Intensity Ratio of Resonant Raman Modes for (n,m) Enriched

Intensity Ratio of Resonant Raman Modes for (n,m) Enriched Semiconducting Carbon Nanotubes Yanmei Piao,† Jeffrey R. Simpson,†,‡ Jason K. Streit,§ Geyou Ao,§ Ming Zheng,§ Jeffrey A. Fagan,*,§ and Angela R. Hight Walker*,† †

Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States Department of Physics, Astronomy and Geosciences, Towson University Towson, Maryland 20899, United States § Material Measurement Laboratory, National Institute of Standards and Technology Gaithersburg, Maryland 20899, United States ‡

S Supporting Information *

ABSTRACT: Relative intensities of resonant Raman spectral features, specifically the radial breathing mode (RBM) and G modes, of 11, chirality-enriched, single-wall carbon nanotube (SWCNT) species were established under second-order optical transition excitation. The results demonstrate an under-recognized complexity in the evaluation of Raman spectra for the assignment of (n,m) population distributions. Strong chiral angle and mod dependencies affect the intensity ratio of the RBM to G modes and can result in misleading interpretations. Furthermore, we report five additional (n,m) values for the chiralitydependent G+ and G− Raman peak positions and intensity ratios; thereby extending the available data to cover more of the smaller diameter regime by including the (5,4) second-order, resonance Raman spectra. Together, the Raman spectral library is demonstrated to be sufficient for decoupling G peaks from multiple species via a spectral fitting process, and enables fundamental characterization even in mixed chiral population samples. KEYWORDS: single-wall carbon nanotubes, resonance Raman, Raman intensity ratio, chiral angle, aqueous two phase separation

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availability of laser lines at the electronic transitions, and difficulties in determining the abundance of a specific chiral index in the sample hamper the unambiguous experimental validation of the predictions. In spite of separate studies on RBM and G band strength,2,4,6,8,18 the relative intensity ratio between the two modes within the same chiral species has yet to be studied. Lacking such important information could result in misinterpretation of the Raman spectra of samples. For example, the G band is frequently thought to be the strongest peak in SWCNTs. This is because in the Raman spectra of nonsorted SWCNTs, RBM peaks from multiple species are distributed across tens of cm−1, while all of the G+ mode contributions are confined within a more narrow region (∼5 cm−1) at approximately 1600 cm−1 and, thus, in sum, appear as a single massive peak. Moreover, the presence or absence of different RBM peaks is often used in the literature to validate existence

uch effort has been devoted to understanding the structure-dependent Raman intensity of single-wall carbon nanotubes (SWCNTs),1−11 both as a characterization method for the material, and as a window into fundamental optical phenomena such as electron−phonon coupling. This effort stems from the desire to utilize the essential structural information encoded in the resonance Raman scattering of SWCNTs, particularly the radial breathing mode (RBM) and the graphite G band.12,13 For instance, the intensity ratio of G− to G+, two submodes of the G band due to quantum confinement, has recently been demonstrated to be strongly dependent on the chiral index (n,m),14 which defines the diameter and chiral angle of each nanotube. Both understanding intrinsic nanotube behavior and enabling accurate characterization are critical for many applications, including optoelectronics and sensors.15−17 A challenge, however, is that each SWCNT has a unique electronic structure. Theories based on an empirical nearest-neighbor tight-binding model,8 ab initio calculations,6 and extended tight binding,2 all predict the Raman intensity of SWCNTs to be strongly chiral index dependent. While the theoretical work is rich, challenges such as multiple chiralities in the sample, © XXXX American Chemical Society

Received: February 9, 2016 Accepted: April 29, 2016

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DOI: 10.1021/acsnano.6b01031 ACS Nano XXXX, XXX, XXX−XXX

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Table 1. Structural Information, Surfactant, Raman RBM, G− and G+ Peak Positions, IG−/IG+ and IRBM/IG+ Ratio of Studied SWCNTs Ordered by Diameter observed dominant (n,m) a

(5,4)* (6,4) (7,3) (6,5) (8,3) (10,0)*a (9,2)*a (7,5) (8,4) (7,6)*a (11,1)

mod

surfactant

dtb (nm)

θ (deg)

E22c (nm)

RBM (cm−1)

G−(cm−1)

G+ (cm−1)

IG−/IG+

IRBM/IG+e

1 2 1 1 2 1 1 2 1 1 1

DOC DOC DNA DNA DNA DOC DOC DNA DNA DOC DNA

0.620 0.692 0.706 0.757 0.782 0.794 0.806 0.829 0.840 0.895 0.916

26.33 23.41 17.00 27.00 15.30 0.00 9.83 24.50 19.11 27.46 4.31

488 586 511 573 670 541 556 654 597 650 620

372 338 332 311 300 295 293 285 281 265 259

1497 1527 1526 1528 1541 1542 1541 1543d 1555

1583 1589 1585 1590 1589 1583d 1586d 1591 1589 1591d 1586

0.20 0.12 0.13 0.13 0.048 0.076 0.094 0.068 0.026

0.75 1.4 0.021 0.24 2.4 0.37 0.49 0.88 0.0049 0.12 0.75

a

Few-chiralities enriched with (n,m) being the major species. bDiameter of the SWCNTs are obtained from ref 51. cE22 values are the peak intensity positions of the E22 transitions as measured in the absorption spectra and these values are then used as the single resonant Raman excitation. dData acquired from spectral decoupling of few-chirality enriched samples which will be discussed in text below. eThe experimental uncertainty of the ratio is estimated to be ±14% (Methods section). The ratio values are corrected to the instrument response (Figure S3).

dependence of the RBM to G+ mode intensity ratio upon E22 resonant excitation, as predicted.19

and imply relative concentration of individual chiralities in a sample. As will be discussed in this paper, RBM cross sections from different (n,m) show significant relative intensity differences upon E22 excitation, substantial enough to affect even qualitative evaluation of chirality distribution in SWCNT samples. Calculations by Popov et al. on the chiral dependence of the RBM intensity have occasionally been used to evaluate the chirality distribution of samples.19−21 However, unambiguous experimental validation of the prediction is critical for applications and is detailed within. As implied above, the challenge of obtaining the resonance Raman spectra for each SWCNT (n,m) arises from sample heterogeneity and the lack of laser excitation wavelengths commonly available in Raman instruments throughout the visible range. Although resonance Raman spectroscopy probes only the tubes whose optical transitions match the incident laser energy, due to the extended resonance window of the RBM (≈ 0.2 eV) and G bands (≈ 0.4 eV) caused by phonon energy and incoming/outgoing Raman process,22,23 detecting Raman signals from additional species is almost inevitable unless extremely high-quality, single-chirality-enriched samples are available. Additionally, nanotube optical properties are known to be affected by chemical surface defects and the endohedral environment, both areas in which the literature is developing quickly.17,24−27 Fortunately, prior work indicates that morphologically defective nanotubes can be removed via a modification of the widely used ultracentrifugation method for SWCNT chirality sorting.28 Similarly, filled- and empty-core SWCNTs can also be separated.29−31 Therefore, obtaining intrinsic Raman spectra of specific chiralities relies on highquality SWCNT samples and knowledge of parameters such as the endohedral environment. In this work, we probe 11, single- or few-chirality-enriched semiconducting SWCNT solutions to determine their resonant Raman behavior, including the RBM to G+ and G− to G+ intensity ratios. We utilize resonance Raman spectroscopy to probe the second electronic transition, E22, of seven singlechirality-enriched SWCNTs, and establish a spectral library that allows decoupling of Raman spectra from few-chirality enriched SWCNT populations. These results vastly expand the scope of reported measurements and most importantly provide strong experimental evidence of significant chiral angle and mod

RESULTS AND DISCUSSIONS Eleven, chirality-enriched, SWCNT samples were probed optically. To compare with the literature on commonly produced samples, each of the populations used here was water-filled and in an aqueous dispersion. Single-chiralityenriched SWCNTs (6,4), (7,3), (6,5), (8,3), (7,5), and (11,1) and few-chirality enriched SWCNTs (5,4)*, (7,6)*, (10,0)*, and (9,2)*, with major species denoted with an asterisk, are purified via aqueous two-phase (ATP) separation.32−35 In each few-chirality-enriched sample, minor species are identified as follows: (5,4)* contains (8,3) and (10,2) as minor; (7,6)* contains (7,5) as minor; (10,0)* contains (6,5) and (8,3) as minor; (9,2)* contains (8,3), (7,5), and (8,4) as minor. Among the 11 SWCNT samples, (7,3), (6,5), (7,5), (8,4), (8,3), and (11,1) are dispersed with DNA (details will be published in a separate article by G. Ao, J. K. Streit, J. A. Fagan, and M. Zheng), and the others are dispersed in sodium deoxycholate (DOC). The detailed information on the samples can be found in the Methods section and summarized in Table 1. Isolation quality of the single-chirality-enriched SWCNTs (n,m), as well as species in few-chirality-enriched SWCNTs (n,m)*, were evaluated and identified with absorption, photoluminescence excitation (PLE), and resonance Raman spectroscopies. Figure 1a presents absorption spectra in the ultraviolet to near-infrared (UV−vis-NIR) region. In the singlechirality-enriched SWCNTs (n,m), sharp and sole optical transition peaks in the E22 and E11 regions, respectively, are observed. Additionally, weak, broad, and asymmetric electron− phonon coupling peaks can be observed to the higher energy side of both electronic transitions.36−38 A photograph of the 11 samples in clear glass tubes is shown in Figure 1b. These SWCNT (n,m) species also show extremely clean PL maps [Figure 1c], further confirming the exceptional separation quality. An additional PL map is presented in Figure S1. The minor chiralities of SWCNTs (n,m)* are identified via optical spectroscopies. Because the (5,4) has a low abundance in the as-produced nanotube sample, we note that the absorption spectrum of this sample does contain additional features in the E11 region other than from the (5,4). The PL map (Figure S1) B


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Figure 1. (a) Absorption spectra, (b) photograph, and (c) PLE of the samples used in this work. PL map for (5,4)* is shown in the Supporting Information. The black vertical arrows in (a) indicate the E22 excitation wavelength for each sample. In (c) major (yellow label) and minor (white labels) species observed in the populations are labeled in PL maps. Note the emission wavelength axis for the (6,4) sample starts from 850 nm, while all the others start from 900 nm.

Figure 2. E22 resonance Raman spectra of RBM, G−, and G+ bands of 11, semiconducting SWCNT samples with labeled chiral indices. The chiral indices with asterisk (*) represent the major species in few-chirality enriched sample. All spectra are scaled such that the G+ peak is unity, and are offset for clarity. The Raman E22 excitation wavelength for each sample is matched to the peak intensity position in the absorption spectrum, and identified by the number adjacent to the spectra. A figure with enlarged scale for (8,4) RBM is available in the Supporting Information. For (11,1), the G− peak was enlarged by 10 times and plotted as a dotted line. The spectra are ordered by diameter from small to large.

indicates the minor semiconducting species in (5,4)* are (8,3) and (10,2). Fortunately, these minor peaks occur at wavelengths significantly longer and out of resonance with the E22 Raman excitation range of the (5,4) as will be discussed below. Resonance Raman spectroscopy was performed on all 11 samples within the wavelength range of 488 nm to 670 nm. In all cases, the excitation wavelength was chosen to probe the second electronic transition, E22, as determined by the wavelength corresponding to the maximum E22 absorption value [Figure 1a]. The Raman spectra (Figure 2) show the RBM, G−, and G+ modes of the SWCNT samples collected at each of the 11 E22 excitations using multiple laser lines. For clarity, spectra were normalized to the G+ peak intensity then offset vertically, with each spectrum labeled in a different color. The single-chirality-enriched SWCNT (n,m)s show RBMs from 250 cm−1 to 400 cm−1. For some species, e.g., (8,4), the RBM is too small to clearly see at this scale, so an enlarged scale is provided in Figure S2. In the (9,2)* and (10,2)* sample, no significant RBMs from minor species are observed. In SWCNT (7,6)*, RBMs from both (7,6) and (7,5) are observed because the two species have almost identical E22 transition positions. The minor species in (5,4)* are (8,3) and (10,2), whose E22 transition energies differ from that of (5,4) by 0.69 and 0.86 eV, respectively. These values are well outside of the standard G band resonance window of approximately 0.4 eV, and therefore, these tubes do not contribute to the resonance Raman measurements of (5,4) reported here. With a carbon-center to carbon-center calculated diameter of only 0.62 nm, the (5,4) species is the smallest diameter nanotube yet separated at bench scale (i.e., several milliliters) and studied via resonant Raman spectroscopy. The resonance Raman spectra of SWCNTs with different chiralities reveal remarkable variance in the intensity ratio of the RBM to G+ modes. Most of the tested samples including the

(5,4), (7,3), (6,5), (10,0), (9,2), (8,4), and (7,6) exhibit a RBM/G+ intensity ratio smaller than 1. In a few cases, such as the (8,4) and (7,3), upon E22 resonance excitation, the RBM relative to the G+ intensity is barely observable at this scale, though measurable within the instrument signal-to-noise level. For example, the (8,4) RBM intensity is 2 orders of magnitude lower than that of its G+. Such an extremely low RBM intensity explains why an earlier Raman study to analyze the chirality distribution in CoMoCAT SWCNTs did not identify the (8,4).39 Other chiralities, however, possess a more intense RBM compared with its G+ mode; for example, the (8,3) species yielded a RBM to G+ intensity ratio as large as 2.4 (Table 1). Quantitative comparison of the RBM to G+ intensity ratio requires characterization of the intensity response of the spectrometer system. Details of the procedure and resulting spectrometer correction factors are discussed in the Methods section and in the Supporting Information. Upon careful analysis of resonant Raman spectra of 11 samples, we obtained unambiguous results indicating a strong mod and chiral angle dependence of the RBM to G+ mode intensity ratio, as was predicted by Popov et al.19 The intensity ratio observed for each species is included in Table 1. The spectral information for the major (n,m) in the few-chiralityenriched samples is obtained via spectral fitting, which will be discussed later in this paper. Semiconducting SWCNTs are divided into two classes by whether the (n−m) modulus 3 = 1 or 2, denoted as mod1 or mod2, respectively.40 In reciprocal space, as shown in Figure 3a, the cutting lines of E11 for mod1 C


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In contrast, the mod1 tubes show a nonmonotonic trend with a definitive valley at approximately 20°. Previous literature indicates that the electron−phonon matrix element of the G+ mode is significantly less sensitive to the chirality of the SWCNT when compared with that of the RBM.7,19,41,42 It has been calculated that the distribution range of the matrix elements of the G+ mode for different (n,m)s is much narrower than that of RBM. The reason is that G+ mode of semiconducting SWCNTs arises from the LO phonon vibration along the tube axis and thus is not substantially affected by the SWCNT structure, such as, diameter or chiral angle. Therefore, the RBM/G+ ratio is expected to follow a similar trend to that of RBM itself. We compared our experimental results to the theoretical RBM strength calculated using a symmetry-adapted, nonorthogonal tight-binding model developed by Popov et al. (see Table S1 for the theory values).18,19,43 Figure 3b shows that our trend obtained with both mod1 and mod2 tubes is in good agreement with the calculation. Impressively, our experimental results even capture the anticipated valley for chiral angles of ≈ 20° for the mod1 tubes. This valley is explained by the trigonal warping effect of the graphene band-structure that causes a shift in the SWCNT transition, as indicated in Popov’s theory,19 and observed by Telg et al.3 While the deviation of the (10,0) is not well understood, SWCNTs with chiral angles at either of the two extreme values, 0° or 30°, are anticipated to have unusual Raman behavior, which has been experimentally observed in the latter case.44 Note that the cutting lines for E11 of a particular nanotube are on the opposite side from the K point to E22, suggesting that a reversal in behavior for RBM strength and thus RBM/G+ intensity ratio should be expected for E11 excitation. We also compare and extend the data set for G− and G+ mode positions with our separated (n,m) species. Our results expand the previously reported general trend that smaller diameters result in lower G− and G+ frequencies.14 Figure S4a compares our observations with previous experimental results and the predicted trend for G− and G+ positions with SWCNT diameter.14 Aside from providing clear consistency for the experiment results in both cases, our inclusion of 5 additional chiralities, specifically the (5,4), significantly expands the experimental range compared to the predicted trend. The other four species are (7,3), (10,0), (9,2), and (11,1). The G− position for (5,4) is downshifted 29 cm−1 from that of the (6,4), which was the lower limit of the previous report. Our results also show that the ratio of G− to G+ intensity decreases from near-armchair tubes (chiral angles of ≈ 30°) to nearzigzag tubes (≈ 0° chiral angle), as shown in Figure S4b, agreeing well with an earlier report.14 Notably, the excitation energy used in the measurement substantially impacts the intensity ratio of RBM to G+ even in these monodispersed samples due to the difference in the resonance Raman excitation profile (REP) of the RBM and G. This is shown in Figure 4a,b, using (7,5) as a representative example. When the energy of an incoming or outgoing photon matches with the electronic transition of a SWCNT, the Raman signal will be significantly enhanced. The energy difference between the incoming or outgoing excitation is the energy of a phonon. A typical RBM phonon carries energy of around 0.03 eV (equivalent to 242 cm−1) which is too small to be resolved in E22 REP; thus, it appears as a single peak. A G+ phonon, however, carries approximately 0.2 eV of energy and can be resolved easily in the REP and appears as two, asymmetric

Figure 3. (a) The first Brillion zone (BZ) of graphene unit cell with marked Γ, M, K, and K′ points. The dashed circle is the projection of the Dirac cone on the BZ. The bottom three figures represent top-down views of the dashed circle. In mod1, E11 and E22 transitions lie on different side of K, with K at the one-third point between the two cutting lines. The last circle represents a simplified version of how the chiral angle relates to the cutting line direction in BZ. (b) RBM/G+ intensity ratio vs chiral angle plotted as blue diamonds and marked with (n,m). Filled and empty symbols representing mod1 and mod2, respectively. The pink crosses are the as-predicted RBM intensity from Popov et al.18,19 The points corresponding to each mod are connected with lines to guide the eye. Error bars result from estimate of experimental uncertainty (Methods section).

and mod2 SWCNTs are located outside and inside of the first Brillouin zone (BZ) of graphene, respectively. The cutting line for E22 is on the opposite side from that for E11 relative to the K point and with the K point always located at one-third of the distance between the two lines. Due to different electron− phonon coupling strengths near the vicinity of the K point, mod1 and mod2 SWCNTs exhibit very different optical properties. Indeed, our results indicate that upon selecting a Raman excitation matched to the E22 resonance excitation measured via absorption, mod2 tubes have an overall higher Raman RBM/G+ value than that of mod1 tubes, which is consistent with an earlier report using mixed-chirality samples.11 Values for the ratio of the RBM to G+ mode are plotted for both sets of species vs their chiral angle in Figure 3b. As the chiral angle increases from 0° to 30°, the RBM/G+ of mod2 tubes gradually decreases over the range of measurement. D


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Figure 4. (a) RBM and (b) G+ Raman excitation profile of the (7,5) SWCNT probed in E22. (c) RBM/G+ intensity ratio (red stars), intensity of G+ (black circles) and RBM (open circles) are plotted vs excitation wavelength. The symbols are connected with lines to guide the eye.

Figure 5. Raman spectra showing subtraction of the contribution of minor species to the G+ band of few-chirality enriched samples. The black solid lines denote spectra from mixture of [(n,m) + (n′,m′)], and the dashed color lines represent different components that are fitted with a Voigt function. The (n′,m′) spectra, serving as standard spectra, are obtained from single chirality enriched samples. (a) and (b) are E22 resonance Raman of (7,6)* and (10,0)*, respectively.

peaks.23,45 The evident difference between the RBM and G+ mode functionalities in the resonance profiles results in the highly excitation energy dependent IRBM/IG+ ratio, with the largest IRBM/IG+ ratio expected to appear at slightly higher energy than the E22 transition (Figure S5). Figure 4c shows the IRBM/IG+ ratio vs excitation, and it roughly follows the trend of the RBM intensity, reaching a maximum value at 654 nm excitation. We note that the RBM/G+ intensity ratio provided in Table 1 is obtained via a single, Raman excitation selected to match the E22 resonance. The frequency of the E22 resonance and therefore the Raman excitation was determined from the peak intensity position in the absorption spectra and not from a full REP. Additionally, the reported intensity ratios result from the collective behavior of the incoming and outgoing resonances for both the RBM and G+ modes; that is, the resonance overlapping effect on RBM and G+ is not deconvoluted in our intensity ratio analysis. To approximate the expected contributions to the ratio, we examined the effects of the broadening factor Γ, the frequency position of the RBM and G+ modes, and the non-Condon parameter C (see Figure S7).45 The combined effects from the Γ and the RBM frequency on the RBM/G+ intensity ratio are corrected and shown in Figure S8 to highlight that they do not alter the trend we report for the RBM/G+ intensity ratio determined at E22 excitation. Unlike RBM modes which span hundreds of cm−1 depending on diameter, the G+ mode of SWCNTs blend together at approximately 1600 cm−1. Therefore, obtaining a G+ mode of a specific chirality from a mixture is challenging. A library of E22 resonance Raman spectra of seven (n,m)s made it possible to isolate chirality-specific G+ mode information from fewchirality-enriched samples without requiring single chiralityenriched separations. E22 resonant Raman spectra of SWCNTs (7,6)* and (10,0)* are used to demonstrate the process as shown in Figure 5. In SWCNT (7,6)*, excitation simultaneously probes the E22 transitions of (7,6) and (7,5). Thus, the (7,6) spectrum can be obtained by subtraction of the (7,5) contribution from (7,6)* using the pure (7,5) RBM as a reference. This method, however, is not applicable to the (10,0)* sample, because the minor (6,5) is not resonantly probed by E22 transition of (10,0). In fact, the RBM of (6,5) is barely seen in the (10,0)* sample spectra upon 541 nm excitation (Figure 2), but its G+ contribution can be seen clearly. Alternatively, the G+ mode of (6,5) was fitted with a Voigt peak. Another Voigt peak representing the G+ of (10,0) was superimposed on the G+ peak of (6,5) in order to produce the G+ of the (10,0)* sample. On the basis of our knowledge of the shape and position of the G+ mode, it stays constant along the resonance window with only peak intensity changes.45

Fortunately, only the (6,5) contributes to the SWCNT (10,0)* Raman spectra, because the (8,3) is out of resonance upon 541 nm excitation. Additional examples of the decoupling process using the G− mode of (7,6)* and the G+ mode of (9,2)* are reported in Supporting Information Figures S9 and S10. These results suggested that the E22 resonance Raman spectra library of the single-chirality enriched samples can be utilized to characterize samples with less purity, which is advantageous to nanotube metrology and the community at large.

CONCLUSIONS In this contribution, we show that the E22 resonant Raman RBM to G+ mode intensity ratio is highly chiral angle and mod dependent. By taking the intensity ratio, possible variations in exciton−photon coupling elements cancel, so our results reflect how the ratio of the RBM to the G+ mode exciton−phonon coupling varies as a function of mod and chiral angle. This ratio vs chiral angle shares the trend of the RBM strength as predicted by a symmetry-adapted, nonorthogonal tight-binding model.19 We determine that while the RBM to G+ intensity ratio depends on excitation energy, broadening factor Γ, and RBM frequency, the above-mentioned trends remain valid. Additionally, based on our results, we compared and expanded the chirality dependent G+ and G− positions by including the E22 resonance Raman spectra of (5,4) and four additional species. Finally, a Raman spectral library is demonstrated to be useful in decoupling multiple species via a spectral fitting process to isolate the Raman spectrum of a certain species in mixed chiral population samples. Our study highlights the complexity of resonance Raman spectra of SWCNTs with different chiral indices, enables accurate characterization of carbon nanotubes, and provides fresh insight into the photophysics of SWCNTs. METHODS Single and Few Chirality Enriched Sample Preparation. Refined populations of SWCNTs were generated via aqueous twopolymer phase extraction (ATPE) as previously described.32−35 Nanotube powders (SG65i, SG76 or EG150 from Southwest Nanotechnologies) were dispersed using either ssDNA or sodium deoxycholate (DOC) in water via tip sonication as appropriate for the utilized separation method, multistage gradient ATPE for DOC dispersed SWCNTs, or recognition based separation for DNA dispersed SWCNTs. Prior to our standard ATPE separation step, E


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ACS Nano SWCNT dispersions were purified of large/dense impurities via centrifugation. After ATPE separation, nanotube fractions were concentrated and partially dialyzed using either ultrafiltration or precipitation/redispersion to remove residual polymer(s) and adjust the background electrolyte or surfactant concentration.46 Raman Spectroscopy. Raman spectra were obtained from aqueous solutions in clear glass tubes. Excitation wavelengths ranging from 510 nm to 670 nm were obtained by using a tunable dye laser with kiton red, rhodamine, and coumarin dyes and producing approximately 25 mW of power at the sample. The E22 transition of (5,4) was probed with the 488 nm line from an Ar+ laser. Raman spectra were collected in a 180° backscattering geometry through a triple-grating Raman spectrometer coupled to a CCD detector cooled with liquid nitrogen. Integration times varied between 60 s and 100 s. Each spectrum was collected 2 times. To obtain the Raman excitation profiles and Raman contour plots, intensities of all spectra were corrected by normalizing to the benzonitrile, a calibration sample, peak intensity at 1599 cm−1. Positions of RBM, G−, and G+ peaks, and the intensities of the three peaks used for ratio calculation in Table 1 are acquired from spectral fitting. A Voigt function is used for the spectral fitting process (decoupling of multiple species) after subtraction of a linear background. The experimental uncertainty (±14%) of the RBM/G+ ratio in Table 1 and error bars of Figure 3b are estimated through the addition in quadrature of the independent uncertainties in laser power (±10%) and intensity calibration procedure (±10%). Raman Intensity Calibration. Correction standards for Raman spectroscopy provide calibration of the intensity response function for the triple-grating spectrometer and CCD detector instrumentation,47−49 which is critical for a quantitative comparison of RBM to G+ intensity. These standard reference materials (SRMs) consist of optical glasses exhibiting a broad photoluminescence. Exciting the photoluminescent SRMs in the same optical geometry as the SWCNT samples studied affords measurement of the instrument’s sensitivity across a broad spectral range.49 By splicing together SRM 2242 (532.2 nm) and SRM 2245 (632.8 nm), we cover an excitation region from 536.48 nm (532.2 nm − 150 cm−1) to 847.29 nm and (632.817 nm − 4000 cm−1).47,48 Comparison of the measured SRM response with the calibrated values then provides a polynomial correction factor,47−49 which may be applied to the measured Raman scattering intensity. Figure S3 shows the correction factor obtained for the triple grating spectrometer and CCD detector used in this work. For excitation wavelengths outside the range of the SRMs, we reference the RBM and G+ peak intensities to nearby benzonitrile peaks at 460.9 cm−1 and 1598.9 cm−1, respectively.50 Correcting the ratio of these benzonitrile peak areas to the measured average value of 0.449 determines the correction factor for each excitation outside the range of the SRMs. This correction factor is then applied to the corresponding RBM to G+ ratio. See Figure S3 for benzonitrile peak area ratios in the reported spectral range. Absorption and Photoluminescence Spectroscopy. UV−visNIR absorbance spectra of the samples were collected in 1 nm increments through a 1 mm, 2 mm, or 10 mm quartz cuvette. The integration time is 0.1 s/nm with 2 nm slit width. During data analysis, the corresponding blank surfactant or polymer solution spectra were linearly subtracted. NIR fluorescence measurements were acquired using a spectrofluorometer equipped with a liquid nitrogen cooled InGaAs array detector. Nanotube samples were excited using a 450 W xenon lamp, and a dual 1200 mm grating monochromator was applied to select a specific excitation wavelength. Emission was collected at 90° and dispersed onto the detector array with a 150 mm grating. For each contour map, integration time was typically 40 s at each excitation wavelength. Instrumental resolution was 6 nm for excitation and 8 nm for emission. All emission spectra were corrected for lamp excitation, filter transmission, and detector efficiency. Samples were diluted with DI water for DNA-wrapped SWCNTs or with a mass fraction of 1% DOC for DOC-coated nanotubes to achieve an E11 absorbance peak value of about 0.2 in a 1 cm cuvette. In the special case of (5,4)*, a second contour plot was acquired using a thermoelectrically cooled silicon PMT to collect the E11 emission at approximately 835 nm.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b01031. The following information is available in the supporting information: PL map of (5,4); E22 resonance Raman spectra of (8,4) with enlarged RBM feature; spectrometer intensity correction factors as a function of photon excitation energy; G+ and G− position vs diameter, intensity ratio of G−/G+ vs chiral angle; full REPs of (7,5), (8,4), and (6,5) with fitting parameters and equations; modeling results of RBM/G+ intensity ratio as a function of various parameters (Γ, C, and Raman mode frequencies); experimental and RBM & Γ corrected RBM/G+ intensity ratios vs chiral angle in log scale; spectral decoupling of (7,6)* and (9,2)*; tabulated theory prediction of E22 excited RBM intensity (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: jeff[email protected]. Notes

The authors declare no competing financial interest.

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