Synthesis and characterization of small size, water soluble, and intense ultraviolet upconversion emission of β-NaYF ₄:Yb, Tm nanocrystals

Uniform NaYF ₄:18%Yb, 0.5%Tm nanocrystals were synthesized via thermal decomposition method following high temperature, and then a change of the nanocrystals was made from oil-soluble to water-soluble by ligand-exchanged method. X-ray diffraction (XRD) analyses showed that the samples were β-NaYF ₄ nanocrystals. The TEM images exhibited that the nanocrystals had an average size of 35 nm. Under 980 nm near-infrared laser excitation, the samples showed intense ultraviolet upconversion emission. 5-photon upconversion fluorescence of the Tm ³⁺ ions in the nanocrystals was stronger than 4-photon and 3-photon upconversion fluorescence. Due to special optical properties and small size, the water-soluble nanocrystals would show potential applications in the biomedical field.