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.