Luminescent Properties in Relation to Controllable Phase and Morphology of LuBO3:Eu3+ Nano/Microcrystals Synthesized by Hydrothermal Approach

The calcite and vaterite type LuBO3:Eu3+ nano/microcrystals with various morphologies were synthesized by the hydrothermal (HT) approach through controlling the pH values of the precursor solutions. The calcite type LuBO3:Eu3+ shows the dominant magnetic dipole D-5(0)-F-7(1) emission due to S-6 inversion symmetry of Lu. It also shows a symmetrical O-Eu charge transfer (CT) excitation band, which, however, is asymmetrical and broad in the material prepared by the solid-state (SS) reaction method. The effect of the residual OH- groups introduced in HT synthesis is discussed on O-Lu exciton annihilation and local environment distortion. Two different Eu3+ centers, one of which is with inversion symmetry, are observed in vaterite type LuBO3:Eu3+ The HT sample with nanostructures, such as vaterite type flowerlike LuBO3:Eu3+, demonstrates a red shift of the CT band and appearance of a long excitation tail at the long wavelength side of the CT band. XRD, selective excitation and fluorescence decay measurements indicate that the red shift is induced by an increase of crystal lattice parameters, and the excitation tail is responsible to the Eu3+ ions located at lower symmetric sites. These sites are attributed to the positions at or close to the surface of the nanostructures in the sample. As the lower symmetric sites are selectively excited, the red emission of D-5(0)-F-7(2) of Eu3+ enhanced twice relatively to the orange emission of D-5(0)-F-7(1), exhibiting superior color chromaticity in red for display application. The luminescence properties of interior Eu3+ and outside Eu3+ are studied and discussed in detail.