Decrease in particle size and enhancement of upconversion emission through Y3+ ions doping in hexagonal NaLuF4:Yb3+/Er3+ nanocrystals

The upconversion (UC) enhancement with size decrease has been realized through Y3+ ions doping in beta-NaLuF4:Yb3+/Er3+ nanocrystals (NCs) by a solvothermal process. X-Ray diffraction (XRD), scanning electron microscopy (SEM) and UC luminescence spectra were used to characterize the resulting samples. With an increase of the Y3+ doping concentration, the NCs size is continuously decreased with no phase transition. Meanwhile, the UC intensity is firstly increased and then it decreases when the Y3+ ions concentration is over 30 mol%. The NCs (beta-NaLu0.48Y0.3Yb0.2Er0.02F4) with the optimum Y3+ ions doping concentration (30 mol%) for UC emission have a diameter of about 80 nm. Compared with beta-NaLu0.78Yb0.2Er0.02F4 and beta-NaY0.78Yb0.2Er0.02F4 prepared under the same conditions, the green UC emission is enhanced by a factor of 1.8 and 16, respectively, for beta-NaLu0.48Y0.3Yb0.2Er0.0 F-2(4). The variation of the UC intensity with the increasing Y3+ ions concentration is attributed to the changing of the symmetry of the local crystal field induced by Y3+ ions doping, which has been proved by the structural probe Eu3+ ions. For beta-NaLu0.78Yb0.2Er0.02F4 and beta-NaLu0.48Y0.3Yb0.2Er0.02F4, a three-photon green UC process and a three-photon red UC process occur simultaneously at high pump power density, which can be described as (4)G(11)/(2) (Er3+) + I-4(15)/(2) (Er3+) -> H-2(11)/(2)/S-4(3)/(2) (Er3+) + I-4(13)/(2) (Er3+) and (4)G(11)/(2) (Er3+) + F-2(7)/(2) (Yb3+). F-4(9)/(2) (Er3+) + F-2(5)/(2) (Yb3+), respectively.