Differences of up-conversion and temperature sensing behaviors of NaYF4: 2 mol % Er3+, 20 mol % Yb3+ Phosphors with cubic and hexagonal phases

In this work, NaYF4: 2 mol % Er3+, 20 mol % Yb3+ phosphors with cubic and hexagonal phases were prepared by co-precipitation method with different reaction temperature. The multi-photon up-conversion processes and optical temperature sensing properties are discussed. The emission spectra of hexagonal phase show higher emission intensity and lower red-to-green ratio (R/G), which are attributed to the decrease of local symmetric. And the number of photons (n), participated in the up-conversion processes, declines as the phase changes from cubic to hexagonal. This can be originated to the exhaustion of electrons on ground state level: 4I15/2 in hexagonal phase. And the fitting n of green emissions decline more obviously than red emission with temperature increasing. The temperature dependent FIR is fitted and the fitted function implies that the energy gap ∆E increases as the phase changes to hexagonal. And the max relative sensitivity reaches 0.012 K−1 at 300 K. Thus, NaYF4: 2 mol % Er3+, 20 mol % Yb3+ phosphors with hexagonal phase have higher SR and can be promising in FIR-based temperature sensing.