Dichroic tomography is a 3D imaging technique in which the polarization of the incident beam is used to induce contrast due to the magnetization or orientation of a sample. The aim is to reconstruct ...not only the optical density but the dichroism of the sample. The theory of dichroic tomographic and laminographic imaging in the parallel-beam case is discussed as well as the problem of reconstruction of the sample’s optical properties. The set of projections resulting from a single tomographic/laminographic measurement is not sufficient to reconstruct the magnetic moment for magnetic circular dichroism unless additional constraints are applied or data are taken at two or more tilt angles. For linear dichroism, three polarizations at a common tilt angle are insufficient for unconstrained reconstruction. However, if one of the measurements is done at a different tilt angle than the other, or the measurements are done at a common polarization but at three distinct tilt angles, then there is enough information to reconstruct without constraints. Possible means of applying constraints are discussed. Furthermore, it is shown that for linear dichroism, the basic assumption that the absorption through a ray path is the integral of the absorption coefficient, defined on the volume of the sample, along the ray path, is not correct when dichroism or birefringence is strong. This assumption is fundamental to tomographic methods. An iterative algorithm for reconstruction of linear dichroism is demonstrated on simulated data.
Do corals form their skeletons by precipitation from solution or by attachment of amorphous precursor particles as observed in other minerals and biominerals? The classical model assumes ...precipitation in contrast with observed “vital effects,” that is, deviations from elemental and isotopic compositions at thermodynamic equilibrium. Here, we show direct spectromicroscopy evidence in Stylophora pistillata corals that two amorphous precursors exist, one hydrated and one anhydrous amorphous calcium carbonate (ACC); that these are formed in the tissue as 400-nm particles; and that they attach to the surface of coral skeletons, remain amorphous for hours, and finally, crystallize into aragonite (CaCO₃). We show in both coral and synthetic aragonite spherulites that crystal growth by attachment of ACC particles is more than 100 times faster than ion-by-ion growth from solution. Fast growth provides a distinct physiological advantage to corals in the rigors of the reef, a crowded and fiercely competitive ecosystem. Corals are affected by warming-induced bleaching and postmortem dissolution, but the finding here that ACC particles are formed inside tissue may make coral skeleton formation less susceptible to ocean acidification than previously assumed. If this is how other corals form their skeletons, perhaps this is how a few corals survived past CO₂ increases, such as the Paleocene–Eocene Thermal Maximum that occurred 56 Mya.
The quantitative determination of the carbon hybridization is critical for establishing processing-structure-properties relationships for carbon-based materials, including amorphous carbon coatings. ...While several techniques have been employed to characterize the amount of sp2 and sp3 carbon in these materials, direct comparisons between analytical results are limited. Here, we compare near edge X-ray absorption fine structure (NEXAFS) spectra of a silicon- and oxygen-containing hydrogenated amorphous carbon (a-C:H:Si:O) coating acquired in synchrotron-based scanning transmission X-ray microscopy (STXM) mode with electron energy loss spectra (EELS) obtained from the same a-C:H:Si:O lamella. While the fractions of sp2 carbon computed from STXM and EELS spectra are in close agreement, the comparison of NEXAFS spectra acquired in STXM mode with NEXAFS spectra collected in partial electron yield mode on a flat a-C:H:Si:O surface indicated that the destructive preparation of thin lamellae for STXM analyses induces variations in the structure of a-C:H:Si:O, namely the breakage of carbon-silicon and carbon-hydrogen bonds, a change in ordering of sp2-bonded carbon, and an increase in the sp2 carbon fraction. These findings can help scientists in the careful interpretation of spectroscopic results obtained from the analysis of samples made of metastable materials after the destructive preparation of specimens for analytical purposes.
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The valence states of Mn in mixed-valent layer and tunnel structure manganese dioxides (MnO2), usually referred to as phyllomanganates and tectomanganates, can be measured by X-ray absorption ...near-edge structure (XANES) spectroscopy with a precision and accuracy that are difficult to estimate owing to the paucity of well-characterized standards. A compilation of the Mn K-edge XANES spectra of most naturally occurring manganates, synthetic analogs of known structure and chemical composition, and pure-valence phase species is presented and made available as an open source. We intend this compilation to serve as a basis for the spectroscopic determination of the fractions of the Mn 2+, 3+, and 4+ valences in mixed-valent manganates and phase mixtures. The XANES derivatives of tectomanganates and phyllomanganates with no or little Mn3+ in the MnO2 layer exhibit intensities, shapes, and relative energy positions of the main features characteristics of a particular valence composition. For these compounds, valence fractions can be derived using linear combination fitting analysis. Best quantitative results are obtained when the unknown spectrum is fit to a weighted sum of all reference spectra in the database with the fractions of species constrained to be non-negative (Combo method). The accuracy of the average valence is estimated to 0.04 v.u. in the range of 3+ to 4+, and decreases when the proportion of divalent Mn is higher than 15%. The accuracy of the method is also lower in (layer Mn3+, Mn4+) manganates, because the XANES features are affected non-additively by the amount and distribution of the Jahn-Teller Mn3+ cations. The merit of the Combo method for the determination of manganese valence sums relative to the methods based on calibration curves is discussed.
Here efficient organic photovoltaic devices fabricated from water‐based colloidal dispersions with donor:acceptor composite nanoparticles achieving up to 9.98% power conversion efficiency (PCE) are ...reported. This high efficiency for water processed organic solar cells is attributed to morphology control by surface energy matching between the donor and the acceptor materials. Indeed, due to a low interfacial energy between donor and the acceptor, no large phase separation occurs during the nanoparticle formation process as well as upon thermal annealing. Indeed, synchrotron‐based scanning transmission X‐ray microscopy reveals that the internal morphology of composite nanoparticles is intermixed as well as the active layer morphology after thermal treatment. The PCE of this system reaches 85% that of devices prepared from chlorinated solvent. The gap between water‐based inks and organic solvent‐based inks gets narrower, which is promising for the development of eco‐friendly processing and fabrication of organic photovoltaics.
This article describes the fabrication of organic photovoltaic devices from water‐based nanoparticle dispersions. By careful control of the surface energies, optimal nanoparticle morphology is achieved, leading to devices with up to 9.98% power conversion efficiency. This study opens the route for environmentally friendly processes for organic photovoltaics.
Analyzer‐free linear dichroic ptychography Chang, Huibin; Marcus, Matthew A.; Marchesini, Stefano
Journal of applied crystallography,
October 2020, 2020-10-01, 20201001, Volume:
53, Issue:
5
Journal Article
Peer reviewed
Open access
Linear dichroism is an important tool to characterize the transmission matrix and determine the crystal or orbital orientation in a material. In order to achieve high‐resolution mapping of ...transmission properties, the linear‐dichroism scattering model in ptychographic imaging is introduced, and an efficient two‐stage reconstruction algorithm is developed. Using the proposed algorithm on a uniaxial material, the dichroic transmission matrix can be recovered without an analyzer by using ptychography measurements with as few as three different polarization angles, with the help of an empty region to remove phase ambiguities.
To characterize the transmission matrix and determine the crystal or orbital orientation in a material with high‐resolution mapping of transmission properties, the linear‐dichroism scattering model in ptychographic imaging is introduced. Via the proposed efficient two‐stage reconstruction algorithm, the dichroic transmission matrix can be recovered without an analyzer by using ptychography measurements with as few as three different polarization angles.
Solution-processed films of colloidal aliovalent niobium-doped anatase TiO2 nanocrystals exhibit modulation of optical transmittance in two spectral regionsnear-infrared (NIR) and visible lightas ...they undergo progressive and reversible charging in an electrochemical cell. The Nb-TiO2 nanocrystal film supports a localized surface plasmon resonance in the NIR, which can be dynamically modulated via capacitive charging. When the nanocrystals are charged by insertion of lithium ions, inducing a well-known structural phase transition of the anatase lattice, strong modulation of visible transmittance is observed. Based on X-ray absorption near-edge spectroscopy, the conduction electrons localize only upon lithium ion insertion, thus rationalizing the two modes of optical switching observed in a single material. These multimodal electrochromic properties show promise for application in dynamic optical filters or smart windows.
Layered double hydroxides (LDH) have been extensively investigated for charge storage, however, their development is hampered by the sluggish reaction dynamics. Herein, triggered by mismatching ...integration of Mn sites, we configured wrinkled Mn/NiCo-LDH with strains and defects, where promoted mass & charge transport behaviors were realized. The well-tailored Mn/NiCo-LDH displays a capacity up to 518 C g
(1 A g
), a remarkable rate performance (78%@100 A g
) and a long cycle life (without capacity decay after 10,000 cycles). We clarified that the moderate electron transfer between the released Mn species and Co
serves as the pre-step, while the compressive strain induces structural deformation with promoted reaction dynamics. Theoretical and operando investigations further demonstrate that the Mn sites boost ion adsorption/transport and electron transfer, and the Mn-induced effect remains active after multiple charge/discharge processes. This contribution provides some insights for controllable structure design and modulation toward high-efficient energy storage.
Highly reactive dense Pt single-atoms stabilized on an oxide support can resolve a grand challenge in the economic use of Pt in catalysis. The maximized number density of reaction sites provided by ...dense Pt single-atoms guarantees the improved catalytic performance of Pt combined with high efficiency. By manipulating the chemical nature of multi-component interfaces, we synthesized CO-tolerant dense Pt single-atoms highly reactive for the CO oxidation reaction, which governs the key steps for chemical energy conversion and emission control. The addition of 1 wt% of Ce to TiO
2
support particles creates a CeO
x
-TiO
2
interface that stabilizes Pt single-atoms by strong electronic interactions. Dense Pt single-atoms formed on CeO
x
/TiO
2
oxides exhibit 15.1 times greater specific mass activity toward CO oxidation at 140 °C compared with a bare Pt/TiO
2
catalyst. We elaborate how the CeO
x
-TiO
2
interfaces activate the interface-mediated Mars-van Krevelen mechanism of CO oxidation and protect Pt single-atoms from CO-poisoning. Through a comprehensive interpretation of the formation and activation of dense Pt single-atoms using
operando
X-ray absorption spectroscopy, density functional theory calculations, and experimental catalyst performance tests, we provide a key that enables the catalytic performance of noble metal single-atom catalysts to be optimized by atomic-scale tuning of the metal-support interface.
Catalytic supremacy of Pt-single atoms achieved by CeO
x
-TiO
2
interfaces.
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