  Halide perovskite nanocrystals (NCs) have shown impressive advances,
exhibiting optical properties that outpace conventional semiconductor NCs, such
as near-unity quantum yields and ultrafast radiative decay rates. Nevertheless,
the NCs suffer even more from stability problems at ambient conditions and due
to moisture than their bulk counterparts. Herein, we report a strategy of
employing polymer micelles as nanoreactors for the synthesis of methylammonium
lead trihalide perovskite NCs. Encapsulated by this polymer shell, the NCs
display strong stability against water degradation and halide ion migration.
Thin films comprising these NCs exhibit a more than 15-fold increase in
lifespan in comparison to unprotected NCs in ambient conditions and even
survive over 75 days of complete immersion in water. Furthermore, the NCs,
which exhibit quantum yields of up to 63% and tunability of the emission
wavelength throughout the visible range, show no signs of halide ion exchange.
Additionally, heterostructures of MAPI and MAPBr NC layers exhibit efficient
F\"orster resonance energy transfer (FRET), revealing a strategy for
optoelectronic integration.

Materials Science;Applied Physics