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General Synthesis of Ordered Mesoporous Rare-Earth Orthovanadate Thin Films and Their Use as Photocatalysts and Phosphors for Lighting Applications
No full textThe block copolymer templating sol–gel synthesis of a novel class of ternary oxide nanomaterials is reported. NdVO, EuVO, GdVO, DyVO, YVO, and TmVO have been prepared as open mesoporous films by the dip-coating method using hydrated rare-earth nitrate salt precursors along with vanadium oxytrichloride. All materials crystallize in the tetragonal ZrSiO-type structure with space group I4/amd. Short-term treatment at 550 °C is found sufficient to initiate crystallization. Characterization via X-ray and electron diffraction, Raman and X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry confirms the single-phase nature and uniformity of the different orthovanadates with tailorable crystallite sizes. The integrated results from electron and atomic force microscopy, Kr physisorption, and in situ and ex situ synchrotron-based small-angle X-ray scattering reveal that the porosity persists throughout the thickness of films, and the mesoscopic ordering is retained even after heating in air at 700 °C. Photobleaching experiments indicate that the sol–gel derived materials, showing an indirect band gap transition at 3.8 ± 0.1 eV, exhibit good photocatalytic properties—the activity is highly superior to that of bulk films of the same nominal composition. Moreover, when doping GdVO, YVO, and solid solution GdVO–YVO with trivalent rare-earth ions such as Eu, Dy, Er, or Tm ions, the films hold promise as phosphors for lighting applications, which might pave the way toward development of (3-dimensional) intricate nanocomposites with unprecedented functionalities
Robust and Accurate Computational Estimation of the Polarizability Tensors of Macromolecules
No full textAlignment of molecules through electric fields minimizes the averaging over orientations, e.g., in single-particle-imaging experiments. The response of molecules to external ac electric fields is governed by their polarizability tensor, which is usually calculated using quantum chemistry methods. These methods are not feasible for large molecules. Here, we calculate the polarizability tensor of proteins using a regression model that correlates the polarizabilities of the 20 amino acids with perfect conductors of the same shape. The dielectric constant of the molecules could be estimated from the slope of the regression line based on the Clausius–Mossotti equation. We benchmark our predictions against the quantum chemistry results for the Trp cagemini protein and the measured dielectric constants of larger proteins. Our method has applications in computing laser alignment of macromolecules, for instance, benefiting single-particle imaging, as well as for estimation of the optical and electrostatic characteristics of proteins and other macromolecules
On site-selective optically and thermally induced processes in storage phosphors
No full textPhoto- thermo- and optically stimulated luminescence properties of ceramics sintered at 1700 °C in air were investigated. Low temperature (10 K) excitation and emission spectra using synchrotron excitation in the range of 150–330 nm are also discussed. The effect of the dopant contents on the various luminescence effects and processes was tackled. The ceramics showed intense thermoluminescence (TL) and the glow curve consisted of two main peaks around 170 and 250 °C upon 5 °C/s heating rate. The shape of the glow curve and TL intensity depended strongly on the dopant concentrations. Above 0.1% of their contents the TL quickly lessened to disappear around 1%. This was in contrary to photoluminescence which hardly showed any quenching up to the concentration of 1%. In addition to the regular first order TL kinetics some contribution from tunneling and semi-localized transitions was proved
Trends in Synthesis, Crystal Structure, and Thermal and Magnetic Properties of Rare-Earth Metal Borohydrides
No full textSynthesis, crystal structures, and thermal and magnetic properties of the complete series of halide-free rare-earth (RE) metal borohydrides are presented. A new synthesis method provides high yield and high purity products. Fifteen new metal borohydride structures are reported. The trends in crystal structures, thermal behavior, and magnetic properties for the entire series of RE(BH4)x are compared and discussed. The RE(BH4)x possess a very rich crystal chemistry, dependent on the oxidation state and the ionic size of the rare-earth ion. Due to the lanthanide contraction, there is a significant decrease in the volume of the RE3+-ion with increasing atomic number, which correlates linearly with the unit cell volume of the α- and β-RE(BH4)3 polymorphs and the solvated complexes α-RE(BH4)3·S(CH3)2. The thermal analysis reveals a one-step decomposition pathway in the temperature range from 247 to 277 °C for all RE(BH4)3 except Lu(BH4)3, which follows a three-step decomposition pathway. In contrast, the RE(BH4)2 decompose at higher temperatures in the range 306 to 390 °C due to lower charge density on the rare-earth ion. The RE(BH4)3 show increasing stability with increasing Pauling electronegativity, which contradicts other main group and transition metal borohydrides. The majority of the compounds follow Curie–Weiss paramagnetic behavior down to 3 K with weak antiferromagnetic interactions and magnetic moments in accord with those of isolated 4f ions. Some of the RE(BH4)x display varying degrees of temperature-dependent magnetic moments due to low-lying excited stated induced by crystal field effects. Additionally, a weak antiferromagnetic ordering is observed in Gd(BH4)3, indicating superexchange through a borohydride group
Self-assembly of block copolymers during hollow fiber spinning: an in situ small-angle X-ray scattering study
No full textWe investigated the self-assembly of block copolymers during hollow fiber membrane (HFM) fabricationby conducting in situ small angle X-ray scattering (SAXS) and ex situ scanning electron microscopy (SEM)studies. SAXS enables us to follow the structural rearrangements after extrusion at different distances fromthe spinning nozzle. The kinetics of the spinning process is examined as a function of the composition ofblock copolymer solutions and the spinning parameters. We studied the influence of the extrusion rate onthe block copolymer microdomains and their self-assembly in weakly segregated and ordered solutions.The addition of magnesium acetate (MgAc2) leads to the ordering of micelles in the block copolymersolution already at lower polymer concentrations and shows an increased number of micelles with largerdomain spacing as compared to the pristine solution. The SAXS data show the effect of shear within thespinneret on the self-assembly of block copolymers and the kinetics of phase separation after extrusion.It is observed that the ordering of micelles in solutions is decreased as indicated by the loss of crystallinitywhile high extrusion rates orient the structures perpendicular to the fiber direction. The structural featuresobtained from in situ SAXS experiments are correlated to the structure in the block copolymer solutionsin the absence of shear and the morphologies in flat sheet and HF membranes obtained by ex situ SEM.This allows a systematic and comparative study of the effects varying the microdomain ordering withindifferent block copolymer solutions and the formed membrane structures
Teraelectronvolt emission from the γ-ray burst GRB 190114C
No full textLong-duration γ-ray bursts (GRBs) are the most luminous sources of electromagnetic radiation known in the Universe. They arise from outflows of plasma with velocities near the speed of light that are ejected by newly formed neutron stars or black holes (of stellar mass) at cosmological distances1,2. Prompt flashes of megaelectronvolt-energy γ-rays are followed by a longer-lasting afterglow emission in a wide range of energies (from radio waves to gigaelectronvolt γ-rays), which originates from synchrotron radiation generated by energetic electrons in the accompanying shock waves3,4. Although emission of γ-rays at even higher (teraelectronvolt) energies by other radiation mechanisms has been theoretically predicted5,6,7,8, it has not been previously detected7,8. Here we report observations of teraelectronvolt emission from the γ-ray burst GRB 190114C. γ-rays were observed in the energy range 0.2–1 teraelectronvolt from about one minute after the burst (at more than 50 standard deviations in the first 20 minutes), revealing a distinct emission component of the afterglow with power comparable to that of the synchrotron component. The observed similarity in the radiated power and temporal behaviour of the teraelectronvolt and X-ray bands points to processes such as inverse Compton upscattering as the mechanism of the teraelectronvolt emission9,10,11. By contrast, processes such as synchrotron emission by ultrahigh-energy protons10,12,13 are not favoured because of their low radiative efficiency. These results are anticipated to be a step towards a deeper understanding of the physics of GRBs and relativistic shock waves
Silica-based oxyfluoride glass and glass-ceramic doped with and -VUV-VIS-NIR spectroscopy and optical thermometry
No full textOptical, structural and morphological properties of Tm3+/Yb3+ co-doped NaYF4-based nano-glass-ceramic and a precursor glass have been investigated. The controlled heat-treatment of multicomponent oxyfluoride glass at 640 °C for 2 h was used to obtain crystalline precipitations. The differences in optical characteristics have been discussed analysing spectra recorded within wide VUV–VIS–NIR spectral range and considering relaxation dynamic of involved excited states. Optical spectra and luminescence decay curves were measured as a function of temperature in the range of 295–725 K. Based on up-converted emission spectra excited at 975 nm, temperature sensing properties of glass-ceramic were verified. The fluorescence intensity ratio between the up-converted emission bands located at around 795 nm (Tm3+:3H4–3H6) and 695 nm (Tm3+:3F3,2–3H6) was studied. The maximum relative temperature sensitivity was determined to be 0.35% K−1 at 445 K
Thermodynamics of the S2-to-S3 State Transition of the Oxygen-Evolving Complex of Photosystem II
No full textThe room temperature pump-probe X-ray free electron laser (XFEL) measurements used for serial femtosecond crystallography provide remarkable information about the structures of the catalytic (S-state) intermediates of the oxygen-evolution reaction of photosystem II. However, mixed populations of these intermediates and moderate resolution limit the interpretation of the data from current experiments. Here, we use Boltzmann statistics and Monte Carlo sampling to provide a model for the S2-to-S3 state transition, allowing structural changes and the insertion of an additional water/hydroxide. Based on our model, water/hydroxide addition to the oxygen-evolving complex (OEC) is not thermodynamically favorable in the S2 g = 2 state, but it is in the S2 g = 4.1 redox isomer. Thus, formation of the S3 state starts by a transition from the S2 g = 2 to the S2 g = 4.1 structure. Then, electrostatic interactions support protonation of D1-H190 and deprotonation of the Ca2+-ligated water (W3) with proton loss to the lumen. The W3 hydroxide moves toward Mn4, completing the coordination shell of Mn4 and allowing its oxidation to the Mn(IV) state. In addition, we observe a conformational change of D1-E189 in the S2 g = 4.1 and S3 structures that is caused by binding an additional hydroxide to Mn1 accompanied by an increase in the population of protonated D1-E189 in the S3 state
The Different Regimes of Axion Gauge Field Inflation
No full textIn axion gauge field inflation an axion-like particle driving cosmic inflation is coupled to the Chern-Simons density of an Abelian or non-Abelian gauge group. In the case of a non-Abelian gauge group, this can lead to the formation of a stable, homogeneous and isotropic gauge field background. We study the dynamics of the inflaton and gauge fields in terms of the two effective coupling parameters: the gauge coupling and the axion decay constant. Starting from the Bunch-Davies vacuum in the far past, we find that the non-trivial gauge field background arises only significantly after the cosmic microwave background (CMB) scales have left the horizon. At these scales, the model thus closely resembles Abelian axion inflation, thereby naturally reconciling the tension of non-Abelian axion gauge field inflation with the latest CMB observations. We further consider two exemplary UV-completions of this setup: multiple Peccei-Quinn axions and axion monodromy in string theory. In both cases we find that the majority of the parameter space is excluded by theoretical or observational constraints. The remaining parameter space can be divided into three regimes. (i) For small gauge couplings we recover natural inflation. For large gauge couplings the non-Abelian gauge theory either (ii) mimics the Abelian theory or (iii) non-linear interactions prohibit a linear analysis of the gauge field perturbations
Defect activation and annihilation in CIGS solar cells: an operando X-ray microscopy study
No full textThe efficiency of thin-film solar cells with a CuIn1-xGaxSe2 absorber is limited by nanoscopic inhomogeneities and defects. Traditional characterization methods are challenged by the multi-scale evaluation of the performance at defects that are buried in the device structures. Multi-modal X-ray microscopy offers a unique tool-set to probe the performance in fully assembled solar cells, and to correlate the performance with composition down to the micro- and nanoscale. We applied this approach to the mapping of temperature-dependent recombination for CuIn1-xGaxSe2 solar cells with different absorber grain sizes, evaluating the same areas from room temperature to 100 C. It was found that poor performing areas in the large-grain sample are correlated with a Cu-deficient phase, whereas defects in the small-grain sample are not correlated with the distribution of Cu. In both samples, classes of recombination sites were identified, where defects were activated or annihilated by temperature. More generally, the methodology of combined operando and in-situ X-ray microscopy was established at the physical limit of spatial resolution given by the device itself. As proof-of-principle, the measurement of nanoscopic current generation in a solar cell is demonstrated with applied bias voltage and bias light