events that must occur to transport charges to the electrodes. This defect pathway could become more important with increasing RR. We hypothesize that in the limiting case of single nanowire transistors we would expect the 99% RR nanowires to exhibit an even greater improvement in transport characteristics. Further experiments are ongoing to help us determine whether the plateau in mobility at high RR is intrinsic to the P3HT nanowires or if it is an extrinsic effect brought about by processing and characterization methods.
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CONCLUSION We have shown that increasing the RR of P3HT on relatively small scales can improve the charge transport properties in both amorphous and nanowire thin films. Increasing the RR from 93% to 99% results in an increase in the (010) structural coherence in the nanowire longitudinal domain by more than a factor of 4. This relatively small increase in RR accounted for significant changes in excitonic coupling. Additionally, nanowires showed H-aggregate behavior with low RR and transitioned to J-aggregate behavior for high RR. These structural and excitonic findings can be interpreted as being a result of increased planarity in the polymer backbone and decreased stacking fault behaviors in the longitudinal direction. From this work, it is clear that even very slight changes in RR can significantly impact the resulting physical and electronic properties of P3HT, emphasizing the importance of determining and reporting even small changes in RR values.
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EXPERIMENTAL SECTION Polymer Synthesis. For each polymer, all reactions were carried out under a nitrogen atmosphere using standard Schlenk techniques. All reagents were purchased from commercial sources and used without purification, unless otherwise indicated. The monomer 2-bromo-5-iodo-3-hexylthiophene and the catalyst 2-tolylNi(PPh3)2Br were synthesized according to literature procedures.35 LiCl (127 mg, 3 mmol) was added to a dried Schlenk flask, which was then further dried by heating in vacuo. 2-Bromo-3-hexyl-5-iodothiophene (373 mg, 1 mmol) was added, and the flask was pumped and purged with nitrogen. Dry THF (20 mL) was added via cannula, and the reaction was cooled to 0 °C with stirring. Isopropylmagnesium chloride (2 M, 0.5 mL, 1 mmol) was added dropwise over 5 min, and stirring continued for 1 h. To a second dry Schlenk flask was added bromo(o-tolyl)bis(triphenylphosphine)nickel(II) (6.3 mg, 0.0084 mmol) and 1,3-bis(diphenylphosphino)propane (10.4 mg, 0.0252 mmol). Dry THF (20 mL) was added via cannula, and the nickel catalyst complex was allowed to stir for 2 h at room temperature. The prepared Grignard functionalized thiophene monomer solution was quickly transferred via cannula to the catalyst solution at 0 °C. The reaction was allowed to warm to room temperature and stirred for 2 h. HCl (5 M, 1 mL) was added to quench the polymerization. The reaction mixture was stirred for 15 min, followed by precipitation with methanol. The polymer was isolated by filtration and further washed with methanol. The polymer was purified by Soxhlet extraction with
Ids =
μWC0 (Vg − Vt) 2L
(3)
where Ids is the drain−source current; μ is the field-effect mobility; W is the channel width (1000 μm); L is the channel length (50 μm); C0 is the capacitance per unit area of the insulator (SiO2, 300 nm, 10 nF cm−2); Vg is the gate voltage; and Vt is the threshold voltage. The field-effect mobility (μ) was determined from a linear fit of (Ids)1/2 vs Vgs in the saturation regime. The threshold voltage (Vt) was estimated from the xintercept of the linear region of (Ids)1/2 vs Vgs. AFM. Samples prepared for OFETs were characterized by AFM to examine nanowire size and film quality after device testing. Tapping mode AFM images were taken on a Veeco multimode AFM with a Nanoscope III controller. The AFM tips used for this study were purchased from Veeco, model RTESP (f ≈ 300 kHz, k ≈ 40 N/m), phosphorus-doped Si tips. XRD and UV−vis. For XRD and UV−vis sample preparation, once nanowire self-assembly was complete, the solutions were centrifuged at 10 000 rpm for 30 min, causing F
DOI: 10.1021/acs.jpcc.5b02914 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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the nanowires to separate from solution. The remaining solvent was then decanted. For UV−vis, the separated nanowires were dispersed in anisole, and UV−vis spectra were recorded on a ThermoFisher EVO350 UV−vis spectrometer. For XRD, the isolated nanowires were collected and dispersed in isopropyl alcohol. This new solution was drop cast onto clean, glass substrates to form a thick, dense, nanowire film. X-ray diffractograms were obtained on a Bruker AXS D8 Focus diffractometer using an accelerating voltage of 40 kV and a Cu Kα X-ray source.
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ASSOCIATED CONTENT
* Supporting Information S
Supporting Information includes integrated 1H NMRs and representative thin film and nanowire transfer curves, and a plot detailing the relationship between mobility and RR for dilute NW transistors for each polymer. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02914.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ‡
These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSF DMR-1407815. REFERENCES
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DOI: 10.1021/acs.jpcc.5b02914 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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