Temperature modulation of concentration quenching in lanthanide-doped nanoparticles for enhanced upconversion luminescence

The doping concentration of lanthanide ions is important for manipulating the luminescence properties of upconversion nanoparticles (UCNPs). However, the serious concentration quenching in highly doped UCNPs remains a vital restriction for further enhanced upconversion luminescence (UCL). Herein, we examined the effect of temperature on the concentration quenching of rare-earth UCNPs, an issue that has been overlooked, and we show that it is significant for biomedical or optical applications of UCNPs. In this work, we prepared a series of UCNPs by doping Er³⁺ luminescent centers at different concentrations in a NaLuF₄: Yb³⁺ matrix. At room temperature (298 K), steady-state photoluminescence (PL) spectroscopy showed substantial concentration quenching of the Er³⁺ emission with increasing doping concentrations. However, the concentration quenching effect was no longer effective at lower temperatures. Kinetic curves obtained from time-resolved PL spectroscopy further showed that the concentration quenching dynamics were vitally altered in the cryogenic temperature region, i.e., below 160 K. Our work on the temperature-switchable concentration quenching mechanism may shed light on improving UCL properties, promoting their practical applications.