Comparative Study of Exciton Relaxation Dynamics in C60 and C70

The exciton dynamics of C70 solid has been investigated by the time-resolved photoluminescence (PL) and transient absorption (TA) technique and compar...
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J. Phys. Chem. B 1999, 103, 6504-6508

Comparative Study of Exciton Relaxation Dynamics in C60 and C70 Solid Hyun Sun Cho,† Sung Ik Yang,† and Seong Keun Kim* Department of Chemistry, Seoul National UniVersity, Seoul 151-742, Korea

Eun-joo Shin and Dongho Kim* National CreatiVe Research InitiatiVes Center for Ultrafast Optical Characteristics Control and Spectroscopy Laboratory, Korea Research Institute of Standards and Science, Taedok Science Town, Taejon 305-600, Korea ReceiVed: April 19, 1999; In Final Form: June 1, 1999

The exciton dynamics of C70 solid has been investigated by the time-resolved photoluminescence (PL) and transient absorption (TA) technique and compared with the case of C60 solid. Three distinct PL bands were observed at 685, 715, and 740 nm. The high-energy band at 685 nm displayed an almost purely singleexponential decay profile, while the other two exhibited a slow-decaying tail as well, especially at low temperatures. Laser fluence dependence of PL decay profiles shows that the decay in C70 solid, in contrast to the case of C60, is predominantly governed by fast (20 ps) nonradiative exciton-trap annihilation, with the intersystem crossing playing a minor role in lower energy bands at 715 and 740 nm. On the other hand, laser fluence dependence of transient absorption signals shows that the decay in C70 is much faster than PL decay, which indicates the fast component arises from the exciton-exciton annihilation process under extremely high laser fluence condition.

Introduction The vigorous research activities on fullerenes accelerated by the discovery of a macroscopic production method1 have been mainly focused on C60, in the expectation that this molecule should serve as a reasonably good prototype for most of molecules in the fullerene family. Consequently, there exists surprisingly little information on the properties of even a relatively common molecule such as C70. As for photophysics in particular, a certain number of optical properties of C70 have been examined so far in solution, but not much is known about C70 in solid form. The relaxation dynamics of photoexcited C60 solid has been extensively investigated under various conditions, and a wide range of mechanisms have been proposed to explain the decay dynamics.2-11 The exciton model2-7 has often been employed to explain the laser fluence dependent relaxation dynamics of photoexcited C60 solid. Models based on Frenkel-type excitons were used because weak intermolecular forces induce little change in the electronic structure of the molecule upon formation of the solid phase, thereby allowing the exciton states to almost retain their molecularity. But since real crystals contain various chemical impurities as well as crystal defects, local exciton centers can form around these sites. The concentration of such excitons is strongly affected by the growth method of the solid and its local structure. In our previous paper,2 we reported exciton decay dynamics in C60 solid in terms of free exciton and self-trapped exciton to explain the dependence of decay behavior on excitation fluence, emission wavelength, and temperature. In the present study, qualitatively different features were found for C70 solid when * To whom correspondence should be addressed. E-mail: seongkim@ plaza.snu.ac.kr; [email protected]. † Also members of national creative research initiatives center for ultrafast optical characteristics control.

compared with the case of C60. The results were discussed in terms of the annihilation mechanism of excitons. In contrast to the case of C60 solid, the excitonic decay in C70 solid was found to be dominated by fast nonradiative exciton-trap annihilation. On the other hand, the decay of transient absorption signals in C70 solid is governed by an exciton-exciton annihilation, as in the case of C60. Experimental Section Time-resolved PL spectra of C70 solid film were measured with the same TCSPC (time-correlated single photon counting) setup used in our previous study.2 Laser fluences ranging from 1 to 10 µJ/cm2 and a temperature range of 10-300 K were used. The dual beam femtosecond time-resolved transient absorption spectrometer consisted of a self-mode-locked femtosecond Ti:sapphire laser (Clark MXR, NJA-5), a Ti:sapphire regenerative amplifier (Clark MXR, CPA-1000) pumped by a Qswitched Nd:YAG laser (ORC-1000), a pulse stretcher/ compressor, an OPG-OPA system, and an optical detection system. A femtosecond Ti:sapphire oscillator pumped by a cw Nd:YVO4 laser (Spectra Physics, Millenia) produces a train of 60 fs mode-locked pulses with an average power of 250 mW at 800 nm. The seed pulses from the oscillator were stretched (∼250 ps) and sent to a Ti:sapphire regenerative amplifier pumped by a Q-switched Nd:YAG laser operating at 1 kHz. The femtosecond seed pulses and Nd:YAG laser pulses were synchronized by adjusting an electronic delay between the Ti: sapphire oscillator and the Nd:YAG laser. Then, the amplified pulse train inside the Ti:sapphire regenerative amplifier was cavity-dumped by using the Q-switching technique, and about 30000-fold amplification at 1 kHz was obtained. After recompression (100 fs), the amplified pulses were color-tuned by the optical parametric generation and optical parametric amplifica-

10.1021/jp991252t CCC: $18.00 © 1999 American Chemical Society Published on Web 07/16/1999

Exciton Relaxation Dynamics in C60 and C70 Solid

Figure 1. (a) Time-integrated PL spectrum of C70 solid collected over 1 µs after a picosecond dye laser excitation at 590 nm at 10 K. (b-f). Time-resolved PL spectra of C70 solid measured at a given time delay with a wavelength interval of 5 nm over the spectral region of emission. Each spectrum is fitted with three Gaussian peaks centered at 685, 715, and 740 nm. The monotonically decaying luminescence intensities from (b) to (f) were magnified by different factors to better show spectral features.

tion (OPG-OPA) technique. The resulting laser pulses had a pulse width of ∼150 fs and an average power of 5-30 mW at 1 kHz repetition rate in the range of 550-700 nm. The pump beam was focused to a 1 mm diameter spot, and the laser fluence was adjusted from 0.11 to 1.2 mJ/cm2 by using a variable neutral density filter. The fundamental beam remaining in the OPGOPA system was focused onto a flowing water cell to generate a white light continuum, which was again split into two parts. One part of the white light continuum was overlapped with the pump beam at the sample to probe the transient, while the other part of the beam was passed through the sample without overlapping the pump beam. The monitoring wavelength was selected by using an appropriate interference filter (fwhm ) 10 nm). By chopping the pump pulses at 43 Hz, the modulated probe pulse as well as the reference pulse were detected by photodiodes. The output current was amplified with a homemade fast preamplifier, and then the resultant voltage of the probe pulses was normalized by a boxcar averager with pulse-to-pulse configuration. The resultant signal modulated by a chopper was measured by a lock-in amplifier and then fed into a personal computer for further signal processing. The C60 and C70 were thermally evaporated and deposited uniformly on the quartz substrate under vacuum. All the experiments were carried out in a vacuum in order to avoid possible reaction with oxygen. Results Figure 1a shows the PL spectrum of C70 solid following a picosecond pulse excitation at 590 nm, integrated over 1 µs. This spectrum was reconstructed by a combination of three

J. Phys. Chem. B, Vol. 103, No. 31, 1999 6505

Figure 2. PL decay profiles at (a) 300 and (b) 10 K for the three emission bands under the high laser fluence condition (10 µJ/cm2). IRF stands for the instrumental response function of the TCSPC system.

Gaussian bands centered respectively at 685 (PL1), 715 (PL2), and 740 nm (PL3). Figure 1b-f shows a series of PL spectra measured at various time delays after the excitation pulse. At zero delay time, the emission spectrum is dominated by the PL1 band at 685 nm with a weak PL2 band at 715 nm. Upon increasing the delay time, the intensity of the PL1 band decreases rapidly, while the PL2 band becomes a major component after 0.3 ns time delay. Although the number of PL bands is different from that of C60 solid,2 the time evolution of spectral shape shown in these figures suggests that these PL bands have different origins, as in the case of C60. To find out the nature of photoexcited states and the decay mechanism for each PL band, we examined the dependence of the PL decay profile on the emission wavelength, laser fluence, and temperature. Figure 2a shows the PL decay profile at various emission wavelengths under high laser fluence condition (ca. 10 µJ/cm2) at room temperature, where all the decays are shown to be very fast. But at 10 K (Figure 2b), the decay of the lower energy bands becomes significantly slow. This type of wavelength-dependent PL decay time behavior was also observed in C60 solid,2 but the most notable difference in the present case of C70 solid is that there exists a very fast and dominant decay component in the beginning of decay for all three PL bands. Also to be noted is the decay of the high-energy PL1 band with virtually no long-lived component even at 10 K. To compare the decay times, the experimental decay curve was fitted to a biexponential function with two time constants (τ1 and τ2) via a nonlinear least-squares procedure. Table 1 lists the fitting parameters for all three PL bands. We also examined the effect of exciton concentration on decay rates by measuring the PL decay profiles under different laser fluence conditions. The laser fluence was varied from 1 to 10 µJ/cm2 by using various ND filters. Special attention was paid to avoid heating of the sample with laser shots that can

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Figure 3. PL decay profiles at (a) 685 and (b) 740 nm at various laser fluences at 10 K. The laser fluence was varied between 1 and 10 µJ/cm2 by using the ND filters.

TABLE 1: Fitting Parameters for PL Decay Profiles for Three Emission Bands wavelength (nm)

temperature (K)

decay time (ps)

relative amplitude (%)

685

10

715

10

740

10

20 600 20 600 20 600 20 600

99.9 0.1 96.9 3.1 93.8 6.2 99.7 0.3

300

cause artifact fluence dependence stemming from underlying temperature dependence, if the latter exists. The results in Figure 3 show a rather small change in the PL decay profile arising from the different amount of excitons produced by photoexcitation. In the case of C60, the laser fluence of 10 µJ/cm2 was estimated to produce a maximum of ca. 10-3 photoexcitation per molecule in the solid.2 Since the absorption cross section of C70 is significantly larger (ca. 4 times) at the excitation wavelength, it is expected that we produce a correspondingly higher concentration of excitons (ca. 10-3(0.5 exciton/molecule) in C70 solid between 1 and 10 µJ/cm2 of laser fluence. But even with such an increase in exciton concentration, the decay is basically monoexcitonic in C70 solid with only a small fraction of decay due to exciton-exciton annihilation.4,6 The temperature dependence of PL decay is shown in Figure 4, which displays the decay profile of the PL1 and PL3 bands at various temperatures. We note that both the entire PL1 band and the fast component of the PL3 band show virtually no temperature dependence, while the slow component of the PL3 band decays faster as the temperature is increased. To elucidate the origin of the fast component in PL decay more in detail, we measured the decay of femtosecond transient

Figure 4. Temperature dependence of PL decay profiles at (a) 685 and (b) 740 nm under the high laser fluence condition (10 µJ/cm2). IRF stands for the instrumental response function of the TCSPC system.

absorption (TA) signal as a function of laser fluence. It turns out that the transient absorption measurement, which inevitably employs much higher laser fluence, demonstrated a much faster decay profile of photoexcited C70 solid. Parts a and b of Figures 5 respectively show the decay curves of TA signals of C60 and C70 solids at 660 nm as a function of laser fluence of 614 nm photoexcitation. The laser fluence was varied between 0.11 and 1.2 mJ/cm2 by using a variable ND filter. Table 2 shows the fitting results with a triexponential function of TA decay profiles of C70 solid under various laser fluences at 300 K. Discussion A summary of the observed PL and TA decay features of C70 solid and their major differences from those of C60 solid2 are given below. (1) C70 solid shows three PL bands at 685, 715, and 740 nm, whereas C60 solid was found to exhibit two PL bands at 680 and 725 nm.2 (2) All three PL bands can be fitted to a biexponential decay function with fast and slow decay times, τ1 and τ2. The values of these time constants are widely different (20 and 600 ps),12 in contrast to those for C60 solid (250 ps and 1 ns). (3) Contrary to the case of C60 solid, the decay of all three PL bands of C70 is predominantly governed by the fast time component τ1 (Table 1). In particular, the high-energy PL1 band shows a nearly single-exponential decay, independent of temperature. On the other hand, the lower energy PL2 and PL3 bands contain a small portion of slow time component τ2, which becomes increasingly more important (up to ∼6.2% at 10 K) at lower temperatures. (4) All PL bands at all temperatures show little dependence on laser fluence, suggesting that the major decay channel is monoexcitonic in character.2 In contrast to C60 solid, which

Exciton Relaxation Dynamics in C60 and C70 Solid

Figure 5. Temporal decay profiles of transient absorption signals for (a) C60 and (b) C70 solid at different laser fluences after photoexcitation at 614 nm and probe at 660 nm. Insets show TA decay profiles of C60 and C70 solid in a larger time window. The symbols stand for the laser fluence: (0) 1.2; (]) 0.63; (4) 0.28; (3) 0.11 mJ/cm2.

TABLE 2: Fitting Parameters for Transient Absorption Decay Profiles for Different Laser Fluences laser fluence (mJ/cm2)

decay time (ps)

relative amplitude (%)

0.11

1 20 600 1 20 600 1 20 600

37 30 33 52 25 23 74 23 3

0.28 1.2

exhibited a certain degree of exciton-exciton decay feature for its fast decay component, the decay dynamics of C70 solid appears to be almost purely monoexcitonic. Candidates of such monoexcitonic decay channels include internal conversion, intersystem crossing, and exciton-trap collision. (5) The TA decay of C70 solid can be fitted to a triexponential decay function with time constants of τ1 ) 1 ps, τ2 ) 20 ps, and τ3 ) 600 ps (Table 2). The portion of τ2 and τ3 components becomes larger at lower laser fluence (Figure 5b and Table 2), indicating a reduced contribution from exciton-exciton annihilation. The TA decay profile of C60 shows the excitonexciton annihilation feature clearly (Figure 5a), which is very similar to previous results.5 As in the C70 case, the transient absorption decay of C60 solid can be fitted to a triexponential

J. Phys. Chem. B, Vol. 103, No. 31, 1999 6507 function with three decay times, τ1, τ2, and τ3 (3 ps, 25 ps, and 1 ns). The τ1 component of C60 (3 ps) being slower than that of C70 (1 ps) might reflect the lower exciton concentration of C60 than that of C70 solid due to the lower absorption cross section. First of all, we want to discuss the nature of the fast and slow decay components. In the case of C60, it was suggested that a good candidate for the observed monoexcitonic decay was intersystem crossing.2 The observed decay time of 1 ns at room temperature was taken to be supporting such an interpretation, in view of the known intersystem crossing rate of 1.2 ns in room-temperature solution.13,14 Likewise, the slow decay time component of 600 ps measured in our C70 solid may also be indicative of an intersystem crossing process since it is very close to the intersystem crossing time of 650 ps for C70 in roomtemperature solution.13-16 Although the time profile seems to display a significantly faster decay as the temperature increases (Figure 4b), this is not due to an increased intersystem crossing rate, which typically does not depend on temperature much, but due to the decreased contribution of the slower component (from 0.3% at 300 K to 6.2% at 10 K (Table 1)). The mostly predominant fast decay component must be also mainly due to some kind of monoexcitonic process, and it is likely that collision between exciton and trap is responsible for this. Because of lower symmetry in molecular geometry of C70 compared with C60, the C70 solid is known to contain more crystal defects and disorder,17-20 which act as exciton trap sites. The nonradiative decay of excitons at these trap sites is typically very facile. Furthermore, the decay is not to be significantly affected by laser fluence, since the exciton concentration is rather low (∼10-3/molecule) so that exciton-exciton annihilation process is going to be less important than exciton-trap collision as long as there is sufficiently large concentration of trap sites. Both of these aspects of the exciton-trap quenching process appear to be in accord with the observed temperature and laser fluence dependence. At this point, it is perhaps worth noting that the fast decay component of C60 solid which was originally attributed solely to S1-S1 annihilation despite its weak laser fluence dependence might in fact have been due in part to the exciton-trap mechanism. The vast difference between the values of fast decay time constant τ1 for C60 solid (250 ps) and for C70 solid (20 ps) could reflect the large difference in their trap concentration. As for the character of the observed PL bands, the highenergy PL1 band is believed to be due to free exciton luminescence, whose major decay channel is exciton-trap collision. On the other hand, the lower energy bands (PL2 and PL3) are assumed to be due to self-trapped exciton states, which still decay through exciton-trap collisions at room temperature, due perhaps to the relatively free nature of these self-trapped excitons at room temperature. But at low temperatures, the mobility of these self-trapped excitons becomes so low that the only remaining effective decay channel is the slower (600 ps) intersystem crossing, in contrast to the case of the free-exciton PL1 band which decays through exciton-trap collision even at 10 K. Femtosecond TA decay of C70 solid shows there exists a very fast decay component (1 ps) which is not observed in the PL experiment. As shown in Figure 5b and Table 2, the fast decay component increases upon increasing the laser fluence, indicating the fast decay process is related to the exciton-exciton annihilation process. Contrary to the case of TCSPC measurement, the vastly increased exciton concentration (ca. 10-1(0.5 exciton/molecule) under TA measurement condition is estimated to be comparable to or even larger than the trap concentration

6508 J. Phys. Chem. B, Vol. 103, No. 31, 1999 in C70 solid. Consequently, the exciton-exciton annihilation process is considered as a major decay channel in the overall exciton decay dynamics in TA measurement. On the other hand, the slower decay time of 20 ps is taken to be due to the excitontrap interaction which becomes more significant as the laser fluence decreases. The fast decay component of C70 does not completely disappear as the laser fluence decreases, in contrast to the case of C60 solid (Figure 5a), showing the characteristic exciton-exciton annihilation feature. As shown in the insets of Figure 5, which was taken at the laser fluence of 0.28 mJ/ cm2, there is also a long decay time component (1 ns and 600 ps), which is due to the intersystem crossing of C60 and C70 as also observed in the PL experiment. In summary, the decay dynamics of C70 solid is mainly governed by exciton-trap annihilation under the relatively low fluence condition for PL measurement, with a minor additional channel due to intersystem crossing, which becomes more significant at lower temperatures. Under the extremely high fluence condition employed in the TA measurement, the exciton concentration is vastly increased and leads to efficient excitonexciton collisions, in addition to exciton-trap annihilation, which becomes less important at higher laser fluences. Acknowledgment. The present work was supported by the Basic Science Research Institute program (BSRI-95-3413), the KOSEF fund through the Center for Molecular Science (S.K.K.), and the Creative Research Initiatives of the Ministry of Science and Technology (D.K.). References and Notes (1) Kratschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167. (2) Yang, S. I.; Suh, Y. D.; Jin, S. M.; Kim, S. K.; Park, J.; Shin, E.-j.; Kim, D. J. Phys. Chem. 1996, 100, 9223. (3) Brorson, S. D.; Kelly, M. K.; Wenschuh, U.; Buhleier, R.; Kuhl, J. Phys. ReV. B 1992, 46, 7329.

Cho et al. (4) Thomas, T. N.; Taylor, R. A.; Ryan, J. F.; Mihailovic, D.; Zamboni, R. Europhys. Lett. 1994, 25, 403. (5) Dexheimer, S. L.; Vareka, W. A.; Mittleman, D.; Zettl, A.; Shank, C. V. Chem. Phys. Lett. 1995, 235, 552. (6) Flom, S. R.; Pong, R. G. S.; Bartoli, F. J.; Kafafi, Z. H. Phys. ReV. B 1992, 46, 15598. (7) Hess, B. C.; Forgy, E. A.; Frolov, S.; Dick, D. D.; Vardeny, Z. V. Phys. ReV. B 1994, 50, 4871. (8) Ebbesen, T. W.; Mochizuki, Y.; Tanigaki, K.; Hiura, H. Europhys. Lett. 1994, 25, 503. (9) Byrne, H. J.; Maser, W.; Ruhle, W. W.; Mittelbach, A.; Honle, W.; von Schnering, H. G.; Movaghar, B.; Roth, S. Chem. Phys. Lett. 1993, 204, 461. (10) Farztdinov, V. M.; Lozovik, Y. E.; Matveets, Y. A.; Stepanov, A. G.; Letokhov, V. S. J. Phys. Chem. 1994, 98, 3290. (11) Rosker, M. J.; Marcy, H. O.; Chang, T. Y.; Khoury, J. T.; Hansen, K.; Whetten, R. L. Chem. Phys. Lett. 1992, 196, 427. (12) It is generally accepted that one can obtain reasonably accurate temporal information if the overall decay time constant to be measured is not smaller than about one-fifth of the IRF time.21 In other words, one can determine a 20 ps decay time with an IRF time of up to 100 ps (in our case the IRF time is 60 ps). In any case, it is not necessary for the IRF to be much faster than the decay function itself. (13) Kim, D.; Lee, M.; Suh, Y. D.; Kim, S. K. J. Am. Chem. Soc. 1992, 114, 4429. (14) Lee, M.; Song, O.-K.; Seo, J.-C.; Kim, D.; Suh, Y. D.; Jin, S. M.; Kim, S. K. Chem. Phys. Lett. 1992, 196, 325. (15) Tanigaki, K.; Ebbesen, T. W.; Kuroshima, S. Chem. Phys. Lett. 1991, 185, 189. (16) Wasielewski, M. R.; O’Neil, M. P.; Lykke, K. R.; Pellin, M. J.; Gruen, D. M. J. Am. Chem. Soc. 1991, 113, 2774. (17) Shin, E.-j.; Park, J.; Lee, M.; Kim, D.; Suh, Y. D.; Yang, S. I.; Jin, S. M.; Kim, S. K. Chem. Phys. Lett. 1993, 209, 427. (18) Shin, E.-j.; Song, O.-K.; Kim, D.; Suh, Y. D.; Yang, S. I.; Kim, S. K. Chem. Phys. Lett. 1994, 218, 107. (19) Verheijen, M. A.; Meekes, H.; Meijer, G.; Bennema, P.; de Boer, J. L.; van Smaalen, S.; van Tendeloo, G.; Amelinckx, S.; Muto, S.; van Landuyt, J. Chem. Phys. 1992, 166, 287. (20) Tomita, M.; Hayashi, T.; Gaskell, P.; Maruno, T.; Tanaka, T. Appl. Phys. Lett. 1992, 61, 1171. (21) Lakowicz, J. R. Topics in Fluorescence Spectroscopy; Plenum Press: New York, 1991.