1s Core-level Shifts Research Articles

Real-Time In-Situ Investigation of the Neutron Irradiation Resistance Ability of Nd3+-Doped Gd3Sc2Al3O12 Laser Crystal

In optical crystals, photodarkening will occur after they were irradiated with high-energy particles, and such induced optical loss generally results in significant performance degradation whether they are used as passive or active optical elements. In the present study, the effects of neutron irradiation on the optical response of the Nd3+-doped Gd3Sc2Al3O12 (Nd:GSAG) single crystal has been revealed in real-time and in-situ. Transient and permanent transmittance reduction in the crystal induced by neutron radiation has been observed and the reduction mechanisms have been analyzed. The XRD characterization method demonstrated that the crystal structure remained constant both before and after neutron irradiation. Importantly, the X-ray photoelectron peak of the O 1s core level shifts to high binding energy, indicating that oxygen vacancies were produced in the crystal after irradiation with neutrons. Thus, the permanent reduction in the transmittance of the crystal after irradiation with neutrons can be attributed to the generation of oxygen vacancies in the crystal. To the best of our knowledge, it is the first time the damage types in rare earth oxide laser crystals caused by neutron irradiation were revealed.

Atomistic Insight into Nitrogen-Terminated Diamond(001) Surfaces by the Adsorption of N, NH, and NH2: A Density Functional Theory Study.

Layer-by-layer construction of diamond devices for spin-sensing calls for the atomistic understanding of the nitrogen species on diamond surfaces. Motivated by recent experiments, we used density functional theory simulations to examine the adsorption of nitrogen species (N, NH, and NH2) on bare and hydrogenated diamond(001) surfaces. On the bare substrate, we find that nitrogen species favor to attack the C═C dimers at low coverages, forming N(ad) and NH(ad) in a bridge configuration and NH2(ad) in a terminal configuration. At increasing coverages up to one full monolayer, the computed adsorption geometries and energetics suggest that the adsorbate-adsorbate interactions are attractive for N(ad), but repulsive for NH(ad) and NH2(ad). On the hydrogenated substrate, the adsorbed nitrogen species are subject to structural modification, as resulted from the weakened adsorbate-substrate interactions. Further, we calculated the vibration of nitrogen species and the 1s core-level shift of surface carbons, providing atomistic insights into the nature of surface bonding. Lastly, we simulated images of representative nitrogen species adsorbed on diamond(001), guiding future work using scanning tunneling microscopy.

Tuning graphene doping by carbon monoxide intercalation at the Ni(111) interface

Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epitaxial graphene monolayer grown on Ni(111), getting trapped into the confined region at the interface. On the basis of ab-initio density functional theory calculations, we provide here a full investigation of the intercalated CO pattern, highlighting the modifications induced on the graphene electronic structure. For a CO coverage as low as 0.14 monolayer (ML), the graphene layer is spatially decoupled from the metallic substrate, with a significant C 1s core level shift towards lower binding energies. The most relevant signature of the CO intercalation is a clear switching of the graphene doping state, which changes from n-type, when strongly interacting with the metal surface, to p-type. The shift of the Dirac cone linearly depends on the CO coverage, reaching about 0.9 eV for the saturation value of 0.57 ML. Theoretical predictions are compared with the results of scanning tunnelling microscopy, low-energy electron diffraction and photoemission spectroscopy experiments, which confirm the proposed scenario for the nearly saturated intercalated CO system.This result opens the way to the application of the graphene/Ni(111) interface as gas sensor to easily detect and quantify the presence of carbon monoxide.

Initial Steps in Forming the Electrode–Electrolyte Interface: H2O Adsorption and Complex Formation on the Ag(111) Surface from Combining Quantum Mechanics Calculations and Ambient Pressure X-ray Photoelectron Spectroscopy

The interaction of water with metal surfaces is at the heart of electrocatalysis. But there remain enormous uncertainties about the atomistic interactions at the electrode-electrolyte interface (EEI). As the first step toward an understanding of the EEI, we report here the details of the initial steps of H2O adsorption and complex formation on a Ag(111) surface, based on coupling quantum mechanics (QM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) experiments. We find a close and direct comparison between simulation and experiment, validated under various isotherm and isobar conditions. We identify five observable oxygen-containing species whose concentrations depend sensitively on temperature and pressure: chemisorbed O* and OH*, H2O* stabilized by hydrogen bond interactions with OH* or O*, and multilayer H2O*. We identify the species experimentally by their O 1s core-level shift that we calculate with QM along with the structures and free energies as a function of temperature and pressure. This leads to a chemical reaction network (CRN) that we use to predict the time evolution of their concentrations over a wide range of temperature (298-798 K) and pressure conditions (10-6-1 Torr), which agree well with the populations determined from APXPS. This multistep simulation CRN protocol should be useful for other heterogeneous catalytic systems.

Initial oxidation of Cu(100) studied by X-ray photo-electron spectroscopy and density functional theory calculations

Density functional theory calculations and ambient pressure X-ray photoelectron spectroscopy are used to investigate initial oxidation of Cu(100). Surface stability with respect to oxygen coverage is calculated together with O 1s core level shifts. Oxidation of Cu(100) is found to occur via the formation of a p(2 × 2) overlayer (0.25 ML) followed by a reconstructed (22 × 2)R45° -O missing-row (MR) structure (0.50 ML). A c(4 × 6) structure with a 0.3 ML coverage is close in stability for intermediate oxygen chemical potentials. The relative stability is found to be weakly dependent on the applied exchange-correlation functional. The calculated shifts in the O 1s binding energy are in good agreement with the measured evolution of the binding energy. The shift to higher O 1s binding energies with increasing oxygen coverage is found to correlate with the charge on neighboring copper atoms. The O 1s core-level shifts here obtained with CO2 as oxidant, are similar to previous measurements of Cu(100) oxidation with O2.

Core level shifts of intercalated graphene

Through intercalation of metals and gases the Dirac cone of graphene on Ir(111) can be shifted with respect to the Fermi level without becoming destroyed by strong hybridization. Here, we use x-ray photoelectron spectroscopy to measure the C 1s core level shift (CLS) of graphene in contact with a number of structurally well-defined intercalation layers (O, H, Eu, and Cs). By analysis of our own and additional literature data for decoupled graphene, the C 1s CLS is found to be a non-monotonic function of the doping level. For small doping levels the shifts are well described by a rigid band model. However, at larger doping levels, a second effect comes into play which is proportional to the transferred charge and counteracts the rigid band shift. Moreover, not only the position, but also the C 1s peak shape displays a unique evolution as a function of doping level. Our conclusions are supported by intercalation experiments with Li, with which, due to the absence of phase separation, the doping level of graphene can be continuously tuned.

Chemical potential shift in organic field-effect transistors identified by soft X-ray operando nano-spectroscopy

A chemical potential shift in an organic field effect transistor (OFET) during operation has been revealed by soft X-ray operando nano-spectroscopy analysis performed using a three-dimensional nanoscale electron-spectroscopy chemical analysis system. OFETs were fabricated using ultrathin (3 ML or 12 nm) single-crystalline C10-DNBDT-NW films on SiO2 (200 nm)/Si substrates with a backgate electrode and top source/drain Au electrodes, and C 1s line profiles under biasing at the backgate and drain electrodes were measured. When applying −30 V to the backgate, there is C 1s core level shift of 0.1 eV; this shift can be attributed to a chemical potential shift corresponding to band bending by the field effect, resulting in p-type doping.

Computational and experimental evidence for a new TM–N3/C moiety family in non-PGM electrocatalysts

In the present work, we demonstrate that the formation energies of previously unexplored single and double carbon vacancy based TM-N3/C defect moieties are favourable. This prediction suggests that these defect motifs, in particular DV-Fe-N3/C can form during high-temperature catalyst synthesis. Defect specific N 1s core-level shifts were computed from first-principles for the deconvolution of XPS observations.

硬 X 線光電子分光による酸化膜の評価

We report the applications of a hard X-ray photoelectron spectroscopy to the characterization of SiO2/Si(001) systems. Large escape depth of high-energy photoelectron enables us to probe buried layers and their interfaces in multilayer structures. Estimation of SiO2 overlayer thicknesses up to 25 nm by angle resolved XPS was possible in SiO2/Si(001) samples. Determination of the thickness profile of a wedged shape SiO2 buried layer was successfully done in Ir (8 nm)/HfO2 (2.2 nm)/thickness graded-SiO2 (0-10 nm)/Si(100). The Si 1s core level showed a SiO2 thickness dependent shift, which was ascribed to fixed charge at the SiO2-Si interface. Energy distribution of interface states at ultrathin thermal oxide/Si(100) interfaces were determined by Si 1s core level shift by applying gate bias in metal-oxide-semiconductor (MOS) structure with 5 nm Au gate electrodes.

Intrinsic Atomic Orbitals: An Unbiased Bridge between Quantum Theory and Chemical Concepts

Modern quantum chemistry can make quantitative predictions on an immense array of chemical systems. However, the interpretation of those predictions is often complicated by the complex wave function expansions used. Here we show that an exceptionally simple algebraic construction allows for defining atomic core and valence orbitals, polarized by the molecular environment, which can exactly represent self-consistent field wave functions. This construction provides an unbiased and direct connection between quantum chemistry and empirical chemical concepts, and can be used, for example, to calculate the nature of bonding in molecules, in chemical terms, from first principles. In particular, we find consistency with electronegativities (χ), C 1s core-level shifts, resonance substituent parameters (σR), Lewis structures, and oxidation states of transition-metal complexes.

Exploring large O 1s and N 1s core level shifts due to intermolecular hydrogen bond formation in organic molecules

Core level shifts (CLSs) induced by intermolecular hydrogen bond (H-bond) formation are studied with a recent implementation based on density functional theory using pseudopotentials and localized atomic orbitals, as applied to the SIESTA code. By calculating different CLSs for a set of representative simple systems containing O and/or N atoms as proton donors and/or acceptors, we are able to determine the role of the core hole screening, from the difference of CLS values calculated in the final and initial state approximations. Our calculations show that CLSs are dominated by electrostatic effects, and that the final magnitude of the CLSs, which are positive (higher binding energy) for the proton acceptor and negative for the proton donor, can be larger than 1eV for strong H-bonds. We also find that core hole screening contribution to final CLS absolute values is always negative, thus being responsible for the difference in the magnitude of the CLS of the proton donor and proton acceptor.

Nature of visible-light responsive fluorinated titanium dioxides

The origin of the visible-light activity of fluorinated TiO2 has been systematically investigated via comprehensive theoretical studies. The 3-coordinated surface F atoms with higher 1s binding energy are identified to be the origin of the visible light activity of the fluorinated TiO2 by systematically analysing 1s core-level shifts of various types of F atoms in fluorinated TiO2. Further analysing the electronic structures of the fluorinated TiO2 using semi-local density functional theory and non-local hybrid density functional theory calculations demonstrates that the introduction of the 3-coordinated surface F atoms leads to the formation of Ti3+ ions in the sub-surface, which is the cause for the bandgap shrinking, increasing the visible-light activity. The results also reveal that the presence of 3-coordinated surface F atoms strengthens the adsorption of hydroxyl groups in their vicinity to enhance the photocatalytic activity. The findings of this work address a long-standing scientific question, providing useful guidance for the design and development of high performance VL photocatalysts.

Surface band bending and electron affinity as a function of hole accumulation density in surface conducting diamond

Simultaneous measurements of work function (ϕ) and C 1s core level shift were employed to determine the change in electron affinity (χ) and band bending as a function of hole sheet density on H-terminated diamond for atmospheric and fullerene (C60F48) induced surface conductivity. Contrary to earlier investigations, it is shown that changes in work function do not reflect variations in the position of the surface Fermi level in response to surface transfer doping. Instead, with a transition from −0.96 to −0.33 eV, χ accounts for a significant amount of the change in ϕ for hole densities between 5×108 and 4×1013 cm−2.

Ammonia on Ni(1 1 1) surface studied by first principles: Bonding, multilayers structure and comparison with experimental IR and XPS data

DFT calculations have been performed on the adsorption of NH 3 on Ni(1 1 1) to obtain information on the structure of the absorbed species, the nature of the chemical interactions between the adsorbate and the surface and the structure of multilayers formed at high coverage. A cluster model, using localized basis functions as well as an approach based on plane waves and periodic boundary conditions have been considered. The two approaches lead to similar results for the relative stabilities of investigated adsorption sites (atop > fcc > hcp) with an adsorption energy of about 15–24 kcal/mol (depending on the coverage). On the atop site, α-ammonia adsorbs molecularly with an equilibrium distance between the nitrogen atom and the surface of 2.03 ± 0.02 Å and a geometry close to the one of the molecule in the gas phase. The good agreement between the two DFT approaches clearly underlines the local nature of the adsorption reaction. The vibrational frequencies computed for NH 3 adsorbed in this site are in good agreement with experimental values. They show that the interaction with the surface leads to a weakening of the strength of the N–H bond while the angular stretching is stronger. Both orbital and topological analyses were used to investigate chemical interactions between the cluster and the molecule. The results strongly suggest an electrostatic (non-covalent) interaction between the substrate and the molecule. Calculations with NH 3 coverages above 0.25 confirm that saturation occurs at a coverage of 0.25. Above the saturation coverage, ammonia molecules in excess form multiple layers (β and γ ammonia) bonded to the first layer by intermolecular hydrogen bonds. N 1s core level shifts calculations performed for the several investigated coverages are also in good agreement with experimental XPS data. It is shown that the H-bond network more than the bond to the surface allows to understand the N 1s core level variations.

Electrical and Chemical Properties of the HfO2 / SiO2 / Si Stack: Impact of HfO2 Thickness and Thermal Budget

In this paper, we investigate the impact of thermal budget and thickness on the chemical and electronic properties of the stack. High temperature anneal at induces both the regrowth and reoxidation of the interfacial layer. A bias drop of 1.1 eV is observed along the whole stack via the C 1s core-level shift and is ascribed both to the interfacial dipole and to fixed charges in . Electrical measurements suggest a dipole strength of 0.2 eV. Ellipsometry and UV photoelectron spectroscopy are combined to deduce the electron affinity . This value does not change with increasing thermal budget or dielectric thickness. The barrier height is estimated to be in agreement with previous internal photoemission results.

Oxidation of deposited Au n ( n = 2–13) on SiO 2/Si: Influence of the NaOH(aq) treatment

Au cluster anions consisting of 2–13 atoms were soft-landed on native-oxide-covered Si wafers. Reaction of soft-landed clusters with an atomic oxygen atmosphere was studied using X-ray photoelectron spectroscopy (XPS). Au 5, Au 7, and Au 13 turned out to show pronounced inertness for Au-oxide formation. When the samples with deposited Au clusters were treated with aqueous NaOH, the inert Au 5, Au 7, and Au 13 clusters became reactive towards Au-oxide formation, whereas the other originally reactive clusters became inert. This result can be interpreted in terms of electronic modification of Au clusters by Na, which was also evidenced by Au 4f and Na 1s core level shifts.

D-Alaninol Adsorption on Cu(100): Photoelectron Spectroscopy and First-Principles Calculations

The adsorption of a single molecule of the D-enantiomer of alaninol (2-amino-1-propanol) on the surface of Cu(100) is investigated through density functional theory calculations. Different possible adsorption sites for D-alaninol are tested, and it is found that the most stable configuration presents both amino and hydroxyl group covalently interacting with "on top" copper atoms. The electronic structure is analyzed in detail and compared with experimental photoelectron spectra. Another adsorption structure in which a dehydrogenation process is assumed to occur on the amino group is analyzed and provides a possible explanation of the valence band electronic structure and of the experimentally observed N 1s core-level shift at full coverage, where a self-assembled ordered chiral monolayer is formed on the copper surface.

X-ray photoelectron spectroscopy of nitromethane adsorption products on Si(100): A model for N 1s core-level shifts in silicon oxynitride films

N 1s core-level shifts from x-ray photoelectron spectra (XPS) are reported for the adsorption products of nitromethane (CH3NO2) on Si(100). Three spectral peaks are identified and these are associated with specific bonding environments for nitrogen by comparison to predicted core-level shifts from density functional calculations on a range of energetically feasible chemical structures. These species can be classified according to the number of N–O bonds (zero, one, or two) that they contain and, in this sense, they are comparable to the species believed to exist in oxynitride films on Si. Since the energetically feasible products of room-temperature CH3NO2 adsorption can be identified with more confidence than those resulting from ion bombardment and high-temperature processing in oxynitride films, nitromethane provides a model system that can aid in correlating spectral features with specific atomic-scale structures. This work supports an earlier proposal that the XPS peak of weakest binding energy is due to species with a dangling bond on nitrogen, while the most intense peak is due to the energetically preferred NSi3 species.

C 60 bonding to graphite and boron nitride surfaces

The present study focuses on the interaction of C60 with the surfaces of highly oriented pyrolitic graphite (HOPG) and sp2-bonded boron nitride (BN). The nanocrystalline BN film was deposited by mass selected ion beams and features an sp2-bonded surface layer, which covers a cubic phase BN film. The first part of the experiment is the sequential deposition of C60, which is monitored by photoelectron spectroscopy in the x-ray (XPS) and ultraviolet (UPS) regime. The growth of the C60 layer on HOPG is close to a layer-by-layer growth mode, but on the BN surface island growth is favored. No charge transfer or chemical reaction (e.g., carbide formation) between the fullerene layer, and the underlying substrate is observed in either case. In the second part of the experiment the samples are heated at a rate of 10 K/min while simultaneously recording the UPS VB spectra. The complete desorption of C60 from the HOPG surface occurs in a small temperature interval between 510–530 K. For the sp2 BN surface the majority of C60 desorbs around 493 K, about half a monolayer (ML) remains, and the C60 concentration decreases gradually with increasing temperature; less than a tenth of a ML can be detected even at 1000 K. The first desorption event at 493 K is attributable to the multilayer desorption from islands. The remaining C60 directly in contact with the BN surface is then removed in a large temperature interval between 500 and 1000 K which indicates the presence of a multitude of adsorption sites. The presence of C60 on the BN film surface also induces a band bending and related B 1s and N 1s core level shifts. An upward band bending is present in the C60 overlayer, which indicates that defects are responsible for the pinning of the Fermi level at the interface.

Atomic processes of NO oxynitridation on Si(100) surfaces

Atomic processes of NO oxynitridation on Si(100) surfaces were studied by the first-principles calculation technique. NO is molecularly chemisorbed at very low temperatures. A channel toward dissociative chemisorption opens at slightly elevated temperatures. The N atom is chemisorbed mostly in between dimer atoms and the O atom in a backbond. The energy barrier for the secondary NO chemisorption increases and further dissociative chemisorption is suppressed in accordance with the observed self-limiting process below room temperature. At high temperatures the N atom substitutes for an adjacent Si atom, forming threefold coordination. The ejected Si atoms may occupy interstitial sites. The calculated N 1s core level shifts at each stage of chemisorption correspond well with the observed variation of core-level shifts upon temperature.