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

Abstract

Long-term thermal quenching of upconversion (UC) limits the usage of UC materials in nanothermometers, lighting phosphors, etc. Although UC thermal enhancement has been achieved through thermal alleviation of surface quenching, the regulation of the balance between internal multiphonon nonradiative relaxation-induced thermal quenching and external surface quencher release-induced thermal enhancement is still a big challenge. Here, we demonstrate a new strategy to achieve huge UC thermal enhancement by constructing surface-dominant quasi-2D KLu2F7:20%Yb3+/2%Er3+@KLu2F7 core–shell nanoflakes with an ultrathin thickness of 1.5 nm. The surface-dominant design enables enhancements of ∼820-fold at the 523 nm band and ∼304-fold in total as the temperature increases from 303 to 463 K. Similar huge UC thermal enhancements are also observed in Ho3+ (∼383-fold)- and Tm3+ (∼324-fold)-doped nanoflakes. Higher relative and absolute sensitivities of the thermally coupled state pairs (Er3+:2H11/2/4S3/2) with respective plateau values of 0.940% K–1 (303 K) and 0.509% K–1 (463 K) are achieved, which obviously outperform the thermal-quenching NaYF4:20%Yb3+/2%Er3+@NaYF4 nanoparticles. Moreover, the highly thermally enhanced nanoflakes enable the development of temperature-dependent anticounterfeiting inks and encryption, favoring the high-temperature usage of luminescence materials.