Submicron CsPbBr3 crystallites prepared through in situ moisture-driven decomposition of Cs4PbBr6 microcrystals

Optical gain in submicron crystals is difficult to sustain because light confinement and radiative recombination are highly sensitive to particle size and local structure. Here we show that restricting the moisture-induced transformation of non-emissive Cs4PbBr6 microcrystals produces 300–700 nm CsPbBr3 crystallites that already support lasing at the single-particle level, before larger and more regular microcrystals develop. Temperature-dependent photoluminescence shows enhanced radiative emission and spectral narrowing at low temperature. Under strong excitation, isolated submicron crystallites display a clear excitation threshold, abrupt linewidth narrowing, quality factors of about 103, linear polarization, and shortened emission dynamics across the threshold, together supporting lasing. Measurements across multiple isolated crystallites show that this behavior is reproducible despite substantial threshold variation. These results identify restricted phase transformation as a route to generating submicron optical-gain units and reveal that regular micrometer-scale cavity structures are not required for lasing to emerge. Achieving stable and controllable optical gain at the micro- and nanoscale remains a fundamental challenge for the development of miniature lasers and integrated on-chip light sources. In this work, the authors use optically inert Cs4PbBr6 microcrystals to show that a moisture-induced confined solid-state phase transformation can generate sub-micron gain units capable of supporting stimulated emission, with individual CsPbBr3 submicron crystals exhibiting clear narrow linewidth emission, a strong linear polarization dependence and lasing.