Chirality in bare and passivated gold nanoclusters

Abstract

Chiral structures have been found as the lowest-energy isomers of bare (Au 28 and Au 55 ) and thiol-passivated ͓Au 28 (SCH 3 ) 16 and Au 38 (SCH 3 ) 24 ] gold nanoclusters. The degree of chirality existing in the chiral clusters was calculated using the Hausdorff chirality measure. We found that the index of chirality is higher in the passivated clusters and decreases with the cluster size. These results are consistent with the observed chiroptical activity recently reported for glutahione-passivated gold nanoclusters, and provide theoretical support for the existence of chirality in these compounds.Detailed knowledge of the lattice structure, shape, morphology, surface structure, and bonding of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. This information is essential to optimizing their utilization as novel nanocatalysts, 1 and as building-blocks of new molecular nanostructured materials, 2 with potential applications in nanoelectronics 3 and biological diagnostics. 4 An effective theoretical approach to determine gold cluster structures is to combine genetic algorithms and many-body potentials ͑to perform global structural optimizations͒, and first-principles density functional theory ͑to confirm the energy ordering of the local minima͒. Using this procedure we recently found many topologically interesting disordered gold nanoclusters with energy near or below the lowestenergy ordered isomer. 5-8 The structures of these clusters showed low spatial symmetry or no symmetry at all, opening the possibility of having distinct electronic and optical properties in such systems. In other studies on passivated gold nanoclusters, 9,10 we also found that the effect of a methylthiol monolayer ͑24 SCH 3 molecules͒ on a truncatedoctahedron ͑with fcc geometry͒ Au 38 cluster is strong enough to produce a dramatic distortion on the gold cluster, resulting in a disordered geometry for the most stable Au 38 (SCH 3 ) 24 passivated cluster.Although the calculated structure factors of the disordered gold clusters were in qualitative agreement with the data obtained from x-ray powder diffraction on experimental samples, 6,7 the direct confirmation of the existence of bare and thiol-passivated gold nanoclusters with low or no spatial symmetry had not been possible due to the lack of enough experimental resolution for clusters in the size range of 1-2 nm. 11-13 Nevertheless, in a recent study using circular dichroism, Shaaff and Whetten ͑SW͒ 14 found a strong optical activity in the metal-based electronic transitions ͑across the near-infrared, visible and near ultraviolet regions͒ of sizeseparated glutathione-passivated gold clusters in the size range of 20-40 Au atoms. SW pointed out that the most plausible interpretation of these results is that the structure of the metal-cluster core of the gold-glutathione cluster compounds would be inherently chiral. Moreover, since the most abundant cluster in the experimental samples corresponds...

Au40: A large tetrahedral magic cluster

“…1a) was transformed into much less symmetric configurations of lower energy. The energy lowering is quite substantial, more than 2 eV, instead of the 0.3 to 0.6 eV lowering found by Garzón et al for their C s model 22,23 . After a local minimum with one adatom sticking out (Fig.…”

“…The authors found that the global minima are amorphous in nature with an Au 4 tetrahedron core and a much bigger outer-shell for Au N with N > 32. Using an empirical potential for global minimum search followed by density functional theory (DFT) calculations, Garzón et al found that the most stable structure of Au 38 is of C s symmetry with an Au 5 core 22,23 . This structure was found to be slightly lower in energy than the high-symmetry truncated octahedron (by about 0.3 to 0.6 eV, depending on the choice of DFT functional 22,23 ).…”

Fluxionality of gold nanoparticles investigated by Born-Oppenheimer molecular dynamics

“…Also this topology is cage-like as the previous one, but it loses symmetry and drops to point group C 1 being therefore chiral. Chirality seems a frequently occurring phenomenon for gold clusters and has been observed by several researchers [82][83][84][85][86][87][88][89][90]. (Table I).…”

Asymmetric Synthesis of Chiral Bimetallic [Ag28Cu12(SR)24]4– Nanoclusters via Ion Pairing

“…The majority of chiral metal nanoclusters known to date are of types (a) [27][28][29][30][31][32][33][34][35][36] and (b) 16,17,[37][38][39] , with a few belonging to types (c) and (d). [40][41][42] As prepared, many of these chiral metal nanoclusters (types b-d) are racemic mixtures that require separation and purification by various optical resolution methods. Enantioseparation of Au nanoclusters (e.g., Au28, Au38, Au40) has been reported by using chiral HPLC columns.…”