Article pubs.acs.org/JPCC
Ultrafast Electronic Relaxation and Vibrational Cooling Dynamics of Au144(SC2H4Ph)60 Nanocluster Probed by Transient Mid-IR Spectroscopy Satu Mustalahti,† Pasi Myllyperkiö,† Tanja Lahtinen,† Kirsi Salorinne,† Sami Malola,‡ Jaakko Koivisto,† Hannu Hak̈ kinen,†,‡ and Mika Pettersson*,† †
Nanoscience Center, Departments of Chemistry and ‡Physics, University of Jyväskylä, P.O. Box 35, Jyväskylä FI-40014, Finland S Supporting Information *
ABSTRACT: Energy relaxation dynamics of a gold nanocluster with atomically precise composition, Au144(SC2H4Ph)60, is studied by transient mid-IR spectroscopy. The experiment is designed to simultaneously probe electronic and vibrational dynamics by using excitation at 652 nm to prepare an electronic state localized on the gold core (as shown by high level DFT calculations) and by probing a stretching vibration localized on phenyl ring of the ligand molecules. We found that electronic relaxation proceeds with a time constant of 1.5 ps simultaneously heating the phonon bath of the cluster. The heat is further dissipated to solvent with a time constant of 29 ps. The electronic relaxation time increases with increasing pump power. Absence of long-lived electronic states and power dependence of relaxation time indicate metallic behavior. The metal−ligand interface modes are strongly anharmonically coupled to the probed mode which provides connection between the cluster core temperature and the vibrational shift of the ligand molecules. The obtained results are relevant for understanding energy relaxation dynamics of nanoclusters and together with the measured absolute molar absorption coefficient of the cluster allow designing experiments for controlled heating of the cluster by continuous wave irradiation.
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INTRODUCTION Metal nanoparticles are used extensively in fundamental and applied research due to their interesting optical, chemical, and mechanical properties. The majority of research in this area is devoted to studies of relatively large particles having a size from several nanometers up to hundreds of nanometers. These particles can be considered as small pieces of bulk metal and their properties closely follow the bulk properties. However, a very distinct size region is accessed when the diameter falls below ∼3 nm. In this size range quantum effects play a role and the properties of the clusters depend sensitively on small details in the structure.1−3 In this size range the clusters change from metallic type to molecular type which is evidenced by distinct phenomena such as opening of the HOMO−LUMO gap and disappearance of a plasmon band in the optical spectrum.4,5 A very interesting class of such compounds is formed by gold clusters protected by thiolate ligands. Following the seminal work by the Kornberg group6 it has become possible to synthesize and purify thiolate-protected clusters to the extent that they can be crystallized and their structure can be determined with atomic accuracy. Currently, the crystal structures of a few thiolate-protected cluster species are known: Au 2 5 (SR) 1 8 , Au 3 6 (SR) 2 4 , Au 3 8 (SR) 2 4 , and Au102(SR)44.6−9 In addition, several other clusters have been purified and their atomic composition has been determined although the attempts to determine their crystal structure have © 2014 American Chemical Society
not yet been successful. One such example is Au144(SR)60 for which high-level DFT calculations predict an icosahedral-based structure which is supported by NMR and powder XRD studies.10,11 Au144(SR)60 can be considered to represent the borderline between molecular and metallic behavior. It has essentially no HOMO−LUMO gap, but a small optical gap of ∼0.27 eV.10,12−15 Much emphasis has been put on studies of electron and phonon dynamics of metal particles.16 The central issue is relaxation processes occurring after photon absorption. These include electron−electron (e-e) scattering, electron−phonon (e-ph) scattering, and heat transfer and dissipation. It is now well established that by using ultrafast excitation it is possible to create a nonequilibrium state between electron and lattice temperatures.17 After excitation, e-e scattering equilibrates fast the electron gas after which lattice is heated via e-ph scattering.18 For Au nanoparticles, the e-ph relaxation time is ∼1 ps and a size effect has been observed whereby smaller particles (1 eV.23 Further ultrafast measurements for Au nanoclusters have indicated that the transition to molecular behavior occurs at the size range below 2.2 nm.24−26 In recent study by Yi et al.27 Au144(SR)60 nanocluster was found to exhibit optical properties associated with metallic behavior and to have an electronic relaxation time constant of ∼1 ps. An interesting application of gold nanoparticles is their use as local heaters in photothermal cancer therapy.28,29 Excitation of strong plasmon resonance of large particles leads to heating of the particle which can be used to locally heat various targets. Combining optical heating capability with suitable surface functionalization can lead to highly specific targeting of certain cellular structures. However, using small nanoclusters may allow targeting even individual biological structures within cells such as functional proteins and enzymes. The specific local heating may allow control of the function of such small organelles by light. In order to reach this interesting target it is important to establish fundamental energy relaxation and dissipation dynamics of small nanoclusters. The Hamm group has studied energy transport through capping layers of small clusters with a core size of 1−2 nm by using isotopically labeled peptides as reporters of local temperature.30,31 These studies have revealed the time scales for electronic and vibrational relaxation which range from ∼1 ps (electronic) to several tens of ps (vibrational), in accordance with typical properties of metal nanoparticles.16 They found that the relaxation times depend on cluster size, but since the used samples were not monodisperse, no detailed correlations could be established. In this work we establish an exact relationship between the relaxation dynamics and atomic composition for one specific nanocluster, Au144(SC2H4Ph)60. Via careful synthesis and sample purification combined with characterization we confirm that only one type of cluster is present in the experiments. The electronic and vibrational energy relaxation and dissipation dynamics are measured. Also, absolute molar absorption coefficient is determined which allows accurate modeling of the cluster temperature under CW irradiation. The results of this paper establish a benchmark of relaxation properties of a structurally well-defined nanocluster which can be used to compare structure−function relationships when more data becomes available. In addition, the results can be applied to designing experiments which aim at controlled heating of target molecular structures using gold nanoclusters.
Figure 1. Au144(PET)60 crystals grown from the toluene-acetonitrile solvent system. The crystals were imaged with an optical microscope from the convex surface of the crystallization vial. The approximated sizes for the crystals are 200−300 μm for large crystals and ∼100−120 μm for medium sized crystals. The thickness of crystal plates is about 20−50 μm.
To confirm that the used sample is atomically monodisperse, most experiments were performed for purified synthesis product and also for recrystallized sample. Spectroscopic Measurements. For mid-IR region measurements the solid sample was dissolved in deuterated dichloromethane (DCM-D2, Aldrich & Eurisotop, 99.9% atom D), which provides a transparent window in the studied spectral region. To determine the molar absorption coefficient for the studied cluster, UV/vis absorption spectrum in the UV/ vis/NIR region (300−900 nm) was measured in four different concentrations (∼0.0001−0.01 mM) for a chlorobenzene (Acros Organics, >99% pure) solution of the sample. Measured absorbance values were plotted as a function of concentration at different wavelengths, and a linear fit was performed to obtain the absorption coefficient. Error was estimated based on errors in concentration and linear fits. 2-Phenylethanethiol (PET, Sigma-Aldrich, >99% pure) used in cluster synthesis was used in the spectroscopic characterization of the ligand molecule. Steady-state spectra in mid-IR and NIR regions were measured with a Nicolet Magna-IR 760 FTIR spectrometer with 1 cm−1 resolution. All measurements in the IR region were performed by using a flow cell with CaF2 windows and optical path length fixed to 80 μm by a Teflon spacer.34 The temperature dependence of FTIR spectrum was studied by using a thermally isolated flow cell holder and a temperature controlling unit (Termo Haake DC10-K10). UV/vis-spectra of the samples were measured with PerkinElmer Lambda 850 spectrometer in 1 mm quartz cuvette. For molar absorption coefficient measurements, the sample was weighed by using a Mettler MT-5 precision scale. The laser setup (UV/vis pump−mid-IR probe spectrometer) used in this work is based on femtosecond Ti:sapphire laser amplifier (Quantronix, Integra-C). Mid-IR pulses used in probing were generated from the fundamental laser beam by using a home-built double-pass optical parametric amplifier
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EXPERIMENTAL METHODS Synthesis. Thiolate monolayer protected Au144(SC2H4Ph)60 clusters were prepared according to a previously published synthesis procedure using 2-phenylethanethiol (PET) as the protecting ligand.32 All reagents used were commercially available. The obtained Au144(PET)60 clusters (for simplicity PET abbreviation is used also for the ligand in the cluster) were analyzed by ESI-MS spectrometry 18234
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(OPA) combined with difference frequency mixing of obtained signal and idler beams in a AgGaS2 crystal. The visible pump beam was generated from laser fundamental by a TOPAS (traveling-wave optical parametric amplifier of superfluorescence, LightConversion Inc.). More detailed description of the used setup is presented in ref 35. Spectral resolution of the experiment was 1.3 cm−1 and the time resolution was estimated to be ∼250 fs based on rise time of electronic absorption in a thin Si wafer. Excitation wavelength centered at 652 nm and excitation energy of approximately 3 × 1015 photons/cm2 per pulse were used (∼300 nJ per pulse, spot size ∼3 × 10−4 cm2). Transient absorption measurements were performed for parallel, perpendicular, and magic angle pump pulse polarization relative to the probe pulse polarization. Different polarizations were obtained by using a λ/2-wave plate. To prevent sample degradation during measurements, the sample solution was moved back and forth in the flow cell by using MFCS-FLEX Fluigent microfluidic flow control device. This technique also enabled the measurements to be performed with small total sample volumes (50−70 μL). A separate set of experiments was performed to study the excitation energy dependence of relaxation dynamics. In these measurements energies from ∼200 to ∼800 nJ were used. After transient absorption measurements the steady-state measurements of FTIR and UV/vis spectra were repeated in order to confirm that no sample degradation has occurred during laser spectroscopy experiments.
Figure 2. Molar absorption coefficient of Au144(PET)60. The excitation wavelength used in the transient absorption measurements is shown in the figure.
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COMPUTATIONAL METHODS Vibrational modes of an isolated 2-phenylethanethiol were calculated with density-functional theory in real-space grids as implemented in the code-package GPAW.36 To analyze the nature of energy specific 652 nm excitations we used a simplified Au144(SH)60 model cluster instead of full 2phenylethanethiolate-ligands and Casida’s formulation of the linear response time-dependent DFT (TDDFT) as implemented in GPAW.37 Induced electron and hole densities were calculated by using time-dependent density-functional perturbation theory similarly as described in ref 38. For structure relaxation we have used convergence criterion of 0.05 eV/Å for the residual forces on atoms and 0.20 Å grid spacing for electron density and furthermore LDA xc-functional to obtain reliable Au−Au distances. For the analysis of electron and hole densities we used 0.25 Å grid spacing and PBE xc-functional.39 GPAW uses projector augmented waves and the setups include relativistic corrections for gold.
Figure 3. FTIR spectra of purified sample (black), recrystallized sample (red), and PET (blue) in DCM-D2 solution. The inset shows an enlarged view of the probed absorption band.
Transient absorption (TA) spectrum of the Au144(PET)60 cluster was measured with a UV/vis pump−mid-IR probe spectrometer by using a probe pulse centered at 1500 cm−1. The probe wavelength was chosen so that a single IR absorption band of Au144 cluster centered at 1496 cm−1 (see inset in Figure 3) was probed. A contour plot of the TA spectrum is shown in Figure 4. TA spectrum shows a negative ground state bleach signal of IR absorption and a positive hot band red-shifted from the ground state signal. In addition, a strong positive signal with instant rise is observed in the entire spectral region, even outside the vibrational band at short time delays. The polarization of exciting light was found to have no effect on TA spectrum. Since no polarization effect was observed, data obtained with magic angle polarization was used to perform a more detailed analysis of the results. Time evolution of the transient bands was analyzed by studying the kinetics at different wavelengths by performing a global fit to obtained data. Kinetics could be sufficiently well fitted by using a sum of two exponentials convoluted with Gaussian instrument response function. The fitted kinetics for ground state bleach, hot band, and early time positive absorption are shown in Figure 5. Time constants obtained from the fits were 1.5 and 29 ps.
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RESULTS Absorption spectrum of Au144(PET)60 cluster in the UV/vis/ NIR region (300−900 nm) is shown in Figure 2. The Y axis is given as absolute molar absorption coefficient. FTIR spectrum was measured in DCM-D2 solution for both purified and recrystallized cluster samples and compared with the spectrum of PET also dissolved in DCM-D2. These spectra are shown in Figure 3. Negative peaks in the spectrum are induced by background correction that was performed to eliminate solvent absorption. As can be seen from Figure 3, cluster spectra closely resemble the spectrum of PET. The electronic absorption of the cluster is seen as constantly rising background absorption in Figure 3. 18235
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Figure 6. Temporal evolution of the hot band center (left) with a decay time constant τ = 0.79 ps, and fwhm of the hot band (right) with a rise time constant τ = 0.61 ps. Figure 4. Contour plot of transient absorption spectrum of a Au144(PET)60 sample.
Figure 5. Kinetic traces for bleach signal at 1497 cm−1 (blue) and hot band signal at 1492 cm−1 (red) with obtained fits (black). Inset shows a kinetic trace for early time positive TA signal at 1510 cm−1 (red) and obtained fit (black).
with respect to characteristic times. It can also be seen that both hot band center position and fwhm return to their initial values as the system has relaxed back to its initial state. To analyze the time constants for growth rate of spectral shifts, a single exponential function was fitted to the data at early delays (see Figure 6). Obtained time constants were ∼0.6−0.8 ps and were found to be quite similar for hot band center position and fwhm. To get further insight into spectral shifts and band broadening induced by increasing temperature, an FTIR spectrum of Au144(PET)60 and PET was measured at different temperatures between −5 °C and +24 °C in DCM-D2 solution. The probed absorption band at 1496 cm−1 was then fitted with a Lorentzian function to analyze changes in peak position and fwhm. The obtained spectra and peak positions obtained from the fits are shown in the Supporting Information (Table S1, Figure S2). It was observed that shifts in the studied temperature range were very small in both samples. However, several times larger shifts as a function of temperature were obtained for cluster sample than for PET, 2.5 × 10−3 and 6.2 × 10−4 cm−1/K, respectively.
Similar analysis was performed for the kinetics obtained with different excitation energies. No excitation energy dependence was observed for the longer time constant. The faster time constant however was found to depend approximately linearly on the excitation energy, and a linear fit for the obtained data was performed. By extrapolating the fit to zero excitation energy a time constant of approximately 1.5 ps was obtained. The faster time constants obtained for different excitation energies and the obtained fit are shown in the Supporting Information (Figure S1). From the data obtained with different excitation energies, it could also be seen that the transient signal raises linearly with increasing pump power. To analyze the time evolution of the spectrum of the probed vibration, a linear background correction was performed to the TA spectrum to remove the strong positive signal at short delays. Transient spectra, obtained at several time delays were then fitted with sum of two Lorentzian functions. Fitted values for hot band center position and fwhm were then plotted as a function of time delay. Obtained plots are presented in Figure 6. This analysis shows that as a function of time the hot band center position first decreases and reaches its minimum value at a time delay of about 3−4 ps after which it increases finally reaching the original value. The fwhm behaves in a similar way
DISCUSSION Steady State Spectra. Au144(PET)60 shows continuous absorption with weak features in the entire UV/vis region. The shape of the spectrum coincides well with previously published spectrum.32 The molar absorption coefficient is also in agreement with earlier work on the 29 kDa cluster.40 There is a bump just above 500 nm which could be a sign of an emerging plasmon absorption. The FTIR spectrum of Au144(PET)60 in DCM-D2 solution coincides well with the spectrum of PET indicating that the vibrational modes in this region are not strongly affected by binding to the gold core. The onset for electronic absorption is observed at ∼2200 cm−1 (0.27 eV) as previously reported.15 The steady-state spectra were analyzed prior to transient absorption measurements to find suitable spectral regions for excitation and probing. Since the main idea of the experiment was to follow energy flow in the system both temporally and spatially, an excitation wavelength that prepares an excited state which is localized in the gold core and a vibration localized in the ligands were chosen as pump and probe. The choice for the excitation wavelength was relatively simple since the Au144(PET)60 cluster has strong and continuous electronic absorption in the visible spectral range. An excitation
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be assigned to electronic relaxation of the gold core which happens via electron−phonon coupling. The obtained electronic energy relaxation time constant coincides relatively well with the previously reported time constant ∼1 ps, also the excitation energy dependence of the time constant is fairly consistent with the previous results.27 These results further support the previously reported conclusion of the metallic nature of the Au144 nanocluster. It can also be noted that for the studied cluster the method used in this work to determine the time constant for electronic excitation seems to confirm the previously reported values determined with a different method (transient absorption in the visible spectral region). The longer decay time constant (29 ps) can be assigned to heat dissipation into the solvent. Also this result seems reasonable when compared with previously published work.30 Heating of the phonon bath is associated with shift of the probed vibration due to anharmonic couplings between modes evidenced by the appearance of a hot band which evolves in time.42 In essence, the probed vibration acts as a local thermometer and its center position and width are good indicators of the temperature of the system. Interestingly, analysis of the shift of the hot band center position and hot band fwhm shows that changes are not instantaneous at early times. These processes were analyzed to proceed with a time constant of 0.6−0.8 ps reflecting the finite time taken to convert electronic energy into heat. The fact that the time constants are not exactly the same as the electronic decay constant may be explained by the complexity of processes determining the bandwidth and position. Qualitatively, however, the observations are in good agreement. An important conclusion from the observation of nonzero rise time is that it is indeed conversion of electronic energy into vibrational heating of the system that it responsible for the hot band. Part of the bleach and the hot band close to time zero may also arise from direct electron−phonon coupling of the electronic excitation and the probed mode but it is not possible to quantify this contribution. From the experiments a unified picture of dynamical events after excitation at 652 nm can be formed. After instant electronic excitation, a nonequilibrium distribution of electrons and holes relaxes with a time constant of 1.5 ps. Simultaneously, phonon bath is heated via e-ph coupling. Heat is further dissipated into the environment with a time constant of 29 ps and there are no long-lived intermediates formed using this excitation wavelength indicating that there is no electronic energy gap in the system. Temperature Dependence. Based on the linear rise in the signal as a function of excitation energy and calculations based on the determined absorption coefficient and approximate concentration of the sample, we can deduce that our excitation is mainly in one-photon absorption regime. After establishing this, we can make estimates of the temperature rise in the cluster after excitation. We consider two limits: (i) heat localization in the inner metal core (114 Au atoms) and (ii) heat evenly distributed in the whole cluster. For (i) we used heat capacity of bulk gold from the literature,43 25.418 J mol−1 K−1 at 25 °C, and for (ii) we summed the heat capacities of bulk gold for the core and 30 RS-Au-SR units. This division is based on the structural model of Au144 from theoretical modeling.10 The (vibrational) heat capacity of one RS-Au-SR unit was calculated according to statistical thermodynamics from the harmonic vibrational spectrum which was obtained from a DFT calculation. The obtained total heat capacity (for
wavelength of 652 nm was chosen to ensure that absorbance of the sample does not significantly exceed 1 with the used concentration. The absorption band at 1496 cm−1 was chosen as a probe because it has high intensity and it does not overlap with the solvent absorptions. Additionally, there is no electronic absorption from the ground state in the probed region, but upon excitation a transient electronic absorption is induced which allows monitoring of electronic relaxation dynamics in the gold core. TDDFT calculations were performed to confirm that upon excitation the prepared state is localized in the gold core and that the probed vibration is localized in the ligands. Results of DFT calculations are shown in Figure 7. According to these
Figure 7. Calculated eigenvectors of C−C double bond stretching mode of the phenyl ring for isolated 2-phenylethanethiol at the upper left corner, the full model structure of the Au144(PET)60 cluster at the upper right corner. Induced electron and hole densities for the 652 nm wavelength excitation are represented in the lower panel in white color as a thin slice taken from the center of the cluster. The surface of the cluster enclosing Au-core and S−Au−S protecting units without the organic part has been drawn as a yellowish ring. Protecting units that fit inside the thin slice (neglecting the organic part) have been also drawn for better visualization of the structure. Electron and hole densities are calculated for simplified Au144(SH)60 model structure.
calculations upon excitation at 652 nm both hole density and electron density are localized in the cluster core, and there is no significant change of electron density in the ligand molecules. The probed absorption band is a highly localized stretching vibration in the phenyl rings of the ligands according to the literature.41 Transient Absorption Experiments. Strong positive transient absorption signal that is observed in the entire probed spectral region can be assigned to a transient electronic absorption arising from excited electrons of the gold core. This feature allows monitoring of dynamics of electronic excitation and relaxation. Rise of the absorption signal from electrons is instant, which can be seen from the TA spectrum. The fact that an instant rise is observed indicates that the initial relaxation process in which the energy of excited electrons is spread over the cluster via the electron−electron scattering cannot be resolved with the time resolution of our laser setup. The short relaxation time constant (1.5 ps), obtained for this feature can 18237
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one cluster) is equal to 2 × 10−20 J K−1 at 25 °C. This model is certainly not exactly correct since it does not describe properly the interface between the gold core and the ligand units but it is sufficient for our purposes. The temperature dependence of heat capacity was taken into account by calculating the total heat capacity at different temperatures (between 250 and 400 K) and fitting the values with a linear function. Temperature dependence for Au heat capacity was obtained from ref 44. Finally, the temperatures were calculated from the photon energy and heat capacity by integrating the change in temperature over the photon energy. This resulted in temperature change ΔT = 63 K for case (i) and 15 K for case (ii). These values should be considered as the upper and lower limits for the system temperature rise. From the dynamical point of view the heat is initially localized in the metal core and then dissipated to the ligands and further to the solvent. In order to further elucidate the role of metal−ligand interface to the dissipation dynamics we attempted to measure vibrational relaxation time of PET by a single color IR-IR pump−probe experiment. The pump and probe frequency was tuned either to the probed mode or to the C−H stretching mode. Interestingly, we were not able to observe any reliable transient signal. In order to confirm that the reason was not due to an experimental problem we repeated the same measurement under the same conditions for Cr(acac)3 (acac = acetylacetonate) which gives a strong response in IR-IR transient absorption in the same spectral region. Cr(acac)3 gave a proper transient signal while the PET ligand did not yield any signal. We conclude that the mode couplings and anharmonicity of the probed mode in the bare PET are so small that within our spectral resolution of 1.3 cm−1 the shift cannot be resolved. This is interesting since the estimated ΔT is much higher than in the cluster experiments, 136 K with 1500 cm−1 excitation. The difference can be explained if we assume that the low-frequency modes in the metal−ligand interface are more strongly anharmonically coupled to the probed mode than the intramolecular modes of bare ligand. We tested this hypothesis by measuring the temperature dependence of IR spectrum for neat PET and for the Au144 cluster. The temperature was varied between −5 and +24 °C because of the low boiling point of the solvent. The shift in the peak position in the cluster sample was found to be about four times larger than the change observed for pure PET. Also the shift in fwhm for the cluster sample was observed to be twice the value obtained for PET. This result is in agreement with our hypothesis and it indicates that gold cluster has low frequency vibrational modes in the gold−ligand interface that are strongly anharmonically coupled to probed ligand vibrations. The observed shifts for PET are also sufficiently small to explain why transient signals could not be seen in the IR−IR experiments. On the other hand, the absolute magnitude of the shifts in the cluster spectra are also too small compared to the shifts observed in the transient absorption measurements. One possible explanation is that the shift increases strongly when going to higher temperatures. The other possibility is that after electronic relaxation the modes are highly nonstatistically excited in the cluster leading to anomalously large shifts compared to the equilibrium situation.
measurements where electronic excitation was induced by 652 nm radiation in the metal core and vibrational probe was localized in the ligand molecule. The energy flow from gold core to ligands and further to solvent was monitored in a single experiment by taking advantage of observation of transient electronic absorption and vibrational absorption in the same mid-IR spectral region. Electronic relaxation and simultaneous heating of the phonon bath takes place with a time constant of 1.5 ps and the time constant increases with pump power. The heat dissipation to the solvent occurs with a time constant of 29 ps. Fast electronic relaxation and power-dependence of the time constant indicate metallic behavior. The metal−ligand interface modes are strongly anharmonically coupled to the probed mode which provides connection between the cluster temperature and the vibrational shift of ligand molecules. The presented results establish a correlation between exact composition and energy relaxation and dissipation dynamics of small nanoclusters by confirming the previously reported electronic relaxation time constant and providing new information on the vibrational cooling. These results also establish a benchmark for comparison with other cluster sizes.
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ASSOCIATED CONTENT
S Supporting Information *
The excitation energy dependence of electronic relaxation time constant, temperature dependent FTIR spectra, and a table of temperature-dependent peak positions. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mika.j.pettersson@jyu.fi. Tel: +358 50 3109969. Fax: +358 14 260 4756. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the Academy of Finland and the NGS-NANO and LASKEMO graduate schools. Computational resources were provided by the CSC in Espoo, Finland. Heikki Häkkänen is thanked for help in the transient absorption measurements and Janne Ihalainen is thanked for discussions regarding the experimental setup for the sample cell.
(1) Qian, H.; Zhu, M.; Wu, Z.; Jin, R. Quantum Sized Gold Nanoclusters with Atomic Precision. Acc. Chem. Res. 2012, 45, 1470− 1479. (2) Tsukuda, T. Toward an Atomic-Level Understanding of SizeSpecific Properties of Protected and Stabilized Gold Clusters. Bull. Chem. Soc. Jpn. 2012, 85, 151−168. (3) Häkkinen, H. The Gold−sulfur Interface at the Nanoscale. Nat. Chem. 2012, 4, 443−455. (4) Philip, P.; Chantharasupawong, P.; Qian, H.; Jin, R.; Jayana, T. Evolution of Nonlinear Optical Properties: From Gold Atomic Clusters to Plasmonic Nanocrystals. Nano Lett. 2012, 12, 4661−4667. (5) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. Optical Absorption Spectra of Nanocrystal Gold Molecules. J. Phys. Chem. B 1997, 101, 3706−3712. (6) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution. Science 2007, 19, 430−433.
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CONCLUSIONS Relaxation dynamics of electronic excitation for the Au144(SC2H4Ph)60 cluster was studied by transient absorption 18238
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The Journal of Physical Chemistry C
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(7) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of a Thiol-protected Au25 Cluster and Optical Properties. J. Am. Chem. Soc. 2008, 130, 5883−5885. (8) Qian, H.; Eckenhoff, W. T.; Zhu, Y.; Pintauer, T.; Jin, R. Total Structure Determination of Thiolate-protected Au38 Nanoparticles. J. Am. Chem. Soc. 2010, 132, 8280−7281. (9) Zeng, C.; Qian, H.; Li, T.; Rosi, N. L.; Yoon, B.; Barnett, R. N.; Whetten, R. L.; Landman, U.; Jin, R. Total Structure and Electronic Properties of the Gold Nanocrystal Au36(SR)24. Angew. Chem. Int. Ed. 2012, 51, 13114−13118. (10) Lopez-Acevedo, O.; Akola, J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. Structure and Bonding in the Ubiquitous Icosahedral Metallic Gold Cluster Au144(SR)60. J. Phys. Chem. C 2009, 113, 5035− 5038. (11) Wong, O. A.; Heinecke, C. L.; Simone, A. R.; Whetten, R. L.; Ackerson, C. J. Ligand Symmetry-equivalence on Thiolate Protected Gold Nanoclusters Determined by NMR Spectroscopy. Nanoscale 2012, 4, 4099−4102. (12) Koivisto, J.; Malola, S.; Kumara, C.; Dass, A.; Häkkinen, H.; Pettersson, M. Experimental and Theoretical Determination of the Optical Gap of the Au144(SC2H4Ph)60 Cluster and the (Au/ Ag)144(SC2H4Ph)60 Nanoalloys. J. Phys. Chem. Lett. 2012, 3, 3076− 3080. (13) Hicks, J. F.; Miles, D. T.; Murray, R. W. Quantized DoubleLayer Charging of Highly Monodisperse Metal Nanoparticles. J. Am. Chem. Soc. 2002, 124, 13322−13328. (14) Quinn, B. M.; Liljeroth, P.; Ruiz, V.; Laaksonen, T.; Kontturi, K. Electrochemical Resolution of 15 Oxidation States for Monolayer Protected Gold Nanoparticles. J. Am. Chem. Soc. 2003, 125, 6644− 6645. (15) Koivisto, J.; Salorinne, K.; Mustalahti, S.; Lahtinen, T.; Malola, S.; Häkkinen, H.; Pettersson, M. Vibrational Perturbations and LigandLayer Coupling in a Single Crystal of Au144(SC2H4Ph)60 Nanocluster. J. Phys. Chem. Lett. 2014, 5, 387−392. (16) Hartland, G. V. Optical Studies of Dynamics in Noble Metal Nanostructures. Chem. Rev. 2011, 111, 3858−3887. (17) Fujimoto, J. G.; Liu, J. M.; Ippen, E. P.; Bloembergen, M. Femtosecond Laser Interaction with Metallic Tungsten and Nonequilibrium Electron and Lattice Temperatures. Phys. Rev. Lett. 1984, 53, 1837−1840. (18) Schoenlein, R. W.; Lin, W. Z.; Fujimoto, J. G.; Eesley, G. L. Femtosecond Studies of Nonequilibrium Electronic Processes in Metals. Phys. Rev. Lett. 1987, 58, 1680−1683. (19) Arbouet, A.; Voisin, C.; Christofilos, D.; Langot, P.; Del fatti, N.; Vallee, F.; Lerme, J.; Celep, G.; Cottancin, E.; Gaudry, M.; et al. Electron-Phonon Scattering in Metal Clusters. Phys. Rev. Lett. 2003, 90, 177401. (20) Smith, B. A.; Zhang, J. Z.; Giebel, U.; Scmid, G. Direct Probe of Size-dependent Electronic Relaxation in single-sized Au and Nearly Monodisperse Pt Colloidal Nano-particles. Chem. Phys. Lett. 1997, 270, 139−144. (21) Link, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. Transition from Nanoparticle to Molecular Behavior: a Femtosecond Transient Absorption Study of a Size-selected 28 Atom Gold Cluster. Chem. Phys. Lett. 2002, 356, 240−246. (22) Miller, S. A.; Womick, J. M.; Parker, J. F.; Murray, R. W.; Moran, A. M. Femtosecond Relaxation Dynamics of Au25L18− MonolayerProtected Clusters. J. Phys. Chem.C 2009, 113, 9440−9444. (23) Akola, J.; Walter, M.; Whetten, R. L.; Häkkinen, H.; Grönbeck, H. On the Structure of Thiolate-Protected Au25. J. Am. Chem. Soc. 2008, 130, 3756−3757. (24) Varnavski, O.; Ramakrishna, G.; Kim, J.; Lee, D.; Goodson, T. Critical Size for the Observation of Quantum Confinement in Optically Excited Gold Clusters. J. Am. Chem. Soc. 2010, 132, 16−17. (25) Yau, S. H.; Varnavski, O.; Gilbertson, J. D.; Chandler, B.; Ramakrishna, G.; Goodson, T., III. Ultrafast Optical Study of Small Gold Monolayer Protected Clusters: A Closer Look at Emission. J. Phys. Chem. C 2010, 114, 15979−15985.
(26) Yau, S. H.; Varnavski, O.; Goodson, T., III. An Ultrafast Look at Au Nanoclusters. Acc. Chem. Res. 2013, 46, 1506−1516. (27) Yi, C.; Tofanelli, M. A.; Ackerson, C. J.; Knappenberger, K. L., Jr. Optical Properties and Electronic Energy Relaxation of Metallic Au144(SR)60 Nanoclusters. J. Am. Chem. Soc. 2013, 135, 18222−18228. (28) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Noble Metals on the Nanoscale: Optical and Photothermal Properties and Some Applications in Imaging, Sensing, Biology, and Medicine. Acc. Chem. Res. 2008, 41, 1578−1586. (29) Lal, S.; Clare, S. E.; Halas, N. J. Nanoshell-enabled Photothermal Cancer Therapy: Impending Clinical Impact. Acc. Chem. Res. 2008, 41, 1842−1851. (30) Schade, M.; Moretto, A.; Donaldson, P. M.; Toniolo, C.; Hamm, P. Vibrational Energy Transport through a Capping Layer of Appropriately Designed Peptide Helices over Gold Nanoparticles. Nano Lett. 2010, 10, 3057−3061. (31) Donaldson, P. M.; Hamm, P. Gold Nanoparticle Capping Layers: Structure, Dynamics, and Surface Enhancement Measured Using 2D-IR Spectroscopy. Angew. Chem., Int. Ed. 2013, 52, 634−638. (32) Qian, H.; Jin, R. Ambient Synthesis of Au144(SR)60 Nanoclusters in Methanol. Chem. Mater. 2011, 23, 2209−2217. (33) Salorinne, K.; Lahtinen, T.; Koivisto, J.; Kalenius, E.; Nissinen, M.; Pettersson, M.; Häkkinen, H. Nondestructive Size Determination of Thiol-Stabilized Gold Nanoclusters in Solution by Diffusion Ordered NMR Spectroscopy. Anal. Chem. 2013, 85, 3489−3492. (34) Bredenbeck, J.; Hamm, P. Versatile Small Volume Closed-cycle Flow Cell System for Transient Spectroscopy at High Repetition Rates. Rev. Sci. Instrum. 2003, 74, 3188−3189. (35) Maçôas, E. M. S.; Kananavicius, R.; Myllyperkiö, P.; Pettersson, M.; Kunttu, H. Ultrafast Electronic and Vibrational Energy Relaxation of Fe(acetylacetonate)3 in Solution. J. Phys. Chem. A 2007, 111, 2054− 2061. (36) Enkovaara, J.; Rostgaard, C.; Mortensen, J. J.; Chen, J.; Dułak, M.; Ferrighi, L.; Gavnholt, J.; Glinsvad, C.; Haikola, V.; Hansen, H. A.; et al. Electronic Structure Calculations with GPAW: a Real-space Implementation of the Projector Augmented-wave Method. J. Phys.: Condens. Matter 2010, 22, 253202. (37) Walter, M.; Häkkinen, H.; Lehtovaara, L.; Puska, M.; Enkovaara, J.; Rostgaard, C.; Mortensen, J. J. Time-dependent Density-functional Theory in the Projector Augmented-wave Method. J. Chem. Phys. 2008, 128, 244101. (38) Yang, H.; Wang, Y.; Huang, H.; Gell, L.; Lehtovaara, L.; Malola, S.; Häkkinen, H.; Zheng, N. All-thiol-stabilized Ag44 and Au12Ag32 Nanoparticles with Single-crystal Structures. Nat.Commun. 2013, 4, 2422. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (40) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. Properties of a Ubiquitous 29 kDa Au:SR Cluster Compound. J. Phys. Chem. B 2001, 105, 8785−8796. (41) Gardner, A. M.; Wright, T. G. Consistent Assignment of the Vibrations of Monosubstituted Benzenes. J. Chem. Phys. 2011, 135, 114305. (42) Hamm, P.; Ohline, S. M.; Zinth, W. Vibrational Cooling after Ultrafast Photoisomerization of Azobenzene Measured by Femtosecond Infrared Spectroscopy. J. Chem. Phys. 1997, 106, 519−529. (43) CRC Handbook of Chemistry and Physics, 85th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2004; pp 4−135. (44) Takahashi, Y.; Akiyama, H. Heat Capacity of Gold from 80 to 1000 K. Thermochim. Acta 1986, 109, 105−109.
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