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Fundamental Importance of Background Analysis in Precise Characterization of Single-Walled Carbon Nanotubes by Optical Absorption Spectroscopy Shigekazu Ohmori,†,§ Takeshi Saito,*,†,‡,§ Masayoshi Tange,† Bikau Shukla,† Toshiya Okazaki,† Motoo Yumura,† and Sumio Iijima† Nanotube Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1 Higashi Tsukuba, Ibaraki 305-8565, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan ReceiVed: December 20, 2009; ReVised Manuscript ReceiVed: April 26, 2010
For precise characterizations of single-walled carbon nanotubes (SWCNTs) by optical absorption spectroscopy, the background extinction that originated from precipitable impurities and SWCNT bundles has been experimentally determined by the interval centrifugation and difference spectrum (IC-DS) technique. The baseline correction using the line shape of the obtained background extinction revealed the actual absorption spectrum of absolutely debundled SWCNTs with detailed features. The chirality distribution, including both semiconductive and metallic SWCNTs, was evaluated by deconvoluting the corrected absorption spectrum into multiple Lorentzian lines, which was well consistent with the result of photoluminescence (PL) mapping measurements. Another peculiarity is that the UV absorption characteristics of SWCNTs hidden in the original observed spectra by overlapping the background extinction also appeared. The baseline correction by the IC-DS technique provides a useful analysis method for characterizing SWCNTs, a complement/alternative to the similar analysis done by PL spectroscopy. 1. Introduction Because optical properties of single-walled carbon nanotubes (SWCNTs) potentially provide fruitful information on their characteristics, a considerable amount of their spectroscopic research has been carried out1-5 to date. Especially, because of its availability and versatility, the optical absorption property of SWCNTs has been frequently used for evaluating the purity,6 mean diameter,7 length,8 and chiralities.9 Recently, accompanying rapid research progress in the precise separation of SWCNTs,10,11 the optical absorption spectroscopy has been utilized as one of the characterization tools for the quantitative estimation of the metal/semiconductor ratio.12 In the optical absorption property of SWCNTs, uniqueness is the coincidence of both molecular- and bulklike features in their spectrum shape, that is, sharp interband electronic transition peaks1,13 and broad absorption from π-plasmon collective excitation,1 respectively. Such uniqueness arises from the ideal one-dimensional (1D) system of SWCNTs, and therefore, analyzing their absorption property is also highly important for a fundamental understanding of photophysics in 1D systems.1 A recent study using twophoton excitation spectroscopy reveals that their visible and near-infrared (vis-NIR) absorption peaks originate from excitons in nature3 as one of the characteristics of 1D systems. However, to date, analyzing the optical absorption property of SWCNTs has suffered from uncertainty due to the broad background that was conventionally attributed to a simple scattering.14 Especially, quantitative characterization for the characteristics of SWCNTs from their optical absorption peaks is extremely difficult because of the large background intensity * To whom correspondence should be addressed. E-mail: takeshi-saito@ aist.go.jp. Tel: +81-29-861-4863. Fax: +81-29-861-3392. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ PRESTO, Japan Science and Technology Agency. § S.O. and T.S. have equally contributed to this work.
overlapping with them. Recently, it has been reported that the intensity ratio between the excitonic absorption and the overlapping background extinction (absorption and scattering) increases as the degree of SWCNTs’ isolation is enhanced,15,16 indicating the considerable influence of bundled SWCNTs on the background. Thus, for background correction in the optical absorption spectroscopy of SWCNTs, such an effect of their bundles is worth considering besides that of the extinction from graphitic particles as one of the major origins of the background extinction. For preparing suspension solutions of debundled SWCNTs as specimens for optical absorption spectroscopy, the ultracentrifugation process is indispensable. The appropriate ultracentrifugation precipitates bundled SWCNTs and graphitic impurities, leaving debundled SWCNTs in the supernatant due to the slight difference in their buoyant densities.2 However, even after the ultracentrifugation process, the featureless and broad background extinction was still superposed upon the observed vis-NIR absorption peaks,1 probably due to the remaining unseparable few-bundled SWCNTs and/or the insufficient centrifugation interval. For an accurate and quantitative analysis of absorption properties of debundled individual SWCNTs, such a background extinction needs to be eliminated completely. So far, although a simple method for background correction for the optical absorption analysis of SWCNTs has been suggested by using a scattering model,9,14 detailed background correction considering the effects of both graphitic impurities and bundled SWCNTs has not been reported. In this work, we have investigated the variation in optical absorption spectra of SWCNTs based on the degree of debundled SWCNTs by the interval centrifugation and difference spectrum (IC-DS) technique. Namely, optical absorption spectra of specimens with a degree of debundled SWCNTs modulated by different intervals of centrifugation were analyzed by the difference spectrum technique. We have clearly distinguished
10.1021/jp9120172 2010 American Chemical Society Published on Web 05/17/2010
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the observed optical absorption into the contribution of debundled SWCNTs and that of the background. By curve fitting of the former one with multiple Lorentzian line profiles, assignments of SWCNTs’ chiralities, including both semiconducting and metallic ones, have been demonstrated, which was confirmed to be comparable and consistent with the result of photoluminescence (PL) mapping measurements. From the chirality distribution, ratios of the first-second interband transitions and metal-semiconductor have been also discussed. 2. Experimental Section SWCNTs synthesized by the gas-phase CVD growth, enhanced direct-injection pyrolytic synthesis (e-DIPS) method,17 were dispersed in D2O containing 1 wt % sodium cholate18 by using a tip ultrasonic homogenizer (SONICS VCX500) equipped with a titanium alloy tip (TI-6AL-4 V). Pulsed sonication was applied (on 1 s, off 2 s) with a power of 200 W for 0.5 h. The dispersion solution of SWCNTs was then settled in six centrifuge tubes and centrifuged at 127 600g (Hitachi CP 100MX with a P56ST swing rotor) to precipitate large bundled SWCNTs and graphitic impurities. For preparing specimens with different degrees of debundled SWCNTs, the interval centrifugation processes were carried out as follows: At first, the dispersed solutions in six centrifuge tubes were centrifuged for 0.5 h and the supernatant (upper 80% of the volume) in two centrifuge tubes was collected, which was denoted as sample A. The remaining four centrifuge tubes were again centrifuged at the same condition for 0.5 h, and similarly, the supernatant in two centrifuge tubes was collected, which was denoted as sample B. Finally, the remaining two centrifuge tubes were again centrifuged at the same condition for 1.5 h, and similarly, the supernatant was collected, which was denoted as sample C. Hence, prepared samples A, B, and C were centrifuged totally for 0.5, 1.0, and 2.5 h, respectively. Samples A-C were then subjected to optical absorption measurements (Hitachi, U-4000). Spectral data have been acquired in the wavelength region between 190 and 1800 nm, corresponding to the photon energy region between 6.53 and 0.689 eV. As a reference in each measurement, a solution of sodium cholate with the same concentration (1 wt %) was used. Sample C was also subjected to PL mapping measurements with a spectrofluorometer (Horiba Spex, Fluorolog 3-2 TRIAX) equipped with a near-infrared photomultiplier module (Hamamatsu H9170-75). The slit widths were set to 5 nm, and scan steps were 5 and 10 nm for excitation and emission, respectively. The accumulation time was set to 0.5 s at each point. The emission was collected in a backscattering geometry. The raw data were corrected for wavelength-dependent instrumental factors and excitation lamp intensities. 3. Results and Discussion Optical absorption spectra for samples A-C with different centrifugation intervals, 0.5, 1.0, and 2.5 h, respectively, are shown in Figure 1a. These observed spectra clearly show electronic transitions in the vis-NIR region, that is, absorption peaks resulting from the first and second interband transitions of semiconducting SWCNTs (S11 and S22) and from the first interband transition of metallic SWCNTs (M11). As shown in the inset of Figure 1a, absorption peaks in the NIR region became sharper with longer centrifugation times without showing any distinctive change in their shapes. Because the centrifugation precipitates SWCNT bundles, leaving debundled SWCNTs in the supernatant,2 sharpening of absorption peaks in the NIR region can be attributed to the increase in the degree
Figure 1. (a) Optical absorption spectra of samples A-C prepared by different centrifugation times, 0.5, 1.0, and 2.5 h, respectively. The inset shows their NIR absorptions of the first interband transition of semiconducting SWCNTs normalized at 1.1 eV. (b) Difference spectra derived from subtracting B or C from A or B, respectively, of absorption spectra in (a). (c) The corrected absorption spectrum that originated from debundled SWCNTs derived by subtracting the background that is proportional to the difference spectrum (B-C) derived from (b). The proportionality constant of the background has been deduced to be 0.43 so as to limit the intensity of the background under the observed spectrum.
of debundled SWCNTs. In other words, the percentage of bundled SWCNTs remaining in the supernatant is directly correlated to the broadening of NIR peaks and vice versa. On the other hand, the analysis of the variation in absorption peaks in the UV-vis region with centrifugation intervals is more vague than that in NIR due to their extremely broad spectrum shapes. To clarify the variability of the optical absorption property with the centrifugation interval, we have analyzed observed absorption spectra by using the difference spectrum technique. In Figure 1b, two difference spectra obtained by subtracting the absorption spectrum B or C from the spectrum
Background Analysis in Characterization of SWCNTs A or B, respectively, are shown. These difference spectra (A-B and B-C) represent the extinction originated from absorption and scattering of the precipitable dispersoid, including SWCNT bundles and graphitic impurities, between these intervals. Particularly the fact that vis-NIR peaks attributed to interband transitions near the Fermi level are clearly observed even in these difference spectra supports the inclusion of a considerable amount of bundled SWCNTs in the precipitated dispersoid. Figure 1a,b shows that the precipitable dispersoid gradually precipitates with increasing degree of debundled SWCNTs in the supernatant for over several hours at the present centrifugation condition. Therefore, some precipitable bundles of SWCNTs would still remain in the supernatant even after 2.5 h of ultracentrifugation. Interestingly, these two difference spectra of A-B and B-C were nearly identical in spectrum shape, as shown in Figure 1b, which suggests that the qualitative extinction property of the precipitable dispersoid is invariable in the present ultracentrifugation processes. Therefore, assuming that the background in the optical absorption spectrum of SWCNTs originated from the extinction of precipitable dispersoids, the spectrum shape of the background can be expressed by the appropriate proportion of the difference spectrum in the present analysis. In other words, the optical absorption of absolutely debundled SWCNTs without any background can be semiexperimentally determined by subtracting the estimated background extinction of the further precipitable dispersoid from the observed spectrum. Actually, Figure 1c shows the optical absorption spectrum of absolutely debundled SWCNTs extracted form the observed spectrum of sample C, as mentioned above. The proportionality constant to the difference spectrum of B-C has been estimated appropriately so as to limit the intensity of the generated background under the observed spectrum. In Figure 1c, all absorption peaks are clearly well-resolved in the whole measured range. These peaks can be identified with the series of S11, S22, M11, and higher interband transitions from various chiralities. In addition, two broad peaks in the UV region were also observed at 4.0 and 5.6 eV with relatively larger line widths ranging from 0.5 to 0.8 eV in full width half-maximum (FWHM) than the above-mentioned vis-NIR absorption peaks. These UV absorption peaks can be attributed to π plasmons19 or near M point transitions,20 which was recently suggested theoretically, due to their characteristics different from vis-NIR interband transitions. By our precise background correction, Figure 1c clearly shows that these peaks in absolutely debundled SWCNTs are basically localized in the UV region, in contrast to previous reports.1,6 Furthermore, the above-mentioned complete spectrum correction by subtraction of the background has brought about the precise and consistent deconvolution analysis for the optical absorption spectrum of the debundled SWCNTs. The results of curve fitting with multiple Lorentzian line profiles were demonstrated, as shown in Figure 2a,b, for the S11 peaks and the others, respectively. Optimized parameters of peak positions and line widths and their chiralities assigned based on the reported data9,21 are summarized in Table 1. Note that assigned chiralities include both semiconducting and metallic SWCNTs. Namely, three peaks in the vis region have been also assigned as totally eight metallic chiralities. This ability to investigate metallic chiralities is one of the advantages of this method for the characterization of SWCNTs’ chirality distribution over the reported method using vis-NIR PL mapping, although the drawback is the limitation of one-by-one assignments in some chiralities. To support the validity of assignments, it was
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Figure 2. Fitted curves (green and blue solid lines) to the corrected absorption spectrum derived in Figure 1c (plotted by open squares 0) by multiple Lorentzian functions (dotted lines) in the regions of the first interband transition (S11) peaks (a) and the others (b).
confirmed that the assigned peak positions and absorption spectrum shape for S11 and S22 transitions were nearly identical with the PL peak positions and emission/excitation spectra, respectively, obtained from the PL mapping measurement (Figures S1-S3; see the Supporting Information). Figure 3 shows the chirality distribution profiles determined from the results of optical absorption and of PL mapping for comparison. In the former profile, the amounts of SWCNTs were represented by areas of corresponding peaks, whereas the PL peak intensities were adopted as the amounts in the latter one. The relative amounts normalized by the largest peak area of (7,5) and spectroscopic data for assigned chiralities are summarized in Table S1 (see the Supporting Information). Some areas of the peaks assigned to several chiralities were divided equally to candidates. Despite such rough treatments, however, these chirality distributions obtained by this method and PL are in good agreement. In addition, the obtained chirality distribution is also consistent with the diameter analysis by resonance Raman spectra of as-grown SWCNT samples (see Figure S6 in the Supporting Information), supporting that the optical absorption and PL spectra are representative of the original composition of the sample, and it also verifies that there is no change in composition of the sample by the centrifugation. The S11/S22 ratio calculated from the relative amounts obtained by the optical absorption analysis is about 1.15, which is approximately equal to that of a previous report,12 1.2, obtained in SWCNTs with a different diameter distribution ranging from 1.1 to 1.3 nm. The slight difference would be caused by an error in the background correction when the dependency of the S11/S22 ratio on the tube diameter distribution can be negligible.
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TABLE 1: Spectral Data Obtained by the Curve Fitting of the Absorption Spectrum of Debundling SWCNTs with Multiple Lorentzian Functions (Figure 3a,b) peak peak position (eV) position (nm) FWHM (meV) peak area 0.74235 0.80122 0.82244 0.87353 0.88711 0.89757 0.94519 0.96547 0.97657 0.98716 0.99517 1.05362 1.08963 1.10653 1.11785 1.1736 1.20766 1.27138 1.29932 1.42597 1.55507 1.68465 1.71776 1.84449 1.91594 2.09736 2.25083 2.46573 2.75116 3.0205 3.24823 3.31223 3.6618 3.84818 4.0387 4.28094 4.49659 5.38769 5.62875 a
1670.2 1547.4 1507.5 1419.3 1397.6 1381.3 1311.7 1284.2 1269.6 1256.0 1245.9 1176.7 1137.9 1120.5 1109.1 1056.4 1026.6 975.2 954.2 869.5 797.3 736.0 721.8 672.2 647.1 591.1 550.8 502.8 450.7 410.5 381.7 374.3 338.6 322.2 307.0 289.6 275.7 230.1 220.3
29.59 26.71 22.27 25.43 14.83 10.63 26.69 17.4 15.52 13.28 10.97 19.85 23.35 24.95 16.87 17.73 26.92 14.89 41.6 70.81 56.08 62.61 58.43 77.95 84.78 81.8 122 124.94 99.71 100.06 70.08 63.93 211.46 72.86 822.34 166.79 112.54 215.73 535.96
0.00025 0.00024 0.00032 0.00056 0.00036 0.0003 0.00021 0.00046 0.00093 0.00069 0.00038 0.00074 0.00148 0.0027 0.00126 0.00084 0.00238 0.00023 0.00144 0.00077 0.00096 0.00177 0.00176 0.0009 0.00434 0.00231 0.00234 0.00236 0.00129 0.00148 0.00109 0.00118 0.00251 0.001 0.05653 0.00246 0.00146 0.01846 0.09736
origin
assigned semiconductive chiralities
assigned metallic chiralitiesa
S11 (11,9) (14,6) S11 (10,9) (12,7) S11 (13,5) (10,8) S11 (9,8) S11 (11,6) (10,6) S11 (11,4) S11 (9,7) (12,4) (13,2) S11 (8,7) S11 (11,1) S11 (10,5) (10,3) S11 (9,5) S11 (8,6) (11,3) (12,1) S11 (9,2) S11 (7,6) (8,4) S11 (9,4) S11 (10,2) S11 (7,5) S11 (6,5) S11 (8,3) S22 (10,8) (11,6) (12,4) (13,2) S22 (9,7) (10,5) (11,3) (12,1) (9,8) (12,5) S22 (10,2) (10,6) S22 (9,4) (8,6) (8,7) (11,4) S22 (8,3) (9,5) S22 (7,5) (7,6) (10,3) S22 (8,4) (11,1) S22 and M11 (9,2) (6,5) (8,5)+ (12,0)+ (11,2)+ (10,4)+ - (9,6)+ M11 (7,7) (8,5)- (12,0)- (11,2)M11 (6,6) (7,4)+ S33 (8,7) S33 (9,5) S33 (7,6) S33 (7,5) S44 (7,6) π plasmon S44 (7,5) N/A N/A π plasmon
The signs “+” and “-” represent the branches.
Figure 3. Chiral distribution of debundled SWCNTs, in which the amounts were normalized by the largest peak (7,5). Spectroscopic data for assigned chiralities are summarized in Table S1 (see the Supporting Information).
Furthermore, from these relative amounts, the metal/semicondoctor ratio can be also evaluated to be 0.33, which is considerably smaller than 0.5 that can be expected from a random growth model.22-24 This can be explained from the narrow tube diameter of the present SWCNT samples. In the SWCNTs with a narrower diameter25 less than 1 nm, such a discrepancy of the metal/semiconductor ratio from the statistical estimation has been also reported. 4. Conclusion The contribution of the background extinction in the optical absorption spectrum of SWCNTs, which comes from precipi-
table impurities and bundling of SWCNTs, has been explored by the interval centrifugation and difference spectrum (IC-DS) technique. The baseline correction using the line shape of the background extinction clarified the detailed features in the optical absorption spectrum of absolutely debundled SWCNTs. The obtained absorption spectrum curve of debundled SWCNTs has been successfully fitted by using multiple Lorentzian lines without further baseline correction, which leads to consistent assignments of most probable candidates of semiconducting and metallic chiralities. It was confirmed that assigned semiconducting chiralities agree well with the result of PL mapping, which supports not only the validity of assignments but also the fundamental importance of analyzing the background extinction for the characterization of SWCNTs by optical absorption spectroscopy with a great degree of certainty. Acknowledgment. This work is partially supported by the New Energy and Industrial Technology Development Organization (NEDO). S.O. and T.S. would like to thank K. Yanagi, Y. Takagi, T. Nakanishi, and Y. Sato for valuable discussions and Ms. T. Owada, Ms. A. Kobayashi, and Mr. Hashimoto for their experimental help. Supporting Information Available: PL spectrum mapping of sample C (Figure S1), plots of S11 and S22 transition energies (Figure S2), and excitation/emission spectra (Figure S3) determined from Figure S1, showing the consistency between the results of PL and curve fitting for the corrected absorption spectrum in the present work. Absorption spectra of C recorded with and without integrated sphere (Figure S4), absorption
Background Analysis in Characterization of SWCNTs spectra after long centrifugation (Figure S5), and Raman spectra of as-grown SWCNTs (Figure S6) are also presented. Results of the curve fitting sorted by chiralities are summarized in Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kataura, H.; Kumazawa, Y.; Maniwa, Y.; Umezu, I.; Suzuki, S.; Ohtsuka, Y.; Achiba, Y. Synth. Met. 1999, 103, 2555–2558. (2) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C.; Ma, J.; Hauge, R. H.; Weisman, R. B.; Smalley, R. E. Science 2002, 297, 593–596. (3) Wang, F.; Dukovic, G.; Brus, L. E.; Heinz, T. F. Science 2005, 308, 838–841. (4) Chen, Y.-C.; Raravikar, N. R.; Schadler, L. S.; Ajayan, P. M.; Zhao, Y.-P.; Lu, T.-M.; Wang, G.-C.; Zhang, X.-C. Appl. Phys. Lett. 2002, 81, 975–977. (5) Gabor, N. M.; Zhong, Z.; Bosnick, K.; Park, J.; McEuen, P. L. Science 2009, 325, 1367–1371. (6) Itkis, M. E.; Perea, D. E.; Jung, R.; Niyogi, S.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 3439–3448. (7) Saito, T.; Ohmori, S.; Shukla, B.; Yumura, M.; Iijima, S. Appl. Phys. Express 2009, 2, 095006. (8) Sun, X.; Zaric, S.; Daranciang, D.; Welsher, K.; Lu, Y.; Li, X.; Dai, H. J. Am. Chem. Soc. 2008, 130, 6551–6555. (9) Nair, N.; Usrey, M. L.; Kim, W.-J.; Braatz, R. D.; Strano, M. S. Anal. Chem. 2006, 78, 7689–7696. (10) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60–65.
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