  The luminescent mechanism of nano-sized materials with indirect bandgap and
parity-forbidden transition has always been a critical issue in breaking
through bottlenecks in traditional luminescent materials. The lack of
understanding has resulted in great disputes regarding the origin of
fluorescence in weak-transition nanocrystals (NCs). This paper proposes a new
physical luminescence model, named as localized state-induced enhanced emission
(LIEE), i.e., localized processes induced by size-limitation or partial doping,
to explain the anomalous luminescence from non-luminescent state to luminescent
state in various perovskite nanocrystals with indirect/direct bandgap or
parity-forbidden transition. These findings provide a theoretical viewpoint to
design efficient lead-free perovskite NCs and promote the development of fast
optical transitions in luminescent materials.
