Ultrafast Energy Transfer from Fluorene Polymers to Single-Walled

Feb 19, 2016 - ABSTRACT: Conjugated polymers can selectively wrap around single- walled carbon nanotubes (SWNTs) with specific chiral angles and...
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Ultrafast Energy Transfer from Fluorene Polymers to Single-Walled Carbon Nanotubes in Wrapped Carbon Nanotube Bundles Arao Nakamura,*,† Takeshi Koyama,‡ Yasumitsu Miyata,§ and Hisanori Shinohara∥ †

Toyota Physical and Chemical Research Institute, Nagakute, 480-1192, Japan Department of Applied Physics, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan § Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo192-0397, Japan ∥ Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan ‡

ABSTRACT: Conjugated polymers can selectively wrap around singlewalled carbon nanotubes (SWNTs) with specific chiral angles and diameters. Such a hybrid material containing both SWNTs and polymers is attractive for applications such as organic photovoltaic devices and photosensors. In this study, we investigate excitation energy transfer (EET) in a paper form of semiconducting SWNTs wrapped by poly(9,9dioctylfluorenyl-2,7-diyl) (PFO) by means of photoluminescence (PL) excitation spectroscopy and time-resolved luminescence spectroscopy. Photoluminescence excitation maps and spectra show PL peaks when the 0−0 singlet transition of PFO is excited at 2.88 eV, as well as peaks corresponding to EET between semiconducting SWNTs wrapped by PFO. The PL decay time measured for a drop-cast PFO film is 82 ps, whereas the decay time due to PFO in the PFO-wrapped SWNT sample is as short as 0.38 ps. From these results, the EET rate between PFO and SWNTs is estimated to be 2.6 × 1012 s−1. Comparison of the highest occupied molecular orbitl and lowest unoccupied molecular orbital energy levels for PFO with the band edge energies of SWNTs indicates that EET can occur from the PFO to SWNTs with band gap energies in the range of 0.8 to 1.4 eV.

1. INTRODUCTION Single-walled carbon nanotubes (SWNTs) and composite systems of SWNTs with molecules and polymers are attractive materials for use in optoelectronic devices such as solar cells and photosensors because of the unique optical and electrical properties of SWNTs, including diameter-dependent optical gaps, strong optical response due to exciton effects, and high carrier mobilities. Wrapping SWNTs with polymers has been used for isolation of individual nanotubes and for selective sorting of SWNTs with different chiralities.1−3 Preferential wrapping of semiconducting SWNTs from as-synthesized materials is achieved using fluorene-based polymers, and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) is widely used to selectively wrap SWNTs with high chiral angles.2,3 In polymer-wrapped SWNTs and polymer-SWNT composites, carrier doping in the ground state,4−6 charge transfer (CT),7−12 and excitation energy transfer (EET)12−15 under photoexcitation can occur across heterointerfaces. Field effect transistor devices consisting of PFO-wrapped SWNTs exhibit p-type characteristics.4−6 Schuettfort et al.7 investigated energy level alignment between SWNTs and conjugated polymers, including poly(3-hexylthiophene) (P3HT) and poly(indenofluorene), to explore CT processes in these composites and showed that P3HT and SWNTs with small diameters form a type II heterojunction. Their group observed ultrafast electron transfer (∼430 fs) from photoexcited P3HT to © 2016 American Chemical Society

SWNTs in a sample of SWNTs coated with a monolayer sheath of P3HT.8 In contrast, for large diameter SWNTs the alignment becomes type I, and EET from P3HT to SWNTs was observed as the dominant process.13 EET processes in PFO-SWNT composites have also been studied by several groups.14,15 Chen et al.15 investigated the polymer concentration dependence of the EET process in PFO-SWNT composites in a toluene solution. The observed photoluminescence excitation (PLE) spectra and photoluminescence (PL) decay times indicate EET from PFO to the semiconducting SWNTs, which demonstrates that the polymer acts as a light-harvesting antenna for the SWNTs. However, the interface morphology is crucial for EET efficiencies, and detailed studies using carefully prepared composite samples are required in order to quantitatively understand EET processes between polymers and SWNTs. In this paper, we report an investigation of the EET process from PFO to SWNTs in paper form samples of PFO-wrapped SWNTs by means of PLE spectroscopy and time-resolved PL spectroscopy. The morphology of the PFO-wrapped SWNT paper sample was characterized by transmission electron microscopy (TEM) observations. Each SWNT is wrapped Received: December 13, 2015 Revised: February 8, 2016 Published: February 19, 2016 4647

DOI: 10.1021/acs.jpcc.5b12191 J. Phys. Chem. C 2016, 120, 4647−4652

Article

The Journal of Physical Chemistry C with a thin layer of PFO, and the wrapped SWNTs form bundled structures. A PLE map of the PFO-wrapped SWNTs in the ultraviolet−visible range shows the fingerprints of the 0− 0 exciton transition energy of PFO, suggesting EET between PFO and SWNTs. The PL decay time measured at the 0−0 transition energy for the PFO-wrapped SWNT sample is shortened to 0.38 ps compared with the decay time of 82 ps for a PFO film. These results yield an EET rate of ∼2.6 × 1012 s−1. Energy level alignment between PFO and SWNTs is discussed considering the diameter distribution of the SWNTs used in this study.

Figure 1. (a) TEM image of PFO-wrapped and bundled SWNTs in a paper sample. (b) Network structures of bundled SWNTs.

2. EXPERIMENTAL METHOD We used paper-form samples of PFO-wrapped SWNTs that were also used in our previous work on exciton energy transfer between wrapped SWNTs.16 SWNTs were produced by the high-pressure carbon monoxide process. PFO-wrapping of the SWNTs was carried out in toluene. Details of the preparation procedures are described in refs 1, 2, and 16. The PFOwrapped SWNTs were aggregated by mixing methanol into the solution. The aggregation was filtered and washed to remove excess PFO polymer. To thoroughly remove residual PFO, a series of procedures, including aggregation, filtration, washing, and redispersion, was repeated three times. Paper form samples were prepared by filtration of the solution sample. PFO films on quartz substrates were prepared by drop-casting from the toluene solution; these drop-cast films were then dried in air. Absorption and PLE spectra were measured using a Shimadzu spectrometer and a Horiba Jobin-Yvon NanoLog spectrofluorometer/Shimadzu CNT-RF system, respectively. PL spectra in the ultraviolet−visible region were measured using a TRIAX 320 (Horiba Jobin-Yvon) spectrometer with a charge-coupled device detector and a semiconductor laser with a wavelength of 404 nm. Luminescence kinetics measurements in the picosecond to nanosecond range were carried out by the time-correlated single photon counting (TCSPC) method using a mode-locked Ti:sapphire laser with a pulse duration of 100 fs and a wavelength of 400 nm operating at 82 MHz as the excitation light source. Luminescence kinetics in the femtosecond to picosecond time range was measured using the frequency up-conversion method.17 The light source was a mode-locked Ti:sapphire laser with a pulse duration of 80 fs. The excitation wavelength was 400 nm and the wavelength of the gate pulse for frequency up-conversion was 800 nm. The instrument response function had a Gaussian shape with a full width at half-maximum of 195 fs, and the spectral resolution was ∼0.03 eV. TEM observations were carried out in a JEOL JEM-2100F instrument at 80 keV.

Figure 2. Absorption (black curve) and emission (blue curve) spectra of the PFO film and emission spectrum (red curve) of PFO-wrapped SWNTs in the paper sample. The vertical arrow indicates the photon energy of the excitation laser.

SWNTs. In the absorption spectrum (black curve), a sharp absorption band at 2.85 eV and a broad absorption band with small peaks at 3.09 and 3.24 eV are observed. The sharp band and the small peak structures can be ascribed to the 0−0 singlet exciton transition (S0−S1) and the vibronic bands, respectively.18−20 The appearance of the 0−0 transition band, related to the β-phase, and its strength depend on the conjugation length.18,19 The observed PL spectrum (blue curve in Figure 2) exhibits a characteristic vibronic structure with peaks at 2.819, 2.661, 2.484, and 2.305 eV. These peaks are assigned to the 0− 0 exciton transition (S1−S0) and the 0−1, 0−2, and 0−3 vibronic replica, respectively.19,20 The 0−0 exciton band in the PL spectrum is red-shifted by ∼0.03 eV from the 0−0 absorption band. The PL spectrum (red curve) observed for the paper sample of PFO-wrapped SWNTs also exhibits four peaks at 2.832, 2.671, 2.487, and 2.325 eV. These peaks are slightly blue-shifted compared to the peaks observed for the PFO film; the first peak is assigned to the 0−0 transition (S1− S0) and the other three to the vibronic replica, respectively. Because the thickness of the paper sample of the PFO-wrapped SWNTs was as large as ∼40 μm, we could not measure its absorption spectrum. Instead, the absorption spectrum of the PFO-wrapped SWNTs in toluene showed sharp absorption bands corresponding to E11 and E22 excitons of SWNTs in the near-infrared and visible regions with the absorption onset of PFO at ∼2.9 eV. No absorption band due to metallic SWNTs was observed (see Figure 1a of ref 16). A PLE map of the PFO-wrapped SWNTs is shown in Figure 3a. Five major fingerprints are observed at emission energies of

3. RESULTS TEM observations revealed the morphology of the PFOwrapped and bundled SWNTs. As shown in Figure 2 of ref 16, each SWNT is well-coated with PFO and they contact each other at a distance of ∼0.9 nm between the SWNTs forming small bundles. Additional TEM images are shown in Figure 1a,b. The PFO-wrapped SWNTs form bundled structures aligned along the tube axes, and these bundles form network structures. In addition, no residual PFO polymer is seen in the TEM images, confirming that excess PFO polymer was largely removed from the sample. Figure 2 shows absorption and PL spectra of a PFO film and a PL spectrum of a paper form sample of PFO-wrapped 4648

DOI: 10.1021/acs.jpcc.5b12191 J. Phys. Chem. C 2016, 120, 4647−4652

Article

The Journal of Physical Chemistry C

Figure 4. PLE spectra probed at different E11 exciton energies in PFOwrapped SWNTs in the paper sample. Red curve, (7,5); green curve, (7,6); black curve, (8,6); blue curve, (8,7); and pink curve, (9,7) SWNTs. Dashed curve: absorption spectrum of the PFO film. Emission intensity and absorbance are shown on a logarithmic scale.

To quantitatively investigate the EET process between PFO and SWNT, we measured PL decay kinetics in the femtosecond and picosecond time regimes. A PL decay curve for the PFO film is shown in Figure 5. The decay curve was measured at

Figure 3. (a) PLE map of PFO-wrapped SWNTs in the paper sample. (b) PLE map on a magnified scale in the excitation energy range of 2.55−3.05 eV. The arrows indicate emission energies of E11 excitons of (7,5), (7,6), (8,6), (8,7), and (9,7) SWNTs from the high energy side.

1.19, 1.08, 1.04, 0.91, and 0.95 eV. From a comparison of the absorption spectrum and the PLE map measured for the PFOwrapped SWNTs in toluene (Figure 1a,b of ref 16), these fingerprints can be assigned to the E11 exciton luminescence of (7,5), (7,6), (8,6), (8,7), and (9,7) SWNTs for excitation of their E22 exciton energies, respectively. Red-shifting of emission photon energies due to the change in environment was considered.16,21 When we excited the paper-form sample at 1.88 eV, corresponding to the E22 exciton energy of (7,5), (7,6) SWNTs, weak fingerprints were also observed at the E11 exciton energies of (8,6), (8,7), and (9,7) SWNTs. As reported in our previous paper,16 these fingerprints result from EET from (7,5), (7,6) SWNTs to (8,6), (8,7), and (9,7) SWNTs since the PFOwrapped SWNTs make contact with each other in bundled structures with an interwall distance of ∼0.9 nm. At an excitation photon energy of ∼1.74 eV, corresponding to the E22 exciton energy of (8,6) SWNTs, a weak fingerprint due to E11 excitons of (9,7) SWNTs was observed at 0.91 eV. This indicates EET from (8,6) SWNTs to (9,7) SWNTs. In our previous study,16 the EET rate between adjacent wrapped SWNTs was found to be 2.7 × 1011 s−1. Figure 3b shows a PLE map on a magnified scale of emission intensity in the excitation energy range between 2.55 and 3.05 eV. At an excitation energy of 2.88 eV, five fingerprints are observed at emission energies corresponding to E11 excitons of (7,5), (7,6), (8,6), (8,7), and (9,7) SWNTs. The PLE spectra probed at the E11 exciton bands of these SWNTs are shown in Figure 4 together with the absorption spectrum of the PFO film. The PLE spectra exhibit a peak at 2.88 eV and this photon energy coincides with the 0−0 transition energy of PFO. This result suggests EET between the PFO polymer and SWNTs. All the PLE spectra probed at the E11 exciton energies exhibit a peak at ∼1.9 eV, corresponding to the E22 exciton energy of (7,5) and (7,6) SWNTs, indicating EET from (7,5) and (7,6) SWNTs to (8,6), (8,7), and (9,7) SWNTs.

Figure 5. Semilogarithmic plot of decay curve of emission intensity in the PFO film. Black dots and red curve represent experimental data and a double-exponential fit. Instrument response function of the laser pulse is shown by blue dots.

2.667 eV, corresponding to the 0−1 vibronic band under femtosecond pulse excitation of 3.10 eV. The decay curve measured at the 0−0 transition energy showed the same decay characteristics. This decay behavior suggested both fast and slow decay and could be fitted to a double exponential function. Convolution analysis with an instrument function (shown by blue dots in Figure 5) of the TCSPC system yielded fast and slow time constants of 82 ± 4 and 600 ± 280 ps, respectively. Because the weighting factor for the fast component is 0.99, the decay is approximately single exponential with a time constant of 82 ps; this value is consistent with values for PFO in film form reported in the literature.20,22−24 We also measured PL decay curves for the paper-form sample of PFO-wrapped SWNTs. Figure 6a shows a decay curve measured at 2.82 eV (0−0 transition energy) with 3.10 eV excitation. The PL decay behavior exhibits fast decay within ∼1 ps and slow decay lasting ∼30 ps. The observed curve was analyzed by convolution using a double-exponential function with an instrument function (inset of Figure 6b). As shown in 4649

DOI: 10.1021/acs.jpcc.5b12191 J. Phys. Chem. C 2016, 120, 4647−4652

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Here, g, γr, and γnr are the generation rate and radiative and nonradiative decay rates for e-h pairs (excitons) in PFO, respectively, and γt is the energy transfer rate from PFO to SWNTs. G, Γr, and Γnr are the generation rate and radiative and nonradiative decay rates of excitons in SWNTs, and Γt is the energy transfer rate between wrapped SWNTs. After pulse excitation, the solution to eq 1 is n(t ) = no exp[−(γr + γnr + γt)t ]

where no is the initial e-h pair density created by the laser pulse. Equation 3 indicates exponential decay of the e-h pairs (excitons) in PFO with a time constant of (γr + γnr + γt)−1. If we use the fast decay constant of 0.38 ps for the PL decay of PFO in the paper-form sample of PFO-wrapped SWNTs, the decay time for the e-h pair density, (γr + γnr + γt)−1 is ∼0.38 ps. Because the PL decay time for the PFO film is ∼82 ps, we can consider (γr + γnr)−1 ∼ 82 ps; this indicates that the PL decay of PFO-wrapped SWNTs is governed by EET from PFO to SWNTs. Therefore, the EET rate is estimated to be γt ∼ 2.6 × 1012 s−1. This value is about 1 order of magnitude larger than the EET rate of 2.7 × 1011 s−1 between PFO-wrapped SWNTs at a wall-to-wall distance of 0.9 nm. Here, we mention energy transfer from PFO to PFO in the paper-form sample of PFO-wrapped SWNTs. The slow component of the observed PL decay is ascribable to PFOs adjacent to the PFO polymers wrapping around SWNTs, and the most probable decay process of e-h pairs (excitons) excited in such polymers is energy transfer between PFOs with different conjugation lengths. Therefore, the energy transfer rate is estimated to be 2.0 × 1011s−1 using the slow decay constant of 4.7 ps and the PL decay time of 82 ps for the PFO film and is 1 order of magnitude smaller than the EET rate from PFO to SWNTs. We can compare the EET rate obtained in this study with the reported EET value between PFO polymers and SWNTs in a toluene solution. Chen et al. measured PLE spectra and PL decay kinetics to investigate the EET process in PFO-SWNT solutions.15 The observed PLE spectrum in their study probed at the E11 exciton energy of (7,5) SWNTs showed a peak at 3.20 eV (388 nm), corresponding to the absorption peak for PFO,, with a decay time of ∼395 ps. This decay time is shorter than the value of 555 ps for the PFO polymer solution. Using these values, we can estimate the EET rate to be (1.4 ns)−1 (= 7.1 × 108 s−1). This value is more than 3 orders of magnitude smaller than the value obtained in the present study. A possible reason for this small value in their study could be the contribution of thick PFO layers wrapped around the SWNTs or residual PFO polymer in the solution sample to the PL intensity measured for the PFO-SWNT composite in solution (the PL decay time for PFO not in contact with SWNTs is longer because there is no decay channel via energy transfer to SWNTs in this case). The EET rate in PFO-wrapped SWNTs can be compared to the charge transfer rate observed for P3HT-wrapped SWNT films. The PL decay time for P3HT in the wrapped SWNTs is 0.43 ps, while that for P3HT polymer is 689 ps.8 Such shortening of the decay time results from electron transfer from P3HT to SWNTs with a rate of 2.3 × 1012 s−1. This value is close to the EET rate obtained in our study. As shown in Figure 1 of ref 8, SWNTs are coated with a monolayer P3HT sheath, which is similar to the morphology of PFO-wrapped SWNTs in the present work. These results indicate that the EET and charge transfer processes from polymers to SWNTs in

Figure 6. (a) Decay curve of emission intensity of PFO in the paper sample of PFO-wrapped SWNTs. (b) Decay curve in the short time range of 0.5−3.0 ps. Closed circles and solid curve represent experimental data and double-exponential fit, respectively. Dashed and dot-dashed curves are decay curves for the fast and slow components, respectively. The inset shows a cross-correlation trace between the gate pulse and excitation pulse corresponding to the instrument response function.

Figure 6a,b, the curves are well fitted to double-exponential functions (solid curve) with fast and slow time constants of 0.38 (dashed curve) and 4.7 ps (dot-dashed curve). The weighting factors for the fast and slow components are 0.82 and 0.18, respectively. These results indicate that the decay time for luminescence from a paper-form sample of PFO-wrapped SWNTs is shortened by about 2 orders of magnitude compared with that of PFO in film-form samples. We can ascribe the fast component to luminescence from PFOs in contact with SWNTs, suggesting fast energy transfer from PFO to SWNTs. The slow component presumably comes from PFO polymers that are not in contact with SWNTs, because there are PFO polymers that touch the wrapping polymers in the bundle structure as shown in Figure 1.

4. DISCUSSION To analyze the dynamical behavior of luminescence from PFOwrapped SWNTs, we used a rate equation model considering EET from PFO to SWNTs. The rate equations for the electron−hole pair (e-h pair) or exciton density n(t) in PFO and the exciton density N(t) in SWNTs are written as dn(t ) = g − γrn(t ) − γnrn(t ) − γtn(t ) dt

(1)

d N (t ) = G + γtn(t ) − ΓrN (t ) − ΓnrN (t ) − ΓtN (t ) dt

(2)

(3)

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DOI: 10.1021/acs.jpcc.5b12191 J. Phys. Chem. C 2016, 120, 4647−4652

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Figure 7. (a) Energy diagram of HOMO and LUMO levels for PFO and semiconducting SWNTs. The energy range for the band gaps due to the diameter distribution of SWNTs contained in the sample is illustrated by the hatched areas. (b) Energy level alignment between PFO and (7,6) SWNT. The energy scale is given with respect to the vacuum level.

for the HOMO and LUMO levels (dashed lines), EET occurs to the C3 and V2 bands. Because the density of states for SWNTs has a sharp structure at the band edge due to the van Hove singularity, more efficient EET takes place when the PFO energy levels match the SWNT band edges.

polymer-wrapped SWNTs show similar transfer rates. This similarity implies that the EET mechanism is likely to be electron exchange by the Dexter mechanism25 due to overlap of electron wave functions between a donor and an acceptor rather than the Förster mechanism26 due to interactions between transition moments. A further detailed study is needed for understanding the exact EET mechanisms between a polymer and SWNT at short distances. We next discuss energy level alignment between PFO and semiconducting SWNTs. In the Förster -Dexter formalism, EET occurs when there is spectral overlap between donors and acceptors. Figure 7a shows the energy levels for PFO and semiconducting SWNTs. We take the values of the highest occupied molecular orbital (HOMO) level (−5.6 eV) and the lowest unoccupied molecular orbital (LUMO) level (−2.5 eV) from a study carried out using photoelectron spectroscopy on a PFO film (showing the 0−0 transition peak at 2.85 eV).27 The HOMO and LUMO levels are illustrated as solid lines in Figure 7a. Another study used cyclic voltammetry measurements28 and obtained different values for the HOMO level (−5.8 eV) and the LUMO level (−2.85 eV), shown by dotted lines in Figure 7a. The work function for the SWNTs is −4.5 eV,7,29 and the first conduction band (C1) energies and the first valence band (V1) energies for the semiconducting SWNTs with various chiralities contained in the sample studied here are illustrated as hatched bands. This energy diagram shows that the PFO HOMO level falls below the V1 energies for SWNTs, and the PFO LUMO level is higher than the C1 energies. Because the HOMO and LUMO levels are well within the SWNT bands considering the higher bands, there is overlap between the PFO HOMO/LUMO levels and the density of states for the conduction and valence bands of SWNTs. Therefore, energy transfer of e-h pairs (excitons) excited in PFO to the conduction and valence bands of SWNTs can occur. To specify the energy bands of SWNTs to which EET can occur from the PFO HOMO and LUMO levels, we show the energy levels for PFO and (7,6) SWNT in Figure 7b. The band edge energies for the conduction bands (C1, C2, C3, and C4) and valence bands (V1, V2, V3, and V4) are taken from the transition energies of E11, E22, E33, and E44 excitons.30 The uncertainty in the band edge estimation due to the effects of excitons and environmental dielectric constants is on the order of one-tenth of an electronvolt. From comparison of the HOMO and LUMO levels of PFO, illustrated by solid lines in Figure 7b with the higher energy bands for the (7,6) SWNT, we find that the excitation energy for an e-h pair (exciton) can transfer to the C4 and V2 bands. If we take the alternative values

5. CONCLUSIONS We investigated EET in a paper form of semiconducting SWNTs wrapped by PFO using PLE spectroscopy and PL kinetics measurements in the femtosecond and picosecond time ranges. The photoluminescence excitation map and spectra show PL peaks corresponding to EET between semiconducting SWNTs wrapped by PFO. In addition, PL peaks can be observed when the 0−0 transition of PFO is excited at 2.88 eV, which suggests EET from PFO polymer to the SWNTs. The PL decay time for the drop-cast PFO film is 82 ps, while the decay time for PL from PFO in the paper form sample of PFO wrapped SWNTs is as short as 0.38 ps. The EET rate is estimated to be 2.6 × 1012 s−1, which is 3 orders of magnitude higher than that observed for PFO-SWNT composites in a toluene solution. From the energy level alignment between the band edge energies of SWNTs and the HOMO/LUMO energy levels of PFO, it is found that EET from PFO to SWNTs can occur for the SWNTs with band gap energies in the range of 0.8 to 1.4 eV. In the PFO-wrapped SWNT bundles, multistep energy transfer, PFO → small diameter SWNT → large diameter SWNT, can occur, indicating the light harvesting over a wide range of wavelengths in the visible and near-infrared region.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors thank Dr. K. Yamanaka of Toyota Central R&D Laboratories for assistance with measurements by the TCSPC method and Dr. H. Kataura and Dr. Y. Asada of the National Institute of Advanced Industrial Science and Technology (AIST) for preparation of the PFO-wrapped SWNT samples. This work has been supported by KAKENHI (Grant 25400332) from the Japan Society for the Promotion of Science. 4651

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DOI: 10.1021/acs.jpcc.5b12191 J. Phys. Chem. C 2016, 120, 4647−4652