Breaking Concentration Limits: High‐Efficiency Tm 3+ ‐Doped Double Perovskite UCL Enhanced by Plasmon Semiconductor Nanoparticles

ABSTRACT Upconversion luminescence (UCL) materials exhibit significant potential for applications in biological imaging, solar cells, and optical security. However, their efficiency is constrained by the low absorption cross‐sections and nonradiative energy losses intrinsic to rare earth ions. In this study, remarkable UCL enhancement was achieved for Cs 2 NaYCl 6 : Yb 3+ /Tm 3+ core–shell nanoparticles (csUCNPs) via surface passivation and the local surface plasmon resonance (LSPR) effect. At a high Tm 3+ doping concentration (10%), the system exhibits the highest fluorescence intensity, surpassing the low‐doping limits reported in conventional systems with larger lattice parameters. This marks the first report of intense fluorescence induced by high‐concentration Tm 3+ doping in this system. Surface defects and non‐radiative transitions were further suppressed via a dual strategy of double perovskite coating and SiO 2 encapsulation, simultaneously enhancing both material stability and luminescence efficiency. Furthermore, the LSPR effect of Cs x WO 3 significantly enhanced the UCL efficiency of csUCNPs, with luminescence intensity amplified by approximately twofold through the synergistic interplay of localized electromagnetic field enhancement and energy transfer, providing experimental and theoretical insights into the role of semiconductor LSPR effects in rare earth luminescence. This work demonstrates further promising potential in multicolor anti‐counterfeiting applications for advanced encryption technologies and cell imaging for biological detection.