Enhanced two-step two-frequency upconversion luminescence in a core/shell/shell nanostructure

Hexagonal-phase NaYF4:Er@NaGdF4:Yb@NaYF4:Er core/shell/shell upconversion nanoparticles (UCNPs) were synthetized using the solvothermal method and first applied in enhancing two-step two-frequency upconversion luminescence (TSTF UCL). In comparison to NaYF4:0.5%Er UCNPs, the UCL intensities and contrast of NaYF4:0.5%Er@NaGdF4:2%Yb@NaYF4:1%Er UCNPs under 850 & 1550 nm excitation increase by about 3.4 and 2.8 times, respectively. By analyzing the TSTF UCL processes, the amplified UCL was mainly attributed to the reduction of surface quenching and the improvement of 1550 nm absorption. The designed nanostructure provides a novel strategy for studying the mechanism of TSTF UCL and would drive the adoption of UCNPs in a volumetric three-dimensional (3-D) display field.