2,065 research outputs found
The Phosphatic chalk of the Mons Basin, Belgium:petrography and geochemistry of the Ciply Phosphatic Chalk and implications on its genesis
Abstract. The Ciply Phosphatic Chalk (CPC) has been exploited for its enrichment in phosphorus in the early part of the twentieth century to produce fertilisers. Regained interests stimulated new research to characterise the potential for rare earth elements endowment and propose a genetic model for the formation of this phosphate deposit. This work studied the CPC using scanning electron microscopy, X-ray diffraction, cathodoluminescence and geochemical analyses. New insights into the formation of the deposits have been obtained regarding the mode of formation of the deposit. First, the deposit clearly shows evidence of alternating phase of phosphatisation and reworking giving rise to the formation of phosintraclasts which are the dominant phosphatic grains of the deposit. Weak or moderate upwellings brought nutrients to the Mons Basin during a period of sea-level highstand. Negative Ce-anomaly and the presence of bioturbation strongly argue against the development of an important oxygen-minimum zone indicating a low-productivity system. In these conditions, Fe-oxyhydroxides might have played an important role in scavenging phosphorus from the water column to the sediment. Phosphatisation seems to have occurred in the sediment when supersaturation relative to francolite was reached in the sediment pore water. Reworking processes probably consisted of wave action during storms. The REE are hosted in francolite and possess an average ΣREE of 350 ppm for the Hyon borehole. Their shale-normalised patterns are similar to other Cretaceous phosphate deposits but also to Cambrian deposits of China which typically display negative Ce-anomaly and HREE depletion. Post-depositional processes are only evident for the “phosphatic sands” which are strongly enriched in both phosphorus and REE. Further studies should aim to better constrain the paleoenvironmental conditions of deposition using stable isotopic studies. LA-ICP-MS studies should be able to give interesting insights into the compositional variations of the phosintraclasts thus giving elements to better characterise the genetic model of the CPC
Excited states with selected CI-QMC: chemically accurate excitation energies and geometries
We employ quantum Monte Carlo to obtain chemically accurate vertical and
adiabatic excitation energies, and equilibrium excited-state structures for the
small, yet challenging, formaldehyde and thioformaldehyde molecules. A key
ingredient is a robust protocol to obtain balanced ground- and excited-state
Jastrow-Slater wave functions at a given geometry, and to maintain such a
balanced description as we relax the structure in the excited state. We use
determinantal components generated via a selected configuration interaction
scheme which targets the same second-order perturbation energy correction for
all states of interest at different geometries, and we fully optimize all
variational parameters in the resultant Jastrow-Slater wave functions.
Importantly, the excitation energies as well as the structural parameters in
the ground and excited states are converged with very compact wave functions
comprising few thousand determinants in a minimally augmented double-
basis set. These results are obtained already at the variational Monte Carlo
level, the more accurate diffusion Monte Carlo method yielding only a small
improvement in the adiabatic excitation energies. We find that matching
Jastrow-Slater wave functions with similar variances can yield excitations
compatible with our best estimates; however, the variance-matching procedure
requires somewhat larger determinantal expansions to achieve the same accuracy,
and it is less straightforward to adapt during structural optimization in the
excited state.Comment: 11 pages, 4 figure
Assessing Excited State Energy Gaps with Time-Dependent Density Functional Theory on Ru(II) Complexes
A set of density functionals coming from different rungs on Jacob's ladder
are employed to evaluate the electronic excited states of three Ru(II)
complexes. While most studies on the performance of density functionals compare
the vertical excitation energies, in this work we focus on the energy gaps
between the electronic excited states, of the same and different multiplicity.
Excited state energy gaps are important for example to determine radiationless
transition probabilities. Besides energies, a functional should deliver the
correct state character and state ordering. Therefore, wavefunction overlaps
are introduced to systematically evaluate the effect of different functionals
on the character of the excited states. As a reference, the energies and state
characters from multi-state second-order perturbation theory complete active
space (MS-CASPT2) are used. In comparison to MS-CASPT2, it is found that while
hybrid functionals provide better vertical excitation energies, pure
functionals typically give more accurate excited state energy gaps. Pure
functionals are also found to reproduce the state character and ordering in
closer agreement to MS-CASPT2 than the hybrid functionals
Effect of sensitization on the electrochemical properties of nanostructured NiO
Screen-printed NiO electrodes were sensitized with 11 different dyes and the respective
electrochemical properties were analyzed in a three-electrode cell with the techniques of cyclic
voltammetry and electrochemical impedance spectroscopy. The dye sensitizers of NiO were organic
molecules of different types (e.g., squaraines, coumarins, and derivatives of triphenyl-amines and
erythrosine B), which were previously employed as sensitizers of the same oxide in dye-sensitized
solar cells of p-type (p-DSCs). Depending on the nature of the sensitizer, diverse types of interactions
occurred between the immobilized sensitizer and the screen-printed NiO electrode at rest and
under polarization. The impedance data recorded at open circuit potential were interpreted in
terms of two different equivalent circuits, depending on the eventual presence of the dye sensitizer
on the mesoporous electrode. The fitting parameter of the charge transfer resistance through the
electrode/electrolyte interface varied in accordance to the differences of the passivation action exerted
by the various dyes against the electrochemical oxidation of NiO. Moreover, it has been observed that
the resistive term RCT associated with the process of dark electron transfer between the dye and NiO
substrate is strictly correlated to the overall efficiency of the photoconversion () of the corresponding
p-DSC, which employs the same dye-sensitized electrode as photocathode
General Approach To Compute Phosphorescent OLED Efficiency
Phosphorescent organic light-emitting diodes (PhOLEDs) are widely used in the
display industry. In PhOLEDs, cyclometalated Ir(III) complexes are the most
widespread triplet emitter dopants to attain red, e.g., Ir(piq)3 (piq =
1-phenylisoquinoline), and green, e.g., Ir(ppy)3 (ppy = 2-phenylpyridine),
emissions, whereas obtaining operative deep-blue emitters is still one of the
major challenges. When designing new emitters, two main characteristics besides
colors should be targeted: high photostability and large photoluminescence
efficiencies. To date, these are very often optimized experimentally in a
trial-and-error manner. Instead, accurate predictive tools would be highly
desirable. In this contribution, we present a general approach for computing
the photoluminescence lifetimes and efficiencies of Ir(III) complexes by
considering all possible competing excited-state deactivation processes and
importantly explicitly including the strongly temperature-dependent ones. This
approach is based on the combination of state-of-the-art quantum chemical
calculations and excited-state decay rate formalism with kinetic modeling,
which is shown to be an efficient and reliable approach for a broad palette of
Ir(III) complexes, i.e., from yellow/orange to deep-blue emitters
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Comparison of the transition dipole moments calculated by TDDFT with high level wavefunction theory
Time dependent density functional theory (TDDFT) is widely used to simulate the excited states of organic and inorganic molecules. We calculate the transition dipole moments (TDM) for a selection of commonly employed exchange-correlation functionals for a test set of 15 molecules and compare them with both EOM-CC3 and ADC(3) calculated TDMs, which we use as a benchmark. Contemporary range-separated hybrid functionals perform the best for both direction and magnitude, while “pure” local hybrids should be employed with caution
Vibrational fingerprint of the absorption properties of UiO-type MOF materials
The absorption properties of UiO-type metal-organic frameworks are computed using TD-DFT simulations on the organic linkers. A set of nine isoreticular structures, including the UiO-66 and UiO-67 materials and functionalized variants, are examined. The excitation energies from a static geometry optimization are compared with dynamic averages obtained from sampling the ground-state potential energy surface using molecular dynamics. The vibrational modes that impact the excitation energy are identified. This analysis is done using a recently proposed tool based on power spectra of the velocities and the excitation energies. The applied procedure allows including important factors influencing the absorption spectra, such as the periodic framework, linker variation and dynamical effects including harmonic and anharmonic nuclear motions. This methodology allows investigating in detail the vibrational fingerprint of the excitation energy of advanced materials such as MOFs and gives perspectives to tailor materials toward new light-based applications
Synthesis and photochromic properties of a bis(diarylethene)-naphthopyran hybrid
The synthesis and photochromic properties of a triphotochromic molecule consisting of one naphthopyran flanked by two diarylethene units investigated by UV-Visible and NMR spectroscopies are reported. Six different states resulting from the open/closed naphthopyran associated with one or two open/cyclized diarylethenes have been characterized. Switching of the naphthopyran group is possible, independently of the state of the diarylethene groups, permitting the controlled generation of electronically connected diarylethene groups. However, the diarylethene groups cannot be closed if the naphthopyran group is open
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