Observation of negative surface and interface energies of quantum dots

Abstract

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.

A General Nucleation Model for Semiconductor Nanocrystals

“…Furthermore, the CNT model neglects surface chemistry, chemical reactions and oxidation state alterations, which often exert significant control over the nucleation process . Moreover, recent experiments have suggested that the surface energy may be negative, indicating that CNT could be entirely inappropriate . Although other physical theories such as spinodal decomposition, kinetic theory and quantum-mechanical correction methods have been proposed, the field of nonclassical nucleation of NPs remains relatively unexplored.…”

“…27 Moreover, recent experiments have suggested that the surface energy may be negative, indicating that CNT could be entirely inappropriate. 28 such as spinodal decomposition, 29 kinetic theory 30 and quantum-mechanical correction methods 31 have been proposed, the field of nonclassical nucleation of NPs remains relatively unexplored.…”