X-ray Absorption Cross Section Research Articles

Tailored molecular for ultra-stability and biocompatible pseudohalide metal-free perovskite towards X-ray detectors with record sensitivity

The emerging pseudohalide metal-free perovskite (pseudohalide–MFPs) X-ray detector caters to the demands of timely mobile diagnosis owing to its lightweight, flexibility, and cost-effectiveness. However, the performance of these devices is severely limited by poor X-ray absorption, ultra-wide band gap, relative instability, and their unknown biotoxicity. Herein, we construct heavy atom covalent bonds (C–Br/Cl) on the A-site organic cation to reinforce component coordination to modulate X-ray absorption and band gap in pseudohalide–MFPs and further enhance its stability. Molecular dynamics simulations demonstrate that the introduction of halogen atoms can strengthen hydrogen bonding interactions, thereby improving the coordination between different components. The resultant (MDABCOBr)–NH4(BF4)3 (MDABCO = N-methyl-N’-diazabicyclo[2.2.2]octonium) single crystal significantly increases X-ray absorption cross-section and crystalline density (from 1.728 to 1.950 g cm−3), and synergistically realizes the band nature modulation (from 7.4 to 5.5 eV) and enhanced ionic migration inhibition (628 meV) with optimized stability. As such, our X-ray detectors realized a sensitivity of 2377 μC Gyair−1 cm−2, an ultralow detection limit of 50.1 nGyair s−1, and impressive operation stability. Moreover, cytotoxicity assay confirmed the compatibility of pseudohalide metal-free perovskite. Finally, within this framework, we successfully fabricate the (MDABCOBr)–NH4(BF4)3-based flexible device to create an ideal in vitro wearable X-ray detection.

Theoretical study on time response of semiconductor photorefractive effects under subpicosecond ultra-fast X-rays.

A theoretical model that can efficiently calculate the refractive index response of semiconductors under ultrafast X-ray radiation is established based on the photorefractive effect of semiconductors. The proposed model is used to interpret X-ray diagnostics experiments, and the results are in good agreement with experiments. In the proposed model, a rate equation model of free carrier density calculation is adopted with the X-ray absorption cross-sections calculated by atomic codes. The two-temperature model is used to describe the electron-lattice equilibration and the extended Drude model is applied to calculate the transient refractive index change. It is found that faster time response can be achieved for semiconductors with shorter carrier lifetime and sub-picosecond resolution can be obtained for InP and [Formula: see text]. The material response time is not sensitive to X-ray energy and the diagnostics can be used in the 1-10 keV energy range. This article is part of the theme issue 'Dynamic and transient processes in warm dense matter'.

Scanning transmission soft X-ray spectromicroscopy of mouse kidney and liver

Scanning transmission X-ray microscopy (STXM) in the soft X-ray range is well-suited to study ultrastructural features of mammalian soft tissues. Especially at the carbon 1s edge, the imaging contrast varies drastically across the edge due to rapid changes in the X-ray absorption cross-section of functional groups present in the tissue samples enabling label-free soft X-ray spectromicroscopic studies. We present STXM spectromicroscopic imaging of mouse kidney and liver tissues. We especially concentrate on ultrastructural abnormalities in genetically modified Slc17a5 mice. STXM is a promising technique to study storage diseases without chemical alteration due to staining agents, but sample preparation poses a challenge.

Singlet Fission-Based High-Resolution X-Ray Imaging Scintillation Screens.

X-ray imaging technology is critical to numerous different walks of daily life, ranging from medical radiography and security screening all the way to high-energy physics. Although several organic chromophores are fabricated and tested as X-ray imaging scintillators, they generally show poor scintillation performance due to their weak X-ray absorption cross-section and inefficient exciton utilization efficiency. Here, a singlet fission-based high-performance organic X-ray imaging scintillator with near unity exciton utilization efficiency is presented. Interestingly, it is found that the X-ray sensitivity and imaging resolution of the singlet fission-based scintillator are dramatically improved by an efficient energy transfer from a thermally activated delayed fluorescence (TADF) sensitizer, in which both singlet and triplet excitons can be efficiently harnessed. The fabricated singlet fission-based scintillator exhibits a high X-ray imaging resolution of 27.5 line pairs per millimeter (lp mm-1 ), which exceeds that of most commercial scintillators, demonstrating its high potential use in medical radiography and security inspection.

Recent Advances in Metal-Based NanoEnhancers for Particle Therapy.

Radiotherapy is one of the most common therapeutic regimens for cancer treatment. Over the past decade, proton therapy (PT) has emerged as an advanced type of radiotherapy (RT) that uses proton beams instead of conventional photon RT. Both PT and carbon-ion beam therapy (CIBT) exhibit excellent therapeutic results because of the physical characteristics of the resulting Bragg peaks, which has been exploited for cancer treatment in medical centers worldwide. Although particle therapies show significant advantages to photon RT by minimizing the radiation damage to normal tissue after the tumors, they still cause damage to normal tissue before the tumor. Since the physical mechanisms are different from particle therapy and photon RT, efforts have been made to ameliorate these effects by combining nanomaterials and particle therapies to improve tumor targeting by concentrating the radiation effects. Metallic nanoparticles (MNPs) exhibit many unique properties, such as strong X-ray absorption cross-sections and catalytic activity, and they are considered nano-radioenhancers (NREs) for RT. In this review, we systematically summarize the putative mechanisms involved in NRE-induced radioenhancement in particle therapy and the experimental results in in vitro and in vivo models. We also discuss the potential of translating preclinical metal-based NP-enhanced particle therapy studies into clinical practice using examples of several metal-based NREs, such as SPION, Abraxane, AGuIX, and NBTXR3. Furthermore, the future challenges and development of NREs for PT are presented for clinical translation. Finally, we propose a roadmap to pursue future studies to strengthen the interplay of particle therapy and nanomedicine.

Heavy-atom engineering of thermally activated delayed fluorophores for high-performance X-ray imaging scintillators

The architectural design and fabrication of low-cost and reliable organic X-ray imaging scintillators with high light yield, ultralow detection limits and excellent imaging resolution is becoming one of the most attractive research directions for chemists, materials scientists, physicists and engineers due to the devices’ promising scientific and applied technological implications. However, the optimal balance among X-ray absorption capability, exciton utilization efficiency and photoluminescence quantum yield of organic scintillation materials is extremely difficult to achieve because of several competitive non-radiative processes, including intersystem crossing and internal conversion. Here we introduced heavy atoms (Cl, Br and I) into thermally activated delayed fluorescence (TADF) chromophores to significantly increase their X-ray absorption cross-section and maintaining their unique TADF properties and high photoluminescence quantum yield. The X-ray imaging screens fabricated using TADF-Br chromophores exhibited highly improved X-ray sensitivity and imaging resolution compared with the TADF-H counterpart. More importantly, the high X-ray imaging resolution of >18.0 line pairs per millimetre achieved from the TADF-Br screen exceeds most reported organic and conventional inorganic scintillators. This study could help revive research on organic X-ray imaging scintillators and pave the way towards exciting applications for radiology and security screening. Heavy atoms like Cl, Br and I introduced into thermally activated delayed fluorescence chromophores can increase the X-ray absorption cross-section. Light yield of ~20,000 photons MeV–1, detection limit of 45.5 nGy s−1 and imaging resolution of >18.0 line pairs per millimetre is demonstrated.

Triplet-triplet energy-transfer-based transparent X-ray imaging scintillators

<h2>Summary</h2> In this work, we fabricated a highly efficient and reabsorption-free transparent X-ray imaging scintillator with high performance using an efficient triplet-triplet energy transfer strategy between thermally activated delayed fluorophore (TADF-Br) and iridium organometallic complex (Ir-OMC). Steady-state and ultrafast time-resolved experiments—supported by high-level density functional theory calculations—clearly demonstrate that efficient triplet-triplet energy transfer from TADF-Br with a high X-ray absorption cross-section to emissive Ir-OMC can be achieved. Such high efficiency of interfacial triplet energy transfer and the heavy atoms in both the donor and acceptor led to a remarkable enhancement in triplet-state radioluminescence upon X-ray irradiation. The fabricated X-ray imaging scintillator achieved an X-ray imaging resolution of 19.8 lp mm⁻¹, exceeding the resolution of most reported organic and organometallic scintillation screens.

Systematic investigation of relativistic effects in EXAFS data analysis

Modern XAFS (x-ray absorption fine structure) data analysis is based on accurate multiple-scattering (MS) calculations of the x-ray absorption cross section, usually carried out solving the nonrelativistic Schr\"odinger equation for complex effective optical muffin-tin potentials describing the scattering of the atoms. The introduction of relativistic effects in extended XAFS (EXAFS) multiple-scattering calculations has been described in several papers and shown to be important for heavy atoms. However, few examples of applications and detailed studies of relativistic effects were given so far. In this work, we have performed a systematic investigation of relativistic corrections in systems of increasing atomic number, using a reliable simulation scheme recently developed and based on the incorporation of a pseudo-Schr\"odinger equation effectively replacing the Dirac relativistic form. Calculations have been put to a test in 12 different pure-element condensed-state systems, with the atomic number ranging from $Z=10$ for crystalline Ne to $Z=90$ for crystalline Th. The importance of accounting for relativistic effects has been highlighted for elements with $Z\ensuremath{\gtrsim}60$, as ones for which relativistic corrections for amplitudes of calculated XAFS MS signals exceed $10%$. The size of relativistic effects for calculated higher-order XAFS signal (with respect to the dominant single-scattering first-neighbor signal) has been shown, taking as example the ${L}_{3}$-edge spectra of crystalline Au and Pb. The size of relativistic effects for the $K$ and ${L}_{3}$ edges has been also evaluated, showing a slight increase of relativistic corrections for the ${L}_{3}$ edge. The improvement in the accuracy of XAFS simulations has been demonstrated comparing the results obtained for structural refinements of the ${L}_{3}$ edge of crystalline Au at 300 K.

Relativistic Corrections to Phase Shift Calculation in the GNXAS Package

Modern XAFS (X-ray Absorption Fine Structure) data-analysis is based on accurate multiple-scattering (MS) calculations of the X-ray absorption cross-section. In this paper, we present the inclusion and test of relativistic corrections for the multiple-scattering calculations within the GnXAS suite of programs, which is relevant to the treatment of the XAFS signals when atoms with high atomic number are contained into the system. We present a suitable strategy for introducing relativistic corrections without altering the basic structure of the programs. In particular, this is realized by modifying only the Phagen program calculating the atomic absorption cross sections and scattering t-matrices for the selected cluster. The modification incorporates a pseudo-Schrödinger Equation (SE) replacing the Dirac relativistic form. The phase-shift calculations have been put to a test in two known molecular and crystalline cases: molecular bromine Br2 and crystalline Pb. Calculations in an extended energy range have been shown to be very close to the non-relativistic case for Br2 (Br K-edge) while corrections have been found to exceed 25% for amplitude and phases of the XAFS multiple-scattering signals (Pb L3-edge). Benefits in the structural refinement using relativistic corrections are discussed for crystalline Pb at room temperature.

3d-4f Resonant Inelastic X-ray Scattering of Actinide Dioxides: Crystal-Field Multiplet Description.

A theoretical overview of the core-to-core (3d-4f) resonant inelastic X-ray scattering (RIXS) spectra of actinide dioxides AnO2 (An = Th, U, Np, Pu, Am, Cu, Bk, Cf) is provided. The 3d-4f RIXS maps were calculated using crystal-field multiplet theory and turned out to be significantly different at the An M5 vs M4 edges, because of selection rules and final state effects. The results of the calculations allowed for a general analysis of so-called high-energy-resolution fluorescence-detected X-ray absorption (HERFD-XAS) spectra. The cuts of the calculated RIXS maps along the incident energy axis at the constant emitted energy, corresponding to the maximum of the RIXS intensity, represented the HERFD spectra and provided their comparison with calculated conventional X-ray absorption (XAS) spectra with a reduced core-hole lifetime broadening at the An M5 and M4 edges. Although the An M5 HERFD profiles were found to depart from the X-ray absorption cross-section, in terms of appearing additional transitions, the results of calculations for the An M4 edges demonstrate overall better agreement between the HERFD and XAS spectra for most dioxides, keeping in mind a restricted HERFD resolution, because of the core–hole lifetime broadening in the final state. The results confirm the utility of HERFD for the An chemical state determination and indicate the importance of calculating the entire RIXS process in order to interpret the HERFD data correctly.

Interlayer charge transfer in tin disulphide: Orbital anisotropy and temporal aspects

An investigation of the unoccupied electronic structure of the transition-metal dichalcogenide tin disulphide has been conducted using core-hole clock spectroscopy. Polarization-dependent x-ray absorption in the tender x-ray regime at the S K edge and maps of the resonant Auger spectra in the S KLL Auger kinetic energy range have been recorded. Supported with ab initio calculations of the unoccupied electronic structure, these allow us to relate resonances in the absorption cross section to excitations along various directions in the Brillouin zone. We observe anisotropy in the x-ray absorption cross section in polarization directions in plane and out of plane of the crystal. There is also anisotropy in the charge transfer dynamics as inferred from the coherent and noncoherent parts of the resonant Auger spectra. This approach can be generally used to interpret dynamics in unoccupied states, e.g., in layered structure or heterogenous interfaces.

Single-shot time-resolved magnetic x-ray absorption at a free-electron laser

Ultrafast dynamics are generally investigated using stroboscopic pump-probe measurements, which characterize the sample properties for a single, specific time delay. These measurements are then repeated for a series of discrete time delays to reconstruct the overall time trace of the process. As a consequence, this approach is limited to the investigation of fully reversible phenomena. We recently introduced an off-axis zone plate based X-ray streaking technique, which overcomes this limitation by sampling the relaxation dynamics with a single femtosecond X-ray pulse streaked over a picosecond long time window. In this article we show that the X-ray absorption cross section can be employed as the contrast mechanism in this novel technique. We show that changes of the absorption cross section on the percent level can be resolved with this method. To this end we measure the ultrafast magnetization dynamics in CoDy alloy films. Investigating different chemical compositions and infrared pump fluences, we demonstrate the routine applicability of this technique. Probing in transmission the average magnetization dynamics of the entire film, our experimental findings indicate that the demagnetization time is independent of the specific infrared laser pump fluence. These results pave the way for the investigation of irreversible phenomena in a wide variety of scientific areas.

PMAO coated Na(Gd0.5Lu0.5)F4:Nd3+ nanocrystals as multifunctional contrast agent with NIR optical, X-ray and magnetic imaging properties

Nanomaterials with multiple imaging functionalities are nowadays getting tremendous attention due to their several superior features compared to existing contrast agents. By developing a nanomaterial that exhibit multiple functionalities, the possibility to increase the amount of imaging information obtained in a short amount of time is becoming more and more a reality. In this work, we developed a multifunctional nanocrystals (NCs), Na(Gd0.5Lu0.5)F4:Nd3+, that combines multiple rare-earth features as an all-in-one imaging agent comprised of optical imaging, magnetic imaging, and X-ray imaging by utilizing the superparamagnetic features of Gd3+, the high X-ray absorption cross section of Lu3+, and the NIR fluorescence of Nd3+. Morphology, optical properties, and cell viability are shown in detail where the utility of this multifunctional imaging agent was confirmed by optical, X-ray and magnetic imaging experiments. Surface functionalization of the NCs is also presented to highlight the potential application of the NCs as contrast agents in biological imaging.

Inorganic, Organic, and Perovskite Halides with Nanotechnology for High–Light Yield X- and γ-ray Scintillators

Trends in scintillators that are used in many applications, such as medical imaging, security, oil-logging, high energy physics and non-destructive inspections are reviewed. First, we address traditional inorganic and organic scintillators with respect of limitation in the scintillation light yields and lifetimes. The combination of high–light yield and fast response can be found in Ce 3 + , Pr 3 + and Nd 3 + lanthanide-doped scintillators while the maximum light yield conversion of 100,000 photons/MeV can be found in Eu 3 + doped SrI 2 . However, the fabrication of those lanthanide-doped scintillators is inefficient and expensive as it requires high-temperature furnaces. A self-grown single crystal using solution processes is already introduced in perovskite photovoltaic technology and it can be the key for low-cost scintillators. A novel class of materials in scintillation includes lead halide perovskites. These materials were explored decades ago due to the large X-ray absorption cross section. However, lately lead halide perovskites have become a focus of interest due to recently reported very high photoluminescence quantum yield and light yield conversion at low temperatures. In principle, 150,000–300,000 photons/MeV light yields can be proportional to the small energy bandgap of these materials, which is below 2 eV. Finally, we discuss the extraction efficiency improvements through the fabrication of the nanostructure in scintillators, which can be implemented in perovskite materials. The recent technology involving quantum dots and nanocrystals may also improve light conversion in perovskite scintillators.

Unraveling 5f-6d hybridization in uranium compounds via spin-resolved L-edge spectroscopy

The multifaceted character of 5f electrons in actinide materials, from localized to itinerant and in between, together with their complex interactions with 6d and other conduction electron states, has thwarted efforts for fully understanding this class of compounds. While theoretical efforts abound, direct experimental probes of relevant electronic states and their hybridization are limited. Here we exploit the presence of sizable quadrupolar and dipolar contributions in the uranium L3-edge X-ray absorption cross section to provide unique information on the extent of spin-polarized hybridization between 5f and 6d electronic states by means of X-ray magnetic circular dichroism. As a result, we show how this 5f-6d hybridization regulates the magnetism of each sublattice in UCu2Si2 and UMn2Si2 compounds, demonstrating the potentiality of this methodology to investigate a plethora of magnetic actinide compounds.

Detecting element specific electrons from a single cobalt nanocluster with synchrotron x-ray scanning tunneling microscopy

We use a nanofabricated scanning tunneling microscope tip as a detector to investigate local X-ray induced tunneling and electron emission from a single cobalt nanocluster on a Au(111) surface. The tip-detector is positioned a few angstroms above the nanocluster, and ramping the incident X-ray energy across the Co photoabsorption K-edge enables the detection of element specific electrons. Atomic-scale spatial dependent changes in the X-ray absorption cross section are directly measured by taking the X-ray induced current as a function of X-ray energy. From the measured sample and tip currents, element specific X-ray induced current components can be separated and thereby the corresponding yields for the X-ray induced processes of the single cobalt nanocluster can be determined. The detection of element specific synchrotron X-ray induced electrons of a single nanocluster opens an avenue for materials characterization on a one particle at-a-time basis.

Exploring the fabrication of Co and Mn nanostructures with focused soft x-ray beam induced deposition

Focused soft X-ray beam induced deposition of metallic deposits from metal organic precursors is a promising novel technique for additive nanostructure fabrication. In the present work, the authors present a comparative study for deposition and in situ characterization of Co and Mn nanostructures in a scanning transmission x-ray microscope. The authors detect a significant selectivity of the deposition process with respect to the incident photon energy that arises from the enhanced x-ray absorption cross section of the precursor molecules for near-threshold excitation. This effect has been investigated for the L2,3-edges of the respective metal centers of two different precursor molecules as well as the N and O K-edges of the respective ligands. The authors find a photon-limited growth mode for deposition from cobalt tricarbonyl nitrosyl [Co(CO)3NO], while the process is precursor-limited for methylcyclopentadienyl manganese tricarbonyl [MeCpMn(CO)3] possibly due to a comparably low vapor pressure of the latter precursor.

High energy X-ray phase and dark-field imaging using a random absorption mask.

High energy X-ray imaging has unique advantage over conventional X-ray imaging, since it enables higher penetration into materials with significantly reduced radiation damage. However, the absorption contrast in high energy region is considerably low due to the reduced X-ray absorption cross section for most materials. Even though the X-ray phase and dark-field imaging techniques can provide substantially increased contrast and complementary information, fabricating dedicated optics for high energies still remain a challenge. To address this issue, we present an alternative X-ray imaging approach to produce transmission, phase and scattering signals at high X-ray energies by using a random absorption mask. Importantly, in addition to the synchrotron radiation source, this approach has been demonstrated for practical imaging application with a laboratory-based microfocus X-ray source. This new imaging method could be potentially useful for studying thick samples or heavy materials for advanced research in materials science.

Ab initio calculations of X-ray magnetic circular dichroism spectra within the projector augmented wave method: An implementation into the VASP code

X-ray absorption and X-ray magnetic circular dichroism (XMCD) are very powerful tools for probing the orbital and spin moments of each atomic species orbital of magnetic materials. In this work, we present the implementation of a module for computing the X-ray absorption and XMCD spectra into the VASP code. We provide a derivation of the absorption cross-section in the electric dipole approximation. The matrix elements, which make up the X-ray absorption cross-section for a given polarization of light, are then computed using either the momentum operator p or the position operator r, within the projector augmented wave method. The core electrons are described using the relativistic basis-set whereas for the valence electrons, the spin–orbit coupling is added perturbatively to the semi-relativistic Hamiltonian. We show that both the p and the r implementations lead to the same results. The results for the K-edge and L23-edges of bcc-iron are then computed and compared to experiment.

Beyond structure: ultrafast X-ray absorption spectroscopy as a probe of non-adiabatic wavepacket dynamics.

The excited state non-adiabatic dynamics of polyatomic molecules, leading to the coupling of structural and electronic dynamics, is a fundamentally important yet challenging problem for both experiment and theory. Ongoing developments in ultrafast extreme vacuum ultraviolet (XUV) and soft X-ray sources present new probes of coupled electronic-structural dynamics because of their novel and desirable characteristics. As one example, inner-shell spectroscopy offers localized, atom-specific probes of evolving electronic structure and bonding (via chemical shifts). In this work, we present the first on-the-fly ultrafast X-ray time-resolved absorption spectrum simulations of excited state wavepacket dynamics: photo-excited ethylene. This was achieved by coupling the ab initio multiple spawning (AIMS) method, employing on-the-fly dynamics simulations, with high-level algebraic diagrammatic construction (ADC) X-ray absorption cross-section calculations. Using the excited state dynamics of ethylene as a test case, we assessed the ability of X-ray absorption spectroscopy to project out the electronic character of complex wavepacket dynamics, and evaluated the sensitivity of the calculated spectra to large amplitude nuclear motion. In particular, we demonstrate the pronounced sensitivity of the pre-edge region of the X-ray absorption spectrum to the electronic and structural evolution of the excited-state wavepacket. We conclude that ultrafast time-resolved X-ray absorption spectroscopy may become a powerful tool in the interrogation of excited state non-adiabatic molecular dynamics.