Origin of strong red emission in Er3+-based upconversion materials: role of intermediate states and cross relaxation

Among the various upconversion (UC) materials, sodium yttrium fluoride doped with ytterbium and erbium (NaYF4:Yb3+,Er3+) is the most widely studied owing to its high UC efficiency. Nonetheless, UC mechanisms are not yet fully understood and, in particular, near-infrared-to-red UC mechanisms are still under debate. Herein, we examine UC mechanisms in Er3+-based UC materials. Most importantly, the F-4(3/2) and F-4(5/2) states of Er3+ were found to be important intermediate states for strong red emission, for the first time. The cross relaxation between the Er3+ ions, back energy transfer from Er3+ to Yb3+, and relative doping concentrations of Er3+ and Yb3+ in NaYF4:Yb3+,Er3+ were found to play important roles in the relative intensity between red and green emissions. The proposed UC mechanism will provide design principles for various Er3+-based UC materials.