Spatial halide exchange reactions for sculpting light in hybrid perovskites

Hybrid perovskites have emerged as transformative materials in optoelectronics because their halide composition enables wide and controllable changes in bandgap and photoluminescence. Yet, translating this compositional tunability into spatially programmed optical functionality remains challenging. This review focuses specifically on spatially controlled halide exchange in hybrid and related lead-halide perovskites, emphasizing how the location, kinetics, and confinement of ion exchange determine the resulting optical landscape. We critically compare light-mediated/photocatalytic, laser-assisted, lithographic, solution-solid, solid-solid, and vapor-phase strategies in terms of localization mechanism, processing time, spatial resolution, compositional stability, scalability, and demonstrated functionality. Particular attention is given to vacancy-assisted ion migration, concentration gradients, interfacial diffusion, and optically driven precursor accumulation as the mechanistic basis of spatial control. We distinguish experimentally demonstrated outcomes-such as localized multicolor emission, compositionally graded nanowires, and single-crystal heterojunctions-from applications that remain prospective, including fully integrated micro-LED arrays and complex photonic circuits. The review identifies a central trade-off between spatial precision and throughput/stability and proposes standardized reporting of feature size, exchange time, compositional fidelity, and post-exchange stability as priorities for moving spatial halide exchange toward practical devices.