  Surface energy is a fundamental property of materials and is particularly
important in describing nanomaterials where atoms or molecules at the surface
constitute a large fraction of the material. Traditionally, surface energy is
considered to be a positive quantity, where atoms or molecules at the surface
are less thermodynamically stable than their counterparts in the interior of
the material because they have fewer bonds or interactions at the surface.
Using calorimetric methods, we show that the surface energy is negative in some
prototypical colloidal semiconductor nanocrystals, or quantum dots with organic
ligand coatings. This implies that the surface atoms are more thermodynamically
stable than those on the interior due to the strong bonds between these atoms
and surfactant molecules, or ligands, that coat their surface. In addition, we
extend this work to core/shell indium phosphide/zinc sulfide nanocrystals and
show that the interfacial energy between these materials is highly
thermodynamically favorable in spite of their large lattice mismatch. This work
challenges many of the assumptions that have guided thinking about colloidal
nanomaterial thermodynamics, investigates the fundamental stability of many
technologically relevant colloidal nanomaterials, and paves the way for future
experimental and theoretical work on nanocrystal thermodynamics.
