Bright Fraction of Single-Walled Carbon Nanotubes through

Letter pubs.acs.org/JPCL

Bright Fraction of Single-Walled Carbon Nanotubes through Correlated Fluorescence and Topography Measurements Lisa J. Nogaj,†,∥ Julie A. Smyder,† Kathryn E. Leach,†,⊥ Xiaomin Tu,§ Ming Zheng,§ and Todd D. Krauss*,†,‡ †

Department of Chemistry and ‡The Institute of Optics, University of Rochester, Rochester, New York 14627, United States National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States

§

S Supporting Information *

ABSTRACT: Correlated measurements of fluorescence and topography were performed for individual single-walled carbon nanotubes (SWNTs) on quartz using epifluorescence confocal microscopy and atomic force microscopy (AFM). Surprisingly, only ∼11% of all SWNTs in DNAwrapped samples were found to be highly emissive on quartz, suggesting that the ensemble fluorescence quantum yield is low because only a small population of SWNTs fluoresces strongly. Qualitatively similar conclusions were obtained from control studies using a sodium cholate surfactant system. To accommodate AFM measurements, excess surfactant was removed from the substrate. Though individual SWNTs on nonrinsed and rinsed surfaces displayed differences in fluorescence intensities and line widths, arising from the influence of the local environment on individual SWNT optical measurements, photoluminescence data from both samples displayed consistent trends. wrapped SWNTs. After fluorophores are dispersed onto a surface, the number of emissive particles in a fluorescence scan is compared to the total population of particles as determined from AFM measurements. In this way, it is possible to clarify whether the low QY arises from a large population of weak emitters or a small population of bright emitters. 12,13 Uncorrelated measurements of fluorescence and topography can be used to quantify the average fraction of semiconducting SWNTs in a sample,14 but a correlated approach makes it possible to elucidate the relationships between fluorescence intensity and SWNT structure, length, and orientation on the substrate. Here, we present measurements of fluorescence images from individual SWNTs that are correlated with topographic images from the exact area of the emitting SWNTs, thereby enabling direct comparison between the two data sets. We determined that only ∼11% of all DNA-wrapped SWNTs on the surface were highly emissive, which suggests that the ensemble QY is low because the total population is composed of only a small fraction of bright emitters in the environment. Studies on a sodium cholate surfactant system gave qualitatively similar conclusions to the DNA surfactant. While the fluorescence properties from individual SWNTs on quartz varied with the amount of DNA surfactant applied to the surface, data from lower-surfactant-level samples displayed consistent trends with data for SWNT samples having higher surfactant levels.8 A sample of HPLC-enriched CoMoCAT SWNTs wrapped in (GT)20 single-stranded DNA was prepared, 15 and the

S

ingle-walled carbon nanotubes (SWNTs) are near-infrared (NIR) fluorophores1 that exhibit robust and size-tunable emission2−4 but typically suffer from low ensemble fluorescence quantum yields (QYs) of 1 peak was observed in the fluorescence spectrum. (c) Neighbor SWNTs: AFM showed >1 particle in the fluorescence pixel, but only one peak was observed in the fluorescence spectrum; hence, only one of the three SWNTs in this pixel was emissive. Spectra for (a) individual and (c) neighboring SWNTs display the expected Lorentzian line shape, confirming that single nanotubes were observed. Insets: AFM images showing topography of SWNTs; round features are residual surfactant. Scale bar = 100 nm.

Figure 2. (a) Overlaid topography (tan) and fluorescence (blue) scans for a rinsed sample of SWNTs on patterned quartz. Blue lines represent the location of nanotubes and green circles indicate areas for which fluorescence spectra were observed. E is the direction of polarization for the incident excitation. Long, straight features that are >2 μm are not SWNTs, but rather, are etch pits in the quartz. (b) Topography image for the same area in (a) with arrows indicating the locations of nanotubes. (c) Fluorescence image for the same area in (a). 2817

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The Journal of Physical Chemistry Letters

when measured with AFM. Indeed, the average SWNT diameter measured with AFM was ∼1.5 ± 0.4 nm, which is larger than the expected value of ∼0.757 nm for a (6,5) SWNT;20 however, our value is consistent with AFM heights of 1−2 nm reported for a similar sample of DNA-wrapped SWNTs,21 and with calculations of ∼2 nm width for (6,5) SWNTs wrapped with DNA.22,23 Because multiple DNA strands sometimes stack upon the nanotube, we observed variations in height along the same nanotube.22,23 Sodium-cholate-wrapped SWNTs (Supporting Information Figure S7) exhibited slightly different statistics than the DNAwrapped samples, though a much smaller correlated area is available. Bright emitters accounted for 22% of all SWNTs and 3% were photobleached, as summarized in Table 1. Fluorescence line widths and intensity values were comparable to the data from the DNA-wrapped samples (values shown in Supporting Information Tables S1 and S2), indicating that at this level of limited surfactant coverage, sodium-cholatewrapped SWNTs have single molecule QY values similar to DNA-wrapped SWNTs. On the other hand, using our measured ensemble QY of 0.19% and bright fraction of 22% for our sodium cholate sample, we calculated an average QY of 0.9% per bright SWNT.13 In a previous study, our laboratory determined a single-particle QY of 3% for bright SWNTs using the same sodium cholate surfactant system studied on quartz, but in a higher-coverage surfactant environment.8 Therefore, we observed approximately a 3-fold decrease in single-particle QY for SWNTs in our sodium cholate sample in a low-coverage environment. Surprisingly, only ∼11% of all DNA-wrapped SWNTs on quartz displayed fluorescence spectra at detectable levels in correlated measurements, despite high densities of SWNTs on the substrates. Therefore, our data suggest that the low ensemble QYs for carbon nanotubes, especially when deposited onto a quartz substrate, may be largely attributed to a small percentage of bright SWNTs in the ensemble and not to a large population of SWNTs with poor (but detectable) emission yields. Note that the fluorescence intensity is expected to follow a cos2θ curve (θ is the angle of the long axis with respect to laser polarization) for individual straight SWNTs.2,24 Our measurements show that SWNT fluorescence was still detectable in rare cases where the excitation was not wellaligned to the long axis of the nanotube (see Supporting Information Figure S8).24,25 Therefore, it is not expected that SWNTs oriented perpendicular to the laser polarization were incorrectly classified. These correlated measurements enable us to make predictions about typical heterogeneous samples. Our measurements suggest that in typical, unsorted samples, it is possible that up to 90% of all semiconducting SWNTs are dark or weakly emissive, and only ∼10% of isolated SWNTs may be considered bright. We hypothesize that the low percentage of emissive SWNTs in a typical sample results from the presence of processing defects along the SWNT sidewall, which are introduced when the SWNT is separated from the catalyst support system and purified using harsh oxidation conditions and during strong ultrasonic disruption used to separate and isolate aggregated SWNTs. Indeed, spectroscopic evidence suggests that the fluorescence QY can be enhanced by nearly 1 order of magnitude when defect sites are passivated by treatment with mild reducing agents.26 Performing the correlated measurements on a quartz substrate is an unavoidable requirement for correlating

bleached or dark, as presented in Table 1. Bright emitters were associated with bright pixels in the fluorescence image; Table 1. Summary of Fluorescence and Topography Measurements for Correlated Samples quantity total area correlated (μm2) total number of SWNTs present total percentage of bright emitters (%)a total percentage of photobleached particles (%)b total percentage of dark particles (%)

value (in DNA)

value (in sodium cholate)

1478 474 11 19

100 72 22 3

70

75

a

Bright emitters are those for which bright pixels were observed in the fluorescence image with a corresponding fluorescence spectrum. b Photobleached particles are those for which bright pixels were observed in the fluorescence image, but with no corresponding fluorescence spectrum.

corresponding fluorescence spectra were observed. A range of fluorescence intensities was observed for SWNTs grouped as bright emitters, as indicated in Supporting Information Figure S5. We expect some variation in luminescence intensity due to the wavelength of the laser being in resonance with E22 for different (n,m) species. In contrast, fluorescence from photobleached emitters was quenched upon initial exposure to photoexcitation; thus, the spectral fluorescence intensity per nanometer was too low to be subsequently detected. Altogether, only ∼11% of all observed SWNTs displayed bright luminescence, ∼19% of all SWNTs displayed photobleached luminescence (and therefore no fluorescence spectrum was subsequently observed for the corresponding bright pixel in the fluorescence image) and ∼70% of all SWNTs displayed no observable fluorescence at all. Correlated images and data for all SWNTs represented in Table 1 are included in the Supporting Information (Figures S2−S4 and S7; Tables S1 and S2). Altogether, for the DNA-wrapped sample, four nanotube structures were observed: (6,4), (9,1), (8,3), or (6,5) SWNTs. The (n,m) distribution of all bright SWNTs is summarized in Supporting Information Figure S6. On the basis of a Gaussian fit to the ensemble absorption spectrum in Figure 1, only ∼2% of this sample was composed of (7,5) SWNTs that we were unable to observe with our detector. Fluorescence from bundles was observed in the (6,5)-enriched sample, similar to previous studies in which energy transfer occurred between SWNTs in direct contact with one another.17−19 In correlated measurements, nearly one-third of all fluorescence spectra were attributed to bundles of semiconducting SWNTs. Even if all of the individual (7,5) SWNTs that we were unable to observe with our detector were bright or transferred energy to themselves in bundles, we would still only predict a 13% bright fraction in our DNA-wrapped sample. On average, the fluorescence intensity for a SWNT bundle was three times greater than that observed for individual SWNTs, and some displayed fluorescence intensities up to an order of magnitude greater. As expected, line widths for SWNT bundles were typically broader than for individual SWNTs. Ensemble studies suggest that the luminescence intensity varies inversely with SWNT diameter,7 but our single molecule fluorescence and AFM topographical data did not strongly follow this trend, likely because of the large variation in SWNT plus “surfactant” heights inherent in a DNA-wrapped sample 2818

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The Journal of Physical Chemistry Letters topography and fluorescence for nanotubes. Control experiments were conducted to determine the extent to which the quartz substrate influenced the photophysical behavior of SWNTs in this study.25,27 Fluorescence spectra associated with correlated images (Figure 3) displayed lower signal-to-noise and broader line widths than is usually anticipated for individual SWNT spectra.8 As summarized in Supporting Information Table S3, we found that the average fluorescence intensity was nearly five times greater for SWNTs in high surfactant coverage versus low surfactant coverage environments and that spectra from high-coverage samples exhibited narrower fluorescence line widths and larger variations in fluorescence intensities. These observations suggest that direct exposure to the quartz substrate accounts for some decreased brightness for the correlated samples.28 However, the 3-fold lower single-particle QY (based on a bright fraction of 22%) that we observed in a low-coverage environment is in very good agreement with the 3-fold decrease in average fluorescence intensity that we observed for our sample relative to the high-coverage control (Supporting Information Table S3). Thus, our data suggest that the quartz substrate may be dimming the intensity of some SWNTs but that the quenching is not strong enough to cause us to miss a significant amount of SWNTs entirely. As additional evidence, we found that our AFM height measurements suggest that the DNA surfactant wrapped around each SWNT in a regular, organized fashion,22,23 thereby minimizing direct contact between the nanotube and the quartz surface. This consideration is important because SWNT fluorescence varies with surfactant system,28−31 solvent,10 atmosphere,28 temperature,32 and pressure33 and can be quenched when the SWNT is in direct contact with some substrates.34 Note also that SWNT fluorescence was stable and uninterrupted (i.e., did not show any “blinking” or “bleaching”) for the bright SWNTs in our samples, again suggesting a weak influence of the substrate. Although the authors acknowledge that the quartz does influence the observed fluorescence behavior, our control studies indicate that the relatively small amount of quenching in our study was not enough to cause us to mislabel a large fraction of bright emitters. Importantly, direct correlation of fluorescence with topography from the same SWNTs allows us to quantify not only the number of emissive semiconductor nanotubes present, but also the fraction of aggregated particles. We found many SWNTs that are not completely isolated from neighboring particles: 23% of all emissive particles were bundles, 25% were neighboring SWNTs and in control samples (nonrinsed), 31% of all emissive particles were assigned to bundles based on characteristics of their fluorescence spectra. Thus, care must be taken when interpreting single molecule SWNT spectra and images, as data typically assigned to individual SWNTs may actually result from small bundles or nanotubes that are situated too close together to be resolved with most optical microscopes. Like previous measurements performed for ensembles of nanotubes,35 longer SWNT lengths did correlate with brighter luminescence intensities, though the correlation was not strong, perhaps due to limited statistical data.36,37 The average length of all DNA-wrapped SWNTs was ∼332 ± 142 nm and agrees well with lengths found for other samples prepared using ultrasonic processing techniques.37 Similar to the individual SWNTs observed by Cognet et al.,38 fluorescence spectra were acquired for straight, bent, and C-shaped SWNTs. Direct length comparison was complicated by variations in the orientation of

the SWNTs on the substrate with respect to the excitation polarization and differing local surfactant coverage. However, some correlation between length and brightness was observed after correcting the fluorescence intensity values based on the orientation for SWNTs situated linearly on the substrate, although a few outlying points exist contrary to the trend (e.g., short SWNTs with anomalously high corrected intensities as in Supporting Information Figure S9). It is possible that some SWNTs were not highly fluorescent because any exciton produced within half the distance of the exciton diffusion length (i.e., >100 nm) should be dark due to quenching at the nanotube ends. Although fewer than 1% of all SWNTs observed in correlated measurements were shorter than 100 nm, ostensibly mitigating these length effects, defects undetectable with AFM along the SWNT sidewall could limit the exciton diffusion length. We also note the possibility that incoming photons excited a weakly absorbing transition (i.e., E12 or E21). Because the E12 energy is close to the E22 excitation energy, these transitions can be readily observed.39,40 Other factors that could account for the observation of fluorescence from misaligned SWNTs include depolarization effects away from a purely linearly polarized laser spot associated with the high NA objective.41 In 2009, Naumov et al. collected uncorrelated topography and fluorescence data for a variety of as-produced and processed SWNT samples dispersed onto a substrate14 and, based on optical measurements, found that 92.1% of standard grade CoMoCAT SWNTs are semiconducting and thus emissive, whereas the remaining SWNTs were determined to be metallic. In the context of our study performed on purified semiconducting SWNTs only, we find a much lower percentage of emissive SWNTs in our samples. Several possible sources of this discrepancy exist, such as a difference in the relative emission efficiency of the samples. DNA-wrapped SWNTs are generally cited as weaker emitters.42 Although our correlated sodium-cholate-wrapped SWNT sample did have a slightly higher percentage of bright emitters than the DNA-wrapped SWNT samples, the observed population of 22% bright emitters in the control sample still falls considerably short of the assumption that all semiconducting SWNTs should have detectable fluorescence. Other factors include differences in excitation rate and the fact that we had to rinse away some surfactant to correlate the AFM and fluorescence images on quartz, possibly leading to higher rates of dark SWNTs in our experiments. On the other hand, we also found that the surface coverage of SWNTs spun on substrates is highly nonuniform, and the relative concentrations of SWNTs can vary by an order of magnitude across a surface. Thus, we found that large areas of the substrate must be imaged with the AFM in order to obtain accurate surface coverage densities for SWNTs. With correlated AFM and optical images, surface heterogeneity is not an issue. Finally, this study allows us to comment on methods that could be used to improve interpretation of optical studies of SWNTs. First, future optical studies should take into consideration that the vast majority of semiconducting SWNTs in typical surfactant-wrapped samples are not highly emissive, as a result of extrinsic factors such as defect sites introduced during processing. Second, we note that fluorescent SWNT samples may be deposited directly onto a substrate such as quartz either by spin-casting or simple drying (followed with rinsing), provided that the particles are well-insulated from the surface by a surfactant. Although fluorescence quenching can 2819

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The Journal of Physical Chemistry Letters certainly occur upon contact with a surface,9 we studied the influence of the surfactant coverage on SWNT fluorescence for samples dispersed onto quartz and found that the particles are indeed still emissive, even with minimal surfactant between the SWNT and the substrate. Finally, it is possible that some fluorescence spectra in single molecule studies that are usually assigned to individual SWNTs may arise from small bundles or neighboring nanotubes that occupy the same area corresponding to one pixel in a fluorescence scan. In summary, studies of correlated fluorescence and topography clarify important features of SWNT photophysics on quartz, including the fraction of brightly emissive particles in a typical surfactant-wrapped sample, the fraction of bundles that are present and the influence of the local environment on fluorescence behavior. By measuring fluorescence from bright SWNTs compared with topographic information obtained using AFM, we determined that only ∼11% of all DNAwrapped SWNTs in an ensemble are bright and that with careful control of the local environment, it may be possible to achieve QYs that exceed 3% for individual SWNTs. Our studies suggest that the ensemble SWNT QY is low in typical samples because only a small fraction of the total population is highly emissive. These studies help to clarify the understanding of the source of low ensemble SWNT QYs, which should assist in the preparation of brighter NIR fluorescence labels incorporating carbon nanotubes.

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ASSOCIATED CONTENT

S Supporting Information *

Sample preparation protocol, absorbance spectrum for sodiumcholate-wrapped SWNTs, correlated topography and fluorescence images for all four samples, histogram of raw SWNT intensity by (n,m) structure, histogram of SWNT (n,m) distribution observed, data on SWNT length and orientation on the substrate, summary tables of fluorescence and topography data for all samples, and summary table of fluorescence data for uncorrelated samples. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b01032.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 585-275-5093. Present Addresses ∥

Department of Chemistry, Gannon University, Erie, Pennsylvania 16541, United States. ⊥ Department of Chemistry, Valparaiso University, Valparaiso, Indiana 46383, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Lenore Kubie for her assistance with sample preparation. The authors gratefully acknowledge the Camille Dreyfus Teacher−Scholar Awards Program, the Alfred P. Sloan Foundation and the Department of Energy Office of Basic Energy Sciences through Grant DE-FG02-06ER15821 for financial support.

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