Upconversion Emission Color Strongly Dependent on Excitation Power Density

ABSTRACT

Power‐tunable luminescent materials hold significant potential for applications in anti‐counterfeiting, information encryption, and displays. However, existing systems often suffer from limited color gamut, slow response, reliance on multi‐material composites, and poor photostability. To overcome these limitations, we present a novel class of multilayer upconversion nanoparticles featuring spatially separated emitter and sensitizer dopants combined with precise concentration control. In this architecture, high concentrations of Er ³⁺ and Tm ³⁺ are confined within distinct shell layers, effectively suppressing non‐radiative energy loss caused by inter‐ion cross‐relaxation. By systematically tuning the Er ³⁺ ‐doping concentration (10%–100%), we further regulate the cross‐relaxation dynamics and energy transfer from Er ³⁺ to Tm ³⁺ . Under single‐wavelength 980 nm excitation, the 20% Er ³⁺ ‐doped C‐SSS sample exhibits a continuous color transition from red to white to blue over a power density range of 4–277.5 mW/mm ² , which is ascribed to a reversal of interfacial energy transfer, while the 40% Er ³⁺ ‐doped sample shows a shift from red to yellow to green, enabling full‐spectrum color tuning. This structure simultaneously achieves high upconversion quantum efficiency and excitation power sensitivity, overcoming the challenge of efficient broadband color modulation in upconversion systems. We demonstrate applications of the nanoparticles in power‐controlled anti‐counterfeiting tags and dual‐mode fluorescence encryption, which promise advanced information security.