Bright Luminescence and Exciton Energy Transfer in Polymer

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Bright Luminescence and Exciton Energy Transfer in Polymer-Wrapped Single-Walled Carbon Nanotube Bundles )

Takeshi Koyama,*,† Yasumitsu Miyata,‡ Yuki Asada,‡ Hisanori Shinohara,‡ Hiromichi Kataura,§, and Arao Nakamura† †

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Department of Applied Physics, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan, ‡Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan, § Nanosystem Research Institute (NRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan, and Japan Science and Technology Agency ( JST), CREST, Kawaguchi, Saitama 330-0012, Japan

ABSTRACT Selective solubilization of semiconducting single-walled carbon nanotubes (SWNTs) by polymer wrapping leads to bright luminescence even in bundles because of suppression of deexcitation processes via metallic SWNTs. We investigate photoluminescence (PL) properties of polymer-wrapped SWNTs in a paper form by means of PL excitation spectroscopy and time-resolved PL spectroscopy. PL excitation spectra indicate bright luminescence and the existence of exciton energy transfer from wide to narrow gap tubes. Time-resolved measurements show that the luminescence decay becomes slower with decreasing gap energy. The observed decay behavior is analyzed by a simple rate equation model describing both intratube exciton decay and exciton energy transfer to neighboring tubes. It is found that the intratube exciton decay rate and the effective transfer rate per tube at the center-to-center distance of ∼1.9 nm are 1.1 1011 s-1 and 2.7 1011 s-1, respectively. These findings are relevant for understanding of energy-transfer mechanisms of one-dimensional excitons. SECTION Nanoparticles and Nanostructures

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that the rate of energy transfer between spatially extended states cannot be expressed by the well-known F€ orster model based on a simple point dipole approximation; the rate is proportional to RDA-6, where RDA is the donor-acceptor distance.22 The theoretical calculation within a distributed transition monopole approximation20 has shown that the transfer rate between two (7,5) tubes with a separation of 1.4 nm is 9.7 1012 s-1, which is about 3 orders of magnitude smaller than the F€ orster model calculation. Experimentally, it is known that the maximum transfer rate between a single (9,1)/(6,5) pair with a separation of 1.66 nm is 5 1011 s-1.23 Here, we report on exciton energy transfer between PFOwrapped SWNTs in paper-form samples studied by both photoluminescence excitation (PLE) spectroscopy and timeresolved luminescence spectroscopy. Our results indicate bright luminescence and excitation energy transfer whose rate is much lower than the F€ orster model calculation, demonstrating a characteristic feature of energy transfer of quasi-one-dimensional excitons. As the starting material for preparing paper samples, PFO wrapping of the SWNTs in toluene was carried out using the

ecent progress of SWNT isolation techniques has shown that organic polymers accomplish selective solubilization of SWNTs.1 Examples include DNA2-6 and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO).7-10 Wrapping by these polymers has two advantages compared with surfactant encapsulation. (1) The polymers enclose a single SWNT without impurities, and consequently, the quantum yield of luminescence is relatively high, about 1 order of magnitude higher than that in surfactant-encapsulated SWNTs.7,11 (2) The polymers achieve good extraction of semiconducting SWNTs from as-synthesized materials containing metallic SWNTs. Preferential wrapping of semiconducting SWNTs suggests the possibility of strong luminescence even in bundles, which would otherwise exhibit strong quenching of luminescence due to deexcitation via metallic SWNTs. However, the packing of SWNTs in the solid form may bring exciton energy transfer between SWNTs, which governs the luminescence efficiency of SWNTs. In recent years, attention has been given to excitation energy transfer among SWNTs because a quasi-one-dimensional exciton in SWNTs has an extended size equivalent to or larger than the donor-acceptor distance. In semiconducting SWNTs, the excition size is estimated to be ∼1-2 nm,12-15 and the centerto-center distance between nanotubes in bundles is a few nanometers.16,17 Recent theoretical studies18-21 have shown

r 2010 American Chemical Society

Received Date: September 27, 2010 Accepted Date: October 22, 2010 Published on Web Date: October 26, 2010

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DOI: 10.1021/jz101338c |J. Phys. Chem. Lett. 2010, 1, 3243–3248

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Figure 1. (a) Absorption spectrum of PFO-wrapped SWNTs in toluene. (b) PLE map of PFO-wrapped SWNTs in toluene. (c) PLE map of PFOwrapped SWNTs in the paper sample. Downward arrows indicate fingerprints representing exciton energy transfers from wide- to narrowgap SWNTs.

Figure 2. Typical TEM images of PFO-wrapped and bundled SWNTs in a paper sample. (a) Two PFO-wrapped SWNTs contacting with each other. (b) Bundled structure of PFO-wrapped SWNTs.

procedure reported in refs 7 and 8. A typical absorption spectrum of PFO-wrapped SWNTs in toluene (solution sample) is shown in Figure 1a. Five sharp bands are observed in the region of the first exciton band E11 in semiconducting SWNTs; they are assigned to the excitonic absorption of SWNTs with large chiral angles, (7,5), (7,6), (8,6), (8,7), and (9,7). Weak absorption bands due to (6,4), (6,5), and (9,8) SWNTs are also observed. We did not observe any appreciable absorption due to the M11 interband transition of metallic SWNTs, which indicates that the content of metallic SWNTs in the sample is very low. A map of the PLE spectrum is shown in Figure 1b. We observe an emission peak at the E11 band of isolated SWNTs excited at the E22 band. The fingerprints corresponding to five prominent peaks in the absorption spectrum can be clearly seen in the PLE map. Consequently, these results indicate that the solubilization of semiconducting SWNTs with large chiral angles is very good, as reported in the previous studies.7-10 We note that the presence of PFO molecules does not affect PL decay processes of SWNTs. The energy transfer from PFO molecules to SWNTs occurs with the excitation above 3.1 eV,9,24 while the reverse process has not been observed with the excitation at lower energies. Time-resolved luminescence measurements with the 1.7 eV excitation25 showed that the PL decay times of PFO-wrapped


SWNTs in toluene dispersion are rather long compared with those of SWNTs wrapped with other surfactant molecules. We prepared paper samples by filtration of the solution sample. The PLE map of the paper samples is shown in Figure 1c. The five fingerprints observed for the solution sample in Figure 1b are also well-resolved in Figure 1c, and red shifts of emission photon energies by several tens of millielectronvolts due to the change in environment are observed.26 In addition, four other fingerprints indicated by downward arrows are also observed. Considering the correspondence between the excitation and emission energies, the four fingerprints result from exciton energy transfer from wide- to narrow-gap SWNTs, for example, from (7,5) and (7,6) tubes to (8,6), (8,7), and (9,7) tubes. In the paper samples, the SWNTs are so close to each other that more efficient energy transfer is possible. A typical TEM image is shown in Figure 2a, where two PFOwrapped SWNTs make contact with each other, and the interwall distance is ∼0.9 nm in the closest region (upper part of Figure 2a). In a recent report,10 a TEM image showed that one SWNT is surrounded by PFOs and that they form a composite structure 6-7 nm across. The sample used in that experiment was SWNTs dispersed by PFO in toluene without any further processing except for the preparation for TEM

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Figure 3. (a) Decay kinetics of luminescence intensity in PFO-wrapped SWNTs in the paper sample. Circles and solid lines represent experimental and fitted results, respectively. For a clear display, decay curves are normalized at maxima (normalization factors are indicated next to the chiralities), and baselines are shifted. (b) Spectra of steady-state luminescence (solid line) and the integrated intensity of the decay curve (circles). (c) Mean number of neighboring acceptor SWNTs with (n,m) against the photon energy.

(F€ orster mechanism)23,27,28 or distributed transition monopole approximation.20 In these mechanisms, an exciton is transferred to an adjacent tube with an overlapping density of states, that is, a semiconducting SWNT with a narrow energy gap. For a SWNT with a large E11 energy, excitons can be transferred to almost all of the adjacent SWNTs acting as acceptors. As the E11 energy of the donor SWNT decreases, the number of the acceptor SWNTs decreases. As a result, the luminescence decay becomes slower with decreasing energy gap, that is, with increasing tube diameter. Let us now estimate the rate of exciton energy transfer. As the exciton decay process, we take into account both internal and external decay channels.29 The internal decay channel originates from radiative and/or nonradiative decay within a tube. Theoretical studies predicted the radiative lifetime to be ∼1-10 ns30,31 while experiments showed that the exciton decay time in isolated SWNTs is on the order of ∼10-1000 ps, which is independent of chirality.32-35 Thus, the internal decay is governed by nonradiative processes such as exciton trapping associated with defects, and we take the internal decay rate γin as a constant for all of the SWNTs. The external decay channel is the exciton energy transfer to acceptor SWNTs. Although the rate of exciton energy transfer depends on spectral overlap and separation between donor and acceptor SWNTs, we assume that the transfer rate per tube γt is one effective value because we are interested in the material's ensemble characteristics. When the number of acceptor tubes for a certain (n,m) donor tube is denoted by j, the external decay rate for the (n,m) donor tube is jγt. The rate equation of exciton density X(t) after pulse excitation is written in the form

observation. In contrast, our paper sample consisted of SWNTs concentrated by removing PFO molecules; therefore, most of the residual PFO molecules were taken off of the composite structure. Another typical TEM image in Figure 2b shows bundled structures of PFO-wrapped SWNTs with tubes aligned along their axes. As the interwall distance between PFO-wrapped SWNTs is ∼0.9 nm, as shown in Figure 2a, the typical distance between adjacent tubes in bundled structures is considered to be ∼0.9 nm. To quantitatively investigate exciton energy transfer between adjacent tubes, we measured luminescence decay kinetics in the femtosecond time region. The observed luminescence decay kinetics of (7,5), (7,6), (8,6), (8,7), (9,7), (9,8), (10,8), and (10,9) tubes under femtosecond pulse excitation at 1.55 eV are shown in Figure 3a. The exciation pulse resonantly excites (9,7) tubes, as shown in Figure 1b. Semiconducting SWNTs with other chiralities can also be excited because the photon energy of 1.55 eV is situated at the high-energy tails of E11 and E22 bands of SWNTs contained in the sample. For a more convenient display of the decay behavior, the intensities are normalized, and the baselines are shifted. The relative intensity of each decay curve is given by multiplying the inverse of the factor indicated in the figure. By integrating the decay curve intensity, we obtained the luminescence spectrum, which is plotted by open circles in Figure 3b together with the steady-state luminescence spectrum (excitation energy of 1.55 eV). The agreement between the spectra is fairly good, which confirms that the decay curves in Figure 3a originate from the exciton recombination of the corresponding SWNTs. As shown in Figure 3a, the decay behavior becomes slower with increasing tube diameter. We interpret this behavior in terms of the exciton energy transfer, as suggested in the PLE map in Figure 1c. The exciton energy transfer between SWNTs has been discussed on the basis of the point dipole approximation


dXðtÞ ¼ - γin XðtÞ - jγt XðtÞ dt

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Here, a term providing the exciton transfer from neighboring wider-gap tubes is neglected because the inflow excitons subsequently experience cascade-like transfer to narrowergap tubes through jγt and the instantaneous number of them is small enough to be negligible compared with that of initially formed excitons. Therefore, the overall decay rate for the concerning (n,m) tube is given by γin þ jγt, and the luminescence kinetics is proportional to a single-exponential function with a time constant of (γin þ jγt)-1. In order to derive the expression for luminescence kinetics of a tube ensemble, the acceptor number j is statistically treated. The TEM images in Figure 2 indicate that the number of adjacent tubes varies in the range from 1 to 6. Six is the maximum value because closely packed SWNTs organize into a two-dimensional triangular lattice.16,17 We assume that the probability of finding the (n,m) donor tubes with j neighboring acceptors obeys the Poisson distribution. When the mean value of j is denoted by N, the probability PN(j) of finding the (n,m) donor tubes is given by PN(j) = (Nj/j!)exp(-N). Therefore, for a large ensemble of SWNTs, the total luminescence intensity I(n,m)(t) from (n,m) tubes is written in the form Iðn, mÞ ðtÞ µ

P

PN ð jÞexp½ - ðγin þ jγt Þt

F€ orster model is inappropriate for calculations of energytransfer rates of quasi-one-dimensional excitons in SWNTs with a short distance. In summary, we investigated exciton energy transfer in a paper form of polymer-wrapped single-walled carbon nanotubes by luminescence spectroscopy. Photoluminescence excitation spectra showed exciton energy transfer to semiconducting tubes with narrower energy gaps. Time-resolved measurements showed that the luminescence decay became slower with decreasing energy gap, which supported the exciton energy transfer and allowed us to analyze transfer rates. From the simple rate equation analysis taking into account both intratube exciton decay and exciton energy transfer to adjacent tubes, it is found that the intratube decay rate and the effective transfer rate per tube are 1.1 1011 and 2.7 1011 s-1, respectively. Interestingly, efficient energy transfer to narrow-gap tubes was observed even in the presence of polymers on the tube surface. The bright luminescence and the potential tunability of the energy transfer will lead to applications of polymer-wrapped SWNTs to nanotubeand polymer-based solid-state optical devices.

EXPERIMENTAL METHODS

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SWNTs used in this study were produced by the highpressure carbon monoxide (HiPco) process. PFO wrapping of the SWNTs in toluene was carried out using the procedure reported in refs 7 and 8. The nanotubes (5-6 mg) were dispersed in toluene (20 mL) with PFO (5-6 mg) by sonication at 15 °C for 20 h. The dispersion was then immediately centrifuged at 10 000g for 5 min, and the upper 90% supernatant was collected. This was then followed by ultracentrifugation at 197 000g for 15 min, and the upper 90% supernatant was collected. The supernatant was used as a solution sample and starting material for preparing paper samples. The solution was mixed with methanol to aggregate the PFO-wrapped SWNTs. The aggregation substances were filtered and washed with toluene to remove excess PFOs, and then, they were redispersed in toluene. A series of procedures, that is, aggregation, filtration, wash, and redispersion, were repeated three times, and the SWNTs were collected on a membrane (Omnipore membrane, Millipore, 1.0 μm pore). This material was used as the paper samples. Absorption spectra were taken using a Shimadzu SolidSpec-3700DUV spectrophotometer. PLE measurements were carried out using a Horiba Jobin-Yvon NanoLog spectrofluorometer and a Shimadzu CNT-RF system. Luminescence kinetics was measured by time-resolved luminescence measurements based on the frequency up-conversion method. The light source was a mode-locked Ti:sapphire laser operating at 82 MHz. The pulse width and the wavelength were 80 fs and 800 nm, respectively (corresponding photon energy: 1.55 eV). The excitation intensity was ∼1 10-6 J cm-2 per pulse, which is well below the threshold (∼10-5 J cm-2 per pulse) of the nonlinear processes of exciton decay.36,37 The instrumental response function of the measurement system was determined by measuring the cross-correlation trace, which had a Gaussian form with a full width at half-maximum of 120 fs. The spectral resolution was about 0.03 eV. To get

¼ expf - γin t - N½1 - expð - γt tÞg The decay curves in Figure 3a are fitted to the function I(n,m)(t) convoluted with the instrumental response function. All of the curves are well-reproduced by choosing a single set of the parameters, as shown by solid curves in Figure 3a. The obtained values of γin and γt are 1.1 1011 and 2.7 1011 s-1, respectively, and the N values are plotted as a function of photon energy in Figure 3c. N decreases with increasing tube diameter, which is reasonable behavior because SWNTs with larger diameters have narrower gaps and the number of adjacent SWNTs potentially acting as acceptors decreases. The effective lifetime 1/γin of 9.1 ps is in agreement with the experimental value of ∼10 ps.32-34 The internal decay rate γin is probably governed by exciton trapping associated with defects in SWNTs. The effective rate of exciton energy transfer γt of 2.7 1011 -1 s at a center-to-center distance of ∼1.9 nm in this study is consistent with the transfer rate of (0.3-5) 1011 s-1 between a single pair of DNA-wrapped (9,1) and (6,5) SWNTs with a center-to-center distance of ∼1.66 nm,23 that is, an interwall distance of ∼0.9 nm. The transfer rate obtained in our study is about 10 times larger than the rate (1.3-1.7) 1010 s-1 reported by the temperature-dependent study.29 The temperature-independent fast decay observed in this study is attributed to the energy transfer between tubes within a small bundle remaining in the sample consisting of SDS-wrapped SWNTs embedded in a gelatin matrix. The lower transfer rate may be due to the difference in the donor-acceptor distance RDA because the rate of exciton energy transfer strongly depends on RDA. The comparison with the theoretical calculation20 shows that the transfer rate obtained in this study is smaller than the theoretical value of 9.7 1012 s-1 for the center-to-center distance of 1.4 nm and is much smaller than the F€ orster model calculation. This result indicates that the


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information about the morphology of the PFO-wrapped SWNTs, we carried out transmission electron microscopy (TEM) observations on the paper sample by employing a JEOL JEM-2100F operating at 80 keV.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. E-mail: koyama@ nuap.nagoya-u.ac.jp. (16)

ACKNOWLEDGMENT This work has been supported by a Grant-

in-Aid for Young Scientists (B) from the Japan Society for the Promotion of Science and a Grant-in-Aid for Scientific Research on Priority Areas Carbon Nanotube Nanoelectronics from the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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Bright Luminescence and Exciton Energy Transfer in Polymer

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