Adjusting upconversion properties in core-shell alloy-like nanoparticles of Er3+ via Yb3+ content for optical anti-counterfeiting.

High-concentration doping of lanthanide ions generally leads to severe cross-relaxation, which affects the optical properties of rare earth fluorides. Introducing Yb3+ ions for a rational alloy-like structure design is an effective method for regulating the cross-relaxation process of Er3+ ions. In this study, core-shell structured nanoparticles were synthesized via a hydrothermal method. The non-radiative transitions caused by surface defects are significantly suppressed with the inert shell coating, greatly enhancing the luminescence intensity. Furthermore, an alloy-like ErYb structure was constructed, effectively suppressing cross-relaxation between Er3+ ions. Additionally, the samples exhibit high power sensitivity under 980 nm excitation and demonstrate multi-color response characteristics under multi-wavelength excitation. This study reveals a new approach to controlling upconversion luminescence in ErYb co-doped systems and provides potential materials for applications in optical anti-counterfeiting and information encryption.