Phase-Dependent Enhancement of the Green-Emitting Upconversion Fluorescence in LaVO4:Yb(3+), Er(3+).

The phase-dependent upconversion luminescence properties of LaVO4:Er(3+) were studied to provide new insights into the design of new upconversion materials with high efficiency. Er(3+)-, Yb(3+)/Er(3+)-doped t-LaVO4 microcrystals were successfully synthesized by the disodium ethylenediaminetetraacetic acid (Na2EDTA)-assisted hydrothermal method. X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometer (ICP-OES), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, luminescence spectroscopy, and thermogravimetric analysis (TGA) were used to characterize the samples. The results indicated that t-LaVO4 presents sheaf-like morphology, and the possible formation mechanism for these sheaves was proposed on the basis of time-dependent experiments. Furthermore, the phase-dependence (i.e., monoclinic- and tetragonal-type) upconversion luminescence properties were systematically studied, and the upconversion mechanisms were proposed according to spectral, pump power, and the concentration of Yb(3+) dependence analyses. It is worthwhile pointing out that the Er(3+)-doped t-LaVO4 exhibits a brighter green emission, which is approximately 10 times that of m-LaVO4:Er(3+) using a continuous 980 nm laser diode as the excitation source. This remarkable improvement was rationally analyzed on the basis of the composition, crystal structures, Raman spectra, morphology, and size. The comparative experiments suggest that the local structure of Er(3+) was considered as an important reason for the higher fluorescence intensity of t-LaVO4:Er(3+), which was also confirmed by the results of density functional theory (DFT) calculations.