Controllable energy transfer in fluorescence upconversion of NdF_3 and NaNdF_4 nanocrystals

The synthesized Nd fluoride nanocrystals exhibited different upconversion behaviors as dispersed and aggregated samples due to the different energy transfer mechanisms. When they were dispersed in water, the NaNdF(4) nanocrystals exhibited approximately 400 times stronger upconversion fluorescence than the NdF(3) nanocrystals. Remarkable upconversion behaviors were found when the nanocrystals were aggregated in the films. For the NdF(3) nanocrystals, the energy transfer processes (4)I(13/2)<--(4)F(3/2)-->(4)G(7/2) in the films generated avalanche upconversion emissions with a high slope of approximately 12.0, which could be due to the large avalanche cross relaxation rates and spectral broadening effect. In contrast, the spectral broadening effect in the NaNdF(4) NCs films increased the energy transfer (4)I(15/2)<--(4)F(3/2)-->(4)G(5/2) of the Nd(3+) ions, and induced a new upconversion emission at approximately 680 nm with the slope increased from 1.0 to 3.2.