Metallic bonding and cluster structure

Abstract

Knowledge of the structure of clusters is essential to predict many of their physical and chemical properties. Using a many-body semiempirical Gupta potential ͑to perform global minimizations͒, and first-principles density functional calculations ͑to confirm the energy ordering of the local minima͒, we have recently found ͓Phys. Rev. Lett. 81, 1600 ͑1998͔͒ that there are many intermediate-size disordered gold nanoclusters with energy near or below the lowest-energy ordered structure. This is especially surprising because we studied ''magic'' cluster sizes, for which very compact-ordered structures exist. Here, we show how the analysis of the local stress can be used to understand the physical origin of this amorphization. We find that the compact ordered structures, which are very stable for pair potentials, are destabilized by the tendency of metallic bonds to contract at the surface, because of the decreased coordination. The amorphization is also favored by the relatively low energy associated to bondlength and coordination disorder in metals. Although these are very general properties of metallic bonding, we find that they are especially important in the case of gold, and we predict some general trends in the tendency of metallic clusters towards amorphous structures.

First-principles study of the atomic and electronic structure of theSi(111)−(5×2)−Ausurface reconstructio

“…We refer to this basis set as DZPs-SZd. This basis set was already used in calculations for several gold clusters, where it was shown to lead to results in very good agreement with more complete basis sets [53].…”

On the structure of small lead clusters

“…The inherent strain in these clusters results in pair distances between surface atoms that are longer than r eq , and so the compression needed to increase n i for the surface atoms is particularly large. Therefore, these traditional structural forms are expected to become increasingly disfavoured by potentials for which the pair separation depends strongly on coordination number [17]. Instead, novel forms that are able to obtain large n i values for the surface atoms, whilst not having too large an energetic penalty for compression of the cluster interior, could potentially be lowest in energy.…”

“…* jpkd1@cam.ac.uk; http://www-wales.ch.cam.ac.uk/~jon Secondly, the many-body character makes it increasingly difficult to relate the observed structure back to the interactions, even when the assumed form for the manybody potential is relatively simple. There has been some interesting progress recently in this area, namely into the causes of the disordered structures [17], and the effect of the range of the attraction and repulsion on the competition between icosahedral, decahedral and close-packed clusters [18], but there is much still to be discovered. This task is particularly important because of the difficulty in producing good empirical metal potentials (it is not feasible to study the sizes in which we are interested in any other way).…”

Density-functional study ofAun(n=2–20)clusters: Lowest-energy structures and electronic properties

“…In Fig.4b, the HOMO-LUMO gap for Au 2 , Au 6 and Au 8 is particularly large (1.94 eV, 2.06 eV, 2.09 eV), which compare well with previous calculations by Häkkinen (1.96 eV, 2.05 eV and 2.04 eV). 21 For n = 10 − 20, substantial HOMO-LUMO gap is found in the Au 10 , Au 14 and Au 18 clusters (1.03 eV, 1.58 eV and 0.92 eV). The high stability of Au 8 and Au 18 can be understood by the effect of s-electron shell, which is also found in silver clusters 44 .…”

“…[1][2][3][4] During the past two decades, the structures of gold clusters have been studied both experimentally [5][6][7][8][9][10] and theoretically. [11][12][13][14][15][16][17][18][19][20][21][22] Experiments suggest that gold nanoclusters with diameters of 1 − 2 nm are amorphous. 5,6 Theoretical results from empirical molecular dynamics simulations or first-principles calculations also support this argument.…”