Emission X-ray Photoelectron Spectroscopy Research Articles

The enhancement of photodegradation efficiency using Pt-TiO2 catalyst.

This study investigates the mechanism of photosensitization and the recombination of excited electron-hole pairs affected by depositing platinum (Pt) on the surface of titanium dioxide (TiO2). A new catalyst of Pt-TiO2 was prepared by a photoreduction process. Being model reactions, the photocatalytic oxidation of methylene blue (MB) and methyl orange (MO) in aqueous solutions using the Pt-TiO2 catalyst was carried out under either UV or visible light irradiation. The experimental results indicate that an optimal content of 0.75%Pt-TiO2 achieves the best photocatalytic performance of MB and MO degradation and that the Pt-TiO2 catalyst can be sensitized by visible light. The interaction of Pt and TiO2 was investigated by means of UV-Vis absorption spectra, photoluminescence emission spectra, and X-ray photoelectron emission spectroscopy. The Pt0, Pt2+ and Pt4+ species existing on the surface of Pt-TiO2, and the Ti3+ species existing in its lattice may form a defect energy level. The Pt impurities, including Pt, Pt(OH)2, and PtO2, and the defect energy level absorb visible light more efficiently in comparison with the pure TiO2 and hinder the recombination rate of excited electron-hole pairs.

Photocatalytic activity of WO x-TiO 2 under visible light irradiation

With an attempt to extend light absorption of the TiO 2-based photocatalysts toward the visible light range and eliminate the rapid recombination of excited electrons/holes during photoreaction, new photocatalyst (WO x -TiO 2) powder was prepared by a sol–gel method. The photooxidation efficiency of WO x -TiO 2 catalyst was also evaluated by conducting a set of experiments to photodegrade methylene blue (MB) in aqueous solution. The photocatalytic activity of WO x -TiO 2 was examined by X-ray diffraction (XRD), UV–VIS absorption spectra, X-ray photoelectron emission spectroscopy (XPS), photoluminescence spectra (PL), surface photovoltage spectra (SPS) and electron-field-induced surface photovoltage spectra (EFISPS). The experiments demonstrated that the MB in aqueous solution was successfully photodegraded using WO x -TiO 2 under visible light irradiation. It was found that an optimal WO x dosage of 3% in WO x -TiO 2 achieved the highest rate of MB photodegradation in this experimental condition. It has been confirmed that WO x -TiO 2 could be excited by visible light ( E<3.2 eV) and the recombination rate of electrons/holes in WO x -TiO 2 declined due to the existence of WO 3 doped in TiO 2. The order of its photoactivity from weak to strong had a good agreement with that of PL intensity and that of EFISPS intensity from strong to weak.

Electronic structure of the mixed-valent system V2−xMoxO5

We have investigated the electronic structure of the mixed-valent system V2−xMoxO5 with varying doping concentration by means of X-ray photoelectron and X-ray emission spectroscopy. A series of V2−xMoxO5 compounds was obtained by the hydrochemical method. By comparing the XPS valence band with the V Lα, O Kα and Mo Lβ2.15 XES spectra we could localize the V3d, Mo4d and O2p states in the valence band. It has been found that in the course of the Mo doping, the density of states just below the Fermi level increases. The enhancement is due to both Mo4d and V3d states and seems to be relatively unaffected by the hybridization with O2p states situated at higher binding energies. The trends in the measured spectra are discussed in comparison to band structure calculations performed for defect-containing supercells.

Study of Au/Au(3+)-TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment.

With an attempt to extend light absorption of TiO2-based photocatalyst toward the visible light range and eliminate the rapid recombination of excited electrons/holes during photoreaction, a new type of photocatalysts (Au/Au(3+)-TiO2) powder was prepared by a photoreduction/sol-gel process. The crystal phase composition, surface structure, and light absorption of the new photocatalysts were comprehensively examined by X-ray differential detection (XRD), UV-visible absorption spectra, X-ray photoelectron emission spectroscopy (XPS), and photoluminescence (PL) spectra. The photooxidation efficiencies of the photocatalysts were also evaluated in the photodegradation of methylene blue (MB) in aqueous solutions under visible light irradiation from a high-pressure sodium lamp (lambda > 400 nm). The results of PL analyses in this study indicated that the gold/gold ion-doping on the surface of TiO2 could eliminate the electron/holes recombination and also increase the light absorption in the visible range. The analytical results of UV-visible diffuse reflection spectra (DRS) and optical absorption spectra indicated that a new energy level below 3.2 eV generated in the Au/Au(3+)-TiO2 promoted the optical absorption in the visible region and made it possible to be excited by visible light (E < 3.2 eV). The experiment demonstrated that the photooxidation efficiency of MB using the Au/Au(3+)-TiO2 powder were significantly higher than that using conventional TiO2 powder and an optimum molar content of gold doping/deposition in the TiO2 was 0.5%. The development of such photocatalysts may be considered a breakthrough in large-scale utilization of solar energy to address environmental needs.

Gadolinium silicate gate dielectric films with sub-1.5 nm equivalent oxide thickness

GdSi x O y gate dielectric films were deposited on Si(001) substrates using ultra-high-vacuum electron-beam evaporation from pressed-powder targets. Transmission electron microscopy showed that the films were amorphous as deposited and remained amorphous when annealed to temperatures up to 900 °C. Capacitance–voltage measurements indicate an equivalent oxide thickness (EOT) of 13.4 Å for a film with composition GdSi0.56O2.59 determined by in situ x-ray photoelectron emission spectroscopy. After forming gas annealing at 500 °C the EOT was reduced to 11.0 Å, at a physical thickness of 45 Å. The same film has a low leakage current of approximately 5.7×10−3 A cm−2 at +1 V, a reduction of 8.7×104 compared to current density estimates of SiO2 films with the same specific capacitance.

Comparative investigations of surface structure, photoluminescence and its excitation in silicon wires

The dependence of the photoluminescence (PL) and PL excitation spectra on the porous silicon top surface structure and the oxide composition on it has been investigated. Researches were carried out using the following methods: PL, PL excitation, electron paramagnetic resonance, atomic force microscope and X-ray photoelectron emission spectroscopy. Results indicate a direct correlation between the suboxide content and roughness structure on the surface with PL intensity. No correlation was noted between the PL intensity and the concentration of Si dangling bonds (non-radiative recombination centers). These results have given further support to a suboxide-related color center on the Si/SiOx interface as the source of the intense red luminescence of silicon wires.

XPS and XES emission investigations of d–p resonance in some copper chalcogenides

Abstract Investigations of binary and ternary copper tellurides and selenides electron energy spectra are of interest because of the considerable sensitivity of their electrophysical and optical properties to the stoichiometry of these compounds. Ab initio theoretical calculations for these non-stoichiometric AI2−xBVI compounds are difficult due to the existence of 3d-electrons in copper. Thus investigations of their band structure and electron density by means of X-ray photoelectron (XPS) and X-ray emission (XES) spectroscopy are important. In this work XPS of binary Cu2Se, Cu1.84Se, Cu1.82Se, Cu2Te, Cu1.95Te, Cu1.9Te, Cu1.7Te, and ternary CuInSe2 are obtained and matched on a common binding energy scale with the corresponding XES. The results of this matching show that copper 3d-states appear to be in the center of the tellurium or selenium p-band, splitting it into two components nearly symmetrically relatively to the d-bands. The splitting value is proportional to the content of copper in all compounds and inversely proportional to the gaps in binary chalcogenides, which corresponds with the d–s, p-resonance model. Comparative analysis of the electronic structure of binary and ternary compounds raises questions about the role of indium in determining the optical properties of CuInSe2.

The role of oxidation on porous silicon photoluminescence and its excitation

The effect of preparation regimes on the oxide composition, the number of dangling bonds, the photoluminescence and its excitation spectra have been investigated. The influence of the oxidation process during aging of porous silicon at ambient atmosphere and annealing in dry oxygen has been investigated via photoluminescence (PL), PL excitation (PLE), electron paramagnetic resonance (EPR) and X-ray photoelectron emission spectroscopy (XPS). Results indicate a direct correlation between the suboxide content and the PL intensity, while no correlation was noted between the PL intensity and the concentration of Si dangling bonds (non-radiative recombination centers). These results given further support to a suboxide-related color center as the source of the intense red luminescence.

Electronic Structure of Doped La-Mn-O Perovskites

The electronic structure of the doped perovskite manganese oxides La1-x Srx MnO3 and La1xBax MnO3 has been investigated by X-ray photoelectron and X-ray emission spectroscopy. By comparing the O Kα and Mn Lα X-ray emission spectra with the valence band X-ray photoelectron spectra we could localize the Mn 3d and O 2p states into the valence band. Our data are compared with previous band structure calculations. The value of the Mn 3s splitting does not change with Sr-doping in the concentration region from x = 0.0 till x = 0.3. This shows that the doping holes have mainly oxygen p character. The large value of the splitting indicates that the 3d electrons are in the high-spin state.

Suboxide-related centre as the source of the intense red luminescence of porous Si

The effect of the preparation regime on the oxide composition as well as the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of porous silicon have been investigated. The influence of the oxidation process during ageing at ambient atmosphere and annealing in dry oxygen of porous silicon has been studied via PL, PLE, infrared (IR) absorption and X-ray photoelectron emission spectroscopy (XPS). Results indicate a correlation between the suboxide concentration and the PL intensity. These results give further support to a suboxide-related colour centre as the source of the intense red luminescence.

Electronic structure of LiMnO : X-ray emission and photoelectron spectra and band structure calculations

The electronic structure of LiMnO2 and Li2MnO3 was studied by means of X-ray photoelectron and soft X-ray emission spectroscopy. For LiMnO2, LSDA and LSDA+U calculations were carried out. The LSDA+U calculations are in rather good agreement with the measured valence-band structure as well as with the magnetic and electrical properties of LiMnO2. It is shown that the band gap in LiMnO2 is determined by the charge-transfer effect.

Electronic structure of transition metal dicarbollide complexes

This paper reports on the results of our electronic structure study of bisdicarbollide complexes of transition metals Fe, Co, Ni, and Cu (36 compounds) by X-ray photoelectron and X-ray emission spectroscopy.

Copper-induced changes in the properties of arsenic chalcogenides

The influence of the presence of Cu in the amount between 1% and 6% in arsenic chalcogenide glasses is examined through a study of the electronic energy levels by means of x-ray photoelectron and x-ray emission spectroscopy, through an investigation of the low-energy tunneling systems by means of phonon echoes at 0.37 K, and through an examination of the photodarkening and the photoinduced dichroism caused by polarized Ar+ laser irradiation. Possible links between the various effects are examined. The Cu atoms become an integral part of the amorphous lattice structure and strongly influence the photodarkening, but they do not have a significant effect on the tunneling systems or the dichroism. It is concluded that the tunneling levels and the dichroism involve only local configurations, while the photodarkening involves larger-scale areas of the lattice.

Electronic structure of ternary transition metal oxides and sulphides: X-ray photoelectron and X-ray emission spectroscopy study

Results of measurements of X-ray photoelectron spectra (XPS) and X-ray emission spectra (XES) of constituents atoms (excited by electrons and tunable synchrotron radiation) of ternary transition metal oxides (EuMnO 3, Sr 2RuO 4) and sulphides (BaCo 1−xNi x S 2 are presented. They are compared with LDA and LDA + U band structure calculations. For Sr 2RuO 4 and BaCo 1− x Ni x S 2 energy excitation dependence of O Kα and S L 2,3 XES near the O 1s and S 2p threshold is found which is attributed to excitation of inequivalent oxygen and sulphur atoms in plane and apical sites, and corresponding O 2p and S 3s3d DOS are mapped.

Schottky barrier formation at the Ca/poly( p-phenylene vinylene) interface and its role in tunneling at the interface

We have studied the formation of the Ca/poly( p-phenylene vinylene) (PPV) interface using X-ray photoelectron emission spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). The rigid shift seen by XPS in the C(ls) core level spectra of PPV points clearly to charge transfer and Schottky barrier formation at the interface as a result of Ca deposition. The NEXAFS spectra suggest that charge transfer at the metal/polymer interface induces the formation of a bipolaron lattice at the interface. Taken together, these results imply that the metal/polymer interface is not rigid and that triangular barrier tunneling fails to take into account the effect of barrier formation. We propose a band bending modified tunneling model to explain charge transfer at the Ca/polymer interface.

Electrical and ellipsometric characterization of the removal of silicon surface damage and contamination resulting from ion beam and plasma processing

The silicon surface damage resulting from low-energy Ar ion beam and plasma bombardment under a variety of bombardment conditions has been measured and compared. Techniques used to characterize the damage and its removal included x-ray photoelectron emission spectroscopy and reflection ellipsometry to determine the type of incorporated impurities and the damage layer depth, respectively; and high-frequency and quasistatic capacitance-voltage measurements of interface trapped and fixed oxide charge to determine the electrical nature of the damage. While the ellipsometrically measured damage could be removed using various described treatments, the electrical damage persisted. Both electrical and optical differences resulting from lattice damage versus impurity-related damage associated with the different dry etching environments could be discerned.

An X-ray photoelectron and X-ray emission study of the electronic structure of Nb trichalcogenides and their lithium intercalates

The electronic structure of Nb trichalcogenides and their lithium intercalates was studied using X-ray photoelectron and X-ray emission spectroscopy. It was found that the metal 4d levels mainly contribute to the upper part of the NbSe 3 and NbS 3 valence zones whereas the deeper part of the valence zones is filled by the chalcogen levels. The intercalation was found to change the charge state of both Nb and chalcogen atoms.

Formation of silicon carbide in silicon substrates during CF 4/H 2 dry etching

Silicon specimens which had been reactive ion etched in CF 4/ X%H 2 (0≤ X ≤40) and subsequently air exposed have been characterised by X-ray photoelectron emission spectroscopy. Angular rotation was used to study films deposited by the plasma process onto the Si surface. In agreement with previous studies it is found that plasma exposure of Si specimens leads to the deposition of a fluorocarbon film. An intriguing new finding was the discovery of subsurface silicon carbide. The existence of this carbide layer was found to be independent of gas composition from 0–40% H 2 for a one-minute plasma exposure. Helium ion channeling studies of the same specimens show Si near-surface disorder. A silicon-carbide formation mechanism is suggested according to which carbon is deposited below the Si surface by the bombardment of carbon containing ions, thus enabling silicon-carbon bonding.

Anisotropic Dry Etching of S1O2 on Si and its Impact on Surface and Near-Surface Properties of the Substrate.

AbstractIn the present paper structural and chemical changes which can occur in the surface and near-surface properties of the substrate during anisotropic dry etching of SiO2 on Si will be reviewed.Silicon specimens which had been etched in CF4/X%H2 (X≤40) have been characterized by X-ray photoelectron emission spectroscopy, He ion channeling, H profiling and Raman scattering techniques.Key results of our studies are summarized as follows: Plasma exposure of a Si surface leads to the deposition of a thin (≤50Å thick) C,F-film.A Si-carbide containing Si region is formed during RIE which is localized near the fluorocarbon-film/Si interface.The near-surface region (∼30–50Å) of the Si substrate is also heavily disordered as found by ion channeling and Raman scattering.A modified, less damaged Si region has been found in the case of hydrogen-based etching gases, which extends from about 30–50Å from the surface to a depth in extent of 250Å and contains a high concentration (∼ 5 at.%) of H as shown by hydrogen profiling techniques.From the observation of Si-H and Si-H2 vibrational modes by Raman scattering it has been shown that some of the H is bonded to the Si lattice.

Formation of a silicon-carbide layer during CF4/H2 dry etching of Si

Silicon specimens which had been reactive ion etched in CF4/x% H2 (0≤x≤40) have been characterized by x-ray photoelectron emission spectroscopy. Angular rotation was used to study films deposited by the plasma process onto the Si surface. In agreement with previous studies it is found that plasma exposure of Si specimens leads to the deposition of a fluorocarbon film. An intriguing new finding was the discovery of a silicon-carbide layer localized near the fluorocarbon-film/Si interface. The existence of this carbide layer was found to be independent of gas composition from 0–40% H2 for a 1-min plasma exposure.