Metastability in pressure-induced structural transformations of CdSe/ZnS core/shell nanocrystals

TL;DR

This study uses molecular dynamics simulations to demonstrate how shell thickness in CdSe/ZnS core/shell nanocrystals influences metastability and phase transformations under pressure, revealing size-dependent behaviors not seen in bulk materials.

Contribution

It shows that tuning shell thickness can stabilize high-pressure phases at ambient conditions and induce unique phase transformations in nanocrystals.

Findings

Core high-pressure structure can be metastable at ambient conditions.

Thick shells induce a wurtzite to NiAs transformation.

Transformation mechanism shifts from heterogenous to homogenous nucleation.

Abstract

The kinetics and thermodynamics of structural transformations under pressure depend strongly on particle size due to the influence of surface free energy. By suitable design of surface structure, composition, and passivation it is possible, in principle, to prepare nanocrystals in structures inaccessible to bulk materials. However, few realizations of such extreme size-dependent behavior exist. Here we show with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high pressure structure can be made metastable under ambient conditions by tuning the thickness of the shell. In nanocrystals with thick shells, we furthermore observe a wurtzite to NiAs transformation, which does not occur in the pure bulk materials. These phenomena are linked to a fundamental change in the atomistic transformation mechanism from heterogenous nucleation at the surface to homogenous nucleation in the crystal core. Our results suggest a new route towards expanding the range of available nanoscale materials.