Size-dependent lattice symmetry breaking determines the exciton fine structure of perovskite nanocrystals

Kavli Affiliate: David T. Limmer

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| Summary:

The ordering of optically bright and dark excitonic states in lead-halide
perovskite nanocrystals has been a matter of some debate. It has been proposed
that the unusually short radiative lifetimes in these materials is due to an
optically bright excitonic ground state, a unique situation among all
nanomaterials. This proposal was based on the influence of the Rashba effect
driven by lattice-induced inversion symmetry breaking. Direct measurement of
the excitonic emission under magnetic fields has shown the signature of a dark
ground state, bringing the role of the Rashba effect into question. Here, we
use a fully atomistic theory to model the exciton fine structure of perovskite
nanocrystals accounting for the realistic lattice distortion at the nanoscale.
We calculate optical gaps and exciton fine structure that compare favorably
with a wide range of experimental works. We find a non-monotonic dependence of
the exciton fine structure splittings due to a size dependence structural
transition between cubic and orthorhombic phases. In addition, the excitonic
ground state is found to be dark with nearly pure spin triplet character
resulting from a small Rashba coupling. We additionally explore the intertwined
effects of lattice distortion and nanocrystal shape on the fine structure
splittings, clarifying observations on poly-disperse nanocrystals.

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