Direct Growth of Rectangular 2D Layered Metal‐Halide Perovskite Microcrystal Photonic Cavities on Functional Substrates

Abstract 2D layered perovskites (2DLPs) are emerging as versatile semiconductors for optoelectronic and photonic applications thanks to their intrinsic electronic confinement, high excitonic binding energy, and large compositional tunability. However, most fabrication methods yield either grainy 2D perovskite films, microcrystalline powders, or microcrystals with highly irregular shapes. Here, a rapid and tunable recrystallization‐based approach is presented to fabricate single‐crystalline 2DLP microcrystals with lateral dimensions from few to hundreds of microns, rectangular shape and excellent thickness homogeneity, with which lateral size and thickness can be adjusted by the synthesis parameters. This method involves dissolving 2DLP powders in a solvent–antisolvent mixture carefully tuned close to saturation, followed by crystallization controlled via a mild thermal gradient and solvent evaporation. This protocol enables the straightforward synthesis of 2DLPs of diverse compositions and on a variety of substrates. Thanks to their regular shape, extremely flat surface and uniform thickness, these microcrystals show homogeneous emission over their entire surface, and behave as photonic cavities, sustaining confined modes and exciton‐polaritons. Such advantageous properties, which in the future could be combined with size and position control, have the potential to boost the development of optoelectronic microdevices, where the single crystals are implemented as emitting or sensing pixels.