A new color-adjustable self-emitting in SrGa₂Ge₂O₈ substituted by Eu³⁺, energy transfer and the determination of luminescence center

Highlights

• The luminescence characteristics and crystal structure of Eu in SrGa₂Ge₂O₈ matrix.

• The dual-excitation emission can be achieved.

• The lattice occupancy and energy transfer was studied.

Abstract

To explore a new color-adjustable self-emitting in SrGa₂Ge₂O₈:Eu phosphors, a series of experiments and characterization were carried out in this paper. Through a classic solid state method, we successfully synthesized the SrGa_2-yGe_2-zO₈:xEu (0 ≤ x, y, z ≦ 0.2) phosphors. The resulting SrGa_2-yGe_2-zO₈:xEu (0 ≤ x, y, z ≦ 0.2) phosphors had been structurally characterized by X-ray Powder Diffraction Refinements (XRD) and Energy Disperse Spectrometer (EDS). In the present study, it was found that under the excitation of 277 nm, by changing the concentration of doped europium ions, a series of color changes from cyan to white are obtained in SrGa₂Ge₂O₈. The energy transfer from the host to the Eu ions occurs. And the energy transfer ability from the host to Eu³⁺ were shown to be induced when aluminum was substituted to the nominal composition. The bond energy calculation result shows that the doped Sr²⁺, Ga³⁺, Ge⁴⁺ ions are substituted by Eu³⁺, Al³⁺ and Si⁴⁺, respectively, which is consistent with the XRD Rietveld refinement analysis. Also, the EDS analysis shows that atomic ratio is 10:19:20 for Sr/Ga/Ge in SrGa₂Ge₂O₈, which is consistent with the XRD result. Diffuse reflection (DR) spectra and electronic structure display the energy band gap is 4.49 and 3.487eV, respectively. Excited by the 260 nm light, it presents a broad band peaks from 350 to 650 nm with the peak at 441 nm. When Al³⁺/Si⁴⁺ ions are doped in it, the red-shift of the matrix and the photoluminescence improvement of SrGa₂Ge₂O₈: Eu can be found.

Introduction

The 4f orbital of rare earth ions provides very rich electron transition energy levels, so its luminescence range is extremely wide. And its physical properties are exceedingly stable, therefore rare earth ions interact with the matrix as activators will get many interesting phenomena [[1], [2], [3], [4], [5], [6], [7]]. For europium ions, the stability of trivalent europium ion is better than that of divalent europium ion [[8], [9], [10], [11], [12]]. The f-f transition of Eu³⁺ comes from a narrow peak of about 612 nm [[13], [14], [15], [16], [17]]. Recent studies have identified Eu³⁺ doped AB₂C₂O₈(including SrAl₂Si₂O₈, SrGa₂Ge₂O₈) host has good fluorescence properties, including a strong excitation emission, thermal stability, color adjustment, etc. [[18], [19], [20], [21], [22], [23]] In SrAl₂Si₂O₈ system, all Sr, Al and Si atoms occupy 8f lattice. Sr atoms were surrounded by seven oxygen atoms, forming a SrO₇ polyhedron. Four oxygen atoms surround the Al and Si atoms and form (Al/Si)O₄ tetrahedron. And in SrGa₂Ge₂O₈ crystal structure, Al and Si can be considered to be replaced by Ga and Ge, respectively [24,25].The crystal structure of SrGa₂Ge₂O₈ has the similarly crystal structure as SrAl₂Si₂O₈. However, the photoluminescence of Eu³⁺ doped or undoped SrGa₂Ge₂O₈ phosphor has not been reported as a potential phosphor.

Our work is to synthesize a series of Eu³⁺ doped or undoped SrGa₂Ge₂O₈ phosphors. The photoluminescence properties of Eu³⁺ doped or undoped SrGa₂Ge₂O₈ phosphor are investigated. An ultra-wide broadband emission from the violet to the orange region is found in SrGa₂Ge₂O₈ phosphor. After doping Eu³⁺, white light emission can be achieved by combining a broad blue-green band at 441 nm and some red emission bands at about 618 nm. In addition, we first performed first-principles display density-functional-theory to understand the origins of the broadband emission of SrGa₂Ge₂O₈ phosphor. It is proved by the analysis of experimental data that the photoluminescence properties of Si⁴⁺ no Al³⁺ substituted SrGa₂Ge₂O₈ phosphor also are detected to analyze the luminescent origin of the SrGa₂Ge₂O₈ phosphor. And the bond valence model (BVM) has been used to evaluate the bond energy calculation, which can help us to judge the dopant preferential occupies.