Melting of size-selected aluminum nanoclusters with 84-128 atoms

Heat capacities have been measured as a function of temperature for isolated aluminum nanoclusters with 84-128 atoms. Most clusters show a single sharp peak in the heat capacity which is attributed to a melting transition. However, there are several size regimes where additional features are observed; for clusters with 84-89 atoms the peak in the heat capacity is either broad or bimodal. For Al(115) (+), Al(116) (+), and Al(117) (+) there are two well-defined peaks, and for Al(126) (+), Al(127) (+), and Al(128) (+) there is a dip in the heat capacity at lower temperature than the peak. The broad or bimodal peaks for clusters with 84-89 atoms are not significantly changed by annealing to 823 K (above the melting temperature), but the dips for Al(126) (+), Al(127) (+), and Al(128) (+) disappear when these clusters are annealed to 523 K (above the temperature of the dip but below the melting temperature). Both the melting temperatures and the latent heats change fairly smoothly with the cluster size in the size regime examined here. There are steps in the melting temperatures for clusters with around 100 and 117 atoms. The step at Al(100) (+) is correlated with a substantial peak in the latent heats but the step at Al(117) (+) correlates with a minimum. Since the latent heats are correlated with the cluster cohesive energies, the substantial peak in the latent heats at Al(100) (+) indicates this cluster is particularly strongly bound.