Enhanced near-infrared upconversion luminescence of GdF₃:Yb ³⁺, Tm³⁺ by Li⁺

GdF₃:0.23Yb³⁺, 0.005Tm³⁺, x%Li⁺ (x =0-7) NIR to NIR upconversion nanocrystals (UCNPs) were synthesized by a hydrothermal method. Their XRD patterns show that they are all orthorhombic phase despite of different Li⁺ ion concentrations. The detailed analysis indicates that lithium ions substitute Gd³⁺ sites at x <3. As the Li⁺ content increases, more Li⁺ ions enter host lattice interstitially. The doped Li⁺ ions affect the crystal field symmetry around Tm³⁺ ions, which results in the change of the irradiation transition probabilities between their corresponding transition levels. Compared with GdF₃:0.23Yb³⁺, 0.005Tm³⁺, the NIR to NIR upconversion emission intensity of GdF₃:0.23Yb ³⁺, 0.005Tm³⁺, 0.03Li⁺ nanocrystals (excitation at 980 nm, emission at 808 nm) increases 2.2 times.