Core Level Research Articles

Relativistic Prolapse-Free Gaussian Basis Sets of Double- and Triple-ζ Quality for s- and p-Block Elements: (aug-)RPF-2Z and (aug-)RPF-3Z.

This study presents two new relativistic Gaussian basis sets without variational prolapse of double- and triple-ζ quality, RPF-2Z and RPF-3Z, along with augmented versions including additional diffuse functions, aug-RPF-2Z and aug-RPF-3Z, which are available for all s and p block elements from Hydrogen to Oganesson. The exponents of the Correlation/Polarization (C/P) functions are obtained from a polynomial version of the generator coordinate Dirac-Fock method (p-GCDF). The choice of C/P functions was guided by multireference configuration interaction calculations with single and double excitations (MR-CISD) based on a valence active space. Finally, calculations of fundamental properties done for atomic and molecular systems (bond lengths, vibrational frequencies, dipole moments, and electron affinities) ensure the expected quality of these new basis sets, which may also exhibit some computational efficiency advantages. Additionally, the prolapse-free feature of these sets must provide a reliable description of properties more dependent on core electron distributions, as well.

From Gas to Solution: The Changing Neutral Structure of Proline upon Solvation.

Liquid-jet photoelectron spectroscopy (LJ-PES) and electronic-structure theory were employed to investigate the chemical and structural properties of the amino acid l-proline in aqueous solution for its three ionized states (protonated, zwitterionic, and deprotonated). This is the first PES study of this amino acid in its biologically relevant environment. Proline's structure in the aqueous phase under neutral conditions is zwitterionic, distinctly different from the nonionic neutral form in the gas phase. By analyzing the carbon 1s and nitrogen 1s core levels as well as the valence spectra of aqueous-phase proline, we found that the electronic structure is dominated by the protonation state of each constituent molecular site (the carboxyl and amine groups) with small yet noticeable interference across the molecule. The site-specific nature of the core-level spectra enables the probing of individual molecular constituents. The valence photoelectron spectra are more difficult to interpret because of the overlapping signals of proline with the solvent and pH-adjusting agents (HCl and NaOH). Yet, we are able to reveal subtle effects of specific (hydrogen-bonding) interaction with the solvent on the electronic structure. We also demonstrate that the relevant conformational space is much smaller for aqueous-phase proline than for its gas-phase analogue. This study suggests that caution must be taken when comparing photoelectron spectra for gaseous- and aqueous-phase molecules, particularly if those molecules are readily protonated/deprotonated in solution.

Electronic structure of 2(5H)-thiophenone studied by UPS and soft X-ray spectroscopy

Abstract The electronic structure of 2(5H)-thiophenone in the gas phase was investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C 1s, O 1s, and S 2p edges. All assignments of the experimental results are supported by both ab-initio electron propagator OVGF calculations for the valence photoemission bands and density functional theory (DFT) and relativistic time dependent DFT calculations for the core levels XPS and NEXAFS spectra. Overall good agreement between experiment and theory is observed; this is especially true for core electron excitations which has permitted an unambiguous assignment of the observed spectral features in terms of single-particle excitations to virtual molecular orbitals. The assignment of the valence band spectra based on OVGF calculations, although satisfactory, points to the importance of electron correlations effects that partially break the single particle picture of ionization.

Changes in electronic structure within NiSx (0.60 < x < 1.53) compound series

In this article, changes in electronic properties of NiSx series of films with different sulfur content (x = S/Ni) are studied using X-ray (XPS) and ultra-violet (UPS) photoelectron spectroscopy techniques. NiSx (0.60 < x < 1.53) films are synthesized on native oxide of Si(111) substrates via reactive deposition of Ni and S in an ultra-high vacuum chamber by varying the sulfur pressures and substrate temperatures. The binding energy (BE) of Ni2p3/2 core level, the separation between the Ni2p3/2 main peaks and satellite peaks, and the Ni3d valence band binding energies are all found to vary nearly linearly with the sulfur content in NiSx series of compounds, suggesting a direct dependence of Ni electronic charge on the sulfur content. The shift of the Ni3d and S3p states (to higher and lower BE, respectively) with an increase in the S content provides experimental confirmation of the theoretical prediction previously reported (Wen et al., 2020) [1]. All NiSx films showed density of states (DOS) at the Fermi level, indicating their metallic nature. The Ni2p3/2 binding energy shift of NiSx films are much smaller than that of oxide-based compounds, confirming the significantly more covalent nature of the sulfides. NiSx film grown on Si (111) substrate showed nickel silicide interface formation. This study provides information on the growth and trends in the electronic properties of NiSx relevant for their applications in catalysis, energy storage, and electronic devices.

Effect of oxygen vacancies on enhancing the photo-catalytic activity, photo-luminescence and electronic structure properties of nanostructured Y-doped CeO2

The exceptional characteristics of CeO2 and Ce1-xYxO2 (x = 0.03, 0.05 and 0.07) nanoparticles were reported in the manuscript supporting that Y3+replaces Ce3+/Ce4+leads to oxygen vacancies formation. The results of XRD measurements revealed FCC structure of decreased crystallite size of CeO2 with improved crystallinity. To investigate the surface morphology, HRTEM and SAED patterns were performed. EDX analyses were undertaken to discuss the elemental and compositional characteristics. The absorption spectra using UV–Vis–NIR spectroscopy were analyzed and red shifted absorbance was found to enhance with decreasing band gap values for increased Y doping. The Photoluminescence spectra depicted various emissions representing the development of various defects and oxygen vacancy with incorporation of Y content in the lattice with CCT values below 4000 K to be classified as warm yellow light for indoor applications. The development of oxygen vacancies in the CeO2 lattice was further supported by XPS measurements for core levels Ce 3d, O 1s and Y 3d. Furthermore, the XPS measurements also reported the valence states of elements, Ce with 3+ and 4+, Y with 3+ and O with 2- along with charged oxygen vacancies. The photo-catalytic analysis revealed that Y-doped CeO2 nanoparticles show better degradation using a variety of characterization. A degradation mechanism that illustrates the impact of oxygen vacancies created by Y-doping on the photo-degradation process has been proposed. The novelty of Y-doped CeO2 nanoparticles stems from their improved photo-catalytic activities, which are linked to structural changes and the formation of oxygen vacancies. This doping considerably affects the electrical structure, resulting in better light absorption and less electron-hole recombination. The detailed outcomes of present study suggested the use of Y-doped CeO2 nanoparticles in optoelectronics, spintronics devices and photo-catalyst applications.

Design and fabrication of tetradymites-based vertically oriented single and multilayer thin-film thermoelectric generators

Tetradymites-based thermoelectric materials and devices have received renewed attention due to their engineering and design flexibility, large scalability, and commercial viability in producing electricity from waste heat for niche applications in small power generation and micro refrigeration. In fact, most commercially available bulk thermoelectric generators (TEGs) are made from tetradymites. In contrast to their bulk counterparts, thin-film vertical TEGs have not been widely adopted. This can be attributable to complexities in design and fabrication methodologies, device measurement challenges, and the hurdle of maintaining a large enough temperature gradient for optimal device performance. In this study, we utilize a facile approach for the design, fabrication, and characterization of tetradymite-based n-type Bi2Te3 and p-type Sb2Te3 single-layer thermoelectric generators, as well as n-type Bi2Te3/Bi2Te2.83Se0.17 and p-type Sb2Te3/Bi0.4Sb1.6Te3 alternating multilayer superlattice TEGs. State-of-the-art characterization techniques were employed to investigate the structural, chemical, and thermoelectric properties of the materials. XRD analysis showed a preferential orientation along the (100) plane with a high intensity peak at 2θ = 25.5°, and XPS spectra exhibited a high-resolution peak at 531.5 eV corresponding to the Bi 4f7/2 core level. The structural data analysis confirmed the dominant metallic phase of the materials as well as their high crystalline nature. Device characterization showed that the multilayer device performed better than the single-layer devices with a recorded voltage, power, and power density of 11 mV, 12 pW, and 15.87 mW/m3 at ΔT = 18 °C, respectively, in comparison to 9.4 mV, 7.8 pW, and 10.31 mW/m3 for the most performing single-layer devices.

The shift in chemical potential of electrons upon intercalation of ZrSe2 by copper

The effective valence of copper in the region of its concentration up to x = 0.25 in CuxZrSe2 has been determined based on an analysis of binding energies of the Zr3d and Se3d core levels and the electromotive forces from an electrochemical experiment for CuxZrSe2, as a function of Cu concentration x. It has been shown that the domination of covalent bonding between copper and the ZrSe2 host lattice within this concentration range results in a notable reduction in the effective valence of copper in comparison to its nominal value.

Band Alignment and Surface Photo-Voltage Analysis of C-Si/Siox/Poly-Silicon Passivating-Contact Stacks Through X-Ray Photoelectron Spectroscopy.

Ultrathin SiOx layers and c-Si/SiOx interfaces find application in tunnel-oxide passivated contacts (TOPcon) for high-efficiency silicon solar cells. Here, we investigate their detailed microscopic properties, with specific attention for the case of c-Si(100) substrates, capped either by p-type or n-type poly-silicon layers [c-Si/SiOx/poly-Si (p+) or c-Si/SiOx/poly-Si (n+)]. Our focus is on the effects of the substrate preparation conditions (either by a dry-plasma or wet SiOx process) and the high-temperature annealing step (as required for the poly-Si crystallization) on the SiOx stoichiometry and its microscopic structure. Through advanced photoemission techniques, we find a clearly decreased valence band offset between the c-Si and SiOx (from 4.5 eV to 4.15 eV) when comparing the dry SiOx with the wet SiOx process, independent of the SiOx film thickness, but correlating with the relative fraction of sub-stochiometric Si states. We lastly examine the magnitude of band-bending of the contact structure through controlled in-situ exposure to light of the surfaces and subsequent tracking of core and valence band levels via a surface photovoltage and a junction photo-voltage (JPV) effect. By analyzing this JPV effect qualitatively, we find it to be proportional to the expected quasi fermi level splitting within the c-Si wafer.

Electron Temperatures in the Venusian Ionosphere From Parker Solar Probe Using Quasi‐Thermal Noise Spectroscopy

AbstractParker Solar Probe (PSP) uses Venus gravity assists (VGA) to achieve the closest orbits to the Sun by a spacecraft. During the third (VGA3) and fourth (VGA4) Venus gravity assists, the PSP entered the Venusian ionosphere. The core electrons could not be detected as they were below the SWEAP/SPAN electrostatic analyzer instrument energy threshold. However, there is another way to estimate the core temperature using quasi‐thermal noise (QTN) data measured by the PSP/FIELDS Radio Frequency Spectrometer instrument. QTN spectroscopy offers an effective tool for measuring electron temperature and density when the electrons are too cold for other instruments to measure, as is the case with VGA3 and VGA4. Low‐frequency plasma wave data from the closest approach during VGA3 and VGA4 was analyzed with the QTN spectroscopy technique to determine the density and first‐ever in‐situ thermal electron temperature of the Venusian ionosphere at solar minimum.

Ambient pressure x-ray photoelectron spectroscopy study on the initial atomic layer deposition process of platinum

The initial adsorption of MeCpPtMe3 is investigated using synchrotron-based ambient pressure x-ray photoelectron spectroscopy (XPS). The experiments are done on a native oxide-covered Si substrate. In addition, a reaction with O2 and the created Pt surface was investigated. Inspiration for the reaction studies was found from atomic layer deposition of metallic Pt, process that uses the same compounds as precursors. With time-resolved XPS, we have been able to observe details of the deposition process and especially see chemical changes on the Pt atoms during the initial deposition of the Pt precursor. The change of the binding energy of the Pt 4f core level appears to occur on a different timescale than the growth of the active surface sites. The very long pulse of the Pt precursor resulted in a metallic surface already from the beginning, which suggest chemical vapor deposition-like reactions occurring between the surface and the precursor molecules in this experiment. Additionally, based on the XPS data measured after the Pt precursor pulse, we can make suggestions for the reaction pathway, which point toward a scenario that leaves carbon from the MeCpPtMe3 precursor on the surface. These carbon species are then efficiently removed by the subsequent coreactant pulse, leaving behind a mostly metallic Pt film.

Interface-resolved photovoltage generation dynamics and band structure evolution in a PbS quantum dot solar cell.

For directed development of solar cells using nanomaterials such as quantum dots, there is a need to understand the device function in detail. Understanding where photovoltage is generated in a device and where energy losses occur is a key aspect of this, and development of methods which can provide this information is needed. We have previously shown that time-resolved photoelectron spectroscopy of core levels can be used to follow the photovoltage dynamics at a specific interface of a lead sulfide quantum dot solar cell. Here, we use the method's selectivity and sample design to investigate the photovoltage generation in different parts of this solar cell and determine how the different layers (including the absorber layer thickness) contribute to charge separation. We show that all layers contribute to photovoltage generation and that a gold contact deposited on the quantum dots is necessary for full photovoltage generation and slow charge recombination. By combining the information obtained, we are able to experimentally follow the time evolution of the solar cell band structure during the charge separation process. Furthermore, we can identify which specific layers need to be optimized for an overall improvement of quantum dot cells. In the future, this methodology can be applied to other types of devices to provide insights into photovoltage generation mechanisms.

Significant improvement in the removal performance of decimeter-sized zero-valent iron plates for mixed heavy metal ions in wastewater by enhancing surface sharpness

This study aims to lower the work function (Wa) of a decimeter-sized zero-valent iron plate (DZVIP) by introducing surface sharpness (TDZVIP) during stirring. Sharpness was achieved by cutting isosceles triangles with heights of 1 cm and varying apex angles (α) of 10˚, 20˚, 30˚, 40˚, and 50˚. A mixed simulated wastewater containing Cu(II), Te(IV), Se(IV), As(III), Bi(III), and Pb(II) at concentrations exceeding emission standards was prepared. Results indicate that the removal process exhibits mixed kinetic behavior, and sharpness significantly enhances the removal capacity (Re), mean removal rate (Vm), recycling performance, and final discharge quality. The Vm values for Cu(II), Te(IV), Se(IV), As(III), Bi(III), and Pb(II) over TDZVIP with optimized α are 2.34, 1.97, 1.51, 1.59, 4.09, and 1.53 times higher, respectively, than those over PDZVIP (without sharpness). UPS measurements provide direct evidence of Wa reduction, while XRD offers indirect confirmation. This is the first time the Wa of the amorphous shell has been identified and verified as a critical potential barrier for core electrons transferring from the DZVIP surface to wastewater. Additionally, it has been successfully reduced by incorporating surface sharpness and stirring, leading to enhanced removal efficiency, improved recycling performance, and higher final discharge quality.

Investigation on electronic and magnetic properties of Cr doped V2O5 thin films prepared by chemical solution deposition method

The present study examines the role of Cr doping on the electronic and magnetic behaviour of V2O5i.e. V2-xCrxO5 (x = 0, 0.05, and 0.10) thin films, prepared by chemical-solution deposition technique on Si (111) substrate. X-Ray diffraction technique shows the purity of the films and confirms no secondary phase formation. The atomic force microscopy was used to confirm the roughness of the films. A morphological investigation is done using High-resolution scanning electron microscopy associated with Energy dispersive X-Ray mapping of each element of the film. Fourier transform Infrared spectroscopy is utilised to identify functional groups that are present in obtained samples. The band-gap of these obtained samples is in the range of 0.8–0.6 eV, as examined by UV–Vis i.e., its value is decreasing with Cr doping. Using vibrating sample magnetometer, it is clear that samples shows ferromagnetic (FM) behaviour with saturation magnetization 0.3–0.6 μB/cc consistent with Langevin function fitting. It concludes the presence of FM behaviour of films and magnetization increases with carrier density, consistent with the carrier-induced origin of the ferromagnetism. X-ray photoemission spectroscopy (XPS) is utilised to understand its electronic properties. Core level spectra measurements suggest that V is in mixed state of 5+ and 4+. However Cr is in 3+ states. XPS and UV–Vis measurements imply that oxygen vacancies significantly contribute to the declination of the band gap and the promotion of FM-like behaviour. This discovery offers a promising pathway for engineering novel devices.

Ground and Excited Electronic Structure Analysis of FeH with Correlated Wave Function Theory and Density Functional Approximations.

FeH is one of the most challenging diatomic molecules to study under electronic structure theory. Here, we have successfully studied 22 electronic states of FeH using ab initio multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), and coupled cluster singles, doubles, and perturbative triples [CCSD(T)] levels of theory. We report their potential energy curves (PECs), excitation energies, dissociation energies, equilibrium electronic configurations, and a series of spectroscopic constants with the use of augmented triple-ζ, quadruple-ζ, and quintuple-ζ quality correlation consistent basis sets. The scalar relativistic effects and active space and core electron correlation contribution on the properties of FeH are also explored. The use of a large CASSCF active space that includes 4s, 4p, 3d, and 4d orbitals of Fe and the 1s of H is critical for producing accurate full PECs with proper dissociations and predicting the exact order of the electronic states. Our findings are in harmony with the experimental results available in the literature and will serve as reference values for future studies of FeH. Furthermore, with the use of PECs, the total internal partition function sum (TIPS) of FeH was calculated across a range of temperatures. Finally, we exploited the single-reference nature of the a6Δ of FeH and its ionized product FeH+ (X5Δ) to evaluate the associated density functional theory (DFT) errors on their dissociation energies and spectroscopic parameters.

Structure and properties of mixed valent CeFe2Al8

Temperature dependent magnetic, electrical transport and thermal properties of polycrystalline orthorhombic CeFe2Al8intermetallic compound have been studied along with its isostructural La counterpart, LaFe2Al8. For the cerium compound, low fielddcmagnetization exhibits an antiferromagnetic like ordering (TN) ∼ 4 K. The main feature of the magnetic susceptibility plot is a broad hump spanning a large temperature range, indicating mixed valence of Ce in the compound, in good agreement with reported literature. However, contrary to the reported observations we find that the mixed valence state is very robust and was evident even up to very high magnetic fields (> 2 T). Further, in this work we report 3d core level photoemission spectra of cerium in CeFe2Al8, to understand the valence state of cerium ions in this system. Additionally, from resistivity measurements it is found that, CeFe2Al8is metallic with no indication of any anomaly, till the lowest temperature of measurement. Specific heat measurements show very low value of heat capacity and electronic contribution. The isostructural La analogue, LaFe2Al8compound shows broadness in susceptibility with maxima around 44 K which may be attributed to ordering of Fe moments. The comparison of Ce and La compounds brings out the role of Fe magnetic moments which may be responsible for competing with cerium moments and resulting in the dilution of long-range magnetic order, also contributing to magnetic frustration in CeFe2Al8.

Elucidating temperature-dependent local structure change and optical properties in GeTe phase-change material

Phase-change memory emerges as a top contender for non-volatile data storage applications. We report here a systematic change in local structure and crystallization kinetics of binary GeTe thin films using temperature-dependent resistivity measurements, which offers single-stage crystallization at around 187 °C, corroborated with x-ray diffraction. Furthermore, the change in chemical bonding upon crystallization is determined through x-ray photoelectron spectroscopy core level spectra, which reveals the existence of Ge and Te components that align with the GeTe crystal structure. Also, an investigation was carried out employing a UV–Vis–NIR spectrophotometer to explore the evolution of optical bandgaps (Eg), Tauc parameter (B) representing the local disorder, and Urbach energy (Eu) of the GeTe material, as it undergoes the transition from a disordered amorphous state to a crystalline state. As crystallization progresses, a consistent shift of Eg from 0.92 to 0.70 eV corresponds to as-deposited amorphous at room temperature and crystalline at 250 °C, respectively. In addition, the reduction in Eu (from 199.87 to 141.27 meV) and a sudden increase of B around crystallization temperature is observed upon increasing temperature, indicating direct observation of enhanced medium-range order and distortion in short-range order, respectively, in GeTe thin films, revealing improved structural and optical properties. These enhancements make the GeTe material ideal for data storage applications of phase-change memory for next-generation computing technology.

Local and Symmetry-Resolved Electronic Structure of Liquid Dimethyl Sulfoxide from Resonant Inelastic Soft X-ray Scattering.

Dimethyl sulfoxide (DMSO) is an important polar solvent that derives its unique properties from the lone pair and the strong polar bond at the sulfinyl functional group. To derive the local and symmetry-resolved electronic structure of liquid DMSO, we have used resonant inelastic soft X-ray scattering (RIXS) maps at the S L2,3, C K, and O K edges. The experimental data are compared to calculations of spectra based on density functional theory, which allows a detailed analysis of the molecular orbitals throughout the molecule. In the RIXS maps, we find the signature of molecular-field splitting of the S 2p core levels, vibronic coupling, and ultrafast nuclear dynamics on the time scale of the RIXS process.

Resonant Raman Auger spectroscopy on transient core-excited Ne ions

Abstract Short-lived core-ionized neon atoms were investigated by measuring under resonant Raman conditions the Auger decay following the excitation of a second core electron into a Rydberg state. Making use of intense and narrow bandwidth x-ray free-electron laser pulses, the photoexcitation spectrum of the femtosecond-lived Ne+1s02s22p6np series was characterized. Energy position and lifetimes of the lower-lying Rydberg states were determined and the final state configurations following the decay of the Ne+1s02s22p63p double-core hole resonance were partially resolved.

Does Positron Attachment Take Place in Water Solution?

We performed a computational study of positron attachment to hydrated amino acids, namely glycine, alanine, and proline in the zwitterionic form. We combined the sequential quantum mechanics/molecular mechanics (s-QM/MM) method with various levels of any particle molecular orbital (APMO) calculations. Consistent with previous studies, our calculations indicate the formation of energetically stable states for the isolated and microsolvated amino acids, in which the positron localizes around the carboxylate group. However, for the larger clusters, composed of 7 to 40 water molecules, hydrogen bonding between the solute and solvent molecules disfavors positron attachment to the amino acids, giving rise to surface states in which the positron is located around the water-vacuum interface. The analysis of positron binding energies, positronic orbitals, radial probability distributions, and annihilation rates consistently pointed out the change from positron-solute to positron-solvent states. Even with the inclusion of an electrostatic embedding around the aggregates, the positrons did not localize around the solute. Positron attachment to molecules in the gas phase is a well-established fact. The existence of hydrated positronic molecules could also be expected from the analogy with transient anion states, which are believed to participate in radiation damage. Our results indicate that positron attachment to hydrated biomolecules, even to zwitterions with negatively charged carboxylated groups, would not take place. For the larger clusters, in which positron-water interactions are favored, the calculations indicate an unexpectedly large contribution of the core orbitals to the annihilation rates, between 15 and 20%. Finally, we explored correlations between positron binding energies (PBEs) and dipole moments, as well as annihilation rates and PBEs, consistent with previous studies for smaller clusters.

Low-temperature oxidation of methane mediated by Al-doped ZnO cluster and nanowire: a first-principles investigation.

First-principles calculations are performed to investigate the catalytic oxidation of methane by using N2O as an oxidizing agent over aluminum (Al)-doped Zn12O12 cluster and (Zn12O12)2 nanowire. The impact of Al impurity on the geometry, electronic structure, and surface reactivity of Zn12O12 and (Zn12O12)2 is thoroughly studied. Our study demonstrates that Al-doped ZnO systems have a better adsorption ability than the corresponding pristine counterparts. It is found that N2O molecule is initially decomposed on the Al site to provide the N2 molecule, and an Al-O intermediate which is an active species for the CH4 oxidation. The conversion of CH4 into CH3OH over AlZn11O12 and (AlZn11O12)2 requires an activation energy of 0.45 and 0.29eV, respectively, indicating it can be easily performed at normal temperatures. Besides, the overoxidation of methanol into formaldehyde cannot take place over the AlZn11O12 and (AlZn11O12)2, due to the high energy barrier needed to dissociate C-H bond of the CH3O intermediate. Dispersion-corrected density functional theory calculations were performed through GGA-PBE exchange-correlation functional combined with a numerical double-ζ plus polarization (DNP) basis set as implemented in DMol3. To include the relativistic effects of core electrons of Zn atoms, DFT-semicore pseudopotentials were adopted. The DFT + D scheme proposed by Grimme was used to involve weak dispersion interactions within the DFT calculations. The reaction energy paths were generated by the minimum energy path calculations using the NEB method.