Two‐Step Sequential Process for Fabricating Graphene Nanosheets Decorated with Copper Halide Nanocrystals and Their Optoelectronic Applications

Abstract Nanocrystals of Cs₃Cu₂I₅, a metal halide with promising potential as a perovskite substitute, are successfully grown on graphene nanosheets. An intermediate stage of physically exfoliated graphene nanosheets, concurrent with the formation of copper‐based seeds on graphene surface, is developed, for the first time, allowing for the subsequent growth of Cs₃Cu₂I₅ nanocrystals (CsNCs) to obtain CsNCs‐decorated graphene nanohybrids (CsGNHs). The morphology and properties of the CsGNHs depend on the input amount of graphene nanosheets during the reaction. The photoluminescence intensity of the CsGNHs decreases with increasing graphene content, indicating that graphene facilitates nonradiative energy transfer from the CsNCs in the CsGNHs. The inherent self‐trapping exciton emission of the CsNCs is also affected by the graphene substrate. As the graphene content increases, the emission peaks of the CsGNHs broaden and diminish. The CsGNHs emit a stable solid‐state luminescence with a large Stokes shift (zero self‐absorption) under a high‐energy ultraviolet (UV) light. Consequently, the CsGNHs can act as optoelectronic transducers and selectively generate a turn‐on photocurrent response under specific‐wavelength UV light. Additionally, a new anticounterfeiting strategy is developed using the CsGNHs as a luminescent black ink to fabricate luminescent quick‐response codes with fake pixels.