Green upconversion luminescence of Er3+ and Yb3+ codoped Gd2Mo4O15 for optical temperature sensing

Er3+ and Yb3+ codoped Gd2Mo4O15 upconversion (UC) phosphors are synthesized by high temperature solid state reaction. The crystalline phase and morphology are studied in detail by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). Under 980 nm laser excitation, the phosphors show a significant green UC luminescence contributed from 2H11/2/4S3/2 → 4I15/2 transitions of Er3+, while the popular red 4F9/2 → 4I15/2 emission is quite weak. The temporal evolutions of Er3+ and Yb3+ emissions after 980 nm pulse excitation are studied as a function of Er3+ and Yb3+ concentrations. It is revealed that the weak red UC emission is due to its low emission efficiency in Gd2Mo4O15 that has a high cutoff phonon energy of 960 cm−1. The temperature sensing behaviors based on the thermally coupled 2H11/2 and 4S3/2 levels have been explored. A high absolute sensitivity of 0.00853 K−1 at 554 K is achieved and an excellent thermal durability is observed. The relative absorptance of 4I15/2 → 2H11/2 transition to the 4I15/2 → 4S3/2 transition is proposed as a good measure for evaluating the absolute sensitivity based on the comparative study of some familiar materials. Our results imply that Gd2Mo4O15 is a promising green UC phosphor for optical thermometry.