Na3Gd(VO4)2: Yb3+/(Er3+, Tm3+, Ho3+) and Na3Gd(VO4)2: Yb3+/Er3+/(La3+, Lu3+, Y3+): upconversion luminescence and temperature sensitivity under single- and multi-wavelength excitation

It is a crucial factor for enhancing upconversion luminescence (UCL) by doping different metals under irradiation. In this article, a series of Na3Gd(VO4)2:Ln3+ (NGVO:Ln3+) were prepared by sol–gel means with citrate and characterized using X-ray diffraction. Their UCL properties were systematically investigated. A wide multi-colour emission corresponding to different Ln3+ (Yb3+/Er3+/Tm3+/Ho3+) doping was obtained under single-wavelength excitation. It is worth pointing out that among all the studied samples, only the UCL intensity of NGVO:15%Yb3+/2%Er3+ was 1.74 times more than that of the sum under the excitation at 980 and 1550 nm due to the synergistic effect under simultaneous 980 and 1550 nm excitation. Interestingly, a higher Sa (5.54 × 10−3 K−1) for NGVO:15%Yb3+/2%Er3+ excited at 1550 nm could be obtained, which had an advantage over that at 980 nm excitation. The possible energy transfer mechanism of NGVO:15%Yb3+/2%Er3+ under single- and multi-wavelength excitation was also discussed in detail using an energy level diagram. To further improve the UCL intensity, NGVO:15%Yb3+/2%Er3+ doped with other rare-earth ions (La3+/Lu3+/Y3+) were synthesized, and their UCL intensity and Sa values were also improved to a certain extent.