Size dependence of up-conversion luminescence of NaYF 4 :Yb 3+ /Tm 3+

Tm3+/Yb3+ codoped NaYF4 microcrystals were synthesized using a hydrothermal method.The bright upconversion light was observed under 980 nm excitation.The upconversion luminescence was systematically investigated at different Yb3+ concentrations and different reaction temperatures and time.The sample with 60% Yb3+ concentration and reacting at 180 oC for 24 h possessed the highest luminescent efficiency.The higher luminescent efficiency was contributed to a large surface area.The large surface area induced the large vibration mode by absorbed H2O and CO2.The larger vibration mode could enhance the energy transfer efficiency from the excited Yb3+ to Tm3+ by the process of phonon assisted energy transfer.