Enhancing negative thermal quenching effect via low-valence doping in two-dimensional confined core–shell upconversion nanocrystals

Luminescent materials with negative thermal quenching effects show superior potentials in temperature sensing and anti-counterfeiting fields. Normally, smaller nanocrystals (NCs) with higher surface-to-volume ratios are used to achieve larger thermal-induced intensification of upconversion (UC) emission intensity. Herein, confining sensitizers and activators to the shell layer with two-dimensional space and introducing defect energy levels via doping low-valence ions are simultaneously applied to enhance the negative thermal quenching effect. By increasing the temperature from 293 K to 413 K, the integral UC emission intensity of 20Yb/2Er:NaGdF4 increases only by ∼2.2 times, whereas that of NaGdF4@20Ca/20Yb/2Er:NaGdF4 core–shell NCs with a similar particle size increases by ∼10.9 times.