Solvothermal synthesis and upconversion properties of about 10 nm orthorhombic LuF3: Yb3+, Er3+ rectangular nanocrystals

Abstract

The Yb3+ and Er3+ codoped orthorhombic LuF3 rectangular nanocrystals (NCs) with the size of about 10nm were synthesized by a facile and effective solvothermal process. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), upconversion (UC) luminescence spectra and decay curves were used to characterize the resulting samples. Compared with YF3 and α-NaYF4 NCs, owning the similar size and the same doping levels of Yb3+ ions and Er3+ ions as LuF3 NCs, the green UC emission of LuF3 NCs is 18.7 times and 5.1 times stronger than that of YF3 and α-NaYF4 NCs respectively; the red UC emission of LuF3 NCs is 13.2 times and 0.6 times stronger than that of YF3 and α-NaYF4 NCs respectively. Under 980nm wavelength excitation, the decay curves of both 4S3/2→4I15/2 transition and 4F9/2→4I15/2 transition exhibit a single exponential function, resulting from the fast energy migrations among Yb3+ ions caused by the high concentration of Yb3+ ions (20mol%). Meanwhile, at relatively low power density, the slopes of the linear plots between log(I) and log(P) for green UC and red UC are 1.7 and 1.9 respectively, which are less than 2 due to the quenching of the thermal effect, indicating a two-photon process for them. At high power density, the slopes are decreased caused by the saturation effect. In addition, we proved the existence of the thermal effect by the pump power dependence of the intensity ratio of 2H11/2→4I15/2 transition to 4S3/2→4I15/2 transition.