Photoluminescence from Exciton Energy Transfer of Single-Walled

Sep 21, 2012 - band gap tubes to smaller band gap tubes through intertube .... the PLE map of HiPco-SDS, all in good agreement with previously reporte...
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Photoluminescence from Exciton Energy Transfer of Single-Walled Carbon Nanotube Bundles Dispersed in Ionic Liquids Juan Yang, Nuoya Yang, Daqi Zhang, Xiao Wang, Yilun Li, and Yan Li* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Key Laboratory for the Physics and Chemistry of Nanodevices, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ABSTRACT: Single-walled carbon nanotubes (SWNTs) can be dispersed into fine bundles in imidazolium-based ionic liquids (ILs) by simple mechanical grinding. Photoluminescence (PL) of the exciton energy migration from larger band gap semiconducting donor nanotubes to smaller band gap semiconducting acceptor nanotubes within the same SWNT bundle is clearly observed and can be explained by the Förster resonance energy transfer (FRET) mechanism. This offers a simple way to relatively brighten up the PL of those less populated, large diameter, small band gap SWNT species. Taking surfactant sodium dodecyl sulfate (SDS)-dispersed samples as a control of individually dispersed SWNTs, incomplete thermalization before exciton recombination is demonstrated in IL-dispersed SWNT bundles.

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INTRODUCTION Ever since its discovery, photoluminescence (PL) of semiconducting single-walled carbon nanotubes (SWNTs)1−3 has attracted significant attention and has become the commonly used method for distinguishing the chirality of semiconducting SWNTs. The near-infrared (NIR) PL emission of SWNTs has potential applications in bioimaging,4−6 electroluminescent devices,3 and other optical and optoelectronic aspects. Attributed to the dependence of PL on chirality1 and environmental factors,7,8 fluorescence spectra of SWNTs can serve as a sensitive method for carbon nanotube population analysis,9 for characterization of different SWNT species,10,11 and for the fundamental study of SWNT band structures.12,13 Most previous studies have focused on suspended SWNTs14 and surfactant-wrapped individual SWNTs dispersed in solution.1,8,15,16 In those cases, SWNT bundles were just treated as byproducts or impurities that need to be debundled or removed through rigorous sonication and centrifugation processes before the PL spectra could be taken since it was believed that the PL signals would be quenched in bundles. However, a few later studies have successfully observed the PL from bundled SWNTs in aqueous solutions17,18 and from suspended SWNT bundles.19,20 The study of PL from bundled SWNTs is especially interesting and important because all the bulk SWNT samples produced with various methods are always synthesized as bundles. Since it is possible to examine directly the optical properties of the component SWNTs within bundles through PL spectra, easy manipulation of the SWNT samples without irreversible damage of the electronic structures caused by extensive sonication and centrifugation processes can be expected. PL of isolated semiconducting SWNTs is emitted when excitons, i.e., electron−hole pairs, which are created by the incident photons, recombine through radiative emission. Light © 2012 American Chemical Society

absorption at higher excitation energies corresponding to the optical transitions such as v2 → c2 and v3 → c3 will be followed by PL emission at lower energy of the c1 → v1 transition for the same nanotube. Isolated metallic nanotubes, on the other hand, will not fluoresce owing to the continuous density of states at the Fermi level and the consequent nonradiative recombination of excitons. In the case for bundles consisting of all semiconducting nanotubes, excitons can migrate from larger band gap tubes to smaller band gap tubes through intertube exciton energy transfer (EET). In the case for bundles containing at least one metallic nanotube, PL will quench due to the photoexcited exciton migration from radiative semiconducting tubes to nonradiative metallic tubes. The PL quench effect will become dramatic as the bundle size increases. Assuming a random chirality distribution with 1/3 metallic and 2/3 semiconducting nanotubes, it can be calculated that the probability of a bundle consisting of all semiconducting SWNTs is less than 2% when the number of nanotubes in the bundle is 10 and is only about 0.03% for 20. Therefore, clear PL emission spectra can only be observed for isolated individual semiconducting SWNTs or for fine SWNT bundles. It was reported that by mixing together and mechanically grinding the bulk SWNT samples with imidazolium-based ionic liquids (ILs) a thermally stable bucky gel with fine SWNT bundles can be formed.21 The IL-dispersed SWNTs are a good system for studying the PL characteristics and the EET of fine SWNT bundles with the following advantages: First, this dispersion method does not involve any sonication, centrifugation, or chemical reaction, thus the molecular integrity of the SWNT samples is retained. Second, imidazolium-based ILs are Received: July 2, 2012 Revised: August 30, 2012 Published: September 21, 2012 22028

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Figure 1. TEM images of (a) IL-dispersed and (b) SDS-dispersed SWNTs. Individual SWNTs covered by SDS are indicated by arrows. The scale bar is 50 nm.

Figure 2. PLE maps of (a) IL-dispersed and (b) SDS-dispersed HiPco SWNTs. (c)−(e) are the emission slices taken from (a) and (b) at the corresponding excitation wavelengths indicated by red arrows, where the (8,4), (7,6), and (9,4) tubes are excited with highest emission intensities, respectively.

the Förster resonance energy transfer (FRET) mechanism.23 Incomplete thermalization before exciton recombination is demonstrated in IL-dispersed fine SWNT bundles. This gives a way to relatively brighten up the PL of those less populated, large diameter, small band gap SWNT species.

totally transparent in the 800−1600 nm near-IR spectral region where SWNTs fluoresce. The absorption does not overlap with that of SWNTs either. Third, it was reported that there is no strong interaction but only weak van der Waals interaction between SWNTs and ILs,22 so the electronic structures and properties of SWNTs could be kept intrinsically. ILs are ideal media for the study and application of SWNTs. In this work, the photoluminescence excitation (PLE) contour maps of IL [BMIM][PF6]-dispersed SWNTs are reported and compared with those of sodium dodecyl sulfate (SDS)-dispersed SWNTs. Emission spectra with excitations of the populated (6,5), (7,5), (7,6), and (8,4) chiralities are deconvoluted into individual fluorescence peaks. Clear EET is observed in the IL dispersion system and can be explained by

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EXPERIMENTAL SECTION

SWNTs produced by both high-pressure decomposition of carbon monoxide (HiPco) and decomposition of CO on cobalt−molybdenum catalyst (CoMoCAT) were utilized in this work. The IL [BMIM][PF6] (99% purity) was purchased from Henan Lihua Pharmaceutical Co. Ltd., China. 22029

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HiPco-IL suspension, due to the largely decreased PL intensities, 14 nm excitation and 50 nm emission slits are used, and the integration time is prolonged to 60 s to obtain a clear PLE map. Even so, the observed overall PL intensities are still much lower than those for HiPco-SDS. The significantly decreased yet still observable PL intensities are evidence that by simple mechanical grinding bulk SWNTs are dispersed into fine bundles in ILs. If most SWNTs are dispersed individually, the PL quench effect will not be so obvious, but if the bundles are relatively large almost all the PL should be quenched. This PL quench mainly results from energy transfer from semiconducting SWNTs to the neighboring metallic SWNTs inside the same bundle and the consequent successive nonradiative decay. Taking into account the different slit width and accumulation time, it can only be roughly estimated from the overall PL intensity ratio that the average bundle size in HiPcoIL is likely between 10 and 20. Second, broad and red-shifted emission bands. The individual PL emission peaks of HiPco-IL are generally redshifted by about 20−50 nm from those of HiPco-SDS, and the fwhm of the peaks are broadened from 30 to 70 meV, noticing that part of this broadening effect is arising from the larger emission slit width used for HiPco-IL. By carefully examining Figures 2a and 2b, it is found that the excitation wavelengths of HiPco-IL are also red-shifted by about 5−10 nm from those of HiPco-SDS. These results as well as the corresponding energy shifts ΔE22 and ΔE11 for each identifiable SWNT chirality are summarized in Table 1. Since the accuracy of ΔE22 is highly

The SWNT-IL suspension was prepared by carefully grinding ∼0.20 mg of SWNT sample in an agate mortar and pestle with 0.5 mL of [BMIM][PF6] for 30 min, and the mixture was then washed off from the mortar and pestle by 9.5 mL of raw [BMIM][PF6]. Absolutely no sonication and centrifugation were used for the SWNT-IL suspension. The SWNT-SDS suspension was prepared by mixing ∼0.20 mg of SWNT sample with 10 mL of 1 wt % SDS-D2O solution. The mixture was treated in a tip sonicator (JY92-2D, Xin Zhi Co.) at 200 W for 10 min (on 1 s, off 1 s) and then centrifuged at 11 000 rpm for 30 min. The resulting supernatant was used for absorption and fluorescence characterization. The PLE measurements were performed on a Horiba Jobin Yvon NanoLog-3 spectrofluorometer equipped with a 450 W xenon arc lamp and a liquid nitrogen-cooled InGaAs detector. An 830 nm filter was set in front of the detector to cut off the Rayleigh scattering. The excitation wavelength was varied from 500 to 750 nm in 5 nm steps, and the emission was collected in the 900−1550 nm region. The ultraviolet−visible-near-infrared absorption spectra were collected in a 1.0 cm path length cell with a PerkinElmer Lambda 950 spectrophotometer. A scan rate of 140 nm/min with a step of 0.5 nm was typically used. The transmission electron microscopy (TEM) was taken on a Tecnai-G2-20-S-Twin (FEI Company) with an acceleration voltage of 120 keV.

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RESULTS AND DISCUSSION Figure 1 shows the TEM images of IL-dispersed and SDSdispersed SWNTs. As can be seen, the SWNTs dispersed in SDS are individual nanotubes covered by surfactant, while the SWNTs in ILs are bundles with an average size of ∼15 nm. In this work, we use the SWNT-SDS suspension as a control of individually dispersed nanotubes and use the SWNT-IL suspension with fine bundles for spectroscopic measurements to study the exciton energy transfer in SWNT bundles. SWNTs have sharp van Hove singularities arising from quasione dimensionality. The energy values of E11, E22, and E33, which are the energy differences between the corresponding valence and conduction bands, are dependent mainly on tube chirality. Therefore, by analyzing the PLE maps with both excitation and emission wavelengths, one can distinguish different semiconducting nanotube species and make the proper chirality assignments. Figures 2a and 2b exhibit the PLE contour maps of HiPco SWNTs dispersed in IL [BMIM][PF6] and in SDS-D2O, respectively, in the same E22 excitation and E11 emission spectral regions for comparison purposes. The bold solid line at the upper right corner in Figure 2a represents secondary diffraction bands, the wavelength of which is twice the excitation wavelength, arising from the scattering caused by nanotube bundles even in the presence of an 830 nm filter. As can be seen, 14 different chiralities are clearly observed in the PLE map of HiPco-SDS, all in good agreement with previously reported data.1 Proper (n,m) assignment can be made accordingly. Of these 14 chiralities, all except (8,3), which has the highest E11 emission, are also shown in the PLE map of HiPco-IL. Comparing to the SDS suspension, three distinct features can be observed in the PLE map of IL suspension. First, largely decreased PL intensities. For HiPco-SDS, the slit widths for both excitation and emission are fixed at 10 nm, and the integration time for each emission is 20 s. The recorded PLE map shows distinct and well-structured PL peaks, indicating most SWNTs are suspended individually. For the

Table 1. Excitation and Emission Wavelengths of HiPco-IL and HiPco-SDS, Respectively, and the Corresponding Energy Shifts for Each Identifiable Chirality in the PLE Maps

(n,m)

dt (nm)

λ22 in HiPcoIL (nm)

(6,5) (8,3) (7,5) (8,4) (10,2) (7,6) (9,4) (11,1) (10,3) (8,6) (9,5) (8,7) (12,2) (11,4)

0.757 0.782 0.829 0.840 0.884 0.895 0.916 0.916 0.936 0.966 0.976 1.032 1.041 1.068

570 -650 590 740 650 725 615 640 725 680 735 695 720

λ22 in HiPcoSDS (nm) 566 663 645 587 735 645 720 609 632 715 670 726 685 712

ΔE22 (meV)

λ11 in HiPcoIL (nm)

λ11 in HiPcoSDS (nm)

ΔE11 (meV)

15 -15 11 11 15 12 20 25 24 27 21 26 19

1000 -1054 1140 1080 1148 1132 1304 1291 1214 1285 1302 1432 1421

976 950 1022 1111 1052 1120 1101 1263 1249 1173 1243 1262 1376 1368

30.5 -35.7 28.4 30.6 27.0 30.8 30.9 32.3 35.7 32.6 30.2 35.2 33.8

limited by the 5 nm excitation measurement steps, the ΔE11 values are expected to be more accurate than ΔE22. It is interesting to notice that although the measured red shifts of λ11 vary from 20 to 50 nm in wavelength for different chirality, when converting into energy units all the energy shifts of E11 are actually close to 30 meV and do not show any clear dependence on tube chirality or diameter. As this red shift is mainly attributed to the surrounding dielectric environment changes caused by the neighboring nanotubes inside the same bundle, the chirality-independent red shift suggests a nearly 22030

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Figure 3. (a) Optical absorption spectra of HiPco-IL and HiPco-SDS-D2O. (b) Deconvolution of the optical absorption spectra of HiPco-IL and HiPco-SDS-D2O in the spectral region of E11S. Deconvoluted peaks indicated by * have been assigned to more than one chirality.

Figure 4. (a) Schematic view of EET between donor and acceptor tubes in a SWNT bundle dispersed in IL [BMIM][PF6]. (b) Schematic band structure diagram for EET between donor and acceptor tubes within the same SWNT bundle.

uniform dielectric environment around the nanotubes in IL suspensions. Third, higher relative intensities for smaller bandgap nanotubes. For HiPco-SDS, nanotubes with lower E11 emission energy and larger diameter such as (8,7), (9,5), and (10,3) have lower PL intensities than those with smaller diameter and emission wavelengths in the 1000−1200 nm region such as (8,4), (7,6), (9,4), and (8,6). In contrast, HiPco-IL shows a significant increase in the relative PL intensities for (8,7), (9,5), and (10,3) tubes. The relative PL intensities for higher E11 emission energy and smaller tubes including (8,4), (7,5), (7,6), (9,4), and (10,2) decline, and the PL of the (8,3) tube, which has the highest E11 emission energy of all 14 chiralities, even vanishes. This relative PL intensity enhancement in nanotubes with higher E11 emission energy is even more distinct in the individual emission spectra, as given in Figures 2c−2e. These are the PL emission slices taken from Figures 2a and 2b at the position indicated by red arrows, where the (8,4), (7,6), and (9,4) tubes are excited with highest emission intensities at 590, 650, and 725 nm for HiPco-IL and at 585, 645, and 720 nm for HiPco-SDS, respectively. The intensities in Figures 2c−2e are normalized with respect to the highest peak in the region for

better comparison purposes and thus do not reflect the absolute PL intensities. The emission spectra are deconvoluted into individual fluorescence peaks using Lorentzian band profiles. All peak wavelengths, heights, and widths are allowed to vary freely for the best fitting of the spectra, and the corresponding chiralities are assigned. As can be seen, the emission peaks for tubes with higher E11 disappear, and the deconvoluted peak area for tubes with lower E11 increases dramatically for HiPcoIL. For example, in Figure 2c where the (8,4) tube is excited, the emission peak of (8,4) is predominant in the SDS suspension as expected, while the (10,3) tube gives even higher PL intensity over the (8,4) tube in the IL suspension. One possible aspect for the difference in relative PL intensities with respect to tube diameter that needs to be taken into account is the change in SWNT chirality distribution due to the different sample preparation method for SDS and IL suspensions. Because the SDS dispersion method requires intense sonication and subsequent centrifugation processes, the relative abundance of different chirality might be different from that in the IL suspension, which involves only simple mechanical grinding. To check for this, we measured the optical absorption spectra for both HiPco-IL and HiPco-SDS, 22031

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relative PL emission for the small bandgap larger diameter acceptor tubes. Although excitons most likely migrate from donor to acceptor, it is also reported that they can be transferred back from acceptor to donor if providing sufficient thermal energy.25 J. Lefebvre and P. Finnie have studied the PL and FRET in elemental bundles of SWNTs, and they have shown the PL peak intensity ratio between donor and acceptor as an exponential function of ΔE11, giving a slope corresponding to a simple Boltzmann distribution factor at room temperature. Although the quantum yield of different chirality is not considered in their case and significant scatter of data points is observed, their data still demonstrate efficiently thermalized excitons before recombination in elemental SWNT bundles and 100% FRET transfer efficiency.25 In this present study, the ratio of the deconvoluted PL peak area between the donor and acceptor

as shown in Figure 3a. In the optical absorption spectra, it is already well recognized that the v1 → c1 and v2 → c2 transitions for semiconducting HiPco SWNTs are usually observed in the ranges of 900−1600 and 600−900 nm, respectively.24 It can be seen in Figure 3a that the two absorption bands centered at 741 and 818 nm in the E22S region of HiPco-IL are red-shifted by about 8 nm from those in HiPco-SDS. In the spectral region of E11S, a direct peak-to-peak correspondence cannot be easily established at the first glance. However, after deconvolution of each broad band in this region with respect to the component semiconducting SWNT chiralities, about 30 meV red shift is observed. These data are in good agreement with results summarized in Table 1 from PL measurements. The deconvolution is carried out with the absorption spectra converted into energy unit. The initial band energies are calculated from the corresponding PL emission wavelength. During the iteration, the peak height and width are allowed to vary freely, but the individual peak energies are only allowed to shift from the initial values by ±10 meV. The deconvoluted spectra are given in Figure 3b, with the solid black curves denoting the background-subtracted absorption spectra in the E11S region, the dashed red and blue curves denoting the fitting spectrum, and the solid red and blue curves denoting the deconvoluted individual peaks, for HiPco-IL and HiPco-SDS, respectively. From the deconvoluted spectra, it can be concluded that the relative abundance for different chiralities does differ from one another in these two suspensions; however, there is no clear trend that IL suspension is favored in larger diameter nanotubes. Therefore, the difference in relative abundance could not possibly account for the dramatic enhancement in relative PL intensities for larger diameter nanotubes in IL suspension. The loss in absolute PL intensity but enhancement in relative PL intensity for certain chiralities are also observed in many other cases for SWNT bundles, and this can be explained by EET.17−20 Förster resonance energy transfer (FRET) is an efficient EET mechanism that can be attributed to SWNT bundles. Figure 4a illustrates the schematic view of EET between donor and acceptor tubes in a SWNT bundle dispersed in IL [BMIM][PF6], and Figure 4b shows the schematic band structure diagram for EET between donor and acceptor tubes within the same SWNT bundle. Excitons in a fluorescent donor (D) nanotube with larger band gap can transfer to a smaller band gap acceptor (A) nanotube, which will fluoresce at lower energy, through resonant, near-field, dipole−dipole coupling. The FRET efficiency is dependent on the D−A distance, the relative orientation of emission and absorption dipoles, and the spectral overlap between the donor and the acceptor. For a SWNT bundle since the tubes are parallel to each other and the wall-to-wall distance between tubes is on the order of graphite stacking, ∼0.34 nm, the FRET efficiency is expected to be high and mainly dependent on the corresponding D−A coupling, which is a function of the energy splitting ΔE11 = ED11 − EA11. At a particular excitation wavelength, only SWNTs with a certain (n,m) can be excited. The photoexcited excitons can migrate from this particular (n,m) tube to the neighboring semiconducting nanotubes inside the same bundle. Many migrations may happen before the excitons finally recombine. In each migration, the tube with larger E11 serves as the donor, while the tube with smaller E11 is the acceptor. Excitons tend to hop from donor tubes to acceptor tubes, resulting in higher

η=

A(E11A − IL) D A(E11 − IL)

(1)

is plotted with respect to the corresponding energy splitting ΔE11, denoted by the black triangles in Figure 5. A logarithm

Figure 5. Deconvoluted PL peak area ratio between donor and acceptor tubes in HiPco-IL as a function of the energy difference between the peaks. Black triangles and red dots are data points before and after normalization with the corresponding deconvoluted PL peak area in SDS. Red line is a linear fit to the red dots. Note that the peak area ratio is in logarithm scale.

scale is used for the peak area ratio. A broad scatter of the data points is displayed. Data points denoted by red dots in Figure 5 distribute much narrower scatter and are derived by normalization of the peak area in the IL with the corresponding (n,m) peak area in SDS for both donor and acceptor, as η′ = =

A(E11A − IL)/A(E11A − SDS) D D A(E11 − IL)/A(E11 − SDS) D A(E11A − IL)A(E11 − SDS) D A(E11 − IL)A(E11A − SDS)

(2)

Given that different nanotube chirality has different quantum yield and different relative abundance, both factors will affect the relative PL intensity for the corresponding chirality. As the deconvoluted peak area in SDS can serve as a control for the PL of the individually dispersed SWNTs, the values in SDS can 22032

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Table 2. Excitation and Emission Wavelengths of CoMoCAT-IL and CoMoCAT-SDS, Respectively, and the Corresponding Energy Shifts for Each Identifiable Chirality in the PLE Maps (n,m)

dt (nm)

λ22 in CoMoCAT-IL (nm)

λ22 in CoMoCAT-SDS (nm)

ΔE22 (meV)

λ11 in CoMoCAT-IL (nm)

λ11 in CoMoCAT-SDS (nm)

ΔE11 (meV)

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

0.757 0.782 0.806 0.829 0.840 0.884 0.895 0.916 0.916 0.936 0.966 0.976 1.032

570 670 555 650 590 -650 720 -640 715 675 --

565 665 550 645 585 735 645 720 610 635 715 670 725

19 0 20 15 18 -15 0 -15 0 14 --

1003 971 1165 1048 1138 -1148 1148 1302 1297 1223 1296 --

980 952 1138 1024 1111 1054 1121 1108 1262 1250 1175 1242 1260

29.0 25.5 25.3 27.7 26.5 -26.0 39.0 30.2 35.9 41.4 41.6 --

Figure 6. PLE maps of (a) IL-dispersed and (b) SDS-dispersed CoMoCAT SWNTs. (c) and (d) are the emission slices taken from (a) and (b) at the corresponding excitation wavelengths indicated by red arrows, where the (6,5) and (7,5) tubes are excited with the highest emission intensities, respectively.

thus account for the difference in both quantum yield and relative abundance for the corresponding chirality. By taking into account this correction for both donor and acceptor tubes, we believe η′ reflects the PL intensities arising simply from the EET between different semiconducting SWNTs in bundles. Log η′ can then be fitted nicely by a linear relationship with respect to ΔE11, as shown by the red fitting line in Figure 5. All data points are close to the fitting line, and the fitted intercept is close to 1 as expected. The fitted slope is 0.0047 meV−1, which is much less than the value of 0.017 meV−1 calculated directly from the Boltzmann distribution factor and also reported in elemental bundles of SWNTs25 at room temperature. This indicates significant competition between exciton thermalization and recombination in IL suspension. In other words,

excitons are not completely thermalized before they recombine, resulting in more donor emission and less acceptor emission than in the case of complete thermalization. The incomplete exciton thermalization before recombination can be attributed to two major reasons. First, the lifetime of excitons will be dramatically decreased in IL suspension. Due to the large dielectric constant and high viscosity of the surrounding ILs, the exciton recombination times for both donor and acceptor will be reduced. Second, the average SWNT bundle size in IL suspension is estimated to be in the order of 10−20, much larger than the elemental bundles reported by Lefebvre and Finnie.25 Excitons may transfer and relax multiple times among the SWNTs in one bundle before they finally recombine. Although the recombination time for an 22033

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Figure 7. (a) Optical absorption spectra of CoMoCAT-IL and CoMoCAT-SDS-D2O. (b) Deconvolution of the optical absorption spectra of CoMoCAT-IL and CoMoCAT-SDS-D2O in the spectral region of E11S. Deconvoluted peaks indicated by * have been assigned to more than one chirality.

800−1400 and 550−800 nm, respectively, the latter also exhibiting a ∼30 meV red-shift after individual peak deconvolution, as shown in the optical absorption spectra in Figure 7. By plotting the relative ratio of the deconvoluted PL peak area η′ between donor and acceptor in logarithm scale with respect to the energy splitting ΔE11, data points for CoMoCAT can also be fitted into a linear relationship with a slope of 0.0021 meV−1. This slope is of the same order of magnitude as that of HiPco and indicates that incomplete exciton thermalization before recombination is a common feature in ILsuspended SWNT bundles. The smaller slope in CoMoCAT than that in HiPco can be explained by lifetime and abundance variation in different (n,m) species. CoMoCAT contains less diverse (n,m) distribution than HiPco, and the most populated chiralities are larger bandgap smaller diameter tubes such as (6,5), (7,5), and (7,6), which usually serve as donor tubes during EET. On one hand, the radiative rate is reported to increase with increasing emission energy.26 Thus, the average donor recombination time is shorter for CoMoCAT than for HiPco. On the other hand, the higher abundance of donor tubes in CoMoCAT gives rise to a higher probability for the case that more than one donor tube with the same (n,m) is inside the same bundle simultaneously. Both factors will result in higher donor emission and lower fitted slope.

individual SWNT is typically much longer than the EET time and the intraband relaxation time, the accumulated exciton thermalization time in bundles may reach the order of the reduced recombination time. Therefore, competition between thermalization and recombination will happen in IL-suspended SWNT bundles. Similar research has been carried out on CoMoCAT-IL and CoMoCAT-SDS. As is well recognized, CoMoCAT contains smaller-diameter nanotubes and less diverse (n,m) distribution than HiPco. The dielectric screening effect of ILs is expected to be similar; however, the EET features are expected to be different to some extent. The corresponding data and spectra for CoMoCAT-IL and CoMoCAT-SDS are given in Table 2 and Figures 6−8. Comparing the PLE contour map and emission slices of CoMoCAT-IL to that of CoMoCAT-SDS, the emission bands are broad and red-shifted by about 30 meV. The absolute PL intensity is largely decreased, but the relative intensity for smaller bandgap tubes is enhanced dramatically. The v1 → c1 and v2 → c2 transitions of semiconducting nanotubes for CoMoCAT are usually observed in the ranges of

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CONCLUSION In this paper, photoluminescence from the exciton energy transfer of the IL-dispersed HiPco and CoMoCAT bundles is clearly observed, with loss in absolute PL intensity and enhancement in relative PL intensity for small bandgap large diameter acceptor tubes, and can be explained by the FRET mechanism. Taking surfactant SDS-dispersed samples as a control of individually dispersed SWNTs, incomplete exciton thermalization before recombination is demonstrated in ILdispersed SWNT bundles and can be attributed to the high dielectric constant of ILs and large SWNT bundle size in IL suspension. The SWNTs-IL suspension offers a simple way to relatively brighten up the PL signal for those nanotubes with higher E11 emission energy and larger diameter, the PL intensity of which is normally covered up by the intense signal of largely populated smaller diameter tubes.

Figure 8. Deconvoluted PL peak area ratio between donor and acceptor tubes in HiPco-IL and CoMoCAT-IL, respectively, after normalization with the corresponding deconvoluted PL peak area in SDS as a function of the energy difference between the peaks and the linear fits. Note that the relative peak area ratio is in logarithm scale. 22034

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-62756773. Fax: +8610-62756773. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank NSFC (Projects 21005004, 21125103, 11179011, J1030413), SRFDP of China, and MOST (Project 2011CB933003) of China for support.

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