Bright UV-C Phosphors with Excellent Thermal Stability-Y1-xScxPO4 Solid Solutions.

The structural and luminescence properties of undoped Y1-xScxPO4 solid solutions have been studied. An intense thermally stable emission with fast decay (τ1/e ~ 10-7 s) and a band position varying from 5.21 to 5.94 eV depending on the Sc/Y ratio is detected and ascribed to the 2p O-3d Sc self-trapped excitons. The quantum yield of the UV-C emission, also depending on the Sc/Y ratio, reaches 34% for the solid solution with x = 0.5 at 300 K. It is shown by a combined analysis of theoretical and experimental data that the formation of Sc clusters occurs in the solid solutions studied. The clusters facilitate the creation of energy wells at the conduction band bottom, which enables deep localization of electronic excitations and the creation of luminescence centers characterized by high quantum yield and thermal stability of the UV-C emission.

Deep-Ultraviolet Emitter: Rare-Earth-Free ZnAl2O4 Nanofibers via a Simple Wet Chemical Route.

Deep-UV (180-280 nm) phosphors have attracted tremendous interest in tri-band-based white light-emitting diode (LED) technology, bio- and photochemistry, as well as various medical fields. However, the application of many UV-emitting materials has been hindered due to their poor thermal or chemical stability, complex synthesis, and environmental harmfulness. A particular concern is posed by the utilization of rare earths affected by rising price and depletion of natural resources. As a consequence, the development of phosphors without rare-earth elements represents an important challenge. In this work, as a potential UV-C phosphor, undoped ZnAl2O4 fibers have been synthesized by a cost-efficient wet chemical route. The rare-earth-free ZnAl2O4 nanofibers exhibit a strong UV emission with two bands peaking at 5.4 eV (230 nm) and 4.75 eV (261 nm). The emission intensity can be controlled by tuning the Zn/Al ratio. A structure-property relationship has been thoroughly studied to understand the origin of the UV emission. For this reason, ZnAl2O4 nanofibers have been analyzed by X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and Raman spectroscopy techniques showing that a normal spinel structure of the synthesized material is preserved within a wide range of Zn/Al ratios. The experimental evidence of a strong and narrow band at 7.04 eV in the excitation spectrum of the 5.4 eV emission suggests its excitonic nature. Moreover, the 4.75 eV emission is shown to be related to excitons perturbed by lattice defects, presumably oxygen or cation vacancies. These findings shed light on the design of UV-C emission devices for sterilization based on a rare-earth-free phosphor, providing a feasible alternative to the conventional phosphors doped with rare-earth elements.

NASICON-type Na3.6Lu1.8-x(PO4)3:xEu3+ phosphors: structure and luminescence.

Na3.6Lu1.8-x(PO4)3:xEu3+ phosphors were synthesized by a high-temperature solid-state reaction. A powder X-ray diffraction study revealed that homogeneous solid solutions with a NASICON-type structure were formed at 0 ≤ x ≤ 0.7. The Na3.6Lu1.8(PO4)3 structure was refined from the powder X-ray diffraction data and the cation distribution in the lattice sites of the NASICON-type structure was revealed. The refinement indicates structural disorder caused by the displacement of a part of Lu cations along the c axis inside the (Lu/Na)O6 octahedra that is confirmed by the broadened emission lines of Eu3+, which substitutes Lu cations. The highest Eu3+ luminescence intensity is found in Na3.6Lu1.8-x(PO4)3:xEu3+ for x = 0.5, whereas a further increase of the Eu3+ content leads to concentration quenching that is shown to occur due to the dipole-dipole interaction. An enhanced temperature stability of the Eu3+ emission was observed at the excitation energy of 3.23 eV. At this excitation energy, thermal quenching of the emission caused by the 7F0 → 5L7 transitions is compensated by the intensity increase of the emission related to the 7F1 → 5GJ transitions, which occurs due to the increase of the 7F1 level population, induced by a temperature rise.

Revised crystal structure and luminescent properties of gadolinium oxyfluoride Gd4O3F6 doped with Eu³âº ions.

The structure of gadolinium oxyfluoride nanoparticles was revised. Extensive studies including X-ray diffraction and Rietveld refinement as well as Fourier transform infrared spectroscopy and Raman spectroscopy confirmed the monoclinic P12/c1 crystal structure of Gd4O3F6. Morphological analysis using transmission electron microscopy showed the nanocrystallinity of the materials prepared via the sol-gel Pechini's method. The luminescent properties of the prepared materials with different concentrations of Eu(3+) ions were characterized by emission spectroscopy. The phosphors obtained were investigated in the vacuum ultraviolet range using synchrotron radiation. The Judd-Ofelt parameters (Ω2, Ω4) and emission efficiencies η were calculated and are discussed in detail.

Hydrothermal synthesis and structural and spectroscopic properties of the new triclinic form of GdBO3:Eu3+ nanocrystals.

Triclinic Gd1-xEuxBO3 nanophosphors have been prepared by a hydrothermal method without using additional coreagents and prior precipitation of precursor (in situ). The formation of the borate nanorods and their crystal structure was refined on the basis of X-ray diffraction patterns (XRD) and well confirmed using various techniques such as infrared spectroscopy (IR), Raman spectroscopy, transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX). The new triclinic crystal structure (space group P1) for the GdBO3 nanocrystals and detailed structure parameters were determined with the help of the Rietveld analysis. The spectroscopic characteristics of the synthesized nanomaterials with different concentrations of Eu(3+) ions were defined with the use of luminescence excitation spectra as well as emission spectra and decay kinetics. The Judd-Ofelt parameters (Ω2, Ω4) and quantum efficiency, η, were also calculated for the more detailed analysis of Eu(3+) spectra in the GdBO3 host.