Study on upconversion luminescence and luminescent dynamics of 20%Yb3+, 0.5%Tm3+ co-doped YF3 and GdF3 nanocrystals

20%Yb3+, 0.5%Tm3+ co-doped YF3 and GdF3 were synthesized through a facile hydrothermal method. After annealing under an argon atmosphere, the sizes and morphologies of the two samples were characterized by field emission scanning electron microscopy, and the phase and crystallization were analyzed by X-ray diffraction. With a 980 nm continuous wave laser diode as the excitation source, blue and ultraviolet upconversion emissions in the wavelength range of 260-510 nm of Tm3+ and Gd3+/Tm3+ ions were recorded. Under the same excitation conditions, the upconversion emission spectra of the two nanocrystals were compared and analyzed. Gd3+ in the ground state cannot absorb 980 nm photons directly because of the large energy gap between the ground state 8S7/2 and the first excited state 6P7/2. In the 20%Yb3+, 0.5%Tm3+ co-doped GdF3 nanocrystals, the excited states 6I(J) of Gd3+ can be populated through the energy transfer 3P2 --> 3H6 (Tm3+): 8S7/2 --> 6I(J)(Gd3+), meaning that Yb3+ acted as primary sensitizers and Tm3+ acted as secondary sensitizers, transferred energies to host material Gd3+ and resulted in the ultraviolet upconversion emission of the host ions. In this article, the upconversion luminescent dynamics were studied at the onset of a 980 nm pulsed laser from an optical parametric oscillator pumped by a 10 ns pulsed Nd:YAG laser, too.