NANO LETTERS
Photoluminescence from Intertube Carrier Migration in Single-Walled Carbon Nanotube Bundles
2006 Vol. 6, No. 12 2864-2867
O. N. Torrens,† D. E. Milkie,† M. Zheng,‡ and J. M. Kikkawa*,† Department of Physics and Astronomy, UniVersity of PennsylVania, 209 South 33rd Street, Philadelphia, PennsylVania 19104, and DuPont Central Research and DeVelopment Experimental Station, Wilmington, Delaware 19880 Received September 1, 2006; Revised Manuscript Received November 14, 2006
ABSTRACT Photoluminescence (PL) identifies spectroscopic signatures of intertube transfer of optically pumped carriers in single-walled carbon nanotube (SWNT) ensembles. Resonant photoexcitation of large band gap SWNTs produces strong PL from smaller band gap SWNTs. Magnetic alignment measurements associate the energy-transfer PL peaks with the formation of SWNT bundles, suggesting that efficient coupling results from physical contact.
The first observation of bright PL from aqueous suspensions of SWNT ensembles1 established optical emission as a crucial probe of isolated nanotube physics. The key to this experimental discovery was the exfoliation of SWNT ropes, accomplished through a combination of high-shear mixing, ultrasonication in the presence of surfactant, and centrifugation. Debundling is thought to improve PL efficiency by preventing migration of photoexcited carriers from bright semiconducting SWNTs to nonradiative metallic SWNTs. This hypothesis implies that carrier transfer between semiconducting SWNTs in a bundle should not only be possible but likely. Direct observations of such migration would not only support popular explanations of why isolated SWNTs tend to be brighter but would also create opportunities for dynamical studies of intertube exciton migration and coupling tunability between various semiconducting SWNT species. In this letter, efficient transfer of photoexcited carriers is directly demonstrated through optical spectroscopy of semiconducting SWNTs in aqueous suspension. Two samples were prepared: a well-dispersed sample of DNA-wrapped SWNTs2 and a sample in which the DNA had been partially replaced with the surfactant sodium dodecyl benzene sulfonate (NaDDBS).3 PL obtained from the former shows expected features, while the latter displays new energytransfer (ET) resonances in which photoexcited carriers migrate from larger to smaller band gap SWNTs and then recombine radiatively. Magnetic alignment studies further show that partial removal of DNA produces SWNT bundles. * Corresponding author. E-mail:
[email protected]. † Department of Physics and Astronomy, University of Pennsylvania. ‡ DuPont Central Research and Development Experimental Station. 10.1021/nl062071n CCC: $33.50 Published on Web 11/24/2006
© 2006 American Chemical Society
Hence the ET resonances are most naturally interpreted as occurring between semiconducting SWNTs within the same bundle. These results suggest a paradigm for improving SWNT quantum efficiency in which the absorptive crosssection of populous SWNTs is used to enhance emission from less-populous, smaller band gap SWNTs. Our first sample (DNA-dispersed) is a room-temperature aqueous suspension of DNA-wrapped CoMoCAT4 SWNTs purified by size exclusion chromatography.5 The SWNTs were dispersed in water with d(GT)20 ssDNA, Tris (50 mM), EDTA (0.5 mM), and NaCl (0.2 M) at pH 7. A second sample (NaDDBS substituted) was made from this first sample by a partial substitution of DNA with NaDDBS (C12H25C6H4SO3Na, Acros 88%). Surfactant exchange was accomplished by repeated concentration of the DNAdispersed sample through a centrifugal filter device (Millipore Microcon YM-50, 2000 g acceleration) and dilution with aqueous solutions of NaDDBS, yielding a SWNT sample with NaDDBS concentration of 0.01 g/mL. Low-power, high-frequency bath sonication3 for a total of 6 h resulted in a suspension that was stable for months at room temperature. Optical absorbance of the DNA-dispersed and NaDDBSsubstituted samples (Figure 1) shows that the relative abundance of various SWNT species is generally comparable and not heavily influenced by choice of surfactant here. Optical characterization of these samples include PL and linear dichroism (LD) spectroscopies. For the PL measurement, a Xenon arc lamp illuminated the sample through a monochromator (12.5 nm bandwidth), and sample emission was dispersed with 8 nm resolution onto a TE-cooled InGaAs photodiode array. The PL emission was collected at a right
Figure 1. (A) Optical absorbance of the DNA-dispersed and (B) NaDDBS-substituted samples.
angle to the excitation, with the measured volume chosen to minimize absorption losses in the excitation and emission beams. Additional magnetic-field-dependent LD measurements obtained the difference in sample optical absorption polarized parallel and perpendicular to the field of a splitcoil superconducting magnet. These LD measurements were made with near-infrared light from the Xe lamp using a Glan-Thompson broadband polarizer, a photoelastic modulator, a spectrometer with 8 nm resolution, and an InGaAs photodiode. Standard lock-in techniques were used to collect the LD data. In both PL and LD measurements, appropriate wavelength filtering avoided second-order grating effects in the monochromator and spectrometer. The DNA-dispersed sample served as a control, and PL resonance peaks from this sample (Figure 2A) were unambiguously identified with specific (n,m) SWNT chiralities.6 Over the range of excitation and detection wavelengths shown, resonances correspond to excitation of the secondlowest optical exciton (E22 exciton) and near-infrared emission at the lowest optical exciton (E11 exciton). While intrinsic PL efficiencies are chirality dependent,7,8 one qualitatively observes that this sample is enriched in only a few species. The (8,3), (6,5), and (7,5) resonances are dominant, with an observed pattern of intensities that compares well with prior measurements on CoMoCAT SWNTs.9 In contrast, broad and intense PL features appear in the NaDDBS-substituted sample occurring from intertube energy transfer (Figure 2B). These ET peaks cannot be identified with the nearby (7,6), (8,4), and (9,2) resonances because they do not match the trend in surfactant-related shifts10,11 established by the (8,3), (6,5), and (7,5) resonances. Figure 2C shows that the latter chiralities undergo small shifts Nano Lett., Vol. 6, No. 12, 2006
Figure 2. (A) PL intensity versus excitation and emission wavelengths for the DNA-dispersed sample, with chiral assignments, and (B) the partially NaDDBS-substituted sample with energy-transfer (ET) peaks labeled. The data have been normalized to unity, corrected for wavelength-dependent excitation and detection efficiencies, and plotted on the same color scale. (C) Equal intensity contours from (B) of 0.4 and 0.9 (solid lines). Blue circles show peak locations for indicated chiralities of the DNA-dispersed sample from (A), and black squares show published values of peak locations for surfactant dispersion with no residual DNA.12 Dashed lines in (C) show the excitation couplings discussed in the text, with arrows in indicating the direction of carrier migration.
appropriate to partial NaDDBS substitution, appearing midway between the positions for pure DNA (Figure 2A) and pure NaDDBS12 dispersion. With the scope of the ET resonances indicated by the shaded region in Figure 2C, one can see that the most intense portion (0.9 intensity contour) does not match the expected locations for (8,4) and (9,2) peaks and cannot be assigned to any individual SWNT chirality. The intensity of the ET peaks makes chiral assignments even less plausible. Substantial chiral-specific changes in SWNT radiative efficiencies and/or populations would be necessary to explain the strength of the ET peaks.13 Both appear unlikely because SWNTs from the DNAdispersed sample are transferred to the NaDDBS-substituted sample and none of the processing steps are expected to be highly species selective.14 Finally, the absorbance data 2865
Figure 3. (A) PL spectra from the NaDDBS-substituted sample at the (8,3), (7,5), and (6,5) excitation resonances, rescaled to show a common line shape. (B) Average of the data from (A) (dashed line), modeled as a linear combination (black line) of PL from the (8,4), (9,2), and (7,6) chiralities. The latter are obtained from resonant excitation of the DNA-dispersed sample. All the spectra in (B) are shown after a linear background subtraction. Additionally, a blue-shift of 12.2 nm was applied to the DNA-dispersed emission axis to account for differences in surfactant, as determined from Figure 2C.
(Figure 1) show no dramatic change in populations and/or oscillator strength that would support such arguments. Instead, the PL data suggest that photon absorption by one species leads to luminescence in another. The ET excitation resonances align well with those of the most populous SWNT species, namely the (8,3) (7,5), and (6,5) chiralities (dashed lines, Figure 2C), and their emission coordinates align well with those of the (8,4), (9,2), and (7,6) species (Figure 3). Such intertube excitonic or electron-hole transfer would most likely occur in bundles, where photoexcited carriers could quickly migrate from populous large band gap tube species into less-common smaller band gap SWNTs. These latter tubes would thus have selectively enhanced emission, while the absorbance characteristics for the entire ensemble would remain relatively unchanged, as observed in our data (Figures 2B and 1, respectively). Also consistent with carrier transfer within a bundle is the broadened emission line shape of the ET resonances, which is interpreted as an overlap of PL bundle emission from the (7,6), (8,4), and (9,2) species. Figure 3 shows that the ET emission line shape is the same for all resolved excitation resonances and can be modeled as a linear combination of emission resonances from the aforementioned chiralities (obtained from the DNA-dispersed sample and blue-shifted by 12.2 nm to account for the change in surfactant). To see whether the extent of bundling indeed differs between our samples, we used LD to study their magnetic alignment. The latter is driven by magnetic anisotropy and is sensitive to bundling, which increases the magnetic anisotropy per particle. The LD method probes nearly the entire suspension in situ, an important advantage over atomic force microscopy, where relatively few tubes are profiled after substrate deposition (a process that can change the degree of aggregation). In our work, magnetic alignment is quantified by the nematic order parameter, S(B) ) 〈(3 cos2 φ - 1)/2〉, where φ is the angle between the long axis of the tube and an applied magnetic field, B B, and angle brackets denote ensemble averaging.15 Because SWNTs have aniso2866
Figure 4. Magnetic-field-dependent alignment where ∆R/3R j ∼ S(B) for each sample. The DNA-dispersed sample (blue) data were taken at 990 nm, and the NaDDBS-substituted sample (pink) data were taken at 978 nm.
tropic polarized absorption features, alignment in a magnetic field results in a macroscopic polarization of the ensemble absorption. This linear absorptive dichroism is measured to obtain S.16 The differences in absorption between light polarized parallel and perpendicular to the magnetic field, R| and R⊥, respectively, are related to S according to ∆R(B) ) S∆Rmax, where ∆R ≡ R| - R⊥ is the absorptive anisotropy and ∆Rmax is the absorptive anisotropy in the limit of complete uniaxial alignment (S ) 1). For SWNTs in the S ) 1 limit, R| . R⊥ due to the depolarization effect of SWNTs.16 Therefore, where the E11 exciton resonance dominates absorption, the quantity ∆R/3R j approximates S, where R j ≡ (1/3)(R| + 2R⊥) is the isotropic absorbance. Figure 4 compares the alignment of DNA and NaDDBSsubstituted SWNTs as a function of magnetic field. Data in Figure 4 were taken at the (6,5) absorption peak, which measures the alignment primarily of that chirality. For the DNA-dispersed sample, we observe a weak parabolic response in B, which is expected for a sample free of large bundles or torques due to ferromagnetic catalyst impurities.15 In contrast, the NaDDBS-substituted sample shows much greater alignment. This latter behavior is consistent with the formation of SWNT bundles because the magnetic-to-thermal energy ratio that drives alignment is, to lowest order, an additive property of constituent SWNTs within a bundle. The nonparabolic shape implies a heterogeneous distribution of bundle sizes, with larger bundles approaching alignment saturation (S ∼ 1) more rapidly. We note that catalyst-assisted alignment is unlikely to account for the observed behavior because the NaDDBS-substituted sample is created directly from the DNA-dispersed sample, which is free of attached ferromagnetic impurities. The spectral distribution of the linear dichroism measurement provides further evidence of bundling. Figure 5 compares ∆R and R j for both samples over a range of wavelengths sufficient to highlight the dominant (6,5) and Nano Lett., Vol. 6, No. 12, 2006
that the goal of researchers seeking to exploit SWNT optical properties may not require complete isolation of individual SWNTs. Future studies of “SWNT heterostructures” might therefore seek ways of deliberately introducing bundling in a controlled manner so as to tune the bundle size and perhaps composition. Acknowledgment. We thank Eugene J. Mele for helpful discussions and T. D. Gierke for supporting our collaboration with DuPont Central Research and Development. We acknowledge support from NSF MRSEC DMR-0520020 and NSF CAREER DMR-0094156, and partial support from DARPA through ONR N00015-01-1-0831. References
Figure 5. Linear dichroism ∆R induced at B ) 7 T (connected squares) and zero-field (isotropic) absorbance R j (solid lines) for (A) the DNA-dispersed sample and (B) the NaDDBS-substituted sample.
(7,5) species. In the DNA-dispersed sample (Figure 5A), the peak positions in ∆R and R j coincide, as expected from the relationship ∆R ∼ 3R j S. In the NaDDBS-substituted sample (Figure 5B), however, ∆R is red-shifted relative to R j , which implies that S is greatly enhanced on the low-energy side of the absorption resonances. This observation is to be expected from bundling because bundles should align more strongly and absorb at longer wavelengths than isolated SWNTs.1,17 Note that a broadening of absorbance features after NaDDBS substitution is additionally consistent with bundling.1,18,19 Although NaDDBS is an effective surfactant, in our case, the presence of residual DNA appears to lower its efficacy, perhaps by means of hydrophobic interactions.20 In conclusion, we observe ET resonances arising from a redistribution of nonequilibrium carriers to small band gap SWNTs within SWNT bundles. This process has been implicated in the quenching of PL emission from bundles by metallic SWNTs, and in this case, one must ask why metallic tubes do not also quench all bundle emission. Prior studies indicate that CoMoCAT SWNTs have a high (11:1) semiconducting-to-metallic ratio.21 Hence, the formation of bundles without any metallic SWNTs, or for which the time for an exciton to diffuse into a metallic tube exceeds the recombination time, is much more likely here than in other popular SWNT materials, where the ratio of semiconducting to metallic SWNTs is thought to be on the order 2:1. Additionally, our preparation conditions may yield smaller bundles for which the occurrence of metallic quenching is minimized. Our findings imply that interesting photophysics can be probed with metal-free dispersed SWNT bundles and suggest Nano Lett., Vol. 6, No. 12, 2006
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