Size dependence of up-conversion luminescence of NaYF4:Yb3+/Tm3+

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 °C 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.