Laser heating effect on the power dependence of upconversion luminescence in Er3+-doped nanopowders

Under 980-nm laser diode excitation, a phenomenon on an over-2-slope in log(I)-versus-log( P) plot for red upconversion (UC) luminescence in Er3+-doped nanopowders is usually observed. Due to the existence of many energy-trapping centers on their surfaces, the laser heating effect is shown very prominently in these nanopowders. In this submission, the influence of laser heating on the power dependence of UC luminescence in NaGdF4:Er3+/Yb3+@ Citrate nanopowders is investigated. In the experimental works, sub-20-nm beta-NaGdF4:Er3+/Yb3+@Citrate nanoparticles were synthesized at 185 degrees C through a solvothermal route. Then we found that UC luminescence at 524 and 657 nm displayed an over-2-slope in log(I)-versus-log( P) plot. The over-2-slope for 524-nm UC luminescence can be well explained from the greater proportion of population at the H-2(11/2) level, which is thermally populated from the S-4(3/2) level, with increase of the excitation power. Cross relaxation of H-2(11/2) + I-4(13/2) -> F-4(9/2) + I-4(11/2) is proposed to feed F-4(9/2) from H-2(11/2) levels, which explains the over-2-slope in log(I)-versus-log( P) plot for red two-photon UC luminescence.