Bright White Upconversion Luminescence under Low Excitation Power Density with Sensitive Temperature Monitoring

Generating white luminescence at the nanoscale is highly desirable for applications in cell imaging and optical sensing, where nanoscale photon sources are essential. However, achieving efficient white upconversion luminescence (UCL) remains a substantial challenge. In this study, we propose a method to achieve ultrastrong white UCL by spatially separating Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ ion pairs into distinct layers within a core/multishell nanoparticle (NaYF₄@NaYbF₄:1.375%Tm@NaYF₄@NaYbF₄:20%Er@NaYF₄, denoted as C-SSSS). The introduction of an inert NaYF₄ interlayer (second shell) is critical, as it controls energy transfer between Er³⁺ and Tm³⁺ ions and suppresses nonradiative cross-relaxation. Under 980 nm excitation, the C-SSSS nanoparticles exhibit white emission intensity 37.1 times greater than that of core-only nanoparticles (NaYbF₄:0.5%Tm, 0.5%Er). Furthermore, optimizing the inert core size to 85 nm maximizes the effective excitation volume of the Yb-Tm-doped active layer, enabling precise control of luminescence intensity and strong white light emission. The C-SSSS nanoparticles also demonstrate exceptional thermal sensitivity, with a thermometry sensitivity 2.5 times higher than that of core-only nanoparticles, attributed to lattice distortion at the NaYF₄@NaYbF₄ interface. This work highlights the dual functionality of ultrastrong white UCL and high-performance luminescent thermometry in a single-nanomaterial system.