11 research outputs found

    Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics

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    Cerium substituting gadolinium in Ca<sub>2</sub>Gd<sub>8</sub>(SiO<sub>4</sub>)<sub>6</sub>O<sub>2</sub> occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce<sup>3+</sup> at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce<sup>3+</sup> sites. The calculated eigenstates of Ce<sup>3+</sup> at the 6H site suggest a weak Ce–O covalent bond formed between Ce<sup>3+</sup> and one of the coordinated oxygen ions not bonded with Si<sup>4+</sup>. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On the basis of the calculated results, we propose a model of charge transfer and vibronic coupling for interpretation of the anomalous thermal quenching of the Ce<sup>3+</sup> luminescence. The combination of experimental and theoretical studies in the present work provides a comprehensive understanding of the spectroscopy and luminescence dynamics of Ce<sup>3+</sup> in crystals of ionic–covalent coordination

    Consequences of ET and MMCT on Luminescence of Ce<sup>3+</sup>-, Eu<sup>3+</sup>-, and Tb<sup>3+</sup>-doped LiYSiO<sub>4</sub>

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    Ce<sup>3+</sup>, Eu<sup>3+</sup>, and Tb<sup>3+</sup> singly doped, Ce<sup>3+</sup>-Tb<sup>3+</sup>, Tb<sup>3+</sup>-Eu<sup>3+</sup>, and Ce<sup>3+</sup>-Eu<sup>3+</sup> doubly doped, as well as Ce<sup>3+</sup>-Tb<sup>3+</sup>-Eu<sup>3+</sup> triply doped LiYSiO<sub>4</sub> phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed to determine the structure of host compound. The cross-relaxation (CR) of Tb<sup>3+</sup> is quantitatively analyzed with the Inokuti–Hirayama model of energy transfer (ET), and the site occupancy is confirmed by emission spectra of Eu<sup>3+</sup>. ET and metal–metal charge transfer (MMCT) are systematically investigated in Ce<sup>3+</sup>-Tb<sup>3+</sup>, Tb<sup>3+</sup>-Eu<sup>3+</sup>, and Ce<sup>3+</sup>-Eu<sup>3+</sup> doubly doped systems. The combined effects of ET and MMCT on luminescence and emission color of Ce<sup>3+</sup>-Tb<sup>3+</sup>-Eu<sup>3+</sup> triply doped samples are discussed in detail, showing that the photoluminescence emission is tunable in a large color gamut

    Charge Transfer Vibronic Transitions in Uranyl Tetrachloride Compounds

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    The electronic and vibronic interactions of uranyl (UO<sub>2</sub>)<sup>2+</sup> in three tetrachloride crystals have been investigated with spectroscopic experiments and theoretical modeling. Analysis and simulation of the absorption and photoluminescence spectra have resulted in a quantitative understanding of the charge transfer vibronic transitions of uranyl in the crystals. The spectra obtained at liquid helium temperature consist of extremely narrow zero-phonon lines (ZPL) and vibronic bands. The observed ZPLs are assigned to the first group of the excited states formed by electronic excitation from the 3σ ground state into the <i>f</i><sub>ÎŽ,ϕ</sub> orbitals of uranyl. The Huang–Rhys theory of vibronic coupling is modified successfully for simulating both the absorption and luminescence spectra. It is shown that only vibronic coupling to the axially symmetric stretching mode is Franck–Condon allowed, whereas other modes are involved through coupling with the symmetric stretching mode. The energies of electronic transitions, vibration frequencies of various local modes, and changes in the OUO bond length of uranyl in different electronic states and in different coordination geometries are evaluated in empirical simulations of the optical spectra. Multiple uranyl sites derived from the resolution of a superlattice at low temperature are resolved by crystallographic characterization and time- and energy-resolved spectroscopic studies. The present empirical simulation provides insights into fundamental understanding of uranyl electronic interactions and is useful for quantitative characterization of uranyl coordination

    Tunable Yellow-Red Photoluminescence and Persistent Afterglow in Phosphors Ca<sub>4</sub>LaO­(BO<sub>3</sub>)<sub>3</sub>:Eu<sup>3+</sup> and Ca<sub>4</sub>EuO­(BO<sub>3</sub>)<sub>3</sub>

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    In most Eu<sup>3+</sup> activated phosphors, only red luminescence from the <sup>5</sup>D<sub>0</sub> is obtainable, and efficiency is limited by concentration quenching. Herein we report a new phosphor of Ca<sub>4</sub>LaO­(BO<sub>3</sub>)<sub>3</sub>:Eu<sup>3+</sup> (CLBO:Eu) with advanced photoluminescence properties. The yellow luminescence emitted from the <sup>5</sup>D<sub>1,2</sub> states is not thermally quenched at room temperature. The relative intensities of the yellow and red emission bands depend strongly on the Eu<sup>3+</sup> doping concentration. More importantly, concentration quenching of Eu<sup>3+</sup> photoluminescence is absent in this phosphor, and the stoichiometric compound of Ca<sub>4</sub>EuO­(BO<sub>3</sub>)<sub>3</sub> emits stronger luminescence than the Eu<sup>3+</sup> doped compounds of CLBO:Eu; it is three times stronger than that of a commercial red phosphor of Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup>. Another beneficial phenomenon is that ligand-to-metal charge transfer (CT) transitions occur in the long UV region with the lowest charge transfer band (CTB) stretched down to about 3.67 eV (∌330 nm). The CT transitions significantly enhance Eu<sup>3+</sup> excitation, and thus result in stronger photoluminescence and promote trapping of excitons for persistent afterglow emission. Along with structure characterization, optical spectra and luminescence dynamics measured under various conditions as a function of Eu<sup>3+</sup> doping, temperature, and excitation wavelength are analyzed for a fundamental understanding of electronic interactions and for potential applications

    Investigation of Transport Properties of Water–Methanol Solution through a CNT with Oscillating Electric Field

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    Molecular dynamics simulations were used to investigate the transport properties of water–methanol solution getting through a carbon nanotube (CNT) with an oscillating electric field. Eight alternating electric fields with different oscillation periods were used in this work. Under the oscillating electric field, water molecules have the advantage of occupying a CNT over methanol molecules. Meanwhile, the space occupancy of water–methanol solution in the CNT increases as the oscillating period increases. More importantly, we found that the oscillating period of electric field affects the van der Waals interaction of the solution inside the CNT and the shell of the CNT, which results in the change in the number of hydrogen bonds in the water–methanol solution confined in the CNT. And the change in the hydrogen-bond network leads to the change in transport properties of water–methanol solution

    In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids

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    Coordination polymers (CPs) of metal ions are central to a large variety of applications, such as catalysis and separations. These polymers frequently occur as amorphous solids that segregate from solution. The structural aspects of this segregation remain elusive due to the dearth of the spectroscopic techniques and computational approaches suitable for probing such systems. Therefore, there is a lacking of understanding of how the molecular building blocks give rise to the mesoscale architectures that characterize CP materials. In this study we revisit a CP phase formed in the extraction of trivalent lanthanide ions by diesters of the phosphoric acid, such as the bis­(2-ethylhexyl)­phosphoric acid (HDEHP). This is a well-known system with practical importance in strategic metals refining and nuclear fuel reprocessing. A CP phase, referred to as a “third phase”, has been known to form in these systems for half a century, yet the structure of the amorphous solid is still a point of contention, illustrating the difficulties faced in characterizing such materials. In this study, we follow a deductive approach to solving the molecular structure of amorphous CP phases, using semiempirical calculations to set up an array of physically plausible models and then deploying a suite of experimental techniques, including optical, magnetic resonance, and X-ray spectroscopies, to consecutively eliminate all but one model. We demonstrate that the “third phase” consists of hexagonally packed linear chains in which the lanthanide ions are connected by three O–P–O bridges, with the modifying groups protruding outward, as in a bottlebrush. The tendency to yield linear polynuclear oligomers that is apparent in this system may also be present in other systems yielding the “third phase”, demonstrating how molecular geometry directs polymeric assembly in hybrid materials. We show that the packing of bridging molecules is central to directing the structure of CP phases and that by manipulating the steric requirements of ancillary groups one can control the structure of the assembly

    Fabrication of Smart pH-Responsive Fluorescent Solid-like Giant Vesicles by Ionic Self-Assembly Strategy

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    A fluorescent solid-like giant vesicle was prepared by using an anionic dye methyl orange (MO) and an oppositely charged surfactant 1-tetra­decyl-3-methyl­imida­zolium bromide (C<sub>14</sub>mimBr) on the basis of the ionic self-assembly (ISA) strategy. The properties of MO/​C<sub>14</sub>mim­Br complexes were comprehensively characterized. The results indicated that the giant vesicle was formed by the fusion of small vesicles and could keep its original structure during the evaporation of solvent. Besides, the giant vesicles exhibit luminescent property owing to the break of intermolecular π–π stacking of MO, which achieves the transformation from aggregation-caused quenching to aggregation-induced emission by noncovalent interaction. Moreover, MO/​C<sub>14</sub>mim­Br complexes also exhibit smart pH-responsive characteristics and abundant thermic phase behavior. That is, various fluorescent structures (polyhedron, giant vesicle, chrysanthemum, peony-like structure) were obtained when pH ≄ 4, whereas a simple nonfluorescent structure (microflake) was obtained when pH = 2 due to the changes of MO configuration. Thus, the fluorescence behavior can be predicted with the color change directly visible to the naked eye by changing the pH. It is expected that the facile and innovative design of supramolecular material by the ISA strategy could be used as pH detection probes and microreactors

    Thermal/Water-Induced Phase Transformation and Photoluminescence of Hybrid Manganese(II)-Based Chloride Single Crystals

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    Mn(II)-based hybrid halides have attracted great attention from the optoelectronic fields due to their nontoxicity, special luminescent properties, and structural diversity. Here, two novel organic–inorganic hybrid Mn(II)-based halide single crystals (1-mpip)MnCl4·3H2O and (1-mpip)2MnCl6 (1-mpip = 1-methylpiperazinium, C5H14N2+) were grown by a slow evaporation method in ambient atmosphere. Interestingly, (1-mpip)2MnCl6 single crystals exhibit the green emission with a PL peak at 522 nm and photoluminescence quantum yields (PLQYs) of ≈5.4%, whereas (1-mpip)MnCl4·3H2O single crystals exhibit no emission characteristics. More importantly, there exists a thermal-induced phase transformation from (1-mpip)MnCl4·3H2O to emissive (1-mpip)2MnCl6 at 372 K. Moreover, a reversible luminescent conversion between (1-mpip)MnCl4·3H2O and (1-mpip)2MnCl6 was simply achieved when heated to 383 K and placed in a humid environment or sprayed with water. This work not only deepens the understanding of the thermal-induced phase transformation and humidity-sensitive luminescent conversion of hybrid Mn(II)-based halides, but also provides a guidance for thermal and humidity sensing and anticounterfeiting applications of these hybrid materials

    Spectroscopy and Luminescence Dynamics of Ce<sup>3+</sup> and Sm<sup>3+</sup> in LiYSiO<sub>4</sub>

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    The lithium yttrium silicate series of LiY<sub>1–<i>x</i></sub>Ln<sub><i>x</i></sub>SiO<sub>4</sub> exhibits superb chemical and optical properties, and with Ln = Ce<sup>3+</sup>, Sm<sup>3+</sup>, its spectroscopic characteristics and luminescence dynamics are investigated in the present work. Energy transfer and nonradiative relaxation dramatically influence the Ln<sup>3+</sup> luminescence spectra and decay dynamics, especially in the Ce<sup>3+</sup>–Sm<sup>3+</sup> codoped phosphors. It is shown that thermal-quenching of the blue Ce<sup>3+</sup> 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 Sm<sup>3+</sup> ions is the leading mechanism that quenches the red Sm<sup>3+</sup> luminescence. In the codoped systems, Ce<sup>3+</sup>–Sm<sup>3+</sup> energy transfer in competing with the thermal quenching enhance the emission from Sm<sup>3+</sup>. 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

    Tuning of Photoluminescence by Cation Nanosegregation in the (CaMg)<sub><i>x</i></sub>(NaSc)<sub>1–<i>x</i></sub>Si<sub>2</sub>O<sub>6</sub> Solid Solution

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    Controlled photoluminescence tuning is important for the optimization and modification of phosphor materials. Herein we report an isostructural solid solution of (CaMg)<sub><i>x</i></sub>(NaSc)<sub>1–<i>x</i></sub>Si<sub>2</sub>O<sub>6</sub> (0 < <i>x</i> < 1) in which cation nanosegregation leads to the presence of two dilute Eu<sup>2+</sup> centers. The distinct nanodomains of isostructural (CaMg)­Si<sub>2</sub>O<sub>6</sub> and (NaSc)­Si<sub>2</sub>O<sub>6</sub> contain a proportional number of Eu<sup>2+</sup> ions with unique, independent spectroscopic signatures. Density functional theory calculations provided a theoretical understanding of the nanosegregation and indicated that the homogeneous solid solution is energetically unstable. It is shown that nanosegregation allows predictive control of color rendering and therefore provides a new method of phosphor development
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