Near-Infrared to Visible Upconversion in Er3+ and Yb3+ Codoped Lu2O3 Nanocrystals: Enhanced Red Color Upconversion and Three-Photon Process in Green Color Upconversion

The synthesis and upconversion luminescence properties upon a 980 nm pump of cubic Lu1.88Yb0.1Er0.02O3 nanocrystals with various shapes, e.g., nanorods, nanosheets, and nanoparticles, are studied. It is observed that with decreasing size of the nanocrystals, the relative intensity of the upconverted red emission (Er3+ : F-4(9/2) -> I-4(15/2)) to the green one (Er3+: (H-2(11/2), S-4(3/2)) -> I-4(15/2)) is increased, and a three-photon process involved in the green upconversion, as described by F-4(9/2) (Er) + F-2(5/2) (Yb) -> H-2(9/2) (Er) + F-2(7/2) (Yb), is synchronously enhanced. An analysis based on steady-state rate equations indicates that the results can be induced by a large I-4(11/2) -> I-4(13/2) nonradiative relaxation rate with a small F-4(9/2) -> I-4(9/2) nonradiative relaxation rate. The large I-4(11/2) -> I-4(13/2) nonradiative relaxation rate is attributed to the occurrence of efficient cross energy transfer to OH- surface group due to the good energy match. As the size of the nanocrystals decreases, the relative surface area is increased, increasing the number of OH- group that can attach to the surface, therefore, enhancing the I-4(11/2) -> I-4(13/2) nonradiative relaxation rate through cross energy transfer to OH- surface group.