Synchrotron-based X-ray Photoemission Spectroscopy Research Articles

Ion irradiation induced modification in the electrical properties and the electronic structures of vanadium dioxide thin films

Ion irradiation modifies the physical properties of oxide materials. The present study focuses on the effect of 150 keV Fe and Ni ion irradiations in VO2 thin films. These films were deposited by pulsed laser on an r-cut sapphire substrate (1 1‾ 02). The range of ion fluences was from 1 × 1014 to 1 × 1016 ions/cm2 and these samples were characterized by using grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, Temperature-dependent resistivity measurements (R-T), and Hall effect measurement. While the presence of monoclinic phases is confirmed by GIXRD and Raman measurements, the ion irradiation on VO2 modifies the metal to insulator transition temperature (MIT). The R-T measurement with supporting analysis of carrier concentration authenticates a decrease in transition temperature and an increase in conductivity with an increment of ion-fluences. Synchrotron-based X-ray photoemission spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) revealed that Vanadium ions are in mixed oxidation states of 4+ and 5+ after ion irradiation. Further, the Photo-Electron Spectroscopy (PES) measurement indicates the metallization of VO2 after ion irradiation consistent with R-T measurements.

Anti-corrosion and erosion properties of multilayers with diamond-like carbon and silicon films deposited on low-carbon steel

A multilayer of diamond-like carbon (DLC) and silicon (Si) films were deposited on low-carbon steel (CS) using a combination of bipolar-pulsed magnetron sputtering (MS) and the radio frequency plasma enhanced chemical deposition (RF-PECVD) methods. Spectroscopic techniques such as Raman spectroscopy, synchrotron-based X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were used to investigate the chemical and structural properties of the film. The mechanical properties were measured using nanoindentation and nano-scratch testers. The films' electrochemical performance and corrosion behavior were evaluated in the 3.5 wt% NaCl solution at room temperature. The erosion resistance of the films was studied under the sand jet impingement apparatus. The results indicate that the 2 Si-layers in the DLC film and CS substrate increase the C‒sp3 content at the DLC film surface resulting in high mechanical hardness. The adhesion of the DLC film on the CS substrate was also improved by adding the Si layer between the CS substrate and DLC film. Consequently, multilayer Si/DLC film resists the elastic deformation of films caused by graphitization.

Intense and stable room-temperature photoluminescence from nanoporous vanadium oxide formed by in-ambient degradation of VI3 crystals

Vanadium oxides have attracted research interest because their optoelectronic properties make them optically active with room-temperature photoluminescence (PL) emission, which, however, is not sufficiently intense for real applications. For this reason, many nanostructured vanadium oxides are currently fabricated through several precursors and different treatments to improve the PL efficiency and enhance the PL intensity. Herein, we propose an alternative and facile route to the fabrication of nanoporous vanadium oxide flakes through the spontaneous in-ambient degradation of layered van der Waals VI3 crystal, which is composed of a mixture of V2O5 and V3O7 phases. The as-grown VI3 crystals and the formed nanostructured vanadium oxide have been thoroughly studied using X-ray diffraction (XRD) and Raman spectroscopy to access the structural properties, X-ray photoemission spectroscopy (XPS) and synchrotron-based XPS to analyze the electronic core levels and valence bands, scanning electron microscopy (SEM) to access the morphology, and PL spectroscopy to grasp the optoelectronic properties. The nanoporous vanadium oxide system reveals an intense room-temperature PL emission in the red light visible range between 1.7 and 2.0 eV (620 and 730 nm), which is consistent with the V2O5 PL response. Remarkably, the PL emission reaches high intensity compared with those of different V2O5 nanostructures and is stable for months without intensity quenching and energy shifting. This discovery easies the integration of nanostructured vanadium oxides in optoelectronic nanodevices. Besides, the facile methodology proposed here promises to be applied to realize other nanostructured transition metal oxides.

CO adsorption on Co(0001) revisited: High-coverage CO superstructures on the close-packed surface of cobalt

The adsorbate overlayer structures formed by CO on highly covered Co(0001) were investigated using temperature programmed desorption, low energy electron diffraction, reflection absorption infrared spectroscopy and synchrotron-based x-ray photoemission spectroscopy, with the purpose to clarify some recent confusion on CO adsorption at high coverages. TPD shows that the coverage of the (2√3 × 2√3)R30o structure (abbreviated as 2√3 structure) is 7/12 (0.583) ML. Its unit cell contains one COtop and six CObridge species that form three ‘bridge dicarbonyls’ in which two CObridge adsorbates share the same cobalt surface atom. Between 0.58 and 0.63 ML the 2√3 structure breaks up into small 2D domains separated by antiphase domain boundaries that consist of ‘double bridge dicarbonyls’ in which three CObridge adsorbates share two cobalt surface atoms. A phase transition that occurs around 0.63 ML creates previously unknown c(8 × 2) and c(12 × 2) domain boundary structures which consist of high density strips with a (2 × 2)–3CO structure and top/hollow site occupation which are separated by antiphase domain boundaries with a lower local coverage. Up to 0.63 ML the structures found at low temperature in UHV are very similar to those observed by others using in-situ STM at 300 K under CO pressure. This changes above 0.63 ML, where STM shows that moiré structures form instead of the c(n × 2) domain boundary structures. We propose that the moiré and c(n × 2) structures have a very similar enthalpy of formation so that small variations of temperature and CO chemical potential can lead to different structures for the same CO coverage. We show that these high-density phases that exist above 0.5 ML do not form at the typical pressures and temperatures used in applied FTS, but they do need to be considered when CO adsorption at room temperature is used as a diagnostic tool to characterize the catalyst surface.

In situ study on the evolution of atomic and electronic structure of LaTiO3/SrTiO3 system

As with other transition-metal oxide interfaces, ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces exhibit interesting electronic properties, such as a two-dimensional electron gas. While the available Mott states and multiple pathways to metallicity in ${\mathrm{LaTiO}}_{3}$ can lead to a variety of applications in oxide electronics, the origin of the different properties remains uncertain. Utilizing real-time, in situ x-ray diffraction at the synchrotron, robust but thin ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures were fabricated using oxide molecular beam epitaxy. A thickness of six unit cells was determined to be critical, at which point the ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ heterostructures begin to sustain sharp, well-defined interfaces. The layers of ${\mathrm{LaTiO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ were sufficiently thin to benefit from the finite escape length of electrons at resonant soft x-ray photoemission, and the heterostructures were transferred in vacuo after growth to another chamber for synchrotron-based x-ray photoemission spectroscopy studies. With the x-ray energy tuned to the Ti-2p resonance, the ${\mathrm{LaTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ interfaces could be probed with enough penetration depth and selectivity. It is shown that all of the heterointerfaces exhibit the two-dimensional electron gas.

Water/methanol solutions characterized by liquid μ-jet XPS and DFT—The methanol hydration case

The advent of liquid μ-jet setups as proposed by Faubel and Winter – in conjunction with X-ray Photoemission Spectroscopy (XPS) – has opened up a large variety of experimental possibilities in the field of atomic and molecular physics. In this study, we present first results from a synchrotron-based XPS core level and valence band electron spectroscopy study on water (10-4 M aqueous NaCl solution) as well as a water/methanol mixture using the newly commissioned ALBA liquid μ-jet setup. The experimental results are compared with simulations from density functional theory (DFT) regarding the electronic structure of single molecules, pure molecular clusters, and mixed clusters configurations as well as previous experimental studies. We give a detailed interpretation of the core level and valence band spectra for the vapour and liquid phases of both sample systems. The resulting overall picture gives insight into the water/methanol concentrations of the vapour and liquid phases as well as into the local electronic structure of the pertinent molecular clusters under consideration, with a special emphasis on methanol as the simplest amphiphilic molecule capable of creating hydrogen bonds.

InAs-oxide interface composition and stability upon thermal oxidation and high-k atomic layer deposition

Defects at the interface between InAs and a native or high permittivity oxide layer are one of the main challenges for realizing III-V semiconductor based metal oxide semiconductor structures with superior device performance. Here we passivate the InAs(100) substrate by removing the native oxide via annealing in ultra-high vacuum (UHV) under a flux of atomic hydrogen and growing a stoichiometry controlled oxide (thermal oxide) in UHV, prior to atomic layer deposition (ALD) of an Al2O3 high-k layer. The semiconductor-oxide interfacial stoichiometry and surface morphology are investigated by synchrotron based X-ray photoemission spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. After thermal oxide growth, we find a thin non-crystalline layer with a flat surface structure. Importantly, the InAs-oxide interface shows a significantly decreased amount of In3+, As5+, and As0 components, which can be correlated to electrically detrimental defects. Capacitance-voltage measurements confirm a decrease of the interface trap density in gate stacks including the thermal oxide as compared to reference samples. This makes the concept of a thermal oxide layer prior to ALD promising for improving device performance if this thermal oxide layer can be stabilized upon exposure to ambient air.

Understanding the SEI Formation at Pristine Li‐Ion Cathodes: Chemisorption and Reaction of DEC on LiCoO2 Surfaces Studied by a Combined SXPS/HREELS Approach

AbstractReactions and solid electrolyte interface (SEI) formation at electrode–electrolyte interfaces are crucial for the stability and performance of Li‐ion batteries, but are still not fully understood on a fundamental level. For improving battery properties, a detailed understanding of these degradation processes is needed. In this contribution, the interface formation between a thin film LiCoO2 cathode material and diethyl carbonate (DEC) as typical battery electrolyte solvent is presented. A surface‐science approach is used performing a stepwise adsorption of DEC onto LiCoO2 at low temperatures. The interface is studied after each step by synchrotron‐based X‐ray photoemission spectroscopy (SXPS) and high‐resolution electron energy loss spectroscopy. The results demonstrate that the decomposition of carbonate solvents in contact with fully lithiated cathode materials as observed in adsorption experiments is complex, including the reduction of solvent, subsequent decomposition reactions, and also catalytic effects. In the present case, lithium ethyl carbonate, lithium ethoxide, and lithium oxides are assigned as reaction products. The spectra provide indications for partial electron transfer coupled to covalent interaction involving surface oxygen O2p orbital and DEC lowest unoccupied molecular orbital (LUMO) states.

CARBON-DOPEDTiO2NANOTUBES: EXPERIMENTAL AND COMPUTATIONAL STUDIES

C-doped TiO2nanotubes (NTs) with anatase structure, prepared by anodizing the polished Ti foils, were characterized using X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and synchrotron-based X-ray photoemission spectroscopy (XPS). XPS results show electron losses in C atoms, no electron change in Ti atoms, and two doping energy levels appeared in band gaps. Structural geometries, DOSs, PDOSs, and Bader charge analyses of C -doped TiO2anatase are predicted by periodic DFT calculations. Eight doping positions were taken into consideration: two substitutional cases (in oxygen and titanium sites) and six interstitial cases. We found that the interstitial carbon doping type is the most stable one, whereas the substitutional cases are rather unstable. Band-gap modifications can also be found in oxygen substitution, but not in titanium substitution. Both band-gap modification and non-band-gap modification are found in the interstitial carbon doping. In these eight C -doping systems, only the C atom in the oxygen substitution case gains electrons, 1.14 e, and others present electron losses within 0.5–4.00 e. The results of XPS measurements, DOSs calculations, and Bader charge analyses show that carbon interstitial is the most likely doping type for the C -doped TiO2NTs.

Fe-INDUCED CHANGE OF ELECTRON AFFINITY AND SECONDARY ELECTRON YIELD ON DIAMOND

Fe films grown on clean diamond (100) have been studied using synchrotron-based X-ray photoemission spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS). The work function and secondary electron yield (SEY) in UPS of the sample were found to change strongly as Fe film thickness increases. After the deposition of about one monolayer Fe, which was found to react with diamond by XPS, work function reaches its minimum while SEY reaches its maximum due to the formation of a dipole layer between the chemisorbed Fe and C atoms at the interface. This study could help the development of high emittance spin-polarized electron source based on magnetic thin films grown on diamond with low or negative electron affinity.

Investigation on interface barrier of Au–CdZnTe contacts

The interface barrier between Au contact and p-CdZnTe was studied by synchrotron-based X-ray photoemission spectroscopy (SXPS), where the interface barrier was determined by the discrepancy between E V−C deduced by the Cd 4d core level with valence band region and E B deduced by the Cd 4d core level with the Fermi edge. The interface barrier height was determined to be 0.88±0.02 eV for Au–CdZnTe without passivation and 1.17±0.02 eV for Au–CdZnTe after passivation. Schottky barrier height was 0.85±0.02 eV without passivation and 0.96±0.02 eV with passivation by current–voltage method. However, 1.39±0.02 eV without passivation and 1.51±0.02 eV with passivation were measured according to the capacitance–voltage method.

Reaction of SO 2 with Cu/TiO 2(1 1 0): Effects of size and metal-oxide interactions in the chemical activation of Cu nanoparticles

The reaction of SO 2 with stoichiometric TiO 2(1 1 0), partially reduced TiO 2 − x (1 1 0) and Cu/TiO 2(1 1 0) was investigated using synchrotron based X-ray photoemission spectroscopy. SO 2 adsorbs on perfect TiO 2(1 1 0) forming SO 4 species at room temperature, while SO 2 dissociatively adsorbs on partially reduced TiO 2 − x (1 1 0) forming SO 4, SO 3 as well as two sulfide species. SO 2 exposure to Cu particles supported on perfect TiO 2(1 1 0) can lead to the formation of SO 4, SO 3 and sulfide species. When depositing Cu on SO 4/TiO 2(1 1 0) at room temperature, the dissociation efficiency of Cu atoms is much higher than that of Cu deposited on TiO 2(1 1 0) prior to SO 2 dosing. The post-deposited Cu atoms can efficiently contact and react with SO 4 species before they form Cu–Cu bonds and big clusters. Small Cu nanoparticles supported on TiO 2(1 1 0) are more reactive towards SO 2 than surfaces of bulk copper. The chemical reactivity of the Cu/TiO 2(1 1 0) system increases with Cu coverage until reaching a maximum at θ Cu = 0.5–0.8 ML. After this point, an increase in Cu coverage leads to the formation of big Cu particles and the reactivity of the system decrease to that typical of bulk Cu. A comparison with results for SO 2/Cu/MgO(0 0 1) indicates that the effects of size and metal ↔ oxide interactions are important for the chemical activation of Cu nanoparticles on titania.

Inhibition of the reduction of Cr(VI) at the magnetite?water interface by calcium carbonate coatings

The effect of calcium carbonate coatings on the reduction of aqueous chromate on the magnetite(1 1 1) surface has been investigated using a combination of synchrotron based X-ray photoemission spectroscopy (PES) and X-ray absorption near edge structure (XANES) spectroscopy, along with laboratory-based powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaCO 3 coatings (dominantly calcite with minor quantities of aragonite and vaterite) of thicknesses ranging from 10 Å to ∼20 m were grown on magnetite(1 1 1) surfaces by exposure to supersaturated aqueous solutions followed by evaporation of the solution—a process that mimics pore-water evaporation in vadose zones leading to the formation of caliche and calcium carbonate coatings on mineral grains. Coating thicknesses were determined from attenuation of the Fe 2p photoemission signal by the carbonate coating. For coatings less than 15 Å thick, Cr 2p photoemission and Cr L II , L III -edge XANES spectra show that chromate is reduced by the underlying magnetite surface; however, as the minimum coating thickness increases beyond 15 Å, the magnetite surface becomes passivated and further chromate reduction ceases. Our findings suggest that carbonate coatings on natural magnetite grains can significantly reduce or eliminate their ability to reduce Cr(VI), which is a toxic and highly mobile environmental contaminant.

Inhibition of the reduction of Cr(VI) at the magnetite–water interface by calcium carbonate coatings

The effect of calcium carbonate coatings on the reduction of aqueous chromate on the magnetite(1 1 1) surface has been investigated using a combination of synchrotron based X-ray photoemission spectroscopy (PES) and X-ray absorption near edge structure (XANES) spectroscopy, along with laboratory-based powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). CaCO 3 coatings (dominantly calcite with minor quantities of aragonite and vaterite) of thicknesses ranging from 10 Å to ∼20 m were grown on magnetite(1 1 1) surfaces by exposure to supersaturated aqueous solutions followed by evaporation of the solution—a process that mimics pore-water evaporation in vadose zones leading to the formation of caliche and calcium carbonate coatings on mineral grains. Coating thicknesses were determined from attenuation of the Fe 2p photoemission signal by the carbonate coating. For coatings less than 15 Å thick, Cr 2p photoemission and Cr L II, L III-edge XANES spectra show that chromate is reduced by the underlying magnetite surface; however, as the minimum coating thickness increases beyond 15 Å, the magnetite surface becomes passivated and further chromate reduction ceases. Our findings suggest that carbonate coatings on natural magnetite grains can significantly reduce or eliminate their ability to reduce Cr(VI), which is a toxic and highly mobile environmental contaminant.