Uniform barium fluoride (BaF2) micro-crystals with Rubik's cube-like structure have been prepared through a facile and environmentally friendly hydrothermal approach assisted by citric acid as the chelating agent. The crystal structures, morphologies and optical properties of the obtained samples were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. The influential factors on the morphologies and size of BaF2 particles, including pH values and the molar ratio of Ba2+/Cit(3-), were systematically investigated and the possible growth mechanism for BaF2 three-dimensional (3D) Rubik's cube-like architectures was proposed on the basis of time-dependent experiments. Moreover, the BaF2 micro-cubes prepared with different amounts of Cit(3-) exhibit an intense blue band emission centered at around 423 nm with the excitation of 336 nm ultraviolet (UV) light and the origin of this strong blue light was analyzed in detail. Results provide an effective strategy to fabricate uniform micro-crystals with controllable morphology and excellent optical properties.

Ce3+/Pr3+ codoped Li2SrSiO4 (LSS) phosphors with blue, red, and near‐infrared (NIR) tri‐emission have been prepared via a high‐temperature solid‐state reaction method. Under the excitation of 200 to 400 nm near‐ultraviolet (n‐UV), the photoluminescence (PL) spectra of phosphors are composed of visible and NIR two parts. The former exhibits blue and red emission bands centered at around 428 nm from 5d–4f transition of Ce3+ and 611 nm from 1D2 → 3H4 transition of Pr3+, those overlap with photosynthesis action spectra of plants and absorption spectra of chlorophylls and carotenoids. While the later presents a broad NIR emission band peaking near 1039 nm caused by the 1G4 → 3H4 of Pr3+, matching with the absorption of bacteriochlorophyll. Their emission intensity ratios (B: R: NIR) could be tuned by altering the relative ratios of Ce3+ and Pr3+ concentration in the phosphors to meet the requirements of multifarious plants and bacteria. The efficient energy transfer from Ce3+ to Pr3+ takes place in the LSS host, which ascribed to an exchange interaction according to PL spectra and decay curves of phosphors. Results suggest that the present LSS: Ce3+, Pr3+ phosphors have great potential applications in plant growth n‐UV LED.

A color-tunable BaCa2MgSi2O8:1%Eu2+, 20%Mn2+ phosphor demonstrates CIE coordinates of (0.3384, 0.2176) and CRI of 82. The results indicate that the as-synthesized phosphor could be a single-phased and white-emitting phosphor for use in UV LEDs.

An optical temperature sensor and optical heater based on Yb3+/Tm3+ co-doped Ba5Gd8Zn4O21 phosphors.

SEM of shuttle-like NaLa(MoO4)2:Yb3+,Er3+ microcrystals, temperature-dependent UC spectra and sensitivity of samples without (A, C) and with (B, D) calcinations.

Yb/Tm co-doped Ba5Gd8Zn4O21 up-conversion (UC) phosphors with thermometry and optical heating properties were successfully prepared by a sol-gel process, crystal structures of all samples were examined by X-ray diffraction (XRD). The phosphors show an intense near-infrared (NIR) and several weak visible emission peaks with 980 nm excitation. The possible UC mechanisms and processes were proposed based on the power dependence of up-conversion luminescence (UCL) intensities, and the lifetimes of 1G4→ 3 H6 blue emissions were also measured to confirm the occurrence of energy transfer (ET). Temperature sensing performances based on the stark levels (G4(1), G4(2)) of Tm 3+ were evaluated by analyzing temperature-dependent UCL spectra in the range of 300-510 K. The maximum sensitivity for phosphors with different UCL intensity was discussed in detail and approached to approximately 0.0061 K at 300K. Furthermore, the heating property produced by laser excitation was also measured, which caused temperature of sample rising from 278.8 to 321.8 K as increasing pump power from 638 to 1802 mW. Results indicate that Yb/Tm co-doped Ba5Gd8Zn4O21 phosphors could be considered as potential candidates for temperature sensor and optical heater.

CaF2:Ho3+/Yb3+ nano-particles with intense green up-conversion (UC) luminescence are successfully synthesized via a facile hydrothermal approach by using NH4F as the fluoride source and Na2EDTA as a chelating reagent. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), and UC emission spectra are used to characterize the structures, shapes, and luminescent properties of the samples. The effects from fluoride sources and chelating reagents on the formations of CaF2 nano-particles are investigated, and the formation process is also deduced. Under the excitation of a 980-nm laser diode, the samples each show a green up-conversion emission centered at 540 nm corresponding to the 5S2/5F4→5I8 transitions of Ho3+. Moreover, the UC mechanisms of Ho3+/Yb3+ co-doped CaF2 nano-particles are also discussed.

Structural and spectroscopic characterizations of the Ce3+/Tb3+(Mn2+) solely and Ce3+-Tb3+(Mn2+) doubly doped phosphate compound Ca9ZnLi(PO4)(7) with beta-Ca-3(PO4)(2) structure have been performed by powder X-ray diffraction and photoluminescence spectra measurements. The weak green emission from Tb3+ and red emission from Mn2+ are significantly enhanced by introduction of sensitizer Ce3+ ions due to an efficient resonant-type energy transfer from Ce3+ to activators Tb3+ or Mn2+. The energy transfer efficiency and the mechanism have been estimated based on spectroscopic data. Meanwhile, the critical distances for energy transfer between the Ce3+ and Tb3+ or Mn2+ ions are also calculated by the method of spectral overlapping.

Ce3+-doped Ba2Ln(BO3)2Cl (Ln = Gd, Y) phosphors were synthesized through a conventional high-temperature solid state method in CO atmosphere. Structural and spectroscopic characterizations of the samples have been performed by X-ray diffraction and photoluminescence spectra measurements. The phosphors can be efficiently excited by near ultraviolet (n-UV) light resulting in blue emission. The optimal Ce3+ dopant concentrations in both compounds were determined, and the concentration quenching mechanisms were also discussed. The photoluminescence excitation (PLE) and emission (PL) spectra, and decay curves at liquid helium temperature were measured to analyze the crystallographic occupancy sites of Ce3+ in the Ba2Ln(BO3)2Cl (Ln = Gd, Y) hosts. The thermal stabilities of the phosphors Ba2Ln(BO3)2Cl:Ce3+ (Ln = Gd, Y) were studied using the dependence of the luminescence intensities on temperature (300–500 K), and their luminescence quenching temperatures and thermal activation energies were also determined. The results indicate that the phosphor Ba2Gd(BO3)2Cl:Ce3+ offers excellent optical properties as a potential blue-emitting phosphor candidate for n-UV LEDs, such as a higher thermal stability and a stronger luminescence intensity, than those of the phosphor Ba2Y(BO3)2Cl:Ce3+.