Self-Assembled Lead Halide Perovskite Nanocrystals in a Perovskite Matrix

Hybrid metal halide perovskite materials are produced with facile routes, but their morphology is sensitive to water, oxygen, temperature, and exposure to light. While phase separation and self-assembly of perovskite nanostructures have been demonstrated, the realization of controlled perovskite–perovskite heterostructures has been limited up to now. We demonstrate here the growth of stable CH3NH3PbI3–x Br x nanocrystals in a CH3NH3PbBr3 matrix. Optical emission from the nanocrystals can be reversibly activated upon illumination through a photobrightening process. Optical microscopy images show that nanocrystals are stable in time, through several illumination cycles. Ultrafast photoluminescence measurements imply that optical excitations are funneled from the matrix into the lower bandgap nanocrystals. Because the nanocrystals represent