Effects of K+ and Gd3+ concentration on up-conversion luminescence and optical thermometric properties in NaYF4:Yb/Ho microcrystals

Abstract A series of NaYF4:0.2Yb/0.02Ho microcrystals co-doped with various K+ and Gd3+ concentrations were synthesized by hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), and upconversion (UC) spectroscopy were used to study the effects of co-doped ions on the phase structure, morphology, size and up-conversion luminescence (UCL) properties of microcrystals in detail. The results show that the K+ and Gd3+ co-doping do not evoke the change in crystal structure, but the UCL intensities of NaYF4:0.2Yb/0.02Ho are significantly enhanced. Compared to K+ and Gd3+-absent sample, UCL intensities of green emission (541 nm) and red emission (643 nm) in NaYF4:0.2Yb/0.02Ho microcrystals via 15 mol% K+ and 15 mol% Gd3+ co-doping are increased by 2.4 times and 3.3 times, respectively. The temperature sensing characteristics and thermal quenching properties of the microcrystals were also explored. Significantly, the maximum relative sensitivity of the studied sample reaches 0.0127 K-1 at 298 K, which indicates that NaYF4:0.2Yb/0.02Ho phosphors can be suitable for temperature sensing.