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Apr 28, 2016 - Fujitsu Laboratories Ltd., Atsugi 243-0197, Japan. •S Supporting Information. ABSTRACT: In this study, high-purity single-chirality (...
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Single-Chirality Separation and Optical Properties of (5,4) Single-Wall Carbon Nanotubes Xiaojun Wei,† Takeshi Tanaka,† Naoto Akizuki,‡ Yuhei Miyauchi,‡,§,∥ Kazunari Matsuda,‡ Mari Ohfuchi,⊥ and Hiromichi Kataura*,† †

Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan ‡ Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan § PRESTO, Japan Science and Technology Agency, Honcho Kawaguchi, Saitama 332-0012, Japan ∥ Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan ⊥ Fujitsu Laboratories Ltd., Atsugi 243-0197, Japan S Supporting Information *

ABSTRACT: In this study, high-purity single-chirality (5,4) and (6,4) single-wall carbon nanotubes (SWCNTs) were efficiently extracted using gel chromatography, and their optical properties were investigated. In the photoluminescence spectrum, the (5,4) SWCNTs exhibited multiple emission peaks, such as E*11 at 968 and E*11− at 1096 nm, in addition to the original E11 peak at 829 nm for the E22 excitation. Although the (6,4) SWCNTs also exhibited similar emission peaks, the intensity of the additional peaks relative to the E11 peak was approximately four times lower than that of (5,4) SWCNTs, even though the same separation procedure was used for both species. The results from density functional theory calculations indicated that both the E11 * and E11 *− emissions originated as emissions from the oxide states. Because no oxidation treatments were performed on the samples, these oxides were most likely spontaneously produced during the sample preparation process. Theoretical results indicated that the (5,4) SWCNT possesses a more stable ether-a-O structure that could cause the E*11 emission. The difference in the stability of the oxide states for (5,4) SWCNT and (6,4) SWCNT strongly suggests that the (5,4) SWCNT is more likely to be oxidized by weak stimulation. These results indicate that the (5,4) SWCNT remained near the outside of the robust structure due to its strong curvature effects.

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no report about the smaller SWCNT to date.15−17 As the gel column has a higher affinity to smaller diameter semiconducting SWCNTs, our aim herein was to separate singlechirality SWCNTs with smaller diameters than in our previous studies and discuss their stability. In this study, we optimized the separation parameters of the previous method to obtain small diameter SWCNTs with high resolution. As a result, highpurity (5,4) and (6,4) SWCNTs were efficiently separated, despite their very low concentration in the pristine SWCNT dispersion. The (5,4) SWCNTs exhibited some interesting optical properties, such as highly softened transverse optical (TO) phonons18−20 in the Raman spectrum and bright PL emissions from naturally produced oxide states, which may be due to the strong curvature effects in (5,4) SWCNTs. Density functional theory (DFT) calculations were performed to gain insight into the adsorption of oxygen molecules onto the (5,4)

INTRODUCTION Single-wall carbon nanotubes (SWCNTs) are considered one of the most fascinating materials for various potential applications because of their unique electrical, optical, and physical properties.1,2 For example, semiconducting SWCNTs exhibit bright near-infrared E11 photoluminescence (PL) via E22 excitation.3,4 The E11 energy is inversely proportional to the diameter of the SWCNT.5−7 Very small diameter SWCNTs8,9 could be used in visible electroluminescence device applications. However, such thin SWCNTs cannot be isolated from the template due to their chemical instability. To the best of our knowledge, the thinnest single-chirality SWCNTs that can be prepared are (5,4) SWCNTs (0.62 nm in diameter)10 using density-gradient ultracentrifugation (DGU)11 and DNA-controlled aqueous two-phase extraction (ATP).12 Previously, we developed a gel chromatography method that was employed to successfully separate single-chirality semiconducting SWCNTs.13,14 At that time, the smallest diameter SWCNT was the (6,4) SWCNT. After that, gel-based singlechirality separation was well continued and improved, but still © 2016 American Chemical Society

Received: March 30, 2016 Revised: April 27, 2016 Published: April 28, 2016 10705

DOI: 10.1021/acs.jpcc.6b03257 J. Phys. Chem. C 2016, 120, 10705−10710

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The Journal of Physical Chemistry C

excitation spectrum of the E11 emission, a charge-coupled device (CCD) was used to improve the signal/noise ratio. The Raman spectra were measured using a single monochromator (CRP-200MS for (5,4) SWCNTs and M331-TP for (6,4) SWCNTs, Bunkokeiki, Japan) equipped with a CCD detector and appropriate edge filters. The (5,4) and (6,4) SWCNTs were excited at 488 and 561 nm, respectively. The excitation power density dependence of the PL spectra was measured using a continuous wave Ti-sapphire laser at an excitation wavelength of 738 nm for the (5,4) SWCNTs and 838 nm for the diazonium-functionalized (6,5) SWCNTs. The PL signal intensity was recorded using a monochromator (SP2500, Princeton Instruments) equipped with a liquid nitrogen-cooled CCD for (5,4) SWCNTs and an InGaAs array for (6,5) SWCNTs.

and (6,4) SWCNTs. Interestingly, the results explained our experimental observations of the oxide state-derived PL characteristics and suggested that the (5,4) SWCNT is more likely to be oxidized than the (6,4) SWCNT.

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EXPERIMENTAL SECTION Preparation of SWCNT Dispersion. SWCNTs that were produced by high-pressure catalytic CO decomposition (HiPco, Raw, Batch #: R1−831 (@ 5.2 wt % solid), 0.8−1.2 nm in diameter, NanoIntegris) were used as the starting material without additional treatment. A net 100 mg of SWCNT powder (1923 mg wet cake) was dispersed in 100 mL of a 2.75 wt % sodium dodecyl sulfate (SDS, ultrapure ≥99.0%, SigmaAldrich) aqueous solution using a tip-type ultrasonic homogenizer (Branson, Sonifier 250D, output power of 29 ± 1% W) for 20 h in a 20 °C water bath (the determination of SDS concentration, see Supporting Information, Figure S1). After ultrasonication, the solution was ultracentrifuged to remove bundles and impurities (Hitachi Koki, CS150GX, S50A rotor, 210000 × g for 2 h at 25 °C). The upper 80% of the supernatant was recovered as an SWCNT dispersion. For a high-yield separation, the following separation procedure was performed within 48 h of ultracentrifugation. Separation of (5,4) SWCNTs. Five open columns filled with 1.4 mL of Sephacryl S200 gel (GE Healthcare) were vertically connected (totaling 7 mL) for the first round of multicolumn separation (Supporting Information, Figure S2). Here a 5 mL plastic syringe with a diameter of 12 mm was used as a column. After equilibration of the multicolumn using a 2.75 wt % SDS solution, 80 mL of the SWCNT dispersion was loaded into the top of the multicolumn. Because the volume of SWCNTs was much greater than the total capacity of the gel columns, excess SWCNTs flowed through the bottom column and were recovered for the next round of multicolumn separation. After washing with the 2.75 wt % SDS solution, the multicolumn was separated into individual columns. The SWCNTs that were adsorbed to each column were eluted with a 5 wt % SDS solution. The four new columns were filled with 1.4 and 0.7 mL of the fresh gel for the second and third rounds, respectively, of multicolumn separation. All separation procedures in the second and third round are identical with the first round. The reduction of the gel volume in the third round was to get higher purity (6,4) SWCNTs. The (5,4) and (6,4) SWCNTs were obtained in the first (columns 1−5) and third rounds of separation (columns 10−12), respectively (Supporting Information, Figure S3). The (5,4) SWCNT dispersion was further concentrated and purified using another column. The fractions from columns 1−5 were combined and diluted with the same amount of water. The resulting (5,4) enriched SWCNT dispersion in 2.5 wt % SDS was injected into a column (2 mL Sephacryl S200 gel) equilibrated with 2.5 wt % SDS, and the adsorbed SWCNTs were then eluted using a 5 wt % SDS solution. The unadsorbed materials were confirmed as the impurities, such as amorphous carbon (Supporting Information, Figure S4). For the (6,4) SWCNTs, the same procedure was performed using the fractions from columns 10−12. Optical Measurements. The optical absorption spectra were measured using an ultraviolet−visible-near-infrared spectrophotometer (UV-3600, Shimadzu). Excitation−emission mapping of the PL spectra was performed using a spectrofluorometer (Nanolog, HORIBA) equipped with a liquid nitrogen-cooled InGaAs array detector. To measure the

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COMPUTATIONAL METHODS DFT calculations were carried out using pseudoatomic orbitals (PAOs)21 implemented in the OpenMX code. The exchangecorrelation potential was processed using the Perdew−Burke− Ernzerhof generalized gradient approximation (GGA-PBE).22 The electron−ion interaction was treated using normconserving pseudopotentials23 with partial core correction.24 We used the PAOs specified by C-s2p2d1 and O-s2p2d1, where C and O are atomic symbols for carbon and oxygen, respectively, and s2p2d1 indicates the employment of two, two, and one orbitals for the s, p, and d components, respectively. The cutoff radii of the PAOs were set to 7.0 Bohr for both the C and O atoms. The convergence criterion of the forces on atoms for all of the geometric optimizations was set to 0.1 eV/ nm. The adsorption energy (Ea) for the O2 molecules is defined by Ea = E(CNT + 2O) − E(CNT) − E(3O2), where E(CNT + 2O) is the total energy of the CNT adsorbed with two O atoms and E(CNT) and E(3O2) are the energies of the CNTs and the triplet O2, respectively. The nudged elastic band (NEB) method25 was used to calculate the reaction path. Additional details for the computational method are provided in ref 26.

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RESULTS AND DISCUSSION We have improved the diameter selectivity of the multicolumn gel chromatography method by increasing the SDS concentration in the SWCNT solution and the total amount of SWCNTs injected into the columns. Consequently, the (5,4) SWCNTs, which have a smaller diameter than that of C60 (Supporting Information, Figure S5), were easily separated. We also separated high-purity (6,4) as a reference. Figure 1a shows the optical absorption spectra of the obtained (5,4) and (6,4) solutions. A series of sharp absorption peaks corresponding to the E11, E22, E33, and E44 transitions were observed in both spectra. Additionally, some weak peaks were observed at 728 nm for (5,4) and 760 nm for (6,4), which are well-known as phonon sidebands27,28 of the E11 transition. The same peaks were also observed in the excitation spectra of the E11 emission. For the (6,4) SWCNTs, the E33 transition energy is higher than the E44 transition energy due to trigonal warping effects in small diameter SWCNTs.29 Figure 1b shows the Raman spectra of the (5,4) and (6,4) SWCNTs. For the (5,4) SWCNTs, the excitation wavelength was selected as 488 nm, which is near the E22 transition, for the best resonance effect. Only one radial breathing mode (RBM) peak at 369 cm−1 and a highly softened TO phonon (G− band) at 1495 cm−1 were observed. Similar Raman peaks were also observed for the (6,4) SWCNTs at 339 10706

DOI: 10.1021/acs.jpcc.6b03257 J. Phys. Chem. C 2016, 120, 10705−10710

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Importantly, because the PL excitation signal contains the PL efficiency, this good agreement between the spectra indicates that the E11 emission efficiencies for the E22, E33, and E44 excitations are nearly the same, even though the relaxation processes should be different. Further, the phonon sideband peak E′22 that corresponds to the E22 excitation was also observed in both the optical absorption and E11 PL excitation spectra, which has rarely been reported to date. The purity of each (n,m) species was estimated as an integrated intensity ratio of the absorption peaks of the target chirality to the sum of all of the other peaks using the PeakFit software, where Lorentzian function was adopted (Supporting Information, Figure S6). As a result, the purities of separated (5,4) and (6,4) SWCNTs were estimated to be 94 and 91%, respectively. The purity of (5,4) SWCNTs is higher than that in previous studies, such as those using DGU11 and DNA-controlled ATP12 methods. Both samples are more than 90% pure, which enables us to reliably investigate their detailed characteristics. Figure 2a,b shows the PL excitation−emission mappings of the (5,4) and (6,4) SWCNTs measured using an InGaAs array

Figure 1. (a) Optical absorption spectra of the separated (5,4) SWCNTs (upper panel) and (6,4) SWCNTs (lower panel) by multicolumn gel chromatography. Inset: photographs of two samples (2 mL). (b) Raman spectra including the RBM region and the G band region at an excitation wavelength of 488 nm for the (5,4) SWCNTs (upper panel) and 561 nm for the (6,4) SWCNTs (lower panel). (c, e) PL contour maps of the (5,4) and (6,4) SWCNTs. (d, f) Comparison of the absorption spectra (blue) and the excitation spectra (red) for the E11 emission at 837 nm for the (5,4) SWCNTs and at 879 nm for the (6,4) SWCNTs.

and 1529 cm−1, respectively, which indicated good agreement with the results from (6,4) SWCNTs in a previous study.20 The frequency of the G− peak of the (5,4) SWCNTs was lower than that of the other single-chirality SWCNTs.13,20 Because the direction of the atomic displacement of the TO phonon is perpendicular to the SWCNT axis, the curvature effect predominantly softened the force constant of the TO phonon. The unusually softened TO phonons of the (5,4) SWCNTs directly indicates a strong curvature effect, which has not been reported previously. Figure 1c,e show the PL excitation−emission maps of the (5,4) and (6,4) SWCNTs, which were recorded using a CCD detector. In general, the Eii (E11, E22, E33, E44∼) excitation E11 emission peaks dominate the PL maps. In addition, the excitation peak that corresponds to the phonon sideband E′11 was also as expected.27,30 If the separated sample contains only one species of SWCNTs, the optical absorption spectrum should be consistent with the excitation spectrum of the E11 emission. Figure 1d and f for the (5,4) and (6,4) SWCNTs, respectively, show a comparison of the E11 excitation and absorption spectra. Very good agreement between the optical absorption and PL excitation was observed for each sample, except for a few peaks, which occurred due to impurities.

Figure 2. (a,b) PL contour maps of the (5,4) and (6,4) SWCNTs. The color scales are modulated using the maximum intensity of E22 excitation−E11 emission peaks.

detector. The bright PL peaks result in a rectangular pattern in both maps. The E11 emission peaks for the E11/E22 excitation and the phonon sidebands were observed at 829 nm for the (5,4) SWCNTs and 877 nm for the (6,4) SWCNTs. In addition to the E11 emission, a similar series of emission peaks were observed at 968 nm for the (5,4) SWCNTs and 1028 nm for the (6,4) SWCNTs (denoted as E11 * ). The intensity of these peaks for the (5,4) SWCNTs was much higher than that of the (6,4) SWCNTs. The energy differences between E11 and E*11 were 215 meV for the (5,4) SWCNTs and 208 meV for the (6,4) SWCNTs. Only for the (5,4) SWCNTs were additional 10707

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The Journal of Physical Chemistry C emission peaks observed at 1096 nm (364 meV lower than E11, denoted as E*11−). However, the intensity of E*11− was much lower than that of E11 * . For the (6,4) SWCNTs, the presence of a E11 *− peak was not firmly established due to its expected weakness. Figure 3a shows the PL spectra of the (5,4) SWCNTs with various excitation power densities (0.087−522 kW/cm2), where

Figure 4. Adsorption energy for the reaction path from physi-b to the ether-a-O + epoxy-b-O structure. The directions of bonds “a”, “b”, and “c” are described using the three black lines shown in the physi-b structure. Inset of (a): the structure of ether-a-O2.

Figure 3. (a) Excitation power density dependence of the emission spectra of the (5,4) SWCNTs, where the excitation wavelength was fixed at 738 nm. (b) Plots of the integrated PL intensities of E11 (blue) * (red) as functions of the excitation power density. Dashed and E11 lines indicate the linear power density dependence. Inset: plots of the * intensities of the oxygen-doped (6,5) SWCNTs.34 E11 and E11

to the cyclo-b structure (50.4 and 42.4 kcal/mol for the (5,4) and (6,4) SWCNTs, respectively), which indicates that the epoxy-a-O2 state is stable for both the (5,4) and the (6,4) SWCNTs. In addition, the energy gain between the forward and reverse reactions for the (5,4) SWCNT (16.6 kcal/mol for epoxy-a-O2 and 21.1 kcal/mol for ether-a-O2 (shown in Figure 4a, inset)) is larger than that for the (6,4) SWCNT (8.9 kcal/ mol for epoxy-a-O2). This result indicates that the (5,4) SWCNT is more likely to be oxidized than the (6,4) SWCNT under the same conditions. After reaching the epoxy-a-O2 structure, the oxygen atoms most likely proceed to the ether-aO + epoxy-b-O structure for both SWCNTs because the Eb from epoxy-a-O2 to the ether-a-O + epoxy-b-O structure is not high. The ether-a-O and epoxy-b-O structures can generate the respective emission due to their different band gaps (1.06 and 0.69 eV for ether-a-O and epoxy-b-O structures, respectively).26 Therefore, our observed E*11 and E*11− in the (5,4) SWCNTs may be due to the ether-a-O and epoxy-b-O structures, respectively. This result is in good agreement with previous results obtained for oxygen-doped (6,5) SWCNTs.35,36 To visualize the detailed PL emission spectral structure, a conventional spectral analysis was performed. Figure 5a,b shows the emission spectra of (5,4) and (6,4) excited by the E22 wavelength (486 nm/2.55 eV for (5,4) and 585 nm/2.12 eV for (6,4)). Each spectrum was fitted using Voigt functions. Both fitted spectra (dashed curves) are consistent with the experimental spectra (solid curves). Both emission spectra of the (5,4) and (6,4) SWCNTs consist of seven peaks, and the details of the individual peaks are shown in Table 1. Five PL peaks (denoted as E11, E−11, E*11+, E*11, and E*11−) were identified as emissions from the intrinsic E11 state and related oxide products based on their energy shifts (ΔE) from E11 as defined using DFT results for oxygen-doped (6,5) SWCNTs at low temperatures from a previous study.35 However, two peaks remain unclear (denoted as “?”). Because the same multiple PL peaks were observed for both the (5,4) and (6,4) SWCNTs, the main oxide products are the same for both SWCNTs. However, the PL intensity ratio between the pristine state and the oxide state are quite different. The integrated PL intensity of each PL peak was normalized to the corresponding E11 intensity, where

the E11 and E*11 peaks are observed. Because the PL intensity was normalized by its E11 peak, the relative intensity of E11 * gradually decreased as the excitation power density increased. The integrated PL intensities of E11 and E*11 were plotted as a function of the excitation power density and are shown in Figure 3b. The PL intensities increased nearly linearly as the excitation power density increased for both E11 and E11 * in the low power density regime and were saturated in the high power density regime. A similar nonlinear PL characteristic was also observed for oxygen-doped (6,5) SWCNTs (Figure 3b, inset) and diazonium-functionalized SWCNTs (Supporting Information, Figure S7). The PL saturation of E*11 was more significant than that of E11 based on the relative intensity decrease of E*11 shown in Figure 3a. This difference in the saturation behavior of E11 * and E11 can be explained by the difference in the exciton states between a localized (oxide) state and the free states in SWCNTs.31−36 To confirm that E*11 originates from a naturally produced oxide in the sonication process, we treated the (5,4) SWCNTs with hydrogen peroxide and measured the PL map again. We observed a higher E*11 intensity than the original one (Supporting Information, Figure S8). These results strongly suggest that the E11 * emission in our (5,4) SWCNTs is the emission from a localized O-doped exciton state, although an oxidation process was not explicitly performed. DFT calculations for O2 physisorbed onto the (5,4) and (6,4) SWCNTs were performed to gain insight into the oxidation reaction of SWCNTs. Figure 4a,b shows the adsorption energy (Ea; see computational methods) for the reaction path from physi-b to the ether-a-O + epoxy-b-O structure as a function of the reaction procedure. For both SWCNTs, the reaction barriers (the adsorption energy difference, which is defined as Eb) of the forward reaction from the physi-b structure to the cyclo-b structures (33.8 and 33.5 kcal/mol for the (5,4) and (6,4) SWCNTs, respectively) were lower than those of the reverse reaction from epoxy-a-O2 10708

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CONCLUSION We separated single-chirality (5,4) SWCNTs using single-step multicolumn gel chromatography to study the optical properties of small-diameter SWCNTs. In addition to the E11 emission, multiple emissions from the oxide states were observed for the (5,4) and (6,4) SWCNTs without any additional oxidation treatment. Importantly, the integrated intensity of each oxide state-derived emission peak of the (5,4) SWCNTs was much higher than that of the (6,4) SWCNTs. The results from DFT calculations confirmed that smalldiameter SWCNTs with highly curved side walls are more likely to be oxidized. Currently, (5,4) is the smallest diameter SWCNT that can be obtained in single-chirality form. We propose that the (5,4) structure is most likely near the border of the robust structure of SWCNTs under normal ambient conditions. SWCNTs smaller than (5,4) may be easily oxidized in air without special treatment. Therefore, to achieve visible light emitting devices using very small diameter SWCNTs, their air stability is an important consideration.

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Figure 5. (a,b) Emission spectra of the (5,4) and (6,4) SWCNTs fitted by peak decomposition procedures using a Voigt function. The excitation wavelengths are 486 nm (E22) and 585 nm (E22) for the (5,4) and (6,4) SWCNTs, respectively. The red solid and black dashed curves correspond to the experimental and fitted spectra, respectively. *Second-order Rayleigh scattering.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03257. Details of (5,4) SWCNT separation, diameter comparison with C60, purity evaluation, excitation power dependence of PL intensity, and H2O2-doping treatment (PDF).

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the E11 intensity values are represented as 100. The relative PL intensity from the oxide states of the (5,4) SWCNTs was much higher than that of the (6,4) SWCNTs. For example, the *− for the (5,4) SWCNTs was 7.8× higher relative intensity of E11 than that for the (6,4) SWCNTs. This difference strongly suggests that (5,4) SWCNTs possess a more oxidative nature. We examined the oxidation probability of the chiralityseparated small-diameter SWCNTs by considering the unexpected emission from the oxide states. The results from the experimental observations and DFT calculations demonstrated that the oxide state-derived PL characteristics of the (5,4) SWCNTs differed from those of both the (6,4) SWCNTs and other larger-diameter (n,m) species that have been separated in our previous studies. The fundamental difference in the stability between the (5,4) and (6,4) SWCNTs suggests that the (5,4) chirality is near the boundary of the robustness of the SWCNT structure due to its strong curvature effects.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: + 81 29 861 2551. Fax: + 81 29 861 2786. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI, Grant No. 25220602. REFERENCES

(1) De Volder, M. F.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon nanotubes: present and future commercial applications. Science 2013, 339, 535−539. (2) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing. Chem. Soc. Rev. 2013, 42, 2824−2860.

Table 1. Assignment of the Fitted Peaks as Well as Their Energy, the Shifted Energy (ΔE), and the Integrated PL Intensity

a

peak number

1

2

3

4

5

6

7

(5,4)

peak name energy (eV) ΔE from E11 relative integrated PL intensitya peak number

E11 1.496 0 100 1

E−11 1.468 0.028 79.3 2

E*11+ 1.364 0.132 35.4 3

E*11 1.282 0.214 73.1 4

? 1.232 0.264 47.2 5

E*11− 1.134 0.362 9.4 6

? 1.106 0.390 6.6 7

(6,4)

peak name energy (eV) ΔE from E11 relative integrated PL intensitya

E11 1.417 0 100

E−11 1.381 0.036 24.9

E*11+ 1.277 0.140 10.8

E*11 1.209 0.208 13.4

? 1.168 0.249 12.8

E*11− 1.115 0.302 1.2

? 1.035 0.382 1.1

These values contain approximately ±5% error due to the deviations from environmental effect, sensitivity of detector, and peak fitting. 10709

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DOI: 10.1021/acs.jpcc.6b03257 J. Phys. Chem. C 2016, 120, 10705−10710