O1s Core Research Articles

The effect of water molecule configurations on the type of core ionized site: O1s core ionization of ascorbic acid in water

The analysis of photoelectron spectra can be a complex and uncertain process, primarily due to the overlap of the peaks in the spectra. This overlap can lead to inaccuracies in the reported ionization energies derived from the binding energy positions of the visible peaks. This constraint explains the absence or inaccurate reporting of ionization energy values for certain biomolecules in the articles. Reliable theoretical methods are required for the interpretation of experimental results. In the present work, the overlap of photoelectron spectrum peaks is of particular importance, given the similarity between the molecular oxygen signatures of ascorbic acid (AA) and water. We used the SAC-CI method, based on the ONIOM model, to examine how the first coordination shell of water molecules affected the O1s core ionization energy of AA at 298 and 350 K temperatures. Our investigation shows that water’s photoelectron spectrum covers the O1s spectrum of AA.

Characterization of peroxide on coal surface: A combined △SCF and XPS study

X‐ray photoelectron spectroscopy (XPS) is widely used to determine the composition of elements and oxygen‐containing functional groups. Accurate determination of core‐electron binding energies (CEBEs) is essential for identifying the types and contents of oxygen‐functional groups present on the surface of coal. The △SCF method has been carried out on CEBEs for an extensive series of oxygen‐containing organic systems encompassing the most common functionalities. The polymers for which experimental values are available for comparison demonstrate the adequacy of the treatment for describing CEBEs for the O1s core level. Therefore, a sound basis is provided for the discussion of CEBEs (O1s) for functionalities for which experimental values are not accurate or available, such as carbonyl, carboxyl, and peroxide. The results are applied to the XPS study of oxidized coal. The change law of peroxide and oxygen‐containing functional groups was discussed. The calculation method and results in this paper can provide a guide for readers to use XPS to study the oxidation of coal surfaces.

A step toward correct interpretation of XPS results in metal oxides: A case study aided by first-principles method in ZnO.

Metal oxide semiconductors constitute a vast group of materials whose physical properties are greatly affected by native defects. For decades, x-ray photoelectron spectroscopy (XPS) has been widely used in defect analysis. However, correct interpretation of XPS results remains a difficult task. In this work, we present a detailed first-principles study on the core-level shift of the most stable and commonly cited crystal imperfections in ZnO, including O and -OH species at the surface with different coverages and bulk defects, including O interstitial (Oi), O vacancy in the +2 charge state (Vo2+), and the neutral vacancy (Vo0). The O1s core level spectrum is simulated and compared with experiments to understand the correlation between local atomic structures and features in the O1s spectrum. In particular, our results indicate that the widely adopted assignment in the defect analysis of ZnO, which links the defect peak in XPS to Vo, the most stable defect, is very likely a misinterpretation. Theoretical analysis indicates that there are no distinguishable XPS features arising from the Vo defect. Furthermore, we show that the commonly observed defect-related peak instead arises due to Oi or specific surface configurations. Given the importance of native defects in materials performance, misinterpretation of XPS results may lead to erroneous conclusions regarding materials properties. This work provides a first-principles basis for the analysis of oxide defects through XPS.

Single-Layered Biosynthesized Copper Oxide (CuO) Nanocoatings as Solar-Selective Absorber

Herein, spectrally selective single-layered CuO nanocoatings were successfully demonstrated via green synthesis and deposited on stainless steel (SS) substrates using a spin coater at 700, 800, 900, and 1000 rpm. The morphological, structural, and compositional analyses of the obtained nanocoatings were studied using SEM, XRD, EDX, and Raman spectroscopy. The SEM images show nanorod-like structure surfaces with dense surface morphology. The XRD patterns confirmed the presence of peaks indexed to a monoclinic structural phase of CuO. The EDX spectra clearly revealed the presence of Cu and O elements, and XPS spectra showed peaks of Cu2p and O1s core levels, which are typical characteristics of Cu (II) and O(II), respectively, in CuO. The Raman spectra showed peaks at 305, 344, and 642 cm−1 attributed to Raman active (Ag+2Bg) modes for Cu-O stretching. Rutherford backscattering spectrometry (RBS) determined the content of the elements and the changes in the thicknesses of the coatings with the rotational speed (RS) of the spin coater. The elemental content of Cu and O atoms were, respectively, 54 and 46%. The thicknesses were calculated to be 1.406 × 1018 atoms/cm2 (296.3 nm), 1.286 × 1018 atoms/cm2 (271.0 nm), 1.138 × 1018 atoms/cm2 (239.8 nm), and 0.985 × 1015 atoms/cm2 (207.5 nm) at 700, 800, 900 and 1000 rpm, respectively. The optical properties of the CuO nanocoatings were characterized using UV–Vis–NIR and FTIR spectrometers; its vital solar selectivity parameters of solar absorptance (α) and emissivity (ε) were evaluated in the ranges of 0.3–2.5 and 2.5–20 µm wavelengths, respectively. The obtained coatings exhibited solar parameters (α = 0.90, and ε = 0.31) associated with 700 rpm due to an intrinsic and interference-induced absorption as well as higher attenuation of light.

Determining surface-specific Hubbard-U corrections and identifying key adsorbates on nickel and cobalt oxide catalyst surfaces.

NiO is a popular transition metal oxide (TMO) with high thermal and chemical stability and Co3O4 is a relatively more reducible TMO due to weaker metal-oxygen bonds. Both are often used as catalysts in a variety of chemical transformations. Density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) are used to investigate catalysis on TMO surfaces, yet both techniques have their own limitations. The accuracy of DFT highly depends on the choice of Hubbard U correction. The bulk-property optimized U value of 5.3 eV for NiO and different U values for Co3O4, without any consensus, are often used in the literature to simulate surface catalysis. However, U values optimized using bulk properties often fail to reproduce surface-adsorbate interactions on TMOs. Similarly, there exists arbitrariness in assigning observed XPS shifts to different surface species on these metal oxides. Hence, a synergistic application of XPS and DFT+U is implemented to determine the surface specific U values for NiO and Co3O4, and to identify adsorbed surface moieties corresponding to experimentally observed XPS shifts. For the NiO (100) surface, the U value of ∼2 eV is able to reproduce the experimentally observed XPS O1s core level binding energy shifts correctly, instead of the bulk property optimized and commonly used U value of 5.3 eV. Using this surface specific U value of 2 eV, the experimentally observed XPS shifts are assigned. Similarly, for Co3O4 (100) surface, ∼3 eV of U value could successfully predict the experimentally observed XPS shifts and corresponding adsorbates. The surface adsorbates and configurations suggested in this work will help analyze experimental XPS data and the surface specific U values will ensure accurate predictions of adsorption and reaction energetics on these catalysts.

XPS, UV–Vis, XRD, and PL spectroscopies for studying nickel nanoparticle positioning effect on nanocomposite film properties

AbstractThis work studies the impact of positioning nickel nanoparticles (Ni‐NPs) on the surface of doped (P84‐PEO) nanocomposite films by using an ultra‐uniform magnetic field as a model for positioning magnetic NPs inside a polymeric matrix. This process is used to achieve desired and adjustable optoelectronic properties and to obtain new optical and electrical characteristics. P84‐PEO copolymer doped with Ni‐NPs exposed to a 450 mT magnetic field during the preparation give rise to a momentous increase in surface roughness, surface reactivity, and modified optical parameters, besides forming Ni oxides on the surface. Based on X‐ray photoelectron spectroscopy experiments, substantial extra shifts occurred in the binding energies of photo‐excited electrons from the O1s and C1s core levels. Additionally, the deconvoluted O1s spectra displays an additional photoelectron peak at 530.0 eV, most likely attributed to oxidized Ni‐NPs at the surface. Moreover, optical and structural properties were studied using UV–Vis and PL spectroscopy and X‐ray diffraction.

Excitation density in time-resolved water window soft X-ray spectroscopies: Experimental constraints in the detection of excited states

Time-resolved X-ray spectroscopies have the potential of unveiling ultrafast processes with chemical sensitivity, but their widespread application is still withheld by technical and experimental constraints on two levels: the count rate and the amount of signal to be measured. In this paper, we will give a brief overview of the available pulsed X-ray sources focusing in particular on those delivering photons with energies inside the water window (280–550 eV), thus allowing to access the C1s, N1s and O1s core levels which are relevant for the characterization of thin organic films and small molecules adsorbed on surfaces. We will mainly discuss the photon fluxes delivered by such sources in relation to their repetition rates, and we will see how these factors affect time-resolved measurements. The main purpose of this work is to discuss the most crucial parameter to adjust in pump-probe spectroscopies: the excitation density, which corresponds to the fraction of photoexcited molecules/atoms. We show that such quantity may be increased up to roughly 25% in gas phase and other robust samples, however, due to a lower damage threshold, in organic films it is typically constrained to be in the order of 1–5%. Despite the initially limited population of excited states in the latter case, we show that the evolution of the system may lead to a collective response of the material, which entirely modifies the measured core level line shape, thus providing a clear signal that may nonetheless offer valuable insights into the dynamics of the studied process.

Impact of high temperatures on aluminoceladonite studied by M\xf6ssbauer, Raman, X-ray diffraction and X-ray photoelectron spectroscopy

Mössbauer, Raman, X-ray diffraction and X-ray photoelectron spectroscopies were used to examine the effects of temperature on the structure of two aluminoceladonite samples. The process of oxidation of Fe2+ to Fe3+ ions started at about 350 °C for the sample richer in Al and at 300 °C for the sample somewhat lower Al-content. Mössbauer results show that this process may be associated with dehydroxylation or even initiate it. The first stage of dehydroxylation takes place at a temperature > 350 °C when the adjacent OH groups are replaced with a single residual oxygen atom. Up to ~500 °C, Fe ions do not migrate from cis-octahedra to trans-octahedra sites, but the coordination number of polyhedra changes from six to five. This temperature can be treated as the second stage of dehydroxylation. The temperature dependence on the integral intensity ratio between bands centered at ~590 and 705 cm−1 (I590/I705) clearly reflects the temperature at which six-coordinated polyhedra are transformed into five-coordinated polyhedra. X-ray photoelectron spectra obtained in the region of the Si2p, Al2p, Fe2p, K2p and O1s core levels, highlighted a route to identify the position of Si, Al, K and Fe cations in a structure of layered silicates with temperature. All the measurements show that the sample with a higher aluminum content and a lower iron content in octahedral sites starts to undergo a structural reorganization at a relatively higher temperature than the less aluminum-rich sample does. This suggests that iron may perform an important role in the initiation of the dehydroxylation of aluminoceladonites.

Influence of magnetron configurations on the structure and properties of room temperature sputtered ZnO thin films

Under the unbalanced magnetron (UBM) sputtering process, not only the plasma is confined near the target like in the conventional balanced magnetron (BM) sputtering process, but also extends towards the substrate and support the ion-assisted deposition (surface of thin films is bombarded by energetic Ar+ ions during the sputtering process). Here, we report the influence of magnetron configurations on the structure and properties of room temperature sputtered ZnO thin films while keeping other process parameters fixed. The UBM configuration has significantly improved various properties of ZnO thin films in comparison to the BM configuration. The crystalline quality with dominant orientation (002) and uniform distribution of grains is observed while an increase in the band gap from 3.25 eV (BM) to 3.33 eV (UBM) is obtained. The lower defects as investigated from Zn2p and O1s core level XPS spectra, which is well supported by Photoluminescence measurements. In addition to that, surface hydrophobicity has been increased from 121.2° (BM) to 125.5° (UBM). Thus, the unbalanced magnetron configuration in the sputtering process significantly enhanced the structural, optical and surface properties of ZnO thin films even at room temperature and low plasma power without any post annealing treatments, which is highly desired for the device fabrication.

On the signatures of oxygen vacancies in O1s core level shifts

Density functional theory calculations are used to investigate O1s surface core level shifts for MgO(100), ZnO(101¯0), In2O3(111) and CeO2(111). Shifts are calculated for the pristine surfaces together with surfaces containing oxygen vacancies and dissociated H2. Pristine surfaces show small negative shifts with respect to the bulk components and vacancies are found to have a minor effect on the O1s binding energies of neighboring oxygen atoms. OH-groups formed by H2 dissociation yield binding energies shifted to higher energies as compared to the oxygen atoms in the bulk. The results stress the difficulties in assigning core-level shifts and suggest that assignments of shifts in O1s binding energies to neighboring oxygen vacancies for the explored oxides should be reconsidered.

Hybridization of ellipsometry and energy loss spectra from XPS for bandgap and optical constants determination in SiON thin films

In this study, we first compare the bandgap determination methods in SiON thin films using two established techniques: Ellipsometry and the Energy Loss Spectrum from X-ray Photoelectron Spectroscopy. In the ellipsometry case, we modelled the optical properties using a single Tauc-Lorentz oscillator model, in a range from 1.5 to 6 eV, while for the XPS case, we used the threshold energy of the Energy Loss Spectrum from the O1s and N1s main core levels to determine the bandgap. We observed a consistent difference of the energy bandgap values obtained between the two methods, reaching up to 1.6 eV. Therefore, we combined the ellipsometry and Energy Loss Spectrum from XPS measurements, creating a hybrid metrology method using a triple Tauc-Lorentz oscillator model. This methodology respected the optical relations of each technique and the complex dielectric constant of the material, creating an overlap of the two measurements between 3.5 and 6 eV. The combination method brings the advantage of a more robust determination of the bandgap in the SiO 2 –Si 3 N 4 system, while creating a way to measure the refractive index and coefficient of extinction for intermixed thin films of SiON, from 1.5 up to 30 eV. • Comparison of bandgap determination using ellipsometry and ELS-XPS. • Large energy bandgap difference between the two methods, up to 1.6 eV. • Combination of the two techniques, using a triple Tauc-Lorentz oscillator model. • Robust determination of the bandgap in the SiO 2 –Si 3 N 4 system. • Method to measure n and k in a large energy range, from 1.5 up to 30 eV.

Magnetron configurations dependent surface properties of SnO2 thin films deposited by sputtering process

The effect of balanced magnetron (BM) and unbalanced magnetron (UBM) configurations, in RF sputtering process, on the surface properties of SnO2 thin films has been investigated. X-ray photoelectron spectroscopy (XPS) Sn3d and O1s core spectra reveal that the films deposited at RF power of 250 W under BM configuration consist of Sn4+ oxidation states, while those deposited under UBM configuration consist of Sn4+ and Sn2+ oxidation states. This has been attributed to the migration of oxygen atoms from SnO2, resulting in the formation of Sn interstitial and oxygen vacancies. The contact angle (θ) recordings reveal that the UBM configuration results in more hydrophobic surface (140.6°) of SnO2 thin films than that under BM configuration (129.6°). Further, Atomic Force Microscopy (AFM) and Field Emission Scanning Electron Microscopy (FESEM) results indicate that the SnO2 thin films deposited under UBM configuration have better density with granular grains in comparison to that under BM configuration. The present studies establish the fact that magnetron configurations in sputtering process have significant impact on the surface properties of SnO2 thin films.

An electron beam induced study in fluorine doped ZnO nanostructures for optical filtering and frequency conversion application

Influence of high energy electron beam treatment on fluorine doped ZnO (FZO) nanostructures and its role in modifying structural, optical, morphological and nonlinear optical properties was studied. FZO nanostructures were grown with different fluorine concentration using an air assisted chemical spray pyrolysis technique. The prepared nanostructures were treated with 8 MeV electron beam line at pre-determined dosages (5 kGy. 10 kGy, 15 kGy and 20 kGy). Compositional and chemical state analysis of FZO films were analyzed by x-ray photoelectron spectroscopy (XPS). The XPS analysis conveys that the percentage area ratio of O1s core level spectra which attributes to oxygen vacancy defects are reduced from 28.9% to 13.7% which endorses a fact that e-beam treatment suppresses the generation of oxygen related defects. The glancing angle X-ray diffraction (GAXRD) study confirms that the deposited films exhibit a single phase which point towards the higher order structural stability and phase purity of FZO nanostructures in intense radiation environment. The ambient temperature PL spectra show quenching of radiative defect centers upon electron beam irradiation which infers that non radiative recombination predominates the radiative recombination in the nanostructures upon e-beam treatment. Open aperture Z-scan analysis shows a magnitude of nonlinear absorption coefficient βeff in the order of 10−1 esu. Enhanced third harmonic generation signal (THG) shown by the films due to photoexcitation and relaxation process endorses the credibility of the grown films for application as UV light emitters.

Impact of barrier layer on HfO2-based conductive bridge random access memory

In this letter, we report the impact of the barrier layer on the performance of HfO2-based conductive bridge RAM (CBRAM) devices with TeTiW as the source layer. The considerable improvement of retention in the CBRAM device using TiW as the barrier layer is attributed to the lower amount of oxygen vacancies in the switching layer, which is justified from the O1s core level in X-ray photoelectron spectroscopy analyses. Therefore, the diffusion of Te in the resistive layer of the device with the TiW barrier layer is limited even at high temperature. The TeTiW/TiW/HfO2/TiN CBRAM device provides an excellent endurance of more than 104 cycles, with an on/off ratio of 200. Such a device also features long retention for up to 104 s at 200 °C.

Biofunctionalization of TiO2 surfaces with self-assembling oligopeptides in different pH and Ionic Strength conditions: Charge effects and molecular organization

Self-assembling peptides (SAPs) were investigated by means of XPS and Angular Dependent NEXAFS spectroscopies, with the aim to probe the influence of pH and Ionic Strength conditions on the chemical structure and molecular organization of SAPs anchored on titania surfaces. XPS at the C1s, N1s, O1s core levels allowed to study surfaces and biomolecule/substrate interfaces. NEXAFS data allowed ascertaining that SAPs molecular structure is preserved upon grafting to the titania surface. Angular Dependent NEXAFS was used to investigate the influence of environmental conditions on the molecular organization behaviour. The objective of our study was to establish a set of methodologies for obtaining arrangements of well-organized biomolecules on scaffolds surfaces as a basic technology to develop and optimize cells adhesion and proliferation for tissue engineering applications.

Geometry-dependent DNA-TiO2 immobilization mechanism: A spectroscopic approach

DNA nucleotides are used as a molecular recognition system on electrodes modified to be applied in the detection of various diseases, but immobilization mechanisms, as well as, charge transfers are not satisfactorily described in the literature. An electrochemical and spectroscopic study was carried out to characterize the molecular groups involved in the direct immobilization of DNA structures on the surface of nanostructured TiO2 with the aim of evaluating the influence of the geometrical aspects. X-ray photoelectron spectroscopy at O1s and P2p core levels indicate that immobilization of DNA samples occurs through covalent (POTi) bonds. X-ray absorption spectra at the Ti2p edge reinforce this conclusion. A new species at 138.5eV was reported from P2p XPS spectra analysis which plays an important role in DNA-TiO2 immobilization. The POTi/OTi ratio showed that quantitatively the DNA immobilization mechanism is dependent on their geometry, becoming more efficient for plasmid ds-DNA structures than for PCR ds-DNA structures. The analysis of photoabsorption spectra at C1s edge revealed that the molecular groups that participate in the C1s→LUMO electronic transitions have different pathways in the charge transfer processes at the DNA-TiO2 interface. Our results may contribute to additional studies of immobilization mechanisms understanding the influence of the geometry of different DNA molecules on nanostructured semiconductor and possible impact to the charge transfer processes with application in biosensors or aptamers.

Core electron binding energies of adsorbates on Cu(111) from first-principles calculations.

Core-level X-ray Photoelectron Spectroscopy (XPS) is often used to study the surfaces of heterogeneous copper-based catalysts, but the interpretation of measured spectra, in particular the assignment of peaks to adsorbed species, can be extremely challenging. In this study we present a computational scheme which combines the use of slab models of the surface for geometry optimization with cluster models for core electron binding energy calculation. We demonstrate that by following this modelling strategy first principles calculations can be used to guide the analysis of experimental core level spectra of complex surfaces relevant to heterogeneous catalysis. The all-electron ΔSCF method is used for the binding energy calculations. Specifically, we calculate core-level binding energy shifts for a series of adsorbates on Cu(111) and show that the resulting C1s and O1s binding energy shifts for adsorbed CO, CO2, C2H4, HCOO, CH3O, H2O, OH, and a surface oxide on Cu(111) are in good overall agreement with the experimental literature.

Effects of gamma irradiations on reactive pulsed laser deposited vanadium dioxide thin films

Vanadium oxide films are considered suitable coatings for various applications such as thermal protective coating of small spacecrafts because of their thermochromic properties. While in outer space, such coating will be exposed to cosmic radiations which include γ-rays. To study the effect of these γ-rays on the coating properties, we have deposited vanadium dioxide (VO2) films on silicon substrates and subjected them to extensive γ-irradiations with typical doses encountered in space missions. The prevalent crystallographic phase after irradiation remains the monoclinic VO2 phase but the films preferential orientation shifts to lower angles due to the presence of disordered regions caused by radiations. Raman spectroscopy measurements also evidences that the VO2 structure is slightly affected by gamma irradiation. Indeed, increasing the gamma rays dose locally alters the crystalline and electronic structures of the films by modifying the V–V inter-dimer distance, which in turns favours the presence of the VO2 metallic phase. From the XPS measurements of V2p and O1s core level spectra, an oxidation of vanadium from V4+ towards V5+ is revealed. The data also reveal a hydroxylation upon irradiation which is corroborated by the vanishing of a low oxidation state peak near the Fermi energy in the valence band. Our observations suggest that gamma radiations induce the formation of Frenkel pairs. Moreover, THz transmission measurements show that the long range structure of VO2 remains intact after irradiation whilst the electrical measurements evidence that the coating resistivity decreases with gamma irradiation and that their transition temperature is slightly reduced for high gamma ray doses. Even though gamma rays are only one of the sources of radiations that are encountered in space environment, these results are very promising with regards to the potential of integration of such VO2 films as a protective coating for spacecrafts.

Electron emission relevant to inner-shell photoionization of condensed water studied by multi-electron coincidence spectroscopy

Multi-electron coincidence spectroscopy using a magnetic-bottle electron spectrometer has been applied to the study of the Auger decay following O1s photoionization of condensed H2O molecules. Coincidence Auger spectra are obtained for three different photoelectron energy ranges. In addition, the energy distribution of the slow electrons ejected in the Auger decay of the O1s core hole is deduced from three-fold coincidences.

Electronic properties of atomic layer deposition films, anatase and rutile TiO2 studied by resonant photoemission spectroscopy

The TiO2 films are prepared by atomic layer deposition (ALD) method using titanium isopropoxide precursors at 250 °C and analyzed using resonant photoemission spectroscopy (resPES). We report on the Ti2p and O1s core levels, on the valence band (VB) spectra and x-ray absorption spectroscopy (XAS) data, and on the resonant photoelectron spectroscopy (resPES) profiles at the O1s and the Ti3p absorption edges. We determine the elemental abundance, the position of the VB maxima, the partial density of states (PDOS) in the VB and in the conduction band (CB) and collect these data in a band scheme. In addition, we analyze the band-gap states as well as the intrinsic states due to polarons and charge-transfer excitations. These states are found to cause multiple Auger decay processes upon resonant excitation. We identify several of these processes and determine their relative contribution to the Auger signal quantitatively. As our resPES data allow a quantitative analysis of these defect states, we determine the relative abundance of the PDOS in the VB and in CB and also the charge neutrality level.The anatase and rutile polymorphs of TiO2 are analyzed in the same way as the TiO2 ALD layer. The electronic properties of the TiO2 ALD layer are compared with the anatase and rutile polymorphs of TiO2. In our comparative study, we find that ALD has its own characteristic electronic structure that is distinct from that of anatase and rutile. However, many details of the electronic structure are comparable and we benefit from our spectroscopic data and our careful analysis to find these differences. These can be attributed to a stronger hybridization of the O2p and Ti3d4s states for the ALD films when compared to the anatase and rutile polymorphs.