10 research outputs found
Is the largest aqueous gold cluster a superatom complex? Electronic structure & optical response of the structurally determined Au146(pMBA)57
The new water-soluble gold cluster Au146(pMBA)57, the structure of which has
been recently determined at sub-atomic resolution by Vergara et al. [1], is the
largest aqueous gold cluster ever structurally determined and likewise the
smallest cluster with a stacking fault. The core presents a twinned truncated
octahedron, while additional peripheral gold atoms follow a C2 rotational
symmetry. According to the usual counting rules of the superatom complex (SAC)
model, the compound attains a number of 92 SAC electrons if the overall net
charge is 3- (three additional electrons). As this is the number of electrons
required for a major shell closing, the question arises if Au146(pMBA)57 should
be regarded as a superatom complex. Starting from the experimental coordinates
we have analyzed the structure using density-functional theory. The optimized
(relaxed) structure retains all the connectivity of the experimental
coordinates, while removing much of its irregularities in interatomic
distances, thereby enhancing the C2-symmetry feature. Analyzing the
angular-momentum projected states, we show that, despite a small gap, the
electronic structure does not exhibit SAC model character. In addition, optical
absorption spectra are found to be relatively smooth compared to the example of
the Au144(SR)60 cluster. The Au146(SR)57 cluster does not derive its stability
from SAC character; it cannot be considered a superatom complex
Optical properties of Ag(BDT)(TPP) in the VIS and UV and influence of ligand modeling based on real-time electron dynamics
International audienceWe study the optical properties of the Ag29(BDT)12(TPP)4 cluster, the geometry of which is available from experimental structure determination, by means of Fourier-transformed induced densities from real-time (time evolution) calculations of time-dependent density-functional theory. In particular, we demonstrate the influence of the ligands on the optical spectra in the visible region and, even more, in the UV. A strong peak in the UV reminiscent of the spectrum of isolated benzene is found to be caused by the phenyl rings of the TPP ligand molecules. Nonetheless, their absence in the modeling also impacts the absorption in the visible region substantially. By contrast, the aromatic rings of the BDT ligands are more strongly coupled to the silver core and loose the character of independent oscillators; they contribute a much less peaked UV absorption. Our results underline the importance of properly accounting for the full ligands for precise and reliable modeling
Visualizing screening in noble-metal clusters: static vs. dynamic
International audienceThe localized surface-plasmon resonance of metal nanoparticles and clusters corresponds to a collective charge oscillation of the quasi-free metal electrons. The polarization of the more localized d electrons opposes the overall polarization of the electron cloud and thus screens the surface plasmon. By contrast, a static electric external field is well screened, as even very small noble-metal clusters are highly metallic: the field inside is practically zero except for the effect of the Friedel-oscillation-like modulations which lead to small values of the polarization of the d electrons. In the present article, we present and compare representations of the induced densities (i) connected to the surface-plasmon resonance and (ii) resulting from an external static electric field. The two cases allow for an intuitive understanding of the differences between the dynamic and the static screening
Identifying Electronic Modes by Fourier Transform from δ-Kick Time-Evolution TDDFT Calculations
International audienc
Is the largest aqueous gold cluster a superatom complex? Electronic structure & optical response of the structurally determined Au 146
MicroED Structure of Au 146 (p-MBA) 57 at Subatomic Resolution Reveals a Twinned FCC Cluster
International audienceSolving the atomic structure of metallic clusters is fundamental to understanding their optical, electronic, and chemical properties. Herein we present the structure of the largest aqueous gold cluster, Au 146 (p-MBA) 57 (p-MBA: para-mercaptobenzoic acid), solved by electron diffraction (MicroED) to subatomic resolution (0.85 Å) and by X-ray diffraction at atomic resolution (1.3 Å). The 146 gold atoms may be decomposed into two constituent sets consisting of 119 core and 27 peripheral atoms. The core atoms are organized in a twinned FCC structure whereas the surface gold atoms follow a C 2 rotational symmetry about an axis bisecting the twinning plane. The protective layer of 57 p-MBAs fully encloses the cluster and comprises bridging, monomeric, and dimeric staple motifs. Au 146 (p-MBA) 57 is the largest cluster observed exhibiting a bulk-like FCC structure as well as the smallest gold particle exhibiting a stacking fault