Enhancing upconversion emissions of NaTm0.02YbxY0.98−xF4 nanocrystals through increasing Yb3+ doping

By increasing the content of Yb3+ ions from 20% to 98% in NaTm0.02YbxY0.98−xF4 (x=0.2−0.98) nanocrystals with size about 10 nm, the intensities of near infrared (800 nm) and blue (470 nm) upconversion (UC) luminescence can be enhanced by orders of 45 and 49 times, respectively, under 970 nm diode laser excitation. Pump power dependence illustrated that the 800 and 470 nm radiations are still two- and three-photon processes, respectively. TEM imaging showed that the enhancement is not from the change of the crystal size. Steady-state equation and the measured lifetimes indicated that the enhanced 800 nm radiations can induce the enhancement of the 470 nm emissions, which is in good agreement with the experimental data.