  Dark ground state exciton in semiconductor nanocrystals has been a subject of
much interest due to its long lifetime attractive for applications requiring
long-lived electronic or spin states. Significant effort has been made recently
to explore and access the dark exciton level in metal halide perovskite
nanocrystals, which are emerging as a superior source of photons and charges
compared to other existing semiconductor nanocrystals. However, the direct
observation of long-lived photoluminescence from dark exciton has remained
elusive in metal halide perovskite nanocrystals. Here, we report the
observation of the intense emission from dark ground state exciton with 1-10 us
lifetime in strongly quantum confined CsPbBr3 nanocrystals, which contrasts the
behavior of weakly confined system explored so far. The study in CsPbBr3
nanocrystals with varying degree of confinement has revealed the crucial role
of quantum confinement in enhancing the bright-dark exciton level splitting
which is important for accessing the dark exciton. Our work demonstrates the
future potential of strongly quantum-confined perovskite nanocrystals as a new
platform to utilize dark excitons.
