Preparation and photoluminescent properties of near-UV broadband-excited red phosphor (Gd1-xEux)6(Te1-yMoy)O12 for white-LEDs

Abstract

Generally, the Eu3+-activated red phosphors suffer narrow 4f-4f excitation lines ranging from near-UV to blue part of the spectrum, resulting in poor spectral overlapping with the emission spectrum of the pumping LED and low energy conversion efficiency. In this paper, the strategy of Te6+/Mo6+ mixing is adopted to enhance the excitation bandwidth of Eu3+ via the energy transfer from Mo6+-O2- charge transfer state to Eu3+, which is crucial for LED applications. A series of (Gd1-xEux)6(Te1-yMoy)O12 red phosphors are synthesized by the solid state method at 1200 ℃. The crystal structure, morphology and luminescent properties are investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescent spectrum. The XRD patterns of (Gd1-xEux)6(Te1-yMoy) O12 (x = 0.2, y = 0, 0.4) match well with that of Gd6TeO12 (JCPDS No. 50-0269), but differ from that of Gd6MoO12 (JCPDS No. 24-1085). The phosphor consists of irregular particles with an average size of 10 m. Upon excitation at 393 nm, the (Gd1-xEux)6TeO12 phosphors emit red light corresponding to the intraconfigurational 4f-4f transitions of Eu3+, and the color coordinates are calculated to be (0.647, 0.353). The 5D07F2 electron-dipole transition dominates the emission spectrum, which reveals that Eu3+ occupies a crystallographic site without an inversion center. Moreover, this transition gives rise to three distinguishable emission lines situated at 605, 618, and 632 nm, respectively. This unusual spectral splitting is supposed to originate from the strong interaction exerted by the crystal field of host on the 4f electrons. The optimum doping content of Eu3+ in (Gd1-xEux)6TeO12 phosphor is 20% (mole fraction), the critical distance for energy transfer is 0.75 nm, and the concentration quenching is confirmed to be induced by the dipole-dipole interaction from the linear relationship between lg(I/x) and lg x (I represents the luminescence intensity, and x represents the doping concentration of Eu3+). As the temperature increases, the emission intensity decreases gradually due to thermal quenching. The integrated emission intensity at 423 K is 70% of the initial value at ambient temperature. The thermal activation energy is determined to be 0.1796 eV from the temperature dependence of luminescence intensities. The partial substitution of Te6+ by Mo6+ does not change the emission position nor intensity significantly, but promotes the excitation bandwidth and conversion efficiency remarkably. Compared with (Gd0.8Eu0.2)6TeO12, the compositionoptimized (Gd0.8Eu0.2)6(Te0.6Mo0.4)O12 presents a relatively flat excitation spectrum in the near-UV region. It also provides more intense emission since (Gd0.8Eu0.2)6MoO12 undergoes the strong concentration quenching arising from the high density of [MoO6] groups. In conclusion, the results indicate that (Gd0.8Eu0.2)6(Te0.6Mo0.4)O12 can serve as a broadband-excited red phosphor for near-UV-based white LEDs.