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Why can small clusters of atoms have lower melting points than the same material in bulk?

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Diagram showing four physical states of matter: solid with tightly packed molecules, liquid with loosely packed molecules, gas with widely spaced molecules, and plasma with free electrons and nuclei, each labeled with descriptions and an arrow indicating increasing energy. — phase States of matter.

Small clusters of atoms often exhibit lower melting points than the same material in bulk primarily because of the high proportion of surface atoms in a cluster.12

In a body of bulk matter, the vast majority of atoms are located in the interior, where they are surrounded by and bonded to other atoms on all sides. In contrast, a significant fraction of the atoms in a small cluster reside on the surface. These surface atoms are bonded only to atoms in their own layer and to those directly beneath them, meaning they have fewer neighbors to hold them to the main body of the material.2

Because these surface atoms have fewer bonds, the average binding energy per atom in a cluster is typically much lower than that of bulk matter. Since less energy is required to overcome these weaker average bonds, the cluster transitions to a liquid phase at a lower temperature than the corresponding bulk material.21