Excitation-wavelength-dependent anti-thermal quenching of upconversion luminescence in hexagonal NaGdF4:Nd3+/Yb3+/Er3+ nanocrystals

Although much effort has been focused on thermally enhanced upconversion (UC) luminescence, it remains a challenge to maintain or even enhance the UC luminescence of NaLnF4-based phosphors (Ln = Y, Gd, Lu) using a surface-independent strategy. In this work, we demonstrate that the effect of temperature on UC luminescence in NaGdF4:Nd3+/Yb3+/Er3+ nanocrystals is strongly dependent on the exciting wavelength. When excited using 808 nm radiation, the UC luminescence exhibits good resistance to thermal quenching. The reason is that, in this situation, the negative effect of multi-phonon decay can be fully compensated by the thermal enhancement in the transition cross-section and in energy-transfer rate. Our research reports a novel strategy for the anti-thermal quenching of upconversion luminescence, which is potentially applicable in the design of dynamic UC anti-counterfeiting as well as in NIR-I-responsive thermometers and nanoheating.