Syntheses, crystal structures, electronic structures and photoluminescence properties of molybdate phosphors for white LEDs

Abstract

The molybdate based luminescent materials, a series of SrMoO4:RE and NaY(MoO4)2:RE (RE=Eu3+ and Tb3+) were fabricated both as powder and thin film phosphors through different synthesis methods: solid state reaction (SSR), sol-gel, hydrothermal and pulsed laser deposition (PLD). Morphological control, best crystallinity, optimized crystal structure and resultant optimal phosphor with efficient emission were obtained with control of experimental conditions, eg. different pH values and heat treatment temperatures. Novel red emitting Eu2O3@SrMoO4:Eu3+ hybrid phosphor was developed by simple sol-gel method without any further manipulation. The surface coated phosphor can be introduced to be greatly improved and modified luminescent properties (in chapter 4). A band model for SrMoO4:Tb3+ phosphor was theoretically analyzed and suggested. The band gap energy gradually decreased and luminescence intensity also increased with increasing heat treatment temperature. It was concluded that an oxygen vacancy positioned in a donor state, resulting in a band gap that is highly reduced at 1300 °C (in chapter 5). NaY(MoO4)2:Eu3+/Tb3+ double molybdate phosphors were synthesized by using different synthesis methods and morphologies were compared with each other. For the hydrothermal method, a double molybdate precursor was tuned by controlling pH and adding the organic additive trisodium citrate. To improve the luminescent properties of the NaY(MoO4)2:Eu3+ phosphor, phosphor samples were mixed with LiF addition by solid state reaction method using planetary ball milling. The highest photoluminescent enhancement was observed in the sample added with 3 mol% LiF (in chapter 6). The photoluminescence characteristics and optimization conditions for the thin films phosphors of NaY1-x(MoO4)2:Eux3+ were investigated. The optimized thin film exhibited excellent surface form, exceptional crystal quality and higher quantum efficiency compared to powder type phosphors. The overlapped charge transfer transitions of Mo6+ - O2- and Eu3+ - O2- were separately extracted using Gaussian fitting (in chapter 7). The structural, morphological, optical and photoluminescence properties of all phosphor samples were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric and differential thermal analysis (TG-DTA), atomic force microscopy (AFM), field-emission scanning electron microscopy and energy dispersive X-ray spectroscopy (FE-SEM/EDS), high-resolution transmission electron microscopy (TEM), ultraviolet-visible (UV-Vis) absorbance analysis and photoluminescence (PL) analysis and decay measurements. The crystal structural parameters, electronic structure and orbital population of molybdates were determined by means of Rietveld refinement, absorbance analysis and density functional theory (DFT) calculation.

This graphical abstract show that (a) the crystal system of Eu2O3@SrMoO4:Eu3+ hybrid phosphor and (b) the excitation and emission spectra of with its resonance energy transfer model. The energy transfer occurs between 1. (MoO4)2- and Eu3+ polyhedron and 2. direct energy transfer from oxygen to europium ion, which are depicted also (c) band model