Dependence of upconversion emission intensity on Yb3+ concentration in Er3+/Yb3+ co-doped flake shaped Y2(MoO4)3 phosphors

Yttrium molybdate phosphors with fixed Er3+ and various Yb3+ concentrations were synthesized via a co-precipitation method. The crystal structure and the morphology of the phosphor were characterized by means of x-ray diffraction and field-emission scanning electron microscopy. Under 980 nm excitation, red and green upconversion emissions centred at 660, 553 and 530 nm were observed. Quantitative analyses on the dependence of upconversion emission intensity on the working current of a laser diode (LD) indicated that two-photon processes are responsible for both red and green upconversion emissions in both cases of low and high Yb3+ concentrations. The relationship between the emission intensity ratio of H-2(11/2) -> I-4(15/2) to S-4(3/2) -> I-4(15/2) and the working current of the LD was studied for the samples doped with low and high Yb3+ concentrations. Finally, a set of rate equations was established based on the possible upconversion mechanism, and an empirical formula was proposed to describe the Yb3+ concentration dependence of upconversion emission intensity; the empirical formula fits well with the experimental data.