Fig. 1. SEM photos of fluoride microcrystals synthesized by hydrothermal method. (a) NaYF₄ microcrystals co-doped with Yb³⁺ and Tm³⁺ were synthesized by oleic acid alcohol; (b) NaYF₄ microcrystals co-doped with Yb³⁺ and Tm³⁺, (c) NaLuF₄ microtubules co-doped with Yb³⁺ and Tm³⁺, and (d) Yb³, Tm³⁺, and Ho³⁺ co-doped with NaLuF₄ microcrystals

Fig. 2. Up-conversion emission spectra of NaYF₄∶Yb³⁺, Tm³⁺ under 980 nm excitation

Fig. 3. Comparison of the fluorescence properties of NaYF₄∶Yb³⁺,Tm³⁺ microcrystals at different annealing temperatures under 980 nm excitation. (a) Up-conversion fluorescence emission spectra; (b) integral diagram of fluorescence intensity peak; (c) fluorescence photograph

Fig. 4. SEM images of NaLuF₄∶Yb³⁺,Tm³⁺(X%/0.5%) prepared by sodium citrate assisted method

Fig. 5. Spectrograms. (a) NaLuF₄∶Yb³⁺,Tm³⁺(X%/0.5%) up-conversion fluorescence emission spectra prepared by sodium citrate method; (b) color coordinate diagram and photo of samples under 980 nm light

Fig. 6. Spectrograms. Up-conversion fluorescence emission spectra, color coordinates, and photos of samples under 980 nm excitation with the change of (a) (b) Tm³⁺ doping amount and (c) Ho³⁺ doping amount

Fig. 7. Up-conversion fluorescence emission spectra, color coordinates, and photo of samples under 980 nm excitation of NaLuF₄∶Yb³⁺,Ho³⁺,Tm³⁺(40%/1%/0.5%) microrods

Fig. 8. Photo of safety ink made of NaLuF₄∶Yb³⁺, Tm³⁺ microtubes

Fig. 9. Anti-counterfeiting photo of flowers printed with NaLuF₄∶Yb³⁺, Tm³⁺ microtubes security ink