Coherent Vibrational Dynamics of Au144(SC8H9)60 Nanoclusters.

The coherent vibrational dynamics of Au144(SC8H9)60, obtained from femtosecond time-resolved transient absorption spectroscopy, are described. Two acoustic modes were identified and assigned, including 2.0 THz breathing and 0.7 THz quadrupolar vibrations. These assignments are consistent with predictions using classical mechanics models, indicating that bulk models accurately describe the vibrational properties of Au144(SC8H9)60. Coherent phonon signals were persistent for up to 3 ps, indicating energy dissipation by the nanocluster was the primary dephasing channel. The initial excitation phases of the breathing and quadrupolar modes were π-phase-shifted, reflecting differences in the displacive nuclear motion of the vibrations. The combined agreement of the vibrational frequencies, relative phases, and decoherence times supported predictions based on classical models. The vibrational frequencies were insensitive to silver substitution for gold but did show increased inhomogeneous damping of the coherent phonons. The ability to predict the vibrational properties of metal nanoclusters can have an impact on nanoresonator and mass sensing technologies.

The Evolution from Superatom- to Plasmon-Mediated Magnetic Circular Dichroism in Colloidal Metal Nanoparticles Spanning the Nonmetallic to Metallic Limits.

The magneto-optical absorption properties of colloidal metal nanoclusters spanning nonmetallic to metallic regimes were examined using variable-temperature variable-field magnetic circular dichroism (VTVH-MCD) spectroscopy. Charge neutral Au25(SC8H9)18 exhibited MCD spectra dominated by Faraday C-terms, consistent with expectations for a nonmetallic paramagnetic nanocluster. This response is reconciled by the open-shell superatom configuration of Au25(SC8H9)18. Metallic and plasmon-supporting Au459(pMBA)170 exhibited temperature-independent VTVH-MCD spectra dominated by Faraday A-terms. Au144(SC8H9)60, which is intermediate to the metallic and nonmetallic limits, showed the most complex VTVH-MCD response of the three nanoclusters, consisting of 19 distinguishable peaks spanning the visible and near-infrared (3.0-1.4 eV). Variable-temperature analysis suggested that none of these transitions originated from plasmon excitation. However, evidence for both paramagnetic and mixed (i.e., nondiscrete) transitions of Au144(SC8H9)60 was observed. These results highlight the complexity of gold nanocluster electronic transitions that emerge as sizes approach metallic length scales. Nanoclusters in this regime may provide opportunities for tailoring the magneto-optical properties of colloidal nanostructures.

Superatom spin-state dynamics of structurally precise metal monolayer-protected clusters (MPCs).

Electronic spin-state dynamics were studied for a series of Au25(SC8H9)18 q and Au24Pd(SC8H9)18 monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = -1, 0, +1. These clusters were chosen for study because Au25(SC8H9)18 -1 is a closed-shell superatomic cluster, but Au25(SC8H9)18 0 is an open-shell (7-electron) system; Au25(SC8H9)18 +1 and PdAu24(SC8H9)18 0 are isoelectronic (6-electron) closed-shell systems. Carrier dynamics for electronic fine structure spin states were isolated using femtosecond time-resolved circularly polarized transient-absorption spectroscopy (fs-CPTA). Excitation energies of 1.82 eV and 1.97 eV were chosen for these measurements on Au25(SC8H9)18 0 in order to achieve resonance matching with electronic fine structure transitions within the superatomic P- and D-orbital manifolds; 1.82-eV excited an unpaired Pz electron to D states, whereas 1.97-eV was resonant with transitions between filled Px and Py subshells and higher-energy D orbitals. fs-CPTA measurements revealed multiple spin-polarized transient signals for neutral (open shell) Au25(SC8H9)18, following 1.82-eV excitation, which persisted for several picoseconds; time constants of 5.03 ± 0.38 ps and 2.36 ± 0.59 ps were measured using 2.43 and 2.14 eV probes, respectively. Polarization-dependent fs-CPTA measurements of PdAu24(SC8H9)18 clusters exhibit no spin-conversion dynamics, similar to the isoelectronic Au25(SC8H9)18 +1 counterpart. These observations of cluster-specific dynamics resulted from spin-polarized superatom P to D excitation, via an unpaired Pz electron of the open-shell seven-electron Au25(SC8H9)18 MPC. These results suggest that MPCs may serve as structurally well-defined prototypes for understanding spin and quantum state dynamics in nanoscale metal systems.

Electrophoretic Mechanism of Au25(SR)18 Heating in Radiofrequency Fields.

Gold nanoparticles in radiofrequency (RF) fields have been observed to heat. There is some debate over the mechanism of heating. Au25(SR)18 in RF is studied for the mechanistic insights obtainable from precise synthetic control over exact charge, size, and spin for this nanoparticle. An electrophoretic mechanism can adequately account for the observed heat. This study adds a new level of understanding to gold particle heating experiments, allowing for the first time a conclusive connection between theoretical and experimentally observed heating rates.

Composition-dependent electronic energy relaxation dynamics of metal domains as revealed by bimetallic Au144-xAgx(SC8H9)60 monolayer-protected clusters.

We examined the electronic relaxation dynamics for mono and bimetallic Au144-xAgx(SC8H9)60 monolayer-protected clusters (MPCs) using femtosecond time-resolved transient extinction spectroscopy. MPCs provide compositionally well-defined model systems for structure-specific determination of nanoscale electronic properties. Based on pulse-energy-dependent transient extinction data, we quantified electron-phonon coupling constants for three distinct Au144-xAgx(SC8H9)60 MPC samples, where x = 0, 0 < x < 30, and x ∼ 30, as Gx=0 = (1.61 ± 0.1) × 1016 W m-3 K-1, Gx<30 = (1.74 ± 0.1) × 1016 W m-3 K-1 and Gx∼30 = (2.07 ± 0.15) × 1016 W m-3 K-1, respectively. These results reflect a trend of greater electron-phonon coupling efficiency with increasing silver content. Based on these data, we conclude that gold-atom replacement by silver occurs at surface sites of the 114-atom metal core of the MPC. Definitive determinations of functional response to nanoscale "alloy" formation and dopant inclusion are critical to establishing predictive models for the development of materials that feature nanoparticles as active components.

Polymorphism in magic-sized Au144(SR)60 clusters.

Ultra-small, magic-sized metal nanoclusters represent an important new class of materials with properties between molecules and particles. However, their small size challenges the conventional methods for structure characterization. Here we present the structure of ultra-stable Au144(SR)60 magic-sized nanoclusters obtained from atomic pair distribution function analysis of X-ray powder diffraction data. The study reveals structural polymorphism in these archetypal nanoclusters. In addition to confirming the theoretically predicted icosahedral-cored cluster, we also find samples with a truncated decahedral core structure, with some samples exhibiting a coexistence of both cluster structures. Although the clusters are monodisperse in size, structural diversity is apparent. The discovery of polymorphism may open up a new dimension in nanoscale engineering.

Crystal Structure of the PdAu24(SR)18(0) Superatom.

The single-crystal X-ray structure of Pd-doped Au25(SR)18 was solved. The crystal structure reveals that in PdAu24(SR)18, the Pd atom is localized only to the centroid of the Au25(SR)18 cluster. This single-crystal X-ray structure shows that PdAu24(SR)18(0) is well conceptualized with the superatom theory. The PdAu24(SR)18(0) charge state is isoelectronic with Au25(SR)18(+1) as determined by a first order Jahn-Teller effect of similar magnitude and by electrochemical comparison. The previously reported increased stability of PdAu24(SR)18 can be rationalized in terms of Pd-Au bonds that are shorter than the Au-Au bonds in Au25(SR)18.

Jahn-Teller effects in Au25(SR)18.

The relationship between oxidation state, structure, and magnetism in many molecules is well described by first-order Jahn-Teller distortions. This relationship is not yet well defined for ligated nanoclusters and nanoparticles, especially the nano-technologically relevant gold-thiolate protected metal clusters. Here we interrogate the relationships between structure, magnetism, and oxidation state for the three stable oxidation states, -1, 0 and +1 of the thiolate protected nanocluster Au25(SR)18. We present the single crystal X-ray structures of the previously undetermined charge state Au25(SR)18+1, as well as a higher quality single crystal structure of the neutral compound Au25(SR)180. Structural data combined with SQUID magnetometry and DFT theory enable a complete description of the optical and magnetic properties of Au25(SR)18 in the three oxidation states. In aggregate the data suggests a first-order Jahn-Teller distortion in this compound. The high quality single crystal X-ray structure enables an analysis of the ligand-ligand and ligand-cluster packing interactions that underlie single-crystal formation in thiolate protected metal clusters.

Structural basis for ligand exchange on Au(25)(SR)(18).

The single-crystal X-ray structure of Au25(SC2H4Ph)16(pBBT)2 is presented. The crystallized compound resulted from ligand exchange of Au25(SC2H4Ph)18 with pBBT as the incoming ligand, and for the first time, ligand exchange is structurally resolved on the widely studied Au25(SR)18 compound. A single ligand in the asymmetric unit is observed to exchange, corresponding to two ligands in the molecule because of the crystallographic symmetry. The ligand-exchanged Au25 is bonded to the most solvent-exposed Au atom in the structure, making the exchange event consistent with an associative mechanism. The apparent nonexchange of other ligands is rationalized through possible selective crystallization of the observed product and differential bond lengths.

Optical properties and electronic energy relaxation of metallic Au144(SR)60 nanoclusters.

Electronic energy relaxation of Au144(SR)60(q) ligand-protected nanoclusters, where SR = SC6H13 and q = -1, 0, +1, and +2, was examined using femtosecond time-resolved transient absorption spectroscopy. The observed differential transient spectra contained three distinct components: (1) transient bleaches at 525 and 600 nm, (2) broad visible excited-state absorption (ESA), and (3) stimulated emission (SE) at 670 nm. The bleach recovery kinetics depended upon the excitation pulse energy and were thus attributed to electron-phonon coupling typical of metallic nanostructures. The prominent bleach at 525 nm was assigned to a core-localized plasmon resonance (CLPR). ESA decay kinetics were oxidation-state dependent and could be described using a metal-sphere charging model. The dynamics, emission energy, and intensity of the SE peak exhibited dielectric-dependent responses indicative of Superatom charge transfer states. On the basis of these data, the Au144(SR)60 system is the smallest-known nanocluster to exhibit quantifiable electron dynamics and optical properties characteristic of metals.

Superatom electron configuration predicts thermal stability of Au25(SR)18 nanoclusters.

The exceptional stability of ligand-stabilized gold nanoclusters such as Au(25)(SC(6)H(13))(18)(-), Au(39)(PR(3))(14)X(6)(-), and Au(102)(SR)(44) arises from the total filling of superatomic electron shells, resulting in a "noble-gas superatom" electron configuration. Electrochemical manipulation of the oxidation state can add or remove electrons from superatom orbitals, creating species electronically analogous to atomic radicals. Herein we show that oxidizing the Au(25)(SR)(18)(-) superatom from the noble-gas-like 1S(2)1P(6) electron configuration to the open-shell radical 1S(2)1P(5) and diradical 1S(2)1P(4) configurations results in decreased thermal stability of the compound, as measured by differential scanning calorimetry. Similar experiments probing five oxidation states of the putatively geometrically stabilized Au(144)(SR)(60) cluster suggest a more complex relationship between oxidation state and thermal stability for this compound.