2p Photoemission Spectra Research Articles

The ST component in the Si 2p photoemission spectrum from H-terminated and oxidized Si (001) surfaces

One doublet is usually employed to fit the Si0 substrate species in the Si 2p photoemission spectra from Si (001) H-terminated (after piranha treatment) and oxidized surfaces. However, there is a second substrate-top component (ST) with a binding energy of 0.3 eV higher than the bulk component; its intensity varies from ∼10% at normal emission (i.e., 90° from the surface) to ∼20% at 35°. It is present even for oxidized surfaces and does not correspond to any of the suboxide species. It corresponds to the first layers of the substrate and is responsible for the decrease in the signal dip between the two S–O branches of the Si 2p spectra for glancing electron takeoff angles. Although it is resolvable for monochromatized sources, the ST component is absent in the literature on Si 2p spectra.

Peak-fitting of Cu 2p photoemission spectra in Cu0, Cu1+, and Cu2+ oxides: A method for discriminating Cu0 from Cu1+

The photoemission spectra of Cu 2p are one of the most studied. Its analysis is especially challenging because, among others, the peaks for Cu0 and Cu1+ overlap, and Cu1+ and Cu2+ coexist under many conditions. The chemical state determination of metallic copper and the initial oxidation mechanism of copper require state-of-the-art peak fitting analysis and background modeling. This work presents a detailed and quantitative analysis of the Cu 2p photoemission spectra for a metallic ultra-thin film and the initial oxidation stages. Additionally, annealed films at 200 °C for up to 5 min were analyzed to assess the full oxidation of the copper oxides and assert the validity of the proposed peak-fitting procedures. For the initial oxidation, it was possible to accurately identify Cu1+ and Cu2+ states and, alongside, quantify the corresponding O 1s species, resulting in compositions close to stoichiometric values. Using the active background, it was possible to achieve close reproduction of all the photoemission spectra; three coexisting copper species were resolved during the fitting procedure. An accurate peak-fitting analysis allowed for quantifying the relative proportion of Cu1+ to Cu0 states despite their strong overlap.

Spectroscopic imaging of photoexcited states at a polycrystalline copper metal surface via two-photon photoelectron emission microscopy

The local photoexcited surface states of a polycrystalline-copper (Cu) substrate were observed via two-photon photoelectron emission microscopy (2P-PEEM) using a femtosecond laser. The 2P-PEEM results revealed local distributions of the photoexcited electrons, strongly depending on the crystal domains. With the energy-filtered images, the local 2P-photoemission spectra were selectively obtained for (111)- and (110)/(100)-crystal domains. Furthermore, the 2P-PEEM could visualize propagating Cu surface plasmon polaritons with wavevector of 7.61 × 106 m−1. These results indicate that the 2P-PEEM technique is a promising tool for studying the distribution of nonequivalent photoexcited surface states for various inhomogeneous functional nanomaterials.

Reconstruction of pyrrhotite fracture surfaces

The interpretation of core level Synchrotron photoemisison spectra (SPES) collected from a pristine fracture surface of monoclinic pyrrhotite (Fe7S8), as a function of photon energy, has identified the three distinct surface species. The Fe 2p photoemission spectra show the Fe in pyrrhotite is high spin FeII in octahedral co-ordination. Comparison of bulk and surface sensitive spectra did not identify any changes due to the broad nature of the high spin Fe 2p spectra. However, the sulfur spectrum shows an undercoordinated monosulfide at 160.8 eV, a disulfide (161.7 eV) and polysulfide species (163.2 eV) at the surface. This indicates stabilisation of the surface involves reconstruction and results in chemical transformation of sulfur species. A high binding energy tail that extends to 167 eV has been attributed to ligand to metal charge transfer satellites (LMCT) and supported by the pre-edge features observed in Fe L2,3 and S L2,3 near edge X-ray absorption fine structure (NEXAFS). An accurate fit for the bulk pyrrhotite sulfide spectrum has been estimated by comparing spectra with different surface sensitivities, and thereby an accurate fitting method has been developed for the monoclinic pyrrhotite S 2p spectrum. This method may be used for future photoemission spectra to improve the analysis of similar samples and provide a basis for interpretation of pyrrhotite samples during mineral processing and environmental conditions.

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr2MnNiO6 was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+ and 2+ states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U − J 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p − d type charge transfer insulator with an intermediate covalent character according to the Zannen–Sawatzy–Allen phase diagram.

Detailed peak fitting analysis of the Zn 2p photoemission spectrum for metallic films and its initial oxidation stages

The metallic Zn 2p photoemission spectra hold a complex background that requires individual assignment of Shirley background for each peak comprising the spectra. For this reason, a close fit requires the use of the Shirley‐Vegh‐Salvi‐Castle background‐type under the active background approach. We found that the intensity of the plasmon peaks and their associated background cannot be described through existing energy loss (intrinsic and extrinsic) formalisms. We also analyzed the Zn 2p and O 1s spectra for the initial stages of oxide formation at various oxygen exposures. We found that the composition of the oxide layer is ZnO1.00±0.10 for all exposures, suggesting that our assessment of the primary function of metallic Zn is accurate and can be employed for quantitative studies. We also present a set of parameters to accurately fit and resolve the metallic and oxide Zn 2p peaks.

High‐Pressure‐Induced Pseudo‐oxidation of Copper Surfaces by Carbon Monoxide

AbstractInstantaneous electronic and structural modifications of Cu surfaces by high‐pressure CO were explored. At pressures over a millibar range the coverage of CO on the Cu(1 1 1) surface reached approximately one monolayer, and we observed several shake‐up peaks in the Cu 2p photoemission spectra for the first time by using ambient pressure X‐ray photoelectron spectroscopy. These shake‐up peaks resembled those obtained for oxidized surface Cu atoms. We propose that these changes are related to the generation of a copper–carbonyl complex and subsequent Cu surface restructuring owing to lateral CO–CO repulsions. The generality of this finding was investigated in two model reactions, the water‐gas shift and CO oxidation, on a Cu(1 1 1) surface, and we clearly identified the pseudo‐oxidation of Cu. These reversible changes were found to be independent of the orientation of the Cu surface and are of general importance for Cu‐based heterogeneous catalysis.

Quantitative analysis of Ni 2p photoemission in NiO and Ni diluted in a SiO2 matrix

In X-ray excited photoelectron emission (XPS), besides the initial excitation process, the shape and intensity of photoelectron peaks are strongly affected by extrinsic excitations due to electron transport out of the surface (including bulk and surface effects) and to intrinsic excitations due to the sudden creation of the static core hole. To make an accurate quantitative interpretation of features observed in XPS, these effects must be included in the theoretical description of the emitted photoelectron spectra. It was previously shown [N. Pauly, S. Tougaard, F. Yubero, Surf. Sci. 620 (2014) 17] that these three effects can be calculated by means of the QUEELS-XPS software (QUantitative analysis of Electron Energy Losses at Surfaces for XPS) in terms of effective energy-differential inelastic electron scattering cross-sections. The only input needed to calculate these cross-sections is the energy loss function of the media which is determined from analysis of Reflection Electron Energy Loss Spectra (REELS). The full XPS spectrum is then modeled by convoluting this energy loss cross-section with the primary excitation spectrum that accounts for all effects which are part of the initial photo-excitation process, i.e. lifetime broadening, spin–orbit coupling, and multiplet splitting. In this paper we apply the previously presented procedure to the study of Ni 2p photoemission in NiO and Ni diluted in a SiO2 matrix (Ni:SiO2), samples being prepared by reactive magnetron sputtering at room temperature. We observe a significant difference between the corresponding Ni 2p primary excitation spectra. The procedure allows quantifying the relative intensity of the c3d9L, c3d10L2, and c3d8 final states contributing to the Ni 2p photoemission spectra of the Ni2+ species in the oxide matrices. Especially, the intensity ratio in NiO between the non-local and local contributions to the 3d9L configuration is determined to be 2.5. Moreover the relative intensity ratio of the c3d9L/c3d10L2/c3d8 configurations is found to be 1.0/0.83/0.11 for both the NiO and Ni:SiO2 samples.

Cysteine on TiO2(110): a theoretical study by reactive dynamics and photoemission spectra simulation.

Owing to the importance of bioinorganic interface properties for the biocompatibility of implants and for biosensing technology, it has become indispensable to gain understanding of their crucial structure-property relations at the atomistic level. Motivated by this fact, we use cysteine amino acid on perfect and defective TiO2(110) surfaces as model systems and study adsorption by means of classical all-atom reactive molecular dynamics and ab initio O 1s, N 1s, and S 2p photoemission spectra (XPS) simulations of the most relevant adsorbate structures. By analysis of the dynamics results and a detailed comparison with spectra recently collected for this adsorbate, we obtain conclusions of both general and particular character. It is shown that the interaction of cysteine with the TiO2(110) surface has multipoint character involving the carboxylic group as well as the amino and sulfur groups. The proton-transfer reactivity of cysteine is enhanced by the presence of the surface, and different forms of cysteines are confirmed to be present in the adsorbate. A general conclusion is that reactive force field dynamics combined with selected spectroscopy provides a viable path to understanding bioinorganic surfaces with ramifications for the design of such surfaces for future technological applications.

Analysis of the effects of annealing on the electron-doped cuprate superconductorNd1.85Ce0.15CuO4−δ using x-ray photoemission spectroscopy

We have studied the Cu 2p, Nd 3d and valence band x-ray photoemission spectra obtained forNd1.85Ce0.15CuO4−δ single crystal under different annealing conditions. It is found that both the2p3/23d10 andthe 2p1/23d10 peaks of the Cu 2p core level appear obviously upon oxygen reduction. Nd 3d spectra movetowards lower binding energy upon reduction. The valence band spectra show anincrease in itinerant electrons with 3d character upon reduction. All the changes ofthe Cu 2p, Nd 3d and valence band spectra can be explained in terms of thedelocalization of itinerant electrons. We suggest that an annealing process could removeexcess oxygen and the associated disordered potential making the conductiveelectrons delocalized with the result that superconductivity becomes possible.

Evolution of the 2p satellite of Ni nano-clusters onTiO2(001) surfaces

Correlation enhancement of electrons in Ni nano-clusters due to confinementin reduced dimensions has been observed. Both the size and shape of thenano-clusters have a strong influence on the Ni 2p satellite structures. For smallNi clusters, apart from the 6 eV satellite, a 3 eV satellite structure emerges inNi 2p3/2 photoemission spectra due to the existence of the3F atomic state. However, the intensity of the 3 eV satellite decreases with increasing cluster sizebecause the Ni 3d level becomes increasingly occupied in large clusters. The increased electronpopulation in the 3d level is achieved through electron transfer from the free-electron-like4sp states to the unhybridized pure 3d spin down states; the latter is lower in energy thanthe 3d–4sp hybridized states. The correlation-induced 6 eV satellite energy is enhanced byAr+ bombardment. These observations make it feasible to gauge the Ni 3d electron populationby probing the 2p photoemission spectra and to modify the supported Ni nano-clustersthrough thermal treatment and/or ion bombardment.

X-ray nanospectroscopic characterization of a molecularly thin ferromagnetic Ti1−xCoxO2 nanosheet

We investigated the local electronic structures of a molecularly thin ferromagnetic Ti0.8Co0.2O2 nanosheet by means of x-ray nanospectroscopy with spectroscopic photoemission and low energy electron microscopy. By analyzing the Co 2p x-ray absorption and photoemission spectra, the chemical state of doped Co ions in individual Ti0.8Co0.2O2 nanosheet was found to be Co2+ low-spin state, which is consistent with previous magnetization data and first-principles calculation [M. Osada et al., Phys. Rev. B 73, 153301 (2006)]. Furthermore, we employed photoelectron emission microscopy to image the variation in chemical states in Ti0.8Co0.2O2 nanosheets with different stacking structures and clarified the identical Co2+ low-spin state in monolayer and overlapped cases.

Mechanisms of concurrent SiO desorption with oxide layer formation at Si(001) surface

AbstractOxidation reactions at Si(001) surfaces have been studied via real‐time in situ photoemission spectroscopy with synchrotron radiation for chemical bonding states of Si and O atoms and mass spectrometry for desorption of SiO molecules with supersonic O2 molecular beams in a temperature region from 900 K to 1300 K. In our previous studies, the SiO desorption yield decreased with increasing incident energy in a temperature region from 900 K to 1000 K. In that case, the time evolutions of Si 2p photoemission spectra showed that SiO2 structure on the surface was easily formed by the action of larger incident energy and the increased SiO2 coverage correlated with the decreased SiO desorption yield. In this study, simultaneous measurements of Si 2p photoemission spectra and SiO desorption yield revealed that the decrease of SiO correlated with the increase of Si2+ component, and the SiO desorption was terminated at the oxide thickness of 0.22 nm. These facts suggest that the SiO desorption takes place at the topmost Si dimers and a precursor for SiO desorption is a so‐called T site, in which O atoms are bonding with the dangling bonds of the dimers. Consequently, M1 and M2 in the Dual‐Oxide‐Species (DOS) model have been clarified to be a T site and a Si2+ state, respectively. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(3): 60–68, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20678

Depth profiling of chemical states and charge density in HfSiON by photoemission spectroscopy using synchrotron radiation

We have investigated chemical states and charge density in HfSiON films as a function of depth using x-ray irradiation time-dependent photoemission spectroscopy. N 1s core-level photoemission spectra deconvoluted into three components depend on HfSiON thickness, indicating the component, which is attributed to the N atoms bonded to Hf atoms, has peak near the surface. On the other hand, charge density estimated from band bending in Si from Si 2p photoemission spectra is also distributed mainly near the surface. These results indicate that the origin of the negative charge trapping can be directly related to the presence of Hf–N bonds.

Enhanced Reactivity of NiO/Pd(100) Ultrathin Films toward H2: Experimental and Theoretical Evidence for the Role of Polar Borders

NiO layers in the ultrathin regime exhibit an enhanced reactivity toward hydrogen with respect to the typical chemical inertness of bulk-like thicker samples. Such a behavior has been studied by means of photoemission (from both core and valence band levels) and quantum mechanical calculations. It is found that after H2 dosing in mild conditions (from PH2 = 6.5 × 10-7 Pa and T = 330 K) ultrathin films (thickness ≤6MLE, MLE = monolayer equivalent) quickly react forming metal nickel and water. The kinetic of the reaction has been followed in situ recording the intensity of the O 1s and Ni 2p photoemission spectra under different reaction conditions (PH2 ranging from 5 × 10-7 to 2 × 10-5 Pa and T from 330 up to 453 K), and a first-order dependence of the reaction rate on the PH2 and the activation energy of the rate determining step (0.16 ± 0.02 eV) have been determined. A 8 MLE thick film recovers the behavior of bulk-like NiO(100) surfaces, where more drastic reduction conditions are needed, and the kineti...

The Mg 2pcore-level in Zn2Mg

We present a synchrotron radiation photoemission (PE) spectroscopy study of the Mg 2p shallow core-level in Zn2Mg Laves phase. The PE measurements were performed on in situ cleaved single-grain samples at 10 i10 mbar pressure and low, 90i100 K, temperature. Analysis of Mg 2p PE spectra indicates the surface shift of the core-level of i0.12 eV. Parameters of the bulk component of Mg 2p level, the binding energy of 49.98 eV, the spin-orbit splitting ¢sio = 0.28 eV, and the lifetime broadening ∞ = 0.06 eV, are comparable to their values determined previously for pure crystalline Mg.

Si(001)表面における酸化膜形成とSiO脱離の共存機構

Oxidation reactions at Si(001) surfaces have been studied via real-time in-situ photoemission spectroscopy for chemical bonding states of Si and O atoms, and mass spectrometry for desorbing SiO molecules with synchrotron radiation and supersonic O2 molecular beams in the temperature range from 900 K to 1300 K. In our previous studies, the SiO desorption yield decreased with increasing incident energy in the temperature range from 900 K to 1000 K,. In that case, the time evolutions of Si 2p photoemission spectra showed that SiO2 structure on the surface was easily formed by the action of larger incident energy and the increased SiO2 coverage correlated with the decreased SiO desorption yield. In this study, coincidence measurements of Si 2p photoemission spectra and SiO desorption yield revealed that the decrease of SiO correlated with the increase of Si2+ component, and the SiO desorption was terminated at the oxide thickness of 0.22 nm. These facts indicate that the SiO desorption takes place at the topmost Si dimmers and its precursor is so called T site, in which O atoms are bonding with the dangling bonds of the dimmers. Consequently, M1 and M2 in the Dual-Oxide-Species (DOS) model were clarified to be T sites and Si2+ states, respectively.

Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si(0 0 1)

To investigate the initial growth of Fe films on Si(0 0 1) and the Fe/Si interface, Fe films at various thicknesses have been systematically studied by soft X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS). The Fe L edge XAS spectrum shows a strong thickness dependence with broader line-width for thinner films. Detailed analysis of the Fe absorption signal as a function of the thickness shows that the broad linewidth of Fe L edge XAS spectra is mostly contributed by the first Fe layer at the Fe/Si interface. In contrast to XAS, Fe 2p photoemission spectra for these films are identical. However, valence band photoemission also shows a strong thickness dependence. Comparing the valence band photoemission spectra of the thin Fe/Si(0 0 1) films with that of pure Si and the thickest Fe film, the difference spectra at all thicknesses show almost identical shape indicating the same origin: the Fe/Si interface. Thus, it is mainly the first Fe layer at Fe/Si layer that is reactive with the Si substrate changing its electronic structure.

Magnetic dichroism in angular resolved XPS on the Fe(110) surface

Magnetic dichroism has been observed in the angular distribution of Fe 2p photoemission spectra from Fe(110) surface by unpolarized Al Kα X-rays. The dichroism asymmetry varies strongly around low-index forward-scattering direction in the Fe thin film. Photoelectron diffraction theory provides explanation for these unpolarized dichroism effects, which should also be generally useful in surface magnetism studies.

UHV-MOCVD growth of TiO 2 on SiO x/Si(1 1 1): Interfacial properties reflected in the Si 2p photoemission spectra

Metal–organic chemical vapour deposition growth of titanium oxide on moderately pre-oxidised Si(1 1 1) using the titanium(IV) isopropoxide precursor has been studied for two different growth modes, reaction-limited growth at 300 °C and flux-limited growth at 500 °C. The interfacial properties have been characterized by monitoring synchrotron radiation excited Si 2p photoemission spectra. The cross-linking from oxidised Si to bulk Si after TTIP exposure has been found to be very similar to that of SiO x /Si(1 1 1). However, the results show that the additional oxidation of Si most probably causes a corrugation of the SiO x /Si interface. Those conclusions are valid for both growth modes. A model is introduced in which the amorphous interface region is described as (TiO 2) x (SiO 2) y where x and y changes linearly and continuously over the interface. The model quantifies how (TiO 2) x (SiO 2) y mixing changes the relative intensities of the signals from silicon oxide and silicon. The method can be generalised and used for the analyses of other metal-oxides on silicon.