Z-Scan studies of KYW, KYbW, KGW, and Ba(NO3)2 crystals

We present the studies of nonlinear refraction and nonlinear absorption in promising crystals which are extensively used in Raman lasers or as solid-state laser host materials: Ba(NO₃)₂, KGW, KYW, and KYbW. The single-beam z-scan technique with 1 ps laser pulses at 790 and 395 nm has been applied for the study. Nonlinear refraction-index intensity-coefficients and two-photon absorption coefficients have been determined for the crystals. The considerable enhancement of nonlinear refraction is observed in the crystals at 395 nm.     

Interfaces analysis of the HfO2/SiO2/Si structure

The physical and chemical properties of the HfO₂/SiO₂/Si stack have been analyzed using cross-section HR TEM, XPS, IR-spectroscopy and ellipsometry. HfO₂ films were deposited by the MO CVD method using as precursors the tetrakis 2,2,6,6 tetramethyl-3,5 heptanedionate hafnium—Hf(dpm)₄ and dicyclopentadienil-hafnium-bis-diethylamide—Сp₂Hf(N(C₂H₅)₂)₂.The amorphous interface layer (IL) between HfO₂ and silicon native oxide has been observed by the HRTEM method. The interface layer comprises hafnium silicate with a smooth varying of chemical composition through the IL thickness. The interface layer formation occurs both during HfO₂ synthesis, and at the annealing of the HfO₂/SiO₂/Si stack. It was concluded from the XPS, and the IR-spectroscopy that the hafnium silicate formation occurs via a solid-state reaction at the HfO₂/SiO₂ interface, and its chemical structure depends on the thickness of the SiO₂ underlayer.     

Al2O3/SiO2 films prepared by electron-beam evaporation as UV antireflection coatings on 4H-SiC

Al₂O₃/SiO₂ films have been deposited as UV antireflection coatings on 4H-SiC by electron-beam evaporation and characterized by reflection spectrum, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The reflectance of the Al₂O₃/SiO₂ films is 0.33% and 10 times lower than that of a thermally grown SiO₂ single layer at 276 nm. The films are amorphous in microstructure and characterize good adhesion to 4H-SiC substrate. XPS results indicate an abrupt interface between evaporated SiO₂ and 4H-SiC substrate free of Si-suboxides. These results make the possibility for 4H-SiC based high performance UV optoelectronic devices with Al₂O₃/SiO₂ films as antireflection coatings.     

Characterization of a thin CeO2-ZrO2-Y2O3 films electrochemical deposited on stainless steel

In this paper, we report for the first time formation of a thin CeO₂-ZrO₂-Y₂O₃ films electrodeposited on a stainless steel substrate. The samples have been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The XRD and XPS data indicate formation of a solid solution and additional existence of Ce³⁺ states near the surface. After annealing, SEM examination has shown a microstructure formed by dispersed spherical agglomerates having a size between 20 and 60 nm.     

The XPS and XAES spectra of Na0.88Mn0.56PS3

Na_0.88Mn_0.56PS₃compounds have been synthesized by starting from MnPS₃ polycrystalline powders by means of a two-step cation-exchange process at a higher intercalation time than the one used for producing the literature reported Na_0.62Mn_0.69PS₃ compounds. The obtained samples have been characterized by X-ray photoelectron spectroscopy (XPS). The resulting spectra of the so-synthesized compounds have been compared with those observed in Na_0.62Mn_0.69PS₃ in our previous paper and similar electronic properties have been noted. In particular by XPS it has been shown that greater sodium content affects neither the core-level binding energies of the host matrix elements nor the type of link between Na⁺ and (Mn_1−xPS₃)⁻.     

Metal-insulator transition and superconductivity in Hg-doped BaPb0.75Bi0.25O3

The effects of mercury doping on the superconductivity, crystal structure, and electronic structure have been investigated in Hg-doped BaPb_0.75Bi_0.25O₃ (BaPb_0.75−xHg_xBi_0.25O₃, BPHBO) by magnetic measurement, powder X-ray diffraction (XRD), and X-ray photoemission spectroscopy (XPS). At lower doping levels, the system is metallic and superconducting. However, the superconductivity is fully suppressed by Hg doping at x>0.25 and recovered with further increase in Hg content at x>0.3, showing a superconductivity reentrant phenomenon. XPS analysis reveals that BPHBO experiences dual metal-insulator transitions (MITs) at these two superconductivity points, which are accompanied by lattice distortions, suggesting that they may be driven by Peierls transitions. The first MIT may be a Mott-transition, while the second may be due to competition between the band filling effect and modification of the charge-disproportionate state.     

Characterization of Pd-CeOx interaction on α-Al2O3 support

The Pd-Ce interaction was studied over CeO₂ (0.3-2.5 wt.%)-Pd (1 wt.%)/α-Al₂O₃ catalysts used in the reforming reaction of CH₄ with CO₂. The samples were characterized by using high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The activity and selectivity behavior was in good agreement with that of other supported metal catalysts (Ni and Pd) modified with different promoters. The preliminary results of HRTEM would indicate that the CeO_x forms small crystallites around the Pd particle. The XPS analysis for the regions of Ce 3d and Pd 3d, gives an account of Ce being present mostly as Ce³⁺ and a high binding energy for Pd 3d_5/2 (335.3 eV), an evidence of Pd-Ce chemical interaction. The Pd/Al XPS intensity ratios vs. the Pd average particle size, determined by TEM, show an excellent correlation for fresh and used catalyst. These results indicate that the diminution of the Pd/Al ratios was due to Pd sintering. Consequently, the small amounts of CeO_x species do not cover the Pd particle, in agreement with the HRTEM results. The overall results stand for the promoter action mechanism of the CeO_x for the reforming reaction with CO₂.     

Embedded structure of silicon monoxide in SiO2 films

The structure of SiO_x (x = 1.94) films has been investigated using both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The SiO_x films were deposited by vacuum evaporation. XPS spectra show that SiO_1.94 films are composed of silicon suboxides and the SiO₂ matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si₃O⁺ ion species were found in the TOF-SIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO₂, and this structure most likely exists as a predominant form of Si₃O₄. The existence of Si-Si structures in the SiO₂ matrix will give rise to dense parts in loose glass networks.     

Photophysical and enhanced daylight photocatalytic properties of N-doped TiO2/g-C3N4 composites

N-doped TiO₂/C₃N₄ composite samples were synthesized by heating the mixture of the hydrolysis product of TiCl₄ and C₃N₄ at different weight ratios. The samples were characterized by X-ray diffraction (XRD), Raman spectrum, UV–vis absorption spectrum, photoluminescence spectrum, X-ray photon electron spectrum (XPS) and surface photovoltage spectrum (SPS). The XRD and Raman results indicate that the introduction of C₃N₄ could inhibit the formation of rutile TiO₂. The composite samples show slight visible light absorption due to the introduction of C₃N₄. The XPS result reveals that some amount of nitrogen is doped into TiO₂, and C₃N₄ exists in the composite sample. The intensities of the SPS signal in the composite samples decrease with the rise in the amount of C₃N₄ in the samples. The photocatalytic activity was evaluated from the Rhodamine B (RhB) degradation under fluorescence light irradiation. The composite samples show significantly enhanced photocatalytic activities and the RhB self-sensitized photodegradation in this system was observed by measuring the photocurrent in the dye sensitized solar cell using the composite as the working electrode.     

Photoelectron spectroscopic investigation of transformation of trifluoroacetate precursors into superconducting YBa2Cu3O7−δ films

X-ray photoelectron spectroscopy (XPS) has been used to investigate the evolution of surface chemistry of YBa₂Cu₃O_7−δ (Y123) films prepared by the metalorganic deposition (MOD) process using trifluoroacetate (TFA) precursors. Detailed XPS core-level spectra obtained from the samples quenched from various points during the calcining and firing stages have been reported for the first time and are used to identify surface species. The XPS data show evidence of formation of intermediate phases such as Y-O-F, BaF₂, and CuO during the calcining process, which are the decomposition products of yttrium, barium, and copper trifluoroacetates, respectively. The TFA precursors are completely decomposed at the end of calcination. The change of binding energies for Y 3d_5/2, Ba 3d_5/2, and O 1s during the firing process indicates that Y123 starts to form at 800 °C after 0.5 h firing. Based on the experimental results, an alternative mechanism of the chemical evolution from precursor to final film in the TFA-MOD process is proposed.     

Electronic structure and magnetic properties of Gd3Co11B4 compound

The electronic structure of hexagonal Gd₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy (XPS) and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. We have found a good agreement between the experimental XPS valence band spectra and theoretical LMTO calculations. Results showed that the Gd₃Co₁₁B₄ compound is ferrimagnetic with the calculated total magnetic moment M=14.29 μ_B/f.u. The values of the magnetic moments on Co atoms strongly depend on the local environment. We have also compared the electronic structure and magnetic properties of Gd₃Co₁₁B₄ compound with those of Nd₃Co₁₁B₄ compound.     

Core level spectroscopy and RHEED analysis of KGd0.95 Nd0.05(WO4)2 surface

A study of the surface structure and electronic properties of (010) KGd_0.95Nd_0.05(WO₄)₂ (Nd:KGW) using RHEED analysis and XPS is presented. It is shown that Nd doping has a negligible effect on the core levels of the basic elements. A bombardment of the Nd:KGW crystal with 3-keV Ar ions results in surface amorphization accompanied by the generation of tungsten ions in lower valence states.     

Aqueous electrochemical insertion of Na into the tungsten oxide hybrid WO3(4,4′-bipyridyl)0.5

Aqueous electrochemical insertion of M⁺ (Na⁺ and H⁺) species into WO₃(4,4′-bipyridyl)_0.5 has been carried out. The chemical states and structure of the resulting product were analysed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS showed the presence of W⁶⁺ as well as the usual reduced W species (W⁵⁺) which is responsible for a change in colour. Moreover, the presence of these intercalates correlates with the evolution of the reduced W species. The bulk structure of the layered hybrid, as determined by powder X-ray diffraction, showed no alteration after electrochemistry, in contrast to the same measurements on tungsten trioxide (WO₃). This however concurs with single-crystal X-ray studies, which show little change in lattice parameters with Na⁺ insertion. Four-probe resistance measurements of the layered hybrid coated film display a drop in resistance after electrochemistry, which can be attributed to the injection of charge-carriers into the conduction band.     

A facile one-pot solvothermal route to tubular forms of luminescent polymeric networks [(C3N3)2(NH)3]n

A facile one-pot solvothermal route has been developed for the synthesis of tubular luminescent polymeric networks [(C₃N₃)₂(NH)₃]_n, structurally related to the proposed g-C₃N₄. XRD patterns showed a characteristic 002 basal plane diffractions, indicating an interlayer d spacing of 3.23 Å. XPS spectra show that the C1s and N1s have a symmetric peak and an asymmetric peak at 288.10 and 399.00 eV, respectively. The bulk composition C₆N_8.9H_4.5 determined by elemental analysis is comparable to the calculated value C₆N₉H₃ for this proposed polymer. FTIR spectra indicated the presence of s-triazine ring, which was further supported by the luminescent and UV-vis absorption characteristics probably depending on π→π^* electronic transition. The tubular structure has been studied by TEM, SAED, and HREM.     

Magnetic,electronic and structural properties of ZnxFe3−xO4

A series of samples Zn_xFe_3−xO₄ have been prepared by the chemical coprecipitation technique and characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and X-ray photoelectron spectroscopy (XPS). XRD demonstrates all the samples of Zn_xFe_3−xO₄ have a spinel structure same as Fe₃O₄. The magnetic hysteresis loops of Zn_xFe_3−xO₄ obtained from VSM indicate that the saturation magnetization has a maximum when x is ∼1/3. The chemical states of Fe atoms and Zn atoms in zinc ferrites have been measured using XPS and Auger electron spectroscopy (AES). The Fe 2p core-level XPS spectra and Zn L₃M₄₅M₄₅ Auger peaks have been analyzed and the results have been discussed in correlation with the samples’ magnetic properties. These results suggest most of Zn atoms occupy the tetrahedral sites and a small amount of them occupy the octahedral sites.     

Characterization of Cr/SiO2 catalysts and ethylene polymerization by XPS

Cr/SiO₂ catalysts with 1 or 3 wt.% Cr loadings and different chromium precursors were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A method to determine chromium species in the sample was developed through the decomposition of the Cr 2p XPS spectrum in Cr⁶⁺ and Cr³⁺ standard spectra. The results of the binding energy from the Cr 2p region and of the distribution of chromium species allowed to evaluate the dynamic photo-reduction of the surface chromium species during XPS analysis. Photo-reduction of surface Cr⁶⁺ to Cr³⁺ species was verified for all samples supported in silica, depending on the precursor and chromium content. Bulk CrO₃ and Cr₂O₃ standards did not reveal variation in the binding energy of Cr 2p_3/2, but a physical mixture of CrO₃ with SiO₂ presented photo-reduction. The behavior of this mixture resembled to the catalysts and suggests the participation of the surface hydroxyls of silica in the photo-reduction process. XPS intensity measurements for assessing dispersion of chromium oxide were used to compare the calcined and reduced catalysts to different chromium precursors. Polyethylene chains were detected by in situ XPS, while oligomerization products were not observed.     

Preparation of superparamagnetic γ-Fe2O3 nanoparticles in nonaqueous medium by γ-irradiation

Superparamagnetic γ-Fe₂O₃ nanocrystallites have been prepared by γ-irradiating ferrocene in the presence of isopropyl alcohol to get Fe nanoparticles in nitrogen atmosphere and at room temperature, followed by oxidization in air to obtain γ-Fe₂O₃. The final black powder was characterized with X-ray powder diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). From XRD pattern and XPS spectrum, we can confirm to get γ-Fe₂O₃. The particle size is several nanometers as shown in TEM image. Magnetic hysteresis loop measurements exhibited that the γ-Fe₂O₃ nanoparticles display superparamagnetism. However, a trace black powder was obtained in kerosene oil using the same method. A possible formation mechanism of the γ-Fe₂O₃ nanoparticles was suggested.     

Surface characterization study of a Pd/Co3O4 methane oxidation catalyst

A surface characterization study using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) has been performed on a 5 wt.% Pd/Co₃O₄ methane oxidation catalyst before and after exposure to a mixture of CH₄ and O₂ in N₂ at 250 °C for a period of 6 days. The primary peaks observed in the XPS survey spectra are the Co 2p, Pd 3d, O 1s and C 1s, along with Co, Pd and O Auger peaks. High-resolution Pd 3d spectra reveal that Pd exists on the surface predominantly as PdO, with no apparent change in chemical state during reaction. High-resolution XPS Co 2p and O 1s spectra reveal an accumulation of CoOOH and a depletion of CoO in the near-surface region during reaction. ISS analysis with intermittent 1-keV Ar⁺ sputtering was used to obtain depth profiles from the catalyst before and after reaction. The results indicate that the Pd/Co concentration ratio decreases with sputtering and that this ratio is larger for the as-prepared catalyst indicating that morphological changes occur during reaction. The ISS depth profile spectra obtained from the catalyst after reaction indicates the presence of an oxyhydroxide layer throughout the near-surface region. This observation is consistent with the XPS data indicating accumulation of hydroxide and oxyhydroxide species at the surface during reaction.Based on these data and the results of related studies, a reaction mechanism is proposed. In this mechanism, methane dissociatively chemisorbs to form a surface methoxy species and CoOOH. The remaining hydrogen atoms are stripped from the methoxy species leaving an active adsorbed C species which reacts with surface oxygen and a hydroxyl group to form an adsorbed bicarbonate ion which then decomposes to form CO₂ and a surface hydroxyl group. These hydroxyl groups also react to form H₂O and then more O₂ adsorbs dissociatively at the vacant sites.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

Surface characterization and catalytic CO + H2 reaction on Fe82.2B17.8 amorphous alloy

Fe_82.2B_17.8 amorphous ribbon has been used as a catalyst for the Fischer-Tropsch-type reaction of CO+H₂. Specific activity has been found to be at least an order of magnitude higher than that of either the crystallized ribbon of identical composition or the supported iron catalyst. Before and after the catalytic tests the ribbons were characterized by XRD, XPS, UPS and Mössbauer spectroscopy in transmission and in conversion electron modes. Conversion electron Mössbauer spectroscopy and UPS proved that the surface of the amorphous ribbons is being partially crystallized during 8000 min reaction time at a maximum reaction temperature of 560 K. The superior catalytic activity has been explained by stabilization of the small iron particles and Fe₂O₃ by boron atoms at the surface and by suppressed carbide formation.