Three types of β-NaYF4 nanoparticles, uncoated core (NaYF4:Yb/Ho/Ce), single-layer coated core@shell (NaYF4:Yb/Ho/ Ce@NaYF4:Yb) and double-layer coated core@shell@shell (NaYF4:Yb/Ho@NaYF4:Yb@NaYF4:Yb) with Ce3+ doped in core, first and second shell, respectively, were synthesized through solvothermal method to investigate the cross-relaxation between Ho3+ and Ce3+ for the tunable upconversion luminescence. By doping Ce3+ into different layers with different doping concentrations, a systematical investigation on the tunable upconversion luminescence from green to red was conducted. The results showed that a remarkable color tuning could be achieved from green to red when increasing the doping concentration of Ce3+ in the same layer of Ho3+. And if Ce3+ and Ho3+ were separated in different layers, the color tuning would be depressed significantly due to the reduced cross-relaxation between Ho3+ and Ce3+. Moreover, the UC emission intensity of core@shell and core@shell@shell was enhanced significantly compared with that of unmodified core nanoparticles.(a) UC emission spectra, (b) R/G ratio, and (c) the digital images of colloidal core NaYF4:Yb3+/Ho3+ nanocrystals doped with 0 mol.%, 4 mol.% and 10 mol.% Ce3+ under 980 nm diode laser of 50 W/cm2 successivelyDownload high-res image (95KB)Download full-size image

Tb3+/Eu3+ co-doped transparent glass ceramics containing CaF2 nanocrystals were successfully synthesized by high temperature melt-quenching method and subsequent heating. The structure and morphology of the samples were investigated by X-ray diffraction (XRD), transmittance electron microscopy (TEM), high resolution TEM (HRTEM) and selected area electron diffraction (SAED). The photoluminescence properties and energy transfer process from Tb3+ to Eu3+ of CaF2:Tb3+,Eu3+ phosphors were also investigated through excitation spectra and decay curves. In addition, the emission spectra of the glass ceramics in a wide temperature range from 21 to 320 K were recorded under the excitation of 485 nm. It was found that the fluorescence intensity ratios of Tb3+ at 545 nm (5D4→7F5) to Eu3+ at 615 nm (5D0→7F2) was highly temperature-dependent with an approximate linear relationship, and the temperature sensitivity was about 0.4%/K. It is expected that the investigated Tb3+/Eu3+ co-doped CaF2 glass ceramics may have prospective application in optical thermometry.(a) Normalized emission spectra of GC700 under the excitation of 485 nm in the temperature range of 21–320 K; (b) Dependence of FIR (I541/I620) on temperature for GC700 and the fitted line for experimental dataDownload high-res image (64KB)Download full-size image

In this paper, Bi3+- and Eu3+-co-doped ScVO4 had been investigated as an alternative red-emitting phosphor under near-ultraviolet excitation. Structural and spectroscopic characterizations were carried out. The results show that the samples can efficiently absorb near-ultraviolet light and produce very bright red emission. In particular, the optimized sample shows much stronger emission intensity than the commercial phosphor Y2O2S: 5% Eu3+ in the excitation range of 365–395 nm, and can reach a strength 5.4 times of the latter under 385 nm excitation. It also has better CIE chromaticity coordinates. These suggest ScVO4: Bi3+, Eu3+ as a promising red-emitting phosphor for white light-emitting diodes based on near-ultraviolet light-emitting diode chips.

Dy3+/Tb3+ codoped CaMoO4 phosphors were synthesized by a simple sol–gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence spectroscopy. The energy transfer process of Dy3+→Tb3+ was confirmed by excitation and emission spectra and luminescence decay curves, and the energy transfer efficiency was also estimated. The results verified that the efficient emission of Tb3+ was sensitized by Dy3+ under the excitation of 354 nm, realizing tunable emission in CaMoO4 phosphors. Furthermore, optical thermometry was achieved by the fluorescence intensity ratio between Tb3+: 5D4→7F5 (~546 nm) and Dy3+: 4F9/2→6H13/2 (~575 nm). It is expected that the investigated CaMoO4 nanograins doped with Dy3+/Tb3+ have prospective applications in display technology and optical thermometry.

•The crystal-field energy levels of Eu3+ in orthorhombic KGdF4 nanocrystals have been identified.•The symmetry of the site occupied by Eu3+ has been determined.•A strong temperature dependence of the relaxation of 5D2 leads to the temperature-dependent luminescence.Luminescent properties of Eu3+ ions in orthorhombic KGdF4 nanocrystals are investigated with high resolution emission spectra measured under the selective excitation of a tunable laser at the temperature of 20 K. Only the emission from one kind of site is detected, and the crystal-field levels of Eu3+ ions in this site, up to 5D2 multiplet, have been identified according to the recorded emission spectra. The symmetry of the site occupied by Eu3+ ions is discussed based on the numbers of emission lines of 5D0–7FJ transitions. A temperature-dependent luminescence is observed under the excitation of 5D2 at 465.5 nm. The emission from 5D1 is nearly absent at 20 K, while plays an important role in the emission spectrum at 300 K. This indicates that the direct relaxation from 5D2 to 5D0 dominates at low temperature. The different processes of the relaxation of 5D2 at 20 and 300 K are discussed with the measured decay curves of the emission from 5D0.