Comparative study on upconversion luminescence and temperature sensing of α- and β-NaYF4:Yb3+/Er3+ nano-/micro-crystals derived from a microwave-assisted hydrothermal route

Yb3+/Er3+ co-doped α- and β-phase NaYF4 nano-/micro-crystals were prepared through a microwave-assisted hydrothermal route. The crystal structure and microscopic morphology of the samples were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Frequency upconverted emissions from the two thermally coupled excited state 2H11/2 and 4S3/2 levels of Er3+ in both phases of phosphors were recorded at temperatures ranging from room temperature to 573 K under 980 nm infrared laser excitation. The time scanning upconversion spectra were investigated in detail to reveal the thermal effect induced by laser irradiation and the luminescent thermal stability of the two phases NaYF4 polycrystals. Comparison of the upconversion luminescence and the sensitivity between the two phases NaYF4 polycrystals indicated that β-phase NaYF4 won much stronger luminescent intensity, better luminescent thermal stability, and higher temperature sensitivity.