Optical heterostructure in a two-dimensional organic crystal.

Optical heterostructures, which feature spatially heterogeneous optical properties at the subwavelength scale, represent a key frontier for next-generation integrated photonics and optoelectronics. While typically realized by joining dissimilar materials, achieving such heterogeneity in single-component systems has remained a fundamental challenge. Here, we report an intrinsic optical heterostructure in a uniform organic nanosheet, manifesting as strongly enhanced fluorescence in the inner zone compared to the outer zone. We demonstrate that this emission heterogeneity stems from a spatially localized solid-state transition in the central top layer, which transforms the initial single crystal into an out-of-plane twin structure and significantly enhances the radiative recombination efficiency. This transition is driven by the competitive interplay between molecule-substrate and intermolecular interactions, as corroborated by multiscale structural, optical, and theoretical analyses. Our findings not only establish a platform for realizing optical heterostructures in organic materials but also open avenues for exploring structural-dynamics-governed photonic phenomena, offering broad implications for future materials design and micro-optical applications.