fig5

Figure 5. (A) Flexible ML paper sheet made of SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ under ambient light; (B) Flexible ML paper sheet made of SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ under UV light; (C) ML spectrum of SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ ML paper sheet; (D) SEM of SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ ML paper sheet (The area outlined in blue is paper fiber.); (E) Under night vision conditions, letters were written on SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ ML paper and recorded via time-lapse photography; (F) NIR ML comparison of black boxes in air atmosphere; (G) NIR ML comparison of black boxes under water condition; (H) NIR ML comparison of banknotes; (I) Printed letters with SrAl₁₂O₁₉:2%Cr³⁺, 1%Nd³⁺ sprayer under the condition of ambient light, UV light and night vision. ML: mechanoluminescence; SEM: scanning electron microscope; NIR: near-infrared; UV: ultraviolet.