Single-Component and Warm-White-Emitting Phosphor NaGd(WO4)2:Tm3+, Dy3+, Eu3+: Synthesis, Luminescence, Energy Transfer, and Tunable Color

Tm3+, Dy3+, and Eu3+ codoped NaGd(WO4)(2) phosphors were prepared by a facile hydrothermal process; they were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectrometer (EDS), photoluminescence spectra, and fluorescence lifetime. The results show that the novel octahedral microcrystals with a mean side length of 2 mu m are obtained. Under the excitation of ultraviolet, individual RE3+ ion (Tm3+, Dy3+, and Eu3+ activated NaGd(WO4)(2) phosphors exhibit excellent emission properties in their respective regions. Moreover, when codoping Dy3+ and Eu3+/Tm3+ in the single component, the energy migration from Dy((3+)) to Eu((3+)) has been demonstrated to be a resonant type via a dipole-quadrupole mechanism as well as that from Tm3+ to Dy3+ ions, of which the critical distance (RDy-Eu) is calculated to be 11.08 angstrom. More significantly, in the Tm3+, Dy3+, and Eu3+ tridoped NaGd(WO4)(2) phosphors, the energy migration of Tm3+-Dy3+-Eu3+, utilized for sensitizing Eu3+ ions besides compensating the red component at low Eu3+ doping concentration, has been discussed first. In addition, under 365 nm near-ultraviolet radiation (nUV), the color-tunable emissions in octahedral NaGd(WO4)(2) microcrystals are realized by giving abundant blue, green, white, yellow, and red emissions, especially warm white emission, and could be favorable candidates in full-color phosphors for nUV-LEDs.