Thermally enhanced NIR-NIR anti-Stokes emission in rare earth doped nanocrystals.

Nanoparticles with anti-Stokes emissions have enabled many sensing applications, but their efficiencies are considerably low. The key to enable the process of anti-Stokes emissions is to create phonons that assist the excited photons to be pumped from a lower energy state onto a higher one. Increasing the temperature will generate more phonons, but it unavoidably quenches the luminescence. Here by quantifying the number of phonons being generated from the host crystal and those at the surface of Yb3+/Nd3+ co-doped nanoparticles, we systematically investigated mechanisms towards the large enhancements of the phonon-assisted anti-Stokes emissions from 980 nm to 750 nm and 803 nm. Moreover, we provided direct evidence that moisture release from the nanoparticle surface at high temperature was not the main reason. We further demonstrated that the brightness of 10 nm nanoparticles was enhanced by more than two orders of magnitude, in stark contrast to the thermal quenching effect.