Upconversion properties of Er3+–Yb3+:NaYF4 phosphors with a wide range of Yb3+ concentration

Photoluminescent Er3+–Yb3+:NaYF4 with 2 mol% Er3+ and a wide range of Yb3+ codoping concentration from 0 to 98 mol% were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single hexagonal phase of Er3+–Yb3+:NaYF4 phosphors was investigated with the increase of Yb3+ concentration. Both green and red emissions of Er3+ were observed from the Er3+–Yb3+:NaYF4 phosphors under 976 nm laser diode (LD) excitation, and gained the strongest emissions intensity at 20 mol% Yb3+ codoped. The green and red emissions of 2H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ can be tuned by varying the Yb3+ codoping concentration. The intensity ratio of green emissions for 2H11/2, 4S3/2→4I15/2 transitions of Er3+ was related to Yb3+ codoping concentration and temperature, implying a potential application of Er3+–Yb3+:NaYF4 phosphors for optical temperature sensing. The upconversion properties of Er3+–Yb3+:NaYF4 phosphors were explained and discussed by the steady-state equations.