Plasmon-Induced Selective Oxidation Reaction at Single-Walled

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Plasmon-Induced Selective Oxidation Reaction at Single-Walled Carbon Nanotubes Satoshi Yasuda, Takahiro Yoshii, Shohei Chiashi, Shigeo Maruyama, and Kei Murakoshi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07636 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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ACS Applied Materials & Interfaces

Plasmon-Induced Selective Oxidation Reaction at Single-Walled Carbon Nanotubes Satoshi Yasuda,*† Takahiro Yoshii,† Shohei Chiashi,‡ Shigeo Maruyama,‡ and Kei Murakoshi*†

†

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-

0810, Japan ‡

Department of Mechanical Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-

8656, Japan

Keywords: Carbon nanotube, plasmon, reactive oxygen species, defect, nanoprocessing

ACS Paragon Plus Environment

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ABSTRACT

Local surface plasmon resonance (LSPR)-induced oxidation of semiconducting and metallic single-walled nanotubes (SWNTs) on the nanometer scale was investigated using surface enhanced Raman scattering (SERS) measurements. An isolated SWNT was supported on a well-defined Au nanodimer structure that possesses an LSPR field at the nanogap under light irradiation, and highly intense SERS spectra of the SWNT at the gap region were measured. SERS analysis under O2-saturated solutions and the addition of reactive oxygen species inhibitors demonstrated that condensed singlet oxygen (1O2), which is one of the reactive oxygen species, was efficiently generated from a semiconducting SWNT at the nanogap by the LSPR field, and led to local oxidation of the tube. In contrast to the semiconducting SWNT, no defect formation was observed in a metallic SWNT, probably because of rapid quenching of the photo-excited state. This selective local defect formation by LSPR-induced oxidation of a semiconducting SWNT would provide novel nanoprocessing and nanofunctionalization methods for the fabrication of future SWNT-based nanodevices.


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INTRODUCTION Since their discovery by Iijima,1 single-walled carbon nanotubes (SWNTs) have shown promise as nanomaterials because they possess well-defined electronic states originating from van Hove singularities. In addition, SWNTs can exhibit either metallic or semiconducting characteristics, which depend on the chirality of the tube. These specific properties allow researchers to utilize SWNTs for novel catalytic, nanoelectric and photo-applications.2–9 A significant key step toward the nanofabrication and realization of SWNT-based nanodevices is the development of a local oxidation process, which will allow accurate cutting of the SWNT and localized functionalization at a desired location on the sidewall surface. Until now, the oxidation of ensemble SWNTs has been widely investigated using strong acids10 and electrochemical,11 plasma-treatment,12,13 gas-phase14 and photochemical reaction methods.15–17 The oxidation reactions in these processes lead to the opening of the tube caps and formation of holes in the sidewalls. However, the oxidation reactions generally proceed throughout the entire tube, and control of local oxidation reactions at a desired position on the tube surface is impossible. Nanoprocessing of individual SWNTs was achieved for the first time using scanning tunneling microscopy (STM) at 4 K.18 The perturbation generated by applying a high bias voltage between the STM tip and the SWNT induced local chemical or desorption reactions, and resulted in the cutting of the SWNT and defect formation in the desired location. Reversible defect engineering of an SWNT on the nanometer scale was also successful under ultra-high vacuum at 5K.19 However, despite this vigorous research, there have been few experimental studies on local oxidation methods for SWNTs on the nanometer scale, or on the selectivity between metallic and semiconducting SWNTs. Further development of novel methods based on other principles is thus still desired. One candidate method involves adapting the photochemical


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reaction process based on the local surface plasmon resonance (LSPR) of a nanometal structure. The LSPR is the surface electromagnetic waves generated at a nanometal surface through the oscillation of the free electrons in the metal structure, and is induced by interaction with irradiated light. In particular, metal nanodimer structures with a nanogap produce strong LSPR fields in the gap under light irradiation owing to coupling of the LSPR of each structure. Because the LSPR field corresponds to condensed light in the nanometer vicinity of the metal surface it has received much attention as a key phenomenon that may lead to a breakthrough in novel LSPR-induced photochemical effects on the nanoscale. For example, it enables the detection of strong Raman scattering of a small number of molecules with high sensitivity, known as surface enhanced Raman scattering (SERS).20–22 Nonlinear photo-polymerization by multi-photon photochemical reactions and trapping of matter, such as polystyrene beads, polymer chains and DNA, can also be achieved using the LSPR field.23–25 Recently, strong coupling between the LSPR and individual SWNTs has been reported, and the breakdown of the selection rules in electronic optical transitions of an SWNT in an LSPR field was demonstrated for the first time.26 This study focused on the local characteristics and photochemical reactivity of the LSPR, and the LSPR was applied as a novel method for the oxidation of SWNTs on the nanometer scale. The LSPR also enables in situ SERS monitoring during LSPR-induced photoreactions. In-situ SERS measurements showed that semiconducting SWNTs (s-SWNTs) generated locally condensed singlet oxygen (1O2), a reactive oxygen species (ROS), under the LSPR field, and subsequent local defect formation in the tube, caused by the oxidation reaction, was clearly observed. In contrast to s-SWNTs, no defect formation was observed for metallic SWNTs (m-SWNTs), which revealed the selective local oxidation of s-SWNTs by the LSPR-induced photoreaction.


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RESULTS AND DISCUSSION First, to produce the LSPR field, well-defined Au nanodimer arrays were fabricated on the indium tin oxide (ITO) glass surface using an angle-resolved nanosphere lithography (NSL) technique.22,27 As shown in Figure 1(a), the length of one side of the Au monomer in the dimer was approximately 40 nm with a height of 30 nm, and the gap between the monomers was a few nm. The dimer structures were controlled to obtain maximum extinction at around 800 nm for incident light parallel to the long axis of the dimer (Figure 1(b), solid red line), because a 785 nm light source was used to excite the Raman scattering of the SWNT. Using incident light

Figure 1. (a) Scanning electron microscopy image of Au nanodimer arrays on indium tin oxide (ITO) glass fabricated using an angle-resolved nanosphere lithography technique. (b) Polarized extinction spectra of Au nanodimer arrays on an ITO substrate. (c) Experimental set-up for surface enhanced Raman scattering (SERS) measurement.


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perpendicular to the long axis of the dimer resulted in the disappearance of the extinction at around 800 nm (Figure 1(b), blue dotted line). The strong extinction when using parallel incident light was attributed to the formation of the LSPR field at the nanogap by excitation with 785 nm light. After fabrication of the Au nanodimer arrays on the ITO substrate, individual SWNTs were placed on the arrays using a simple dispersion process.28 To elucidate the LSPR-induced chemical reactivity of SWNTs, SERS measurements were conducted in combination with an electrochemical system (Figure 1(c)) in O2- or N2-saturated electrolyte solutions. (see the Experimental section). Figure 2 shows the dependence of the SERS spectra of an isolated s-SWNT on time and excitation laser intensity in (a) O2- and (b) N2-saturated 0.5 M H2SO4 electrolyte solution. The

Figure 2. Surface enhanced Raman scattering (SERS) spectra measured at 30 s intervals and normalized using the intensity of the G+ mode of an individual semiconducting single-walled nanotube (s-SWNT). The excitation laser intensity was changed every 180 s, and intensities of 0.1, 0.2, 0.4, 0.6, 0.9, 1.3 and 1.7 mW were used. (a) (9, 7) s-SWNT (ωRBM = 216 cm−1) in O2-saturated acidic solution (b) (10, 5) s-SWNT (ωRBM = 235 cm−1) in N2-saturated acidic solution.


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spectra were acquired at 30 s intervals and the excitation laser intensity was changed every 180 s; intensities of 0.1, 0.2, 0.4, 0.6, 0.9, 1.3 and 1.7 mW were used. As shown in Figure 2, three characteristic vibration peaks were clearly observed for both isolated SWNTs: the radial breathing mode (RBM), D mode and G mode.29 The RBMs observed at 216 cm−1 and 223 cm−1 correspond to the in-phase vibration in the radial direction and can be used as a fingerprint to identify the diameter, chirality and nature (s- or m-) of the SWNT.26 From the relationship between the experimentally obtained RBM frequency (ωRBM) and the excitation laser energy, which is an extended Kataura plot,30,31 the observed SWNTs could be identified as (9, 7) and (10, 5) s-SWNTs. The G mode vibration at around 1580 cm−1 stems from the in-plane stretching mode of the C–C bond in the graphene lattice structure. This mode splits into G+ and G− modes at higher and lower Raman frequencies; these are attributed to stretching along the tube axis and along the axis of the tube circumference, respectively.32 The shapes and peak positions provide indicators to allow us to distinguish between s- and m-SWNTs. For s-SWNTs, both peaks exhibit a Lorentzian shape, whereas in m-SWNTs the G− mode is shifted to lower frequencies and is strongly broadened owing to π–plasmon coupling with the G− mode (the Breit-Wigner-Fano (BWF) line shape).33 The Lorentzian shape of the G modes for both SWNTs shown in Figure 2 is clear evidence for their semiconductor character, and this is consistent with the assignment based on the RBM frequencies using the Kataura plot. The D mode (1250–1400 cm−1) is the dominant sp2 Raman signature of disorder, and is observed when the carbon material has a defect structure. The intensity ratio of the D mode to the G mode, or the D/G ratio, is commonly used to evaluate the degree of structural disorder of the honeycomb lattice of graphene.34 Because the SERS spectra were normalized using the G+ mode, the intensity of the D mode is equivalent to the degree of structural disorder. For the O2-saturated solution (Figure 2(a)), no D mode was


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observed on weak laser intensity irradiation (0.1–0.4 mW). However, the D mode appeared with 0.6 mW irradiation, and further increases in the intensity of the irradiation (> 0.6 mW) caused an increase in the intensity of the signal. With the strongest laser intensity irradiation (1.7 mW), the intensity of the D mode reached that of the G mode. However, the trend in the N2-saturated solution was markedly different to that in the O2-saturated solution. Even with intense laser irradiation, no increase in the intensity of the D mode was observed in the N2 saturated solution (Figure 2(b)). To determine the reversibility of the SERS spectrum after intense laser irradiation, the SERS spectrum was measured again at 0.1 mW after the intense irradiation (1.7 mW). Figure 3 shows a comparison of the SERS spectra of an s-SWNT in the O2-saturated solution at the beginning of the experiment (top spectrum, gray line) and after intense laser irradiation (bottom spectrum, black line). It is clear that although the irradiation intensity was reduced to 0.1 mW, a

Figure 3. Surface enhanced Raman scattering (SERS) spectra normalized using the intensity of the G+ band of an individual (10, 5) semiconducting single walled nanotube (s-SWNT) (ωRBM = 235 cm−1) in O2-saturated solution. Each spectrum was obtained with 0.1 mW irradiation intensity. Gray line: SERS spectrum obtained at the beginning of the experiment. Black line: SERS spectrum obtained after intense laser irradiation.


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strong D mode remained and the G+ mode was broadened. Previous studies have shown that the linewidth of the G mode is related to the crystallinity of the carbon material.34,35 Hence, the intense D mode and the broadening of the G+ mode indicate the formation of defects and amorphous structures in the graphene structure.36 Furthermore, a relatively large decrease in the RBM peak intensity was observed, which suggests that the tubular structure was deformed by the introduction of defects.10 Similar measurements were conducted for other chiral s-SWNTs under the same resonance condition, and all samples showed defect formation, suggesting that this is a universal phenomenon for s-SWNTs in O2-saturated solution. These results strongly indicate that LSPR-induced defect formation occurs in s-SWNTs in O2-saturated solution, but does not occur in N2-saturated solution. To elucidate the fundamental reaction mechanism of LSPR-induced defect formation, the effect of the electronic properties of the SWNTs was also investigated. The dependence of the SERS measurements on time and excitation laser intensity was therefore determined for mSWNTs in both O2- and N2-saturated solutions (Figure S1), and the results are summarized with those for s-SWNTs in Figure 4. The bars show the average D/G ratio during intense laser irradiation (1.7 mW). At the beginning of the experiment, the Raman spectrum was measured using weak laser irradiation (0.1 mW), and the D/G ratios were almost zero for all samples. After intense laser irradiation for 3 min, we found that the m-SWNT showed only a limited increase in the D/G ratio under both conditions, unlike the results observed for the s-SWNT in O2-saturated solution. These results provide clear evidence that the electronic character of the SWNT also plays a role in defect formation. The relevance of the LSPR field to the defect formation was also investigated. Without the Au nanodimer array, and therefore in the absence of the LSPR field, the intensity of the resonance Raman scattering (RRS) spectrum was clearly weaker than that of


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Figure 4. Bar graph of the average D/G ratio during 3 min laser irradiation at 1.7 mW. Samples were (9, 7) and (10, 5) semiconducting single-walled nanotubes (s-SWNTs) in O2and N2-saturated solutions, and (14, 8) metallic SWNTs (m-SWNTs) in O2- and N2-saturated solutions. The D/G ratios were almost zero for all samples prior to the intense laser irradiation. The error bars represent the maximum and minimum values of the D/G ratio for six spectra measured over 3 min.

the SERS spectrum (Figure S2). Raman measurement was carried out for isolated s- and mSWNTs in O2-saturated electrolyte solutions and showed no increase in the intensity of the D mode, even after long periods of intense laser irradiation (s-SWNT: 14 mW for 24 min, mSWNT: 15 mW for 21 min), indicating that no defects were formed (Figure S3). These results indicate that the LSPR field also plays a crucial role in defect formation. From these SERS measurements, we have clearly revealed that formation of defect structures occurred only in the s-SWNT in the O2-saturated solution under LSPR excitation. A possible origin of the defect formation is the photo-oxidation reaction caused by activated O2 under the LSPR field. In general, when photons with energies greater than that of


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the band gap are irradiated, the s-SWNT is excited and forms excited electrons and holes. In contrast, the excited states in m-SWNTs are immediately deactivated owing to the relatively fast non-radiative relaxation. Photo-excited electrons in the s-SWNT can transfer their energies to the surroundings. One well-known activation phenomenon is the photo-induced formation of reactive oxygen species (ROS). There are two main types of ROS: singlet O2 (1O2), which is generated by energy transfer, and the superoxide radical (O2·−), generated by electron transfer.37 Both types of ROS are highly reactive and can easily oxidize carbon materials. For example, 1O2 can react on the surface of SWNTs to give a 1,4-endoperoxide in which O2 is added across the benzene structure.38,39 O2·− is also expected to react with nanotubes.40 Murakami et al. reported that these ROS are indeed generated from O2 around photo-absorbed s-SWNTs.41 Considering these results, we can hypothesize that the photo-excited electrons induced by the LSPR field in the s-SWNT efficiently produce ROS at the nanogap, and play an important role in accelerating the local defect formation through oxidation reactions. To confirm our hypothesis and obtain further insight into the reaction mechanism, the effect of ROS on defect formation was evaluated by performing SERS measurements after addition of ROS inhibitors. Two types of ROS inhibitors were used in this study: mannitol and 1,4-diazabicyclo[2.2.2]octane (DABCO). Mannitol and DABCO are known to selectively quench O2·− and 1O2, respectively.41–44 Figure 5 shows a bar graph of the observed D/G ratio under intense laser irradiation (1.7 mW) for the s-SWNT in an O2-saturated electrolyte solution with and without ROS inhibitors. Before subjecting the samples to the intense laser irradiation, we confirmed that the D/G ratios were almost zero for all samples at low laser irradiation (0.1 mW). As shown earlier in Figure 4, a clear increase in the D/G ratio was observed in the absence of inhibitors (labeled ‘No inhibitors’ in Figure 5). When mannitol (O2·− quencher) was added (‘+


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Figure 5. Bar graph of the average D/G ratio during 3 min laser irradiation at 1.7 mW in O2saturated phosphate buffer solution with and without reactive oxygen species (ROS) inhibitors. Samples were (11, 3) semiconducting single walled nanotubes (s-SWNT) (ωRBM = 216 cm−1), (8, 6) s-SWNT (ωRBM = 244 cm−1) and (12, 1) s-SWNT (ωRBM = 241 cm−1) for ‘No inhibitors’, ‘+ Mannitol’ and ‘+ DABCO’, respectively.

Mannitol’ in Figure 5), the D/G ratio increased to a similar level to that observed for the sample containing no ROS inhibitors, indicating that O2·− makes only a minimal contribution to the LSPR-induced oxidation. However, when DABCO (1O2 quencher) was added, only a minor increase in the ratio was observed (‘+ DABCO’ in Figure 5). These results indicate that the generated 1O2 was effectively quenched by DABCO, and this inhibited the oxidation of the sSWNT. These findings provide clear evidence that 1O2 plays an important role in the LSPRinduced oxidation of the s-SWNT. Based on the results of our study, the following overall reaction is suggested. The LSPR field generates a considerable number of photoelectrons in the s-SWNT at the nanogap region, but these photoelectrons are not generated in m-SWNTs owing to rapid deactivation by nonradiative relaxation. The excited photoelectrons in s-SWNTs in the LSPR field in an O2-saturated


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solution transfer their energy to O2 adsorbed on the surface of the tube, efficiently forming 1O2.41 However, no 1O2 is generated in N2-saturated solutions because of the absence of O2. Concentrated 1O2 immediately reacts with the tube surface around the LSPR field and kinetically accelerates the production of 1,4-endoperoxide on the surface through a 1,4-cycloaddition reaction,38 resulting in localized defect formation.39 As described above, Murakami et al. have previously reported 1O2 generation from sSWNTs under light irradiation;41 however, they did not report any defect formation. This disagreement with our result is ascribed to the LSPR field. Because s-SWNT-enriched dispersed solutions were irradiated with near infrared light without an LSPR field in the study by Murakami et al., it is suggested that low concentrations of 1O2 were generated along the entire SWNT, and the subsequent oxidation reactions were too kinetically slow to allow observation of the defect formation. Laser-induced thermal heating might affect the defect formation in sSWNTs. Our previous study evaluated the LSPR-induced thermal heating effect of an SWNT at the nanodimer.45 The dependence of the local temperature increase on the laser power of the LSPR field was estimated from the frequency shift of the G mode of the SWNT. From the results, it is estimated that 1.7 mW laser irradiation (spot size ~1 µm2) corresponds to an approximately 80 °C rise in solution temperature. Although only the temperature rise is insufficient for thermal oxidation of the tube even in an O2 atmosphere, it might contribute to accelerate the chemical oxidation reaction related to 1O2. Quantification of the number of defect sites was attempted based on previous research using monolayer graphene. It is well known that the D/G ratio has an inverse relationship with the crystallite size, and the distance between defects (LD) can be estimated experimentally from the D/G ratio and the full width half maximum (FWHM) of the D and G modes.35 From the


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previous literature, LD was roughly estimated to be 1–2 nm when the D/G ratio was approximately equal to 1 with a D mode FWHM of approximately 40 cm−1, which corresponds to the most defective tube produced by 1.7 mW laser irradiation in our study. Assuming that an SWNT with a diameter of 1 nm is located in a strong LSPR field (approximate spot size of ~5 nm) at the Au pyramid-shaped nanostructure edge,46 the surface area of the graphene sheet in the SWNT is estimated to be approximately 15.7 nm2. This rough estimation suggests that a single or very few defects could be produced in the tube under the LSPR field by the oxidation process. Although further considerations, including tube chirality and the SERS effect,47 must be taken into account for accurate quantification, the present estimation implies the achievement of highly localized SWNT reactions through LSPR excitation.

CONCLUSIONS LSPR-induced local and selective oxidation of s-SWNTs was, for the first time, demonstrated using SERS measurements. Under the LSPR field, an s-SWNT in O2-saturated solution showed a clear increase in the intensity of the D mode peak arising from defect formation whereas the peak from an s-SWNT in a non-O2 containing environment did not show an increase in intensity. In contrast, no increase in the intensity of the D mode peak was observed for m-SWNTs, even in O2-saturated solution. SERS measurements with added ROS inhibitors revealed that condensed 1

O2, which is generated by excited s-SWNTs in the LSPR field, accelerates the chemical

oxidation reaction on the s-SWNT surface and causes local defect formation. Selective local defect formation in s-SWNTs has not been reported to date, and could become a novel method for the nanoprocessing of s-SWNTs in desired locations on the atomic scale. Although


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techniques allowing precise control and installation of plasmon-active nanometal structures on SWNTs, such as scanning probe techniques, are necessary for real nanoprocessing of SWNTs on the nanoscale, our findings provide proof-of-principle for the selective nanoengineering of SWNTs and may serve to open a route towards SWNT-based nanodevices.

EXPERIMENTAL METHODS Material preparation. SWNTs produced by an arc plasma jet method (Meijo Nano Carbon Co., Ltd) or by an alcohol catalytic chemical vapor deposition method (provided by Prof. Maruyama's group) were used.48 Each 10 µg sample of SWNTs was sonicated in 10 ml of 1,2dichloromethane or chloroform for 2 h, and then centrifuged for 2 min to remove excess SWNT bundles. The supernatants were diluted in 30 µl 1,2-dichloromethane or chloroform, and solutions containing individual SWNTs were dispersed onto the AuNSL/ITO substrate. Excess SWNTs were removed by rinsing several times with Milli-Q water. We previously confirmed that this process can support individual SWNTs on atomically flat Au surfaces by using STM.28 This simple process allows an isolated SWNT to be supported around the Au nanodimer.26 Analysis. Inverted backscattering geometry Raman spectroscopy with electrochemical control was used (Nanofinder 30; ×100 objective lens, ~1 µm spot diameter). Figure 1(c) shows a schematic diagram of the electrochemical Raman spectroscopy system. Prior to the experiments, O2 adsorbed on the SWNTs was eliminated by electrochemical reduction at a potential of -0.4 V vs. Ag/AgCl for 10 min. These conditions were required because of the strong affinity of O2 for the SWNT.49,50 Removal of the O2 adsorbed on the tube is necessary before starting the experiments because the presence of adsorbed O2 would influence the detailed analysis of the


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LSPR-induced oxidation reaction in this study. The advantage of SERS measurement in an electrolyte solution is not only the facile O2 elimination using electrochemical techniques, but also the effective thermal dissipation of SWNTs induced by laser irradiation.45 For the Raman spectroscopy measurements, linear polarized incident light parallel to the long axis of the Au dimer was irradiated to obtain a strong interplay between the SWNTs and the LSPR field, and all measurements were conducted without electrochemical control. To avoid effect of different SWNT nature, all SERS measurements were carried out using SWNT supported on AuNSL/ITO system that single RBM peak can be observed. The electrolyte solution was 0.5 M H2SO4. All experiments were conducted in O2- or N2-saturated electrolyte solutions under gas flow. Mannitol and DABCO, which quench O2·− and 1O2, respectively, were used as ROS inhibitors. Mannitol and DABCO were dissolved in standard phosphate buffer (pH 6.86 at 25 °C) at concentrations of 0.1 and 0.05 M, respectively, and the effect of ROS on the defect formation was evaluated by SERS measurements.


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SUPPORTING INFORMATION The supporting information is available free of charge on the ACS Publications website at DOI:???. Dependence of SERS on time and excitation laser intensity for m-SWNTs, effect of LSPR field enhancement on the Raman spectra, RRS time dependence of s- and m-SWNTs without the LSPR field (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail [email protected] *E-mail [email protected]

ORCID Kei Murakoshi : 0000-0003-4786-0115 Satoshi Yasuda : 0000-0002-3694-1641

Present Addresses Satoshi Yasuda


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Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Ibaraki, 319-1195, Japan E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This study was supported by the Japan Science and Technology Agency, PRESTO and JSPS KAKENHI Grant Numbers 15K05465, 26248001, and 16H06506 (in Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation.

ABBREVIATIONS LSPR, Local surface plasmon resonance; SWNTs, single-walled nanotubes; SERS; surface enhanced Raman scattering; STM, scanning tunneling microscopy; ROS, reactive oxygen species; ITO, indium tin oxide; NSL, angle-resolved nanosphere lithography; BWF, BreitWigner-Fano; RRS, resonance Raman scattering; DABCO, 1,4-diazabicyclo[2.2.2]octane; FWHM, full width half maximum.


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SYNOPSIS


27

Plasmon-Induced Selective Oxidation Reaction at Single-Walled

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