Ab Initio Molecular Dynamical Investigation of the Finite Temperature Behavior of the Tetrahedral Au₁₉ and Au₂₀ Clusters

Abstract

Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au19 and Au20 clusters. Au20 has been reported to be a unique molecule having tetrahedral geometry, a large HOMO-LUMO energy gap and an atomic packing similar to that of the bulk gold (J. Li et al., Science, 299 864, 2003). Our results show that the geometry of Au19 is exactly identical to that of Au20 with one missing corner atom (called as vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold cluster exhibit dramatic differences. Au20 undergoes a clear and distinct solid like to liquid like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au19 has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au19 between 650 K to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au20 is destroyed around 800 K, and the cluster is clearly in a liquid like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature which has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.

Behaviour of ‘free-standing’ hollow Au nanocages at finite temperatures: a BOMD study

“…One of the bottlenecks in the synthesis and application of HGCs is the lack of an in-depth understanding on the finitetemperature behaviour of 'free-standing' HGCs in spite of their wide importance and potentiality. In fact, the finitetemperature studies on gold clusters are few and scattered [52][53][54][55][56]. In one of the rare and consolidated study on a gold cage, Manninen et al [55] clearly show that the surface of the doped cage is retained only up to 366 K. In another study, we show that the 32-atom icosahedron gold cluster is stable only up to 300 K [56].…”

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

“…In the case of disordered ground states, a minor rearrangement of the atoms gives rise to many nearly degenerate isomers leading to an almost continuous spectrum [38,41,42]. Finally, we compare the specific heats of these two clusters with those of Au 19 and Au 20 [39]. It may be recalled that a highly symmetric Au 20 cluster shows a sharp melting peak at around 770 K while Au 19 having one vacancy (the missing apex atom of the pyramid) has a broad peak.…”

“…It has been observed that the addition of a few atoms can change the melting temperature as well as shape of the heat capacity curve dramatically. It has been demonstrated that 'ordered' ground state structure results into a well-defined melting peak [37][38][39][40][41][42].…”

“…Soulé et al have simulated finite temperature behaviour of gold clusters with 7, 13 and 20 atoms [45]. Krishnamurty et al carried out extensive first principles MD simulations to investigate finite temperature behaviour of Au 19 and Au 20 [39]. The ground state geometry of Au 19 is the same as that of Au 20 except for the missing corner atom of the pyramid.…”

Molecular dynamical simulations of melting behaviors of metal clusters

“…[7][8][9][10][11][12] The melting properties of clusters are different from bulk materials. [13][14][15][16][17] Usually the melting points of clusters decrease with decreasing the cluster size (number of atoms in a cluster).…”