  The instability of metal halide perovskites upon exposure to moisture or heat
strongly hampers their applications in optoelectronic devices. Here, we report
the large-yield synthesis of highly water-resistant total-inorganic green
luminescent CsPbBr3/CsPb2Br5 core/shell heteronanocrystals (HNCs) by developing
an in situ phase transition approach. It is implemented via water-driven phase
transition of the original monoclinic CsPbBr3 nanocrystal and the resultant
tetragonal CsPb2Br5 nanoshell has small lattice mismatch with the CsPbBr3 core,
which ensures formation of an epitaxial interface for the yielded
CsPbBr3/CsPb2Br5 HNCs. These HNCs maintain nearly 100% of the original
luminescence intensity after immersion in water for eleven months and the
luminescence intensity drops only to 81.3% at 100 {\deg}C. The transient
luminescence spectroscopy and density functional theory calculation reveal that
there are double radiative recombination channels in the core CsPbBr3
nanocrystal, and the electron potential barrier provided by the CsPb2Br5
nanoshell significantly improves the exciton recombination rate. A prototype
quasi-white light-emitting device based on these robust CsPbBr3/CsPb2Br5 HNCs
is realized, showing their strong competence in solid-state lighting and wide
color-gamut displays.
