  Three-dimensional lead halide perovskites have surprised people for their
defect-tolerant electronic and optical properties, two-dimensional lead halide
layered structures exhibit even more puzzling phenomena: luminescent edge
states in Ruddlesden-Popper perovskites and conflicting reports of highly
luminescent versus non-emissive CsPb$_{\text{2}}$Br$_{\text{5}}$. In this work,
we report the observation of bright luminescent surface states on the edges of
CsPb$_{\text{2}}$Br$_{\text{5}}$ microplatelets. We prove that green surface
emission makes wide-bandgap single crystal CsPb$_{\text{2}}$Br$_{\text{5}}$
highly luminescent. Using polarized Raman spectroscopy and atomic-resolution
transmission electron microscopy, we further prove that polycrystalline
CsPb$_{\text{2}}$Br$_{\text{5}}$ is responsible for the bright luminescence. We
propose that these bright edge states originate from corner-sharing clusters of
PbBr$_{\text{6}}$ in the distorted regions between
CsPb$_{\text{2}}$Br$_{\text{5}}$ nanocrystals. Because metal halide octahedrons
are building blocks of perovskites, our discoveries settle a long-standing
controversy over the basic property of CsPb$_{\text{2}}$Br$_{\text{5}}$ and
open new opportunities to understand, design and engineer perovskite solar
cells and other optoelectronic devices.
