Observation of Negative Surface and Interface Energies of Quantum Dots

TL;DR

This study reveals that surface and interface energies in certain quantum dots can be negative, indicating greater stability at the surface and interface than in the interior, challenging traditional thermodynamic assumptions.

Contribution

The paper demonstrates negative surface and interfacial energies in colloidal nanocrystals, providing new insights into nanomaterial thermodynamics and stability.

Findings

Surface energy is negative in some quantum dots.

Interfacial energy between dissimilar nanocrystals is highly favorable.

Challenges traditional views on nanomaterial stability.

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.