Sunlight-activated long persistent luminescence in the ultraviolet-B spectral region from Bi3+-doped garnet phosphors for covert optical tagging

Abstract

Ultraviolet (UV) luminescent materials hold great promise for applications across a wide range of fields, such as environmental, biological and medical fields. However, luminescent materials with the capability to continuously emit UV light for minutes or hours after ceasing the excitation are relatively lacking, although such phosphors are highly desired for some new important applications. Here, we report a series of Bi³⁺-doped garnet persistent phosphors that exhibit strong ultraviolet-B (UVB) persistent luminescence peaking at ∼313 nm and a long persistence time of more than 60 h. Remarkably, these persistent phosphors can be repeatedly charged by natural sunlight under any weather conditions, at any moment between sunrise and sunset during the day, and in various aqueous media. Minutes of natural sunlight irradiation can result in hours (>12 h) of persistent UVB light emission. Benefitting from the exceptional self-sustained UVB luminescence performance, and an optical signal outside of the visual spectrum, together with the avoidance of interference from ambient light, these materials show high promise for covert optical tagging applications. More importantly, the UVB persistent luminescence under the excitation of natural sunlight found in this study is expected to inspire the discovery of more excellent sunlight-activated UV persistent phosphors.