Up-Converting K(2)Gd(PO(4))(WO(4)):20%Yb(3+),Ho(3+) Phosphors for Temperature Sensing

Inorganic luminescent materials that can be excited with NIR radiation and emit in the visible spectrum have recently gained much scientific interest. Such materials can be utilized as anti-counterfeiting pigments, luminescent thermometers, bio-imaging agents, etc. In this work, we report the synthesis and optical properties of K(2)Gd(PO(4))(WO(4)):Ho(3+) and K(2)Gd(PO(4))(WO(4)):20%Yb(3+),Ho(3+) powders. The single-phase samples were prepared by the solid-state reaction method, and the Ho(3+) concentration was changed from 0.5% to 10% with respect to Gd(3+). It is interesting to note that under 450 nm excitation, no concentration quenching was observed in K(2)Gd(PO(4))(WO(4)):Ho(3+) (at least up to 10% Ho(3+)) samples. However, adding 20% Yb(3+) has caused a gradual decrease in Ho(3+) emission intensity with an increase in its concentration. It turned out that this phenomenon is caused by the increasing probability of Ho(3+) → Yb(3+) energy transfer when Ho(3+) content increases. K(2)Gd(PO(4))(WO(4)):20%Yb(3+),0.5%Ho(3+) sample showed exceptionally high up-conversion (UC) emission stability in the 77–500 K range. The UC emission intensity reached a maximum at ca. 350 K, and the intensity at 500 K was around four times stronger than the intensity at 77 K. Moreover, the red/green emission ratio gradually increased with increasing temperature, which could be used for temperature sensing purposes.