Spatiotemporal control of upconversion luminescence through vacancy-induced local structure engineering

Abstract

The escalating proliferation of counterfeit commodities in global markets has emerged as a critical socioeconomic challenge, driving urgent demands for advanced anti-counterfeiting technologies with enhanced security and practicability. Lanthanide-doped upconversion nanoparticles (UCNPs) have demonstrated exceptional potential in fluorescent anti-counterfeiting applications due to their unique anti-Stokes emission characteristics and near-infrared (NIR) excitation capability. Despite significant progress, current multicolor modulation strategies face fundamental limitations including complex doping architectures, inefficient energy transfer, and reliance on multiple excitation wavelengths. This study addresses these challenges through a novel crystal field engineering approach to achieve excitation-responsive color tuning in a simplified single-activator system. We developed a cubic-phase Na_0.5YbF_3.5:Er/Y@CaF₂:Yb nanostructure through precise local crystal field modulation, where strategic adjustment of Na⁺/F⁻ molar ratios enables remarkable enhancement of red-to-green emission ratios compared to conventional counterparts. The engineered CaF₂:Yb sensitization shell effectively suppresses surface-related quenching, resulting in UC enhancement. Crucially, the optimized system demonstrates unprecedented excitation-dependent chromatic switching between green and red emissions through simple laser pulse width modulation under single 980 nm excitation. The unique luminescent properties enable the designed nanocrystals exhibit promising in advanced anti-counterfeiting application. These findings not only overcome existing limitations in multicolor UCNP engineering but also provide fundamental insights into excitation-selective energy transfer mechanisms, with potential extensions to information security and high-resolution bioimaging applications.