SrAl₂O₄:Eu²⁺,Dy³⁺ Long Afterglow Phosphor and Its Flexible Film for Optomechanical Sensing Application

Mechanoluminescence is an unusual phenomenon in which a material emits electromagnetic radiation due to any deformation caused by mechanical force. In the present study, highly flexible mechanoluminescent films of SrAl₂O₄:Eu²⁺,Dy³⁺ have been prepared by incorporating phosphor in a poly(vinyl alcohol) hydrogel using optimized ratios of dimethyl sulfoxide (DMSO):water as solvents. Upon introducing DMSO as a solvent along with water, flexibility and mechanical properties such as tear resistance and hardness of hydrogel were enhanced to a large extent. Samples prepared with DMSO:water (80:20 wt %) exhibit a higher tensile strength of 13.4 MPa, elongation strain of about 620%, and higher transparency. This hydrogel transmits enough energy to SrAl₂O₄:Eu²⁺,Dy³⁺ upon mechanical impact. For the proof of concept, mechanoluminescence (ML) testing was done, and it was found that the emission intensity of samples is linearly dependent on the force of impact. Room temperature photoluminescence (PL) emission from SrAl₂O₄ is attributed to 5d-4f transitions of Eu²⁺ ions with an afterglow lifetime of ∼5 h. Emission intensity was found to persist at ∼90% of its actual value even at a temperature of ∼100 °C, indicating the high thermal stability of phosphor. Furthermore, thermogravimetric analysis was carried out to study the thermal stability of phosphor-incorporated films. Weight loss during TGA occurs in three steps: loss of solvent, decomposition of the cross-linking branches from the PVA backbone, and decomposition of PVA. The as-prepared film showed excellent flexibility, thermal stability, and good mechanical strength, evidencing it as a potential candidate for self-powered flexible impact sensing applications.