Moisture resistance, luminescence enhancement, energy transfer and tunable color of novel core-shell structure BaGeF6:Mn4+ phosphor

Recently, Mn4+-doped narrow band red-emitting fluoride phosphors with exceptional luminescence properties have been widely studied for improving the performance of white light-emitting diodes (WLEDs). However, the poor water stability and single luminescent color limit their wider application. Herein, the design idea of new core-shell structure is proposed to improve the properties of BaGeF6:Mn4+ red phosphors by coating poly propylene glycol (PPG) and NaGdF4:Dy3+ nanoparticles. The phase purity, morphology and elemental composition are confirmed via X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The luminescence properties of all phosphors are researched in detail with excitation spectra, emission spectra and fluorescence lifetime curves. PPG coating layer can not only modify the surface defects of the BaGeF6:Mn4+ phosphor to enhance the luminous intensity, but also successfully improve the water resistance of the phosphor. Therefore, BaGeF6:Mn4+@PPG phosphor as red component is more effective for packaging warm WLED. In addition, PPG is considered as the adsorption media layer to connect BaGeF6:Mn4+ phosphors and NaGdF4:Dy3+ nanoparticles. The as-synthesized BaGeF6:Mn4+@PPG-NaGdF4:Dy3+ composite phosphors can emit blue, yellow and red light simultaneously under the excitation of ultraviolet light. The energy transfer processes from Gd3+ to Dy3+ and from Dy3+ to Mn4+ are systematically discussed. By changing the doping concentration of Mn4+ and excitation wavelengths, the photochromic properties of the composite phosphor are successfully realized and the warm white light emission is obtained under the excitation of ultraviolet light. These findings demonstrate that the BaGeF6:Mn4+@PPG-NaGdF4:Dy3+ composite phosphors have more value for practical application.