Self-recoverable mechanoluminescence in simple oxides: Al<sub>2</sub>O<sub>3</sub>:Cr.

Abstract

Materials exhibiting mechanoluminescence (ML) that directly convert mechanical stimuli into light hold significant potential for real-time stress sensing and intelligent photonic systems. However, most high-performance ML systems rely on complex multicomponent compounds that often suffer from limited intensity, stability, and scalability, largely due to poorly understood mechanisms. Herein, we report a simple Al₂O₃:Cr³⁺ oxide that exhibits unprecedented ML intensity, enabled by a well-defined mechanical-to-optical energy conversion process. The self-recoverable ML arises from stress-induced ionization of electrons from luminescence centers, followed by their recapture upon stress release. By precisely tuning the doping levels, annealing conditions, and heterojunction interfaces, Al₂O₃:Cr³⁺ phosphors achieved intense, reproducible, and thermally stable near-infrared emission. Notably, high-temperature annealing dramatically enhanced the ML intensity, with thermodynamic and kinetic analyses revealing increases in the carrier and defect concentrations by several orders of magnitude, accounting for the exceptional brightness. By leveraging the chemical robustness, abundance, and low cost of alumina, we demonstrated the flexible ML paper for stress visualization and multi-level anti-counterfeiting, as well as in-situ grown Al₂O₃:Cr³⁺ luminescent layers on Cr-Al alloys for passive, real-time stress monitoring. This study establishes Al₂O₃ as a durable and scalable oxide platform for next-generation self-recoverable ML materials, bridging fundamental research and practical sensing technologies.