photoluminescence from exciton energy transfer of...

Photoluminescence from Exciton Energy Transfer of Single-WalledCarbon Nanotube Bundles Dispersed in Ionic LiquidsJuan 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, KeyLaboratory 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 bedispersed into fine bundles in imidazolium-based ionic liquids (ILs) bysimple mechanical grinding. Photoluminescence (PL) of the excitonenergy migration from larger band gap semiconducting donor nanotubesto smaller band gap semiconducting acceptor nanotubes within the sameSWNT bundle is clearly observed and can be explained by the Forsterresonance energy transfer (FRET) mechanism. This offers a simple wayto 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.

■ INTRODUCTIONEver since its discovery, photoluminescence (PL) of semi-conducting single-walled carbon nanotubes (SWNTs)1−3 hasattracted significant attention and has become the commonlyused method for distinguishing the chirality of semiconductingSWNTs. The near-infrared (NIR) PL emission of SWNTs haspotential applications in bioimaging,4−6 electroluminescentdevices,3 and other optical and optoelectronic aspects.Attributed to the dependence of PL on chirality1 andenvironmental factors,7,8 fluorescence spectra of SWNTs canserve as a sensitive method for carbon nanotube populationanalysis,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 insolution.1,8,15,16 In those cases, SWNT bundles were justtreated as byproducts or impurities that need to be debundledor removed through rigorous sonication and centrifugationprocesses before the PL spectra could be taken since it wasbelieved that the PL signals would be quenched in bundles.However, a few later studies have successfully observed the PLfrom bundled SWNTs in aqueous solutions17,18 and fromsuspended SWNT bundles.19,20 The study of PL from bundledSWNTs is especially interesting and important because all thebulk SWNT samples produced with various methods are alwayssynthesized as bundles. Since it is possible to examine directlythe optical properties of the component SWNTs withinbundles through PL spectra, easy manipulation of the SWNTsamples without irreversible damage of the electronic structurescaused by extensive sonication and centrifugation processes canbe expected.PL of isolated semiconducting SWNTs is emitted when

excitons, i.e., electron−hole pairs, which are created by theincident photons, recombine through radiative emission. Light

absorption at higher excitation energies corresponding to theoptical transitions such as v2 → c2 and v3 → c3 will be followedby PL emission at lower energy of the c1 → v1 transition for thesame nanotube. Isolated metallic nanotubes, on the other hand,will not fluoresce owing to the continuous density of states atthe Fermi level and the consequent nonradiative recombinationof excitons. In the case for bundles consisting of allsemiconducting nanotubes, excitons can migrate from largerband gap tubes to smaller band gap tubes through intertubeexciton energy transfer (EET). In the case for bundlescontaining at least one metallic nanotube, PL will quench dueto the photoexcited exciton migration from radiative semi-conducting tubes to nonradiative metallic tubes. The PLquench effect will become dramatic as the bundle size increases.Assuming a random chirality distribution with 1/3 metallic and2/3 semiconducting nanotubes, it can be calculated that theprobability of a bundle consisting of all semiconductingSWNTs is less than 2% when the number of nanotubes inthe bundle is 10 and is only about 0.03% for 20. Therefore,clear PL emission spectra can only be observed for isolatedindividual semiconducting SWNTs or for fine SWNT bundles.It was reported that by mixing together and mechanically

grinding the bulk SWNT samples with imidazolium-based ionicliquids (ILs) a thermally stable bucky gel with fine SWNTbundles can be formed.21 The IL-dispersed SWNTs are a goodsystem for studying the PL characteristics and the EET of fineSWNT bundles with the following advantages: First, thisdispersion method does not involve any sonication, centrifu-gation, or chemical reaction, thus the molecular integrity of theSWNT samples is retained. Second, imidazolium-based ILs are

Received: July 2, 2012Revised: August 30, 2012Published: September 21, 2012

Article

pubs.acs.org/JPCC

© 2012 American Chemical Society 22028 dx.doi.org/10.1021/jp306515a | J. Phys. Chem. C 2012, 116, 22028−22035

pubs.acs.org/JPCC

totally transparent in the 800−1600 nm near-IR spectral regionwhere SWNTs fluoresce. The absorption does not overlap withthat of SWNTs either. Third, it was reported that there is nostrong interaction but only weak van der Waals interactionbetween SWNTs and ILs,22 so the electronic structures andproperties of SWNTs could be kept intrinsically. ILs are idealmedia for the study and application of SWNTs.In this work, the photoluminescence excitation (PLE)

contour maps of IL [BMIM][PF6]-dispersed SWNTs arereported and compared with those of sodium dodecyl sulfate(SDS)-dispersed SWNTs. Emission spectra with excitations ofthe populated (6,5), (7,5), (7,6), and (8,4) chiralities aredeconvoluted into individual fluorescence peaks. Clear EET isobserved in the IL dispersion system and can be explained by

the Forster resonance energy transfer (FRET) mechanism.23

Incomplete thermalization before exciton recombination isdemonstrated in IL-dispersed fine SWNT bundles. This gives away to relatively brighten up the PL of those less populated,large diameter, small band gap SWNT species.

■ EXPERIMENTAL SECTION

SWNTs produced by both high-pressure decomposition ofcarbon monoxide (HiPco) and decomposition of CO oncobalt−molybdenum catalyst (CoMoCAT) were utilized in thiswork. The IL [BMIM][PF6] (99% purity) was purchased fromHenan Lihua Pharmaceutical Co. Ltd., China.

Figure 1. TEM images of (a) IL-dispersed and (b) SDS-dispersed SWNTs. Individual SWNTs covered by SDS are indicated by arrows. The scalebar 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 thecorresponding excitation wavelengths indicated by red arrows, where the (8,4), (7,6), and (9,4) tubes are excited with highest emission intensities,respectively.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp306515a | J. Phys. Chem. C 2012, 116, 22028−2203522029

The SWNT-IL suspension was prepared by carefully grinding∼0.20 mg of SWNT sample in an agate mortar and pestle with0.5 mL of [BMIM][PF6] for 30 min, and the mixture was thenwashed off from the mortar and pestle by 9.5 mL of raw[BMIM][PF6]. Absolutely no sonication and centrifugationwere used for the SWNT-IL suspension. The SWNT-SDSsuspension was prepared by mixing ∼0.20 mg of SWNT samplewith 10 mL of 1 wt % SDS-D2O solution. The mixture wastreated in a tip sonicator (JY92-2D, Xin Zhi Co.) at 200 W for10 min (on 1 s, off 1 s) and then centrifuged at 11 000 rpm for30 min. The resulting supernatant was used for absorption andfluorescence characterization.The PLE measurements were performed on a Horiba Jobin

Yvon NanoLog-3 spectrofluorometer equipped with a 450 Wxenon arc lamp and a liquid nitrogen-cooled InGaAs detector.An 830 nm filter was set in front of the detector to cut off theRayleigh scattering. The excitation wavelength was varied from500 to 750 nm in 5 nm steps, and the emission was collected inthe 900−1550 nm region. The ultraviolet−visible-near-infraredabsorption spectra were collected in a 1.0 cm path length cellwith a PerkinElmer Lambda 950 spectrophotometer. A scanrate 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 accelerationvoltage of 120 keV.

■ RESULTS AND DISCUSSIONFigure 1 shows the TEM images of IL-dispersed and SDS-dispersed SWNTs. As can be seen, the SWNTs dispersed inSDS are individual nanotubes covered by surfactant, while theSWNTs in ILs are bundles with an average size of ∼15 nm. Inthis work, we use the SWNT-SDS suspension as a control ofindividually dispersed nanotubes and use the SWNT-ILsuspension with fine bundles for spectroscopic measurementsto study the exciton energy transfer in SWNT bundles.SWNTs have sharp van Hove singularities arising from quasi-

one dimensionality. The energy values of E11, E22, and E33,which are the energy differences between the correspondingvalence and conduction bands, are dependent mainly on tubechirality. Therefore, by analyzing the PLE maps with bothexcitation and emission wavelengths, one can distinguishdifferent semiconducting nanotube species and make theproper chirality assignments. Figures 2a and 2b exhibit thePLE contour maps of HiPco SWNTs dispersed in IL[BMIM][PF6] and in SDS-D2O, respectively, in the same E22excitation and E11 emission spectral regions for comparisonpurposes. The bold solid line at the upper right corner in Figure2a represents secondary diffraction bands, the wavelength ofwhich is twice the excitation wavelength, arising from thescattering caused by nanotube bundles even in the presence ofan 830 nm filter.As can be seen, 14 different chiralities are clearly observed in

the PLE map of HiPco-SDS, all in good agreement withpreviously reported data.1 Proper (n,m) assignment can bemade accordingly. Of these 14 chiralities, all except (8,3), whichhas the highest E11 emission, are also shown in the PLE map ofHiPco-IL. Comparing to the SDS suspension, three distinctfeatures 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 recordedPLE map shows distinct and well-structured PL peaks,indicating most SWNTs are suspended individually. For the

HiPco-IL suspension, due to the largely decreased PLintensities, 14 nm excitation and 50 nm emission slits areused, and the integration time is prolonged to 60 s to obtain aclear PLE map. Even so, the observed overall PL intensities arestill much lower than those for HiPco-SDS. The significantlydecreased yet still observable PL intensities are evidence that bysimple mechanical grinding bulk SWNTs are dispersed into finebundles in ILs. If most SWNTs are dispersed individually, thePL quench effect will not be so obvious, but if the bundles arerelatively large almost all the PL should be quenched. This PLquench mainly results from energy transfer from semi-conducting SWNTs to the neighboring metallic SWNTs insidethe same bundle and the consequent successive nonradiativedecay. Taking into account the different slit width andaccumulation time, it can only be roughly estimated from theoverall PL intensity ratio that the average bundle size in HiPco-IL is likely between 10 and 20.Second, broad and red-shifted emission bands. The

individual PL emission peaks of HiPco-IL are generally red-shifted by about 20−50 nm from those of HiPco-SDS, and thefwhm of the peaks are broadened from 30 to 70 meV, noticingthat part of this broadening effect is arising from the largeremission slit width used for HiPco-IL. By carefully examiningFigures 2a and 2b, it is found that the excitation wavelengths ofHiPco-IL are also red-shifted by about 5−10 nm from those ofHiPco-SDS. These results as well as the corresponding energyshifts ΔE22 and ΔE11 for each identifiable SWNT chirality aresummarized in Table 1. Since the accuracy of ΔE22 is highly

limited by the 5 nm excitation measurement steps, the ΔE11values are expected to be more accurate than ΔE22. It isinteresting to notice that although the measured red shifts ofλ11 vary from 20 to 50 nm in wavelength for different chirality,when converting into energy units all the energy shifts of E11are actually close to 30 meV and do not show any cleardependence on tube chirality or diameter. As this red shift ismainly attributed to the surrounding dielectric environmentchanges caused by the neighboring nanotubes inside the samebundle, the chirality-independent red shift suggests a nearly

Table 1. Excitation and Emission Wavelengths of HiPco-ILand HiPco-SDS, Respectively, and the CorrespondingEnergy Shifts for Each Identifiable Chirality in the PLEMaps

(n,m)dt

(nm)

λ22 inHiPco-IL

(nm)

λ22 inHiPco-SDS(nm)

ΔE22(meV)

λ11 inHiPco-IL

(nm)

λ11 inHiPco-SDS(nm)

ΔE11(meV)

(6,5) 0.757 570 566 15 1000 976 30.5(8,3) 0.782 -- 663 -- -- 950 --(7,5) 0.829 650 645 15 1054 1022 35.7(8,4) 0.840 590 587 11 1140 1111 28.4(10,2) 0.884 740 735 11 1080 1052 30.6(7,6) 0.895 650 645 15 1148 1120 27.0(9,4) 0.916 725 720 12 1132 1101 30.8(11,1) 0.916 615 609 20 1304 1263 30.9(10,3) 0.936 640 632 25 1291 1249 32.3(8,6) 0.966 725 715 24 1214 1173 35.7(9,5) 0.976 680 670 27 1285 1243 32.6(8,7) 1.032 735 726 21 1302 1262 30.2(12,2) 1.041 695 685 26 1432 1376 35.2(11,4) 1.068 720 712 19 1421 1368 33.8



uniform dielectric environment around the nanotubes in ILsuspensions.Third, higher relative intensities for smaller bandgap

nanotubes. For HiPco-SDS, nanotubes with lower E11 emissionenergy and larger diameter such as (8,7), (9,5), and (10,3) havelower PL intensities than those with smaller diameter andemission wavelengths in the 1000−1200 nm region such as(8,4), (7,6), (9,4), and (8,6). In contrast, HiPco-IL shows asignificant increase in the relative PL intensities for (8,7), (9,5),and (10,3) tubes. The relative PL intensities for higher E11emission 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, whichhas the highest E11 emission energy of all 14 chiralities, evenvanishes. This relative PL intensity enhancement in nanotubeswith higher E11 emission energy is even more distinct in theindividual emission spectra, as given in Figures 2c−2e. Theseare the PL emission slices taken from Figures 2a and 2b at theposition 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 forHiPco-SDS, respectively. The intensities in Figures 2c−2e arenormalized with respect to the highest peak in the region for

better comparison purposes and thus do not reflect the absolutePL intensities. The emission spectra are deconvoluted intoindividual fluorescence peaks using Lorentzian band profiles.All peak wavelengths, heights, and widths are allowed to varyfreely for the best fitting of the spectra, and the correspondingchiralities are assigned. As can be seen, the emission peaks fortubes with higher E11 disappear, and the deconvoluted peakarea for tubes with lower E11 increases dramatically for HiPco-IL. For example, in Figure 2c where the (8,4) tube is excited,the emission peak of (8,4) is predominant in the SDSsuspension as expected, while the (10,3) tube gives evenhigher 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 betaken into account is the change in SWNT chirality distributiondue to the different sample preparation method for SDS and ILsuspensions. Because the SDS dispersion method requiresintense sonication and subsequent centrifugation processes, therelative abundance of different chirality might be different fromthat in the IL suspension, which involves only simplemechanical grinding. To check for this, we measured theoptical absorption spectra for both HiPco-IL and HiPco-SDS,

Figure 3. (a) Optical absorption spectra of HiPco-IL and HiPco-SDS-D2O. (b) Deconvolution of the optical absorption spectra of HiPco-IL andHiPco-SDS-D2O in the spectral region of E11

S. 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 bandstructure diagram for EET between donor and acceptor tubes within the same SWNT bundle.



as shown in Figure 3a. In the optical absorption spectra, it isalready well recognized that the v1 → c1 and v2 → c2 transitionsfor semiconducting HiPco SWNTs are usually observed in theranges of 900−1600 and 600−900 nm, respectively.24 It can beseen in Figure 3a that the two absorption bands centered at 741and 818 nm in the E22

S region of HiPco-IL are red-shifted byabout 8 nm from those in HiPco-SDS. In the spectral region ofE11

S, a direct peak-to-peak correspondence cannot be easilyestablished at the first glance. However, after deconvolution ofeach broad band in this region with respect to the componentsemiconducting SWNT chiralities, about 30 meV red shift isobserved. These data are in good agreement with resultssummarized in Table 1 from PL measurements. Thedeconvolution is carried out with the absorption spectraconverted into energy unit. The initial band energies arecalculated from the corresponding PL emission wavelength.During the iteration, the peak height and width are allowed tovary freely, but the individual peak energies are only allowed toshift from the initial values by ±10 meV. The deconvolutedspectra are given in Figure 3b, with the solid black curvesdenoting the background-subtracted absorption spectra in theE11

S region, the dashed red and blue curves denoting the fittingspectrum, and the solid red and blue curves denoting thedeconvoluted individual peaks, for HiPco-IL and HiPco-SDS,respectively. From the deconvoluted spectra, it can beconcluded that the relative abundance for different chiralitiesdoes differ from one another in these two suspensions;however, there is no clear trend that IL suspension is favored inlarger diameter nanotubes. Therefore, the difference in relativeabundance could not possibly account for the dramaticenhancement in relative PL intensities for larger diameternanotubes in IL suspension.The loss in absolute PL intensity but enhancement in relative

PL intensity for certain chiralities are also observed in manyother cases for SWNT bundles, and this can be explained byEET.17−20 Forster resonance energy transfer (FRET) is anefficient EET mechanism that can be attributed to SWNTbundles. Figure 4a illustrates the schematic view of EETbetween donor and acceptor tubes in a SWNT bundledispersed in IL [BMIM][PF6], and Figure 4b shows theschematic band structure diagram for EET between donor andacceptor tubes within the same SWNT bundle. Excitons in afluorescent donor (D) nanotube with larger band gap cantransfer to a smaller band gap acceptor (A) nanotube, whichwill fluoresce at lower energy, through resonant, near-field,dipole−dipole coupling. The FRET efficiency is dependent onthe D−A distance, the relative orientation of emission andabsorption dipoles, and the spectral overlap between the donorand the acceptor. For a SWNT bundle since the tubes areparallel to each other and the wall-to-wall distance betweentubes is on the order of graphite stacking, ∼0.34 nm, the FRETefficiency is expected to be high and mainly dependent on thecorresponding D−A coupling, which is a function of the energysplitting ΔE11 = E11

D − E11A .

At a particular excitation wavelength, only SWNTs with acertain (n,m) can be excited. The photoexcited excitons canmigrate from this particular (n,m) tube to the neighboringsemiconducting nanotubes inside the same bundle. Manymigrations 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 tohop from donor tubes to acceptor tubes, resulting in higher

relative PL emission for the small bandgap larger diameteracceptor tubes.Although excitons most likely migrate from donor to

acceptor, it is also reported that they can be transferred backfrom acceptor to donor if providing sufficient thermal energy.25

J. Lefebvre and P. Finnie have studied the PL and FRET inelemental bundles of SWNTs, and they have shown the PLpeak intensity ratio between donor and acceptor as anexponential function of ΔE11, giving a slope corresponding toa simple Boltzmann distribution factor at room temperature.Although the quantum yield of different chirality is notconsidered in their case and significant scatter of data pointsis observed, their data still demonstrate efficiently thermalizedexcitons before recombination in elemental SWNT bundles and100% FRET transfer efficiency.25

In this present study, the ratio of the deconvoluted PL peakarea between the donor and acceptor

η =−−

A EA E

( IL)( IL)

11A

11D

(1)

is plotted with respect to the corresponding energy splittingΔE11, denoted by the black triangles in Figure 5. A logarithm

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

η′ =− −− −

=− −− −

A E A EA E A E

A E A EA E A E

( IL)/ ( SDS)( IL)/ ( SDS)

( IL) ( SDS)( IL) ( SDS)

11A

11A

11D

11D

11A

11D

11D

11A

(2)

Given that different nanotube chirality has different quantumyield and different relative abundance, both factors will affectthe relative PL intensity for the corresponding chirality. As thedeconvoluted peak area in SDS can serve as a control for the PLof the individually dispersed SWNTs, the values in SDS can

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



thus account for the difference in both quantum yield andrelative abundance for the corresponding chirality. By takinginto account this correction for both donor and acceptor tubes,we believe η′ reflects the PL intensities arising simply from theEET between different semiconducting SWNTs in bundles.Log η′ can then be fitted nicely by a linear relationship withrespect to ΔE11, as shown by the red fitting line in Figure 5. Alldata points are close to the fitting line, and the fitted intercept isclose to 1 as expected. The fitted slope is 0.0047 meV−1, whichis much less than the value of 0.017 meV−1 calculated directlyfrom the Boltzmann distribution factor and also reported inelemental bundles of SWNTs25 at room temperature. Thisindicates significant competition between exciton thermal-ization and recombination in IL suspension. In other words,

excitons are not completely thermalized before they recombine,resulting in more donor emission and less acceptor emissionthan in the case of complete thermalization.The incomplete exciton thermalization before recombination

can be attributed to two major reasons. First, the lifetime ofexcitons will be dramatically decreased in IL suspension. Due tothe large dielectric constant and high viscosity of thesurrounding ILs, the exciton recombination times for bothdonor and acceptor will be reduced. Second, the averageSWNT bundle size in IL suspension is estimated to be in theorder of 10−20, much larger than the elemental bundlesreported by Lefebvre and Finnie.25 Excitons may transfer andrelax multiple times among the SWNTs in one bundle beforethey finally recombine. Although the recombination time for an

Table 2. Excitation and Emission Wavelengths of CoMoCAT-IL and CoMoCAT-SDS, Respectively, and the CorrespondingEnergy 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) 0.757 570 565 19 1003 980 29.0(8,3) 0.782 670 665 0 971 952 25.5(9,2) 0.806 555 550 20 1165 1138 25.3(7,5) 0.829 650 645 15 1048 1024 27.7(8,4) 0.840 590 585 18 1138 1111 26.5(10,2) 0.884 -- 735 -- -- 1054 --(7,6) 0.895 650 645 15 1148 1121 26.0(9,4) 0.916 720 720 0 1148 1108 39.0(11,1) 0.916 -- 610 -- 1302 1262 30.2(10,3) 0.936 640 635 15 1297 1250 35.9(8,6) 0.966 715 715 0 1223 1175 41.4(9,5) 0.976 675 670 14 1296 1242 41.6(8,7) 1.032 -- 725 -- -- 1260 --

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) atthe corresponding excitation wavelengths indicated by red arrows, where the (6,5) and (7,5) tubes are excited with the highest emission intensities,respectively.



individual SWNT is typically much longer than the EET timeand the intraband relaxation time, the accumulated excitonthermalization time in bundles may reach the order of thereduced recombination time. Therefore, competition betweenthermalization and recombination will happen in IL-suspendedSWNT bundles.Similar research has been carried out on CoMoCAT-IL and

CoMoCAT-SDS. As is well recognized, CoMoCAT containssmaller-diameter nanotubes and less diverse (n,m) distributionthan HiPco. The dielectric screening effect of ILs is expected tobe similar; however, the EET features are expected to bedifferent to some extent. The corresponding data and spectrafor CoMoCAT-IL and CoMoCAT-SDS are given in Table 2and Figures 6−8. Comparing the PLE contour map andemission 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 relativeintensity for smaller bandgap tubes is enhanced dramatically.The v1 → c1 and v2 → c2 transitions of semiconductingnanotubes for CoMoCAT are usually observed in the ranges of

800−1400 and 550−800 nm, respectively, the latter alsoexhibiting a ∼30 meV red-shift after individual peakdeconvolution, as shown in the optical absorption spectra inFigure 7.By plotting the relative ratio of the deconvoluted PL peak

area η′ between donor and acceptor in logarithm scale withrespect to the energy splitting ΔE11, data points for CoMoCATcan also be fitted into a linear relationship with a slope of0.0021 meV−1. This slope is of the same order of magnitude asthat of HiPco and indicates that incomplete exciton thermal-ization before recombination is a common feature in IL-suspended SWNT bundles. The smaller slope in CoMoCATthan that in HiPco can be explained by lifetime and abundancevariation in different (n,m) species. CoMoCAT contains lessdiverse (n,m) distribution than HiPco, and the most populatedchiralities are larger bandgap smaller diameter tubes such as(6,5), (7,5), and (7,6), which usually serve as donor tubesduring EET. On one hand, the radiative rate is reported toincrease with increasing emission energy.26 Thus, the averagedonor recombination time is shorter for CoMoCAT than forHiPco. On the other hand, the higher abundance of donortubes in CoMoCAT gives rise to a higher probability for thecase that more than one donor tube with the same (n,m) isinside the same bundle simultaneously. Both factors will resultin higher donor emission and lower fitted slope.

■ CONCLUSION

In this paper, photoluminescence from the exciton energytransfer of the IL-dispersed HiPco and CoMoCAT bundles isclearly observed, with loss in absolute PL intensity andenhancement in relative PL intensity for small bandgap largediameter acceptor tubes, and can be explained by the FRETmechanism. Taking surfactant SDS-dispersed samples as acontrol of individually dispersed SWNTs, incomplete excitonthermalization before recombination is demonstrated in IL-dispersed SWNT bundles and can be attributed to the highdielectric constant of ILs and large SWNT bundle size in ILsuspension. The SWNTs-IL suspension offers a simple way torelatively brighten up the PL signal for those nanotubes withhigher E11 emission energy and larger diameter, the PLintensity of which is normally covered up by the intense signalof largely populated smaller diameter tubes.

Figure 7. (a) Optical absorption spectra of CoMoCAT-IL and CoMoCAT-SDS-D2O. (b) Deconvolution of the optical absorption spectra ofCoMoCAT-IL and CoMoCAT-SDS-D2O in the spectral region of E11

S. Deconvoluted peaks indicated by * have been assigned to more than onechirality.

Figure 8. Deconvoluted PL peak area ratio between donor andacceptor tubes in HiPco-IL and CoMoCAT-IL, respectively, afternormalization with the corresponding deconvoluted PL peak area inSDS as a function of the energy difference between the peaks and thelinear fits. Note that the relative peak area ratio is in logarithm scale.



■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Tel.: +86-10-62756773. Fax: +86-10-62756773.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe 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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