Absorption spectra of very small metal clusters exhibit individual peaks that reflect the discreteness of their localized electronic states. With increasing size, these states develop into bands and the discrete absorption peaks give way to smooth spectra with, at most, a broad localized surface-plasmon resonance band. The widely accepted view over the last decades has been that clusters of more than a few dozen atoms are large enough to have necessarily smooth spectra. Here we show through theory and experiment that for the ubiquitous thiolate cluster compound Au 144 (SR) 60 this view has to be revised: clearly visible individual peaks pervade the full near-IR, VIS and near-UV ranges of low-temperature spectra, conveying information on quantum states in the cluster. The peaks develop well reproducibly with decreasing temperature, thereby highlighting the importance of temperature effects. Calculations using time-dependent density-functional theory indicate the contributions of different parts of the cluster-ligand compound to the spectra.

It is known that the surface-plasmon resonance (SPR) in small spherical Au nanoparticles of about 2 nm is strongly damped. We demonstrate that small Au nanorods of high aspect ratio develop a strong longitudinal SPR, of intensity comparable to that in Ag rods, as soon as the resonance energy drops below the onset of the interband transitions due to the geometry. We present ab initio calculations of time-dependent density-functional theory of rods with lengths of up to 7 nm. Changing length and width, not only the energy but also the character of the resonance in Au rods can be tuned. Moreover, the aspect ratio alone is not sufficient to predict even the character of the spectrum; the absolute size matters.

Solving 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, Au146(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 C2 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. Au146(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.

The optical response of Au–Ag bimetallic nanoalloys has been studied using pseudopotential time-dependent density-functional theory calculations. The structures included the magic-number icosahedral nanoparticles of 55 and 147 atoms, 37-atom pentagonal rods, and 20-atom tetrahedra. Our results show strong resonances for the pure Ag nanoparticles and strongly broadened spectra with many transitions for the pure gold structures, in qualitative agreement with available experiment and previous calculations. For bimetallic core–shell particles, the outer shell determines the overall character of the optical response; a single outer layer of Ag can produce an Ag-like resonance even in a gold-rich structure. The inclusion of a gold core within a silver shell leads to a distinct red-shift of the silver-like resonances as well as to some damping. The bimetallic nanoparticles are found to be very sensitive to the chemical configuration, the position of the atomic species in some cases outweighing the effect of changing composition. For randomly alloyed configurations of 147-atom nanoparticles, the spectra show a smooth transition between pure Ag and Au and are in good qualitative agreement with available experiment.

This report concerns the remarkable fine structure reported recently in the optical absorption spectrum of the ubiquitous icosahedral Au144(SR)60 cluster compounds when measured under cryogenic conditions. The theoretical explanation of the spectrum relied upon an I-symmetrized variant of the conventional Pd145-type structure-model; real-time TDDFT calculations revealed that, in contradistinction to the prior state of knowledge, the spectrum is profoundly structured and rich in quantum-state information. Reported herein is an investigation of the sensitivity of the theoretical electronic absorption spectra of this compound to variations in the structure. Both I-symmetric as well as asymmetric structure-models are considered; having the same core structure and connectivity, these differ in the mutual configurations about the pyramidal S atoms, which produce significant structure differences penetrating into the gold core. As R-groups, both methyl (R=CH3) and hydrogen (R=H) are considered. The effects on the structure and spectra of local optimizations employing different exchange-correlation (xc-) functionals are also considered. The results may be summarized as follows: all computed spectra show a rich fine-structure when computed at a similar level of resolution (∼0.16 eV, transform limited); the I-symmetric structure with R=CH3 has more pronounced features than the asymmetric structure with the same rest group. This is consistent with the high degree of symmetry-imposed degeneracy in the electronic states of the former. These spectral differences between the I-symmetric and the unsymmetrical models are reduced when the CH3 R group is replaced by the smaller R=H. Many other systematic differences are noted. In particular, we show explicitly the differences caused by changing the R group, the exchange-correlation functional in the geometry relaxation, and the charge state. The present study contains clear indications as to what factors need to be well-controlled in order to achieve good agreement with available experiment. They will be useful for understanding ligand effects on the optical characteristics of thiolate-protected (and other) noble-metal clusters in this interesting size-range where the plasmon (LSPR) emerges.