Melting of the Au₂₀ Gold Cluster: Does Charge Matter?

Abstract

We investigate the dependence upon the charge of the heat capacities of magic gold cluster Au obtained from density functional-based tight binding theory within parallel tempering molecular dynamics and the multiple histogram method. The melting temperatures, determined from the heat capacity curves, are found to be 1102 K for neutral Au and only 866 and 826 K for Au and Au. Both the canonical and the microcanonical aspects of the transition are discussed. A convex intruder, associated with a negative heat capacity, is evidenced in the microcanonical entropy in the case of the neutral cluster but is absent in the melting processes of the ions. The present work shows that a single charge quantitatively affects the thermal properties of the gold 20mer.

Evidencing the relationship between isomer spectra and melting: the 20- and 55-atom silver and gold cluster cases

“…The heat capacity curves as a function of the canonical temperature are shown in figure 2 in the temperature range of the solid-liquid transition. The case of Au 20 was already discussed with respect to charge effects 32 and is given here for comparison. The first observations concern the temperature widths of the transitions and the heights of the associated peaks.…”

“…A number of these pioneering studies were conducted using model Lennard-Jones potentials 4,6,7 . In the case of metal clusters, several experimental [8][9][10][11][12][13][14][15][16][17] and theoretical [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] investigations were achieved to evidence the specific features of finite size melting with a description of the interatomic forces more realistic than the simple Lennard-Jones potentials. While a continuous behaviour of their properties can be expected for relatively large sizes and in the nanoparticle size domain (N>100-1000 depending on the species), a discrete evolution is exhibited for small sizes down to the microcluster domain.…”

“…Obviously, the magic character of a given size can stem either from geometric stability or electronic stability, as it is the case in metal clusters. Both types of stabilities can also interfere and even combine, like in the specific clusters Au 20 (see 32 and references therein) or Al 32 particularly stable and exhibiting sharp caloric curves 17,31 . Not only size but also charge can influence the caloric properties of relatively small metal clusters 17,31,32 , since charge changes the filling of the electronic shells.…”

“…For small clusters, the melting temperature evolution with size is non-monotonous 36 and the addition or removal of an atom can lead to drastic variation of the melting temperature [28][29][30]45 . Moreover, for such small sizes, the melting temperature may also depend on the charge state 32 . The present work is devoted to a detailed comparative study of melting between isovalent noble metal gold and silver clusters for the intermediate sizes 20 and 55, which can be considered as the low limit of the transition between clusters and nanoparticles.…”

Density-functional tight-binding: basic concepts and applications to molecules and clusters

“…Other DFTB thermodynamical studies were concerned with metallic systems and in particular silver and gold clusters. The effect of charge on the doubly magic (electronically closed shell and geometrically a symmetric pyramid) cluster Au 20 was investigated [359] as well as the the correlation between the isomer spectra features and the nature of the solid-to-liquid transition [360], from the comparison between the caloric curves of structurally ordered systems (Au 20 , Ag 55 ) and those of disordered cases (Ag 20 , Au 55 ).…”