Activation of the surface dark-layer to enhance upconversion in a thermal field

Thermal quenching, in which light emission experiences a loss with increasing temperature, broadly limits luminescent efficiency at higher temperature in optical materials, such as lighting phosphors(1-3) and fluorescent probes(4-6). Thermal quenching is commonly caused by the increased activity of phonons that leverages the non-radiative relaxation pathways. Here, we report a kind of heat-favourable phonons existing at the surface of lanthanide-doped upconversion nanomaterials to combat thermal quenching. It favours energy transfer from sensitizers to activators to pump up the intermediate excitedstate upconversion process. We identify that the oxygen moiety chelating Yb3+ ions, [Yb center dot center dot center dot O], is the key underpinning this enhancement. We demonstrate an approximately 2,000-fold enhancement in blue emission for 9.7 nm Yb3+-Tm3+ co-doped nanoparticles at 453 K. This strategy not only provides a powerful solution to illuminate the dark layer of ultra-small upconversion nanoparticles, but also suggests a new pathway to build high-efficiency upconversion systems.