Improved multiphoton ultraviolet upconversion photoluminescence in ultrasmall core-shell nanocrystals

Near-infrared to ultraviolet multiphoton upconversion photoluminescence in ultrasmall Tm3+/Yb3+-codoped CaF2 nanocrystals (∼6.7  nm in size) was observed and further significantly enhanced by growing an active shell of NaYF4:Yb3+. Owing to the active shell, the lanthanide emitters inside the core are effectively prevented from the surface quenchers, and the excitation energy is absorbed more efficiently via the additional luminescence sensitizer Yb³⁺ embedded in the shell. The details of underlying physics were investigated and discussed. The results present a good ultrasmall luminescent material system for achieving efficient multiphoton upconversion, which shows great potential in versatile industrial and biological applications.