X-ray Photoelectron Spectroscopy Core-level Spectra Research Articles

An interpretation for the components of 2p 3/2 core level x-ray photoelectron spectra of the cations in some inverse spinel oxides

In an effort to reconcile the various interpretations for the cation components of the 2p 3/2 observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p 3/2 core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p 3/2 core levels of NiFe2O4 thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles Ni x Co1−x Fe2O4 (x = 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe2O4 thin film. Surface-to-bulk core level shifts are also observed in CoFe2O4 and NiCo2O4 thin films but not as significantly as in NiFe2O4 thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles.

Electronic structure of CeCu4In from band structure calculations and X-ray photoelectron spectroscopy

We present a combined new experimental and theoretical study of the electronic structure for the heavy fermion CeCu4In based on X-ray photoelectron spectroscopy (XPS) data and ab-initio band structure calculations. The compound crystallizes in the orthorhombic CeCu4.38In1.62 type of structure (space group Pnnm). Below the Fermi energy the total density of states contains mainly 3d–states of Cu atoms that hybridized with the Ce 4f electronic states. The Ce core-level XPS spectra point to a stable trivalent configuration of Ce atoms in CeCu4In, consistently with the magnetic susceptibility data. For more detailed information about electronic states the fully relativistic band structure was calculated within the density functional theory (DFT) for the first time. Based on these calculations we present calculated photoemission spectra, which very well reproduce the experimental ones. The Fermi level is located at a deep decrease in the density of electronic states and reaches a value of 3.69 states/(eV f.u.). This value is much smaller than the one obtained experimentally, which indicates the importance of many-body effects, which are not properly taken into account in ab-initio calculations. The valence band is formed mainly by Cu(3d) electrons with a small contribution of Ce(4f) electrons in the immediate vicinity of the Fermi level.

Oxygen vacancies mediated changes in ferromagnetic, optical, Photoluminescent and electronic structure properties of Ho-doped CeO2 nanoparticles

The undoped CeO2 and Ce1-xHoxO2 (x = 0.03, 0.05 and 0.07) nanoparticles prepared using microwave assisted co-precipitation route are investigated using X-Ray Diffraction (XRD), UV–Vis–NIR, Raman, photoluminescence, X-ray Photoelectron Spectroscopy (XPS) and SQUID-VSM magnetometer. The XRD validates the inclusion of Ho3+ at the Ce3+/Ce4+ site while maintaining the crystal structure with development of defects and oxygen vacancies with expansion of the CeO2 nanolattice. The composition of the elements with desired stoichiometry is confirmed by Energy Dispersive X-ray spectra analysis. The absorption spectroscopy analysis reports the Urbach energy along with refractive index increases with Ho3+ doping and the optical band gap is decreased indicating red shift. All the nanoparticles are subjected to PL investigations to ascertain the oxygen vacancy and defect structure of nanoparticles. Raman active modes at ∼500 - 630 cm−1 verifies the existence of oxygen vacancies (Vo)., which are seen to rise with concentration of Ho-doping in the CeO2 lattice. When Ce4+ transforms into Ce3+, oxygen vacancies are generated in the lattice to maintain the charge neutrality, as can be seen by the Ce-3d, O-1s, and Ho-4d core level XPS spectra. To demonstrate that Ho3+ doping favours ferromagnetic ordering in the CeO2 lattice, the magnetic properties using the Bound Magnetic Polaron model and F centre exchange mechanism are discussed. The development of various complexes that involve host and doped cations are mediated by Vo suggests the existence of F0, F+, and F2+ centres. The weak ferromagnetic behaviour in Ho3+-doped CeO2 nanoparticles reinforce the relationship between Vo to explain the observed magnetic properties of nanoparticles.

Antisites disorder mediated magnetization relaxation and polydispersity in La2NiMnO6 crystallites

Antisite disorders play a critical role in determining the magnetic ground state of the La2NiMnO6 double perovskite compounds that show multifunctional behaviours and spin-glass phases at lower temperatures. The spin-glass (SG) state and magnetization relaxation in these compounds have been studied extensively; however, the polydispersity of the SG state remains elusive. Employing ac-magnetic susceptibility χ(ω,T) and temperature-dependent electron spin resonance (ESR) spectroscopy, herein, we probe the polydispersity of the SG phase in La2NiMnO6 crystallites. We employed X-ray photoelectron spectroscopy (XPS) to -investigate the crystallite size-dependent variable fractions of Mn3+/Mn4+ (Ni2+/Ni3+) cations and their influence on completing exchange interactions. Mn-2p core-level XPS spectra suggest a dominant Mn4+ + Ni2+→Mn3+ + Ni3+ charge transfer peak that appeared as a satellite peak at lower binding energy than the Mn core-level. The observed differences in saturation magnetization (MS) as a function of crystallite size indicate the variation in the degree of antisites disorder. Competing magnetic interactions driven by mixed valence and disorder facilitate a spin-glass (SG) phase at lower temperatures. Utilizing ESR measurement across paramagnetic → competing ferro/antiferromagnetic phase, a modified g-factor that ranges from 2.050-2.037 is observed in the paramagnetic region. The line width of the ESR signals is found to be increased across the transition suggesting spin-freezing characteristics. Cole-Cole plots obtained from χ(ω,T) data indicate collective spin relaxation dynamics in the proximity of the freezing temperature. Our findings suggest the importance of the size effect and its relation with the degree of antisites disorders in these crystallites. These experimental results enable the La2NiMnO6 compound to find its possible applications in new areas of material research.

Boron doping of diamond-like carbon films by pulsed laser deposition technique for photovoltaic application

Diamond-like carbon (DLC) films were deposited by pulsed laser deposition technique using a glassy carbon disc as carbon source material. Boron doping in DLC films was carried out by ablating boron disc sequentially during the synthesis of DLC films. Field emission scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and Raman measurements were utilized to characterize the films. Hot-probe study indicated the films to be p-type. Core level XPS spectra of B1s appeared at ∼ 189.5 eV and indicated that boron atoms are in form of B-C network connect. Raman studies indicated a dilatation of host lattice with the incorporation of boron in DLC matrix favouring graphitization. Band gap of the thin films decreased from ∼2.0 eV (pure DLC) to ∼1.76 eV with increased incorporation of boron. A photovoltaic cell structure (glass/indium tin oxide)/B:DLC/ZnO/Ag) conceived here, indicated an open circuit voltage ∼110 mV, short circuit current density ∼7.5 mA/cm2 with a fill factor ∼0.3, and a low conversion efficiency (η) ∼ 0.23%.

Electronic structure of YbFe4Al8 antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe4Al8 using X-ray photoelectron spectroscopy (XPS) and first-principles calculations. The XPS valence band of YbFe4Al8 consists of two contributions coming from divalent (Yb2+) and trivalent (Yb3+) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4d XPS spectra. We study several collinear antiferromagnetic models of YbFe4Al8 from the first-principles and for comparison we also consider LuFe4Al8 with a fully filled 4f shell. We predict that only Fe sublattices of YbFe4Al8 carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mulliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4f orbitals on Yb valence and Fe magnetic moments.

Fabrication of fluoroalkylsilane/zeolitic imidazolate framework composites for highly efficient superhydrophobic coating

Superhydrophobic bagasse paper was successfully engineered by facile spray coating with a zeolitic imidazolate framework composite modified by 3,3,3-trifluoropropyltrimethoxysilane (FAS) and used as a filter membrane with special wettability for oil/water separation. Surface characteristics such as surface morphology, surface moieties, roughness, and wettability were observed. The addition of FAS decreased the apparent surface energy between solid surfaces and liquids with a polar contribution of surface free energy as low as 0 mN m−1. The formation of the FAS layer was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy core-level spectra. Three-dimensional surface topography revealed an increase in the spray coating cycle that enhanced the hydrophobicity of the coated paper. The results confirmed that not only microstructures but also the presence of functional groups with low surface energy (such as FAS) can promote surface hydrophobicity. The spray-coated paper with FAS/ZIF in the presence of a small amount of nanocellulose provided a superhydrophobic surface after the first spray cycle. The separation efficiency was up to 85% and was slightly affected by oil viscosity. Moreover, this can be used as a filter membrane for water-in-oil emulsions. In the present work, simple spray-coated paper is a promising candidate material for gravity-driven oil/water separation and water repellent coating.

Improvement in thermal stability and crystallization mechanism of Sm doped Ge2Sb2Te5 thin films for phase change memory applications

The thin films of (Ge2Sb2Te5)100−xSmx(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2) (Sm-GST) phase change material have been investigated employing X-ray photoelectron spectroscopy (XPS) to examine the nature of chemical bonding in as-deposited thin films of Sm-GST. The composition of as-deposited thin films of Sm-GST has been also analyzed from the peak area ratios of XPS core-level spectra and the morphology of the thin film has been studied using field emission scanning electron microscopy (FESEM). The powder samples obtained from the as-deposited thin films have been utilized for the non-isothermal differential scanning calorimetry (DSC) measurements at the constant heating rate of 10 K/min. The values of glass transition temperature (Tg), onset crystallization (Tc), peak crystallization temperature (Tp) and melting temperature (Tm) obtained from DSC curves of Sm-GST thin films have been used for the evaluation of thermal stability parameters. The activation energy for crystallization (Ec) and avrami exponent (n) for fcc and hexagonal phase of Sm-GST thin films have been evaluated using Henderson’s method and Matusita’s method. The impact of Sm doping on the thermal stability, glass-forming ability and crystallization activation energy of as-deposited thin films have been examined and its possible influence on the memory device performance has been correlated.

The Mechanism of Surface-Radical Generation and Amorphization of Crystalline Quartz Sand upon Mechanochemical Grinding

The structural and morphological changes that occur during the mechanochemical grinding of quartz sand have been characterized by electron paramagnetic resonance (EPR), 29Si solid-state nuclear magnetic resonance (SSNMR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analysis. BET analysis showed that mechanochemical grinding of quartz sand leads to a significant surface area increase within the first 60 min, which gradually tapers off as the grinding continues (up to 600 min). This rapid initial increase in the surface area does not correlate with mechanoradical and silanol formation; EPR and 29Si SSNMR infer that an initial lag phase exists after which the levels of these species increase substantially. Sequential peak broadening of 29Si SSNMR spectra as well as Si 2p and O 1s XPS core level spectra support matrix amorphization over 600 min of mechanochemical grinding. Amorphization was also observed in the SEM and BET data. These findings suggest that weak points on the surface of quartz sand particles initially fracture, causing a rapid particle size reduction but no significant formation of mechanoradicals. However, once a certain surface area limit is reached, continued grinding leads to abrasion of the freshly fractured surfaces, resulting in the formation of silyl radicals (≡Si•), siloxyl radicals (≡Si–O•), and surface silanols (≡Si–OH and ═Si–(OH)2).

Electronic structure of Alq3 and Liq using soft X-ray spectroscopy and density functional theory calculation

The electronic structure of aluminum tris-8-hydroxyquinoline (Alq3) and 8-hydroquinolatolithium (Liq) was investigated using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectral simulation with density functional theory. The chemical bonding states of Alq3 and Liq were analyzed using XPS core-level spectra. The band gaps of Alq3 and Liq were measured by combining the XAS and XES results. Additionally, resonant and non-resonant XES were used to measure the density of states for O sites in the molecules.

High-Performance Ru 2 P Anodic Catalyst for Alkaline Polymer Electrolyte Fuel Cells

High-Performance Ru ₂ P Anodic Catalyst for Alkaline Polymer Electrolyte Fuel Cells

Development of multiple core-level XPS spectra decomposition method based on the Bayesian information criterion

There is a need to develop an automatic spectral analysis method integrated with reference database as the reference database builds up. At the time of spectral analysis, the compound ratio is often estimated by comparing a measured spectrum with reference spectra of known single-phase compound samples. However, it is difficult to automate all processes, and there is the problem that the operator's arbitrariness is included in the analysis results. The present paper proposes a method that analyzes the X-ray photoelectron spectroscopy (XPS) spectrum of a multiphase compound using modeled reference XPS spectra. The proposed method estimates the component ratio of compounds in a fully automatic manner. To use reference spectra measured by different devices or cited from the literatures for spectra analysis, reference spectra are represented by a mathematical function using a peak separation method based on the Bayesian information criterion (BIC). In particular, it is clarified that the estimation accuracy is improved by simultaneously analyzing multiple core-level spectra rather than by independently analyzing only single core-level spectrum.

Mixed Valence of Ce and Its Consequences on the Magnetic State of Ce9Ru4Ga5: Electronic Structure Studies.

We report on X-ray photoelectron spectroscopy (XPS) and ab initio electronic structure investigations of a novel intermetallic material CeRuGa. The compound crystallizes with a tetragonal unit cell (space group I4) that contains three inequivalent Ce atoms sites. The Ce core level XPS spectra indicated an intermediate valence (IV) of selected Ce ions, in line with the previously reported thermodynamic and spectroscopic data. The ab initio calculations revealed that Ce1 ions located at Wyckoff positions possess stable trivalent configuration, whereas Ce2 ions that occupy site are intermediate valent. Moreover, for Ce3 ions, located at different position, a fractional valence was found. The results are discussed in terms of on-site and intersite hybridization effects.

X-ray Photoelectron Spectroscopy Study of Indium Tin Mixed Oxides on the Surface of Silicate Glass

Indium tin oxide (ITO) films grown on silicate glass substrates have been characterized by X-ray photoelectron spectroscopy (XPS). Valence band and core level XPS spectra have been used for qualitative and quantitative elemental analysis of the films before and after Ar+ ion etching to a depth of ~50 nm. It has been found that the densest (best quality) oxide layer was produced on the surface of sample M3, prepared at a lower pressure in comparison with the other samples and a higher discharge voltage in the magnetron plasma. Sample M2 and, to a lesser extent, sample M1 have been shown to contain constituent elements of the silicate glass substrate (Si, Al, Na, K, Ca, Mg, Fe, S, O, and C), which was due to the loose structure (poor quality) of the indium tin oxide coating on the substrate. The surface layer of the as-prepared samples contained mostly In3+ and Sn2+ ions bonded not only to oxygens of the In2O3 and SnO oxides but also to the oxygens of impurity hydroxyl and carbonate groups. Ar+ ion etching of the sample surface led to the removal of the impurities. As a result, there were mostly In3+ and Sn2+ ions bonded to oxygen in In2O3 and SnO.

Core level photoemission line shape selection: Atomic adsorbates on iron

Robust fitting of core level photoemission spectra is often central to reliable interpretation of X‐ray photoelectron spectroscopy (XPS) data. One key element is employment of the correct line shape function for each spectral component. In this study, we consider this topic, focusing on XPS data from atomic adsorbates, namely, O and S, on Fe(110). The potential of employing density functional theory (DFT) for generating adsorbate projected electronic density of states (PDOS) to support line shape selection is explored. O 1s core level XPS spectra, acquired from various ordered overlayers of chemisorbed O, all display an equivalent asymmetric line shape. Previous work suggests that this asymmetry is a result of finite O PDOS in the vicinity of the Fermi level, allowing O 1s photoexcitation to induce a weighted continuum of final states through electron‐hole pair excitation. This origin is corroborated by O DFT‐PDOS generated for an optimised five‐layer Fe(110)(2 × 2)‐O slab. Adsorbate DFT‐PDOS were also computed for Fe(110) ‐S. As, similar to adsorbed O, there is a significant continuous distribution of states about the Fermi level, it is proposed that the S 2p XPS core levels should also have asymmetric profiles. S 2p XPS data acquired from Fe(110) ‐S, and their subsequent fitting, verify this prediction, suggesting that DFT‐PDOS could aid line shape selection.

Electronic Structure of Nitrogen- and Phosphorus-Doped Graphenes Grown by Chemical Vapor Deposition Method.

Heteroatom doping is a widely used method for the modification of the electronic and chemical properties of graphene. A low-pressure chemical vapor deposition technique (CVD) is used here to grow pure, nitrogen-doped and phosphorous-doped few-layer graphene films from methane, acetonitrile and methane-phosphine mixture, respectively. The electronic structure of the films transferred onto SiO2/Si wafers by wet etching of copper substrates is studied by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy using a synchrotron radiation source. Annealing in an ultra-high vacuum at ca. 773 K allows for the removal of impurities formed on the surface of films during the synthesis and transfer procedure and changes the chemical state of nitrogen in nitrogen-doped graphene. Core level XPS spectra detect a low n-type doping of graphene film when nitrogen or phosphorous atoms are incorporated in the lattice. The electrical sheet resistance increases in the order: graphene < P-graphene < N-graphene. This tendency is related to the density of defects evaluated from the ratio of intensities of Raman peaks, valence band XPS and NEXAFS spectroscopy data.

Evaluation of physico-chemical properties and biocompatibility of new surface functionalized Fe3O4 clusters of nanoparticles

In this research, magnetic nanoclusters of Fe3O4 (MNC) were synthesized by solvothermal method using different organic acids as surfactants such malic acid, aspartic acid and sodium tartrate in order to obtain new nanostructured materials with applications in medicine. The interaction process of these acids on the surface of magnetic nanoclusters is important in understanding the change induced on the surface properties of the obtained MNC. Structural analysis and physico-chemical characterization of the new synthesized materials were performed by means o X-ray photoelectron spectroscopy (XPS), transmission electron microscopy, zeta potential and magnetic measurements to provide their spherical shape, stability and specific functional groups and the influence of the surfactants on their magnetic properties. XPS core level and valence band photoemission spectra for all investigated samples is discussed in terms of changes induced in the electronic structure linked to variation of the Fe2+ cations at the samples surface. This is of fundamental importance to better understand the electronic structure and magnetism of the obtained MNC in order to tailor their electronic properties by surface engineering for specific biomedical applications. The biological effects of MNC in rat blood were studied by hemolysis and erythrocyte antioxidant systems (superoxide dismutase and catalase) showing no negative effect on the cells.

Defect assisted improved room temperature ferromagnetism in Ce doped SnO2 nanoparticles

In the present work, pure and Ce doped SnO2 have been prepared via sol-gel method and the effect of Ce doping on the structural, optical and magnetic properties of SnO2 are studied systematically. A detailed structural analysis has been carried out by X-ray diffraction (XRD), field emission scanning microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, Raman scattering, X-ray photoelectron spectroscopy (XPS). These studies reveal that pure and Ce doped SnO2 NPs have a polycrystalline single phase tetragonal rutile structure without any detectable impurity phases and most of the dopant ions occupy the lattice site into the host structure of SnO2. The core level XPS spectrum of Ce 3d reveals that most of the Ce ions are in 3+ valence state. Simulated O K-edge based soft X-ray absorption analysis, high resolution O 1s XPS results and photoluminescence study reveal that oxygen vacancies are generated in the doped samples. Band gap of SnO2 is found to decrease with increase in Ce content. Magnetic measurements show that all the Ce doped samples clearly display the room temperature ferromagnetic behavior while pure SnO2 shows diamagnetic nature. Magnetic parameters have been determined from the fitted M-H loops and transition from diamagnetic to ferromagnetic is observed when Ce ions are doped into SnO2 structure.

Bayesian Hamiltonian Selection in X-ray Photoelectron Spectroscopy

Core-level X-ray photoelectron spectroscopy (XPS) is a useful measurement technique for investigating the electronic states of a strongly correlated electron system. Usually, to extract physical information of a target object from a core-level XPS spectrum, we need to set an effective Hamiltonian by physical consideration so as to express complicated electron-to-electron interactions in the transition of core-level XPS, and manually tune the physical parameters of the effective Hamiltonian so as to represent the XPS spectrum. Then, we can extract physical information from the tuned parameters. In this paper, we propose an automated method for analyzing core-level XPS spectra based on the Bayesian model selection framework, which selects the effective Hamiltonian and estimates its parameters automatically. The Bayesian model selection, which often has a large computational cost, was carried out by the exchange Monte Carlo sampling method. By applying our proposed method to the 3$d$ core-level XPS spectra of Ce and La compounds, we confirmed that our proposed method selected an effective Hamiltonian and estimated its parameters appropriately; these results were consistent with conventional knowledge obtained from physical studies. Moreover, using our proposed method, we can also evaluate the uncertainty of its estimation values and clarify why the effective Hamiltonian was selected. Such information is difficult to obtain by the conventional analysis method.

XPS on Gd1−xCexCo2 Intermetallic Compounds

The electronic structure of Gd1−xCexCo2 compounds with x = 0.2, 0.4, 0.6, 0.75, and 0.9 is investigated by means of X‐ray photoelectron spectroscopy (XPS). All investigated samples are single phase and possess a cubic MgCu2 Laves type structure (C15). XPS core level spectra point out the intermediate valence state of Ce ions and that the Co atoms carry magnetic moments of similar magnitude in all compounds but smaller than in pure Co metal. The XPS valence band spectra of Gd1−xCexCo2 compounds are mainly the result of the Co 3d, Ce 5d, and Ce 4f states superposition, influenced by the 3d–5d and 3d–4f hybridizations.