Marcus A. Tofanelli

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.

Structural Basis for Ligand Exchange on Au25(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.

Crystal Structure of the PdAu24(SR)180 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.

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.

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.

Chapter 5 - Structure Determination by Single Crystal X-ray Crystallography

Abstract X-ray crystal structures of thiolate-protected gold and silver clusters undergird current structural understanding of these clusters. Both small-molecule and macromolecular crystallography traditions prove useful in the practical aspects of solving these single crystal structures. Here we give an overview of structural chemistry methods, with emphasis on single crystal X-ray methods. We highlight areas where the crystallography of clusters poses special challenges, such as in crystal growth and disorder refinement. We present a comprehensive overview of the structures solved so far, and discuss the frontiers of structure determination for thiolate ligand-protected Au, Ag, and bimetallic clusters.