Luminescent monodisperse nanocrystals of lanthanide oxyfluorides synthesized from trifluoroacetate precursors in high-boiling solvents

Abstract

Monodisperse lanthanide oxyfluoride nanocrystals (LaOF:Ce and/or Tb, LaOF:Eu, LaOF:Eu/LaOF (core/ shell), GdOF:Ce,Tb, GdCF:Eu, GdOF:Yb,Er) with cubic structure were prepared by codecomposing the lanthanide trifluoroacetate precursors (Ln(CF3COO)(3)) in oleic acid/oleylamine. The nanocrystals show controlled size (2-7 nm) and shape (nanopolyhedra and elongated nanocrystals) and can form a large-area superlattice on copper grids via self-assembly. They were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photoluminescence (PL) and upconversion (UC) spectroscopies, along with PL lifetime measurement. The formation of well-doped LnOF nanocrystals has been demonstrated by the combined results of XRD, XPS, and luminescence measurements. The luminescent behaviors of differently sized and shaped nanocrystals have been revealed to be largely dependent upon the small size effect in terms of surface-to-volume ratio and the surface-structure effects in terms of the surface site of the luminescent ions and surface bonding of the capping ligands. It seems that LaOF is a better host material for the energy transfer from Ce3+ to Tb3+ ions in emitting green light than GdOF, whereas GdOF is a better host material for the red emissions, from EU3+ ions than LaOF. Under a 980 nm near-IR laser excitation, the GdOF:Yb,Er nanocrystals can emit UC green light through a two-photon process.