An investigation of the thermal sensitivity and stability of the β-NaYF₄:Yb,Er upconversion nanophosphors

The upconversion luminescence (UCL) of the hexagonal (β)-phase NaYF₄:Yb,Er nanophosphors as a function of temperature (300-450 K) was examined under 978 nm light excitation. The thermal sensitivity was evaluated based on the UCL intensity ratio between ²H _11/2-⁴I_15/2 and ⁴S _3/2-⁴I_15/2 transitions (R_HS). Excitation power, particle sizes (37, 65, and 140 nm), ion doping concentrations, and with and without silica coating were studied. It was found that R_HS is only dependent on temperature at the low excitation power, and has no direct relation with the particle size and surface effects. Silica coating was found to enhance the thermal stability significantly without altering the thermal sensitivity.