Synthesis and luminescence properties of rare-earth free narrow band green emitting Na2ZnSiO4:Mn2+ phosphor for white LEDs

Abstract

Novel rare-earth free green-emitting phosphor Na2ZnSiO4:Mn2+ was prepared by conventional high temperature solid state reaction method in the carbon reducing atmosphere at 900 °C. The crystal structure was confirmed by X-ray diffraction (XRD) and the structure information was refined by the Rietveld method. The luminescence properties of Na2ZnSiO4:Mn2+ phosphors as a function of Mn2+ concentrations were investigated. The excitation band peaking at ∼359, 383, 427, 449, and 467 nm are attributed to ground level 6A1(6S) to excited levels 4E(4D), 4T2(4D), [4A(4G), 4E(4G)], 4T2(4G), and 4T1(4G) transitions of Mn2+ ion, respectively, which indicates that Na2ZnSiO4:Mn2+ phosphor may be excited by (near) ultraviolet and blue region light-emitting diode chip. The narrow emission band peaks at ∼515 nm generated by 4T1(4G)→6A1(6S) transition of the Mn2+ can exhibit strong green emission. The luminescence mechanism was explained by the Tanabe-Sugano diagram. The optimum Mn2+ doping concentration was 0.5 mol%. The critical distance between Mn2+ was calculated to be 33.7 Å. The concentration quenching mechanism can be interpreted by the dipole-dipole interaction of Mn2+ ions. With the Mn2+ doping concentration increasing in the range of 0.2–0.8 mol%, the lifetime of this phosphor decreased from 7.51 to 7.39 ms. These phosphors also possessed good thermal stability at high temperature, and the emission intensity at 125 °C was about 74.9% of that at 25 °C. The present work suggests that Na2ZnSiO4:Mn2+ phosphor is a kind of potential green-emitting phosphor to provide a bright green component in the red/green/blue tri-color system w-LEDs.