Metastability in Pressure-Induced
Structural Transformations
of CdSe/ZnS Core/Shell Nanocrystals
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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 heterogeneous nucleation
at the surface to homogeneous nucleation in the crystal core