Spectral Properties and Energy Transfer between Ce³⁺ and Yb³⁺ in the Ca₃Sc₂Si₃O₁₂ Host: Is It an Electron Transfer Mechanism?

The downshifting from Ce³⁺ blue emission to Yb³⁺ near-infrared emission has been studied in the garnet host Ca_{2.8–2<i>x</i>}Ce_0.1Yb_<i>x</i>Na_0.1+<i>x</i>Sc₂Si₃O₁₂ (<i>x</i> = 0–0.36). The downshifting does not involve quantum cutting, but one incident blue photon is transferred from Ce³⁺ to Yb³⁺ with an energy transfer efficiency up to 90% when <i>x</i> = 0.36 for the Yb³⁺ dopant ion. For <i>x</i> ≤ 0.15, a multiphonon-assisted electric dipole–electric quadrupole mechanism of energy transfer dominates, while for the highest concentration of Yb³⁺ employed, the electron transfer mechanism is confirmed. A temperature-dependent increase of the Ce³⁺ → Yb³⁺ energy transfer rate does not exclusively indicate the electron transfer mechanism. The application of the material to solar energy conversion is indicated

Site Occupancies, Luminescence, and Thermometric Properties of LiY₉(SiO₄)₆O₂:Ce³⁺ Phosphors

In this work, we report the tunable emission properties of Ce³⁺ in an apatite-type LiY₉(SiO₄)₆O₂ compound via adjusting the doping concentration or temperature. The occupancies of Ce³⁺ ions at two different sites (Wyckoff 6h and 4f sites) in LiY₉(SiO₄)₆O₂ have been determined by Rietveld refinements. Two kinds of Ce³⁺ f–d transitions have been studied in detail and then assigned to certain sites. The effects of temperature and doping concentration on Ce³⁺ luminescence properties have been systematically investigated. It is found that the Ce³⁺ ions prefer occupying Wyckoff 6h sites and the energy transfer between Ce³⁺ at two sites becomes more efficient with an increase in doping concentration. In addition, the charge-transfer vibronic exciton (CTVE) induced by the existence of free oxygen ion plays an important role in the thermal quenching of Ce³⁺ at 6h sites. Because of the tunable emissions from cyan to blue with increasing temperature, the phosphors LiY₉(SiO₄)₆O₂:Ce³⁺ are endowed with possible thermometric applications

Combined Experimental and ab Initio Study of Site Preference of Ce³⁺ in SrAl₂O₄

Low-temperature photoluminescence properties of Sr_{1–2<i>x</i>}Ce_<i>x</i>Na_<i>x</i>Al₂O₄ (<i>x</i> = 0.001) synthesized by a solid-state reaction method are measured with excitation energies in the vacuum ultraviolet (VUV) to ultraviolet (UV) range. Two distinct activator centers with different emission and excitation intensities are observed and attributed to Ce³⁺ occupying the Sr1 and Sr2 sites of SrAl₂O₄ with different probabilities. Hybrid density functional theory (DFT) calculations within the supercell model are then carried out to optimize the local structures of Ce³⁺ located at the two Sr sites of SrAl₂O₄, on which wave function-based CASSCF/CASPT2 embedded cluster calculations with the spin–orbit effect are performed to derive the Ce³⁺ 4f¹ and 5d¹ energy levels. On the basis of the observed relative spectral intensities, the calculated DFT total energies, and the comparison between experimental and calculated 4f → 5d transition energies, we conclude that, in SrAl₂O₄:Ce³⁺, the dopant Ce³⁺ prefers to occupy the slightly smaller Sr2 site, rather than the larger Sr1 site as proposed earlier. Furthermore, by using an established linear relationship between the lowest 4f → 5d transition energies of Ce³⁺ and Eu²⁺ located at the same site of a given compound, we find that, in SrAl₂O₄:Eu²⁺, the dominant green emission observed at room temperature arises from Eu²⁺ located at the Sr2 site of SrAl₂O₄

Excitation Wavelength Dependent Luminescence of LuNbO₄:Pr³⁺Influences of Intervalence Charge Transfer and Host Sensitization

A series of LuNbO₄:Pr³⁺ phosphors was prepared by a solid-state reaction method at high-temperature. Rietveld refinements were performed based on powder X-ray diffraction (XRD) data. Diffuse reflectance spectra (DRS), UV–vis photoluminescence (PL), time-resolved emission spectra (TRES), and fluorescence decays were utilized to study the luminescence and host sensitization processes of Pr³⁺ in LuNbO₄. Excitation wavelength dependent luminescence of LuNbO₄:Pr³⁺ was investigated and explained in consideration of the processes of nonradiation relaxation via cross-relaxation, multiphonon relaxation, and crossover to the intervalence charge transfer (IVCT) state. Furthermore, the host sensitization of Pr³⁺ emission in LuNbO₄ was confirmed and the energy transfer efficiency from host to Pr³⁺ increased with increasing Pr³⁺ doping concentration/temperature. Because the change of emission intensities for both blue from the host and red from ¹D₂ is sensitive to temperature, a large variation of emission color is observed between RT and 500 K

Cellulose-Based Composite Macrogels from Cellulose Fiber and Cellulose Nanofiber as Intestine Delivery Vehicles for Probiotics

Cellulose-based composite macrogels made by cellulose fiber/cellulose nanofiber (CCNM) were used as an intestine delivery vehicle for probiotics. Cellulose nanofiber (CNF) was prepared by a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation system, and the carboxyl groups in CNF acted as pore size and pH responsibility regulators in CCNMs to regulate the probiotics loading and controlled release property. The macrogel presented a porosity of 92.68% with a CNF content of 90%, and the corresponding released viable <i>Lactobacillus plantarum</i> (<i>L. plantarum</i>) was up to 2.68 × 10⁸ cfu/mL. The porous structure and high porosity benefited <i>L. plantarum</i> cells to infiltrate into the core of macrogels. In addition, the macrogels made with high contents of CNF showed sustainable release of <i>L. plantarum</i> cells and delivered enough viable cells to the desired region of intestine tracts. The porous cellulose macrogels prepared by a green and environmental friendly method show potential in the application of fabricating targeted delivery vehicles of bioactive agents

Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg₂Si₂O₇:Ln (Ce³⁺, Eu²⁺) Phosphors

The host structure and the synchrotron radiation VUV–UV luminescence properties of samples BaMg₂Si₂O₇ (BMSO):Ln (Ce³⁺, Eu²⁺) at different doping levels and different temperatures were investigated in detail. Three important aspects are studied to elucidate the luminescence properties of samples: (1) the vacuum referred binding energy (VRBE) scheme is constructed with the electron binding in the BMSO host bands and in the Ce³⁺ and Eu²⁺ impurity levels with the aim to explain the different thermal stabilities of Ce³⁺ and Eu²⁺ emissions; (2) the electron–vibrational interaction analysis on the narrow Eu²⁺ emission indicates a weak electron–phonon interaction in the current case; (3) by using three models (Inokuti–Hirayama, Yokota–Tanimoto, and Burshteĭn models) at different conditions, the energy transfer dynamics between Ce³⁺ and Eu²⁺ was analyzed. It reveals that the energy transfer from Ce³⁺ to Eu²⁺ via electric dipole–dipole (EDD) interaction is dominant while energy migration between Ce³⁺ is negligible. Finally, the X-ray excited luminescence spectra of samples BMSO:Ce³⁺/Eu²⁺ are collected to evaluate their possible scintillator applications

Spectroscopy and Luminescence Dynamics of Ce³⁺ and Sm³⁺ in LiYSiO₄

The lithium yttrium silicate series of LiY_1–<i>x</i>Ln_<i>x</i>SiO₄ exhibits superb chemical and optical properties, and with Ln = Ce³⁺, Sm³⁺, its spectroscopic characteristics and luminescence dynamics are investigated in the present work. Energy transfer and nonradiative relaxation dramatically influence the Ln³⁺ luminescence spectra and decay dynamics, especially in the Ce³⁺–Sm³⁺ codoped phosphors. It is shown that thermal-quenching of the blue Ce³⁺ luminescence is primarily due to thermal ionization in the 5d excited states rather than multiphonon relaxation, whereas cross-relaxation arising from electric dipole–dipole interaction between adjacent Sm³⁺ ions is the leading mechanism that quenches the red Sm³⁺ luminescence. In the codoped systems, Ce³⁺–Sm³⁺ energy transfer in competing with the thermal quenching enhance the emission from Sm³⁺. The combined influences of concentration quenching, thermal ionization, and energy transfer including cross-relaxation on the luminescence intensity of single-center and codoped phosphors are analyzed based on the theories of ion–ion and ion–lattice interactions

Spectral Properties and Energy Transfer of a Potential Solar Energy Converter

The energy transfer between Ce³⁺ and Eu²⁺ has been investigated in the host Ca₃Sc₂Si₃O₁₂ (CSS), prepared by a modified sol–gel method. Excitation and emission measurements from the near-infrared to the vacuum ultraviolet spectral regions have been performed upon CSS, Ce³⁺-doped CSS, Eu²⁺-doped CSS and Ce³⁺, Eu²⁺-co-doped CSS, at various concentrations, including experiments at temperatures range of 15–460 K. The energy transfer efficiency from Ce³⁺ to Eu²⁺ can approach 90%, and the Ce³⁺ donor decay curves for different Eu²⁺ acceptor concentrations in the codoped system were fitted by the Inokuti–Hirayama method, indicating that it is energy transfer induced by electric dipole interaction. The use of the Ce³⁺, Eu²⁺ couple in the CSS host as a wideband harvester with an emission profile tailored to the response of the silicon solar cell in solar energy conversion suffers from two main drawbacks relating to valence instability and emission quenching of Eu²⁺. Possible solutions are suggested