Simultaneous size adjustment and upconversion luminescence enhancement of β-NaLuF4:Yb3+/Er3+,Er3+/Tm3+ microcrystals by introducing Ca2+ for temperature sensing

Abstract

β-NaLuF₄:Yb³⁺/Er³⁺ microcrystals have been obtained through a facile hydrothermal method at a relatively low temperature (180 °C) within only two hours. By introducing Ca²⁺ ions, the microcrystals can simultaneously be tunable in size and realize upconversion luminescence enhancement. It is found that when the concentration of Ca²⁺ ions increases from 0 to 20 mol%, the average diameter of disks increases while the height decreases. However, with the concentration of Ca²⁺ further increasing, the opposite is true. This result can be ascribed to the capping effect of F⁻ on the different crystal faces. More importantly, the upconversion luminescence intensities of green and red emissions of β-NaLuF₄:Yb³⁺/Er³⁺ with 40 mol% Ca²⁺ ion doping are about 7.5 and 7.9 times higher than those of the sample without Ca²⁺ doping, respectively. Besides, the Ca²⁺ doping can also be extended to β-NaLuF₄:Er³⁺/Tm³⁺ for enhancement of the upconversion luminescence. Eventually, emphasis will be on the temperature-dependent upconversion properties of β-NaLuF₄:20Yb³⁺/2Er³⁺/40Ca²⁺ (mol%) and Ca²⁺-free samples. The maximum sensitivity of β-NaLuF₄:20Yb³⁺/2Er³⁺/40Ca²⁺ (mol%) (0.00040 K⁻¹) is higher than that of β-NaLuF₄:20Yb³⁺/2Er³⁺ (mol%) (0.00021 K⁻¹) at 120 K under the same excitation pump power of 1 W, indicating that the sample with Ca²⁺ has potential for application to temperature sensing.