  Zero dimensional perovskite Cs4PbBr6 has attracted considerable attention
recently not only because of its highly efficient green photoluminescence (PL),
but also its two highly debated opposing mechanisms of the luminescence:
embedded CsPbBr3 nanocrystals versus intrinsic Br vacancy states. After a brief
discussion on the root cause of the controversy, we provide sensitive but
non-invasive methods that can not only directly correlate luminescence with the
underlying structure, but also distinguish point defects from embedded
nanostructures. We first synthesized both emissive and non-emissive Cs4PbBr6
crystals, obtained the complete Raman spectrum of Cs4PbBr6 and assigned all
Raman bands based on density functional theory simulations. We then used
correlated Raman-PL as a passive structure-property method to identify the
difference between emissive and non-emissive Cs4PbBr6 crystals and revealed the
existence of CsPbBr3 nanocrystals in emissive Cs4PbBr6. We finally employed a
diamond anvil cell to probe the response of luminescence centers to hydrostatic
pressure. The observations of fast red-shifting, diminishing and eventual
disappearance of both green emission and Raman below Cs4PbBr6 phase transition
pressure of ~3 GPa is compatible with CsPbBr3 nanocrystal inclusions as green
PL emitters and cannot be explained by Br vacancies. The resolution of this
long-lasting controversy paves the way for further device applications of low
dimensional perovskites, and our comprehensive optical technique integrating
structure-property with dynamic pressure response is generic and can be applied
to other emerging optical materials to understand the nature of their
luminescent centers.
