Heterogeneity of properties in Ar nanoparticles

Molecular dynamic simulations have been used to investigate the structure and the physics of unsupported spherical Ar particles in the size range between 2 and 12 nm. It is shown that the potential energy of atoms increases along the radius from the center of the particle to the free surface, allowing to identify a core region and a surface layer. The atoms included in the core region exhibit bulk-like properties, whereas the properties of the atoms located in the surface layer significantly deviate from bulk ones. Vibrational and elastic properties also suggest for the nanometer-sized particles a heterogeneous structure. However, in this case it seems that the surface layer can be further divided into two different regions. All the observed features can be related to the unsaturation of the coordination of surface atoms, which accounts for the observed values of the free energy per surface unit.