Enhanced upconversion emission of NaGdF4: Yb3+/Tm3+ crystals by introducing Li+ ions for anti-counterfeiting recognition

The upconversion efficiency could be affected by Li⁺ ions with changing the local crystal field of nanoparticles. Therefore, a series of NaGdF₄:Yb³⁺/Tm³⁺ micro-particles with different Li⁺ doping concentrations were synthesized and investigated, respectively. The structure and morphology of the synthesized materials were tested by X-ray diffraction and scanning electron microscope. The photoluminescence properties were studied with different concentrations Li⁺ ions doping under the irradiation of 980 nm laser. The emission peaks of Tm³⁺ ions are 449, 477, 645, 723, 800 nm which are attributed to high excited states to ground state ³H₆ transition through a multiple steps resonance energy transfer from excited Yb³⁺. The enormous increase in upconversion emission in upconversion micro-particles (UCMPs) by introducing Li⁺ ions was observed by the naked eye. The strongest photoluminescence intensity is the sample with 2.5% doped Li⁺ ions. Additionally, the practicality of the resultant UCMPs for photoluminescence visible imaging was systematically investigated. These results suggest that the Li⁺ doped NaGdF₄: Yb³⁺/Tm³⁺ is very promising as a screen printing material for anti-counterfeiting recognition.