Multicolour and up-conversion fluorescence of lanthanide doped Vernier phase yttrium oxyfluoride nanocrystals

Abstract

Multicolour and up-conversion fluorescent nanocrystals of lanthanide doped orthorhombic yttrium oxyfluorides were prepared via a facile precursor route for the first time. The stepwise fabrication procedure involves room-temperature coprecipitation of Y³⁺ ions with CO₃²⁻ and F⁻ in water and subsequent thermal decomposition. Powder X-ray diffraction studies confirmed that all of the undoped and lanthanide ion-doped samples annealed at 400 °C were pure Vernier phase adopting an orthorhombic structure. The crystal structure, synthetic details, morphologies, and luminescence properties of the nanocrystals were investigated by means of TEM, thermal and spectroscopic measurements. In the structure, Y atoms occupy four crystalline sites with different coordination by O²⁻/F⁻, giving rise to complex environments for the doping of rare earth ions. Green emission bands were observed from the Tb³⁺ doped samples under UV excitation (λ_ex = 284, 377 nm), while the Eu³⁺ doped nanocrystals exhibited bright red emission upon both UV and blue irradiation (λ_ex = 381, 465 nm). Those nanocrystals codoped with Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺ and Yb³⁺/Tm³⁺ show intense red, green and blue up-conversion fluorescence respectively upon irradiation at 980 nm with a diode laser. The Vernier phase yttrium oxyfluoride phosphors have the potential for use in applications such as bioprobes, optical nanodevices, and chromatic displays.