Dependence of Formation Energies of Tin Nanoclusters on their Size and Shape

In a novel molecular beam experiment we have calorimetrically investigated the dependence of the formation energies of isolated ${\mathrm{Sn}}_N$ clusters on their size. The experimentally determined size dependence of the formation energy for ${\mathrm{Sn}}_N$ clusters consisting of between 95 and 975 atoms can be explained by the existence of two different types of cluster isomers: One class of isomers is characterized by formation energies proportional to $N^{-1/3\mathrm{}}$, indicating compact spherical-like shapes. The other class has constant formation energies for the investigated size range, which is consistent with quasi-one-dimensional geometries.