Near-infrared to visible upconversion in Er ³⁺ and Yb ³⁺ codoped Lu ₂O ₃ 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 Lu _1.88Yb _0.1Er _0.02O ₃ 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 (Er ³⁺: ⁴F _9/2 f ⁴I _15/2) to the green one (Er ³⁺: ( ²H _11/2, ⁴S _3/2) f ⁴I _15/2) is increased, and a three-photon process involved in the green upconversion, as described by ⁴F _9/2(Er) + ²F _5/2 (Yb) f ²H _9/2 (Er) + ²F _7/2 (Yb), is synchronously enhanced. An analysis based on steady-state rate equations indicates that the results can be induced by a large ⁴I _11/2 f ⁴I _13/2 nonradiative relaxation rate with a small ⁴F _9/2 f ⁴I _9/2 nonradiative relaxation rate. The large ⁴I _11/2 f ⁴I _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 ⁴I _11/2 f ⁴I _13/2 nonradiative relaxation rate through cross energy transfer to OH ^- surface group.