Colloidal β-KYF₄:Yb³⁺,Er³⁺/Tm³⁺ nanocrystals: tunable multicolor up-conversion luminescence from UV to NIR regions

Pure β-KYF₄ nanocrystals codoped with Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ were successfully synthesized via the thermal decomposition of trifluoroacetate precursors using oleic acid and octadecylene as coordinating solvents. The up-conversion optical properties of the β-KYF₄ nanocrystals with different lanthanide (Ln)-doped ions (Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺) were investigated. It is found that the colloidal dispersion of β-KYF₄:Yb,Er/Tm nanocrystals display strong multiple up-conversion emission spanning from the deep UV-to-NIR regions under 980 nm excitation. Pure β-KYF₄ nanocrystals codoped with Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ can emit bright blue, green, yellow, and red emissions visible to the naked eye by adjusting the concentrations of Yb³⁺ sensitizer ion and Er³⁺ or Tm³⁺ activator ions. Moreover, the UC luminescent colors can be tuned for the four colors from blue to green to yellow and finally to red emission in the β-KYF₄:X%Yb³⁺/(0.2%-2%)Er³⁺ and β-KYF₄:25%Yb³⁺/0.2%Tm³⁺ colloidal samples. This work substantially expands our understanding of this category of KYF₄ upconversion nanocrystals.