11 research outputs found
CeâO Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics
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 ionicâcovalent mixed
bonding at a 6H site in the same crystalline compound provides an
ideal system for comparative studies of ionâligand 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 CeâO coordination
at the two Ce<sup>3+</sup> sites. The calculated eigenstates of Ce<sup>3+</sup> at the 6H site suggest a weak CeâO 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
CeâO 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 ionicâcovalent
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>
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 InokutiâHirayama model of energy transfer
(ET), and the site occupancy is confirmed by emission spectra of Eu<sup>3+</sup>. ET and metalâmetal 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
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 3Ï ground state into the <i>f</i><sub>ÎŽ,Ï</sub> orbitals of uranyl. The HuangâRhys 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 FranckâCondon 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 Oî»Uî»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>LaOÂ(BO<sub>3</sub>)<sub>3</sub>:Eu<sup>3+</sup> and Ca<sub>4</sub>EuOÂ(BO<sub>3</sub>)<sub>3</sub>
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>LaOÂ(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>EuOÂ(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 (âŒ330
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 WaterâMethanol Solution through a CNT with Oscillating Electric Field
Molecular
dynamics simulations were used to investigate the transport properties
of waterâmethanol 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 waterâmethanol 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 waterâmethanol
solution confined in the CNT. And the change in the hydrogen-bond
network leads to the change in transport properties of waterâmethanol
solution
In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids
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 bisÂ(2-ethylhexyl)Âphosphoric
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 âthird phaseâ, 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 âthird phaseâ consists of hexagonally packed linear
chains in which the lanthanide ions are connected by three OâPâO
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 âthird
phaseâ, 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
A fluorescent
solid-like giant vesicle was prepared by using an
anionic dye methyl orange (MO) and an oppositely charged surfactant
1-tetraÂdecyl-3-methylÂimidaÂzolium bromide (C<sub>14</sub>mimBr) on the basis of the ionic self-assembly (ISA) strategy.
The properties of MO/âC<sub>14</sub>mimÂBr 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 ÏâÏ stacking of MO, which achieves
the transformation from aggregation-caused quenching to aggregation-induced
emission by noncovalent interaction. Moreover, MO/âC<sub>14</sub>mimÂBr 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 â„ 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
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 organicâinorganic
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 â5.4%, 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>
The lithium yttrium silicate series
of LiY<sub>1â<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>âSm<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 dipoleâdipole
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>âSm<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 ionâion and ionâlattice
interactions
Tuning of Photoluminescence by Cation Nanosegregation in the (CaMg)<sub><i>x</i></sub>(NaSc)<sub>1â<i>x</i></sub>Si<sub>2</sub>O<sub>6</sub> Solid Solution
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>1â<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)ÂSi<sub>2</sub>O<sub>6</sub> and (NaSc)ÂSi<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