Luminescence manipulation of Cu-doped CaAl₂O₄ phosphors via selective Cu⁺/Cu²⁺ valence state regulation

Abstract

Copper ions (Cu⁺ and Cu²⁺) serve as important luminescent centers in various inorganic materials but often coexist within the same host lattice. Both ions exhibit broadband emissions with analogous peak shapes and closely overlapping peak positions. This spectral similarity, however, makes it extremely challenging to distinguish Cu⁺ and Cu²⁺ from each other via conventional luminescence spectroscopy, which hinders in-depth investigations into their distinct luminescent mechanisms and limits the exploration of their application potential in advanced optical systems. In this work, we address this challenge by actively manipulating the valence states of Cu ions doped in CaAl₂O₄ through selective co-doping with mono-valent Na⁺ and trivalent lanthanide ions (La³⁺/Tb³⁺/Dy³⁺). Combined experimental and first-principles studies reveal that Na⁺ doping effectively stabilizes Cu²⁺, leading to an intense blue emission centered at 471 nm. Conversely, co-doping with trivalent lanthanides promotes the formation of Cu⁺, resulting in a bright green emission peak at 501 nm. Leveraging this valence tunability, we demonstrate a prototype optical encoding device based on their cooperative luminescence, highlighting its promising utility in anti-counterfeiting technologies. This study not only provides a fundamental understanding for deciphering and controlling the luminescence of copper ions with different valencies, but also offers valuable guidance for the design of novel multifunctional Cu-activated phosphors.