Sub-10 nm and monodisperse β-NaYF 4 :Yb,Tm,Gd nanocrystals with intense ultraviolet upconversion luminescence

Lanthanide-doped upconversion nanocrystals of β-NaYF 4 :18%Yb,0.5%Tm, X %Gd and core–shell structured β-NaYF 4 :18%Yb,0.5%Tm,20%Gd@NaGdF 4 as small as 8 nm and 12 nm in diameter respectively were synthesized. They display intense and high-order ultraviolet (UV) emissions upon 980 nm excitation.