A time-dependent luminescent phosphor of Na <sub>2</sub>Ba <sub>2</sub>Si <sub>2</sub>O <sub>7</sub>:Eu for multi-level encryption and dynamic information display
Jie Shao, Xiyang Li, Meng Liu, Haiqin Sun, Qiwei Zhang, Dengfeng Peng, Fuchi Liu
Abstract
Photo-stimuli responsive materials show great potential in the fields of information encryption and storage due to their distinctive spatial or temporal color changes. However, the conventional single or multi-color static outputs by light stimulus difficulty meet practical requirements for high-security optical storage technologies. Here, a novel dynamic irradiation-responsive phosphor of Na<sub>2</sub>Ba<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:Eu is demonstrated, exhibiting high storage stability and convenient readout behaviors. The inherent Eu<sup>2+</sup> luminescence can be dynamically tuned, instantly read out, and conveniently erased by controlling irradiation duration of a portable diode laser (365 nm). The modulation mechanism is unraveled by optically induced oxidation reactions of Eu<sup>2+</sup>→Eu<sup>3+</sup> and defects as killer centers. The excellent luminescence modulation degree (Δ<i>R</i><sub>t</sub> = 89.5%) and the accompanying larger color contrast enable the creation of invisible optical codes with multi-level encryption in bright or dark field. These results indicate potential applications of Na<sub>2</sub>Ba<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>-based materials in information encryption and invisible optical storage, and are expected to expand more investigations on optically induced PL modulation behaviors based on mixed valences and defects.