Synthesis and luminescence properties of rare earth doped NaGdF4 upconversion phosphors

A series of NaGdF4:Re(Re=Tm3+, Er3+, Yb3+) upconversion luminescence phosphors with various doping concentrations were prepared by a hydrothermal method. X-ray diffraction (XRD), field emission scanning electron microscope (SEM) and upconversion (UC) emission spectra were used to characterize the samples. The results of XRD indicate that all the samples are hexagonal phase NaGdF4. The average crystallite sizes are estimated to be 41~43 nm. Under 980 nm laser excitation, the Er3+ and Yb3+ codoped NaGdF4 phosphors show green emission from 2H11/ 2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+ ions, the red emissions attributed to 4F9/2→4I15/2 transitions of Er3+ ions, respectively. The Tm3+ and Yb3+ codoped NaGdF4 phosphors show blue emission from 1G4→3H6 transitions of Tm3+ ions, red emission assigned to 1G4→3F4 and 3F2, 3→3H6 transitions of Tm3+ ions, near-infrared corresponding to the 3H4→3H6 transitions of Tm3+ ions, respectively. The upconversion luminescence intensity and a relative intensity of red, green and blue emission are governed by the doping Yb3+ concentrations in Er3+, Tm3+ and Yb3+ tridoped NaGdF4 phosphors. The possible upconversion luminescence mechanisms of the samples are discussed. The calculated CIE color coordinates display that the upconversion luminescence color can be tuned by varying the doping ions concentration.