Size-dependent upconversion luminescence in Er3+/Yb3+-codoped nanocrystalline yttria:: Saturation and thermal effects

In this paper, the upconversion luminescent properties of Y2O3:Er3+(1 %)/Yb3+(4%) nanoparticles with different sizes. (13-55 nm) and its corresponding bulk material as a function of excitation power were studied under 978-nm excitation. Red (F-4(9/2) -> I-4(15/2)), green (H-2(11/2), S-4(3/2) -> I-4(15/2)), and blue (H-2(9/2) -> I-4(15/2)) transitions were observed. The results indicated that the relative intensity of the blue as well as the red to the green increased gradually with decreasing particle size. As a function of excitation power, the slope in the In-In plot for the red emission changed between 2.0 and 1.0 and gradually decreased with increasing particle size, which was attributed to competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the particle size decreased to 13 nm, a three-photon populating process occurred for the green emission. As the excitation power varied in different paths, gradually increasing or gradually decreasing, a hysteresis loop appeared in the power dependence of emission intensity, which was mainly caused by a local thermal effect induced by laser irradiation. The intensity ratio of H-2(11/2) -> I-4(15/2) to S-4(3/2) -> I-4(15/2) (RHs) varied complicatedly with excitation power, which was theoretically explained considering the thermal distribution and relaxation processes. Two novel cross-relaxation paths were proposed on the basis of the variation of R-HS under excitation at different wavelengths, 488 and 978 nm.