Sacrificing trap density to achieve short-delay and high-contrast mechanoluminescence for stress imaging

Trap-controlled mechanoluminescence (ML) enables the direct observation of stress concentration of load-bearing objects through imaging the ML distribution, showing numerous prospects in stress detection, bio-imaging and optical displays. However, the applications of trap-controlled ML materials universally require long-time delay to fade the noise of symbiotic persistent luminescence (PersL) in order to achieve high-contrast ML images. In view of the difficulty to solve the PersL problem through individually eliminating the PersL traps, herein we propose a novel strategy of sacrificing trap density which decreases PersL and ML traps as a whole. By employing Sr 2+ substitution to decrease the trap density of Ca 2 Nb 2 O 7 :Pr 3+ , we identify a novel composition of (Ca 0.5 Sr 0.5 ) 2 Nb 2 O 7 :Pr 3+ displaying short-delay and high-contrast ML images, and evaluate its practicability through a 2-dimensional in-situ imaging experiment of dynamic stress distribution. The underlying mechanism is ascribed to the greater decrease ratio of PersL intensity than ML intensity as a result of the larger detrapping rate of traps due to stress (leading to ML) than that due to thermal energy (PersL). Furthermore, multi-spectral investigations of (Ca,Sr) 2 Nb 2 O 7 :Pr 3+ system reveal a distinctive electron transition process co-regulated by trap levels, charge transfer state and crystal field . The proposed strategy and the associated phosphors are expected to initiate the reconstruction of PersL-type ML materials and bring important implications for real-world stress imaging.