Visible-to-Near-Infrared Mechanoluminescence in Bi-Activated Spinel Compounds for Multiple Information Anticounterfeiting

Mechanoluminescence (ML) is the nonthermal luminescence generated in the process of force-to-light conversion, which has broad prospects in stress sensing, wearable devices, biomechanics, and multiple information anticounterfeiting. Multivalence emitter ions utilize their own self-reduction process to realize multiband ML without introducing another dopant, such as Eu³⁺/Eu²⁺, Sm³⁺/Sm²⁺, and Mn⁴⁺/Mn²⁺. However, self-reduction-induced ML in bismuth-activated materials has rarely been reported so far. In this work, a novel visible-to-near-infrared (vis-NIR) ML induced by the self-reduction of Bi³⁺ to Bi²⁺ in the spinel-type compound (MgGa₂O₄) is reported. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and PL lifetime curves demonstrate that Bi³⁺/Bi²⁺ ions are the main luminescence centers. Notably, the possible self-reduction model is proposed, where a magnesium vacancy (V_Mg^″) is considered as the driving force for the self-reduction of Bi³⁺ to Bi²⁺. Furthermore, an oxygen vacancy (V_O^••) is confirmed by electron paramagnetic resonance (EPR) spectroscopy. Combined with thermoluminescence (TL) glow curves and ML spectra, a plausible trap-controlled ML mechanism is illustrated, where electron-hole (V_O^••/V_Mg^″) pairs play a significant role in capturing electrons and holes. It is worth noting that the proof-of-concept dual-mode electronic signature application is implemented based on the flexible ML film, which improves the capabilities of signature anticounterfeiting for high-level security applications. Besides, multistimulus-responsive luminescence behaviors of the ML film are realized under the excitation of a 254 nm UV lamp, thermal disturbance, 980 nm laser, and mechanical stimuli. In general, this study provides new insights into designing vis-NIR ML materials toward wider application possibilities.