Article pubs.acs.org/Langmuir
Aqueous Nanosilica Dispersants for Carbon Nanotube Takafumi Matsuda,† Daiki Minami,‡ Fitri Khoerunnisa,‡,§ Motoo Sunaga,† Masahiro Nakamura,∥ Shigenori Utsumi,∥ Tsutomu Itoh,⊥ Toshihiko Fujimori,† Takuya Hayashi,# Yoshiyuki Hattori,& Morinobu Endo,% Hiroshi Isobe,† Hiroshi Onodera,† and Katsumi Kaneko*,‡ †
Technical Center, Fuji Chemical Co., Ltd., Nakatsugawa 509-9132, Japan Center for Energy and Environmental Science, Shinshu University, Nagano 380-8553, Japan § Department of Chemistry, Indonesia University of Education, Bandung 40154, Indonesia ∥ Department of Mechanical Systems Engineering, Tokyo University of Science, Suwa, Chino 391-0292, Japan ⊥ Center for Chemical Analysis, Chiba University, Inage, Chiba 263-8522, Japan # Department of Electrical Engineering, Faculty of Engineering, and %Institute of Carbon Science and Technology, Shinshu University, Nagano 380-8553, Japan & Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda 386-8567, Japan ‡
S Supporting Information *
ABSTRACT: Nanosilicas can disperse single-wall carbon nanotube (SWCNT) in aqueous solution efficiently; SWCNTs are stably dispersed in aqueous media for more than 6 months. The SWCNT dispersing solution with nanosilica can produce highly conductive transparent films which satisfy the requirements for application to touch panels. Even multiwall carbon nanotube can be dispersed easily in aqueous solution. The highly stable dispersion of SWCNTs in the presence of nanosilica is associated with charge transfer interaction which generates effective charges on the SWCNT particles, giving rise to electrostatic repulsion between the SWCNTs in the aqueous solution. Adhesion of charged nanosilicas on SWCNTs in the aqueous solution and a marked depression of the S11 peak of optical absorption spectrum of the SWCNT with nanosilicas suggest charge transfer interaction of nanosilicas with SWCNT. Thus-formed isolated SWCNTs are fixed on the flexible three-dimensional silica jelly structure in the aqueous solution, leading to the uniform and stable dispersion of SWCNTs.
1. INTRODUCTION Carbon nanotubes (CNTs) have exceptional electrical, mechanical, thermal, electromagnetic, and interfacial properties.1−8 CNTs have been applied to produce new composites based on combining ceramics and polymers9−11 as well as in scanning tunneling microscope probes12,13 and batteries.14 The applications of CNTs remain quite limited. The most serious obstacle is the lack of a means of efficiently dispersing CNTs in matrixes while maintaining their outstanding properties. An effective dispersion of CNTs provides the innovative composites having the excellent properties of CNTs. Organic compounds have been used to disperse CNTs in water,15−22 but these dispersants tend to remain on the surface of the CNTs and thus attenuate their unique characteristics.23 Herein we report a new silica-based dispersant which shows promise for the dispersion of CNTs. Following dispersion, the silica dispersant could be readily removed from the surface of CNTs. The nanosilica dispersant could make an innovation in the fields of CNTs science because the nanosilica gives great affinities between the surface of CNTs and matrixes. Nonfunctionalized CNTs cannot be dispersed in aqueous phases due to their hydrophobic nature, and so a variety of © 2015 American Chemical Society
surface modification techniques have been investigated in an attempt to convert hydrophobic nature of the CNT surface to hydrophilic one. These surface modification methods can be classified as either covalent or noncovalent approaches. Covalent processes introduce covalent-bonding surface modifying groups onto the CNT surface by chemical or physical oxidation so as to donate a suitable hydrophilicity. This process, however, locally disrupts sp2 hybridization in the CNT structure, which in turn deteriorates the electronic performance of the CNTs.22−26 Noncovalent surface modification methods using surfactants or polymers with hydrophilic moieties can preserve the sp2 hybridization structure and the electronic performance of CNTs. In particular, supramolecular chemistry has provided the promising route for preservation of the key properties of CNT and recyclability of supramolecular dispersant.27−31 However, we need another route for dispersion of CNT in order to extend the application areas of CNT. There is a definite requirement for an inorganic CNT dispersant with Received: November 25, 2014 Revised: February 22, 2015 Published: February 23, 2015 3194
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at room temperature. Freeze-dried samples of SWCNTs and nanosilica were prepared by vacuum drying following the rapid freezing of the solutions with liquid nitrogen. The XPS of the nanosilica-dispersed SWCNTs after drying was measured using the freeze-dried samples with the aid of JEOL-JPS-9200.
excellent thermal and light stabilities. For this reason we must develop an inorganic surface modifier which is both readily removable and thermally stable. Silica nanoparticles have been used to disperse oil droplets in aqueous phases,32 and those are also known to stabilize emulsions in water,33 since they are concentrated at the interface between hydrophobic oils and the aqueous phase. Even micrometer scale silica is efficient for dispersion of carbon fibers in cement.34 SWCNT can be coated with thin SiO2 layer of about 1 nm.35 The homogeneous silica shells are stably formed on CNT, which plays a role of the template for production of hollow silica structures.36 The positive electron affinity of silica suggests a weak charge transfer interaction of silica with CNT because CNT has an amphoteric character.36 The DFT calculation study showed slight electronic structure change in silica and depletion of the π-electron of SWCNT for the model of SWCNT covered by a stack of silica rings.37 Furthermore, the silica particles possess both hydrophilicity and hydrophobicity due to the presence of polar surface hydroxyl groups and nonpolar silicon dioxide, and thus silica-based dispersant should be promising for dispersion of CNTs in water. Silica nanoparticles have the further advantage of being easily removed following dispersion via dissolution treatment with either alkaline or acid solutions. This paper describes the adhesion of size-controlled silica nanoparticles (which we term nanosilica) on the hydrophobic surfaces of CNTs, leading to highly stable dispersions of CNTs in aqueous solutions. Furthermore, we can produce the highly transparent conductive film with SWCNT with the nanosilica dispersants. This nanosilica dispersants should innovate various CNT composite technologies and CNT devices such as thermally stable, flexible, transparent, and electrically conducting film electrodes.
3. RESULTS AND DISCUSSION Dispersion Ability of Nanosilicas. Both single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs) were used in this study. Figure 1 shows the TEM
Figure 1. TEM images of (a, b) SWCNTs and (c, d) MWCNTs.
2. EXPERIMENTAL SECTION Materials. SWCNTs were prepared by the laser ablation method, employing Co and Ni catalysts,38 while MWCNTs were purchased from Showa Denko K. K. The SWCNTs and MWCNTs were dispersed in solution using a direct-probe sonicator (VC-505, Sonics & Materials, Inc.) with settings of 150 W and 20 kHz for 10 min, maintaining the solution temperature at 295.0 ± 0.5 K throughout the sonication. The SWCNT film was produced on a poly(ethylene terephthalate) (PET) substrate heated at 373 K using spray coating of 0.01 wt % SWCNT solutions containing 1 wt % nanosilicas. The SWCNT-coated film was dipped in the mixed acid solution of 1 mol/L HNO3 and 1 mol/L HF at room temperature for 10 min, washed by distilled water, and dried under atmosphere. Characterization. The high-resolution TEM images were obtained using HR-TEM (200 kV, JEM2100, JEOL, Japan). The N2 adsorption isotherms were measured at 77 K using an ASAP2020 instrument (Micromeritics, USA). SWCNT and MWCNT samples were pretreated at 423 K and 10 mPa for 6 h prior to measurement of the adsorption isotherms. The X-ray diffraction (XRD) patterns of the SWCNTs and MWCNTs were measured using a wavelength of 0.154 06 nm (M18XHF, Bruker AXS). Raman spectra were measured using Raman spectrometer of Renishaw equipped with a diode laser (power 0.3 mW, wavelength 785 nm). The optical absorption spectra of the SWCNT on the PET film with and without nanosilicas in the wavelength range of 400−2000 nm using UV−vis−NIR spectrometer (JASCO V-670); the measurement of SWCNT without nanosilica was carried out after removal treatment of the nanosilicas with 1 M HF solution for 20 and 40 min. The SAXS profile of the nanosilica solution was obtained at a wavelength of 0.150 nm (Aichi Synchrotron Radiation Center, BL8S1). The zeta potential was measured with the Doppler laser method by use of Nicomp 380 ZLS. Optical absorption spectra and zeta potentials of the dispersed SWCNTs were measured
images of SWCNTs and MWCNTs. These images clearly show the highly crystalline structures of both materials. The average tube diameters of the SWCNTs and MWCNTs were 1.46 ± 0.04 and 150 ± 10 nm, respectively. The SWCNTs tend to form bundled and entangled structures, which intensively interferes with their ready dispersion in water. The TEM images of the MWCNTs suggest that the sample consists of a mixture of open and closed tubes. Figure 2 shows N2 adsorption isotherms of SWCNTs and MWCNTs at 77 K. The N2 adsorption isotherm of the SWCNTs exhibits a sharp uptake below P/P0 = 0.05, indicating the presence of micropores originating from the interstitial pores in the bundle structures. This adsorption isotherm also shows a vertically positioned adsorption hysteresis above P/P0 = 0.8, resulting from mesopores in the entangled structures of the SWCNT bundles. Correspondingly, the αs plot has a marked upward shift below αs = 0.5, resulting from adsorption enhanced by the overlapped interaction potentials of micropores of width less than 1 nm.39 This enhanced adsorption must be subtracted to accurately evaluate the surface area, using the subtracting pore effect (SPE) method.40 The N2 adsorption isotherm of the MWCNTs exhibits a gradual step around P/P0 = 0.3, indicating the presence of micropores and well-crystallized graphitic walls. As the αs plot has no clear node around αs = 0.5, the contribution by micropores is very limited. Based on the slopes of these plots between the origin and αs = 0.5, the surface areas of the SWCNTs and MWCNTs are 354 and 13 m2/g, 3195
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Figure 2. N2 adsorption isotherms (left) and αs plots (right) of (a) SWCNTs and (b) MWCNTs.
Figure 4 shows small-angle X-ray scattering (SAXS) profiles, indicating that the resulting nanosilica particles were 3.2 nm in size. Figure 5 shows the N2 adsorption isotherm of a nanosilica sample obtained by freeze-drying followed by pretreatment at 423 K and 10 mPa for 10 h. The resulting isotherm exhibits a sharp uptake below P/P0 = 0.05, indicating the presence of micropores formed between the nanosilica particles. The surface area of the sample was determined to be 646 m2/g by αs analysis.40 The average size of nanosilica particles estimated from the surface area and the density, assuming that the particles are spherical, was 4 nm, being somewhat larger than the size in aqueous solution from SAXS. The CNT samples of 1 mg were added to 50 mL portions of aqueous nanosilica solutions of different silica concentrations ranging from 0.001 to 1 wt % under ambient conditions. The CNTs were then dispersed in the nanosilica solutions by ultrasonication for 10 min at 293 K using a homogenizer (20 kHz, 150 W). Following sonication at silica concentrations above 0.005 wt %, the SWCNTs were well dispersed in the solutions, keeping the well dispersion for a month at least, as shown in Figure 6a,b. On the other hand, SWCNTs precipitated within 24 h after sonication with the nanosilica concentration of 0.001
respectively. The total concentration of Co and Ni in the SWCNTs was 0.2 wt %, while the MWCNTs contained Fe of 0.2 wt % with thermal gravimetric analysis (Figure S1 in Supporting Information). The X-ray diffraction patterns are shown in Figure 3. The strong peak at 2θ = 6° in Figure 3a is assigned to the triangular lattice structure of bundled SWCNTs. The associated lattice spacing is 1.47 nm, which is in good agreement with the SWCNT diameter. The average crystallite size of the MWCNT was determined to be 14 nm based on the sharp peak at 2θ = 26° and Scherrer’s formula, indicating a highly ordered layer structure. The well-crystallized structure of the MWCNTs coincides with the presence of a step at P/P0 = 0.3 in the N2 adsorption isotherm. The sharp G bands of Raman spectra of SWCNTs and MWCNTs indicate the highly crystalline structure (Figure S2 in Supporting Information). As it is well-known that highly crystalline carbon nanotube cannot be dispersed in water without surface modification or addition of surfactants, the dispersion of these CNTs in water should be extremely difficult. Nanosilica particles were produced by preparing a 5 wt % sodium silicate aqueous solution, and then sodium ions were exchanged with protons by means of a cation exchange resin. 3196
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dispersed MWCNTs precipitate after the centrifugation treatment due to their heavy particle weight. Figure 6d shows a transparent conductive film fabricated with SWCNTs using the nanosilica dispersants by use of the spray-coating method; the average sheet resistance and transparency at 550 nm of this transparent film are 200 Ω/□ and 90%, respectively, which are close to the required value for application to touch panels. As the nanosilica can be removed by acid dissolution at the mild conditions (1 M HNO3 for several minutes at room temperature), the excellent properties of SWCNT can be preserved after the film formation. Thus, the nanosilica dispersant has a high potential application. The above results show that nanosilica can act as a powerful dispersant for SWCNTs in water. The optical absorption spectra of SWCNTs dispersed with nanosilica in aqueous solution were also measured, as shown in Figure 8. The spectra exhibit broad two peaks at 1.2 and 1.7 eV which are attributed to the semiconducting S22 and metallic M11 transitions, respectively.41 Because of a marked noise, we cannot observe an explicit peak around 0.7 eV which is assigned to S11, although the optical absorption spectrum of SWCNT film shown in Figure 9 has an explicit peak at 0.70 eV. Here, S11 and S22 are the transition between the first and second van Hove singularities of the semiconductive SWCNTs, respectively, whereas M11 is the transition between the first van Hove singularities of the metallic SWCNTs.42,43 The observation of optical absorption spectra with distinct S22 and M11 peaks intrinsic to SWCNTs provides a strong evidence of a microscopically uniform dispersion of SWCNTs in the nanosilica solution. In addition, the intensity of the optical absorption spectrum increases with the silica concentration over the range of 0.001−0.01 wt %, giving a microscopic evidence that silica improves the dispersibility of SWCNTs in the aqueous phase. Interaction between SWCNT and Nanosilicas. The highly stable dispersion of SWCNTs in the presence of nanosilica is associated with charge transfer interaction which generates effective charges on the SWCNT particles, giving rise to electrostatic repulsion between the SWCNTs in aqueous solution. Figure 9 shows optical absorption spectra of SWCNT film with nanosilicas and that treated with 1 M HF. The spectrum of SWCNT treated with HF solution has clear three peaks at 0.71, 1.27, and 1.78 eV, which are assigned to S11, S22, and M11 transitions, respectively.41−43 However, the peaks of SWCNT with nanosilicas are broad due to scattering by nanosilicas; the S11 peak is quite weak and broad in particular. It is well-known that the charge transfer interaction of SWCNT with electron acceptors or donors does not reduce the S22 and M11 peaks, but the S11 peak.41−43 We can compare the peak intensity change of S11 after normalization of the peak intensity of S22 for SWCNT with nanosilica and SWCNT treated with HF solution, as shown in Figure S4. The presence of nanosilicas decreases significantly the intensity of the S11 peak at 0.71 eV, which can be ascribed to the charge transfer interaction between SWCNT and nanosilica. No clear observation of the S11 peak of the dispersed SWCNT in the solution (Figure 8) also should be associated with the depression of the absorbance from the charge transfer interaction between SWCNT and nanosilica. Figure S5 shows Raman spectra of SWCNTs dispersed in nanosilica solutions based on the curve fittings. We can observe a slight higher frequency shift of the G-band in the Raman spectra of SWCNTs dispersed in nanosilica solutions (Figure
Figure 3. X-ray diffraction patterns of (a) SWCNTs and (b) MWCNTs (λ = 0.154 06 nm).
Figure 4. Small-angle X-ray scattering (SAXS) pattern of nanosilica in solution.
wt %. The dispersion states of SWCNTs in solutions with different silica concentrations after centrifugation for 60 min at 4000 rpm are shown in Figure 6c, which indicates that SWCNTs are stably dispersed even after centrifugation. Figure 7 shows the Raman spectra of SWCNTs dispersed in nanosilica solutions following centrifugation. G-band and D-band peaks are clearly observed in all solutions except for the solution of the nanosilica concentration of 0.001 wt %. The intense G-band peaks indicate good dispersion of the SWCNTs even after centrifugation, whereas the disappearance of the G-band indicates ill dispersion of SWCNTs in the case of the nanosilica concentration of 0.001 wt %. The peak at 200 cm−1 is scattered light from the excitation laser. This Raman spectroscopic examination also shows that the 0.001 wt % silica solution is insufficient for stable dispersion. Nanosilica is also effective for dispersion of MWCNTs in the nanosilica concentration range of more than 0.05 wt %, as shown in Figure S3. However, the 3197
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Figure 5. (a) N2 adsorption isotherm and (b) αs plots of nanosilica at 77 K. The nanosilica sample was prepared from a 1 wt % solution by freeze drying.
Figure 6. Dispersion states and a transparent conductive film of SWCNTs. Dispersion states of SWCNTs in aqueous solutions with differing nanosilica concentrations after (a) 30 min, (b) 1 day, and (c) centrifugation for 60 min at 4000 rpm. A transparent conductive film was fabricated from the dispersion liquid of SWCNTs (d). The sheet resistance and transparency at 550 nm of this transparent film are 200 Ω/□ and 90%, respectively. These values are almost satisfied with the requirement for application to touch panels.
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Figure 9. Optical absorption spectra of SWCNT film with nanosilicas and that treated with 1 M HF for 20 and 40 min. SWCNT treated with nanosilica (a); SWCNT treated nanosilica after washing with HF for 20 min (b) and 40 min (c). Figure 7. Raman spectra of SWCNTs dispersed in silica solutions with different concentrations of nanosilica (excitation laser λ = 785 nm).
S6 in Supporting Information), which does not contradict with the charge transfer interaction.44,45 A subsidiary information on adhesion of nanosilicas on the surface of SWCNTs in the solution was obtained by TEM observations of the freeze-dried SWCNTs dispersed in nanosilica solutions. The resulting TEM images show contact adhesion of amorphous silica nanoparticles on the SWCNTs (Figure 10a). Interestingly, some of these particles crystallized during the TEM observations, exhibiting the lattice fringes. The lattice spacing of these crystals indicates the formation of quartz during the TEM observation,46 indicating that the particles adhering on the SWCNTs are noncrystalline silica nanoparticles. TEM images also indicated that nanosilica particles tend to associate with each other in the spaces between SWCNTs (Figure S7 in Supporting Information). The nanosilica in the solution assists debundling of SWCNTs to give rise to the observed highly dispersed aqueous solutions (Figure 6b). X-ray photoelectron spectroscopic (XPS) examination showed that the freeze-dried SWCNTs possessed hydrophilic surface functional groups assisting in their dispersion in the nanosilica solution (Figure S8 in Supporting Information); the content of surface functional groups does not change after the dispersion treatment with nanosilicas. Figure
Figure 10. Mechanism of dispersion of SWCNTs with nanosilicas. (a) TEM image of SWCNTs with nanosilica particles along with the lattice spacings of the lattice fringes and quartz segments.29 The lattice spacings of nanosilica adsorbed on the SWCNTs were determined to be 0.17, 0.198, and 0.21 nm, which are in good agreement with the 200, 021, 201, and 022 planes of quartz crystal. (b) Nanosilica adheres to the surface of SWCNTs, leading to debundling SWCNTs.
11 shows variations in the zeta potential of SWCNTs as a function of the silica concentrations, and it can be seen that the
Figure 8. Evaluation of dispersion stabilities of SWCNTs with different concentrations of nanosilica dispersants. (a) Optical absorption spectra of SWCNTs dispersed in 0.5 wt % aqueous nanosilica solution. The peaks at 680 and 960 nm are assigned to the metallic and semiconducting SWCNTs, respectively. (b) Optical absorbance of SWCNT dispersions at 960 nm at different nanosilica concentrations. 3199
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4. CONCLUSIONS Organic dispersants such as surfactants have been widely used for dispersion of CNTs in aqueous solution, although the dispersion treatment of CNTs with organic dispersants deteriorates the excellent properties of CNTs. Then, the development of a new type of the dispersant for CNTs making up the shortcomings of the organic dispersants has been requested for a long time. The highly efficient nanosilica dispersant as an inorganic dispersant for CNTs in aqueous solution is first developed. The nanosilica dispersant can be easily removable by washing under a mild condition after drying to preserve outstanding properties of carbon nanotubes, offering a new application route. Also, the use of nanosilica offers an environmentally friendly scalable method for dispersing carbon nanotube, which should promote the wider application of CNTs. One of the promising applications of the nanosilica-aided SWCNT dispersion is on production of the transparent conducting films. As nanosilicas are thermally stable and donate a partial hydrophilicity to CNTs, new types of composites containing CNTs can be prepared to offer sustainable materials of sufficient robustness.
Figure 11. Changes in the zeta potential and the pH of SWCNT dispersion with nanosilica concentration. The zeta potential decreases with increasing the concentration of nanosilica up to 0.01 wt % because of the adhesion of nanosilica dispersants on SWCNTs surface. The negative charge is shielded by the protons, increasing the zeta potential in the concentration range above 0.01 wt %.
zeta potential decreases with increasing nanosilica concentration up to 0.01 wt %. The decrease of zeta potential with increasing nanosilica concentration in the range of 0.001−0.01 wt % is caused by negative charges on the nanosilica surface. The adhesion of nanosilica (and the resulting negative charging of the SWCNTs) should stem from the charge transfer interaction between the SWCNTs and nanosilica, as suggested in the optical absorption spectroscopy study. In the higher silica concentration region, the negative charge on the SWCNTs surface is shielded by positively charged protons supplied by the solution of low pH.47 Consequently, increase of the nanosilica concentration decreases the dispersion stability of the SWCNTs at the silica concentration of 1.0 wt %. Thus, the nanosilica concentration is a key factor to control the stability of SWCNTs in solution. High Application Potential of Nanosilica to Transparent Conducting Films. The use of nanosilica offers a simple and scalable method for dispersing SWCNTs and MWCNTs, which should promote the wider application of CNTs. One of promising application of the nanosilica-aided SWCNT dispersion is production of the transparent conducting films. Figure 6d shows the SWCNT film on a PET substrate after the dissolution treatment of nanosilicas. The dissolution treatment improved the SEM image of the SWCNT film remarkably (Figure S9) and gave a predominant reduction of the film resistance from 107 to 102 Ω. The atomic force microscopic observation shows that the SWCNTs form a nanonetwork on the PET film which leads to the considerably high sheet conductivity (Figure S10 in Supporting Information). The sheet resistance was in the range of 150−200 Ω/□ (with a 90% high transparency at 550 nm after subtraction of absorption by the PET film itself). Thus, the novel dispersion technology of SWCNTs with nanosilica improved both the conductivity and transparency. However, still we need to improve the methods of the film production and removal of the nanosilica to attain the application. There should be other possibilities of developing novel hybrid materials from SWCNTs using nanosilica. The hybrid material made by this novel dispersion method should offer superior thermal stability and robustness, since the nanosilica itself has these properties and the charge transfer between CNT and nanosilica can offer excellent strength and flexibility.
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ASSOCIATED CONTENT
S Supporting Information *
Thermogravimetric plots and Raman spectra of SWCNTs and MWCNTs, dispersion state of MWCNTs in nanosilica solution, the normalized optical absorption spectra of SWCNT films treated with nanosilica, curve fitting of the Gband of SWCNTs dispersed in silica solutions, the G-band peak position of SWCNT dispersion in nanosilica solution with varying concentrations, TEM images of SWCNTs dispersed in a 0.05 wt % nanosilica solution, XPS C 1s spectra of SWCNTs and SWCNT freeze dried from a 0.001 wt % nanosilica solution, SEM images of SWCNT film, and atomic force microscope image of SWCNTs on the PET film. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel +81-26-269-5742; Fax +81-26-269-5737; e-mail
[email protected] (K.K.). Author Contributions
T.M., D.M., T.F., M.S., H.I., H.O., and K.K. planned the experimental research; F.K., S.U. and T.I. purified and characterized the CNTs; D.M. and M.N. measured the zeta potential; T.H. and E.M. performed the TEM observation; Y.H. prepared the SWCNT. T.M., D.M., and K.K. drafted the manuscript, and all authors participated in the writing and review of the final draft. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was funded by the Development of Innovative Carbon Nanotube Composite Materials for a Low Carbon Emission Society of NEDO, Japan. D.M., T.F., M.E., and K.K. were supported by Exotic Nanocarbons and by the Japan Regional Innovation Strategy Program by the Excellence (JRISE), JST. 3200
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