Article pubs.acs.org/JPCC
Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes G. Soavi,† F. Scotognella,† D. Brida,‡ T. Hefner,§ F. Spaẗ h,§ M. R. Antognazza,∥ T. Hertel,§ G. Lanzani,*,†,∥ and G. Cerullo†,⊥ †
Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Department of Physics and Center for Applied Photonics, University of Konstanz, Universitätsstraße 10, 78464 Konstanz, Germany § Institute for Physical and Theoretical Chemistry, Department of Chemistry and Pharmacy, University of Wuerzburg, Wuerzburg 97074, Germany ∥ Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli, 70/3, 20133 Milano, Italy ⊥ IFN-CNR, Piazza L. da Vinci, 32, 20133 Milano, Italy ‡
ABSTRACT: We show that excitons are not the unique outcome of photoexcitation in single-walled carbon nanotubes (SWNTs). Our experiments of transient photoinduced absorption suggest that charge carriers are formed with quantum yield of a few percent and that such species strongly affect the long-lived transient spectrum. Photogenerated charge carriers induce strong local electric fields that shift by the Stark effect the second subband exciton absorption in SWNTs, resulting in a characteristic derivative shape of the transient absorption spectra.
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INTRODUCTION Single-walled carbon nanotubes (SWNTs)1 show distinctive physical and optical properties2,3 that are particularly promising for the development of a new generation of advanced devices with enhanced mechanical, electronic, and optical capabilities. SWNTs display large nonlinear susceptibilities and fast carrier relaxation times which make them a key component in ultrafast photonics, allowing all-optical switching in optical communication network devices4−6 and their implementation as saturable absorbers in mode-locked lasers.7 A comprehensive picture of the photoexcitation scenario in semiconducting SWNTs, including elementary excitations, their relaxation dynamics, and their deactivation paths, is, however, a prerequisite to further develop their application in photonics and optoelectronics. According to theory, the elementary photoexcitations in SWNTs are Wannier−Mott singlet excitons.8−10 Such states have a large binding energy, of the order of the optical gap, due to the low dimensionality that reduces Coulomb screening in spite of the high electronic density. The binding energy, exciton size, and diffusion in the incoherent regime were recently estimated by means of two-photon excitation11,12 and transient absorption13 spectroscopies. Energy relaxation from higher lying (Sn) excitons to the first exciton occurs within ∼50 fs.14 This is a remarkably short time compared to inorganic semiconductors, indicative of the 1D nature of the system and strong coupling to high-energy optical phonons. The strong confinement of the electron−hole (e−h) relative motion leads to molecular-like states with strong electron−phonon coupling. This appears evident in the large Raman cross section that can be measured in © XXXX American Chemical Society
standard continuous wave (CW) scattering or in transient coherent Raman experiments. The most strongly active modes in the (6,5) semiconducting tube are the radial breathing and G modes.15,16 There are other photoexcitations that could coexist with neutral singlet excitons in SWNTs. Biexciton formation in SWNTs was first predicted theoretically17 and later found experimentally.18,19 Charge carriers,20−22 trions,23 and triplet states24 have been proposed to play a role in photoexcitations, yet with unknown relative quantum yield. Recently, Crochet et al.25 reported on free carrier photogeneration in aggregated SWNTs following high-energy excitation, at the π−π* transition associated with the M saddle point of the graphene lattice. Intertube interactions in fact may play a role in many SWNT samples because colloidal suspensions are generally metastable and have the tendency to aggregate with time, leading to efficient intertube energy transfer26 and possibly charge separation. Moreover, variations in sample character and quality complicate the interpretation of spectral signatures and dynamics. A mixture of different nanotubes with distinct but overlapping optical transitions may cause large inhomogeneous broadening in the absorption spectra resulting in congestion of spectroscopic features. For this reason the identification of photoexcited charge carriers and triplet states is still elusive. Received: April 23, 2013 Revised: April 29, 2013
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the maximum of the signal below 2%. The temporal resolution was determined to be ∼150 fs, and the probe spectrum covers a region from 2 to 3.1 eV. The pump photon energy was tuned inand out-of-resonance with respect to the S1 and S2 transitions (see Figure 1a). For CW photocurrent experiments we used a device consisting of a film of (6,5) nanotubes sandwiched between indium tin oxide (ITO) and aluminum electrodes. The photocurrent action spectrum has been measured by illuminating the sample with a halogen tungsten light source (Spectral Products AWB030), properly filtered by a Spectral products 1/8 M monochromator, and measuring the current signal by a lock-in technique (STANFORD SR830). Frequency chopper has been set at 230 Hz. Light incidence is from the ITO side. The photocurrent spectrum has been properly corrected by taking into account the system responsivity.
In this paper, we apply ultrafast pump−probe spectroscopy, with tunable pump photon energy and broadband coverage, to a sample of highly enriched (6,5) SWNTs. We demonstrate that the transient absorption signal, following initial ultrafast relaxation, is dominated by a long-lived spectral feature, observed over a broad probing region. This long-lived signal can be well described by the derivative of the ground-state absorption spectrum and is independent of the pump photon energy. This signal is consistent with an energy shift of ground-state absorption, which we assign to trapped photoinduced charge carriers. According to this interpretation, we estimate that in semiconducting (6,5) SWNTs about 1−2% of the absorbed photons generate long-lived e−h pairs.
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EXPERIMENTAL METHODS The SWNT sample used for these investigations is highly enriched in the (6,5) species and imbedded in a gelatin film. This film was prepared from 30 μL of a density gradient ultracentrifugation (DGU) enriched SWNT suspension in a sodium cholate/sodium dodecyl sulfate mixture.27,28 Iodixanol as well as sodium dodecyl sulfate (SDS) residues from the DGU process were removed by dilution with sodium cholate (SC) solution and filtration with a benchtop centrifuge. The resulting SWNT/SC suspension with 30 μL volume was then mixed with 20 μL of 15 wt % gelatin solution and finally drop-cast on a #0 microscope coverslip. The absorption spectrum, displayed in Figure 1a, is
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RESULTS AND DISCUSSION Figure 2 shows transient absorption spectra, ΔA = −ln(1 + ΔT/ T), acquired at different pump−probe delays and the corresponding dynamics for selected probe photon energies following optical excitation at 2.17 eV, i.e., resonant with S2, and 1.27 eV, i.e., resonant with S1. At early times the signal is dominated by exciton dynamics. In the first 10 ps in Figures 2a and 2c, the negative signal around the S2 peak, corresponding to the excitonic photobleaching (PB), undergoes a spectral narrowing and a blue shift. At the same time the signal around 2.07 eV shows a change in sign, which is highlighted by the red colored dynamics in Figures 2b and 2e. Following this initial rapid evolution, all the dynamics shown in Figures 2b and 2d exhibit a slow decay component, with features that remain nearly unchanged until the end of the measurement window. This observation points to the existence of long-lived photoinduced states in SWNTs. We concentrate our study on transient absorption spectra around 30 ps delay (blue transient spectra in Figures 2a and 2c), at which time all the ultrafast components and spectral changes related to the exciton dynamics have almost disappeared, while the signal-to-noise ratio is still good enough to recognize clear spectral features. Figure 3 shows ΔA spectra at 30 ps pump−probe delay for different excitation energies (blue line) in the S2−S3 spectral region. The transient absorption signal can be interpreted either as a superposition of PB and photoinduced absorption (PA) bands or, alternatively, as a derivative modulation of the groundstate absorption spectrum. The origin of the long-lived could be triplet excitons, leading to PA and PB signals, or charge carriers or thermal ef fects, leading to a shift of the absorption spectrum. It is in general difficult to experimentally distinguish between these mechanisms when the spectral signature is not assigned. Regarding the triplet exciton generation mechanisms: (i) Spin flip induced by spin−orbit coupling could yield a significant triplet population when the singlet decay rate is comparable to the intersystem crossing rate.30 We expect this to be true in conjugated systems only in the presence of heavy metallic impurities. (ii) Fission from the relaxed singlet exciton is unlikely because it requires very large exchange energy, not expected in SWNTs. (iii) e−h recombination seems feasible. Following photoexcitation a fraction of the energy may go into e−h pairs (scattering to the continuum). Excluding efficient spin−lattice relaxation the pairs could still undergo nongeminate recombination forming triplet pairs. At low excitation density, however, this mechanism should not be relevant. Charge carriers, instead, could be formed by: (i) decay of second subband excitons into resonant continuum states; (ii) direct excitation into the
Figure 1. (a) Absorption spectrum of the (6,5) semiconducting SWNT sample; arrows indicate the four pump photon energies employed in the experiment. Inset: normalized CW photocurrent for a device made by a film of (6,5) nanotubes sandwiched between ITO and aluminum electrodes. (b) Schematic electronic and excitonic transitions for a semiconducting carbon nanotube.
dominated by the first and second sub-band excitonic transitions of (6,5) SWNTs at 1.27 and 2.17 eV, respectively. For timeresolved experiments the sample was excited with 10 nm bandwidth pump pulses generated by a homemade optical parametric amplifier driven by a regeneratively amplified Ti:sapphire laser (500 μJ, 150 fs, 1 kHz) and probed by a broadband supercontinuum generated in CaF2. We measured the probe transmission through the sample with an optical multichannel analyzer working at the full repetition rate of the laser source.29 The acquisition of the pump-perturbed and pump-unperturbed probe spectra allowed extraction of the sample’s differential transmission (ΔT/T). We worked in a regime that is linear with respect to the pump intensity, keeping B
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Figure 2. (a, c) Transient absorption spectra at different pump−probe delays and dynamics (b, d) for 2.17 and 1.27 eV excitation energy, respectively. In the inset (a, c) a zoom of the probe region between 2.5 and 3.1 eV is shown. From transient spectra (a, c) we notice that the modulation and main spectral features remain unvaried after the first 5−10 ps. This is confirmed by the slow dynamic component (b, d) that we attribute to charges and that is present for the three selected probe energies (2.07, 2.17, and 2.3 eV).
Figure 3. ΔA transient spectra (blue line), simple fit taking into account only Stark effect (red line), and extended fit considering photobleaching from charges and excitons (dotted line) for the four pump photon energies used in the experiments. The probe region between 2.5 and 3.1 eV (b, d, f, h) can be fully explained by a rigid shift of the absorption spectrum, while the probe region between 2 and 2.5 eV (a, c, e, g) needs a more detailed analysis.
continuum of states; or (iii) linear dissociation of first subband excitons.22 Before recombination, free carriers could be stabilized for longer times by localized charge traps. Charge photogeneration in SWNTs is supported by the CW photocurrent31 and by THz pump−probe spectroscopy.22 To confirm this for our specific tube chirality, we acquired a CW photocurrent spectrum, reported in the inset of Figure 1. The peaks in this spectrum are in clear correspondence with the main
excitonic transitions. These data show (i) that the second exciton resonance is more “effective” than the first one (i.e., there is a higher relative quantum efficiency) in charge generation, a typical behavior associated to excess energy and (ii) that charge carriers are generated also when the excitation energy lies in the gap of the semiconducting (6,5) nanotubes (the continuum is approximately 300−400 meV above S1). C
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Figure 3 directly compares the experimental data with our simple numerical model. In the probe region between 2.5 and 3.1 eV (Figures 3b, 3d, 3f, 3h) the simple fit ΔA1fit(ω) = C1(σ*(ω) − σ0(ω))N0 + C2 reproduces very well the experimental data. Here σ0(ω) and σ*(ω) are the unperturbed and 100 meV red-shifted absorption cross sections of the SWNTs, respectively, and N0 is the ground-state population. The constant C2 represents a broad absorption background assigned to electronic transitions of the trapped charge carriers. In the spectral region between 2.0 and 2.2 eV the fitting function ΔA1fit(ω) fails for all the different excitation energies. In this probe region the fit should also take into account the PB of (6,5) transitions due to the photoinduced charges and excitons. Bleaching can arise both from ground-state depletion, as for charge photogeneration where no excitons are created but less electrons are available in the valence band for absorption, and from Pauli blocking, where not only the ground state is depleted but also the excited state is filled. For both resonant and nonresonant excitation the bleaching term due to ground-state depletion induced by charge formation must be present. In our experiments, we control the photon fluence to keep constant the excitation density when we tune the pump pulse, and thus we expect the same charge photogeneration yield. In the case of resonant excitation, an additional bleaching term due to exciton formation and Pauli blocking must be considered. Better fits for nonresonant excitation are obtained (see Figure 4a) by using
The spectral signatures of the trapped charges arise from their electric field that perturbs the local environment, causing Stark effect of the exciton resonance peaks in surrounding tubes. The signal should be a derivative-like modulation of the ground-state absorption spectrum with additional bleaching features due to kstate filling in the excited tubes.20,21 Triplets could instead induce characteristic excited state PA signatures. In particular, the T1− T3 transition could be nearly matching with the S0−S2 transition, giving rise to a positive ΔA signal around 2.3 eV, near the PB signal from S2. However at other energies such correspondence becomes less obvious, making it difficult to assign to triplet PA the additional long-lived positive ΔA bands, peaking at 2.05, 2.55, and 2.9 eV, respectively. On the other hand, such bands coincide very well with the red-shifted replicas of the ground-state absorption, and it would be a coincidence to have such a peculiar sequence of excited state PA bands. The red shift of the ground-state absorption of the SWNTs observed in the transient absorption spectra could also be attributed to a thermal effect, i.e., to a transient heating of the sample induced by absorption of the pump pulse. We exclude this explanation by evaluating the strength of such effect and by comparing our results to those presented in the work of Fantini et al.32 where the temperature dependence of transition energies for carbon nanotubes is studied by mean of Raman spectroscopy. According to this study, (6,5) carbon nanotubes undergo a blue shift of the S2 transition energy with increasing temperature. We have also estimated a transient temperature increase in the order of ΔT ≈ 0.2 K in our experimental condition (100 nJ pulses at 1.55 eV, 200 μm spot size, and 0.1 OD of the sample) that would correspond to a blue shift of the absorption of approximately 20 μeV according to Fantini’s paper. This is completely in contrast with our experimental results that show a red shift of 100 meV, as we discuss in the following. For all these reasons we propose electro-absorption induced by photogenerated charges as a plausible mechanism to explain our experimental evidence. Charge carriers trapped near a tube will induce strong local fields and consequently a Stark shift of the electronic dipole-allowed transitions. It is worth noting that our study covers a broad energy region above 2 eV. In the spectral region of S1 absorption we expect triplet absorption24 overlapping with a weaker Stark effect. As a consequence, it appears that the high-energy spectral region is better suited for detecting charge carrier signatures. Nevertheless, effects of charges on the first exciton S1 (in particular lattice deformation) have been reported in the literature.21 The possibility to qualitatively model the transient absorption spectra, on the femtosecond to picosecond time scale, with a simple red shift of the absorption spectrum has already been discussed by Styers-Barnett et al.,18 who attributed the physical origin of the shift to biexciton formation. In contrast, we propose that this red shift is due to a charge-induced Stark effect. Our assignment is supported by the following observations: (i) as pointed out earlier, the transient spectra undergo a significant change in the first 5−10 ps (Figure 2) and then remain almost unchanged on a longer time scale. The biexciton should instead give an instantaneous signal and show an exponential decay with time constant typical of excitonic lifetime in SWNTs and (ii) we obtain a similar agreement between the fit and experimental data when pumping in and out of resonance with the excitonic peaks. We would expect, on the contrary, to see a more evident signature of biexcitons when pumping at the excitonic resonances.
Figure 4. Explanation of fit residuals in the fitting procedure between 2 and 2.2 eV for pump photon energy at 1.55 eV (nonresonant) and 2.17 eV (resonant). In (a) the blue line is the unperturbed absorption spectrum; the black dotted line is the shifted spectrum; and the light blue line is the shifted and bleached absorption spectrum. In addition, (b) shows the bleached spectrum due to excitons (green line). Transient absorption spectra that take into account the bleaching due to excited charges and bleaching due to excitons are asymmetric (c and d) with respect to a simple derivative of the absorption spectrum and reproduce well our experimental data.
ΔA2fit(ω) = C1(σ*(ω) − σ0(ω))N0 − σ*ΔN* + C2, where ΔN* is the excited state population density. The additional term takes into account the PB of the Stark-shifted excitonic transition (light blue line in Figure 4a,b) due to the charge carrier photogeneration and ground-state depletion. Figure 4c shows the validity of the fit considering this extended model. When pumping at resonance with S1 or S2 (Figures 3a and 3e), the transient absorption spectrum can be modeled as ΔA3fit(ω) = C1(σ*(ω) − σ0(ω))N0 − σ*(ω)ΔN* − σ0(ω)ΔN0 + C2, where the additional term σ0(ω)ΔN0 (see Figure 4b) takes into account PB of the photoexcited exciton transition due to Pauli blocking. D
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Figure 5. Intensity-dependent measurements for 1.55 eV pump photon energy. We observe a linear dependence of the transient absorption amplitude for the main peak at 2.17 eV (a) and the C1 parameter (b) with respect to the pump intensity. The energy shift due to the Stark effect (c, d, and e) is the same at all pump intensities.
The validity of this fit is shown in Figure 4d. It is remarkable how our simple model, including only charge-induced Stark shift and PB, can fit almost quantitatively the transient absorption spectra of SWNTs over a very broad probe window (2−3.1 eV) and for different excitation energies. It remains to be understood why we observe no photobleaching from the ground-state absorption peaks in the spectral region between 2.5 and 3.1 eV, which belong to impurities and tubes of other chiralities. We can explain this by assuming that, after photoexcitation, generation of charges occurs predominantly from (6,5) tube chirality. This can induce a local electric field that shifts the absorption spectrum of adjacent tubes as a sort of external field, thus without depletion of the ground state. On the other hand the excited (6,5) tubes will show both Stark shift and PB due to depletion of their ground state. Finally we investigate the pump intensity dependence of charge photogeneration. Figure 5 shows intensity-dependent measurements for excitation at 1.55 eV. We find that the energy shift suitable to reproduce the transient spectra is always δEb = 100 meV, independent of pump intensity. This shows that the total number of charges is not affecting the spectral energy shift but only the amplitude of the transient signal. In turn, this observation suggests that each carrier acts independently on its local environment. We can compare the measured value of the
for S2, both to the red of the exciton resonance, in good agreement with our experimental results. The C1 parameter increases linearly with the pump intensity. Assuming that C1 = Nabs/N0 (where Nabs is the density of photogenerated charges) under a linear regime this provides an approximate value of 1−2% for the quantum yield of excited charges. This also allows us to better explain why we need to use different C1 parameters for different spectral regions in our fitting model. The free parameters in the fit are, in principle, C1, C2, and the energy shift δEb. The 2.0−3.1 eV region probes different species: the (6,5) chirality is predominant in the 2−2.5 eV region, while other chiralities and possibly metallic impurities are probed in the 2.5−3.1 eV region. Therefore, we do not expect the parameters C1, C2, and δEb to remain constant throughout the probing range, as the different species are expected to respond differently to the electric field; in particular it is reasonable that the density of charges, and thus C1, varies for different species. In our approach we chose to keep constant δEb (for any pump energy over the entire probe region) and C2 (for a given pump energy over the entire probe region) and leave C1 as the only free parameter: this was the most physically meaningful approach that also gave the best agreement with experimental data.
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CONCLUSIONS In conclusion, we have performed an investigation of the transient absorption, with broad spectral coverage and tunable pump photon energy, in (6,5) SWNTs. Our results are consistent with the following photoexcitation scenario in semiconducting nanotubes: after the initial ultrafast exciton relaxation, within the first 10 ps, transient absorption spectra are quantitatively described in terms of energy red shift of the ground absorption spectrum in a broad spectral region. The shift is ascribed to the
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energy shift with theoretical estimates by assuming δEb = kb(edF) / Eb for S133,34 and about 1/3 of this value for S2 (Eb is the binding energy; e is the electron charge; d is the nanotube diameter; F is the electric field; and kb is a parameter obtained from ref 34). If we consider a diameter d ∼ 1 nm, a binding energy Eb ∼ 350 meV,25 and the electric field given by a single charge on a range of ∼3 nm, we obtain a shift δEb ∼ 260 meV for S1 and δEb ∼ 85 meV E
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(12) Maultzsch, J.; Pomraenke, R.; Reich, S.; Chang, E.; Prezzi, D.; Ruini, A.; Molinari, E.; Strano, M. S.; Thomsen, C.; Lienau, C. Exciton Binding Energies in Carbon Nanotubes from Two-Photon Photoluminescence. Phys. Rev. B 2005, 72, 241402. (13) Luer, L.; Hoseinkhani, S.; Polli, D.; Crochet, J.; Hertel, T.; Lanzani, G. Size and Mobility of Excitons in (6, 5) Carbon Nanotubes. Nature Phys. 2009, 5, 54−58. (14) Manzoni, C.; Gambetta, A.; Menna, E.; Meneghetti, M.; Lanzani, G.; Cerullo, G. Intersubband Exciton Relaxation Dynamics in SingleWalled Carbon Nanotubes. Phys. Rev. Lett. 2005, 94, 207401. (15) Gambetta, A.; Manzoni, C.; Menna, E.; Meneghetti, M.; Cerullo, G.; Lanzani, G.; Tretiak, S.; Piryatinski, A.; Saxena, A.; Martin, R. L.; Bishop, A. R. Real-Time Observation of Nonlinear Coherent Phonon Dynamics in Single-Walled Carbon Nanotubes. Nat. Phys. 2006, 2, 515−520. (16) Luer, L.; Gadermaier, C.; Crochet, J.; Hertel, T.; Brida, D.; Lanzani, G. Coherent Phonon Dynamics in Semiconducting Carbon Nanotubes: A Quantitative Study of Electron−Phonon Coupling. Phys. Rev. Lett. 2009, 102, 127401. (17) Pedersen, T. G.; Pedersen, K.; Cornean, H. D.; Duclos, P. Stability and Signatures of Biexcitons in Carbon Nanotubes. Nano Lett. 2005, 5, 291−294. (18) Styers-Barnett, D. J.; Ellison, S. P.; Mehl, B. P.; Westlake, B. C.; House, R. L.; Park, C.; Wise, K. E.; Papanikolas, J. M. Exciton Dynamics and Biexciton Formation in Single-Walled Carbon Nanotubes Studied with Femtosecond Transient Absorption Spectroscopy. J. Phys. Chem. C 2008, 112, 4507−4516. (19) Colombier, L.; Selles, J.; Rousseau, E.; Lauret, J. S.; Vialla, F.; Voisin, C.; Cassabois, G. Detection of a Biexciton in Semiconducting Carbon Nanotubes Using Nonlinear Optical Spectroscopy. Phys. Rev. Lett. 2012, 109, 197402. (20) Gadermaier, C.; Menna, E.; Meneghetti, M.; Kennedy, W. J.; Vardeny, Z. V.; Lanzani, G. Long-Lived Charged States in Single-Walled Carbon Nanotubes. Nano Lett. 2006, 6, 301−305. (21) Sciascia, C.; Crochet, J.; Hertel, T.; Lanzani, G. Long Lived Photo Excitations in (6, 5) Carbon Nanotubes. Eur. Phys. J. B 2010, 75, 115− 120. (22) Beard, M. C.; Blackburn, J. L.; Heben, M. J. Photogenerated Free Carrier Dynamics in Metal and Semiconductor Single-Walled Carbon Nanotube Films. Nano Lett. 2008, 8, 4238−4242. (23) Watanabe, K.; Asano, K. Trions in Semiconducting Single-Walled Carbon Nanotubes. Phys. Rev. B 2012, 85, 035416. (24) Park, J.; Deria, P.; Therien, M. J. Dynamics and Transient Absorption Spectral Signatures of the Single-Wall Carbon Nanotube Electronically Excited Triplet State. J. Am. Chem. Soc. 2011, 133, 17156− 17159. (25) Crochet, J. J.; Hoseinkhani, S.; Lüer, L.; Hertel, T.; Doorn, S. K.; Lanzani, G. Free-Carrier Generation in Aggregates of Single-Wall Carbon Nanotubes by Photoexcitation in the Ultraviolet Regime. Phys. Rev. Lett. 2011, 107, 257402. (26) Luer, L.; Crochet, J.; Hertel, T.; Cerullo, G.; Lanzani, G. Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates. ACS Nano 2010, 4, 4265−4273. (27) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1, 60−65. (28) Crochet, J.; Clemens, M.; Hertel, T. Quantum Yield Heterogeneities of Aqueous Single-Wall Carbon Nanotube Suspensions. J. Am. Chem. Soc. 2007, 129, 8058−8059. (29) Brida, D.; Manzoni, C.; Cirmi, G.; Polli, D.; Cerullo, G. Tracking Ultrafast Energy Flow in Molecules Using Broadly Tunable FewOptical-Cycle Pulses. IEEE J. Sel. Top. Quantum Electron. 2012, 18, 329− 331. (30) In carbon conjugated systems without heavy atom we expect the intersystem crossing rate to be in the order of 1 ns−1. (31) Mohite, A. D.; Gopinath, P.; Shah, H. M.; Alphenaar, B. W. Exciton Dissociation and Stark Effect in the Carbon Nanotube Photocurrent Spectrum. Nano Lett. 2008, 8, 142−146.
Stark effect induced by photogenerated and trapped charge carriers. Our results suggest that about 1−2% of the absorbed photons generate long-lived charges. These charges are trapped and cannot recombine into singlet or triplet excitons, at least in the investigated time window (100 ps). Besides the modulation effect, transient spectra are also affected by selective state filling, according to the contribution of each configuration to the state transitions. Upon tuning the excitation wavelength we observed a rather similar behavior. In a previous study it was shown that pumping at 1.55 eV (out of resonance with the excitonic peak) in a sample with predominant (6,5) population actually leads to excitation of the (6,5) tubes.35 This was assigned to efficient and ultrafast energy transfer in small nanotube bundles. Here we confirm that result, but in addition we show that a fraction of a few percent of the absorbed photons ends up in charged trap states.
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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 G.L. and T.H. acknowledge the ITN project 316633 “POCAONTAS”. G.C. acknowledges support by the European Research Council Advanced Grant “STRATUS” (ERC-2011AdG No. 291198). F.S. acknowledges financial support from Italian Ministry of University and Research (project PRIN 20102011 “DSSCX” No. 20104XET32_007).
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REFERENCES
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The Journal of Physical Chemistry C
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dx.doi.org/10.1021/jp404009z | J. Phys. Chem. C XXXX, XXX, XXX−XXX
