Upconversion luminescence enhancement and photochromic regulation of mononuclear β-NaYF4: Yb, Er

Due to its unique luminescence performance, rare earth-doped upconversion nanoparticles have been widely used in energy, environment, medicine, and other fields. This study adopts a simple and rapid solvothermal method for synthesis β-NaYF4 upconversion luminescent crystal. Firstly, the hydrothermal reaction time and temperature, PH value, PVP dosage, as well as the doping concentration of rare earth ions as sensitizers and activators were optimized through gradient experiments. The most favorable reaction parameters for upconversion luminescence of products were explored. In addition, since there are few reports on optimizing luminescence by adjusting the ratio of NaF/RE3+ as well as the ratios of different solvents during the synthesis of NaYF4 particles. By adjusting NaF/RE3+ (molar ratios) and ethylene glycol/H2O (volume ratio) reasonably, the transformation from cubic phase to hexagonal phase, morphology regulation and upconversion luminescence were achieved. The results indicate that the phase transition and morphology changes of the crystal significantly affect the upconversion luminescence strength of the sample. In addition, we innovatively substituted Y3+ in the matrix with Lu3+ according to the doping gradient, exploring the upconversion luminescence performance of the sample when the matrix contains more than one lanthanide element. The results showed that when more than one lanthanide element was added to the matrix, the upconversion luminescence intensity showed a significant weakening phenomenon. However, it is worth noting that when the substitution of Lu3+ for Y3+ reaches 70 %, the upconversion luminescence color of the sample changes from bright green luminescence to white.