2p Photoemission Research Articles

Orientational anisotropy due to molecular field splitting in sulfur 2p photoemission from CS2 and SF6 - theoretical treatment and application to photoelectron recoil.

Photoelectron recoil strongly modifies the high kinetic energy photoemission spectra from atoms and molecules as well as from surface structures. In most cases studied so far, photoemission from atomic-like inner-shell or core orbitals has been assumed to be isotropic in the molecular frame of reference. However, in the presence of molecular field splitting of p or d orbitals, this assumption is not justified per se. We present a general theoretical treatment, linking the orientational distribution of gas-phase molecules to the electron emission and detection in a certain direction in the laboratory frame. The approach is then applied to the S 2p photoemission from a linear molecule such as CS2 and we investigate, how the predicted orientational anisotropies due to molecular field splitting affect the photoelectron recoil excitations. Lastly, experimental S 2p high-kinetic-energy photoelectron spectra of SF6 and CS2 are analyzed using the modeled recoil lineshapes representing the anisotropy-affected recoil effects.

Surface Analysis Insight Note: Observations relating to photoemission peak shapes, oxidation state, and chemistry of titanium oxide films

It is common practice to describe the coordination of metal atoms in a binding configuration with their nearest neighbors in terms of oxidation state, a measure by which the number of electrons redistributed between atoms forming chemical bonds. In XPS terms, change to an oxidation state is commonly inferred by correlating photoemission signal with binding energy. The assumption, when classifying photoemission signals into distinct spectral shapes, is that a distribution of intensities shifted to lower binding energy is evidence of a reduction in oxidation state. In this Insight note, we raise the prospect that changes in photoemission peak shape may occur without obvious changes, determined by XPS in stoichiometry for a material. It is well known that TiO2 measured by XPS yields reproducible Ti 2p photoemission peaks. However, on exposing TiO2 to ion beams, Ti 2p photoemission evolves to complex distributions in intensity, which are particularly difficult to analyze by traditional fitting of bell‐shaped curves to these data. For these reasons, in this Insight note, a thin film of TiO2 deposited on a silicon substrate is chosen for analysis by XPS and linear algebraic techniques. Alterations in spectral shapes created from modified TiO2, which might be interpreted as the change in oxidation state, are assessed in terms of relative proportions of titanium to oxygen. It is found through detailed analysis of spectra that quantification by XPS, using procedures routinely used in practice, is not in accord with the typical interpretations of photoemission shapes. The data processing methods used and results presented in this work are of particular relevance to elucidating fundamental phenomena governing the surface evolution of materials‐enabled energy processes where cyclic/non‐steady usage changes the nature of bonding, especially in the presence of contaminants.

Electronic properties of MnO(110) film grown on Cu (111)

In this study, we investigated MnO thin films grown on Cu(111) using a soft x-ray and UV based spectroscopy, low energy electron diffraction (LEED), scanning tunneling (STM) and scanning transmission electron microscopy (STEM). MnO thin film assumes (110) preferential plane orientation organized in different spatial domains to better adapt to Cu(111) surface and an in-plane/out-of-plane lattice parameter variation up to 11% with respect to bulk counterpart originating a sizeable XLD signal at Mn L2,3 edges. The formation of oxidized Cu region during the MnO thin film growth is not avoidable due to the copper reactivity to oxygen and at the interface the coexistence of MnO islands alternated with Cu2O regions was observed by STM. Mn 2p and Mn 3s core level photoemission line shapes present a slight change in multiplet peak relative intensity with respect bulk MnO counterpart due to the role of Cu substrate in the core level relaxation process. The photon energy dependent photoemission spectra of valence band discriminate the MnO states and indicate a weak influence of the substrate once the film thickness is reduced to 1 nm. These findings open new route in the tuning oxide thin film properties.

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.

Detailed peak fitting analysis of the Ni 2p photoemission spectrum for metallic nickel and an initial oxidation

A quantitative study of the surface composition of clean metallic and partially oxidized nickel exposed to oxygen was carried out employing X-ray photoelectron spectroscopy (XPS). By fitting both branches of the metallic Ni 2p spectrum, it was possible to resolve two previously unreported satellite peaks in the Ni 2p1/2 portion of the spectrum. For the partial oxidation of the nickel surface, we analyzed the Ni 2p, O 1s, and C 1s angle-resolved XPS spectra. Using state-of-the-art peak-fitting (including the block approach) and background modeling methods (including the active approach) it was possible to discriminate the peaks of Ni2O3 from those of metallic nickel for both branches of the Ni 2p spectra from partially oxidized nickel. The oxidation of metallic nickel is through the formation of a Ni+3 oxide layer together with deep protrusions. The uncertainties are calculated with the covariance matrix method. The strong overlap and shape similarity of the Ni0 L3M45M45 Auger structure with that of the Ni+3 strongly suggests that the modified Auger Parameter does not provide direct additional insight about the chemical state to that provided by the photoemission spectra.

Synchrotron X-ray spectroscopic study of the antiferromagnetic-ferromagnetic transition in Ni-doped FeRh epitaxial thin films

We investigate the change of magnetism and electronic structure across the antiferromagnetic (AFM) -ferromagnetic (FM) transition of Ni-doped FeRh epitaxial thin films by x-ray magnetic circular dichroism (XMCD) and hard x-ray photoemission spectroscopy (HAXPES). The Rh L edge XMCD results indicate that the remnant FM phase at low temperature possesses smaller Rh moment than the normal FM phase which can undergo AFM-FM transition. The HAXPES results confirm an increase of Rh 4d density of state at the Fermi level and a possible “well-screened” state at the Ni 2p photoemission in the FM phase. FM fluctuation of the inter-site Fe-Fe exchange coupling within the AFM phase can be interpreted from the temperature dependence of the Fe 2p HAXPES results.

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.

Separation of surface oxide from bulk Ni by selective Ni 3p photoelectron spectroscopy for chemical analysis in coincidence with Ni M-edge Auger electrons

The chemical shift of core level binding energies makes electron spectroscopy for chemical analysis (ESCA) a workhorse analytical tool for science and industry. For some elements, close lying and overlapping spectral features within the natural life time broadening restrict applications. We establish how the core level binding energy chemical shift can be picked up experimentally by the additional selectivity through Auger electron photoelectron coincidence spectroscopy (APECS). Coincident measurement of Ni 3p photoemission with different MVV Auger regions from specific decay channels, narrows the 3p core-levels to a width of 1.2 eV, resolves the spin–orbit splitting of 1.6 eV and determines the chemical shift of Ni 3p levels of a Ni(111) single crystal and its oxidized surface layer to 0.6 eV.

Site-Dependent Peculiarities of Calcium Bonds in Bone Tissue.

The relationship of the hierarchical organization of the skeleton with the local electronic and atomic structure of bone is investigated. The Ca 2p photoemission from intact and various arthritis-damaged areas was measured and examined to study site-dependent peculiarities of calcium bonds in subchondral femoral bone. The medial and lateral condyles of the femur resected during total knee arthroplasty were used as samples. The Ca 2p3/2,1/2-1 photoelectron spectra demonstrate the distinct hierarchy-induced deviations of calcium bonds on the proximal side of the samples. It is shown that the apatite calcium bonds dominate in intact area, whereas non-apatite bonds dominate in OA-damaged areas, especially near sclerotic area but not inside it. The site dependence is associated with the interaction of broken collagen molecules with hydroxyapatite nanocrystallites at the cartilage-bone interface. The interplay of biomechanical and biochemical processes is examined, and the restoration of calcium bonds in sclerotic bone is discussed.

Synthesis and Characterization of SrFexMn1-x(O,F)3-δ Oxide (δ = 0 and 0.5) and Oxyfluoride Perovskite Films.

We report the synthesis and characterization of as-grown SrFexMn1-xO2.5 epitaxial films, which were also subjected to postgrowth oxidation and topotactic fluorination to obtain SrFexMn1-xO3 and SrFexMn1-xO2.5-δFγ films. We show how both the B-site cation and anion composition influence the structural, electronic, and optical properties of this family of perovskite materials. The Fe substitution of Mn in SrMnO2.5 gradually expands the c-axis parameter, as indicated by X-ray diffraction. With increasing x, the F content incorporated under identical fluorination conditions increases, reaching its maximum in SrFeO2.5-δFγ. In the compounds with mixed B-site occupation, the Fe 2p photoemission peaks are shifted upon fluorination, while the Mn 2p peaks are not, suggesting inductive effects lead to asymmetric responses in how F alters the Mn and Fe bonds. Electronic transport measurements reveal all compounds are insulators, with the exception of SrFeO3, and demonstrate that fluorination increases resistivity for all values of x. Optical absorption spectra in the SrFexMn1-xO2.5 and SrFexMn1-xO3 films evolve systematically as a function of x, consistent with a physical scenario in which optical changes with Fe substitution arise from a linear combination of Mn and Fe 3d bands within the electronic structure. In contrast, the F incorporation induces nonlinear changes to the optical response, suggesting a more complex impact on the electronic structure in materials with concurrent B-site and anion site substitution.

Photoemission Spectroscopy Study on Hydrogen Termination Effect on SiO2/Si Structure Fabricated Using H+-Implanted Si Substrate

Using proton (H+)-implanted silicon (Si) substrates, we clarified the effect of dangling bond termination by hydrogen on the interfacial strain in the silicon dioxide (SiO2)/Si system. The variations of the SiO2/Si interface structure caused by H+ implantation into a SiO2/Si sample and by hydrogen out-diffusion heat treatment were analyzed by high-resolution synchrotron radiation photoemission spectroscopy. We found that H+ implantation into the SiO2/Si sample [intentional generation of the interfacial dangling bonds] can increase the intensity of the strained-Si peaks in the Si 2p photoemission spectrum. In addition, our study revealed that the strained Si atom amount and dangling bond density are reduced by hydrogen out-diffusion heat treatment. These findings suggest that the increase/decrease in the dangling bond density by H atoms results in the increase/decrease in local strain field around a dangling bond, thereby changing the length of the Si–Si bonds beneath the SiO2/Si interface. Out-diffused hydrogen seems to play roles to not only reduce the dangling bond density but also relax the local strain at the SiO2/Si interface. The hydrogen termination effect is expected to have an advantage in structural stability in the SiO2/Si system as compared with the pure thermal termination effect.

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.

Electron Conduction of Nd0.6Sr0.4FeO3−δ Thin Film with Oxygen Vacancies Prepared by RF Magnetron Sputtering

We have firstly studied the electrical conductivity and the electronic structure of the Nd0.6Sr0.4FeO3−δ (NSFO) thin film on Al2O3(0001) substrate deposited by RF magnetron sputtering. The prepared thin film has larger lattice constant than the bulk crystal due to the stress from the substrate and the oxygen vacancies. The Fe 2p photoemission spectrum exhibits the mixed valence states of Fe2+ and Fe3+. The electrical conductivity exhibits thermal activation-type behavior and increases with increasing film thickness. The band gap of charge transfer type is in a good agreement with the activation energy estimated from the Arrhenius plot. These results indicate the conducting carrier of the NSFO thin film is mainly electron, although the conductivity does not depend on oxygen gas partial pressure.

Magnetic decoupling of ferromagnetic metals through a graphene spacer

We study the magnetic coupling between different ferromagnetic metals (FMs) across a graphene (G) layer, and the role of graphene as a thin covalent spacer. Starting with G grown on a FM substrate (Ni or Co), we deposited on top at room temperature several FM metals (Fe, Ni, Co). By measuring the dichroic effect of 3p photoemission lines we detect the magnetization of the substrate and the sign of the exchange coupling in FM overlayer at room temperature. We show that the G layer magnetically decouples the FM metals.

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.

Structure and surface chemistry of Al2O3 coated LiMn2O4 nanostructured electrodes with improved lifetime

Aluminum oxide coatings deposited on LiMn2O4/carbon fiber electrodes by atomic layer deposition (ALD) are shown to enhance cathode performance in lithium-ion batteries. With a thin Al2O3 coating derived from 10 ALD cycles, the electrodes exhibit 2.5 times greater capacity retention over 500 cycles at a rate of 1C as well as enhanced rate capability and decreased polarization resistance. Structural and surface studies of the electrodes before and after cycling reveal that a near-surface phenomenon is responsible for the improved electrochemical performance. The crystal structure and overall morphology of the LiMn2O4 electrode are found to be unaffected by electrochemical cycling, both for coated and uncoated samples. However, evidence of Mn diffusion into the ALD coatings is observed from both transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM-EDS) and X-ray Photoelectron Spectroscopy (XPS) after electrochemical cycling. Furthermore, XPS analysis of the Al 2p photoemission peak for the ALD coated electrodes reveal a significant shift in binding energy and peak shape, suggesting the presence of an Al–O–F compound formed by sequestering HF in the electrolyte. These observations provide new insight toward understanding the mechanism in which ultrathin coatings of amphoteric oxides can inhibit capacity loss for LiMn2O4 cathodes in lithium-ion batteries.

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.

A revisit to ultrathin NiO(0 0 1) film: LEED and valence band photoemission studies

LEED and photoemission measurements have been performed on ultrathin NiO films to reinvestigate its surface quality and valence electronic structures, respectively. On Ag(001) substrate, the best epitaxial order was observed for high temperature deposition with sufficient oxygen flux associated with a post-deposition oxygen annealing. The effect of the substrate vicinity on valence band electronic structure, in case of interfacial NiO layers, has been explained. The variation of Ni 3d to O 2p photoemission cross-section with photon energy (hν) has been demonstrated in this work.