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The synthesis and upconversion luminescence properties upon a 980 nm pump of cubic Lu 1.88 Yb 0.1 Er 0.02 O 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+: 4 F 9/2 → 4 I 15/2 ) to the green one (Er 3+: ( 2 H 11/2, 4 S 3/2 ) → 4 I 15/2 ) is increased, and a three-photon process involved in the green upconversion, as described by 4 F 9/2 (Er) + 2 F 5/2 (Yb) → 2 H 9/2 (Er) + 2 F 7/2 (Yb), is synchronously enhanced. An analysis based on steady-state rate equations indicates that the results can be induced by a large 4 I 11/2 → 4 I 13/2 nonradiative relaxation rate with a small 4 F 9/2 → 4 I 9/2 nonradiative relaxation rate. The large 4 I 11/2 → 4 I 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 4 I 11/2 → 4 I 13/2 nonradiative relaxation rate through cross energy transfer to OH − surface group.
Li et al. (Tue,) studied this question.