Exploring the intra-4f and the bright white light upconversion emissions of GdO:Yb,Er-based materials for thermometry.

Upconversion broadband white light emission driven by low-power near-infrared (NIR) lasers has been reported for many materials, but the mechanisms and effects related to this phenomenon remain unclear. Herein, we investigate the origin of laser-induced continuous white light emission in synthesized nanoparticles (GdYbEr)O and a mechanical mixture of commercial oxides with the same composition 89% GdO, 10% YbO, and 1% ErO. We report their photophysical features with respect to sample compactness, laser irradiation (wavelength, power density, excitation cycles), pressure, temperature, and temporal dynamics. Despite the sensitizer (Yb) and activator (Er) being in different particles for the mechanical mixture, efficient discrete and continuous upconversion emissions were observed. Furthermore, the synthesized nanoparticles were developed as primary luminescent thermometers (upon excitation at NIR) in the 299-363 K range, using the Er upconversion H → I/S → I intensity ratio. They were also operating as secondary ones in the 1949-3086 K, based on the blackbody distribution of the observed white light emission. Our findings provide important insights into the mechanisms and effects related to the transition from discrete to continuous upconversion emissions with potential applications in remote temperature sensing.