Facile Synthetic Route to Atomically Thin Conductive Wires from

Letter pubs.acs.org/JPCL

Facile Synthetic Route to Atomically Thin Conductive Wires from Single-Species Molecules in One-Dimensionally Confined Space: Doped Conjugated Polymers inside Single-Walled Carbon Nanotubes Makoto Sasaki,† Takeshi Koyama,*,† Hideo Kishida,† Koji Asaka,‡ Yahachi Saito,‡ Yukihiro Yoshida,§ and Gunzi Saito§,∥ †

Department of Applied Physics, Nagoya University, Nagoya 464-8603, Japan Department of Quantum Engineering, Nagoya University, Nagoya 464-8603, Japan § Faculty of Agriculture, Meijo University, Nagoya 468-8502, Japan ∥ Toyota Physical and Chemical Research Institute, Nagakute 480-1192, Japan ‡

S Supporting Information *

ABSTRACT: A facile synthetic method for doped conjugated molecules by a heating process is demonstrated. Br-terminated terthiophene precursors are encapsulated in single-walled carbon nanotubes by a vapor-phase reaction, and additional heat treatment promotes the thermal condensation of the precursors. Transmission electron microscopy observations and optical measurements show the successful synthesis of sexithiophenes and their doping (oxidation) by Br dopants generated by the condensation reaction. This study provides a new strategy for the synthesis of the doped conjugated polymers from single-species molecules by only a heating process.

olythiophenes, one of the most widely studied classes of πconjugated polymers, show high conductivity in the doped state and can be easily synthesized in large quantities and formed into arbitrary shapes. Owing to these characteristics, polythiophenes have been studied for their application in various devices, such as solar cells,1 field effect transistors,2 and light-emitting diodes.3 The conventional synthetic methods of polythiophenes are electropolymerization in an electrolyte solution and chemical polymerization using a catalyst.4 In these synthetic processes, however, it is difficult to obtain polythiophenes free of structural defects, such as bending and torsion. The structural defects confine the conjugated region of π-electrons and, therefore, restrict the conductive region on a polymer chain. Hence, linear chains of polythiophenes without structural defects are favorable for achieving high conductivity5 and might ultimately lead to superconductivity in the conjugated polymers.6 For the realization of a linear polymer chain, synthesis of polythiophenes in a one-dimensionally confined space has been proposed. For example, 12-meric oligothiophenes with a linear chain encapsulated in tert-butyldiphenylsilyl groups were obtained by chemical condensation.7 In that report, the doping of the synthesized oligothiophenes was performed by reacting with FeCl3. Recently, thermal condensation of π-electron molecules inside carbon nanotubes has been demonstrated,8−10

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© XXXX American Chemical Society

and the synthesis of polythiophenes inside the nanotubes by the thermal polymerization of α-sexithiophenes (6T; Scheme 1) at 350 °C has been reported.11 Miyaura et al. also Scheme 1. Molecular Structures of (top) 3Ta and (bottom) 6Tb Derivatives

a

X = H: 3T; X = Br: Br2-3T. bX = H: 6T; X = Br: Br2-6T.

successfully achieved electrochemical doping of the polythiophenes 11 using a similar procedure as that reported previously.12 It is noteworthy that chemical condensation by Ie et al.7 and thermal condensation by Miyaura et al.11 needed an additional chemical or electrochemical process for carrier doping of the obtained neutral polythiophenes. Received: March 27, 2017 Accepted: March 28, 2017

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were obtained by subtracting the absorption spectrum for SWNT from that for Br2-3T/SWNT. The obtained ΔA spectra are shown in Figure 2a. The ΔA spectrum for Br2-3T/SWNT

In the present study, we propose a new synthetic route to doped conjugated polymers from single-species molecules inside carbon nanotubes by only a heating process. Halogenterminated oligothiophenes were used as precursors; their thermal condensation simultaneously generates polythiophenes and halogen species, the latter of which are often used as electron acceptors of conjugated polymers owing to their high electron affinity.13,14 Thus, the condensation polymerization of halogen-terminated oligothiophenes inside carbon nanotubes enables the synthesis of oxidized polythiophenes without any additional processes, as described above. The starting materials are the single-walled carbon nanotubes (SWNTs) with tube diameters of 1.0 ± 0.2 nm (NanoIntegris, HiPco Purified) and Br-terminated terthiophene molecules, 5,5″-dibromo-2,2′:5′,2″-terthiophene (Br2 -3T) molecules (Scheme 1),15 as the precursor materials; for comparison, the nonsubstituted 2,2′:5′,2″-terthiophene (3T) molecules were used. We synthesized four types of samples: (1) Br2-3T/ SWNT(T) and (2) Br2-3T/SWNT, which are the composites of SWNTs and the encapsulated Br2-3T molecules, with and without thermal condensation at 265 °C, respectively, and (3) 3T/SWNT(T) and (4) 3T/SWNT, which are the composites of SWNTs and the encapsulated 3T molecules, with and without heating, respectively, using the same procedure as that for Br2-3T/SWNT. First, to investigate molecular encapsulation into the SWNTs, we observed the fabricated samples by transmission electron microscopy (TEM). Figure 1 shows a typical TEM

Figure 2. (a) Differential absorption spectra (ΔA spectra) for Br2-3T/ SWNT (orange solid line) and Br2-3T/SWNT(T) (red solid line) and the absorption spectrum for Br2-3T in chloroform (orange dashed line). Black circles indicate the positions of two absorption bands emerging by thermal treatment. (b) Absorption spectra for 6T molecules in chloroform (purple solid line) and doped 6T (green solid line) (the latter data were taken from Figure 2 in ref 17). (c) ΔA spectra for 3T/SWNT (light blue solid line) and 3T/SWNT(T) (dark blue solid line) and the absorption spectrum for 3T in chloroform (light blue dashed line).

(orange solid line) showed an absorption band with a peak at ∼3.2 eV, which is assigned to the π−π* transition band of Br23T (orange dashed line), indicating the presence of the Br2-3T molecules in the sample. After thermal treatment at 265 °C, the absorption spectrum changed; the ΔA spectrum for Br2-3T/ SWNT(T) (red solid line) showed an apparent red shift of the π−π* transition band to ∼2.6 eV. As seen in Figure 2b, the value is close to that of the 6T peak (2.8 eV, purple solid line), indicating the condensation of Br2-3T, which is in agreement with the TEM image. The packing density of synthesized oligothiophenes in SWNTs is estimated to be 35% using absorbance of the oligothiophenes and SWNTs in the sample based on the reported absorption cross section per carbon atom of the SWNT18 and the molar extinction coefficient of oligothiophene.19 In addition to thermal condensation, we found another feature in the ΔA spectrum for Br2-3T/ SWNT(T), namely, the emergence of two absorption bands at ∼0.7 and ∼1.5 eV (marked by black circles in Figure 2a). As can be seen in Figure 2b, the positively doped 6T (monovalent cation) showed absorption bands at 0.9 and 1.6 eV in the infrared region (green solid line).17 The emergence of the two bands indicated the oxidation of 6T. Thus, it is obvious that at least a portion of the 6T (or Br2-6T) molecules synthesized in the SWNTs were oxidized. It is noted that we fabricated the other dozen Br2-3T/SWNT(T) samples, and above 60% of the samples showed weak but apparent absorption bands of the 6T (or Br2-6T) molecule and its cation.

Figure 1. Typical TEM image of Br2-3T/SWNT(T). The doubleheaded arrows indicate one-dimensional materials inside the SWNTs.

image for Br2-3T/SWNT(T). We can see the one-dimensional materials inside the SWNTs, indicated by double-headed arrows, and the length of the materials was estimated to be ∼2.3 nm, which is almost the same as the length of a 6T (or Br2-6T) molecule. The TEM image suggests the successful encapsulation and condensation of Br2-3T inside the SWNTs. To examine the condensation and carrier doping in the samples, we measured the optical absorption spectra of the samples. It is well-known that the peak photon energy of the π−π* transition of oligothiophene decreases with the extension of the π-conjugation system, that is, the increase in the number of thiophene rings.16 In addition, the oxidation of oligothiophenes gives rise to additional absorption bands in the infrared region.17 Thus, the absorption measurements are useful to investigate the condensation and oxidation of the Br2-3T molecules. The absorption spectra of the Br2-3T/SWNT and the reference SWNT samples are shown in Figure S1 in the Supporting Information. To focus on the encapsulated molecules, the differential absorption spectra (ΔA spectra) 1703

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position is caused by interactions with the SWNTs and/or Br termination. Thus, it was proved that the Br2-3T molecules exist in the uncondensed state in the Br2-3T/SWNT sample, as suggested in the absorption spectrum in Figure 2. A weak peak at 1506 cm−1 indicates that a small number of 6T (or Br2-6T; purple dashed line) molecules were formed in the sample during molecular encapsulation by a vapor-phase reaction at 160 °C (see below). The Raman spectrum for the Br2-3T/ SWNT(T) heat-treated at 265 °C is shown as a red line. A peak of the CC antisymmetric stretching mode was observed at 1506 cm−1, which is close to that of 6T (1507 cm−1). At the low-frequency side of the peak, a weak shoulder was also observed, indicating the formation of a small amount of highly polymerized thiophenes (for example, 1498 cm −1 for polythiophene22). Therefore, the spectrum indicates that the major components present inside the SWNTs were the 6T (or Br2-6T) molecules, and further condensation reaction of the molecules occurred a little. This is probably because the packing density of the oligothiophenes in SWNTs was as low as 35%, as indicated by the absorption spectra. The low density may cause low probability for encounter of the oligothiophenes. A pronounced peak at ∼1460 cm−1 in both spectra is assigned to the CC symmetric stretching mode of the thiophene rings.22 Because the difference in the peak position of this mode in 3T and 6T is very small,22 it is difficult to determine the polymerization degree from this mode. The peak positions at 1457 cm−1 for Br2-3T/SWNT and 1462 cm−1 for Br2-3T/SWNT(T) are consistent with those for 3T (1461 cm−1) and 6T (1460 cm−1), respectively, within the peak widths. To confirm the existence of Br species near the thiophene molecules in the Br2-3T/SWNT(T) sample, we performed elemental analysis using energy-dispersive X-ray spectroscopy (EDS). The EDS spectra for the Br2-3T/SWNT(T) sample and the Br2-3T powder are shown in Figure 4a and the inset, respectively. The peaks at 1.8 and 2.2 keV are assigned to the Br Lα and S Kα lines, respectively, showing that the Br2-3T/ SWNT(T) sample contains Br and oligothiophenes, as expected from the optical data. Figure 4b−d shows a scanning electron microscopy (SEM) image of Br2-3T/SWNT(T) and the corresponding EDS elemental maps for Br and S, respectively. The positions at which Br and S signals were observed match with those of the SWNT bundles in Figure 4b, indicating that Br is located in close vicinity to oligothiophenes. Therefore, it is highly probable that the synthesized oligothiophenes are doped by Br, as expected. Finally, we comment on the doping level in the Br2-3T/ SWNT(T) sample. In Figure 2a, the absorption spectrum for Br2-3T/SWNT(T) shows an intense π−π* transition band at ∼2.6 eV and weak bands at ∼0.7 and ∼1.5 eV. The ratio of absorption intensity of the weak bands at ∼0.7 and ∼1.5 eV to that of the intense π−π* transition band at ∼2.6 eV is similar to that of a polythiophene film doped with ClO4− (doping level, i.e., the number of positive charges per thiophene ring, 3− 6%).23 Therefore, the number of positive charges is roughly 3− 6 per 100 thiophene rings in Br2-3T/SWNT(T). In the EDS spectra, we can see that the number of Br dopants decreased after thermal treatment (see Figure 4a and Figure S3 in Supporting Information). Because a significant fraction of Br dopants seems to be removed from the SWNTs, the Br2-3T/ SWNT(T) sample has a low doping level. Nevertheless, Figure 4c,d indicates an almost homogeneous distribution of Br (dopant) and S (thiophene), suggesting a homogeneous doping

To investigate the effect of Br substitution on 3T on the condensation and oxidation reactions, we measured the absorption spectra of the 3T/SWNT and 3T/SWNT(T) samples (Figure S1) and compared them with those of the Br2-3T/SWNT and Br2-3T/SWNT(T) samples. The ΔA spectrum for 3T/SWNT is shown as a light blue solid line in Figure 2c. The main peak is located at ∼3.3 eV, close to that of 3T in chloroform (3.5 eV; light blue dashed line). Because the red shift of the band by encapsulation in the SWNTs (∼0.2 eV) is comparable to that for the Br2-3T/SWNT samples, it is possible that the C−Br bonds in Br2-3T were not cleaved in the encapsulation step during the preparation of the Br2-3T/ SWNT samples. Although the main peak of the ΔA spectrum for 3T/SWNT(T) (dark blue solid line) is slightly red-shifted to ∼3.1 eV, the peak position is higher than that for the 6T molecules in chloroform (2.8 eV, purple dashed line) in Figure 2b. This implies that most of the 3T molecules remained unreacted at 265 °C, at which most of the Br2-3T molecules in the Br2-3T/SWNT(T) sample condensed to form the 6T (or Br2-6T) molecules. This indicates that Br termination has a positive effect on the condensation reaction at low temperatures. The condensation of 3T molecules in the 3T/SWNT sample started at a considerably higher temperature of 400 °C, where the thermal decomposition of 3T molecules occurred simultaneously (the details are provided in the Supporting Information). In the ΔA spectrum of the 3T/SWNT heattreated at above 400 °C (Figure S2 in the Supporting Information), no absorption structure was observed in the infrared region except for the derivative-like structures due to the shifts of the exciton bands of the SWNTs,20,21 suggesting that the oligothiophenes synthesized inside the SWNTs were inevitably in the undoped state. Comparison of absorption in the infrared region between the 3T/SWNT(T) and Br2-3T/ SWNT(T) samples after thermal treatment indicated that a portion of the synthesized oligothiophenes in the Br2-3T/ SWNT(T) sample are oxidized and suggested that the dopants are the Br ions generated by condensation. Next, we measured the Raman spectra to investigate the polymerization degree of the synthesized oligothiophenes because the Raman peak of the CC antisymmetric stretching mode of the thiophene ring shifts as the number of thiophene rings increases.22 The peak of the mode for 3T is located at 1530 cm−1 (light blue dashed line in Figure 3).22 The Raman spectrum for the Br2-3T/SWNT sample is shown as an orange line in Figure 3. The peak of the mode was observed at 1524 cm−1, close to that of 3T. The slight difference in the peak

Figure 3. Raman spectra for Br2-3T/SWNT (orange) and Br2-3T/ SWNT(T) (red). Peak positions of the CC antisymmetric stretching mode of 3T and 6T are indicated by the light blue and purple dashed lines.22 1704

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Figure 4. (a) Typical EDS spectrum, (b) SEM image, and EDS element maps of (c) bromine and (d) sulfur for Br2-3T/SWNT(T). The inset of (a) shows the EDS spectrum of Br2-3T. The peak at 6.2 keV in the EDS spectrum (a) is attributed to the Fe Kα line from the remnant catalysts for SWNT synthesis.

6T (or Br2-6T) molecules (estimated doping level is 7.5%). These results demonstrated the successful synthesis of the doped 6T (or Br2-6T) molecules from Br2-3T inside the SWNTs. We finally point out the low doping level of the synthesized molecules, which resulted from the decrease in the number of Br dopants inside the SWNTs after thermal treatment. Use of halogen-terminated precursors with heavy halogen atoms (e.g., iodine) might increase the number of dopants remaining in the SWNTs and thus enhance the doping level of the synthesized oligomers and polymers.

of thiophenes. Taking the number ratio of Br and S atoms in the Br2-3T precursors to be 2/3, the ratio in the Br2-3T/ SWNT(T) sample was estimated to be 0.075 using the peak intensities of the Br and S atoms in the EDS spectra in Figure 4a. Assuming that all of the Br atoms were cleaved from the synthesized 6T and the Br dopants exist as monatomic ions, the doping level was calculated to be 7.5%. This value is consistent with that suggested by the absorption spectrum, 3−6%. It is possible that replacing the Br atoms with heavier halogen atoms, that is, iodine,24 will lead to restriction of the eliminated halogen species inside the SWNTs. In addition, because the bond dissociation energy of the C−I bond is lower than that of the C−Br bond,25 the C−I bonds will be cleaved at lower temperature. Thus, an increase in the number of dopants inside SWNTs is expected. These characteristics certainly result in a higher doping level. (Although the radius of the I atom is larger than that of the Br atom, I atoms can be encapsulated in SWNTs with a diameter of 1.0 nm.26) Studies along this line are underway. In the present study, SWNT materials consisted of both semiconducting and metallic SWNTs. The doping behavior of oligothiophenes may be dependent on the electronic properties of surrounding SWNTs. Investigation on this behavior will be a future study. In this study, we used a two-step heating process, consisting of encapsulation of the precursors in SWNTs by a vapor-phase reaction and additional heat treatment for thermal condensation of the precursors, in order to prove the realization of condensation within the SWNTs. However, in principle, the encapsulation and condensation of the precursors in SWNTs can be achieved by a single heating process at a suitable temperature. In fact, we succeeded in a single-step synthesis of doped 6T (or Br2-6T) molecules at 220 °C (see the Supporting Information). Therefore, this study provided a new strategy for the single-step synthesis of doped conjugated polymers from single-species molecules by thermal polymerization at low temperatures in a one-dimensionally confined space. In conclusion, we carried out encapsulation of the Br2-3T precursors in the SWNTs by a vapor-phase reaction and additional heat treatment for the thermal condensation of the precursors. TEM observations proved encapsulation of the 6T (or Br2-6T) molecules inside the SWNTs. The absorption and Raman spectra supported the condensation of Br2-3T inside the SWNTs and proved the doping of the synthesized 6T (or Br26T) molecules. Comparison of the absorption spectra for the samples with those for the Br2-3T and 3T precursors suggested that the dopants were the Br ions generated by condensation of the Br2-3T molecules. EDS analysis supported the Br-doping of

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EXPERIMENTAL METHODS The sample preparation was carried out in accordance with the procedure reported previously.10 3T was purchased from Wako and recrystallized from distilled methanol, whereas Br2-3T was synthesized and purified according to the literature.15 We used the HiPco-SWNTs produced by the high-pressure carbon monoxide method, with a tube diameter of 1.0 ± 0.2 nm (NanoIntegris, HiPco Purified). The SWNTs were heated for 1 h at 650 °C and 10−5 Torr for purification and heated for 1 h at 550 °C in air for cap opening. The SWNTs were dispersed in 1 wt % sodium cholate aqueous solution. The dispersed SWNTs were collected by vacuum filtration, and SWNT films were formed. The SWNT films were transferred onto quartz substrates. Molecular encapsulation was carried out by vaporphase reaction. The precursor molecules were sealed with a SWNT film at 10−5 Torr in a glass tube. The glass tube was heated for 24 h at 160 °C, at which the precursors undergo sublimation. The SWNT film was washed with toluene for 15 min to remove the precursor molecules adsorbed outside the SWNTs. For thermal condensation, the sample thus obtained was heated for 24 h at 10−5 Torr and a higher temperature, typically at 265 °C. We investigated the condensation of the Br2-3T precursors in another type of SWNTs with a tube diameter of 1.4 ± 0.1 nm (see the Supporting Information). TEM observations were carried out employing a JEM-2010 instrument operated at 120 kV.10 The absorption and Raman spectra were measured using a HITACHI U-3500 spectrophotometer and a Renishaw spectrometer with a diode laser at 488 nm (corresponding photon energy of 2.54 eV), respectively.10 The EDS experiments were conducted using a JEOL JSM5510LVN scanning electron microscope.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00724. 1705

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Additional data of absorption and EDS spectra and details of high-temperature treatment of 3T/SWNT, condensation of Br2-3T precursors in a different type of SWNTs, and single-step synthesis of doped 6T (or Br26T) molecules (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-52-7894450. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP22655045, The Ogasawara Foundation for the Promotion of Science and Engineering, the Research Foundation for the Electrotechnology of Chubu, The NAGAI Foundation for Science and Technology, Toyota Physical and Chemical Research Institute Scholars, Toukai Foundation for Technology, and The Murata Science Foundation.

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