Dual-Window Broadband Near-Infrared Mechanoluminescence in MgO-Based Phosphors

Near-infrared (NIR) mechanoluminescence (ML) offers a self-powered and non-invasive route for stress visualization and biomechanical imaging, yet most NIR ML materials remain constrained by narrow emission bandwidths and reliance on dark-field conditions. Here, we report two complementary MgO-based ML systems spanning the NIR-I and NIR-II windows, establishing MgO as a robust platform for broadband mechanoresponsive emission. MgO:Cr³⁺ delivers tunable NIR-I ML (700-1000 nm; full width at half maximum, FWHM, 94-188 nm) under diverse mechanical stimuli, including friction, impact, tension, compression, bending, and twisting, while MgO:Ni²⁺ produces ultrabroadband NIR-II ML (1000-1700 nm; FWHM 225 nm) readily detectable under ambient lighting. Mechanistic studies reveal that ML arises from the synergistic interplay of local piezoelectric polarization, dislocation-mediated charge separation, and triboelectric effects at organic-inorganic interfaces, whereas the observed spectral variations stem from crystal-field modulation and lattice distortions. Co-doping of Cr³⁺ and Ni²⁺ further differentiates optical and mechanical excitation pathways-optical excitation induces Cr³⁺-sensitized Ni²⁺ emission, whereas mechanical excitation directly activates their low-lying levels. These insights unify mechano-to-photon conversion in centrosymmetric oxides and provide design principles for broadband, durable, and environmentally adaptive ML materials, advancing applications in multimodal sensing, structural health monitoring, and noninvasive biomechanical imaging.