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 2H11/2 and 4S3/2. 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.