image: (a) Normalized excitation-wavelength-dependent PersL spectra of CaGa1.97O4:0.5%Bi (irradiation time, 5 min; interval, 1 min). (b) Persistent decay monitored at the corresponding wavelength after 5 min of xenon lamp excitation from 240 to 440 nm. (c) The trajectory of tunable PersL colors recorded when changing the excitation wavelength from 240 to 440 nm. (d) ultraviolet color chart showing the ability to visually detect specific wavelengths in the ultraviolet range using CaGa1.97O4:0.5%Bi.
Credit: by Bo-Mei Liu, Yue Lin, Yingchun Liu, Bibo Lou, Chong-Geng Ma, Hui Zhang, and Jing Wang
Optical anti-counterfeiting technology, as a preventive measure, has deeply permeated our daily lives. Visually readable codes designed based on optical materials are widely used due to their ease of verification, reasonable cost, and difficulty in replication. The rapid development of modern technology and the increasingly rampant activities of counterfeiting pose greater challenges to optical anti-counterfeiting technology. Consequently, optical anti-counterfeiting material systems based on multimodal integrated applications have garnered widespread attention.
In a new paper published in Light: Science & Application, a team of scientists, led by Professor Jing Wang from Ministry of Education Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, China, and co-workers have developed a non-stoichiometric persistent luminescence (PersL) inorganic material, CaGaxO4:Bi (x < 2), which is capable of responding to various ultraviolet light stimuli. This material exhibits a color change in its PersL that is dependent on the excitation wavelength, thereby demonstrating excitation-wavelength-dependent emission properties. Under the stimulation of different ultraviolet wavelengths, it can achieve steady-state luminescence and persistent luminescence in three distinct colors: green, yellow, and orange. As the excitation wavelength varies continuously from about 240 nm to 400 nm, the corresponding emission wavelength shifts from 605 nm to 540 nm, indicating the material's ability to visualize in response to certain ultraviolet wavelengths (Figure 1).
In contrast to conventional inorganic PersL materials, which can only display single-color afterglow images, this study presents a composite film of polydimethylsiloxane (PDMS) prepared from a single-component material system that can display multicolor afterglow patterns. As shown in Figure 2, the response of the material to afterglow at specific excitation wavelengths is exploited by using different optical masks to display different colors and patterns at different UV excitations.
Moreover, by controlling the excitation wavelength and irradiation time, a novel information storage mode encoding afterglow color and duration is realized. By setting the irradiation time and patterns of 254 nm and 365 nm light sources, the display duration of yellow and green digital afterglow patterns in the composite film can be controlled. The afterglow digital patterns of different colors have their observation windows, leading to the development of a new information read-write mode. The researchers have succinctly outlined the design principles and prospective application scenarios:
“We propose the application of color- and time-resolved PersL properties for information writing, reading, and display. The main breakthroughs are as follows: (1) A multicolor PersL material with excitation-wavelength-dependent properties was prepared by a non-stoichiometric design; (2) Experimental and theoretical studies have substantiated the mechanism underlying the distinct luminescence effect and demonstrated the feasibility of the defect engineering strategy in fabricating materials with PersL; and (3) Taking advantage of the distinct multicolor PersL properties, the applications of this material in UV detection, anti-counterfeiting, and information storage and encryption have been explored.”
Article Title
Excitation-wavelength-dependent persistent luminescence from single-component nonstoichiometric CaGaxO4:Bi for dynamic anti-counterfeiting