Evidence for site-sensitive screening of core holes at the Si and Ge (001) surface

Real-Space Pseudopotential Method for the Calculation of Third-Row Elements X-ray Photoelectron Spectroscopic Signatures

X-ray photoelectron spectroscopy (XPS) is a powerful characterization technique that unveils subtle chemical environment differences via core-electron binding energy (CEBE) analysis. We extend the development of real-space pseudopotential methods to calculating 1s, 2s, and 2p3/2 CEBEs of third-row elements (S, P, and Si) within the framework of Kohn-Sham density-functional theory (KS-DFT). The new approach systematically prevents variational collapse and simplifies core-excited orbital selection within dense energy level distributions. However, careful error cancellation analysis is required to achieve accuracy comparable to all-electron methods and experiments. Combined with real-space KS-DFT implementation, this development enables large-scale simulations with both Dirichlet boundary conditions and periodic boundary conditions.

Bridging the gap between surface physics and photonics

Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.

Al-Pt intermetallic compounds: HAXPES study

Intermetallic compounds in the Al-Pt system were systematically studied via hard X-ray photoelectron spectroscopy, focusing on the positions of Pt 4f and Al 2s core levels and valence band features. On one hand, with increasing Al content, the Pt 4f core levels shift towards higher binding energies (BE), revealing the influence of the atomic interactions (chemical bonding) on the electronic state of Pt. On the other hand, the charge transfer from Al to Pt increases with increasing Al content in Al-Pt compounds. These two facts cannot be combined using the standard "chemical shift" approach. Computational analysis reveals that higher negative effective charges of Pt atoms are accompanied by reduced occupancy of Pt 5d orbitals, leading to the limited availability of these electrons for the screening of the 4f core hole and this in turn explains the experimentally observed shift of 4f core levels to higher BE.

Effect of Different In2O3(111) Surface Terminations on CO2 Adsorption

In2O3-based catalysts have shown high activity and selectivity for CO2 hydrogenation to methanol; however, the origin of the high performance of In2O3 is still unclear. To elucidate the initial steps of CO2 hydrogenation over In2O3, we have combined X-ray photoelectron spectroscopy and density functional theory calculations to study the adsorption of CO2 on the In2O3(111) crystalline surface with different terminations, namely, the stoichiometric, reduced, and hydroxylated surface. The combined approach confirms that the reduction of the surface results in the formation of In adatoms and that water dissociates on the surface at room temperature. A comparison of the experimental spectra and the computed core-level shifts (using methanol and formic acid as benchmark molecules) suggests that CO2 adsorbs as a carbonate on all three surface terminations. We find that the adsorption of CO2 is hindered by hydroxyl groups on the hydroxylated surface.

Two-Dimensional Self-Assembly Driven by Intermolecular Hydrogen Bonding in Benzodi-7-azaindole Molecules on Au(111)

The control of molecular structures at the nanoscale plays a critical role in the development of materials and applications. The adsorption of a polyheteroaromatic molecule with hydrogen bond donor and acceptor sites integrated in the conjugated structure itself, namely, benzodi-7-azaindole (BDAI), has been studied on Au(111). Intermolecular hydrogen bonding determines the formation of highly organized linear structures where surface chirality, resulting from the 2D confinement of the centrosymmetric molecules, is observed. Moreover, the structural features of the BDAI molecule lead to the formation of two differentiated arrangements with extended brick-wall and herringbone packing. A comprehensive experimental study that combines scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory theoretical calculations has been performed to fully characterize the 2D hydrogen-bonded domains and the on-surface thermal stability of the physisorbed material.

Long-Term Degradation Mechanisms in Application-Implemented Radical Thin Films

Blatter radical derivatives are very attractive due to their potential applications, ranging from batteries to quantum technologies. In this work, we focus on the latest insights regarding the fundamental mechanisms of radical thin film (long-term) degradation, by comparing two Blatter radical derivatives. We find that the interaction with different contaminants (such as atomic H, Ar, N, and O and molecular H₂, N₂, O₂, H₂O, and NH₂) affects the chemical and magnetic properties of the thin films upon air exposure. Also, the radical-specific site, where the contaminant interaction takes place, plays a role. Atomic H and NH₂ are detrimental to the magnetic properties of Blatter radicals, while the presence of molecular water influences more specifically the magnetic properties of the diradical thin films, and it is believed to be the major cause of the shorter diradical thin film lifetime in air.

An ab initio approach to anisotropic alloying into the Si(001) surface

By employing density functional theory calculations, we explore the initial stage of competitive alloying of co-deposited silver and indium atoms into a silicon surface. In particular, we identify respective adsorption positions and activation barriers governing their diffusion on a dimer-reconstructed silicon surface. Furthermore, we develop a growth model that appropriately describes diffusion mechanisms and silicon morphology with the account of silicon dimerization and the presence of C-type defects. Based on the surface kinetic Monte Carlo simulations, we examine the dynamics of bimetallic adsorption and elaborate on the temperature effects on the submonolayer growth of an Ag-In alloy. A close inspection of adatom migration clearly indicates effective nucleation of Ag and In atoms, followed by the formation of orthogonal atomic chains. We show that the epitaxial bimetallic growth might potentially lead to exotic ordering of adatoms in the form of anisotropic two-dimensional lattices via orthogonally oriented single-metal rows. We argue that this scenario becomes favorable provided above room temperature, while our numerical results are shown to be in agreement with the experimental findings.

Real-Space Pseudopotential Method for the Calculation of 1s Core-Level Binding Energies

We systematically studied a real-space pesudopotential method for the calculation of 1s core–electron binding energies of second-row elements B, C, N, and O within the framework of Kohn–Sham density functional theory (KS-DFT). With Dirichlet boundary conditions, pseudopotential calculations can provide accurate core–electron binding energies for molecular systems, when compared with the results from all-electron calculations and experiments. Furthermore, we report that with one simple additional nonself-consistent calculation as a refinement step using a hybrid exchange-correlation functional, we can generally improve the accuracy of binding energy shifts, promising a strategy for improving accuracy at a much lower computational cost. The specializations in the present approach, combined with our efficient real-space KS-DFT implementation, provide key advantages for calculating accurate core–electron binding energies of large-scale systems.

Insights into the electronic structure of hydroxyl on Ag(110) under near ambient conditions

Adsorbed hydroxyl is a key intermediate present in many catalytic reactions and electrochemical processes. In particular, hydroxyl adsorbed on noble metal surfaces has attracted attention due to its role in water-gas shift, selective oxidation of hydrocarbons and water splitting. In this work, from a well-defined oxygen covered Ag(110) surface with O-p(2 × 1) reconstruction, we prepared a fully hydroxylated surface phase in equilibrium with water and oxygen in the gas phase under near ambient conditions. In situ soft X-ray spectroscopy combined with density functional theory revealed distinctive modifications in the electronic structure of the adsorbate layer upon hydroxylation. We show that both the core and valence electronic states of OH adsorbates have higher binding energies relative to the Fermi level than the states for the O adsorbate. The OH orbitals interact with the d band of Ag giving rise to hybridized orbitals with bonding and anti-bonding symmetry, with larger energy splitting than the oxygen adsorbate.

A Synchrotron Radiation Photoemission Study of SiGe(001)-2×1 Grown on Ge and Si Substrates: The Surface Electronic Structure for Various Ge Concentrations

Beyond the macroscopic perspective, this study microscopically investigates Si_1−xGe_x(001)-2×1 samples that were grown on the epi Ge(001) and epi Si(001) substrates via molecular-beam epitaxy, using the high-resolution synchrotron radiation photoelectron spectroscopy (SRPES) as a probe. The low-energy electron diffraction equipped in the SRPES chamber showed 2×1 double-domain reconstruction. Analyses of the Ge 3d core-level spectra acquired using different photon energies and emission angles consistently reveal the ordered spots to be in a Ge–Ge tilted configuration, which is similar to that in epi Ge(001)-2×1. It was further found that the subsurface layer was actually dominated by Ge, which supported the buckled configuration. The Si atoms were first found in the third surface layer. These Si atoms were further divided into two parts, one underneath the Ge–Ge dimer and one between the dimer row. The distinct energy positions of the Si 2p core-level spectrum were caused by stresses, not by charge alternations.

Can oxygen vacancies in ceria surfaces be measured by O1s photoemission spectroscopy?

X-ray photoemission spectroscopy is a standard technique for materials characterization and the O 1s binding energy is commonly measured for oxides. Here we use density functional theory calculations to investigate how the O 1s binding energy in CeO₂(111) is influenced by the presence of oxygen vacancies. The case with point vacancies in CeO₂(111) is compared to complete reduction to Ce₂O₃. Reduction of CeO₂by oxygen vacancies is found to have a minor effect on the O 1s binding energy. The O 1s binding energy is instead clearly changed when the character of the chemical bond for the considered oxygen atom is modified by, for example, the formation of OH-groups or carbonates.

Phthalocyanine reactivity and interaction on the 6H-SiC(0001)-(3×3) surface by core-level experiments and simulations

The adsorption of phthalocyanine (H2Pc) on the 6H-SiC(0001)-(3×3) surface is investigated using X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and density functional theory (DFT) calculations....

Atomic-Scale Modification of Oxidation Phenomena on the Ge(100) Surface by Si Alloying

Properties of Ge oxides are significantly different from those of widely used Si oxides. For example, the instability of GeO _x at device junctions causes electronic defect levels that degrade the performance of Ge-containing devices (e.g., transistors and infrared detectors). Therefore, the passivating Si layers have been commonly used at Ge interfaces to reduce the effects of Ge oxide instability and mimic the successful strategy of Si oxidation. To contribute to the atomic-scale knowledge and control of oxidation of such Si-alloyed Ge interfaces (O/Si/Ge), we present a synchrotron radiation core-level study of O/Si/Ge, which is combined with scanning probe microscopy measurements. The oxidation processes and electronic properties of O/Si/Ge(100) are examined as functions of Si amount and oxidation doses. In particular, the incorporation of Si into Ge is shown to cause the strengthening of Ge-O bonds and the increase of incorporated oxygen amount in oxide/Ge junctions, supporting that the method is useful to decrease the defect-level densities.

Controlled growth of ordered monolayers of N-heterocyclic carbenes on silicon

N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications.

The Degree of Oxidation of Graphene Oxide

We show that the degree of oxidation of graphene oxide (GO) can be obtained by using a combination of state-of-the-art ab initio computational modeling and X-ray photoemission spectroscopy (XPS). We show that the shift of the XPS C1s peak relative to pristine graphene, ΔEC1s, can be described with high accuracy by ΔEC1s=A(cO-cl)2+E0, where c0 is the oxygen concentration, A=52.3 eV, cl=0.122, and E0=1.22 eV. Our results demonstrate a precise determination of the oxygen content of GO samples.

Final-State Simulations of Core-Level Binding Energies at Metal-Organic Hybrid Interfaces: Artifacts Caused by Spurious Collective Electrostatic Effects

Core-level energies are frequently calculated to explain the X-ray photoelectron spectra of metal-organic hybrid interfaces. The current paper describes how such simulations can be flawed when modeling interfaces between physisorbed organic molecules and metals. The problem occurs when applying periodic boundary conditions to correctly describe extended interfaces and simultaneously considering core hole excitations in the framework of a final-state approach to account for screening effects. Since the core hole is generated in every unit cell, an artificial dipole layer is formed. In this work, we study methane on an Al(100) surface as a deliberately chosen model system for hybrid interfaces to evaluate the impact of this computational artifact. We show that changing the supercell size leads to artificial shifts in the calculated core-level energies that can be well beyond 1 eV for small cells. The same applies to atoms at comparably large distances from the substrate, encountered, for example, in extended, upright-standing adsorbate molecules. We also argue that the calculated work function change due to a core-level excitation can serve as an indication for the occurrence of such an artifact and discuss possible remedies for the problem.

Accurate Absolute and Relative Core-Level Binding Energies from GW

We present an accurate approach to compute X-ray photoelectron spectra based on the GW Green's function method that overcomes the shortcomings of common density functional theory approaches. GW has become a popular tool to compute valence excitations for a wide range of materials. However, core-level spectroscopy is thus far almost uncharted in GW. We show that single-shot perturbation calculations in the G0W0 approximation, which are routinely used for valence states, cannot be applied for core levels and suffer from an extreme, erroneous transfer of spectral weight to the satellite spectrum. The correct behavior can be restored by partial self-consistent GW schemes or by using hybrid functionals with almost 50% of exact exchange as a starting point for G0W0. We also include relativistic corrections and present a benchmark study for 65 molecular 1s excitations. Our absolute and relative GW core-level binding energies agree within 0.3 and 0.2 eV with experiment, respectively.

Ultrafast core-excited electron dynamics in model crystalline organic semiconductors

Resonant photoemission measurements show that ultrafast electron dynamics in core-excited states of large organic semiconductors depends on both the nature of the core-hole and the proximal chemical environment.

Hydrogen adsorption on In2O3(111) and In2O3(110)

DFT calculations are used to explore H₂ adsorption and dissociation on In₂O₃ surfaces.

Operando XANES from first-principles and its application to iridium oxide

Density-functional theory calculations augmented with a continuum description of the electrochemical environment are implemented to simulated X-ray absorption spectra as a function of the applied potential.

A dithiocarbamate anchoring group as a flexible platform for interface engineering

The molecular organization and electronic properties of dithiocarbamate (DTC) anchored self-assembled monolayers (SAMs) linked to Au(111) substrates are studied by a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and state-of-the-art density functional theory calculations. For that, several piperidine/piperazine precursors with different architecture and substitution patterns are selected. The presented data show that the DTC anchor provides a useful building block for monomolecular self-assembly on coinage metals with both sulfur atoms bonded to the substrate in a way similar to what is usually observed for the more commonly applied thiolate docking group. The combination of the DTC group with the quite flexible piperidine/piperazine cyclic linkers results in a dense molecular packing with an upright orientation of the terminal moieties. The latter comprise phenyl rings bearing various substituents, which enables tuning the interfacial dipole over a wide range. Simulations on two prototypical DTC-docked SAMs help to better understand the experimental observations and provide insight into the local origin of the SAM-induced shifts in the electrostatic energy. In particular, a comparison of measured and simulated XP spectra reveals the significant contribution of the DTC group to the interfacial dipole.

Microscopic Views of Atomic and Molecular Oxygen Bonding with epi Ge(001)-2 × 1 Studied by High-Resolution Synchrotron Radiation Photoemission

In this paper, we investigate the embryonic stage of oxidation of an epi Ge(001)-2 × 1 by atomic oxygen and molecular O₂ via synchrotron radiation photoemission. The topmost buckled surface with the up- and down-dimer atoms, and the first subsurface layer behaves distinctly from the bulk by exhibiting surface core-level shifts in the Ge 3d core-level spectrum. The O₂ molecules become dissociated upon reaching the epi Ge(001)-2 × 1 surface. One of the O atoms removes the up-dimer atom and the other bonds with the underneath Ge atom in the subsurface layer. Atomic oxygen preferentially adsorbed on the epi Ge(001)-2 ×1 in between the up-dimer atoms and the underneath subsurface atoms, without affecting the down-dimer atoms. The electronic environment of the O-affiliated Ge up-dimer atoms becomes similar to that of the down-dimer atoms. They both exhibit an enrichment in charge, where the subsurface of the Ge layer is maintained in a charge-deficient state. The dipole moment that was originally generated in the buckled reconstruction no longer exists, thereby resulting in a decrease in the ionization potential. The down-dimer Ge atoms and the back-bonded subsurface atoms remain inert to atomic O and molecular O₂, which might account for the low reliability in the Ge-related metal-oxide-semiconductor (MOS) devices.

Experimental and theoretical 2p core-level spectra of size-selected gas-phase aluminum and silicon cluster cations: chemical shifts, geometric structure, and coordination-dependent screening

We present 2p core-level spectra of size-selected aluminum and silicon cluster cations from soft X-ray photoionization efficiency curves and density functional theory. The experimental and theoretical results are in very good quantitative agreement and allow for geometric structure determination. New ground state geometries for Al₁₂⁺, Si₁₅⁺, Si₁₆⁺, and Si₁₉⁺ are proposed on this basis. The chemical shifts of the 2p electron binding energies reveal a substantial difference for aluminum and silicon clusters: while in aluminum the 2p electron binding energy decreases with increasing coordination number, no such correlation was observed for silicon. The 2p binding energy shifts in clusters of both elements differ strongly from those of the corresponding bulk matter. For aluminum clusters, the core-level shifts between outer shell atoms and the encapsulated atom are of opposite sign and one order of magnitude larger than the corresponding core-level shift between surface and bulk atoms in the solid. For silicon clusters, the core-level shifts are of the same order of magnitude in clusters and in bulk silicon but no obvious correlation of chemical shift and bond length, as present for reconstructed silicon surfaces, are observed.

Platinum single-atom adsorption on graphene: a density functional theory study

Single-atom catalysis, which utilizes single atoms as active sites, is one of the most promising ways to enhance the catalytic activity and to reduce the amount of precious metals used. Platinum atoms deposited on graphene are reported to show enhanced catalytic activity for some chemical reactions, e.g. methanol oxidation in direct methanol fuel cells. However, the precise atomic structure, the key to understand the origin of the improved catalytic activity, is yet to be clarified. Here, we present a computational study to investigate the structure of platinum adsorbed on graphene with special emphasis on the edges of graphene nanoribbons. By means of density functional theory based thermodynamics, we find that single platinum atoms preferentially adsorb on the substitutional carbon sites at the hydrogen terminated graphene edge. The structures are further corroborated by the core level shift calculations. Large positive core level shifts indicate the strong interaction between single Pt atoms and graphene. The atomistic insight obtained in this study will be a basis for further investigation of the activity of single-atom catalysts based on platinum and graphene related materials.

Unusual oxidation-induced core-level shifts at the HfO2/InP interface

X-ray photoelectron spectroscopy (XPS) is one of the most used methods in a diverse field of materials science and engineering. The elemental core-level binding energies (BE) and core-level shifts (CLS) are determined and interpreted in the XPS. Oxidation is commonly considered to increase the BE of the core electrons of metal and semiconductor elements (i.e., positive BE shift due to O bonds), because valence electron charge density moves toward electronegative O atoms in the intuitive charge-transfer model. Here we demonstrate that this BE hypothesis is not generally valid by presenting XPS spectra and a consistent model of atomic processes occurring at HfO₂/InP interface including negative In CLSs. It is shown theoretically for abrupt HfO₂/InP model structures that there is no correlation between the In CLSs and the number of oxygen neighbors. However, the P CLSs can be estimated using the number of close O neighbors. First native oxide model interfaces for III-V semiconductors are introduced. The results obtained from ab initio calculations and synchrotron XPS measurements emphasize the importance of complementary analyses in various academic and industrial investigations where CLSs are at the heart of advancing knowledge.

Fingerprints of sp¹ Hybridized C in the Near-Edge X-ray Absorption Spectra of Surface-Grown Materials

Carbon structures comprising sp 1 chains (e.g., polyynes or cumulenes) can be synthesized by exploiting on-surface chemistry and molecular self-assembly of organic precursors, opening to the use of the full experimental and theoretical surface-science toolbox for their characterization. In particular, polarized near-edge X-ray absorption fine structure (NEXAFS) can be used to determine molecular adsorption angles and is here also suggested as a probe to discriminate sp 1 /sp 2 character in the structures. We present an ab initio study of the polarized NEXAFS spectrum of model and real sp 1 /sp 2 materials. Calculations are performed within density functional theory with plane waves and pseudopotentials, and spectra are computed by core-excited C potentials. We evaluate the dichroism in the spectrum for ideal carbynes and highlight the main differences relative to typical sp 2 systems. We then consider a mixed polymer alternating sp 1 C 4 units with sp 2 biphenyl groups, recently synthesized on Au(111), as well as other linear structures and two-dimensional networks, pointing out a spectral line shape specifically due to the the presence of linear C chains. Our study suggests that the measurements of polarized NEXAFS spectra could be used to distinctly fingerprint the presence of sp 1 hybridization in surface-grown C structures.

Accurate quasiparticle calculation of x-ray photoelectron spectra of solids

It has been highly desired to provide an accurate and reliable method to calculate core electron binding energies (CEBEs) of crystals and to understand the final state screening effect on a core hole in high resolution x-ray photoelectron spectroscopy (XPS), because the ΔSCF method cannot be simply used for bulk systems. We propose to use the quasiparticle calculation based on many-body perturbation theory for this problem. In this study, CEBEs of band-gapped crystals, silicon, diamond, β-SiC, BN, and AlP, are investigated by means of the GW approximation (GWA) using the full ω integration and compared with the preexisting XPS data. The screening effect on a deep core hole is also investigated in detail by evaluating the relaxation energy (RE) from the core and valence contributions separately. Calculated results show that not only the valence electrons but also the core electrons have an important contribution to the RE, and the GWA have a tendency to underestimate CEBEs due to the excess RE. This underestimation can be improved by introducing the self-screening correction to the GWA. The resulting C1s, B1s, N1s, Si2p, and Al2p CEBEs are in excellent agreement with the experiments within 1 eV absolute error range. The present self-screening corrected GW approach has the capability to achieve the highly accurate prediction of CEBEs without any empirical parameter for band-gapped crystals, and provide a more reliable theoretical approach than the conventional ΔSCF-DFT method.

Core Level Spectra of Organic Molecules Adsorbed on Graphene

We perform first principle calculations based on density functional theory to investigate the effect of the adsorption of core-excited organic molecules on graphene. We simulate Near Edge X-ray absorption Fine Structure (NEXAFS) and X-ray Photoemission Spectroscopy (XPS) at the N and C edges for two moieties: pyridine and the pyridine radical on graphene, which exemplify two different adsorption characters. The modifications of molecular and graphene energy levels due to their interplay with the core-level excitation are discussed. We find that upon physisorption of pyridine, the binding energies of graphene close to the adsorption site reduce mildly, and the NEXAFS spectra of the molecule and graphene resemble those of gas phase pyridine and pristine graphene, respectively. However, the chemisorption of the pyridine radical is found to significantly alter these core excited spectra. The C 1s binding energy of the C atom of graphene participating in chemisorption increases by ∼1 eV, and the C atoms of graphene alternate to the adsorption site show a reduction in the binding energy. Analogously, these C atoms also show strong modifications in the NEXAFS spectra. The NEXAFS spectrum of the chemisorbed molecule is also modified as a result of hybridization with and screening by graphene. We eventually explore the electronic properties and magnetism of the system as a core-level excitation is adiabatically switched on.

On the prediction of core level binding energies in molecules, surfaces and solids

Core level binding energies, directly measured by X-ray photoelectron spectroscopy (XPS), provide unique information regarding the chemical environment of atoms in a given system. However, interpretation of XPS in extended systems may not be straightforward and requires assistance from theory. The different state-of-the-art theoretical methods commonly used to approach core level binding energies and their shifts with respect to a given reference are reviewed and critically assessed with special emphasis on recently developed theoretical methods and with a focus on future applications in materials and surface sciences.

An experimental and theoretical investigation of XPS and NEXAFS of 5-halouracils

The C, N and O 1s excitation and ionization processes of 5X-uracil (X = F, Cl, Br, and I) were investigated using near edge X-ray absorption fine structure (NEXAFS) and X-ray photoemission (XPS) spectroscopies. This study aims at the fine assessment of the effects of the functionalization of uracil molecules by halogen atoms having different electronegativity and bound to the same molecular site. Two DFT-based approaches, which rely on different paradigms, have been used to simulate the experimental spectra and to assign the corresponding features. The analysis of the screening of the core holes of the different atoms via electronic charge density plots has turned out to be a useful tool to illustrate the competition between the partially aromatic and partially conjugate properties of this class of molecules.

Are multiple oxygen species selective in ethylene epoxidation on silver?

The nature of the oxygen species active in ethylene epoxidation is a long-standing question. While the structure of the oxygen species that participates in total oxidation (nucleophilic oxygen) is known the atomic structure of the selective species (electrophilic oxygen) is still debated. Here, we use both in situ and UHV X-ray Photoelectron Spectroscopy (XPS) to study the interaction of oxygen with a silver surface. We show experimental evidence that the unreconstructed adsorbed atomic oxygen (Oads) often argued to be active in epoxidation has a binding energy (BE) ≤ 528 eV, showing a core-level shift to lower BE with respect to the O-reconstructions, as previously predicted by DFT. Thus, contrary to the frequent assignment, adsorbed atomic oxygen cannot account for the electrophilic oxygen species with an O 1s BE of 530-531 eV, thought to be the active species in ethylene epoxidation. Moreover, we show that Oads is present at very low O-coverages during in situ XPS measurements and that it can be obtained at slightly higher coverages in UHV at low temperature. DFT calculations support that only low coverages of Oads are stable. The highly reactive species is titrated by background gases even at low temperature in UHV conditions. Our findings suggest that at least two different species could participate in the partial oxidation of ethylene on silver.

Assessing GW Approaches for Predicting Core Level Binding Energies

Here we present a systematic study on the performance of different GW approaches: G₀W₀, G₀W₀ with linearized quasiparticle equation (lin-G₀W₀), and quasiparticle self-consistent GW (qsGW), in predicting core level binding energies (CLBEs) on a series of representative molecules comparing to Kohn-Sham (KS) orbital energy-based results. KS orbital energies obtained using the PBE functional are 20-30 eV lower in energy than experimental values obtained from X-ray photoemission spectroscopy (XPS), showing that any Koopmans-like interpretation of KS core level orbitals fails dramatically. Results from qsGW lead to CLBEs that are closer to experimental values from XPS, yet too large. For the qsGW method, the mean absolute error is about 2 eV, an order of magnitude better than plain KS PBE orbital energies and quite close to predictions from ΔSCF calculations with the same functional, which are accurate within ∼1 eV. Smaller errors of ∼0.6 eV are found for qsGW CLBE shifts, again similar to those obtained using ΔSCF PBE. The computationally more affordable G₀W₀ approximation leads to results less accurate than qsGW, with an error of ∼9 eV for CLBEs and ∼0.9 eV for their shifts. Interestingly, starting G₀W₀ from PBE0 reduces this error to ∼4 eV with a slight improvement on the shifts as well (∼0.4 eV). The validity of the G₀W₀ results is however questionable since only linearized quasiparticle equation results can be obtained. The present results pave the way to estimate CLBEs in periodic systems where ΔSCF calculations are not straightforward although further improvement is clearly needed.

Individual Atomic Imaging of Multiple Dopant Sites in As-Doped Si Using Spectro-Photoelectron Holography

The atomic scale characterization of dopant atoms in semiconductor devices to establish correlations with the electrical activation of these atoms is essential to the advancement of contemporary semiconductor process technology. Spectro-photoelectron holography combined with first-principles simulations can determine the local three-dimensional atomic structures of dopant elements, which in turn affect their electronic states. In the work reported herein, this technique was used to examine arsenic (As) atoms doped into a silicon (Si) crystal. As 3d core level photoelectron spectroscopy demonstrated the presence of three types of As atoms at a total concentration of approximately 10²⁰ cm^-3, denoted as BEH, BEM, and BEL. On the basis of Hall effect measurements, the BEH atoms corresponded to electrically active As occupying substitutional sites and exhibiting larger thermal fluctuations than the Si atoms, while the BEM atoms corresponded to electrically inactive As embedded in the As_nV (n = 2-4) type clusters. Finally, the BEL atoms were assigned to electrically inactive As in locally disordered structures.

Theoretical Insights into Unexpected Molecular Core Level Shifts: Chemical and Surface Effects

A set of density-functional theory based tools is employed to elucidate the influence of chemical and surface-induced changes on the core level shifts of X-ray photoelectron spectroscopy experiments. The capabilities of our tools are demonstrated by analyzing the origin of an unpredicted component in the N 1s core level spectra of metal phthalocyanine molecules (in particular ZnPc) adsorbed on Cu(110). We address surface induced effects, such as splitting of the lowest unoccupied molecular orbital or local electrostatic effects, demonstrating that these cannot account for the huge core level shift measured experimentally. Our calculations also show that, when adsorbed at low temperatures, these molecules might capture hydrogen atoms from the surface, giving rise to hydrogenated molecular species and, consequently, to an extra component in the molecular core level spectra. Only upon annealing, and subsequent hydrogen release, would the molecules recover their nominal structural and electronic properties.

Methanol on Anatase TiO2 (101): Mechanistic Insights into Photocatalysis

The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction.

Absolute Binding Energies of Core Levels in Solids from First Principles

A general method is presented to calculate absolute binding energies of core levels in metals and insulators, based on a penalty functional and an exact Coulomb cutoff method in the framework of density functional theory. The spurious interaction of core holes between supercells is avoided by the exact Coulomb cutoff method, while the variational penalty functional enables us to treat multiple splittings due to chemical shift, spin-orbit coupling, and exchange interaction on equal footing, both of which are not accessible by previous methods. It is demonstrated that the absolute binding energies of core levels for both metals and insulators are calculated by the proposed method in a mean absolute (relative) error of 0.4 eV (0.16%) for eight cases compared to experimental values measured with x-ray photoemission spectroscopy within a generalized gradient approximation to the exchange-correlation functional.

Spectroscopic identification of the chemical interplay between defects and dopants in Al-doped ZnO

Contributions to the spectroscopic response of defects and dopants in Al-doped ZnO films are determined combining X-ray spectroscopies and DFT.

The electronic structure of iridium oxide electrodes active in water splitting

Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with Ir(III). These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts.

Understanding Chemical versus Electrostatic Shifts in X-ray Photoelectron Spectra of Organic Self-Assembled Monolayers

The focus of the present article is on understanding the insight that X-ray photoelectron spectroscopy (XPS) measurements can provide when studying self-assembled monolayers. Comparing density functional theory calculations to experimental data on deliberately chosen model systems, we show that both the chemical environment and electrostatic effects arising from a superposition of molecular dipoles influence the measured core-level binding energies to a significant degree. The crucial role of the often overlooked electrostatic effects in polar self-assembled monolayers (SAMs) is unambiguously demonstrated by changing the dipole density through varying the SAM coverage. As a consequence of this effect, care has to be taken when extracting chemical information from the XP spectra of ordered organic adsorbate layers. Our results, furthermore, imply that XPS is a powerful tool for probing local variations in the electrostatic energy in nanoscopic systems, especially in SAMs.

Skin Bond Electron Relaxation Dynamics of Germanium Manipulated by Interactions with H2 , O2 , H2 O, H2 O2 , HF, and Au

Although germanium performs amazingly well at sites surrounding hetero-coordinated impurities and under-coordinated defects or skins with unusual properties, having important impact on electronic and optical devices, understanding the behavior of the local bonds and electrons at such sites remains a great challenge. Here we show that a combination of density functional theory calculations, zone-resolved X-ray photoelectron spectroscopy, and bond order length strength correlation mechanism has enabled us to clarify the physical origin of the Ge 3d core-level shift for the under-coordinated (111) and (100) skin with and without hetero-coordinated H2 , O2 , H2 O, H2 O2 , HF impurities. The Ge 3d level shifts from 27.579 (for an isolated atom) by 1.381 to 28.960 eV upon bulk formation. Atomic under-coordination shifts the binding energy further to 29.823 eV for the (001) and to 29.713 eV for the (111) monolayer skin. Addition of O2 , HF, H2 O, H2 O2 and Au impurities results in quantum entrapment by different amounts, but H adsorption leads to polarization.