Monodisperse, size-tunable and highly efficient β-NaYF4:Yb,Er(Tm) up-conversion luminescent nanospheres: controllable synthesis and their surface modifications†
Abstract
Monodisperse, oil-dispersible β-NaYF4:Yb,Er(Tm) up-conversion (UC) luminescent nanospheres (NPs) have been successfully synthesized. The as-prepared NPs show high size-uniformity without any size-selection process, and the sizes of the NPs can be tuned in the range of 18 nm–45 nm, which are well suitable for biolabels. The NPs are thoroughly characterized and a growth mechanism is proposed. By co-doping Yb-Er or Yb-Tm, the as-prepared NPs can show bright green or blue UC luminescence under the irradiation of a 980 nm near-infrared (NIR) laser, and the UC luminescence can be obviously enhanced by constructing a NaYF4:Yb,Er@NaYF4 core/shell structure. Furthermore, the hydrophobic UC nanospheres (UCNPs) are converted into water-soluble by a robust