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 Au(SCH) 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 Au(SCH).

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

Electronic spin-state dynamics were studied for a series of Au(SCH) and AuPd(SCH) monolayer-protected clusters (MPCs) prepared in a series of oxidation states, q, including q = -1, 0, +1. These clusters were chosen for study because Au(SCH) is a closed-shell superatomic cluster, but Au(SCH) is an open-shell (7-electron) system; Au(SCH) and PdAu(SCH) 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).

Electrophoretic Mechanism of Au(SR) Heating in Radiofrequency Fields.

Gold nanoparticles in radiofrequency (RF) fields have been observed to heat. There is some debate over the mechanism of heating. Au(SR) in RF is studied for the mechanistic insights obtainable from precise synthetic control over exact charge, size, and spin for this nanoparticle.

Composition-dependent electronic energy relaxation dynamics of metal domains as revealed by bimetallic AuAg(SCH) monolayer-protected clusters.

We examined the electronic relaxation dynamics for mono and bimetallic AuAg(SCH) 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 AuAg(SCH) MPC samples, where x = 0, 0 < x < 30, and x ∼ 30, as G = (1.

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.

Jahn-Teller effects in Au(SR).

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 Au(SR).

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.

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.

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.

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.