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 ²H_11/2 and ⁴S_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 °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.