X-ray Photoelectron Spectroscopy Core-level Spectra Research Articles

Nonstoichiometric Tungsten Oxide Based on Hexagonal WO3

On the initial stage of reduction of hexagonal tungsten trioxide, h-WO3, the nonstoichiometric h-WO2.8 phase was synthesized. The X-ray powder diffraction analysis and subsequent refinement using the Rietveld Full Profile Matching & Integrated Intensities Refinement of X-ray and/or Neutron Data Programs (FullProf Version 3.5 Dec97-LLB-JRC) indicate that the crystal structure of the h-WO2.8 phase is ascribed to the UO3 structure type with lattice parameters a = 0.3625 nm and c = 0.3780 nm. The X-ray photoelectron spectroscopy (XPS) method was applied to study the electronic structure of the nonstoichiometric tungsten trioxide. The XPS valence band and core-level spectra of the h-WO2.8 phase and, for comparison, h-WO3 were derived. The formation of an additional near-Fermi subband, which is absent on the XPS valence-band spectrum of hexagonal tungsten trioxide, was observed on the spectrum of the nonstoichiometric hexagonal WO2.8 compound. Half-widths of the XPS valence band as well as both W 4f and O 1s co...

Particle–hole interaction effect on a core-level XPS spectrum II

Abstract X-ray photoelectron spectroscopy (XPS) spectral behavior of an initial discrete core-hole state embedded or nearly embedded in a continuum is studied by a Green’s function model calculation. The particle–hole interaction U in the final two-hole one-particle state is included in the initial core-hole self-energy by the ladder and ring approximation. With an increase of U, the transition to an unoccupied (particle) state near the double-ionization threshold increases in the final state. With a further increase of U, the transition to an empty bound level below the Fermi level dominates. It is shown that even when the coupling between the initial discrete core-hole state and the final continuum state is strong there can be a sharp resonant state. When the coupling is weak, there is a possibility of the breakdown of the quasi-particle approximation for the initial core-hole state. U can substantially affect the initial core-hole spectral function.

X-ray photoelectron spectroscopy studies of modified surfaces of α-Al2O3, SiO2, and Si3N4 by low energy reactive ion beam irradiation

Reactions of N2+ ion beams with oxide surfaces of α-Al2O3(0001) single crystal and chemical vapor deposition (CVD) SiO2, and reactions of O2+ ion beams with a nitride surface of Si-rich CVD Si3N4 were investigated as a function of ion beam energy (200–1000 V) and dose (1×1015–1×1017/cm2). The thickness modified by the irradiation of a reactive low kinetic energy ion beam was measured using high resolution cross-sectional images of transmission electron microscopy (HR-XTEM), and the formation of new bonding induced by chemical reaction was analyzed by x-ray photoelectron spectroscopy (XPS). New bonding of Al–O–N on α-Al2O3(0001) started to be observed at 600 V N2+ ion energy and a dose of 1×1016/cm2, and Al–N bonding could be found at an ion beam energy of 1 keV. The thickness of the aluminum oxynitride layer after 800 V N2+ bombardment has been determined to be 10–50 Å by HR-XTEM analysis. In the case of CVD SiO2 surface modification, new bonding related to nitrogen was not clearly resolved in the XPS spectra, irrespective of the change of ion beam energy from 200 to 1000 V and ion dose from 1×1015 to 1×1017/cm2. However, widening of the full width at half maximum of Si 2p core-level XPS spectra for the modified SiO2 surface and the peak position of N 1s around 399 eV were evidence of the existence of nitrogen-related bonding like Si–O–N in the modified CVD SiO2 surfaces. Moreover, it was very interesting that the Si 2p peak of elemental Si appeared in the sample irradiated at a dose of 1×1017/cm2. Its occurrence was considered to be due mainly to the preferential sputtering effect, and was found to be largely dependent on the ion beam energy as well as on the ion dose. In the surface modification of low-pressure CVD Si3N4 by direct ken O2+ ion irradiation, Si–O–N bonding could be successfully created at an ion beam energy of 200 V and it evolved significantly at ion beam energies higher than 500 V. From the above results, low energy reactive ion beam irradiation can successfully create new bonding structures on oxide and nitride surfaces due to a surface chemical reaction like nitridation or oxidation, and is expected to be very useful for the formation of new ultrathin functional layers on ceramic surfaces.

Tricyclohexylphosphine Adsorbed on Rhodium

Tricyclohexylphosphine (TCHP) adsorbates on rhodium, prepared both from solution and by sublimation in UHV are studied by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRAS), and temperature-programmed desorption (TPD). XPS and IRAS are used to investigate the molecular orientation of the adsorbates and the coordination to the surface. TPD is used to study the stability of the adsorbates. Molecular surface interactions causing chemical shifts in the core level spectra of the adsorbates on rhodium are investigated using multilayer films and adsorbates on gold as references. In the solution deposition procedure, freshly evaporated rhodium samples are shortly exposed to air, resulting in oxygen covered surfaces. The P(2p) core level XPS spectrum shows only one type of phosphorus with a P(2p3/2) binding energy about 2.7 eV higher than when adsorbed on gold. This is in agreement with molecular adsorption through oxygen to the surface. To study the interaction of TCHP with clean rhodium we also prepared a TCHP adsorbate by an in situ sublimation process in UHV. One dominating state of phosphorus is observed with a P(2p3/2)binding energy about 2.6 eV lower than when adsorbed from solution, but close to what was found when adsorbed on gold. This indicates formation of a soft donor/acceptor bond due to a direct coordination of the molecule through the phosphorus lone pair orbital to the rhodium surface. The soft donor/acceptor and oxygen bridging to rhodium lead to a large electronic reorganization of the phosphorus atom, which manifests itself as substantial infrared peak shifts of the cyclohexyl modes, especially the CH2 stretches. Combined IRAS and TPD measurements show molecular desorption at 465 K for the UHV prepared monolayer, which is consistent with strong chemisorption to the rhodium surface.

Surface modification of α-Al2O3(0001) by N2+ ion irradiation

The surface of sapphire most widely used as a substrate for III-V nitride growth was modified by N2+ ion irradiation with ion-beam energies from 100 to 1000 eV. As the ion-beam energy increased from 100 to 500 eV, the root-mean-square roughness of the surface morphology decreased. However no significant change in surface roughness at energy higher than 500 eV was observed. From the N 1s x-ray photoelectron spectroscopy core-level spectra, no peak related to a nitrogen bond could be found in the samples irradiated with ion energy below 500 eV. An AlON peak began to appear in the samples irradiated with 600–900 eV N2+ ions, and two peaks corresponding to AlON and AlN were distinctively observed in the sample irradiated at 1 keV.

Evidence for the existence of absorbed NO in double-layered cuprate

The NO intercalation properties of layered cuprate La2yxBaxSrCu2O6ya reported by Machida et al. [1] suggest that the desorption process of NO contrasts with conventional NO absorption±desorption systems in the following ways: (a) NO absorption is not associated with the formation of nitrates or nitrites of barium; (b) the desorption temperature of N2 is much higher than that of O2. Also, the chemical states of NO over LaBaSrCu2O6ya were investigated using X-ray photoelectron spectroscopy (XPS) [2]. The obtained results suggested the surface adsorption of NO2 group corresponding to NO trapped by oxygen vacancies [2]. However, no prior measurement on the chemical states of absorbed NO in double-layered cuprates has been reported. Here, we report the chemical states of the intercalated NO in LaBaSrCu2O6ya investigated by the electron-energyloss spectroscopy (EELS). Powdered LaBaSrCu2O6ya was prepared by coprecipitation of the citrates of the elements. After calcination at 1073 K for 5 h in air, the powder was ground and then heated at 1273 K for 10 h in air. The crystal phase of the powder was determined by powder X-ray diffraction (XRD), and the singlephase XRD pro®les have been given in [1]. The NOintercalated samples were prepared by annealing at 627 K for 2 h upon introduction of 0.5 vol % NO± 99.5 vol % N2 gas. The NO-intercalated samples showed a similar XRD pro®le to LaBaSrCu2O6ya and there were no other additional refelctions. These powders were pressed into pellets and analysed by EELS and powder XRD. The electron-energy-loss spectra were acquired using the Gatan model 666 parallel-detection energy-loss spectrometer mounted on a Hitachi HF-2000 transmission electron microscope. The transmission electron microscopy specimens, i.e., ceramic akes, were prepared by crushing the sample pellets. The absorption edges of N 1s and O 1s levels were measured by high-energy EELS in transmission through samples about 200 AE thick. The EELS data from cuprates were recorded as two separate spectral regions. The N 1s absorption edges of NOintercalated LaBaSrCu2O6ya sample is shown in Fig. 1. Electron incidences are parallel to the [0 0 1] directions where diffraction spots are indexed in terms of the tetragonal double-layered cuprate, the so-called ` 326'' type. The features around 401 eV in the NO-intercalated LaBaSrCu2O6ya sample are attributed to N 1s bands similar to the results of BN and air in [3]. Therefore, the N 1s absorption edges indicates the existence of N elements in LaBaSrCu2O6ya. Also, the N 1s absorption edges are not observed for the sample in vacuum for 3 h. Therefore, it is considered that the NO-intercalated sample in vacuum releases N2 gas from the oxide. The O 1s absorption edges of the as-prepared and NO-intercalated LaBaSrCu2O6ya samples are shown in Fig. 2. The O 1s absorption edges of the NOintercalated LaBaSrCu2O6ya samples show a rise in intensity at 528±532 eV with NO intercalation as shown in Fig. 2. Nucker et al. [4, 5] reported that the rise in intensity of the O 1s absorption edges was realized in the low-energy part (E , 531 eV) which was related to holes on O sites. Therefore, the rise in intensity of the O 1s absorption edges below 532 eV as shown in Fig. 2 suggests that holes enter the O 2p states with the NO intercalation. Our previous report showed that the binding energies of Cu 2p3=2 XPS core-level spectra of the NO-intercalated LaBaSrCu2O6ya sample (933.1 eV) are away from that of as-prepared sample (934.0 eV) [2, 6]. As the formation of nitrates or nitrites are not observed in the NO-intercalated sample, the chemical shift of Cu 2p3=2 XPS data in our previous [2, 6] report re ects the intercalation effects of NO molecules into the layer structure. It is known that the core-level binding energy of Cu 2p is lower in an

X-ray photoelectron spectroscopies of Ni-doped Bi-2212 compounds

Samples with the nominal composition Bi 2Sr 2Ca 1Cu 2− x Ni x O 8+δ ( x = 0.00, 0.02, 0.10, 0.20) have been studied by X-ray photoelectron spectroscopy (XPS). From deconvolution of Bi 4f and O 1s XPS core-level spectra, we propose that the main reason leading to the decrease of superconducting transition temperature ( T c ) with increasing Ni content, is the distortion of the Bi-2212 crystal structure by extra oxygen being intercalated between the adjacent Bi-O layers which then affects the hole concentration ( n h ) of the CuO 2 planes.

Sol-gel prepared In 2O 3 thin films

Semiconductor sensors based on nanocrystalline In 2O 3 thin films, which were prepared by a sol-gel method, showed high sensitivity to NO 2. In 2O 3 sensing layers were obtained by calcination of In(OH) 3 sol which was stabilized with nitric acid. These films were investigated in respect to their non-stoichiometry. The effect of annealing in air and in reducing atmosphere was studied by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). In XPS core level spectra taken upon heating in vacuo (620 K) an increase in the full width at half-maximum (FWHM) values of the In 3d doublet and a decrease in the oxygen to indium ratio ( n o/ n In) is found. ESR measurements show the presence of electronic and ‘hole’ centers (F-centers, In 2) in the sol-gel In 2O 3 used for the preparation of the sensing layers. Their concentration is increased markedly by heating in vacuum (620 K) and in H 2 (470 K, H 2 under flow conditions). So the combined ESR and XPS investigation suggests the formation of non-stoichiometric In 2O 3− x upon annealing of stabilized sols and gives evidence for the presence of electronic and hole centers in the sensitive layers.

Dominant role of the surface in photoemission from quasi-one dimensional conductors: K0.3MoO3

Abstract The electronic structure of the quasi-one dimensional oxide conductor K 0.3 MoO 3 has been studied using both X-ray photoelectron spectroscopy (XPS) and angle resolved photoemission (ARP) spectroscopy. We find that the spectra are dominated by surface effects, particularly deviations from stoichiometry. We report changes in the line shape and binding energy of the Mo 3 d core level XPS spectra that are due to variations in core hole screening related to the density of defects in the surface layer. We also discuss the role of surface defects in modifying ARP spectra close to the Fermi level, particularly in the context of recent claims to observe Luttinger liquid behavior in photoemission spectra from K 0.3 MoO 3 .

Core level X-ray photoemission study of amorphous Fe xZr 100− x alloys ( x = 90,91)

Electronic structures of amorphous Fe x Zr 100− x alloys ( x = 90,91) have been investigated by using X-ray photoelectron spectroscopy (XPS) at room temperature. It is found that large energy scale features of the Fe 2p and 3s core level XPS spectra of these alloys are essentially identical to those of Fe metal, suggesting that local electronic structures around Fe atoms in these alloys are similar to those in Fe metal. It is expected that the basic mechanism for the magnetism in a-Fe x Zr 100− x alloys ( x ≈ 90) is same as that in Fe metal. On the other hand, core level shifts and much broader linewidths are observed in the Zr 3d core level XPS spectra of these alloys, as compared to Zr metal. The line shape analysis of the Zr 3d core level XPS spectra reveals that each spin—orbit-split component in a-Fe x Zr 100− x alloys ( x = 90,91) consists of two peaks, causing a broader total linewidth than in Zr metal. Possible origins for such composite structures might be two final states screened by different numbers of localized Zr 4d electrons and the existence of Zr atoms with different local environments.

STUDY OF Ba CORE LEVEL BINDING ENERGIES IN A YBa2Cu3O~7 THIN FILM

A thin film of superconducting YBa 2 Cu 3 O ~7 (YBCO) system (Tc ~ 89 K ) has been studied by x-ray photoelectron spectroscopy (XPS) to investigate the core level electronic structure. The Ba 3d and 4d core level XPS spectra show three binding energy components with the high binding energy component originating from the non-superconducting surface of the system. The role of oxygen ordering/disordering has been discussed to explain the origin of the other two bulk-dependent components. An attempt has been made to resolve some of the discrepancies in the Ba core level spectra reported earlier.

X-Ray Photoelectron Spectroscopy Studies of Photochemical Changes in High-Performance Fibers

The double bonds and aromaticity that impart high thermal stability in fibers often increase the absorption of LTV radiation, resulting in greater susceptibility to photodegradation. This study compares the resistance of three high-performance fibers (Nomex aramid, Kynol novoloid, and PBI) to accelerated light exposure. X-ray photoelectron spectroscopy (XPS) was used to evaluate the photochemical changes on the surfaces of the fibers. On the basis of the C 1 s core-level spectra and oxygen/carbon atomic ratios, the Nomex aramid fibers were photo-oxidized more rapidly than Kynol novoloid and PBI fibers. Similarly, Nomex aramid fibers exhibited more rapid strength losses than the other fibers. We propose that tautomerization inhibited photodegradation in the Kynol novoloid and PBI fibers. XPS core-level and valence-band spectra interpreted by ab initio calculations are shown to be valuable probes of photo-oxidation in fibers.

X-ray photoelectron spectroscopy study of chemically etched Tl 2Ba 2CaCu 2O 8+ x thin film surfaces

X-ray photoelectron spectroscopy (XPS) is used to characterize the surfaces of epitaxial c-axis oriented Tl 2Ba 2CaCu 2 O 8+ x (TBCCO) thin films which have been chemically etched in a solution of Br 2 in absolute ethanol. Comparison of the XPS core level spectra presented here with spectra in the literature obtained from single crystals cleaved in vacuum or polycrystalline pellets fractured or scraped in vacuum shows that the chemically etched thin films exhibit lower levels of nonsuperconducting surface species. Cation disorder is found to be restricted to the Ba sites. XPS valence band spectra show a clear Fermi edge, indicating that the room temperature conductivity near the surface is metallic. To the authors' knowledge, this is the first report of a clear Fermi edge detected on TBCCO. Exposure of the chemically etched films to air results in a significant surface degradation even for exposure times as short as 1 min.

Effect of Precursors on Lithium Infiltrated Silica Gels Studied by Xps

ABSTRACTLithium silicate gels of composition 15 mole % lithia - 85 mole % silica were prepared by soaking highly porous silica gels in aqueous LiNO3 or methanolic LiOH solutions. The fracture surfaces of these samples were studied by X-ray photoelectron spectroscopy (XPS). High resolution XPS core level spectra were collected such that both chemical shift and atomic concentration could be determined. While the average lithium content was slightly higher in LiNO3 gels, only some of the lithium was associated with the silica network. In contrast, most of the lithium in LiOH gels was associated with the network.

Electron-, x-ray-, and ion-stimulated decomposition of nitrate salts

Decomposition of lithium, sodium, and ammonium nitrate stimulated by 500-eV electrons, 1254-eV X-rays, and 4-keV Ar/sup +/ ions is investigated by X-ray photoelectron spectroscopy (XPS). The core level XPS spectra show that a steady-state composition is attained at a total irradiation dose of 10/sup 17/ particles/cm/sup 2/ in which the damaged layer contains predominantly M/sub 2/O and M/sub 2/O/sub 2/ with smaller amounts of MNO/sub 3/ and MNO/sub 2/, where M = Li and Na. Irradiation of NH/sub 4/NO/sub 3/ produces no changes detectable by XPS, suggesting that the decomposition products are all volatile. While the relative atomic concentrations of the constituent elements within the damaged layer change in a similar manner for the three modes of radiation, i.e. there is an increase in alkali metal, decrease in nitrogen, and only minor changes in oxygen concentration, the specific products and relative abundances of these products differ. Consideration of these results in terms of the mechanisms of energy deposition into the atomic lattice indicates that the energy density deposited by all three sources exceeds that required for decomposition and that the excess energy determines the extent of reaction. 31 references, 5 figures, 1 table.

Ion bombardment-induced decomposition of Li and Ba sulfates and carbonates studied by X-ray photoelectron spectroscopy

Radiation damage to the surfaces of lithium and barium sulfates and carbonates under 4 ke V Ar + bombardment has been investigated by X-ray photoelectron Spectroscopy (XPS). Damage is readily observed at a dose of 1 × 10 16 ions cm −2 with saturation occurring over the range 2–8 × 10 17 ions cm −2. Both valence and core level XPS spectra indicate that, at the saturation dose, the basic sulfate and carbonate structures remain along with decomposition products. Both sulfur and carbon are preferentially lost from all four compounds and oxygen is preferentially lost from both Li compounds but not from the Ba compounds as a result of bombardment. The major decomposition products are the metal oxides with smaller quantities of carbides, sulfides, and SO n x− ( n = 3,2,1) species.

Electronic structure of Ce and its intermetallic compounds

We report the core-level x-ray photoelectron spectroscopy spectra of 30 intermetallic compounds of La and Ce. The results are discussed in the light of calculations using the Anderson impurity model of the $4f$ levels and including the degeneracy of the $4f$ levels. The comparison allows us to derive values for the coupling between the $f$ levels and the conduction states $\ensuremath{\Delta}$ and the number of $4f$ electrons ${n}_{f}$. We find $\ensuremath{\Delta}$ to be up to \ensuremath{\sim} 150 meV in some Ce intermetallic compounds, and find ${n}_{f}$ to range from \ensuremath{\sim}0.8-1.1. We cannot reconcile the core-level results with the traditional promotional model in which $\ensuremath{\Delta}$ was assumed to be 10 meV or less and the $f$-electron count could range from 1 down to zero in Ce intermetallic compounds.