Article pubs.acs.org/JPCC
Nanometals: Identifying the Onset of Metallic Relaxation Dynamics in Monolayer-Protected Gold Clusters Using Femtosecond Spectroscopy Chongyue Yi,†,§ Hongjun Zheng,†,§ Laura M. Tvedte,‡ Christopher J. Ackerson,‡ and Kenneth L. Knappenberger, Jr.*,† †
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
‡
ABSTRACT: Electronic relaxation dynamics were studied for a series of gold monolayer-protected clusters (MPCs) whose sizes ranged from 1.5 to 2.4 nm. Au 9 6 (mMBA) 42 , Au 10 2 (pMBA) 44 , Au 115 (pMBA) 4 9 , Au 117 (mMBA) 5 0 , Au144(pMBA)60, Au250(pMBA)98, and Au459(pMBA)170 (pMBA = paramercaptobenzoic acid; mMBA = meta-mercaptobenzoic acid) were selected for study because they bridged the expected transition from nonmetallic to metallic electron behavior. Excitation-pulse-energy-dependent measurements confirmed Au144(pMBA)60 (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron−phonon coupling constant of (1.63 ± 0.25) × 1016 W m−3 K−1. Smaller, nonmetallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantumconfined electronic states. Volume-dependent electronic relaxation dynamics for ≤1.8 nm MPCs were observed and attributed to a combination of large energy differences between electronic states and phonon frequencies and spatial separation of photoexcited electrons and holes. Evidence for the latter was obtained by substituting mMBA for pMBA as a passivating ligand, which resulted in a 4-fold increase in the relaxation rate constant.
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ligands.21 Despite significant advances in describing the electronic structure and steady-state extinction spectra of ultrasmall metal nanoclusters and larger metallic nanoparticles, the inherent structural heterogeneity of this class of nanomaterials has made it difficult to determine the size at which the transition from nonmetallic to metallic electronic behavior occurs. Monolayer-protected nanoclusters (MPCs) are a class of nanomaterials that can be synthesized and isolated with atomic precision, making them useful models for determining the sizeand structure-dependent properties of metal particles in the 1− 2 nm domain.22−25 Here, we provide the first comprehensive report on electronic relaxation dynamics for a range of gold MPCs, Au96(mMBA)42, Au102(pMBA)44, Au115(pMBA)49, Au 117 (mMBA) 50 , Au 144 (pMBA) 60 , Au 250 (pMBA) 98 , and Au459(pMBA)170, where pMBA and mMBA are para- and meta-mercaptobenzoic acid, respectively. The sizes of the inorganic diameters of these MPCs range from 1.5 to 2.4 nm, where the transition from nonmetallic to metallic behavior is expected to occur.26−32
INTRODUCTION Photonic nanomaterials offer great potential for light-driven applications including photocatalysis, solar-to-electric energy conversion, medical therapeutics, sensing, and optical image contrast.1 These unique opportunities arise because materials confined to nanoscale dimensions often display strikingly different chemical, physical, and optical properties than their bulk counterparts. For metal nanoparticles (≥∼ 2 nm), sizeand structure-dependent properties, such as the localized surface plasmon resonance (LSPR), can be used to amplify light−matter interactions, resulting in increased optical signals and nanoparticle-mediated photocatalysis.2−11 In contrast to metallic nanoparticles, quantum-confined metal nanoclusters (≤∼2 nm) exhibit structured absorption spectra and microsecond near-infrared photoluminescence.12−18 These optical properties emerge for clusters because the manifold of electronic states near the Fermi level is nondegenerate, with energy gaps exceeding kT.19 As a result of the large energy mismatch between typical phonon frequencies and the electronic energy gaps of quantum-confined metals, the electronic relaxation dynamics of small nanoclusters differ from the electron−phonon scattering models that accurately describe bulk and nanoparticle metallic systems.20 For Au25(SC8H9)18 and Au38(SC12H25)24, electronic relaxation is determined by the strength of the coupling between specific electronic states and the vibrational modes of protecting © 2015 American Chemical Society
Received: December 4, 2014 Revised: February 14, 2015 Published: February 24, 2015 6307
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Figure 1. (a) Linear extinction spectra for Au96(mMBA)42 (magenta), Au102(pMBA)44 (black), Au115(pMBA)49 (red), Au117(mMBA)50 (orange), Au144(pMBA)60 (blue), Au250(pMBA)98 (green), and Au459(pMBA)170 (olive). A distinct absorption feature corresponding to a LSPR is observed near 530 nm for the larger nanostructures. (b) First derivative spectra for Au102(pMBA)44 (black), Au115(pMBA)49 (red), and Au144(pMBA)60 (blue). The first derivative spectrum shows a feature at 540 nm for Au144(pMBA)60 that is not detected for the smaller MPCs. This feature is attributed to a core-localized plasmon resonance. (c) Comparison of the transient difference spectra (raw data) acquired for Au102(pMBA)44 (black), Au115(pMBA)49 (red), and Au144(pMBA)60 (blue). Spectra were recorded at a pump−probe time delay of 1 ps following 400 nm excitation. (d) Transient difference spectra acquired for, Au250(pMBA)98 (green) and Au459(pMBA)170 (olive) recorded at a pump−probe time delay of 1 ps. The dominant feature of these spectra is transient bleach at the LSPR wavelength.
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shaker (30 °C, 100 rpm) overnight. The Au117(mMBA)50 compound is produced by substituting mMBA for pMBA in the Au102(SR)44 synthesis; briefly, chloroauric acid, metamercaptobenzoic acid, and methanol were added to water and stirred for 1 h. Sodium borohydride was added to reduce to nanoparticles, and the reduction continued overnight. In all cases, final preparation of the MPCs was by gel purification. Femtosecond Time-Resolved Transient Extinction. Femtosecond pump−probe transient extinction experiments were performed on a 1 kHz regeneratively amplified Ti:Sapphire laser system that delivered 800 μJ pulse energies centered at 800 nm. The amplified pulse was characterized by frequency-resolved optical gating (FROG) pulse diagnostics.39 The amplified laser output was frequency doubled to generate 400 nm light (200 μJ/pulse), which was attenuated and used as the excitation pump pulse. Excitation pulse energies used in the work reported here ranged from 400 to 1400 nJ/pulse. A small portion (4%) of the fundamental laser output was passed through a sapphire plate to generate the continuum probe pulse that typically extended from 450 to 850 nm. The pump−probe time delay was controlled using a retroreflecting mirror mounted on a motorized linear translation stage (Newport). Both pulses were spatially overlapped in the sample−laser interaction region. Differential absorption of the probe was measured as a function of the time delay between the pump and probe by mechanically chopping the pump pulse at 500 Hz.
EXPERIMENTAL SECTION Nanocluster Synthesis and Characterization. The Au102(pMBA)44 cluster was synthesized in a direct synthesis (as opposed to a “size-focusing synthesis”) as described previously33 with the formula assigned from a single-crystal X-ray structure.34 The Au144(pMBA)60 cluster is also a “direct” synthesis, as described previously35 with the Au114 core nuclearity assigned by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM).36 The Au115(pMBA)49, Au250(pMBA)98, and Au459(pMBA)170 nanoclusters were synthesized in a size-focusing synthesis recently reported.37 The molecular formulas of each product were assigned by quantitative analysis of polyacrylamide gel electrophoresis, using the method of Tsukuda.38 While these three nanoclusters appear as discrete molecular products as evidenced by their stability to etching conditions and discrete banding on a polyacrylamide gel, there is approximately 10% uncertainty associated with the molecular formulas assigned to these three products. This uncertainty arises primarily from uncertainties inherent in the electrophoretic method used to assign the approximate molecular formulas. The Au96(mMBA)42 and Au117(mMBA)50 compounds are novel in this report, with the product formulas assigned by the electrophoretic method described above.33 The Au96(mMBA)42 compound is synthesized by reduction with sodium borohydride of a 3.4:1 mMBA/Au in 70% n-butanol in an incubating 6308
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Figure 2. Global analysis and SVD results for Au102(pMBA)44 (a), Au115(pMBA)49 (b), Au144(pMBA)60 (c), and Au117(mMBA)50 (d). All data were collected using an excitation wavelength of 400 nm.
amplitude coefficient of the ith component, t is the pump− probe time delay, and τi is the relaxation time of the ith component.
Here, the probe was spectrally dispersed on a silicon diode array to generate a wavelength-resolved differential absorption spectrum that spanned from 450 to 800 nm. Data were acquired for 2 s at each pump−probe delay. The instrument response time (120 fs) was determined from the nonresonant response of the pump and probe pulses in H2O. The full dynamic range of the measurements extended from 10 ps before to 3.2 ns after time zero. Data Analysis Method. In order to determine the temporal evolution of transient absorption spectra, global regression analysis was used. Global regression analysis allowed for deconvolution of individual components that contributed to highly congested time-resolved spectra. The singular value decomposition (SVD) method40 was employed using a program written in-house to convert the raw 3-D data matrix into principle kinetics and spectra of the individual components. The SVD method produced a linearly independent set of eigenvectors, which stored spectral and kinetic information. In particular, the principle kinetics eigenvectors were especially useful for discerning the number of unique electronic energy relaxation mechanisms. In this way, the principle kinetics eigenvectors could be used to characterize the time dependence of the transient spectra. The principle kinetics were fit globally to the sum of multiple exponential decay functions S(t ) = G(t ) ∑ ci e−t/ τi i
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RESULTS AND DISCUSSION
First insights into structure-specific optical properties were obtained from the linear absorption spectrum of each MPC sample. Figure 1a portrays the absorption spectra of all six MPCs studied; the first derivative spectra of Au102(pMBA)44, Au115(pMBA)49, and Au144(pMBA)60 are given in Figure 1b. Nonzero absorption was recorded across the entire visible spectrum, with nearly monotonic increases at shorter wavelengths observed for all MPC samples. For small MPC nanoclusters (Au102−Au144), evidence for discrete electronic absorption transitions was revealed by the first derivative spectra, which showed common peaks at 440 and 670 nm for all three MPCs. Au144(pMBA)60 also exhibited an absorption peak at 540 nm that was not detected for smaller clusters. Absorption peaks in this region are attributed to core-localized plasmon resonances, which are collective electronic excitations with maximum electron density localized on the metal atoms of the nanocluster core.28 In contrast, the Au250(pMBA)98 and Au459(pMBA)170 samples both exhibited a prominent absorption peak at 530 nm as expected based on the known LSPR of gold metallic nanoparticles.19 All MPC samples exhibited interband absorption signals for excitation energies exceeding 2.4 eV (Figure 1a). These size-dependent linear extinction data indicated that only gold domains smaller than 1.8 nm (i.e., Au144(pMBA)60) exhibited peaks arising from the discrete electronic absorption transitions that are consistent with expectations for quantum-confined metal nanoclusters. Ex-
(1)
Here, g(t) is a Gaussian function that deconvoluted the instrument response function (IRF) to the Gaussian pump and probe laser pulses, i is the total number of components, ci is the 6309
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similar feature for Au144(SC8H9)60, which was assigned to transient bleaching of the core-localized plasmon resonance.26 In order to examine if the negative-amplitude component detected for Au144(pMBA)60 was a signature of metallic behavior, the time-dependent amplitude of this transient signal was compared to the LSPR bleach response of the Au250(pMBA)98 and Au459(pMBA)170 MPCs obtained employing a range of excitation pulse energies that spanned from 400 to 1400 nJ/pulse. The basis for this comparison is the wellknown two-temperature model that accurately describes the excitation pulse energy dependence of electron cooling rates for metallic systems.45,46 Electronic excitation of metals using pulsed lasers generates a nonequilibrium Fermi gas that thermalizes by three successive steps: (1) femtosecond electron−electron scattering, (2) picosecond electron−phonon scattering, and (3) energy dissipation to the surroundings on a time scale of tens to hundreds of picoseconds, depending on the volume of the metal domain.19 The second process of electron−phonon scattering exhibits a linear rate dependence on the excitation laser pulse energy, which makes it relevant for identifying and quantifying metallic electronic behavior. As an example of this behavior, the time-dependent magnitude of the transient extinction signal monitored at the bleach maximum for Au459(pMBA)170 is shown in Figure 3a. These data showed an unambiguous linear dependence on laser pulse energy (Figure 3b). Examination of the time-dependent bleach recovery signals for Au250(pMBA)98 and Au144(pMBA) 60 (Figure 2c; component 2) yielded similar responses (Figure
citation spectra for gold domains larger than 1.8 nm were more typical of plasmon-supporting metal nanoparticles. The Au144(pMBA)60 cluster was unique in that, in addition to the low-energy absorption features common to all small gold domains, it also exhibited collective excitations at visible wavelengths. Therefore, the 1.8 nm Au144(pMBA)60 species likely represents a transition from cluster-like to metallic-like nanoparticle behavior. In order to determine if the transition from cluster-like (i.e., nonmetallic) to metallic electronic properties occurs at 1.8 nm, all MPCs were studied using femtosecond time-resolved transient extinction spectroscopy. Laser-based spectroscopies are ideally suited for determining the onset of metallic electron dynamics in colloidal nanoparticles because the time-dependent magnitude of the transient difference signals can be used to quantify electronic relaxation rates.19,41−43 Transient extinction spectra measured using a pump−probe time delay of 1 ps following 400 nm excitation are shown for Au102(pMBA)44, Au115(pMBA)49, and Au144(pMBA)60 in Figure 1c. Similar to linear absorption data, the transient spectra shared some common features, although size-dependent distinctions were observed. All spectra in Figure 1c were dominated by excitedstate absorption (ESA) at long wavelengths. All three samples exhibited broad ESA with resolvable peaks at 575 and 700 nm, which were attributed to electronic excitation between nondegenerate excited states. These findings were consistent with discrete electronic states for small metal nanoclusters. The transient extinction spectrum for Au144(pMBA)60 also included a broad negative-amplitude component that extended from 475 to 550 nm, further distinguishing the 1.8 nm domain from smaller MPCs. In contrast, the primary feature of both the Au250(pMBA)98 and Au459(pMBA)170 MPCs was a transient bleach at the LSPR wavelength (Figure 1d). LSPR bleaching is a characteristic transient signal following metal nanoparticle excitation.19 The similar negative-amplitude transient extinction signal measured for Au144(pMBA)60 at short wavelength provided further evidence that the 1.8 nm MPC exhibits some metallic character. In order to resolve the electronic relaxation dynamics of the Au144(pMBA)60 and smaller MPCs, the femtosecond timeresolved transient data were analyzed using global analysis and SVD methods. Figure 2a portrays the global analysis results for Au102(pMBA)44, which revealed three electronic relaxation components, two with time constants of 1.4 and 160 ps and one persisting for several nanoseconds. In contrast, the Au115(pMBA)49 (Figure 2b) and Au117(mMBA)50 (Figure 2d) MPCs revealed only two components; the nanosecond decay process was not detected for these nanoclusters. Au 144 (pMBA) 60 exhibited three components with time constants of 0.60, 1.70, and 23.7 ps (Figure 2c). For all three nanoclusters, component 1 corresponded to ESA signals that decayed on a time scale of 0.6−2.2 ps, depending on the MPC identity. All four samples also exhibited similar spectral shapes for component two; however, there was sharp cluster dependence for the corresponding component 2 relaxation time constants, which decreased linearly with increasing MPC domain size. The additional components detected for Au102(pMBA)44 and Au144(pMBA)60 were MPC-specific. The nanosecond decay process observed for Au102(pMBA)44 was attributed to low-energy radiative decay, which is known for subnanometer MPCs.18,21,44 For Au144(pMBA)60, the 1.7 ps process corresponded to a transient bleach recovery signal that was not observed for smaller MPCs. We previously reported a
Figure 3. Time-dependent transient bleach magnitude for Au459(pMBA)170 following 400 nm excitation using many different excitation pulse energies (400−1400 nJ/pulse). (b) Electron−phonon scattering relaxation time constants plotted versus the laser excitaiton pulse energy for Au144(pMBA)60 (black), Au250(pMBA)98 (green), and Au459(pMBA)170 (red). The linear trend in the panel (b) data is an indication of metallic electronic relaxation dynamics. 6310
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from high- to low-energy excited states. The component 2 spectra were MPC-independent and arose from excited-state absorption spanning 525−725 nm. Component 2 decay time constants were in the range of 10−150 ps, depending on MPC. Indeed, the time dependence of the transient absorption signal magnitude was unique for each MPC (Figure 4a). When these
3b). This behavior was not detected for the Au102(pMBA)44, Au115(pMBA)49, and Au117(mMBA)50 MPCs. The linear relationship observed for the larger gold MPCs is understood by treating the nonequilibrium electron gas and the metal lattice as two coupled subsystems at different temperatures; the temperature of the electron gas is determined by the excitation laser pulse energy, and the lattice remains at room temperature. The rate of energy flow from the hot electron gas to the lattice is directly dependent upon the nanoparticle electron−phonon coupling constant, G.45,46 Hence, the linear relationship between the excitation laser pulse energy and electron−phonon scattering rates is a hallmark of metallic electron thermalization processes. The room-temperature electron−phonon coupling constant for metals can be determined from these data (Figure 3b) by extrapolating the linear trend in relaxation rates to zero laser pulse energy, which corresponds to τ0 in eq 2 G=
γT0 τ0
(2)
where γ = 66 J m−3 K−2 and T0 is the laboratory temperature.19 The experimentally determined electron−phonon coupling constants for Au 144 (pMBA) 60 , Au 250 (pMBA) 9 8 , and Au459(pMBA)170 were (1.63 ± 0.25) × 1016, (1.99 ± 0.41) × 1016, and (2.26 ± 0.25) × 1016 W m−3 K−1, respectively. We have also determined the electron−phonon coupling constant for Au144(SC6H13)60 [(1.68 ± 0.15) × 1016 W m−3 K−1],26 which agreed well with the value for the pMBA-protected variant. On the basis of these results showing uniform electron−phonon scattering behaviors for MPCs from Au144(pMBA)60 up to Au459(pMBA)170, we conclude that Au144(pMBA)60, which corresponds to a 1.8 nm gold domain, exhibits metallic electronic energy relaxation dynamics. To our knowledge, the Au144 MPC family represents the smallest known cluster to exhibit metallic electron dynamics. Treatment of femtosecond pump−probe data acquired for the Au144(SC6H13)60 MPC requires models based on both metallic electron thermalization and discrete superatom states. Inclusion of both models is necessary because gold MPCs in the region intermediate to clusters and metallic nanoparticles support both types of excitation; absorption of a 400 nm photon is resonant with both superatom electronic and gold interband excitation.27,28 The global analysis results presented in Figure 2 indicated that both discrete excited electronic states and nonequilibrium electron gases were produced by laser pumping of the Au144(pMBA)60 ensemble. Consistent with the current data, infrared absorption measurements and computational studies show that Au144(SR)60 electronic states near the Fermi level are not separated by an energy gap.26−29,46,47 However, absorption peaks detected at approximately 0.27 eV are reconciled by allowed spectroscopic transitions between superatom and ligand-based orbitials.32 Taken together, the femtosecond pump−probe, infrared absorption, and computational results all indicate that the 1.8 nm Au144 MPC represents a structurally precise nanostructure at the onset of metallic behavior. Electronic relaxation dynamics for smaller, nonmetallic MPCs and for superatom states of Au144(pMBA)60 showed different behaviors. The spectra of component 1 from the global analysis (Figure 2) were cluster-specific, resulting from the size-dependent electronic structure of quantum-confined MPCs. By comparison, the time dependence of its decay was similar for all four MPCs and attributed to internal conversion
Figure 4. Normalized time-dependent transient extinction signal truncated to the picosecond time scale for Au102(pMBA)44 (red), Au115(pMBA)49 (blue), Au117(mMBA)50 (green), and Au144(pMBA)60 (orange). The data reflected a strong sensitivity of relaxation rate to nanoparticle size on this time scale. The comparison between Au115(pMBA)49 (blue) and Au117(mMBA)50 (green) also showed the influence of the protecting ligand on electronic dynamics. (b) Summary plot showing the relaxation rates for pMBA-protected (black) and mMBA-protected nanoclusters plotted versus the nanostructure volume.
data were fitted to obtain component 2 decay rate constants and the results were plotted versus the domain volume (Figure 4b, black filled squares), a monotonic increase with domain volume increase was revealed. This size-dependent behavior was similar in nature to a phonon bottleneck, which has been predicted to impede electronic relaxation for colloidal semiconducting nanocrystals.20,48−50 For spatially confined nanocrystals, the size dependence is attributed to inefficient electron−phonon coupling that results when the energy separating electronic states greatly exceeds the energy of phonon modes needed to assist carrier relaxation. Both radiative and nonradiative relaxations of MPCs are mediated by electron−phonon and electron−ligand-vibration coupling efficiency.21 For many nanocrystals, the phonon bottleneck is obscured by strong columbic interactions between electrons and holes, which mediate rapid nonradiative decay. In the case of MPCs, the charge carriers may be spatially separated if the one carrier is localized in the staple motif while another is in the 6311
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relaxation. These data illustrate the ability to tailor the properties of colloidal metal nanoclusters in the 1−2 nm domain size through judicious control of MPC structure and dimensions and, as such, are expected to impact many applications that make use of the nanophotonic material platform.
core. Indeed, this type of electronic excitation is expected for many gold MPCs at ultraviolet and visible pump energies, which involve excitation from metal orbitals into predominantly ligand-based states.28 Specifically for Au144(SH)60, excitation is expected to generate a core-localized hole and a liganddelocalized electron. In order to determine if the size-dependent relaxation dynamics observed for the Aux(pMBA)y nanoclusters could have resulted from spatially separated charge carriers, we investigated the influence of the protecting ligand on the rate constant associated with component 2 decay. The timedependent transient extinction signals plotted in Figure 4a compare the relaxation dynamics for Au115(pMBA)49 and Au117(mMBA)50. The mMBA and pMBA protecting ligands were chosen for this comparison because, although structurally similar, the Hammett constants are 0.15 and 0.25 for mMBA and pMBA, respectively.51 Therefore, the para-substituted version of mercapto-benzoic acid is a much stronger electronwithdrawing species than the meta-substituted analogue. As a result, it was expected that electronic excitation of pMBA would result in increased probability of spatially separated electrons and holes relative to excitation of mMBA. The transient signal decay was markedly accelerated for the Au117(mMBA)50 sample (Figure 4a), reflecting a 4-fold increase in the relaxation rate constant (Figure 4b, red filled square). A similar observation was made upon comparing the Au96(mMBA)42 species with Au102(pMBA)44; the electronic relaxation rate constant for the mMBA-protected nanocluster was 3.3× faster than that for the pMBA-protected variant. Therefore, these data provide direct evidence that the electronic relaxation dynamics of nonmetallic MPCs are affected by the electronic properties of the protecting ligand, which is consistent with expectations for spatially confined electron−hole pairs in semiconducting nanocrystals.50
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions §
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. C.Y. and H.Z. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS An award from the National Science Foundation to K.L.K. supported a portion of this work (Award No. CHE-1150249). C.J.A was supported by Colorado State University and NIH Award R21 EB014520.
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REFERENCES
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CONCLUSION We have presented the first examination of electronic energy relaxation dynamics for a range of monolayer-protected gold nanoclusters that includes the transition from cluster-like to metallic behavior. On the basis of these data, we concluded that the 1.8 nm Au144(pMBA)60 species represents the smallest known gold domain to exhibit metallic electron cooling behavior. Determinations of the electron−phonon coupling constant for the 1.8 nm domain indicated that the cooling dynamics are insensitive to the protecting ligand (i.e., pMBA versus SC6H13). MPCs in the 1.5−1.8 nm range exhibited electronic relaxation dynamics that were consistent with exciton relaxation typical for spatially confined semiconductors. In particular, these smaller metal nanoclusters showed electronic relaxation rate constants that were directly determined by MPC size. The size-dependent trend in electronic relaxation rates was attributed to inefficient electron−phonon scattering and electron−hole interactions. Taking advantage of ligand exchange methods, we were able to confirm that the spatial location of the carriers in the MPC architecture influenced electronic relaxation rates; Au115(pMBA)49 had a decay rate constant that was only 1/4 that determined for Au117(mMBA)50. Because of the strong electron-withdrawing strength of pMBA, we conclude that electrons were localized in the ligand shell, whereas holes were confined to the MPC core. Similar results were observed when the dynamics of Au96(mMBA)42 and Au102(pMBA)44 were compared. In the case of mMBA, the increased density of both electrons and holes in the nanocluster’s core resulted in accelerated carrier 6312
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The Journal of Physical Chemistry C
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DOI: 10.1021/jp512112z J. Phys. Chem. C 2015, 119, 6307−6313
