Stability of gold cages (Au16 and Au17) at finite temperature

Abstract

We have employed ab initio molecular dynamics to investigate the stability of the smallest gold cages, namely Au16 and Au17, at finite temperatures. First, we obtain the ground state structure along with at least 50 distinct isomers for both the clusters. This is followed by the finite temperature simulations of these clusters. Each cluster is maintained at 12 different temperatures for a time period of at least 150 ps. Thus, the total simulation time is of the order of 2.4 ns for each cluster. We observe that the cages are stable at least up to 850 K. Although both clusters melt around the same temperature, i.e. around 900 K, Au17 shows a peak in the heat capacity curve in contrast to the broad peak seen for Au16.

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

“…However, very symmetric structures often correspond to particularly stable equilibrium structures and may therefore be associated with larger structural excitation energies and higher melting temperatures than the disordered ones. The variability of the isomer spectrum is thus determinant on the thermodynamic properties 21,26,45 and will be analyzed in the discussion.…”

“…We will hereafter focus on gold and silver clusters. Their structural and melting properties have been investigated using various experimental setups 34,40,43,44 and theoretical approaches 19,20,[24][25][26][27][28][28][29][30]33,35,36,41,[45][46][47][48][49] , mostly with empirical potentials. The most stable structures reported for different cluster sizes exhibit either very regular shape (e.g.…”

“…As an example, it has been reported that Au 55 could present cavities below its outer shell 33,48,51 and a competition between ordered and disordered geometries has been evidenced in the case of Au 147 33,52 . Beyond the search for the most stable isomer, studies have been conducted to identify structural excited isomers 26,45,47,51 . In small clusters, a premelting involving transition towards these secondary minima has been reported [24][25][26]35,46,49 .…”

“…The melting temperature was reported to increase with the metal nanoparticle size for large clusters 27,34,43 , confirming the pioneer observation of Buffat and Borel 2 . 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 .…”

Application of Molecular Dynamics Simulation to Small Systems

“…In our literature review, we found only one recent example of a configurational search on a cluster using MD simulations, that of Chandrachud et al (Chandrachud, 2009). These researchers employed VASP to study gold cages using Born-Oppenheimer MD to generate initial configurations.…”