J. Phys. Chem. C 2010, 114, 19935–19940
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Ultrafast Relaxation Dynamics of [Au25(SR)18]q Nanoclusters: Effects of Charge State Huifeng Qian,† Matthew Y. Sfeir,*,‡ and Rongchao Jin*,† Department of Chemistry, Carnegie Mellon UniVersity, 4400 Fifth AVenue, Pittsburgh, PennsylVania 15213, United States, and Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973, United States ReceiVed: August 20, 2010; ReVised Manuscript ReceiVed: October 10, 2010
The ultrafast electron relaxation dynamics of anionic and neutral Au25(SR)18 nanoclusters are investigated using broad-band time-resolved optical spectroscopy. From an analysis of the wavelength-dependent transient absorption kinetics, we have obtained valuable information on the spectral features that originate from excitation of “core” and “core-shell” states. In both clusters, photoexcitation occurs into two nondegenerate states near the HOMO-LUMO gap that are derived from the core orbitals. A large difference in the lifetime of the core excitations is observed, with [Au25(SR)18]- exhibiting a decay rate more than 1000 times slower than the neutral cluster. Both clusters show strong coupling to two different coherent phonon modes, which are observed at 2.4 and 1.2 THz. The electron-phonon coupling is analyzed in terms of the spectral distribution and damping of the coherent modes. Introduction The linear and nonlinear optical properties of noble metal nanoparticles (e.g., Au, Ag)1-9 have long been particularly attractive from the perspectives of both fundamental science and technological applications. Recent advances in synthetic chemistry have permitted precise control of the number of atoms in a particle, at least in the ultrasmall size regime (e.g., 1 ns). The linear absorption spectrum (gray dotted line) is plotted on an arbitrary reverse axis for comparison. (B) Three-component global fit for [Au25(SR)18]0 excited at 420 nm.
Figure 3. Transient absorption spectra for (A) [Au25(SR)18]- and (B) [Au25(SR)18]0 as a function of excitation wavelength and time. Note: In (A), data points corresponding to degenerate pump/probe conditions have been excluded from the plot as residual scattering from the pump beam could not be adequately rejected.
The broad-band transient absorption (TA) spectra of both [Au25(SR)18]- and [Au25(SR)18]0 show multiexponential decays across the entire measurement time window (∼3 ns). Both positive transient absorption (e.g., excited state absorption, abbreviated as ESA) and negative transient absorption (e.g., ground state bleach) signals were observed. Using a global fitting procedure, we are able to extract the wavelength-dependent amplitudes (cn) of the characteristic decay components for both [Au25(SR)18]- and [Au25(SR)18]0:
∆A(λ) )
∑ cn(λ) exp[-t/τn] n
Note that in this analysis, the lifetimes (τn) are assumed to be wavelength independent. Below we discuss the detailed analysis of the transient absorption signals for both Au25 anions and neutral particles. For excitation in excess of the HOMO-LUMO gap, the anion shows two distinct decay components that are resolvable in our experiment. As shown in Figure 2A, we are able to extract a fast component, c1, with a lifetime of approximately 1 ps and a long-lived component, c2, that does not significantly decay in our measurement window (∼3 ns). An examination of the relative amplitudes extracted from the global fit shows that the 1 ps amplitude spectra (c1) are dominated by strong photoinduced absorptions (i.e., ESAs), which peak around 520 nm. An examination of the early time full transient absorption spectra (Figure 3A) shows that a portion of this broad positive background overlaps with the negative features that correspond
to bleaches of the linear absorption features; note that data points corresponding to degenerate pump/probe conditions have been excluded from the spectra (Figure 3A) as residual scattering from the pump beam could not be adequately rejected. After ∼1 ps, the initial set of strong ESAs decay, evolving to the long delay time transient spectra which are dominated by the ground state bleach near 670 nm. The photoinduced absorption background associated with the c2 component appears to be broader, allowing for the observation of weak bleach features which can be correlated to higher energy linear absorption resonances that are discernible but not distinct in the linear spectrum (plotted on a reverse y axis, gray dotted line in Figure 2A). We note it has been reported that the spectral position of the bleach minima appears to blue-shift with increasing time,26 a trend which can also be seen in Figure 3. However, from our global analysis, we believe that it is the fast decay of the positive absorption components (ESA) that contributes to this apparent spectral shift rather than any real upconversion effect. In Figure 2B, we show the relative amplitudes corresponding to three extracted decay components observed in the neutral cluster. It exhibits very similar early time behavior to the anion, with a picosecond component (c1) that is dominated by ESA features and which quickly decays into a distinct set of transient features (c2). Similar to the anion, the net result is an apparent “blue-shift” of the bleach minima as a function of time as the overlapping ESAs decay (Figure 3B). However, unlike the anion, the second set of transient features decay on a time scale of ∼5 ps. The subsequent long time scale transient spectra (c3, >1 ns) very closely resemble the reverse of the linear absorption
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spectrum. This limit, not observed in the anion in the available time window (3 ns), corresponds to a long-lived ground state depopulation resulting from decay into a state with featureless absorption in the visible. The change in rate of the c2 component coincides with changes in the spectral position and oscillator strength of the linear absorption features. For example, the relative oscillator strengths of the two linear absorption features are reversed, with the stronger absorption in the neutral cluster appearing at lower energy with a small high-energy shoulder. In addition, we note that the spectral positions of the linear and transient ground state absorption features in the two clusters can be overlapped by introducing a rigid blue-shift of ∼100 meV to the anion (Supporting Information Figure S2). A possible explanation for the difference in the core lifetime may be related to the absolute values of the energy levels. Unlike the anion, the neutral cluster has an unpaired spin in the core orbitals, which should affect both the absolute position of its energy levels (relative to vacuum) as well as the optical transition energy. This fact could change the electronic overlap of the core and shell excited states, leading to a dramatic difference in decay rate and a change in the relative oscillator strengths of the two gap states. In addition, we must also account for the presence of the nearby cation (tetraoctylammonium, TOA+) that stabilizes the anionic cluster. We note that the TOA+ and [Au25(SR)18]- are intimately coupled and move in pair in solution, as evidenced by a separate NMR diffusion experiment (solvent: CD3Cl). In [Au25(SR)18]0, however, there is no Coulomb interaction with a counterion, potentially destabilizing the excited state. At present, it is not fully clear if it is the difference in the absolute energy of the levels that causes the dramatic difference in decay rates or rather the small structural differences between the two clusters.20 Detailed theoretical work will be needed to verify these effects. Furthermore, we can compare the carrier dynamics of these two clusters when the wavelength of the laser pump pulse is varied relative to the HOMO-LUMO gap. Figure 3A shows the transient absorption spectra 0.5 ps after excitation with a pump pulse centered at 390 nm (3.2 eV), 530 nm (2.3 eV), 620 nm (2 eV), and 825 nm (1.5 eV). In addition, the transient spectrum corresponding to 390 nm excitation is shown 1000 ps after excitation. For comparison, all spectra have been normalized at 780 nm. From the global analysis above, we analyze the excitation-dependent transient spectra in terms of a linear combination of amplitude spectra corresponding to the short (1 ps) and long (>1 ns) transient components. We can see that the relative amplitude of the short component increases relative to the long component as a function of increasing photon energy, although the associated kinetic rates have been found to remain constant. This manifests itself as an increase in the ESAs in the range of 500-600 nm and an apparent shift in both the magnitude and wavelength minima of the bleach near 670 nm. We can see that after long times, the UV and visible pumped transient spectra evolve to a state which resembles the transients directly pumped using NIR light (e.g., 825 nm pulse). In contrast, the transient spectra from [Au25(SR)18]0 clusters show little dependence on the photon pump energy (Figure 3B). Excitation with either 420 nm (3 eV) or 780 nm (1.6 eV) shows nearly identical transient spectra. We interpret this data using a core-shell model in which the Au13 icosahedron is electronically coupled to the Au12/ thiolate shell. Recent work by Miller et al. using a transient grating technique to examine the Au25 anion dynamics suggested that the core-to-surface relaxation process occurs in ∼1 ps, with internal conversion processes proceeding at a much faster rate
Qian et al. (not resolvable in our experiment).26 However, our excitationdependent transient data on the anion suggest that the longlived component can be directly photoexcited and that the 1 ps kinetic process is related to a separate transient species. As in previous work, we rule out internal conversion as an explanation of the fast relaxation, as there exists no signature of stateblocking effects developing on a 1 ps time scale at the bleach wavelengths. Although the neutral cluster exhibits no excitationdependent differences in the early time transient spectra, the linear absorption is seen to exhibit a weak high-energy shoulder (Figure 1, arrow). Furthermore, we argue that enough similarities exist in the wavelength dependence of the dynamics to draw similar conclusions as to the nature of these excited states. For example, as in the anion, there is an initial set of strong ESAs that decay in 1 ns kinetic signature) result from direct photoexcitation. In the second, both species result from a slightly nondegenerate HOMO-LUMO transition localized on the core orbitals. By comparing the amplitude spectra of two anion species with different ligand termination, [Au25(SR)18]- (R: CH2CH2Ph) and [Au25(SG)18]- (G: glutathione), we observe that the spectral position of the linear and transient ground state absorption features are largely (although not completely) unaffected by the change in atomic structure. On the other hand, we observe that the position and intensities of the ESA resonances are significantly altered (see Supporting Information Figure S1). For this reason, we suggest that the split HOMO-LUMO transition is the more likely scenario that describes the cluster dynamics. As a result, we now have a means of interpreting the linear spectra of the anion and neutral species. Although the strong peaks (∼670 nm) in both linear absorption spectra occur at roughly the same photon energy, the corresponding electronic
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Figure 4. Kinetic traces for (A) [Au25(SR)18]- (800 nm pump) and (B) [Au25(SR)18]0 (780 nm pump) showing the probe wavelength-dependent coherent response.
Figure 5. Integrated Fourier transform amplitude spectra for coherent response at (A) 2.4 THz and (B) 1.2 THz for both the neutral and anion clusters.
states likely have a different origin. We argue that the apparent spectral alignment of the strong near-gap transition simply reflects the fact that the splitting of the gap excitations is similar in magnitude (∼100 meV) to the renormalization of the optical gap after ionization. As the relative amplitude of the two peak changes also switches in the two clusters, likely due to the small structural differences between the two clusters, spectral coincidence of the strong optical transition occurs. It is also possible that alignment of higher energy transitions may also be a coincidence, as these states should contain more shell character and be more strongly affected by the changes in the ligand bonding. An examination of the wavelength-dependent kinetics in detail reveals that both clusters exhibit a coherent phonon response near time zero. In Figure 4A, a 2.4 THz oscillation (80 cm-1) is seen to dominate when the system is probed at 666 nm. The frequency of this coherent response is similar to what has been previously observed for quantum-confined gold nanoclusters.26,34 However, at higher probe energies, we detect a second coherent mode at approximately 1.2 THz (40 cm-1), seen in Figure 4A at a probe wavelength of 532 nm. A similar response is observed for [Au25(SR)18]0 clusters (Figure 4B). To our knowledge, the observation of this feature (i.e., 1.2 THz) has not been previously reported in discussions of electron-phonon coupling in these types of nanoclusters. In order to gain insight into the origin of this coherent response, we perform the following analysis: For each probe wavelength, we perform a Fourier transform of the early time kinetic data and integrate the resulting frequency spectrum around the finite width of a particular phonon modesin this case around 1.2 and 2.4 THz. In Figure 5, we plot the integrated amplitude of the Fourier transform as a function of wavelength which reveals that [Au25(SR)18]- and [Au25(SR)18]0 couple similarly to the two modes observed in this experiment. Figure 5A shows the Fourier transform amplitude corresponding to the
2.4 THz mode, which couples strongly to electronic states close to the optical gap. Although this feature has been previously assigned to an acoustic, spherically symmetric phonon mode involving the core atoms,34,35 we find that the amplitude spectrum exhibits a strong antiresonance near the peak of the core excitation. Further information can be gained from an examination of the lower energy mode at 1.2 THz (Figure 5B). The most striking difference from the amplitude spectra at 2.4 THz is that the 1.2 THz mode most strongly couples to states that lie well above the band gap of [Au25(SR)18]q. According to DFT calculations, these states are determined to involve the d band electronic transitions, with states closer to the gap corresponding to transitions involving sp orbitals. The origin of the 2× difference of the two phonon modes has not been clear to us. All these results suggest that detailed calculations will be needed to determine the origins of the coherent response in these clusters. It is noteworthy that coherent lattice vibrations were previously observed in metallic Au nanocrystals including nanospheres and nanorods, but in that case a continuum mechanical model was found to interpret well the observed oscillations.8,36,37 The observed phonon energies (5 and 10 meV, respectively) are small compared to the experimental and theoretical level spacing. For example, nearly 0.9 eV of energy must be dissipated through an internal conversion process in going from the HOMO-LUMO+1 state near 2.7 eV (λpeak ∼450 nm) to the HOMO-LUMO state near 1.8 eV (λpeak ∼670 nm). Furthermore, relaxation to the lowest energy core-shell state at ∼1.3 eV (observed using photoluminescence) requires an additional 0.5 eV of energy relaxation. Although coupling to solvent and ligand modes will assist with the internal conversion process, the observed time scales are much shorter than would be expected. Recent work has indicated that fast electronic relaxation can be achieved through a coherent shape deformation
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process rather than through multiphonon emission.38 Although direct evidence for this process is lacking at this time, we note that the damping times of the observed phonons are very similar to the lifetime of one core excitation (∼1 ps). Further analysis of these features is expected to give more insight into the complicated dynamics of [Au25(SR)18]q nanoclusters. Conclusions We have compared the ultrafast electron dynamics of [Au25(SR)18]- and [Au25(SR)18]0 nanoclusters. In both clusters, we observe two different transient species; evidence suggests that both these transitions originate from orbitals localized on the Au13 icosohedron. A fast transient species is found to have a similar lifetime (∼1 ps) in both species, while the second species is found to differ in decay rate by a factor of >103. Unlike the anion, [Au25(SR)18]0 shows transient spectra that are largely independent of the pump wavelengths, although it exhibits transient bleach features that reflect both the 670 and 630 nm energy states in the linear absorption spectrum. These results reveal details of the fine electronic structure of [Au25(SR)18]- and [Au25(SR)18]0 nanoclusters. We speculate that differences in the optical response result from differences in the absolute energy of the transitions, changes in symmetry of the shell atoms, and the presence of a counterion in the anionic cluster. Finally, we determine that the two charge states exhibit similar electron-phonon coupling. The phonon emission of at least two coherent modes (2.4 and 1.2 THz) and the shape deformations which result are likely responsible for the fast internal conversion processes. Theoretical modeling on the ultrafast dynamics of [Au25(SR)18]q is needed to fully understand the observed complicated dynamics. Acknowledgment. We thank Jon A. Schuller for discussions on the coherent phonon response. Research was carried out in part at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. R.J. acknowledges research support by AFOSR and NIOSH. Supporting Information Available: 1H NMR spectrum of the charge neutral Au25(SC2H4Ph)18 clusters in CDCl3 (Figure S1); comparison of the absorption spectrum of the neutral cluster to that of the anionic cluster which has been rigidly red-shifted by 100 meV (Figure S2); comparison of the ultrafast dynamics results of [Au25(SC2H4Ph)]- and [Au25(SG)18]- (Figure S3). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: New York, 1995. (2) Link, S.; El-Sayed, M. A. Annu. ReV. Phys. Chem. 2003, 54, 331.
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