Conventional X-ray Photoelectron Spectroscopy Research Articles

Self-limiting chemical vapor deposition of an ultra-thin silicon oxide film using tri-( tert-butoxy)silanol

Tri-( tert-butoxy)silanol (tBOS) rapidly forms a self-limited ∼10-Å-thick silicon oxide film upon exposure to a Si(100)-2×1 surface at 300 K. The majority of hydrocarbon spontaneously desorbs at this temperature. Heating to ∼700 K removes the remaining tert-butoxy groups. The films were characterized by conventional X-ray photoelectron spectroscopy (XPS), synchrotron XPS of the Si 2p core-level and valence band regions, and reflection absorption infrared spectroscopy (RAIRS).

X‐ray photoelectron spectroscopy studies of Ti–Al and Ti–Al–V alloys using Cr Kβ radiation

AbstractHigh‐energy and conventional x‐ray photoelectron spectroscopy (XPS) using Cr Kβ (hν = 5946.7 eV) and Al Kα (hν = 1486.6 eV) radiation, respectively, were employed to study the alloying behaviour of Ti with Al and V in Ti–Al (Al = 10, 20 and 30 at.%), V–50at.%Al, Ti–35at.%Al–15at.%V, Ti–40at.%Al–20at.%V, Ti–25at.%Al–25at.%V and Ti–50at.%V alloys. The differences of the initial and final state Auger parameters of the elements of interest between unalloyed and alloyed conditions were used to interpret possible charge transfer/redistribution between the elements and to explain subsequent electronic changes occurring upon alloying. The spectroscopic studies were related to thermodynamic predictions and the microstructures of the alloys. The large negative shift of the Al Auger parameter indicated that Al atoms are better screened in the pure metallic environment than in the alloy. This shift was attributed also to the ordering tendency in the Ti–Al and Ti–Al–V alloys, shown by thermodynamic modelling and microstructural studies. Variations of the initial state Auger parameter indicated that initial state effects are significant and may indicate charge transfer from the Al sites towards the transition metal sites. The Auger parameter variations of the transition metals were small, which is in agreement with previous studies. Copyright © 2001 John Wiley & Sons, Ltd.

Calculations of charge transfer in Mg-and Al-transition metal alloys using the Auger parameter

Conventional and high-energy x-ray photoelectron spectroscopy (XPS) with Al Kα and Cr Kβ radiation, respectively, were used to calculate the Auger parameters of the elements in a vapour-deposited Mg-8.9 at.% V alloy and in as-cast V-50 at.% Al and Nb-17 at.% Al alloys. Analysis of variations in the Auger parameters of the elements of interest between unalloyed and alloyed conditions were used to calculate values of charge transfer, occurring upon alloying, using the model developed by Thomas and Weightman. These values were used to investigate the electronic changes that take place upon alloying Mg with V and Al with V and Nb. The results were related to the general alloying behaviour of the Mg-V system as well as to thermodynamic predictions and the microstructures of the other two alloys. An attempt has been made to understand the metallurgical significance of charge transfer in terms of the tendency for ordering or disordering of solid solutions. The ordering tendency in the Nb-17 at.% Al alloy, shown by thermodynamic modelling and microstructural studies, was attributed to substantial electron transfer from Al to Nb. The small charge transfer from Al to V in the V-50 at.% Al alloy was attributed to the lack of ordering in this system.

Calculations of charge transfer in Nb–17Al and V–50Al alloys, using the Auger parameter

Conventional and high energy X-ray photoelectron spectroscopy (XPS) with Al K α and Cr K β radiation, respectively, were used to calculate the Auger parameters of the elements in the V–50 at% Al and Nb–17 at% Al alloys. The shifts of the Auger parameters of the elements of interest between unalloyed and alloyed conditions were used to calculate values of charge transfer occurring upon alloying. The results were related to thermodynamic predictions and the microstructures of the two alloys. The ordering tendency in the Nb–17 at% Al alloy, shown by thermodynamic modelling and the microstructural studies, was attributed to substantial electron transfer from Al to Nb. The small charge transfer from Al to V in the V–50 at% Al alloy was attributed to the lack of ordering in this alloy.

Temperature effects on the Si/SiO 2 interface defects and suboxide distribution

Temperature effects on the Si/SiO 2 (0.5–5 nm thick) interface defects and suboxide distribution are studied by electron paramagnetic resonance and conventional X-ray photoelectron spectroscopy. The experiments are made on porous (66% porosity) silicon samples (10 μm thick) thermally oxidized in the 400–1000°C temperature range. Electron paramagnetic resonance measurements (∼9.6 GHz, 25°C) show a decrease of the density of broken bonds called P b1 when the oxidation temperature increases. However no change of the interfacial structure is observed through the suboxide distribution. We interpret our results in terms of viscoelastic relaxation.

Incipient oxidation of fractured pyrite surfaces in air

Abstract Synchrotron excited and conventional X-ray photoelectron spectroscopy has been used to study the initial oxidation of fractured pyrite surfaces in air. The dramatically enhanced surface sensitivity obtained with synchrotron radiation allowed us to follow the reactivity of each of the different sulphur surface sites and to detect intermediate sulphur oxidation species. It has been shown that the surface sulphur components that are directly exposed to atmospheric gases undergo oxidation to sulphate, whereas bulk sulphur mainly forms `electron deficient' disulphides. Fe 2p spectra were used to investigate the oxidation of Fe(II) and Fe(III) surface sites. The intensity of Fe(III)–S sites does not change significantly within a 14 h oxidation time. The number of Fe(II) surface sites diminishes and FeOOH is formed. The results have been used to propose an oxidation mechanism including the oxidation of different iron as well as sulphur sites.

An XPS Study of Sphalerite Activation by Copper

X-ray photoelectron spectroscopy (XPS) was used to study the kinetics and mechanisms of sphalerite activation in a 10-4 M CuSO4 solution at pH 9.2. The activation was fast during the first 10 min, after which the rate decreased exponentially. The increase in the amount of copper ion uptake was accompanied by the displacement of zinc ions from the sphalerite surface, indicating that the mechanism of copper uptake is one of displacement rather than adsorption. The XPS data show that the kinetics of activation increased considerably in a deoxygenated CuSO4 solution. Both the conventional XPS and synchrotron radiation XPS (SR-XPS) analyses show that copper activation involves a mechanism in which Cu2+ ions are reduced to the Cu(I) state, while the sulfur in ZnS is oxidized. Activation in the absence of oxygen and at an open circuit results in the formation of a CuS-like product, while in an air-saturated solution copper polysulfides are formed. The latter mechanisms are supported by the appearance of two distinct S(2p) doublets representing S- and S0 species at higher binding energies. When copper-activated sphalerite was conditioned in an air-saturated solution of pH 9.2, the CuS-like activation product is oxidized in preference to unactivated ZnS. The oxidation resulted in a loss of copper from the sphalerite surface, which may be detrimental to flotation.

The initial products of the anodic oxidation of galena in acidic solution and the influence of mineral stoichiometry

The oxidation products retained at the surface of galena samples from Broken Hill (lead-rich) and Missouri (sulphur-rich) following mild anodic treatment in acetate solution of pH 4.6 have been determined by X-ray photoelectron spectroscopy (XPS) using excitation both by Al K α X-rays and by synchrotron radiation (SR). Elemental sulphur was found to be the major oxidation product on both galenas. For short anodization times, S 0 could only be detected by the more surface sensitive SR-XPS. Electrochemical studies showed that the quantity of sulphur on a Missouri galena surface did not decrease significantly when the galena was held for 40 min at open circuit. This contrasts with previous studies on Broken Hill galena in which the sulphur coverage was found to decrease to a low value in this time period, a behaviour that was assigned to diffusion of lead atoms from the bulk. The different behaviour of the two galenas can be explained by the different stoichiometry of the Broken Hill and Missouri minerals. According to SR-XPS measurements, the intensity of the S 0 component in S 2p spectra for galena surfaces held for 40 min at open circuit following anodization at 0.3 V for 20 s was less than for the equivalent spectra carried out within 7 min of anodization. The S 0 intensity was negligible for the Broken Hill mineral in agreement with electrochemical studies. A small but significant decrease in the S 2p intensity was also observed for Missouri galena and this was assigned to accumulation of the sulphur product into clusters. S 2p components at the binding energies expected for lead polysulphide were also observed in the spectra for the Missouri mineral. Conventional XPS revealed a significant variability in the S 0 2p intensity following short anodization times and this could also arise from sulphur clustering.

XPS Study of a New Heterogeneous Ruthenium Catalyst Precursor

The catalyst precursor has been synthesized by modifying the silica surface with ClPPh2 followed by a surface reaction with RuCl3. Supports consisting of a thin layer of SiO2 on a silicon single crystal have been used. Characterization of the different stages in the reaction procedure has been performed by a combination of conventional and angle-resolved X-ray photoelectron spectroscopy (XPS) using RuCl3/P(C6H5)2/SiO2/Si(100) model precursors. Different types of ruthenium centra have been identified.

XPS and SR-XPS techniques applied to sulphide mineral surfaces

X-ray photoelectron spectroscopy (XPS) is one of the major techniques in basic research of flotation-related surface studies of sulphide minerals. The advantages of the method, in general, are good surface sensitivity, rather straightforward elemental and chemical state analysis and reliable quantification of the data. The most serious disadvantage is the ex-situ nature of the technique. Surface sensitivity of the XPS technique can be considerably improved if an optimal excitation energy is chosen for a characteristic emission of each different element. This can be done by using a synchrotron radiation (SR) source, providing a continuous energy distribution over large energy region, instead of a fixed excitation energy source (either AlKα or MgKα radiation from sealed off X-ray tube) used in conventional XPS. In the case of identification of sulphur species, which often is the main task in surface analysis of sulphide surfaces, this leads to approxinnately an order of magnitude improvement in surface sensitivity and, hence, essentially better possibility to detect surface species of submonolayer coverage. Air oxidation of PbS and FeS2 has been studied with SR excitation. On PbS cleavage surface, oxidized sulphur (sulphate and/or polysulphide type) and lead species were detected after 10 min exposure time in air at room temperature, indicating congruent oxidation of PbS surface. The oxidation reactions proceeded relatively quickly during the first 4 h of contact with air. SR-XPS experiments with cleaved FeS2 also show the formation of sulphate as a primary oxidation product in air. However, the initial layer, formed immediately after cleavage, seemed to passivate the surface against further oxidation.

Determination of the bonding of alkyl monolayers to the Si(111) surface using chemical-shift, scanned-energy photoelectron diffraction

The bonding of alkyl monolayers to Si(111) surfaces has been studied by conventional x-ray photoelectron spectroscopy (XPS) and chemical-shift, scanned-energy photoelectron diffraction (PED) using synchrotron radiation. Two very different wet-chemical methods have been used to prepare the alkyl monolayers: (i) olefin insertion into the H–Si bond on the H–Si(111) surface, and (ii) replacement of Cl on the Cl–Si(111) surface by an alkyl group from an alkyllithium reagent. In both cases, XPS has revealed a C 1s signal chemically shifted to lower binding energy, which we have assigned to carbon bonded to silicon. PED has shown that both preparative methods result in carbon bonded in an atop site with the expected C–Si bond length of 1.85±0.05 Å. Chemical-shift, scanned-energy photoelectron diffraction is a particularly valuable probe of local structure at surfaces that contain the same element in multiple, chemically distinct environments.

X-ray diffraction and x-ray photoelectron spectroscopy study of partially strained SiGe layers produced via excimer laser processing

Structural properties of graded Si(1−x)Gex layers obtained on Si(100) by pulsed laser induced epitaxy were investigated by means of conventional powder x-ray diffraction and x-ray photoelectron spectroscopy. The Si(1−x)Gex epitaxial layers were formed by pulsed KrF-laser driven rapid melting and crystallization of thin amorphous Ge layers deposited onto the Si(100). The experimental results showed that, by increasing the number of laser pulses, good quality and partially strained epitaxial layers could be attained. A Monte Carlo data evaluation algorithm is proposed, which is capable to determine, by the simultaneous fit of data obtained by x-ray diffraction and x-ray photoelectron spectroscopy, the strain level as a function of Ge concentration.

Olefin metathesis catalyst.

Abstract Model supports consisting of a thin layer of SiO2 on a Silicon single crystal have been used to study the [W]n+/SiO2/Si (100) model catalyst precursor prepared by a controlled reaction of π-C5H5W(CO)3Cl with the SiO2 surface. Characterization of the tungsten surface species has been performed by combination of conventional, angle-resolved and depth-profiling X-ray photoelectron spectroscopy (XPS). The silica surface consists of highly dispersed W-oxide units linked by two Si–O–W bonds. Further, compared with the powder analogues, a drastic increase in spectral resolution and detailed band structure is observed in the XPS spectra.

X-ray photoelectron diffraction investigation of Ge segregation and film morphology during first stage heteroepitaxy of Si on Ge(001)

X-ray photoelectron diffraction (XPD) and its chemical selectivity have been used in conjunction with surface atom titration by ultra-violet photoelectron spectroscopy and conventional X-ray photoelectron spectroscopy to investigate the interplay between Ge segregation and growth morphology in the earliest stages (0–12 monolayers) of Si solid-source-epitaxy on Ge(001)-2 × 1 surfaces held at room temperature (RT) and at 400°C. A detailed examination, as a function of the Si coverage, of the forward scattering peak contrasts in the [111] and [001] directions of the Si 2p and Ge 3d polar angle distributions provides a diagnostic of whether or not a particular atom is in the top layer (segregation) and yields information about the stacking sequences (morphology). These investigations allowed us to conclude in favour of an initial Si bilayer patch-growth-mode exempt of segregation at RT and of a floating Ge surface at 400°C, the Si agglomeration staying rather mild in the first monolayers. Accessorily, the XPD-specific problems of electron defocusing effects through long atomic chains and strain-induced lattice expansion or contraction have also been addressed.

Application of high-energy synchrotron-radiation XPS to determine the thickness of SiO2 thin films on Si(100)

The escape depth of photoelectrons depends on their kinetic energy. We apply this relationship to measure film thicknesses from X-ray photoelectron spectroscopy (XPS) measurements with tunable-energy synchrotron radiation (SR). For this purpose, a “high-energy SR-XPS” instrument has been constructed and used to characterize thermally oxidized thin films on Si(100) single crystals. In order to observe photoelectrons emitted from deeper regions than with conventional XPS, Si 1s photoelectrons with an energy up to 4000 eV were measured with X-ray energies up to 5800 eV. The oxide thickness was estimated from measurements of the relative Si intensities from the oxide and the substrate at various photon energies. Our results suggest that the SR-XPS system is useful for measuring the thickness of thin films.

Identifying chemisorption in the interaction of thiol collectors with sulfide minerals by XPS: adsorption of xanthate on silver and silver sulfide

The adsorption of ethyl xanthate on silver and sulfidised silver surfaces at controlled potentials has been investigated by X-ray photoelectron spectroscopy (XPS). Silver/xanthate and silver sulfide/xanthate were chosen as model systems for investigating the identification of chemisorption of thiol collectors on sulfide minerals by means of core electron energy shifts. There was no discernible effect from submonolayer adsorption on either substrate on the corresponding Ag(MNN) spectrum, even when the presence of adsorbed xanthate was clearly evident from the S 2p spectrum. It was confirmed that the Ag(MNN) electron energies for multilayer silver xanthate were significantly different from those for silver and silver sulfide. The basis for the general proposition, that chemisorption of thiol collectors on sulfide mineral surfaces might not necessarily give rise to discernible substrate core electron energy shifts, was examined in the context of previously reported investigations of chemisorption of adsorbates from the vapour phase. It was concluded from this appraisal, and from the experimental results for the interaction of xanthate with silver and silver sulfide, that substrate core electron energy shifts on chemisorption are not always large enough to be detectable by conventional XPS. By contrast, core electron energy shifts for multilayer metal-collector compound formation are clearly discernible.

Thermal desorption spectroscopic analysis for residual chlorine on Al–Si–Cu after Cl2 electron cyclotron resonance plasma etching

Thermal desorption spectroscopy is used to measure the amount of residual chlorine on Al–Si–Cu. The Al–Si–Cu surface is etched by Cl2 electron cyclotron resonance (ECR) plasma at substrate temperatures of 30 and −60 °C. The surfaces are then cleaned by one of the following methods: de-ionized water rinsing, H2 ECR plasma irradiation, or annealing in H2/N2 mixed gas. AlCl molecules, which may be responsible for aftercorrosion, are detected. Chlorine compounds, however, cannot be detected by conventional x-ray photoelectron spectroscopy or Auger electron spectroscopy. The low substrate temperature etching reduces the amount of residual chlorine. Moreover, the cleaning ability of the H2 ECR plasma treatment is no less than that of the de-ionized water rinse, and is superior to that of annealing in H2/N2 mixed gas.

X-ray photoelectron spectroscopy study of submonolayer native oxides on HF-treated Si surfaces

This article investigates, by a novel spectral analysis technique, the possibility of applying conventional x-ray photoelectron spectroscopy to characterize the initial oxidation process. Quantitative analysis of submonolayer native oxides is made possible by spectral decomposition of Si 2p into Si4+, Six+, and Si0+ based on the O 1s binding energy, and by calculation of the SiO2 and SiOx thicknesses using the decomposition results. Here, the sensitivity for oxide-thickness change is about 1/10 monolayer. Using this technique, the initial oxidation processes of HF-treated Si(111) and Si(100) are precisely characterized. The influence of the dissolved-oxygen concentration in the HF solution is also investigated.

Angle-resolved and depth profiling XPS investigation of a monolayer niobium oxide catalyst

Supports consisting of a thin layer of SiO 2 on a silicon single crystal have been used to study the NbO x /SiO 2/Si(100) model catalyst, obtained by a controlled surface reaction between Nb(OC 2H 5) 5 and silanol groups on the SiO 2 support. Characterization of the niobic acid monolayer has been performed by a combination of conventional, angle-resolved and depth profiling X-ray photoelectron spectroscopy (XPS). Compared to the powder analogues, a drastic increase in spectral resolution and detailed band structure is observed in the XPS spectra. The number of attached Nb atoms on the silicagel (specific surface, S BET = 345 m 2 g −1; pore diameter, 20 nm) has been determined as 6.86 × 10 4 mol g −1 with a surface density of 1.25 atoms nm −2.

Applicability of the Tougaard ratio D in the analysis of nanometric TiO 2 overlayers

Recent investigations have shown that X-ray photoelectron spectroscopy (XPS) spectra of metallic specimens may contain information about the elemental depth distributions in the surface region. The increase in the spectral background on the low kinetic energy side of a photoelectron peak is due to inelastically collided photoelectrons, and is thus strongly dependent on the depth of origin of the photoelectrons. The aim of this investigation was to evaluate the applicability of the simple Tougaard peak-to-background ratio D in the analysis of nanometric TiO 2 and carbon overlayers. Prior to the overlayer evaluations, the ratio D 0 of the Ti 2p emission in a homogeneous TiO 2 matrix was determined from an in situ scraped single crystal TiO 2 specimen. The experimental result obtained for D 0 in the TiO 2 matrix was 13 ± 0.5 eV, i.e. lower than reported the “universal ratio” D 0 of 23 eV. For the overlayer thickness evaluation, the mean path lengths R of the substrate photoelectrons were calculated from the peak-to-background ratios of the substrate photoelectron peaks for test samples with nanometric TiO 2 and carbon overlayers on molybdenum substrates. The results were compared with results from conventional XPS layer calculations, XPS sputter profiling and X-ray microanalysis. The results from the different methods were found to agree within 25%.