Vacuum-Assisted Strong Luminescence Thermal Enhancement in NaYF4:Ho3+/Yb3+ Upconverting Nanocrystals: A Conclusive Evidence for the Effect of Water Desorption

The luminescence thermal enhancement (LTE), in lanthanide-doped upconverting nanocrystals (UCNCs), has attracted extensive attention for its applications in anticounterfeiting, temperature sensing, and so on. Some mechanisms for LTE were proposed, including thermally induced water desorption from the surface of UCNCs, but conclusive evidence is absent for any of the mechanisms. Here, vacuumizing is employed in the experiment for the LTE in NaYF4:Ho3+/Yb3+ UCNCs, and significantly promoted LTE is observed. The 300–450–300 K temperature cycling experiment in vacuum shows continuous luminescence enhancement without luminescence intensity recovery as in the cooling process in moisture air, indicating the effect of water desorption because water readsorption is blocked in vacuum. It is found that the Ho3+ 5I6 level can be deexcited by direct long-range energy transfer to water adsorbed at the surface of nanocrystals due to resonant coupling between the Ho3+ 5I6 → 5I7 transition and the −OH vibration of water. As a result, water desorption suppresses the 5I6 depopulation and thus enhances the luminescence of Ho3+. Finally, the regulation of upconversion LTE is realized by adjusting the size of nanocrystals and Ho3+ or Yb3+ concentrations. Our findings indicate that the vacuumizing technique is an effective method to distinguish water-desorption-induced LTE. This work deepens the understanding of LTE and offers insights into the regulation of LTE.