Ambiently Stable Two-Dimensional β-CuI Monolayers with Self-Trapping Exciton Luminescence

β-phase cuprous iodide (β-CuI)─a transparent p-type semiconductor with a wide bandgap and low thermal conductivity─holds promise for high-temperature electronics, flexible electrodes, light-emitting diodes (LEDs), and thermoelectrics; however, it is experimentally known only as a high-temperature phase within 645-675 K or in surface-engineered composites. Here, we report two-dimensional (2D) β-CuI monolayers prepared from γ-CuI via phase transformation and liquid-phase exfoliation. These monolayers stably exist under ambient conditions and have a hexagonal structure with a lateral size of ∼500 nm and thickness of 1 nm. Theoretical calculations indicate that the preparation of 2D β-CuI monolayers arises from low exfoliation energy along with dynamic and thermodynamic stability. Interestingly, the 2D β-CuI exhibits a unique self-trapped exciton luminescence phenomenon, emitting a broad white light spectrum close to full-spectrum illumination of sunlight. The findings represent a step toward the preparation of 2D β-CuI under ambient conditions and pave the way for the exploration of its unique properties and applications.