J. Phys. Chem. B 1999, 103, 10775-10780
10775
New Transient Absorption Observed in the Spectrum of Colloidal CdSe Nanoparticles Pumped with High-Power Femtosecond Pulses C. Burda, S. Link, T. C. Green, and M. A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: May 6, 1999; In Final Form: October 7, 1999
The power dependence of the transient absorption spectrum of CdSe nanoparticle colloids with size distribution of 4.0 ( 0.4 nm diameter is studied with femtosecond pump-probe techniques. At the lowest pump laser power, the absorption bleaching (negative spectrum) characteristic of the exciton spectrum is observed with maxima at 560 and 480 nm. As the pump laser power increases, two new transient absorptions at 510 and 590 nm with unresolved fast rise (300 particles counted). The particles were thus used without size-selective precipitation.
10.1021/jp991503y CCC: $18.00 © 1999 American Chemical Society Published on Web 11/12/1999
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Figure 1. Transient absorption spectra of CdSe NPs with an average diameter of 4 nm, pumped with 400 nm femtosecond-laser pulses with a laser power of 4 mJ/cm2. The delay times of the spectra are 200 fs up to 110 ps. The measured absorption changes can be explained by the state filling effect and surface trapping of the exciton. The inset shows the corresponding kinetic traces of the bleach decay observed at 480 and 570 nm. The short lifetime (τ1) of the 570 nm transient reflects fast trapping of the charge carriers at the NP surface to the shallow trap states. The longer lifetimes of the bleach decay are assigned to the charge carrier trapping by deeper traps.
The femtosecond transient spectroscopy experiments were carried out as follows. An amplified Ti-sapphire laser system (Clark MXR CPA 1000) was pumped by an argon ion laser (Coherent Innova 300). This produced laser pulses of 80 fs duration (HWFM) and an energy of 1 mJ at 790 nm. The repetition rate was 1 kHz. A small part (4%) of the fundamental was used to generate a white light continuum in a 1 mm sapphire plate. The remaining laser light was frequency doubled by using a 0.2 mm BBO crystal. The excitation power was adjusted with a filter wheel and measured in front of the sample directly before and after the measurement. The measured laser beam diameter of the excitation pulse in the sample was 250 µm in all experiments. For spectral measurements, a CCD camera (Princeton Instruments) attached to a spectrograph (Acton Research) was used. The group velocity dispersion of the white light continuum was compensated. The absolute laser power was adjusted by an appropriate aperture, the relative power with a filter wheel. Femtosecond pump-probe spectra of the colloidal CdSe NPs with 4 nm diameter in toluene were recorded as a function of pump intensity. The excitation wavelength was 400 nm and all other experimental conditions were kept constant during the course of the measurements. The samples were measured as colloidal solutions at room temperature in fast rotating quartz cells. Furthermore, no optical degradation was observed after each experiment, which was checked by UV-vis absorption, fluorescence spectroscopy, and TEM measurements. Results In this section, the laser power dependence of the transient differential spectra is presented for 4, 12, and 32 mJ/cm2 pump power. These three experiments were selected to represent a series of measurements where the pump power was increased in steps of 4 mJ/cm2. The presented new absorption features are the first spectral change observable by increasing the lowest possible pump power.
Figure 1 shows the transient absorption spectra of the CdSe NPs, pumped with 400 nm femtosecond-laser pulses. The laser power was adjusted to 4 mJ/cm2. At this laser power we can resolve the bleach spectra of the two exciton transitions at 560 and 480 nm. Klimov et al. explained the bleaching features using the state filling model.23,32 Corresponding to previous theoretical and experimental work, the lowest energy bleach band at 560 was assigned to the |1se,1sh〉 state.23,32 The second bleach band at 480 nm was assigned to the strongly allowed transition to the |1pe,1ph〉 state.23,32 It might be due to an overlapping number of transitions involving electron and hole excited states.33 The inset in Figure 1 shows the corresponding kinetic traces of the bleach decay observed at the exciton-transition wavelengths of 480 and 570 nm. The spectral diffusion from high to low energy is clearly visible, as the 480 nm band decays faster than the 570 nm band. A more detailed kinetic analysis of the spectral diffusion of CdSe NP is presented in ref 28. For the decay of the second bleached peak, two time components are resolved, one of 2.5 ps and one longer than 30 ps. For the decay of the |1se,1sh〉 transition, we measured two longer time components, one of 5.8 ps and one longer than 70 ps. The short lifetime of the 1se-1sh transient suggests fast trapping by the shallow traps. This is consistent with the observed shallow trap emission in our photoluminescence experiments.28 The slow component of the bleach decay (τ2 in Figure 1) is possibly due to trapping of the charge carriers by deeper traps.28 It needs to be pointed out that in NPs, even after trapping of the charge carriers (which is monitored by the bleach recovery), the electron and hole are still bound by coulomb interactions, due to the spatial confinement. This induces a permanent dipole moment (µNP) within the particle. If the charge carriers are rapidly trapped at the surface, higher laser powers must lead to a higher density of trapped carriers. It was predicted
Spectrum of Colloidal CdSe Nanoparticles
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Figure 2. Transient absorption spectra of CdSe NPs with an average diameter of 4 nm, pumped with 400 nm femtosecond-laser pulses with a laser power of 12 mJ/cm2 in a time range of 60 ps. The bleaching can be explained by state filling. The positive induced absorption feature at longer wavelengths is attributed to the influence of a high density of excited charge carriers to the probe transition. It leads to new high pump power induced absorptions. The two kinetic traces in the inset of the figure show the decay of the bleaching at 500 and 570 nm. It is interesting to note that the slow component of the bleach recovery is reduced from 70 to 27 ps. We assign this acceleration to the higher charge carrier density effect at this laser power. The absorption did not decay in our experimental time range of 100 ps. This long lifetime of the transients provides further evidence that free and trapped charge carriers might be involved in the formation of the observed state.
by Hu et al.34,35 that high (trapped) charge carrier densities lead to additional transient absorptions due to exciton-biexciton transitions. In Figure 2, the power of the pump pulse was adjusted to 12 mJ/cm2 while the beam characteristics and the solution remained unchanged from those used in the 4 mJ/cm2 experiment. The transient spectra still show the bleaching of the 1se-1sh transition at 560 nm. In addition, superimposed transient absorptions are observed on both sides of the bleach. The maxima of these absorptions could not be determined since the bleach recovery is not complete within our time window of 100 ps. However, the kinetic analysis of the bleach decay reveals an apparent acceleration of the slow component from 70 to 27 ps. We therefore conclude that the observed new spectral shape and the accelerated bleach recovery is not due to a mixture of different excited NPs, but the sample is excited homogeneously. As only the intensity of the pump laser pulse is increased, the acceleration of the bleach recovery must be due to a higher charge carrier density. The apparent acceleration of the bleach recovery could then be explained either by a higher probability for the trapping of an excited charge carrier in the field of others or by a superposition of the bleach recovery and the kinetics for the new high power induced wavelength-overlapping absorption feature. Additional Auger processes cannot be excluded, but an additional absorption on the red side (>650 nm) of the transient spectra reminiscent for solvated electrons was not observed. It is possible that the ejected charge carrier could be trapped on the particle surface and does not get completely ejected into the solvent. In Figure 3, the power of the pump pulse was adjusted to 32 mJ/cm2 while the beam characteristics and the solution remained unchanged from those used in the 4 and 12 mJ/cm2 experiments. The bleaching is still visible at short delay times as a minimum in the broad absorption extending over a spectral range from
450 to 650 nm. It disappears almost completely with a characteristic time of τ1 ) 2 ps and τ2 ) 25 ps at 560 nm which is again significantly accelerated compared to the 4 mJ/cm2 experiment (Figure 1). Furthermore, it is also accelerated compared to the experiment with 12 mJ/cm2 excitation energy (Figure 2). This is in agreement with the model proposed above in which a higher density of excited charge carriers leads to faster trapping and/or a higher probability for the Auger process. The spectrum in Figure 3 at long delay times shows an apparent single broad band at 560 nm. The question immediately arises as to whether it is indeed a single band but gave the appearance of two bands at short delay times due to the nondecaying bleach band. To answer this question, quenching experiments were carried out. As demonstrated in ref 28, it is possible to transfer an excited conduction band electron of 4 nm diameter CdSe NPs to an electron acceptor such as benzoquinone (BQ) adsorbed on the surface of the NP. We used this fact to see if this would change the band shape of the observed transient absorption in this region. We found36 that the addition of BQ leads to a quenching of the high-energy side of the absorption, whereas the low-energy portion remains unchanged. This experiment supports unambiguously the assignment that the high pump power induced absorption consists of two bands corresponding to two different transitions. It is also found that the dynamics of the bleach recovery is accelerated by the fast interfacial electron transfer to the adsorbed BQ, as was already demonstrated in ref 28. It is noteworthy that our results in colloidal solutions are in qualitative agreement with the previous studies on CdSe microcrystallites in glasses.37-39 Discussion The two high power induced transient absorptions can result from (1) transient absorptions from the lowest metastable state
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Figure 3. Transient absorption spectra of CdSe NPs, pumped with laser pulses of 32 mJ/cm2 with delay times varying from 200 fs to 110 ps. The bleaching is at this laser power covered by a broad absorption and is only visible by the growth of the broad absorption at 560 nm, where initially an absorption minimum was observed. The change in the spectral shape of the CdSe NPs from a bleaching (Figure 1) to an absorption (Figure 3) is a laser power dependent effect and a higher charge carrier density is responsible for this transient behavior. From the increase at 560 nm a bleach recovery is extracted as shown in the inset. The relaxation times are found to be 900 fs and 17 ps, which is shorter than relaxation times measured at 4 mJ/cm2. The bleaching recovers much faster because of the trapping and possible Auger process of the initially created electrons and holes. The high power induced absorption has a much longer lifetime, as it is due to trapped electrons and holes. The spectral changes of the new transition due to carrier trapping could not be resolved.
of the nanoparticle, (2) red shifted exciton absorptions of the nanocrystal (corresponding to the bleach bands) induced by the internal Stark effect due to the high density of the free or trapped electrons and holes created by the multiple absorption in the particle itself, or (3) transitions to the bound biexciton (the dimer of the electron-hole pair) levels.40 1. Absorption from a Metastable State. The metastable state could be the dark state proposed41 by Bawendi et al. or a surface trap state populated via the exciton level. In this case, the redshifted absorption would be expected to have a rise time, which is equal to the observed decay time of the bleaching spectrum. This is not observed. The transient appears on an ultrashort time range (400 fs,28 and thus cannot explain a much more rapid formation of the biexciton. If the observed transient absorption is due to a “heterobiexciton” resulting from the interaction between the bright exciton and a net dipole of the rapidly surface trapped electrons and holes, then the 120 meV is simply the Stark red shift of the free exciton absorption as discussed above. Acknowledgment. The authors thank the ONR (Grant N00014-95-1-0306) for its continued support of this work. C.B. thanks the Swiss National Science Foundation (SNF) for a postdoctoral fellowship, T.G. and S.L. thank the MDI for partial support from the ONR Molecular Design Institute at Georgia Tech. We thank Dr. A. Franceschetti for helpful discussions on the energy of biexcitons in small quantum dots.
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