Huge Thermal Enhancement of Surface-Dominant Quasi-2D Upconversion Nanoflakes

Quasi-2Dcore-shell nanoflakes enable similar to 820-foldUC thermal enhancement, favoring the sustainable high-temperatureusage of UC nanoprobes. Long-term thermal quenching of upconversion (UC) limitsthe usageof UC materials in nanothermometers, lighting phosphors, etc. Although UC thermal enhancement has been achieved through thermalalleviation of surface quenching, the regulation of the balance betweeninternal multiphonon nonradiative relaxation-induced thermal quenchingand external surface quencher release-induced thermal enhancementis still a big challenge. Here, we demonstrate a new strategy to achievehuge UC thermal enhancement by constructing surface-dominant quasi-2DKLu(2)F(7):20%Yb3+/2%Er3+@KLu2F7 core-shell nanoflakes with an ultrathinthickness of 1.5 nm. The surface-dominant design enables enhancementsof similar to 820-fold at the 523 nm band and similar to 304-fold in totalas the temperature increases from 303 to 463 K. Similar huge UC thermalenhancements are also observed in Ho3+ (similar to 383-fold)-and Tm3+ (similar to 324-fold)-doped nanoflakes. Higher relativeand absolute sensitivities of the thermally coupled state pairs (Er3+:H-2(11/2)/S-4(3/2)) with respective plateau values of 0.940% K-1 (303K) and 0.509% K-1 (463 K) are achieved, which obviouslyoutperform the thermal-quenching NaYF4:20%Yb3+/2%Er3+@NaYF4 nanoparticles. Moreover, thehighly thermally enhanced nanoflakes enable the development of temperature-dependentanticounterfeiting inks and encryption, favoring the high-temperatureusage of luminescence materials.