Noncovalent Modification of Single-Walled Carbon Nanotubes Using

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Functional Nanostructured Materials (including low-D carbon)

Noncovalent Modification of Single-Walled Carbon Nanotubes Using Thermally-Cleavable Polythiophenes for Solution-Processed Thermoelectric Films Pan He, Satoshi Shimano, Krishnachary Salikolimi, Takashi Isoshima, Yohei Kakefuda, Takao Mori, Yasujiro Taguchi, Yoshihiro Ito, and Masuki Kawamoto ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14820 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Materials & Interfaces

Noncovalent Modification of Single-Walled Carbon Nanotubes Using Thermally-Cleavable Polythiophenes for Solution-Processed Thermoelectric Films Pan He,†, Satoshi Shimano,# Krishnachary Salikolimi,† Takashi Isoshima,¶ Yohei Kakefuda,§ Takao Mori,§ Yasujiro Taguchi,# Yoshihiro Ito,*,†,¶ and Masuki Kawamoto*,†,¶,ǁ †Emergent

Bioengineering Materials Research Team, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan School

of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China #Strong Correlation Materials Research Group, RIKEN CEMS, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ¶Nano Medical Engineering Laboratory, RIKEN Center for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan §National Institute for Materials Science (NIMS), WPI-MANA and Center for Functional Sensor & Actuator, 1-1-1 Namiki, Tsukuba 305-0044, Japan ǁGraduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan KEYWORDS: Noncovalent Modification, Polythiophene, Thermal Cleavage, Single-Walled Carbon Nanotubes, Thermoelectric Material ABSTRACT: Four thermally-cleavable polythiophene derivatives containing carbonate and solubilizing groups were synthesized for noncovalent modification of single-walled carbon nanotubes (SWCNTs). A well-dispersed polythiophene/SWCNTs composite was obtained by adsorption of the polymer at the SWCNTs surface. The solution-processed composite film exhibited solid-state thermal cleavage of the insulating solubilizing group through decarboxylation, producing an insoluble composite film. The thermallycleavable composite film was evaluated for potential application as a thermoelectric (TE) material. The electrical conductivity (σ) of the thermally-treated composite film was up to 250 times higher than that of the as-prepared composite film. The increased σ contributed to an increase in the power factor (PF). The ethanol-processed composite film could be applicable for green processing of a TE material using the less-toxic solvent. The substrate-free polythiophene/SWCNTs composite film prepared by simple solvent evaporation yielded a figure-of-merit of 3.1 × 10−2 with a PF of 28.8 μW m−1 K−2 at 25 °C. This solution-processed methodology is beneficial for the development of a flexible TE material.

INTRODUCTION Materials that exhibit thermoelectric (TE) conversion have been of great interest for power generating systems.1-2 Because TE materials directly convert waste heat into electricity, this energy harvesting could be instrumental in realizing a sustainable society. The energy conversion efficiency is defined as the TE figure-of-merit zT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. Inorganic semiconductors such as bismuth telluride and lead telluride have been investigated extensively because of their large zT values (~2).24 In contrast to inorganic TE materials, the one-dimensional carbon allotropes of carbon nanotubes (CNTs) are advantageous for TE materials because of their large σ values, mechanical strength, and light weight.5 Unfortunately, raw CNTs have high κ values (3000 W m−1 K−1 for an individual CNT),6 leading to low zT values of 10−3–10−4.7-8

Recently, conjugated polymer/CNTs composites have generated interest as TE materials because of their costeffective production and mechanical flexibility. For example, polyaniline/single-walled carbon nanotubes (SWCNTs) composites exhibit good TE performance with a high power factor (PF; S2σ) of 2710 μW m−1 K−2.9-10 Flexible TE films have been prepared using poly(3,4-ethylenedioxythiophene)poly(styrene sulfonate) (PEDOT:PSS)/SWCNTs,11 poly(3hexylthiophene)(P3HT)/multi-walled carbon nanotubes (MWCNTs),12 and polypyrrole/SWCNTs13 composites. Though CNTs are good candidates for TE materials, they exhibit poor processability owing to inevitable aggregation. Dispersed CNTs in organic solvents and in water have been investigated using noncovalent surface modifications.14-16 When dispersants such as small molecules, surfactants, polymers, and biomaterials are attached to the CNT surface through hydrophobic or delocalized π stacking interactions, the dispersant/CNTs composite exhibits homogeneous dispersion. We have reported the development of the novel functional

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dispersants of peptide aptamers17 and fullerene-based nanoparticles18 for aqueous dispersion of SWCNTs. The SWCNTs dispersants of conjugated polymers have attracted particular interest because of their specific π–π interactions between the extended hexagonal lattice of sp2-bonded carbon atoms and the conjugated polymer backbone.19 The conjugated polymer requires the solubilizing group in the side chain to induce solubility in solvents and to avoid intermolecular aggregation between the π-conjugated main chains. However, the non-aromatic solubilizing group has some drawbacks. The insulating nature of the solubilizing group gives rise to a decreased density of the chromophore and an interruption of electrical conduction, inducing a decrease in σ. Thermal elimination of the solubilizing group is a fascinating approach, leading to a higher density of the conjugated polymer.20-27 After thermal cleavage of the terminal group using this post-processing step, an increase in power conversion efficiency for photovoltaic cells21-24, 27 and a good semiconducting nature for thin-film transistors20 can be achieved. In this study, we report the first example of SWCNTs dispersants using thermally-cleavable polythiophene derivatives for solution-processed TE films (Figure 1). The conjugated polymer contains a carbonate group as a thermallycleavable unit and a solubilizing group as a flexible unit in a terminal group. For solution-processed methodologies, the polar carbonate group contributes to good solubility in solvents, and various types of the solubilizing group participate in the solubility and miscibility against solvents. If the polymer exhibits a solid-state reaction resulting in thermal cleavage of the terminal group, the resulting polymer is insoluble in any solvent. Dispersed SWCNTs solutions via noncovalent modification by adsorption of the conjugated polymer at the SWCNTs surface would be useful for the solution-processed polymer/SWCNTs composite film. The thermal cleavage of the terminal group in the composite film is expected to increase the σ value, leading to superior TE performance.

Figure 1. Chemical structures of the polythiophene derivatives.

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(Mw/Mn)), the polymers are expected to possess more than 25 thiophene units in the main chain. The thermal behavior was investigated by differential scanning calorimetry and thermogravimetric analysis (TGA). Low glass transition temperatures (Tg) below 5 °C were observed, indicating that flexible solubilizing chains in terminals led to amorphous solids at low temperatures. We also found that the temperature at which the polythiophenes lost 10% of their weight (T10) was around 300 °C under nitrogen. In contrast, PDEA exhibited a lower T10 value of 192 °C. The polymers were readily soluble in conventional organic solvents such as tetrahydrofuran (THF), chloroform, and N,N-dimethylformamide; however, these are insoluble in water. We note that PDEA was dissolved in ethanol owing to the hydrophilic diethylamine unit in the terminal group. Thermal Cleavage Behavior of the Polythiophene Derivatives Thermal cleavage of the carbonate group was investigated using TGA-mass spectrometry (MS) under nitrogen (Figure 2). The TGA profiles exhibited ~50% weight loss at 350 °C, as well as the elimination of the carbonate and solubilizing groups. In contrast, elimination behavior of PDEA occurred at lower temperatures. Evolution of carbon dioxide (m/z: 44) was commonly observed owing to decarboxylation. Other fragmentation depended on the chemical structures of the terminal groups; PTEG exhibited α-cleavage of triethylene glycol, yielding ether-based fragmented cations (Figure S1a). The alkyl carbonate esters in PHEX and POCT induced a McLafferty rearrangement, producing related cationic olefins (Figure S1b and S1c).28 The PDEA exhibited both the McLafferty rearrangement and the α-cleavage of the diethyl amino group (Figure S1d). As mentioned above, the T10 value (192 °C) of PDEA was much lower than that of other polymers (Table 1). This lower T10 value may be owing to effective elimination of the amino group around at 250 °C, as identified from the fragmentation of m/z: 86 (Figure 2d). The thermal behavior of the carbonate ester group in the side chain is also considered. First, we selected P3HT, which includes simple alkyl chains and is one of the standard polythiophene derivatives for bulk heterojunction solar cells.29 Further, because PHEX contains carbonate ester with the same chain length of the hexyl group, we compared PHEX with P3HT using the TGA profiles. We found that weight loss initiates at 293 °C for PHEX, while weight decrease initiates at 491 °C for P3HT.30 These results indicate that the carbonate ester is crucial for the thermally-cleavable group through decarboxylation.

RESULTS AND DISCUSSION Synthesis and Polymer Characterization Full synthesis details are provided in the Supporting Information. A chloroformate derivative containing a different solubilizing group was prepared by reacting the related alcohol with triphosgene. Esterification of 3-thiopheneethanol with the chloroformate derivative in trimethylamine afforded a linkage of carbonate ester in a monomer. The amine-terminated monomer DEA was synthesized by conventional substitution with alkyl bromide and diethylamine. Oxidation coupling of the monomer using iron(III) chloride produced the polythiophene derivatives. The polymer was purified using size-exclusion chromatography to remove oligomers and impurities. The characterization of the polythiophene derivatives is listed in Table 1. From the number-average molecular weight (Mn) and polydispersity index (weight-average molecular weight


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ACS Applied Materials & Interfaces Figure 2. TGA-MS profiles of polythiophene derivatives under nitrogen for (a) PTEG, (b) PHEX, (c) POCT, and (d) PDEA. Heating rate: 10 °C min−1. Asterisk in (d) indicates unknown peaks.

Figure 3 shows representative results of the thermal cleavage behavior of PTEG. Specifically, 13C cross-polarization (CP)/magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra revealed that a peak (peak B in Figure 3a) corresponding to carbon at the 4-position of the thiophene unit broadened after heating to 275 °C for 30 min. However, the carbonate ester (peak A) and the ethylene glycol (peaks C and D) groups still remained after the thermal treatment (Figure 3a(ii)). In contrast, the characteristic peaks except peak B disappeared after heating to 320 °C for 30 min, resulting from thermal cleavage of the terminal group (Figure 3a(iii)). The 13C CP/MAS NMR spectra of POCT also exhibited thermal cleavage of octyl carbonate ester in the solid (Figure S2). Change in the chemical structures of PTEG by thermal cleavage was confirmed by Fourier transform-infrared (FT-IR) spectroscopy (Figure 3b). Before thermal treatment, specific bands corresponding to stretching modes of the carbonyl and the ester groups were observed at 1745 and 1260 cm−1, respectively. Furthermore, C–H and C–O stretching modes of the alkyl and ethylene oxide groups in the solubilizing group were detected at 2876 and 1107 cm−1, respectively. After heating, PTEG exhibited a sharp decrease in the intensities of these stretching modes, and a new broad band was observed around 3650 cm−1 that was consistent with a stretching mode of a hydroxyl group (Figure 3b(ii)). Other polymers also exhibited the related band of the hydroxyl group appearing after thermal cleavage (Figure S3). We also note that the out-of-plane C–H bending mode of the thiophene unit at 789 cm−1 shows broadening after thermal treatment.31 Figure 3c shows the change in absorption spectra of PTEG in a film at various curing temperatures. The maximum absorption wavelength λmax before heating was 433 nm, which was attributed to a π–π* transition of the polythiophene units (Table 1). Furthermore, substantial shoulders present at 550−600 nm signified the vibronic propagation of the π–π* transition of the main chain.32 After the thermal treatment, λmax values shifted to shorter wavelength regions and the vibronic bands gradually reduced. These results suggest that a change in the molecular conformation of the thiophene units is induced by thermal cleavage of the side chain. Orientation of the polymer side chains can be affected by that of the polymer main chain.33 After the thermal treatment, the resulting polymer may have less effect on the orientation of the main chain owing to the shorter side chain. Thus, the blue-shifted absorption spectra are the result of less conjugation of the random conformation of the polymer backbone. We also found that the polythiophenes after thermal cleavage exhibit similar λmax values (Table 1 and Figure S5). This indicates that all of the polymers present the same molecular structure after thermal cleavage.

Figure 3. (a) 13C CP/MAS NMR spectra of PTEG (i) before heating and (ii) after heating at 275 °C for 30 min, and (iii) after heating at 320 °C for 30 min. (b) FT-IR spectra of PTEG on a KBr plate (i) before heating and (ii) after heating at 325 °C for 5 min. (c) Absorption spectra of PTEG in a film at various curing temperatures. Thermal treatments for all samples were carried out under nitrogen. All spectra were measured at 25 °C.

We conclude that the polythiophene derivatives display thermal cleavage of the terminal groups, yielding a hydroxyethyl group in the polymer side chain (Scheme 1). This cleavage behavior is beneficial for chemical modification of a polymer structure in the solid state. Because the functional groups can be eliminated simultaneously, unexpected side reactions do not occur during the thermal treatment. We also expect that the resulting hydroxyethyl group would be useful for further functionalization. Because the terminal hydroxy group forms ether and ester linkages, conjugation of the functional group leads to changes in the morphologies and polarities of the polythiophene derivatives. Scheme 1. Possible Mechanism for Thermal Cleavage of Functional Groups in Polythiophenes

We investigated the surface morphology of a polymer film before and after thermal cleavage using atomic force microscopy (AFM) (Figure 4). Before heating, the spin-coated PTEG film on the fused-silica substrate exhibited a smooth surface with an average roughness (Ra) of 2.7 nm (Figure 4(i)). After heating at 275 °C for 1 min, the film thickness decreased from 50 to 30 nm owing to elimination of the terminal groups (Figure 4(ii)). Interestingly, the Ra value of the resulting film (3.1 nm) did not change significantly, suggesting that thermal cleavage occurred uniformly in the solid state. From the cross-sectional profiles we observe concave structures in the PTEG film before thermal cleavage, while a convex profile is observed after thermal cleavage. The mechanism behind these observed changes in the surface structures is unclear at the present stage. One possibility is that thermal cleavage may alter the surface morphology of PTEG through the elimination of the carbonate ester and tri(ethylene oxide) groups.



The polythiophene film after thermal cleavage was insoluble in any organic solvents. We expect that the morphological properties of the polymer surface changed significantly. To evaluate this hypothesis, water contact angle measurements of the PTEG film were performed. Before thermal cleavage, a contact angle of 22.7 ± 6.7° was obtained because of the hydrophilic side chain of the tri(ethylene oxide) group. In contrast, a contact angle of 89.3 ± 4.1° was observed after thermal cleavage. The large change in the contact angle is owing to only one hydrophilic segment of the hydroxyl group in the repeated thiophene unit. We found that thermal cleavage of the functional groups gives rise to not only a change in the chemical structure but also a change in the morphology of the polymer in the solid state.

Figure 4. AFM images (top) and corresponding height profiles (bottom) of PTEG as a spin-coated film on a fused-silica substrate (i) before and (ii) after heating at 275 °C for 1 min. The horizontal lines in the images indicate from where the height profiles were taken. Thermal treatments were carried out under nitrogen.

Preparation of Solution-Processed SWCNTs Films Using NonCovalent-Bonding Methodologies Next, we investigated the dispersion behavior of SWCNTs using thermally-cleavable polythiophenes. A mixture of raw SWCNTs and a polymer solution were sonicated for 15 min in a water bath at 25 °C, yielding dispersed SWCNTs. We found that PTEG, PHEX, and POCT act as SWCNTs dispersants in THF, and PDEA acts as a polymer dispersant of SWCNTs in ethanol (Figure 5a). The dispersed SWCNTs remained unchanged after one week of storage at 25 °C. Judging from transmission electron microscope (TEM) images collected from the sample, small bundles of SWCNTs 20–30 nm in diameter and 1–2 μm in length exist in the dispersed solution (Figure 5b and S6). Because the SWCNTs diameters are 0.7–1.4 nm, these small bundles of SWCNTs indicate entangled structures. Furthermore, we found PDEA attached to the SWCNTs surface owing to π–π interactions between the conjugated polymer and the SWCNTs.19 A solution-processed SWCNTs film was prepared by dropcasting the dispersed solution on a glass substrate, whereupon the SWCNTs film was formed after drying at 70 °C in a vacuum oven. Thermal cleavage behavior of the polymer in the SWCNTs film was investigated using scanning electron microscopy (SEM) (Figure 5c). Network structures of the PHEX/SWCNTs composite (25:75 w/w) were observed in the as-prepared film, wherein composite bundles 20–50 nm in diameter were found to be present owing to polymer-covered aggregated SWCNTs (Figure 5c(i)). After thermal treatment at 350 °C for 5 min, the diameters of the composite bundles decreased by 30–50 % in the network structures (Figure 5c(ii)). Further, thermal cleavage of the PHEX/SWCNTs composite was confirmed by FT-IR spectra (Figure S7). Stretching modes

of the hydrocarbon, carbonyl, and ester groups in the terminal hexyl carbonate ester were observed at 2924, 1741, and 1251 cm−1, respectively. These peaks were nearly diminished after heating, suggesting that the diameter decrease of the composite bundles resulted from thermal cleavage of the terminal group. We also found that the PHEX/SWCNTs film after the thermal treatment was insoluble in any solvent. This change in solubility is consistent with that expected in the polymer films after thermal cleavage.

Figure 5. (a) Dispersion behavior of SWCNTs using PDEA in ethanol for (i) raw SWCNTs, (ii) PDEA solution (0.8 mg mL−1), and (iii) PDEA/SWCNTs (50:50 w/w) composite. (b) TEM image of PDEA/SWCNTs (50:50 w/w) composite obtained after dropcasting the as-dispersed solution on a carbon-coated copper TEM grid. (c) SEM images of a PHEX/SWCNTs (25:75 w/w) composite film on a glass substrate (i) before heating and (ii) after heating at 350 °C for 5 min under nitrogen.

TE Properties of Polythiophene/SWCNTs Composite Films Non-covalent-bonding modification of SWCNTs using thermally-cleavable polythiophenes yielded solution-processed composite films. Furthermore, insoluble SWCNTs films were obtained by thermal cleavage of the solubilizing group in the polymer. We posit that modification of the SWCNTs film should lead to a change in its electrical properties. Because the non-aromatic chain of the cleavable group acts as an electrical insulator, an increase in electrical conductivity should be induced after thermal cleavage. Moreover, the polymer/SWCNTs composite should tend to exhibit flexibility because of the SWCNTs network mechanical properties. To explore these hypotheses, the TE properties of the polymer/SWCNTs films were investigated. Figure 6 shows the TE behavior of the PHEX/SWCNTs composite films. The ratio of the film thicknesses before and after thermal cleavage (dAfter/dBefore) decreased with increasing concentration of PHEX because the amount of the cleavage group depended on the polymer concentration (Figure 6a). Changes in σ values of the PHEX/SWCNTs films after thermal cleavage were also a function of the polymer concentration. The σ value of the PHEX/SWCNTs (80:20 w/w) film after thermal cleavage (35 S cm−1) was about 250 times higher than that before thermal cleavage (0.14 S cm−1) (Figure 6b). This large change in σ is owing to the elimination of the insulating group of hexyl carbonate ester. The S value (ΔV/ΔΤ, where ΔV is the change in voltage) of the PHEX/SWCNTs film before thermal cleavage decreased with increasing σ value; where the S values of the composite films of 80:20 w/w (σ: 0.14 S cm−1) and 25:75 w/w (σ: 32 S cm−1) were 76.2 and 44.8 μV K−1, respectively (Figure 6c). We also found that the S and σ values of the PHEX/SWCNTs films


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ACS Applied Materials & Interfaces after thermal cleavage did not change significantly, nor depend on the SWCNTs concentrations. One possibility is that percolation of the SWCNTs networks forms even in low SWCNTs concentration. The AFM images reveal that SWCNTs networks form in the PHEX/SWCNTs (80:20 w/w) composite, even though the diameters of the composite bundles in PHEX/SWCNTs (80:20 w/w) are 50–80% smaller than those in PHEX/SWCNTs (25:75 w/w) (Figure S8). This is because the bundle diameter tends to decrease with decreasing SWCNTs concentration. The PF (S2σ) of the composite film increased after thermal cleavage because of increasing σ (Figure 6d). The PF value of the PHEX/SWCNTs (25:75 w/w) film after thermal cleavage reached 15.8 μW m−1 K−2, and thus the thermal treatment as a post-process is beneficial for TE performance enhancement.

Figure 6. TE behavior as given by the change in (a) film thickness, (b) electrical conductivity (c), Seebeck coefficient (d) and power factor of the PHEX/SWCNTs film after thermal cleavage for various weight ratios. Thickness of as-prepared PHEX/SWCNTs films: 0.5, 0.6, 1.2, and 1.9 μm for 80:20, 67:33, 50:50, and 25:75 w/w, respectively. Data for before heating (blue) and after heating (red) at 350 °C for 5 min under nitrogen. All measurements were performed at 25 °C.

Ethanol Processing of SWCNTs-Based TE Films As described prior, the PDEA was soluble in ethanol (Figure 5a) and exhibited a lower T10 value (192 °C) (Figure 2d). We consider that the ethanol-processed PDEA/SWCNTs (50:50 w/w) film can be categorized as a greener approach owing to the use of the less-toxic solvent.16, 34 Figure 7a shows the change in dAfter/dBefore and σ at various curing temperatures. The dAfter/dBefore = 1 at 25 °C because no thermal cleavage took place. In contrast, a decrease in dAfter/dBefore of 0.89 was observed at 200 °C, indicating that thermal cleavage of the terminal diethylaminohexyl carbonate ester proceeded near the T10 value. A curing temperature of 300 °C gave rise to enhancement of thermal cleavage yielding a dAfter/dBefore of 0.69. The σ values of the PDEA/SWCNTs films were affected by the curing temperatures, where the σ values after heating at 25, 200, and 300 °C were 13, 22, and 73 S cm−1, respectively. The PF values increased with increasing curing temperature, though the value of S (~ 40 μV K−1) was almost constant (Figure 7b). These results suggest that the thermal cleavage behavior of

the terminal group at high temperatures induces an increase in the σ value, resulting in the higher PF value. We also found that processability of the SWCNTs films can be controlled by the solubility and cleavage temperature of the terminal group in the polymer side chain.

Figure 7. Changes in the (a) film thickness and electrical conductivity and (b) Seebeck coefficient and power factor for PDEA/SWCNTs (50:50 w/w) films after thermal treatment at various curing temperatures. As-prepared film thickness: 2.7 μm. All measurements were performed at 25 °C.

Solution-Processed Substrate-Free TE Films Finally, we explored the TE properties of the PTEG/SWCNTs composite. The effects of the thermal cleavage behavior in the PTEG/SWCNTs (50:50 w/w) film on a glass substrate are summarized in Figure S9. Characteristics of the PTEG/SWCNTs film were similar to those of the PDEA/SWCNTs film; (i) a decrease in dAfter/dBefore took place at high curing temperatures, (ii) the σ values increased with increasing curing temperature owing to elimination of the insulating terminal group, and (iii) the PF values increased when the curing temperatures were high, resulting from higher σ values. During fabrication of the composite film, we found that PTEG enables a substrate-free SWCNTs film (Figure 8). A dispersed solution of PTEG/SWCNTs (50:50 w/w) in THF was placed in a polytetrafluoroethylene (PTFE) beaker and the THF was evaporated slowly at 25 °C until a solid SWCNTs film was obtained in the beaker bottom (Figure 8(i)). The PTEG/SWCNTs film was cut from the as-prepared shape into a square shape (size: 12 mm × 12 mm, film thickness: 90 μm), whereupon a flexible film was formed after thermal cleavage at 350 °C for 30 min under nitrogen (Figure 8(ii)). The formation of a free-standing SWCNTs film may be impacted by the miscibility of the terminal group and the THF. The Hansen solubility parameter (δ) of the solubilizing group (triethylene glycol dimethyl ether (triglyme)) in PTEG is 9.6.35 In contrast, the δ values of the solubilizing groups of hexane and octane in PHEX and POCT are 7.3 and 7.6, respectively.36 Because the δ value of THF (9.5) is similar to that of triglyme (9.6), the PTEG-coated SWCNTs and THF tend to be miscible in the dispersed solution. This miscibility is likely to contribute to film formation by suppressing the segregation of the polymer at the SWCNTs surface after evaporation of the THF.37



Figure 8. Photographs of the substrate-free (i) as-prepared and (ii) post-thermally-cleaved PTEG/SWCNTs (50:50 w/w) films.

Table 2 shows the TE properties of the substrate-free PTEG/SWCNTs (50:50 w/w) film at 25 °C. Before thermal cleavage, the dimensionless figure-of-merit (zT) reached 0.59 × 10−2 with a σ value of 54 S cm−1. In contrast, the zT value after thermal cleavage (3.1 × 10−2) was five times greater than that before thermal cleavage. Because zT is defined as S2σT/κ, thermal cleavage lead to both increase in the σ value (188 S cm−1) and decrease in the κ value (0.27 W m−1 K−1), yielding enhanced the zT (Table 2).38 Several groups have reported the TE behavior of polythiophene/CNTs composite,11-12, 39-48 but unfortunately the zT values of most of these composites were not estimated. In these works, the P3HT/SWCNTs (19:81 w/w) composite exhibited a zT of > 10−2 with a high PF value of 95 μW m−1K−2.43 Further, a zT value of 0.871 × 10−4 at 120 °C was obtained from mechanical mixing of the polythiophene/MWCNTs (20:80 w/w) composite with a PF value of 2 μW m−1 K−2.42 The zT of the thermally cleaved PTEG/SWCNTs composite is exceptionally high, underlining the effect of our thermal treatment process. The κ value of the PTEG/SWCNTs composite takes similarly low values as those of the composites previously reported (0.15–0.8 W m−1 K−1). Because the polymers intrinsically possess low κ values, SWCNTs covered with the polymer will impede thermal transport by phonon scattering, but will affect electrical transport to a much lesser extent.49 The thermal treatment utilized in this work possesses the large benefit of significantly enhancing the σ value. Our findings indicate that the thermally-cleavable polythiophene is promising for solution-processed SWCNTs films, where an insoluble SWCNTs film can be obtained only by a thermal treatment. After elimination of the solubilizing group, increased σ in the resulting film contributes to the enhancement of the TE performance. Because SWCNTs-based TE films have been previously fabricated using printing and coating methods,50-51 our polymer/SWCNTs composites are also applicable as the solution-processable TE materials for printed electronics.

EXPERIMENTAL SECTION General Procedures for Preparing Dispersed SWCNTs. Raw SWCNTs (10 mg) were added to a THF or ethanol solution of the polymer (1.0 g L−1, 40 mL). The mixture was sonicated for 15 min using a tip-type ultrasonic homogenizer (Branson Sonifer 250, Branson Ultrasonics, Danbury, CT, USA; power output: 40 W) in an ice bath. The resulting polymer/SWCNTs composite (80:20 w/w) was collected as a supernatant without filtration. Binary composites with various SWCNTs concentrations were also prepared at concentration ratios up to polymer/SWCNTs = 25:75 w/w. Preparation of Films. Before preparation of films, all the substrates were treated using a UV-ozone cleaner (ASM1101N, Asumi Giken Ltd., Tokyo, Japan) for 10 min. A polymer film was prepared using a spin coater (A-100, Mikasa Co., Ltd., Tokyo, Japan), by spin-coating the THF or ethanol solution of the polymer (5.0 g L−1) onto a fused-silica substrate (size: 12 mm × 25 mm, thickness: 1 mm) at a rate of 1000 rpm for 30 s. The resulting film was dried using a temperature controller (FP90 Central Processor equipped with a FP-82HT Hot Stage, Mettler TOLEDO, Columbus, OH, USA) under nitrogen. A polymer/SWCNTs composite film was prepared by casting the dispersed SWCNTs solution on a glass substrate (size: 5 mm × 10 mm, thickness: 1 mm). The composite film was dried in a vacuum oven (VM-101, Ikeda Scientific Co. Ltd., Tokyo, Japan) at 70 °C overnight. The substrate-free PTEG/SWCNTs composite film was fabricated by solvent evaporation. Raw SWCNTs (40 mg) were added to the THF solution of PTEG (1.0 g L−1, 40 mL). The mixture was sonicated for 15 min using a tip-type ultrasonic homogenizer in an ice bath. The dispersed SWCNTs solution was poured into a PTFE beaker (50 mL, AS ONE Co., Osaka, Japan), whereupon the THF was evaporated slowly at 25 °C in air. The PTEG/SWCNTs (50:50 w/w) composite film was obtained after drying in a vacuum oven at 70 °C overnight. The thermal cleavage behavior of the polymer and the polymer/SWCNTs composite films was examined using the temperature controller at various temperatures under nitrogen.

ASSOCIATED CONTENT CONCLUSION Thermally-cleavable polythiophene derivatives including carbonate and solubilizing groups were synthesized. The polythiophene film exhibited thermal cleavage of the solubilizing group through decarboxylation, yielding an insoluble polymer film. We found that these polymers allow both chemical modification of the molecular structure and surface modification of the polymer film by the solid-state reaction. Polythiophenes were used as polymer dispersants of SWCNTs using noncovalent bonding methodology, producing the solution-processed polymer/SWCNTs composite films. The PF for the PHEX/SWCNTs (25:75 w/w) film after thermal cleavage was enhanced from 6.4 to 15.8 μW m−1 K−2 owing to the increase in electrical conductivity. Ethanol-dispersed SWCNTs yielded TE responses in the PDEA/SWCNTs film as a potential application of green processing of carbon nanomaterials. The substrate-free PTEG/SWCNTs (50:50 w/w) film obtained by solvent evaporation exhibited a zT of 3.1 × 10−2 and a PF of 28.8 μW m−1 K−2 at 25 °C. This free-standing film will be beneficial for flexible TE materials.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Further details of experimental procedures and characterization including synthetic procedures, solid-state 13C NMR, FT-IR, and absorption spectra, and AFM and TEM images (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.I.) *E-mail: [email protected] (M.K.)

ORCID

Yoshihiro Ito: 0000-0002-1154-253X Masuki Kawamoto: 0000-0003-3101-4416

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI (Grant No. JP15K05639) for M.K. from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We thank RIGAKU Co. for TGA-MS spectrometry and technical support. We thank


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ACS Applied Materials & Interfaces Professor Yu Nagase and Mr. Tomoki Mimura of Tokai University for GPC measurements. We thank Dr. Zhaomin Hou and Dr. Masayoshi Nishiura of the Organometallic Chemistry Laboratory, RIKEN for DSC and TGA measurements. We thank the RIKEN Brain Science Institute for high-resolution mass spectrometry. We thank Dr. Daisuke Hashizume, Mr. Daishi Inoue, and Ms. Tomoka Kikitsu of the Materials Characterization Support Unit, RIKEN Center for Emergent Matter Science, for SEM and TEM measurements and for their comments. We also thank Dr. Takafumi Sassa of the RIKEN Center for Advanced Photonics for film thickness measurements. We thank Dr. Quansheng Guo of WPIMANA and Center for Functional Sensor & Actuator, for specific heat measurements. We thank Sara Maccagnano-Zacher, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

REFERENCES 1. Yang, J.; Yip, H.-L.; Jen, A. K.-Y., Rational Design of Advanced Thermoelectric Materials. Adv. Energy Mater. 2013, 3, 549565. 2. Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G., New and Old Concepts in Thermoelectric Materials. Angew. Chem. Int. Ed. 2009, 48, 8616-8639. 3. Liu, W.; Yan, X.; Chen, G.; Ren, Z., Recent Advances in Thermoelectric Nanocomposites. Nano Energy 2012, 1, 42-56. 4. Heremans, J. P.; Dresselhaus, M. S.; Bell, L. E.; Morelli, D. T., When Thermoelectrics Reached the Nanoscale. Nat. Nanotech. 2013, 8, 471-473. 5. Blackburn, J. L.; Ferguson, A. J.; Cho, C.; Grunlan, J. C., Carbon-Nanotube-Based Thermoelectric Materials and Devices. Adv. Mater. 2018, 30, 1704386. 6. Kim, P.; Shi, L.; Majumdar, A.; McEuen, P. L., Thermal Transport Measurements of Individual Multiwalled Nanotubes. Phys. Rev. Lett. 2001, 87, 215502. 7. Nonoguchi, Y.; Hata, K.; Kawai, T., Dispersion of Synthetic MoS2 Flakes and Their Spontaneous Adsorption on Single-Walled Carbon Nanotubes. Chem. Plus. Chem. 2015, 80, 1158-1163. 8. Wenbin, Z.; Qingxia, F.; Qiang, Z.; Kewei, L.; Le, C.; Xiaogang, G.; Feng, Y.; Nan, Z.; Zhuojian, X.; Huiliang, C.; Shiqi, X.; Yanchun, W.; Huaping, L.; Weiya, Z.; Sishen, X., Ultrahigh-PowerFactor Carbon Nanotubes and an Ingenious Strategy for Thermoelectric Performance Evaluation. Small 2016, 12, 3407-3414. 9. Cho, C.; Wallace, K. L.; Tzeng, P.; Hsu, J. H.; Yu, C.; Grunlan, J. C., Outstanding Low Temperature Thermoelectric Power Factor from Completely Organic Thin Films Enabled by Multidimensional Conjugated Nanomaterials. Adv. Energy Mater. 2016, 6, 1502168. 10. Cho, C.; Stevens, B.; Hsu, J.-H.; Bureau, R.; Hagen, D. A.; Regev, O.; Yu, C.; Grunlan, J. C., Completely Organic Multilayer Thin Film with Thermoelectric Power Factor Rivaling Inorganic Tellurides. Adv. Mater. 2015, 27, 2996-3001. 11. Yu, C.; Choi, K.; Yin, L.; Grunlan, J. C., Light-Weight Flexible Carbon Nanotube Based Organic Composites with Large Thermoelectric Power Factors. ACS Nano 2011, 5, 7885-7892. 12. Dörling, B.; Ryan, J. D.; Craddock, J. D.; Sorrentino, A.; Basaty, A. E.; Gomez, A.; Garriga, M.; Pereiro, E.; Anthony, J. E.; Weisenberger, M. C.; Goñi, A. R.; Müller, C.; Campoy-Quiles, M., Photoinduced p- to n-type Switching in Thermoelectric PolymerCarbon Nanotube Composites. Adv. Mater. 2016, 28, 2782-2789. 13. Liang, L.; Gao, C.; Chen, G.; Guo, C.-Y., Large-Area, Stretchable, Super Flexible and Mechanically Stable Thermoelectric Films of Polymer/Carbon Nanotube Composites. J. Mater. Chem. C 2016, 4, 526-532. 14. Zhao, Y.-L.; Stoddart, J. F., Noncovalent Functionalization of Single-Walled Carbon Nanotubes. Acc. Chem. Res. 2009, 42, 11611171. 15. Fujigaya, T.; Nakashima, N., Non-Covalent Polymer Wrapping of Carbon Nanotubes and the Role of Wrapped Polymers as Functional Dispersants. Sci. Tech. Adv. Mater. 2015, 16, 024802.

16. Kawamoto, M.; He, P.; Ito, Y., Green Processing of Carbon Nanomaterials. Adv. Mater. 2017, 29, 1602423. 17. Li, Z.; Kameda, T.; Isoshima, T.; Kobatake, E.; Tanaka, T.; Ito, Y.; Kawamoto, M., Solubilization of Single-Walled Carbon Nanotubes Using a Peptide Aptamer in Water below the Critical Micelle Concentration. Langmuir 2015, 31, 3482-3488. 18. Li, Z.; He, P.; Chong, H.; Furube, A.; Seki, K.; Yu, H.-h.; Tajima, K.; Ito, Y.; Kawamoto, M., Direct Aqueous Dispersion of Carbon Nanotubes Using Nanoparticle-Formed Fullerenes and SelfAssembled Formation of p/n Heterojunctions with Polythiophene. ACS Omega 2017, 2, 1625-1632. 19. Samanta, S. K.; Fritsch, M.; Scherf, U.; Gomulya, W.; Bisri, S. Z.; Loi, M. A., Conjugated Polymer-Assisted Dispersion of SingleWall Carbon Nanotubes: The Power of Polymer Wrapping. Acc. Chem. Res. 2014, 47, 2446-2456. 20. Sun, B.; Hong, W.; Aziz, H.; Li, Y., DiketopyrrolopyrroleBased Semiconducting Polymer Bearing Thermocleavable Side Chains. J. Mater. Chem. 2012, 22, 18950-18955. 21. Helgesen, M.; Madsen, M. V.; Andreasen, B.; Tromholt, T.; Andreasen, J. W.; Krebs, F. C., Thermally Reactive Thiazolo[5,4d]thiazole Based Copolymers for High Photochemical Stability in Polymer Solar Cells. Polym. Chem. 2011, 2, 2536-2542. 22. Brusso, J. L.; Lilliedal, M. R.; Holdcroft, S., pi-Conjugated Polymers with Thermocleavable Substituents for Use as Active Layers in Organic Photovoltaics. Polym. Chem. 2011, 2, 175-180. 23. Helgesen, M.; Gevorgyan, S. A.; Krebs, F. C.; Janssen, R. A. J., Substituted 2,1,3-Benzothiadiazole- And Thiophene-Based Polymers for Solar Cells-Introducing a New Thermocleavable Precursor. Chem. Mater. 2009, 21, 4669-4675. 24. Gevorgyan, S. A.; Krebs, F. C., Bulk Heterojunctions Based on Native Polythiophene. Chem. Mater. 2008, 20, 4386-4390. 25. Petersen, M. H.; Gevorgyan, S. A.; Krebs, F. C., Thermocleavable Low Band Gap Polymers and Solar Cells Therefrom with Remarkable Stability toward Oxygen. Macromolecules 2008, 41, 8986-8994. 26. Edder, C.; Armstrong, P. B.; Prado, K. B.; Frechet, J. M. J., Benzothiadiazole- and Pyrrole-Based Polymers Bearing Thermally Cleavable Solubilizing Groups as Precursors for Low Bandgap Polymers. Chem. Commun. 2006, 1965-1967. 27. Liu, J.; Kadnikova, E. N.; Liu, Y.; McGehee, M. D.; Fréchet, J. M. J., Polythiophene Containing Thermally Removable Solubilizing Groups Enhances the Interface and the Performance of PolymerTitania Hybrid Solar Cells. J. Am. Chem. Soc. 2004, 126, 9486-9487. 28. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C., Spectrometric Identification of Organic Compounds. Wiley: 1991. 29. Marrocchi, A.; Lanari, D.; Facchetti, A.; Vaccaro, L. Energy Environ. Sci. 2012, 5, 8457-8474. 30. Zhao, J.; Bertho, S.; Vandenbergh, J.; Assche, G. V.; Manca, J.; Vanderzange, D.; Yin, X.; Shi, J.; Cleij, T.; Lutsen, L.; Mele, B. V. Phys. Chem. Chem. Phys. 2011, 13, 12285-12292. 31. Mark, J. E., Polymer Data Handbook. Oxford University Press: 1999. 32. Salikolimi, K.; Kawamoto, M.; He, P.; Aigaki, T.; Ito, Y., Polythiophene Nanoparticles that Display Reversible Multichromism in Aqueous Media. Polym. J. 2017, 49, 429-437. 33. Martín, J.; Nogales, A.; Martín-González, M., The SmecticIsotropic Transition of P3HT Determines the Formation of Nanowires or Nanotubes into Porous Templates. Macromolecules 2013, 46, 14771483. 34. Duan, C. H.; Cai, W. Z.; Hsu, B. B. Y.; Zhong, C. M.; Zhang, K.; Liu, C. C.; Hu, Z. C.; Huang, F.; Bazan, G. C.; Heeger, A. J.; Cao, Y., Toward Green Solvent Processable Photovoltaic Materials for Polymer Solar Cells: the Role of Highly Polar Pendant Groups in Charge Carrier Transport and Photovoltaic Behavior. Energy Environ. Sci. 2013, 6, 3022-3034. 35. Lin, H.-M.; Nash, R. A., An Experimental Method for Determining the Hildebrand Solubility Parameter of Organic Nonelectrolytes. J. Pharm. Sci. 1993, 82, 1018-1026. 36. Barton, A. F. M., Solubility Parameters. Chem. Rev. 1975, 75, 731-753.



37. Yang, X., Semiconducting Polymer Composites: Principles, Morphologies, Properties and Applications. Wiley: 2012. 38. Mori, T., Novel Principles and Nanostructuring Methods for Enhanced Thermoelectrics. Small 2017, 13, 1702013. 39. Lee, W.; Kang, Y. H.; Lee, J. Y.; Jang, K.-S.; Cho, S. Y., Improving the Thermoelectric Power Factor of CNT/PEDOT:PSS Nanocomposite Films by Ethylene Glycol Treatment. RSC Adv. 2016, 6, 53339-53344. 40. Hong, C. T.; Lee, W.; Kang, Y. H.; Yoo, Y.; Ryu, J.; Cho, S. Y.; Jang, K.-S., Effective Doping by Spin-Coating and Enhanced Thermoelectric Power Factors in SWCNT/P3HT Hybrid Films. J. Mater. Chem. A 2015, 3, 12314-12319. 41. Lee, W.; Hong, C. T.; Kwon, O. H.; Yoo, Y.; Kang, Y. H.; Lee, J. Y.; Cho, S. Y.; Jang, K.-S., Enhanced Thermoelectric Performance of Bar-Coated SWCNT/P3HT Thin Films. ACS Appl. Mater. Interfaces 2015, 7, 6550-6556. 42. Wang, L.; Jia, X.; Wang, D.; Zhu, G.; Li, J., Preparation and Thermoelectric Properties of Polythiophene/Multiwalled Carbon Nanotube Composites. Synth. Met. 2013, 181, 79-85. 43. Bounioux, C.; Diaz-Chao, P.; Campoy-Quiles, M.; MartinGonzalez, M. S.; Goni, A. R.; Yerushalmi-Rozene, R.; Muller, C., Thermoelectric Composites of Poly(3-hexylthiophene) and Carbon Nanotubes with a Large Power Factor. Energy Environ. Sci. 2013, 6, 918-925. 44. Moriarty, G. P.; Briggs, K.; Stevens, B.; Yu, C.; Grunlan, J. C., Fully Organic Nanocomposites with High Thermoelectric Power

Page 8 of 9

Factors by using a Dual−Stabilizer Preparation. Energy Tech. 2013, 1, 265-272. 45. Moriarty, G. P.; De, S.; King, P. J.; Khan, U.; Via, M.; King, J. A.; Coleman, J. N.; Grunlan, J. C., Thermoelectric Behavior of Organic Thin Film Nanocomposites. J. Polym. Sci. Part B: Polym. Phys. 2013, 51, 119-123. 46. Song, H.; Liu, C.; Xu, J.; Jiang, Q.; Shi, H., Fabrication of a Layered Nanostructure PEDOT:PSS/SWCNTs Composite and Its Thermoelectric Performance. RSC Adv. 2013, 3, 22065-22071. 47. Du, Y.; Shen, S. Z.; Yang, W. D.; Cai, K. F.; Casey, P. S., Preparation and Characterization of Multiwalled Carbon Nanotube/Poly(3-hexylthiophene) Thermoelectric Composite Materials. Synth. Met. 2012, 162, 375-380. 48. Kim, D.; Kim, Y.; Choi, K.; Grunlan, J. C.; Yu, C. H., Improved Thermoelectric Behavior of Nanotube-Filled Polymer Composites with Poly(3,4-ethylenedioxythiophene) Poly(styrenesulfonate). ACS Nano 2010, 4, 513-523. 49. Yao, Q.; Chen, L.; Zhang, W.; Liufu, S.; Chen, X., Enhanced Thermoelectric Performance of Single-Walled Carbon Nanotubes/Polyaniline Hybrid Nanocomposites. ACS Nano 2010, 4, 2445-2451. 50. Park, S.; Vosguerichian, M.; Bao, Z., A Review of Fabrication and Applications of Carbon Nanotube Film-Based Flexible Electronics. Nanoscale 2013, 5, 1727-1752. 51. Chen, K.; Gao, W.; Emaminejad, S.; Kiriya, D.; Ota, H.; Nyein, H. Y. Y.; Takei, K.; Javey, A., Printed Carbon Nanotube Electronics and Sensor Systems. Adv. Mater. 2016, 28, 4397-4414.

Table 1. Molecular weights, thermal properties, and absorption maxima of polythiophenes Polymer

Mna

PDIb

Tg (°C)c

T10 (°C)d

λBeforemax (nm)e

λAftermax (nm)f

PTEG

7,900

2.1

− 32

320

433

416

PHEX

14,000

2.1

− 17

293

426

400

POCT

14,600

2.7

−8

323

438

411

PDEA

12,500

1.9

5

192

464

412

aThe

number-average molecular weight. bPolydispersity index. cGlass transition temperature. d10% Weight loss. eAbsorption maximum before thermal cleavage. fAbsorption maximum after thermal cleavage.

Table 2. Characteristics of substrate-free PTEG/SWCNTs (50:50 w/w) films at 25 °C Curing temperature (°C)a

σ (S cm−1)b

S (μV K−1)c

PF (μW m−1 K−2)d

κ (W m−1 K−1)e

zT (x 102)f

25

54

41.0 ± 3.6

9.1

0.46

0.59

350

188

38.8 ± 0.7

28.8

0.27

3.1

aHeated

under nitrogen. bElectrical conductivity. cSeebeck coefficient. dPower factor. eThermal conductivity. fFigure-of-merit.

TOC image


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ACS Applied Materials & Interfaces


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Noncovalent Modification of Single-Walled Carbon Nanotubes Using

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