Simultaneous phase and size manipulation in NaYF4: Er3+/Yb3+ upconverting nanoparticles for a non-invasion optical thermometer

A series of Er3+/Yb3+-codoped NaYF4 upconverting nanoparticles were synthesized via a hydrothermal method. By controlling the synthetic temperature, the phase transition and size manipulation of nanoparticles were simultaneously realized in the studied compounds. Upon the irradiation of near-infrared light, the prepared nanoparticles revealed dazzling visible emissions arising from the intra-4f transitions of Er3+ ions. Moreover, the pump power-dependent upconversion (UC) emission spectra were recorded to analyze the involved UC mechanism. Furthermore, the temperature sensing behaviors of the resultant compounds were investigated by analyzing the temperature- dependent green emission intensities from the thermally coupled levels of H-2(11/2) and S-4(3/2). It is found that the sensor sensitivity of the synthesized samples is dependent on the synthetic temperature which is further confirmed by the Judd-Ofelt theory. Additionally, the sensor sensitivity of the prepared nanoparticles is also found to be dependent on the laser pump power. The Er3+/Yb3+-codoped NaYF4 upconverting nanoparticles sintered at 160 degrees C exhibited the optimum thermometric properties with a maximum sensor sensitivity of 0044 K-1 at 637 K when the excitation pump power was 139 mW. The adjustment of the synthetic temperature and excitation pump power is a promising channel to manipulate the sensor sensitivity of the Er3+/Yb3+-codoped NaYF4 upconverting nanoparticles.