Down-converted luminescence and energy transfer of α-KGd1−xEuxF4 nanophosphors with selective excitation

A series of KGd1−xEuxF4 (x=1, 5, 10, 15, 20, 40, 60, 80 and 100 mol%) nanophosphors were prepared by a simple and reproducible hydrothermal route. The structural and morphological properties of the samples were studied by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM), showing uniform nano-sized particles with cubic phase. The excitation spectra disclosed the intrinsic absorption bands of Eu3+5L6 state at 393 nm and of Gd3+6IJ state at 272.4 nm. From the downconversion (DC) emission spectra, it was found that the optimal Eu3+ concentration was 20 mol% and 15 mol% under 393 nm and 272.4 nm excitation, respectively. The nonradiative energy transfer (ET) mechanism is discussed and it turns out that ET of Eu3+–Eu3+ couple and Gd3+–Eu3+ couple occurs mainly via electric dipole–dipole and electric dipole–quadrupole interactions, respectively. From luminescence decay curves, the lifetimes of KGd1−xEuxF4 nanophosphors are seen to decrease with the increase of Eu3+ concentration. Both the emission spectra and luminescence decay curves show that downconversion ET between Gd3+ and Eu3+ ions is more efficient than that between Eu3+ and Eu3+ ions, thus suggesting potential applications in mercury-free fluorescence lamp and plasma display panel under vacuum ultraviolet (VUV) radiation.