Multicolor up-conversion emissions of Tm ³⁺/Er ³⁺/Yb ³⁺ tri-doped YF ₃ phosphors

Tm ³⁺/Er ³⁺/Yb ³⁺ tri-doped yttrium fluoride (YF ₃) phosphors were prepared by a facile hydrothermal method. X-ray topographic analysis found that the phosphors were crystallized products. Their sizes and morphologies were characterized by scanning electron microscopy (SEM, Hitachi S-4800), which indicated that most of the YF ₃ phosphors were hundreds of nanometers in size. Up-conversion (UC) spectra were recorded under 980-nm diode laser excitation at room temperature with a fluorescence spectrometer (Hitachi F-4500). Plenty of UC emissions of Tm ³⁺ and Er ³⁺ were observed from ultraviolet to red. For Tm ³⁺ ions, a five-photon process (∼291 nm and ∼347 nm), a four-photon process (∼362 nm and ∼452 nm), and a three-photon process (∼475 nm) were identified in the UC spectra. The UC emissions from the Er ³⁺ were: ∼380 nm, ∼408 nm, ∼521 nm, ∼537 nm, and ∼652 nm. Therefore, cyan-white light can be observed by the naked eye at 980-nm excitation, even under low excitation power density. By comparing the UC spectra of the phosphors annealed at different temperatures, we found that the intensity of the UC luminescence increased as annealing temperature increased. Furthermore, the spectral dependencies on Tm ³⁺ doped concentrations were studied. The energy transfer processes and fluorescence dynamics in the tri-doped system are currently being investigated.