Prediction and Measurement of Resonant and Nonresonant Shake Effects in the Core-Level X-ray Emission Spectra of 3d Transition Metal Compounds.

Shake effects, resulting from sudden core potential changes during photoexcitation, are well-known in X-ray photoelectron spectroscopy (XPS) and often produce satellite peaks due to many-body excitations. It has been thought, however, that they are negligible in core-to-core X-ray emission spectroscopy (CTC-XES), where the difference in core-hole potentials upon radiative decay is rather small. We demonstrate that shake effects are significant in Kα XES from 3d transition metal systems with nominally zero valence electrons.

The evolution of solvation symmetry and composition in Zn halide aqueous solutions from dilute to extreme concentrations.

The emergence of cation-anion species, or contact ion pairs, is fundamental to understanding the physical properties of aqueous solutions when moving from the ideal, low-concentration limit to the manifestly non-ideal limits of very high solute concentration or constituent ion activity. We focus here on Zn halide solutions both as a model system and also as an exemplar of the applications spanning from (i) electrical energy storage the paradigm of water in salt electrolyte (WiSE) to (ii) the physical chemistry of brines in geochemistry to (iii) the long-standing problem of nucleation. Using a combination of experimental and theoretical approaches we quantify the halide coordination number and changing coordination geometry without embedded use of theoretical equilibrium constants.

Colloidal, Room-Temperature Growth of Metal Oxide Shells on InP Quantum Dots.

We demonstrate colloidal, layer-by-layer growth of metal oxide shells on InP quantum dots (QDs) at room temperature. We show with computational modeling that native InP QD surface oxides give rise to nonradiative pathways due to the presence of surface-localized dark states near the band edges. Replacing surface indium with zinc to form a ZnO shell results in reduced nonradiative decay and a density of states at the valence band edge that resembles defect-free, stoichiometric InP.

Informed Chemical Classification of Organophosphorus Compounds via Unsupervised Machine Learning of X-ray Absorption Spectroscopy and X-ray Emission Spectroscopy.

We analyze an ensemble of organophosphorus compounds to form an unbiased characterization of the information encoded in their X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectra (VtC-XES). Data-driven emergence of chemical classes via unsupervised machine learning, specifically cluster analysis in the Uniform Manifold Approximation and Projection (UMAP) embedding, finds spectral sensitivity to coordination, oxidation, aromaticity, intramolecular hydrogen bonding, and ligand identity. Subsequently, we implement supervised machine learning via Gaussian process classifiers to identify confidence in predictions that match our initial qualitative assessments of clustering.

Conjugated Metal-Organic Macrocycles: Synthesis, Characterization, and Electrical Conductivity.

The dimensional reduction of solids into smaller fragments provides a route to achieve new physical properties and gain deeper insight into the extended parent structures. Here, we report the synthesis of CuTOTP-OR (TOTP = 2,3,6,7-tetraoxidotriphenylene), a family of copper-based macrocycles that resemble truncated fragments of the conductive two-dimensional (2D) metal-organic framework Cu(HHTP) (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene). The planar metal-organic macrocycles self-assemble into ordered nanotubes with internal diameters of ∼2 nm and short interlayer distances of ∼3.

Unsupervised machine learning for unbiased chemical classification in X-ray absorption spectroscopy and X-ray emission spectroscopy.

We report a comprehensive computational study of unsupervised machine learning for extraction of chemically relevant information in X-ray absorption near edge structure (XANES) and in valence-to-core X-ray emission spectra (VtC-XES) for classification of a broad ensemble of sulphorganic molecules. By progressively decreasing the constraining assumptions of the unsupervised machine learning algorithm, moving from principal component analysis (PCA) to a variational autoencoder (VAE) to t-distributed stochastic neighbour embedding (t-SNE), we find improved sensitivity to steadily more refined chemical information. Surprisingly, when embedding the ensemble of spectra in merely two dimensions, t-SNE distinguishes not just oxidation state and general sulphur bonding environment but also the aromaticity of the bonding radical group with 87% accuracy as well as identifying even finer details in electronic structure within aromatic or aliphatic sub-classes.

New Insights into the High-Performance Black Phosphorus Anode for Lithium-Ion Batteries.

Black phosphorus (BP) is a promising anode material in lithium-ion batteries (LIBs) owing to its high electrical conductivity and capacity. However, the huge volume change of BP during cycling induces rapid capacity fading. In addition, the unclear electrochemical mechanism of BP hinders the development of rational designs and preparation of high-performance BP-based anodes.

Surface Functionalization of Black Phosphorus with Nitrenes: Identification of P=N Bonds by Using Isotopic Labeling.

Surface functionalization of two-dimensional crystals is a key path to tuning their intrinsic physical and chemical properties. However, synthetic protocols and experimental strategies to directly probe chemical bonding in modified surfaces are scarce. Introduced herein is a mild, surface-specific protocol for the surface functionalization of few-layer black phosphorus nanosheets using a family of photolytically generated nitrenes (RN) from the corresponding azides.

Probing Sulfur Chemical and Electronic Structure with Experimental Observation and Quantitative Theoretical Prediction of Kα and Valence-to-Core Kβ X-ray Emission Spectroscopy.

An extensive experimental and theoretical study of the Kα and Kβ high-resolution X-ray emission spectroscopy (XES) of sulfur-bearing systems is presented. This study encompasses a wide range of organic and inorganic compounds, including numerous experimental spectra from both prior published work and new measurements. Employing a linear-response time-dependent density functional theory (LR-TDDFT) approach, strong quantitative agreement is found in the calculation of energy shifts of the core-to-core Kα as well as the full range of spectral features in the valence-to-core Kβ spectrum.

Resonant inelastic X-ray scattering using a miniature dispersive Rowland refocusing spectrometer.

X-ray absorption spectroscopy (XAS) beamlines worldwide are steadily increasing their emphasis on full photon-in/photon-out spectroscopies, such as resonant inelastic X-ray scattering (RIXS), resonant X-ray emission spectroscopy (RXES) and high energy resolution fluorescence detection XAS (HERFD-XAS). In such cases, each beamline must match the choice of emission spectrometer to the scientific mission of its users. Previous work has recently reported a miniature tender X-ray spectrometer using a dispersive Rowland refocusing (DRR) geometry that functions with high energy resolution even with a large X-ray spot size on the sample [Holden et al.

A mail-in and user facility for X-ray absorption near-edge structure: the CEI-XANES laboratory X-ray spectrometer at the University of Washington.

There are more than 100 beamlines or endstations worldwide that frequently support X-ray absorption fine-structure (XAFS) measurements, thus providing critical enabling capability for research across numerous scientific disciplines. However, the absence of a supporting tier of more readily accessible, lower-performing options has caused systemic inefficiencies, resulting in high oversubscription and the omission of many scientifically and socially valuable XAFS applications that are incompatible with the synchrotron facility access model. To this end, this work describes the design, performance and uses of the Clean Energy Institute X-ray absorption near-edge structure (CEI-XANES) laboratory spectrometer and its use as both a user-present and mail-in facility.

Rapid evolution of the Photosystem II electronic structure during water splitting.

Photosynthetic water oxidation is a fundamental process that sustains the biosphere. A MnCa cluster embedded in the photosystem II protein environment is responsible for the production of atmospheric oxygen. Here, time-resolved x-ray emission spectroscopy (XES) was used to observe the process of oxygen formation in real time.

Interface Engineering V O Nanofibers for High-Energy and Durable Supercapacitors.

A local electric field is induced to engineer the interface of vanadium pentoxide nanofibers (V O -NF) to manipulate the charge transport behavior and obtain high-energy and durable supercapacitors. The interface of V O -NF is modified with oxygen vacancies (Vö) in a one-step polymerization process of polyaniline (PANI). In the charge storage process, the local electric field deriving from the lopsided charge distribution around Vö will provide Coulombic forces to promote the charge transport in the resultant Vö-V O /PANI nanocable electrode.

An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research.

X-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) are advanced x-ray spectroscopies that impact a wide range of disciplines. However, unlike the majority of other spectroscopic methods, XAFS and XES are accompanied by an unusual access model, wherein the dominant use of the technique is for premier research studies at world-class facilities, i.e.

Pressure Effects on the 4f Electronic Structure of Light Lanthanides.

Using the satellite structure of the Lγ_{1} line in nonresonant x-ray emission spectra, we probe the high-pressure evolution of the bare 4f signature of the early light lanthanides at ambient temperature. For Ce and Pr the satellite peak experiences a sudden reduction concurrent with their respective volume collapse (VC) transitions. These new experimental results are supported by calculations using state-of-the-art extended atomic structure codes for Ce and Pr, and also for Nd, which does not exhibit a VC.

Vacuum formed temporary spherically and toroidally bent crystal analyzers for x-ray absorption and x-ray emission spectroscopy.

We demonstrate that vacuum forming of 10-cm diameter silicon wafers of various crystallographic orientations under an x-ray permeable, flexible window can easily generate spherically bent crystal analyzers and toroidally bent crystal analyzers with ∼1-eV energy resolution and a 1-m major radius of curvature. In applications at synchrotron light sources, x-ray free electron lasers, and laboratory spectrometers, these characteristics are generally sufficient for many x-ray absorption fine structure (XAFS), x-ray emission spectroscopy (XES), and resonant inelastic x-ray scattering applications in the chemical sciences. Unlike existing optics manufacturing methods using epoxy or anodic bonding, vacuum forming without adhesive is temporary in the sense that the bent wafer can be removed when vacuum is released and exchanged for a different orientation wafer.

X-ray Emission Spectroscopy at X-ray Free Electron Lasers: Limits to Observation of the Classical Spectroscopic Response for Electronic Structure Analysis.

X-ray free electron lasers (XFELs) provide ultrashort intense X-ray pulses suitable to probe electron dynamics but can also induce a multitude of nonlinear excitation processes. These affect spectroscopic measurements and interpretation, particularly for upcoming brighter XFELs. Here we identify and discuss the limits to observing classical spectroscopy, where only one photon is absorbed per atom for a Mn in a light element (O, C, H) environment.

Energy-Degeneracy-Driven Covalency in Actinide Bonding.

Evaluating the nature of chemical bonding for actinide elements represents one of the most important and long-standing problems in actinide science. We directly address this challenge and contribute a Cl K-edge X-ray absorption spectroscopy and relativistic density functional theory study that quantitatively evaluates An-Cl covalency in AnCl (An = Th, U, Np, Pu). The results showed significant mixing between Cl 3p- and An 5f- and 6d-orbitals (t*/t* and t*/e *), with the 6d-orbitals showing more pronounced covalent bonding than the 5f-orbitals.

The coordination chemistry of Cm, Am, and Ac in nitrate solutions: an actinide L-edge EXAFS study.

Understanding actinide(iii) (An = Cm, Am, Ac) solution-phase speciation is critical for controlling many actinide processing schemes, ranging from medical applications to reprocessing of spent nuclear fuel. Unfortunately, in comparison to most elements in the periodic table, An speciation is often poorly defined in complexing aqueous solutions and in organic media. This neglect - in large part - is a direct result of the radioactive properties of these elements, which make them difficult to handle and acquire.

A color x-ray camera for 2-6 keV using a mass produced back illuminated complementary metal oxide semiconductor sensor.

There are several reports in the scientific literature of the use of mass-produced charge coupled device or complementary metal oxide semiconductor (CMOS) sensors as x-ray detectors that combine high spatial resolution with significant energy resolution. Exploiting a relatively new especially favorable ambient-temperature back-illuminated CMOS sensor, we report the development of a spectroscopic x-ray camera having particularly impressive performance for 2-6 keV photons. This instrument has several beneficial characteristics for advanced x-ray spectroscopy studies in the laboratory, at synchrotron light sources, at x-ray free electron lasers, or when using pulsed x-ray sources such as for laser plasma physics research.

Sulfur Speciation in Biochars by Very High Resolution Benchtop Kα X-ray Emission Spectroscopy.

The analytical chemistry of sulfur-containing materials poses substantial technical challenges, especially due to the limitations of S NMR and the time-intensive preparations required for wet-chemistry analyses. A number of prior studies have found that synchrotron-based X-ray absorption near edge structure (XANES) measurements can give detailed speciation of sulfur chemistry in such cases. However, due to the obvious access limitations, synchrotron XANES of sulfur cannot be part of routine analytical practice across the chemical sciences community.

Determination of Hexavalent Chromium Fractions in Plastics Using Laboratory-Based, High-Resolution X-ray Emission Spectroscopy.

Cr(VI) is a well-known human carcinogen with many water-soluble moieties. Its presence in both natural and man-made substances poses a risk to public health, especially when contamination of groundwater is possible. This has led the European Union and other jurisdictions to include Cr(VI) in restriction of hazardous substances regulations.