Dopant Induced Stabilization of Silicon Cluster at Finite Temperature

With the advances in miniaturization, understanding and controlling properties of significant technological systems like silicon in nano regime assumes considerable importance. It turns out that small silicon clusters in the size range of 15-20 atoms are unstable upon heating and in fact fragment in the temperature range of 1200 K to 1500 K. In the present work we demonstrate that it is possible to stabilize such clusters by introducing appropriate dopant (in this case Ti). Specifically, by using the first principle density functional simulations we show that Ti doped Si$_{16}$, having the Frank-Kasper geometry, remains stable till 2200 K and fragments only above 2600 K. The observed melting transition is a two step process. The first step is initiated by the surface melting around 600 K. The second step is the destruction of the cage which occurs around 2250 K giving rise to a peak in the heat capacity curve.Comment: 6 pages, 8 Figs. Submitted to PR

Electronic structures, equilibrium geometries, and finite-temperature properties of Na<SUB>n</SUB> (n=39-55) from first principles

Density-functional theory has been applied to investigate systematics of sodium clusters Nan in the size range of n=39-55. A clear evolutionary trend in the growth of their ground-state geometries emerges. The clusters at the beginning of the series (n=39-43) are symmetric and have partial icosahedral (two-shell) structure. The growth then goes through a series of disordered clusters (n=44-52) where the icosahedral core is lost. However, for n&#8805;53, a three-shell icosahedral structure emerges. This change in the nature of the geometry is abrupt. In addition, density-functional molecular dynamics has been used to calculate the specific heat curves for the representative sizes n=43, 45, 48, and 52. These results along with already available thermodynamic calculations for n=40, 50, and 55 enable us to carry out a detailed analysis of the heat capacity curves and their relationship with respective geometries for the entire series. Our results clearly bring out strong correlation between the evolution of the geometries and the nature of the shape of the heat capacities. The results also firmly establish the size-sensitive nature of the heat capacities in sodium clusters

Density functional analysis of the structural evolution of Ga<SUB>n</SUB> (n=30-55) clusters and its influence on the melting characteristics

Recent experimental results have reported surprising variations in the shapes of the heat capacity curves and melting temperatures of gallium clusters in the size range of 30-55 atoms [ G. A. Breaux et al., J. Am. Chem. Soc. 126, 8628 (2004) ]. In the present work, we have carried out an extensive density functional investigation on ten selected clusters in the above mentioned size range. In particular, we have analyzed the ground state geometry and the nature of bonding in these clusters using electron localization function. We demonstrate that the existence or otherwise of a large island of atoms bonded with similar strength (i.e., the local order) in the ground state geometry is responsible for the variation in the shape of the heat capacity curve. We attribute the observed higher melting temperatures of some of the clusters (viz., Ga45-Ga48) to the presence of a distinct core and strong covalent bonds between the core and surface atoms. The present work clearly demonstrates that it is possible to understand the general trends observed in the heat capacity curves across the entire series on the basis of the analysis of their ground state