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Electronic Properties of Conjugated Polyelectrolyte/SingleWalled Carbon Nanotube Composites Yao Li, Cheng-Kang Mai, Hung Phan, Xiaofeng Liu, Thuc-Quyen Nguyen, Guillermo C. Bazan,* and Mary B. Chan-Park* Carbon nanotubes (CNTs)[1] are attractive because of their outstanding chemical,[2] electrical[3] and mechanical[4] properties. As a one-dimensional material, their high aspect ratio allows CNTs to form percolating networks in composite materials, which leads to highly conducting films and to increases in viscosity that allow one to manage film thickness.[5] In order to implement CNTs into applications, the bulk material needs to be well dispersed and methods for achieving such suspensions have been intensively studied due to the potential advantage of solution processing.[6] Among different approaches,[7] dispersing CNTs with functional conjugated and nonconjugated polymers attracts special attention as it does not introduce defects to the nanotubes and avoids contamination from surfactants.[8] The polymer/CNT composites thus obtained have been shown to possess excellent mechanical and electrical properties.[9] Conjugated polyelectrolytes (CPEs) have been found to be good CNT dispersants.[10] CPEs are defined as π-conjugated polymers with ionic side chains.[11] Combining both features of organic semiconductors and polyelectrolytes, they offer a broad field for applications ranging from organic light-emitting diodes (OLEDs),[12] solar cells,[13] to biosensors[14] and bioimaging.[15] When used to disperse CNTs, CPEs have the advantage over neutral conjugated polymers of being soluble in polar solvents, like water and methanol (MeOH), which provides greener alternatives for the replacement of toxic solvents, and allows incorporation into organic semiconductor devices via simple solution processing without disrupting underlying hydrophobic active layers. In this contribution, an anionic narrow band gap CPE, namely PCPDTBTSO3TBA (P1, poly[2,6-(4,4bis-tetrabutylammonium butanylsulfonate-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)], Figure 1a),[16] was used to disperse single-walled carbon nanoY. Li,[+] Prof. M. B. Chan-Park School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore E-mail: [email protected] Dr. C.-K. Mai,[+] H. Phan, X. Liu, Prof. T.-Q. Nguyen, Prof. G. C. Bazan Center for Polymers and Organic Solids Department of Chemistry and Biochemistry University of California Santa Barbara, California 93106, USA E-mail: [email protected] [+]Y. Li and Dr. C.-K. Mai contributed equally to this work.
DOI: 10.1002/adma.201400612
Adv. Mater. 2014, DOI: 10.1002/adma.201400612
tubes (SWNTs) in MeOH and a stable, conductive CPE/SWNT composite dispersion was obtained. A cationic CPE containing the same backbone PCPDTBTPyr+BIm4− (P2, poly[2,6-(4,4bis(6-pyridiniumhexyl)-4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene tetrakis(1-imidazolyl)borate)-alt-4,7-(2,1,3-benzothiadiazole)], Figure 1a)[17] was also studied for comparison. An interesting feature of P1 is that it can be easily doped in aqueous media to provide a stable conductive polymer, whereas P2 is not initially doped. We find that P1 gives higher electrical conductivity than P2 for all the CPE/SWNT composite films as obtained immediately after deposition. Additionally, the electrical conductivity increases after treating CPE/SWNT films with acid vapor for both P1 and P2 composites. The UV/Vis-NIR absorption spectrum of P1 in MeOH reveals that the polymer is doped, as indicated by the low energy transition in the NIR region related with polarons (Figure 1b).[18] These polarons are generated during the purification by dialysis, as described recently in the literature, and are stabilized by the pendant sulfonate groups, presumably via Coulombic stabilization.[16] In contrast, the absence of such electrostatic stabilization in P2 does not favor polaron formation on the conjugated backbone. One therefore observes a similar absorption profile typical of a narrow band gap conjugated polymer (Figure 1b). Thus, the blue shift of P1 compared to that of P2 is consistent with previous literature reports for cationic and anionic CPEs with identical conjugated backbones.[16,17] SWNT dispersions were prepared in 5 mg mL−1P1 solutions in MeOH by applying probe sonication for one hour. Different amounts of SWNTs were added to give P1:SWNT weight ratios of 100:1, 20:1, 5:1, 2:1 and 1:1. Attempts to prepare suspension with higher content of SWNT than that of 1:1 P1:SWNT were not successful, and excessive SWNTs stayed at the bottom as a gel/paste like residue. Optical images of 0.05 mg mL−1P1 solution and P1:SWNT dispersions taken 3 days after preparation show that the dispersions are homogenous and reasonably stable (Figure 2a). For example, no change in the absorption of the dispersions is observed after six weeks. As more SWNTs were added, the color of dispersions gradually became darker, a feature that could also be observed in thin films spun cast from the dispersions (Figure 2b). It is worth noticing that the film thickness increases with increasing SWNT concentration, from 10 nm for pure P1 film to 65–70 nm for P1:SWNT = 1:1, under similar casting conditions. The color change can be quantitatively characterized by examination of the absorption spectra of the dispersions (Figure 2c). The baseline of P1:SWNT dispersion shifts up upon progressive addition of SWNTs. Additionally, the broad absorbance band centered at 1030 nm grows in intensity with
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Figure 1. a) Chemical structures of PCPDTBTSO3TBA (P1) and PCPDTBTPyr+BIm4−(P2). b) UV/Vis-NIR absorption spectra of P1 (ⵧ) and P2 (䉲) in MeOH.
Figure 2. a) Optical images of P1 and P1:SWNT dispersions with weight ratio 100:1, 20:1, 5:1, 2:1 and 1:1 (P1 concentration fixed at 0.05 mg mL−1 for all dispersions). b) Optical images of P1 and P1:SWNT composite films spun coat from each dispersion at 1500 rpm, 60 sec. c) UV/Vis-NIR absorption of P1 and P1:SWNT dispersions at different ratios, and the absorption of 0.05 mg mL−1 SWNT is shown in dash line. d) UV/Vis-NIR absorption data for P1:SWNT = 1:1, SWNT, the P1 contribution to the P1:SWNT complex (subtraction) and free P1.
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Adv. Mater. 2014, DOI: 10.1002/adma.201400612
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Table 1. Sheet resistance and conductivity of P1:, P2: and PSSNa:SWNT composite films before and after TFA vapor treatment. Films
Resistance [Ω/sq]
Conductivity [S/cm]
Resistance (TFA) [Ω/sq]
Conductivity (TFA)[S/cm]
P1
>10 Ma)
< 0.1
>10 Ma)
< 0.1
P1:SWNT = 100:1
a)
>10 M
< 0.1
>10 M
< 0.1
P1:SWNT = 20:1
>10 Ma)
< 0.1
>10 Ma)
< 0.1
P1:SWNT = 5:1
2650 ± 200 k
0.13 – 0.2
1850 ± 200 k
0.2 – 0.25
a)
P1:SWNT = 2:1
35 ± 5 k
8 – 10
14 ± 2 k
20 – 25
P1:SWNT = 1:1
5±1k
25 – 40
2.5 ± 0.1 k
55 – 65
P2:SWNT = 5:1
> 10 Ma)
< 0.1
110 ± 10 k
3–4
P2:SWNT = 2:1
130 ± 10 k
2.5 – 3
15 ± 2 k
25 – 30
P2:SWNT = 1:1
10 ± 1 k
15 – 20
2.8 ± 0.3 k
50 – 60
PSSNa:SWNT = 5:1
> 10 Ma)
< 0.1
> 10 Ma)
< 0.1
PSSNa:SWNT = 2:1
1200 ± 150 k
0.7 – 1
1450 ± 100 k
0.6 – 0.9
PSSNa:SWNT = 1:1
12 ± 1 k
20 – 30
20 ± 2 k
15 – 25
a)10
MΩ/sq is the maximum resistance that can be measured by the four-point resistivity mapper.
increased SWNT concentration. This feature corresponds to absorption of the second van Hove electronic transition of semiconducting nanotubes (S22),[19] which can also be observed in 0.05 mg mL−1 SWNT dispersed in N-methyl-2-pyrrolidone (NMP). We further verified this by dispersing 99% pure metallic SWNTs in a P1 solution using the same method, and we no longer observe the absorption band around 1030 nm (Supporting Information, Figure S1). For the P1:SWNT combination with a weight ratio of 1:1, subtraction of the SWNT spectrum obtained at an equivalent concentration resulted in an absorption profile that is contributed principally by P1 (Figure 2d). Comparison of this spectrum obtained to the absorption of standalone P1 reveals a red shift of λmax in the polymer absorption. This shift provides evidence of SWNT complexation with the CPE.[10,20] A similar λmax shift was observed when all the solution absorptions were taken in NMP (Supporting Information, Figure S2). The sheet resistances of P1 and P1:SWNT composite films were measured by four-point probe method (Table 1). For the P1 and P1:SWNT composites of 100:1 and 20:1 weight ratios, sheet resistances are greater than 10 MΩ/sq; beyond the measurable range of our setup. A sheet resistance of 2650 ± 200 kΩ/ sq was obtained when P1:SWNT = 5:1, at which concentration SWNT exist as bundles, as observed by using transmission electron microscopy (TEM, see Supporting Information, Figure S4). Similar results were obtained for same for P2:SWNT = 5:1. The value decreases by two orders of magnitude to 35 ± 5 kΩ/sq when the weight ratio gets to 2:1 and reaches 5 ± 1 kΩ/sq for the P1:SWNT = 1:1 blend, corresponding to an electrical conductivity of 25 – 40 S/cm. This trend agrees with what one observes in other polymer:SWNT systems, in which the film conductivity increases with increased SWNT loading.[21] Conductivity values of composite films prepared using the same concentration of non-doped cationic CPE P2 with SWNTs follow the same trend but are lower compared with P1:SWNT at similar SWNT loadings. Furthermore, composite films
Adv. Mater. 2014, DOI: 10.1002/adma.201400612
containing the non-conjugated polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSSNa), show even lower conductivity than those with P1 and P2 under similar experimental conditions. For example, when polymer:SWNT = 2:1, the electrical conductivity is 8–10 S/cm for P1:SWNT, compared with 2.5–3.0 S/cm and 0.7–1 S/cm for P2:SWNT and PSSNa:SWNT, respectively. These observations provide evidence of the improvements in conductivity by (a) the conjugated backbone, and (b) the initial doping in the CPE. The strong hydrophobic interaction between CPE and SWNT is further confirmed by absorption and resistance studies of samples before and after centrifugations. We centrifuged a P1:SWNT (1:1, P1 concentration is 5 mg mL−1) dispersion in MeOH, and isolated the supernatant. We observed that SWNT bundles settled down and that the color of supernatant became less dark. The solution absorbance after centrifugation exhibits simultaneous decrease of both CPE and SWNT absorptions to approximately 25% of the one before centrifugation (Supporting Information, Figure S3), indicating that the CPE is effectively complexed with SWNT. Films prepared from the supernatant gave a sheet resistance more than one order higher than those before centrifugation. For instance, with P1:SWNT = 1:1, the sheet resistance of the film after centrifugation is around 235 kΩ/sq (thickness = 10 – 15 nm), compared with 5 kΩ/sq (thickness = 65 – 70 nm) without centrifugation. It has been previously established that doping of P1 can occur via protonation. The quantity of unpaired electrons increases after addition of acid, as indicated by electron paramagnetic resonance and film conductivity measurements.[16] We therefore examined possible additional doping of the P1:SWNT in the solid state by exposing the film to trifluoroacetic acid (TFA) vapor at 50 °C overnight. As shown in Table 1 this treatment led to increases in conductivity. For the composite films with ratios: P1:SWNT = 5:1, 2:1 and 1:1, conductivity values approximately double after TFA treatment. We also exposed P2:SWNT
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films to TFA at the same conditions as P1:SWNT films. It is worth mentioning that the cationic CPE P2 cannot be doped by HCl in polar solvents as the pendant cationic groups hinder the formation of stable (cationic) polaronic states on the conjugated backbone.[16] Surprisingly, the P2:SWNT films treated with TFA also show an enhanced electrical conductivity. By measuring the UV/Vis-NIR absorption for P2:SWNT composite films before and after TFA vapor treatment, one can confirm doping of the conjugated polymer by the emergence of low energy transitions from 900 nm to 1300 nm (Supporting Information, Figure S5). Formation of positive polarons is therefore favored when P2 is mixed in the solid state with SWNTs. In contrast to P1 and P2, the PSSNa:SWNT composite films show no evidence of doping after TFA treatment as discerned from absorption spectra (Supporting Information, Figure S6) and no increase of electrical conductivity is observed. The electrical conductivity values for both P1:SWNT and P2:SWNT composite films after acid treatment can reach up to 60 S/cm. Similar increases of electrical conductivities are also observed when HCl vapor is used instead of TFA (Supporting Information, Figure S7 and Table S1). There is a 5 to 10 fold increase in electrical conductivity of P2:SWNT films after treating with TFA vapor, which is significantly higher than the increase observed with P1:SWNT. However, both composites reach similar conductivities after TFA treatments (see Table 1). We compared the UV/Vis-NIR absorption of neat P1 and P2 films before and after treating with TFA vapor (Supporting Information, Figure S8). In accordance with the change in conductivity, the absorption spectrum of P2:SWNT before/after TFA shows a much higher increase of low energy transitions relative to P1:SWNT. We also note that gel permeation chromatography (GPC) showed the average number of repeat units for both polymers to be approximately 8 (P1: Mn = 7.7 kg mol−1, Mw = 8.2 kg mol−1;[16] P2: Mn = 5.1 kg mol−1, Mw = 9.5 kg mol−1).[17] Considering that P1 is doped before TFA vapor treatment, we ascribe the greater impact by TFA on the conductivity of P2:SWNT blends on the fact that very few, if any, charge carriers are present in the as cast films. The bulk conductivity of films can be calculated based on the sheet resistance and film thickness, as shown in Table 1. To take a closer look at the films at the nanoscale, we characterized the P1 film and P1:SWNT composite films using conductive atomic force microscopy (c-AFM), which is a microscopy technique that maps out current and topography simultaneously.[22] The topographic image of neat P1 film shows that the surface is smooth and continuous with a root mean square roughness of 1.6 nm. The surface roughness is found to increase upon incorporation of SWNTs. For P1:SWNT composite films from ratio 100:1 to 1:1 (Supporting Information, Figure S9), the density of the fiber like features in the image increases gradually and corresponds proportionately to the SWNT weight ratio. One can also see that the SWNTs are homogenously dispersed and have a uniform distribution in all the films. In the current image of P1 film, the CPE itself shows a current of less than 5 pA, even at an applied voltage of –3 V (Figure 3a). When SWNTs were added to P1 at P1:SWNT = 100:1, a distinct current profile could be collected at the much lower applied voltage of –0.75 V (Figure 3b). With
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a darker color on the scale bar indicating a higher current, the SWNTs appear to be black on the images in contrast to the yellow background filled with the less conductive P1. At low weight ratio of SWNT (100:1), current images demonstrate the SWNTs distribution much clearer compared to topography images. In addition, SWNTs with different electronic properties appear similar in topography images but they can be distinguished in current images. It implies that, in general, c-AFM is not only a nanoscale electrical characterization tool for SWNT composite, but also an excellent visualization tool. As the current saturates at 2 nA in the experiment setup, the rest of the films from 20:1 to 1:1 are scanned under different bias voltage (–0.2 V to –0.25 mV) to adjust the range of current of the film in the profile (Figure 3c–f). Nanoscale conductivity is compared by normalizing the average current from c-AFM with respect to the applied voltage. From neat P1 to P1:SWNT = 1:1, the average current increases by 5 orders. By plotting the normalized current versus weight ratio of SWNT, the average current measured at nanoscale from c-AFM is found to be correlated with the bulk conductivity measured by four-point probe (Figure 4). They follow a similar trend as the weight ratio of SWNT increases. We also note that c-AFM exhibits the advantage of being capable to measure a wide range of conductivity when there are certain limits for the four-point probe measurement. In conclusion, the self-doped anionic narrow band gap CPE P1 and its cationic counterpart P2 can be used to disperse SWNT in MeOH. There is significant interaction between the CPE and the SWNT surface, as determined by the bathochromic shift of the CPE absorption band. The resultant P1:SWNT composite films can be made highly conductive, as evidenced by both bulk sheet resistance and c-AFM measurements. The as-cast P1:SWNT composite films are more conductive than P2:SWNT, most reasonably due to a higher density of unpaired free electrons in the doped P1, and both composites have higher electrical conductivity than the non-conjugated polyelectrolyte PSSNa:SWNT composite. SWNT composite films for both P1 and P2 can be further enhanced by treatment with trifluoroacetic acid vapor. With the acid doping phenomenon being absent in PSSNa, these studies highlight the multifaceted function of doped CPEs: dispersant in solution and a medium that allows charge transport in the solid state. With a tunable conductivity, thickness and transmittance controlled by SWNT concentration and acid doping, the CPE:SWNT composite films hold promise to be used as conductive layers in solution deposited electronic devices.
Experimental Section Materials: Purified arc discharge SWNTs were purchased from Carbon Solutions, Inc. (Riverside, CA, USA) and used without any further purification. Surfactant-eliminated 99% metallic-enriched SWNTs (IsoNanotubes-M) were purchased from Nanointegris Inc. (Menlo Park, CA, USA). The two CPEs used in this work, P1 and P2 were synthesized according to the literature.[16,17] TFA was purchased from EMD Chemicals (USA). PSSNa with an average MW of ∼1,000,000 and other solvents were purchased from Sigma-Aldrich (USA). All chemicals and solvents were used as received.
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COMMUNICATION Figure 3. c-AFM dark current images (5 µm × 5 µm) of P1 collected at –3 V (a), P1:SWNT composite films at weight ratio 100:1, collected at –0.75 V (b), 20:1, collected at –0.2 V (c), 5:1, collected at –0.1 V (d), 2:1, collected at –0.5 mV (e) and 1:1, collected at –0.25 mV (f). Scale bars are in nA/V and are shown in different ranges. Preparation of SWNT Dispersions: 5 mg mL−1 P1 and P2 solutions were prepared from MeOH and the same concentration of a PSSNa solution was prepared from deionized water. Different amounts of SWNT powder were added into the polyelectrolyte solutions to give polymer:SWNT weight ratios of 100:1, 20:1, 5:1, 2:1 and 1:1, respectively. SWNTs were dispersed using probe sonication (SONICS, VCX-130) at 85 W in a water−ice bath for 1 h, and same condition was used to prepare SWNT and P1:SWNT dispersions in NMP. Centrifugation was performed at 16873 g (14000 rpm, rotor model: Ependorf FA-45–18–11) for 1 h. Film Preparation: Devices were prepared in ambient on Corning 1737 glass cleaned by detergent, DI water, acetone and isopropanol in bath sonication and UV/ozone treatment. Films were spun cast from above mentioned SWNT dispersions at 1500 rpm for 60 sec. Acid vapor doping
Adv. Mater. 2014, DOI: 10.1002/adma.201400612
was performed by heating TFA (or HCl) and films on a hotplate covered by petri dish at 50 °C overnight. Characterizations: Optical absorbance spectroscopy was performed on the dispersions and thin films using a Perkin-Elmer Lambda 750 UV-Vis spectrometer. Film resistivity was measured with CDE ResMap 4 Point Resistivity Mapper, and conductivity was calculated based on film thicknesses determined using an Ambios XP-100 stylus profilometer and AFM. The relationship between conductivity (σ), sheet resistance (Rs) and film thickness (t) is given by the equation: σ = 1/(Rs × t). TEM measurements were taken with FEI T20 EDX at 200 kV. The TEM samples were prepared by casting a drop of suspension (
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Electronic Properties of Conjugated Polyelectrolyte/SingleWalled Carbon Nanotube Composites Yao Li, Cheng-Kang Mai, Hung Phan, Xiaofeng Liu, Thuc-Quyen Nguyen, Guillermo C. Bazan,* and Mary B. Chan-Park* Carbon nanotubes (CNTs)[1] are attractive because of their outstanding chemical,[2] electrical[3] and mechanical[4] properties. As a one-dimensional material, their high aspect ratio allows CNTs to form percolating networks in composite materials, which leads to highly conducting films and to increases in viscosity that allow one to manage film thickness.[5] In order to implement CNTs into applications, the bulk material needs to be well dispersed and methods for achieving such suspensions have been intensively studied due to the potential advantage of solution processing.[6] Among different approaches,[7] dispersing CNTs with functional conjugated and nonconjugated polymers attracts special attention as it does not introduce defects to the nanotubes and avoids contamination from surfactants.[8] The polymer/CNT composites thus obtained have been shown to possess excellent mechanical and electrical properties.[9] Conjugated polyelectrolytes (CPEs) have been found to be good CNT dispersants.[10] CPEs are defined as π-conjugated polymers with ionic side chains.[11] Combining both features of organic semiconductors and polyelectrolytes, they offer a broad field for applications ranging from organic light-emitting diodes (OLEDs),[12] solar cells,[13] to biosensors[14] and bioimaging.[15] When used to disperse CNTs, CPEs have the advantage over neutral conjugated polymers of being soluble in polar solvents, like water and methanol (MeOH), which provides greener alternatives for the replacement of toxic solvents, and allows incorporation into organic semiconductor devices via simple solution processing without disrupting underlying hydrophobic active layers. In this contribution, an anionic narrow band gap CPE, namely PCPDTBTSO3TBA (P1, poly[2,6-(4,4bis-tetrabutylammonium butanylsulfonate-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)], Figure 1a),[16] was used to disperse single-walled carbon nanoY. Li,[+] Prof. M. B. Chan-Park School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore E-mail: [email protected] Dr. C.-K. Mai,[+] H. Phan, X. Liu, Prof. T.-Q. Nguyen, Prof. G. C. Bazan Center for Polymers and Organic Solids Department of Chemistry and Biochemistry University of California Santa Barbara, California 93106, USA E-mail: [email protected] [+]Y. Li and Dr. C.-K. Mai contributed equally to this work.
DOI: 10.1002/adma.201400612
Adv. Mater. 2014, DOI: 10.1002/adma.201400612
tubes (SWNTs) in MeOH and a stable, conductive CPE/SWNT composite dispersion was obtained. A cationic CPE containing the same backbone PCPDTBTPyr+BIm4− (P2, poly[2,6-(4,4bis(6-pyridiniumhexyl)-4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene tetrakis(1-imidazolyl)borate)-alt-4,7-(2,1,3-benzothiadiazole)], Figure 1a)[17] was also studied for comparison. An interesting feature of P1 is that it can be easily doped in aqueous media to provide a stable conductive polymer, whereas P2 is not initially doped. We find that P1 gives higher electrical conductivity than P2 for all the CPE/SWNT composite films as obtained immediately after deposition. Additionally, the electrical conductivity increases after treating CPE/SWNT films with acid vapor for both P1 and P2 composites. The UV/Vis-NIR absorption spectrum of P1 in MeOH reveals that the polymer is doped, as indicated by the low energy transition in the NIR region related with polarons (Figure 1b).[18] These polarons are generated during the purification by dialysis, as described recently in the literature, and are stabilized by the pendant sulfonate groups, presumably via Coulombic stabilization.[16] In contrast, the absence of such electrostatic stabilization in P2 does not favor polaron formation on the conjugated backbone. One therefore observes a similar absorption profile typical of a narrow band gap conjugated polymer (Figure 1b). Thus, the blue shift of P1 compared to that of P2 is consistent with previous literature reports for cationic and anionic CPEs with identical conjugated backbones.[16,17] SWNT dispersions were prepared in 5 mg mL−1P1 solutions in MeOH by applying probe sonication for one hour. Different amounts of SWNTs were added to give P1:SWNT weight ratios of 100:1, 20:1, 5:1, 2:1 and 1:1. Attempts to prepare suspension with higher content of SWNT than that of 1:1 P1:SWNT were not successful, and excessive SWNTs stayed at the bottom as a gel/paste like residue. Optical images of 0.05 mg mL−1P1 solution and P1:SWNT dispersions taken 3 days after preparation show that the dispersions are homogenous and reasonably stable (Figure 2a). For example, no change in the absorption of the dispersions is observed after six weeks. As more SWNTs were added, the color of dispersions gradually became darker, a feature that could also be observed in thin films spun cast from the dispersions (Figure 2b). It is worth noticing that the film thickness increases with increasing SWNT concentration, from 10 nm for pure P1 film to 65–70 nm for P1:SWNT = 1:1, under similar casting conditions. The color change can be quantitatively characterized by examination of the absorption spectra of the dispersions (Figure 2c). The baseline of P1:SWNT dispersion shifts up upon progressive addition of SWNTs. Additionally, the broad absorbance band centered at 1030 nm grows in intensity with
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Figure 1. a) Chemical structures of PCPDTBTSO3TBA (P1) and PCPDTBTPyr+BIm4−(P2). b) UV/Vis-NIR absorption spectra of P1 (ⵧ) and P2 (䉲) in MeOH.
Figure 2. a) Optical images of P1 and P1:SWNT dispersions with weight ratio 100:1, 20:1, 5:1, 2:1 and 1:1 (P1 concentration fixed at 0.05 mg mL−1 for all dispersions). b) Optical images of P1 and P1:SWNT composite films spun coat from each dispersion at 1500 rpm, 60 sec. c) UV/Vis-NIR absorption of P1 and P1:SWNT dispersions at different ratios, and the absorption of 0.05 mg mL−1 SWNT is shown in dash line. d) UV/Vis-NIR absorption data for P1:SWNT = 1:1, SWNT, the P1 contribution to the P1:SWNT complex (subtraction) and free P1.
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Table 1. Sheet resistance and conductivity of P1:, P2: and PSSNa:SWNT composite films before and after TFA vapor treatment. Films
Resistance [Ω/sq]
Conductivity [S/cm]
Resistance (TFA) [Ω/sq]
Conductivity (TFA)[S/cm]
P1
>10 Ma)
< 0.1
>10 Ma)
< 0.1
P1:SWNT = 100:1
a)
>10 M
< 0.1
>10 M
< 0.1
P1:SWNT = 20:1
>10 Ma)
< 0.1
>10 Ma)
< 0.1
P1:SWNT = 5:1
2650 ± 200 k
0.13 – 0.2
1850 ± 200 k
0.2 – 0.25
a)
P1:SWNT = 2:1
35 ± 5 k
8 – 10
14 ± 2 k
20 – 25
P1:SWNT = 1:1
5±1k
25 – 40
2.5 ± 0.1 k
55 – 65
P2:SWNT = 5:1
> 10 Ma)
< 0.1
110 ± 10 k
3–4
P2:SWNT = 2:1
130 ± 10 k
2.5 – 3
15 ± 2 k
25 – 30
P2:SWNT = 1:1
10 ± 1 k
15 – 20
2.8 ± 0.3 k
50 – 60
PSSNa:SWNT = 5:1
> 10 Ma)
< 0.1
> 10 Ma)
< 0.1
PSSNa:SWNT = 2:1
1200 ± 150 k
0.7 – 1
1450 ± 100 k
0.6 – 0.9
PSSNa:SWNT = 1:1
12 ± 1 k
20 – 30
20 ± 2 k
15 – 25
a)10
MΩ/sq is the maximum resistance that can be measured by the four-point resistivity mapper.
increased SWNT concentration. This feature corresponds to absorption of the second van Hove electronic transition of semiconducting nanotubes (S22),[19] which can also be observed in 0.05 mg mL−1 SWNT dispersed in N-methyl-2-pyrrolidone (NMP). We further verified this by dispersing 99% pure metallic SWNTs in a P1 solution using the same method, and we no longer observe the absorption band around 1030 nm (Supporting Information, Figure S1). For the P1:SWNT combination with a weight ratio of 1:1, subtraction of the SWNT spectrum obtained at an equivalent concentration resulted in an absorption profile that is contributed principally by P1 (Figure 2d). Comparison of this spectrum obtained to the absorption of standalone P1 reveals a red shift of λmax in the polymer absorption. This shift provides evidence of SWNT complexation with the CPE.[10,20] A similar λmax shift was observed when all the solution absorptions were taken in NMP (Supporting Information, Figure S2). The sheet resistances of P1 and P1:SWNT composite films were measured by four-point probe method (Table 1). For the P1 and P1:SWNT composites of 100:1 and 20:1 weight ratios, sheet resistances are greater than 10 MΩ/sq; beyond the measurable range of our setup. A sheet resistance of 2650 ± 200 kΩ/ sq was obtained when P1:SWNT = 5:1, at which concentration SWNT exist as bundles, as observed by using transmission electron microscopy (TEM, see Supporting Information, Figure S4). Similar results were obtained for same for P2:SWNT = 5:1. The value decreases by two orders of magnitude to 35 ± 5 kΩ/sq when the weight ratio gets to 2:1 and reaches 5 ± 1 kΩ/sq for the P1:SWNT = 1:1 blend, corresponding to an electrical conductivity of 25 – 40 S/cm. This trend agrees with what one observes in other polymer:SWNT systems, in which the film conductivity increases with increased SWNT loading.[21] Conductivity values of composite films prepared using the same concentration of non-doped cationic CPE P2 with SWNTs follow the same trend but are lower compared with P1:SWNT at similar SWNT loadings. Furthermore, composite films
Adv. Mater. 2014, DOI: 10.1002/adma.201400612
containing the non-conjugated polyelectrolyte, poly(sodium 4-styrenesulfonate) (PSSNa), show even lower conductivity than those with P1 and P2 under similar experimental conditions. For example, when polymer:SWNT = 2:1, the electrical conductivity is 8–10 S/cm for P1:SWNT, compared with 2.5–3.0 S/cm and 0.7–1 S/cm for P2:SWNT and PSSNa:SWNT, respectively. These observations provide evidence of the improvements in conductivity by (a) the conjugated backbone, and (b) the initial doping in the CPE. The strong hydrophobic interaction between CPE and SWNT is further confirmed by absorption and resistance studies of samples before and after centrifugations. We centrifuged a P1:SWNT (1:1, P1 concentration is 5 mg mL−1) dispersion in MeOH, and isolated the supernatant. We observed that SWNT bundles settled down and that the color of supernatant became less dark. The solution absorbance after centrifugation exhibits simultaneous decrease of both CPE and SWNT absorptions to approximately 25% of the one before centrifugation (Supporting Information, Figure S3), indicating that the CPE is effectively complexed with SWNT. Films prepared from the supernatant gave a sheet resistance more than one order higher than those before centrifugation. For instance, with P1:SWNT = 1:1, the sheet resistance of the film after centrifugation is around 235 kΩ/sq (thickness = 10 – 15 nm), compared with 5 kΩ/sq (thickness = 65 – 70 nm) without centrifugation. It has been previously established that doping of P1 can occur via protonation. The quantity of unpaired electrons increases after addition of acid, as indicated by electron paramagnetic resonance and film conductivity measurements.[16] We therefore examined possible additional doping of the P1:SWNT in the solid state by exposing the film to trifluoroacetic acid (TFA) vapor at 50 °C overnight. As shown in Table 1 this treatment led to increases in conductivity. For the composite films with ratios: P1:SWNT = 5:1, 2:1 and 1:1, conductivity values approximately double after TFA treatment. We also exposed P2:SWNT
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films to TFA at the same conditions as P1:SWNT films. It is worth mentioning that the cationic CPE P2 cannot be doped by HCl in polar solvents as the pendant cationic groups hinder the formation of stable (cationic) polaronic states on the conjugated backbone.[16] Surprisingly, the P2:SWNT films treated with TFA also show an enhanced electrical conductivity. By measuring the UV/Vis-NIR absorption for P2:SWNT composite films before and after TFA vapor treatment, one can confirm doping of the conjugated polymer by the emergence of low energy transitions from 900 nm to 1300 nm (Supporting Information, Figure S5). Formation of positive polarons is therefore favored when P2 is mixed in the solid state with SWNTs. In contrast to P1 and P2, the PSSNa:SWNT composite films show no evidence of doping after TFA treatment as discerned from absorption spectra (Supporting Information, Figure S6) and no increase of electrical conductivity is observed. The electrical conductivity values for both P1:SWNT and P2:SWNT composite films after acid treatment can reach up to 60 S/cm. Similar increases of electrical conductivities are also observed when HCl vapor is used instead of TFA (Supporting Information, Figure S7 and Table S1). There is a 5 to 10 fold increase in electrical conductivity of P2:SWNT films after treating with TFA vapor, which is significantly higher than the increase observed with P1:SWNT. However, both composites reach similar conductivities after TFA treatments (see Table 1). We compared the UV/Vis-NIR absorption of neat P1 and P2 films before and after treating with TFA vapor (Supporting Information, Figure S8). In accordance with the change in conductivity, the absorption spectrum of P2:SWNT before/after TFA shows a much higher increase of low energy transitions relative to P1:SWNT. We also note that gel permeation chromatography (GPC) showed the average number of repeat units for both polymers to be approximately 8 (P1: Mn = 7.7 kg mol−1, Mw = 8.2 kg mol−1;[16] P2: Mn = 5.1 kg mol−1, Mw = 9.5 kg mol−1).[17] Considering that P1 is doped before TFA vapor treatment, we ascribe the greater impact by TFA on the conductivity of P2:SWNT blends on the fact that very few, if any, charge carriers are present in the as cast films. The bulk conductivity of films can be calculated based on the sheet resistance and film thickness, as shown in Table 1. To take a closer look at the films at the nanoscale, we characterized the P1 film and P1:SWNT composite films using conductive atomic force microscopy (c-AFM), which is a microscopy technique that maps out current and topography simultaneously.[22] The topographic image of neat P1 film shows that the surface is smooth and continuous with a root mean square roughness of 1.6 nm. The surface roughness is found to increase upon incorporation of SWNTs. For P1:SWNT composite films from ratio 100:1 to 1:1 (Supporting Information, Figure S9), the density of the fiber like features in the image increases gradually and corresponds proportionately to the SWNT weight ratio. One can also see that the SWNTs are homogenously dispersed and have a uniform distribution in all the films. In the current image of P1 film, the CPE itself shows a current of less than 5 pA, even at an applied voltage of –3 V (Figure 3a). When SWNTs were added to P1 at P1:SWNT = 100:1, a distinct current profile could be collected at the much lower applied voltage of –0.75 V (Figure 3b). With
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a darker color on the scale bar indicating a higher current, the SWNTs appear to be black on the images in contrast to the yellow background filled with the less conductive P1. At low weight ratio of SWNT (100:1), current images demonstrate the SWNTs distribution much clearer compared to topography images. In addition, SWNTs with different electronic properties appear similar in topography images but they can be distinguished in current images. It implies that, in general, c-AFM is not only a nanoscale electrical characterization tool for SWNT composite, but also an excellent visualization tool. As the current saturates at 2 nA in the experiment setup, the rest of the films from 20:1 to 1:1 are scanned under different bias voltage (–0.2 V to –0.25 mV) to adjust the range of current of the film in the profile (Figure 3c–f). Nanoscale conductivity is compared by normalizing the average current from c-AFM with respect to the applied voltage. From neat P1 to P1:SWNT = 1:1, the average current increases by 5 orders. By plotting the normalized current versus weight ratio of SWNT, the average current measured at nanoscale from c-AFM is found to be correlated with the bulk conductivity measured by four-point probe (Figure 4). They follow a similar trend as the weight ratio of SWNT increases. We also note that c-AFM exhibits the advantage of being capable to measure a wide range of conductivity when there are certain limits for the four-point probe measurement. In conclusion, the self-doped anionic narrow band gap CPE P1 and its cationic counterpart P2 can be used to disperse SWNT in MeOH. There is significant interaction between the CPE and the SWNT surface, as determined by the bathochromic shift of the CPE absorption band. The resultant P1:SWNT composite films can be made highly conductive, as evidenced by both bulk sheet resistance and c-AFM measurements. The as-cast P1:SWNT composite films are more conductive than P2:SWNT, most reasonably due to a higher density of unpaired free electrons in the doped P1, and both composites have higher electrical conductivity than the non-conjugated polyelectrolyte PSSNa:SWNT composite. SWNT composite films for both P1 and P2 can be further enhanced by treatment with trifluoroacetic acid vapor. With the acid doping phenomenon being absent in PSSNa, these studies highlight the multifaceted function of doped CPEs: dispersant in solution and a medium that allows charge transport in the solid state. With a tunable conductivity, thickness and transmittance controlled by SWNT concentration and acid doping, the CPE:SWNT composite films hold promise to be used as conductive layers in solution deposited electronic devices.
Experimental Section Materials: Purified arc discharge SWNTs were purchased from Carbon Solutions, Inc. (Riverside, CA, USA) and used without any further purification. Surfactant-eliminated 99% metallic-enriched SWNTs (IsoNanotubes-M) were purchased from Nanointegris Inc. (Menlo Park, CA, USA). The two CPEs used in this work, P1 and P2 were synthesized according to the literature.[16,17] TFA was purchased from EMD Chemicals (USA). PSSNa with an average MW of ∼1,000,000 and other solvents were purchased from Sigma-Aldrich (USA). All chemicals and solvents were used as received.
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COMMUNICATION Figure 3. c-AFM dark current images (5 µm × 5 µm) of P1 collected at –3 V (a), P1:SWNT composite films at weight ratio 100:1, collected at –0.75 V (b), 20:1, collected at –0.2 V (c), 5:1, collected at –0.1 V (d), 2:1, collected at –0.5 mV (e) and 1:1, collected at –0.25 mV (f). Scale bars are in nA/V and are shown in different ranges. Preparation of SWNT Dispersions: 5 mg mL−1 P1 and P2 solutions were prepared from MeOH and the same concentration of a PSSNa solution was prepared from deionized water. Different amounts of SWNT powder were added into the polyelectrolyte solutions to give polymer:SWNT weight ratios of 100:1, 20:1, 5:1, 2:1 and 1:1, respectively. SWNTs were dispersed using probe sonication (SONICS, VCX-130) at 85 W in a water−ice bath for 1 h, and same condition was used to prepare SWNT and P1:SWNT dispersions in NMP. Centrifugation was performed at 16873 g (14000 rpm, rotor model: Ependorf FA-45–18–11) for 1 h. Film Preparation: Devices were prepared in ambient on Corning 1737 glass cleaned by detergent, DI water, acetone and isopropanol in bath sonication and UV/ozone treatment. Films were spun cast from above mentioned SWNT dispersions at 1500 rpm for 60 sec. Acid vapor doping
Adv. Mater. 2014, DOI: 10.1002/adma.201400612
was performed by heating TFA (or HCl) and films on a hotplate covered by petri dish at 50 °C overnight. Characterizations: Optical absorbance spectroscopy was performed on the dispersions and thin films using a Perkin-Elmer Lambda 750 UV-Vis spectrometer. Film resistivity was measured with CDE ResMap 4 Point Resistivity Mapper, and conductivity was calculated based on film thicknesses determined using an Ambios XP-100 stylus profilometer and AFM. The relationship between conductivity (σ), sheet resistance (Rs) and film thickness (t) is given by the equation: σ = 1/(Rs × t). TEM measurements were taken with FEI T20 EDX at 200 kV. The TEM samples were prepared by casting a drop of suspension (
