Mixed adlayer of alkanethiol and peptide on GaAs(100): quantitative characterization by X-ray photoelectron spectroscopy

Homogeneous and mixed adlayers composed of an alkanethiol (1-octadecanethiol, ODT) and a peptide (CGISYGRKKRRQRRR) on GaAs(100) were formed in two different solvent systems: phosphate-buffered saline (PBS) and N,N-dimethylformamide (DMF). The chemical composition of each adlayer was characterized by X-ray photoelectron spectroscopy (XPS). The data showed that the makeup of the adlayer and its stability largely depends on the solvent used. Angle-resolved XPS also revealed that the adlayer thickness and tilt angles were different from values obtained from ellipsometry measurements and vastly varied between the two solvents used. The coverage data extracted from the XPS measurements indicated that homogeneous adlayers of peptide in PBS buffer form a multilayered film. Homogeneous alkanethiol adlayers exhibited monolayer coverage under all solvent treatments. Coadsorbed layers containing both alkanethiol and peptide have fractional monolayer coverage in both solvents.     

Secondary ion mass spectrometry and X-ray photoelectron spectroscopy investigation on chemical vapor deposited CeO(2-)ZrO(2)-TiO(2) thin films

Mixed CeO(2)-ZrO(2) systems have attracted widespread interest for their use in three-way catalyst (TWC) technology for automotive exhaust conversion to non-toxic products. In this work, CeO(2)-ZrO(2) thin films were deposited, via chemical vapor deposition, in order to obtain nanoscale materials with a high surface-to-volume ratio, with precise control of system properties. The addition of TiO(2) as buffer layer was also investigated. Cordierite was chosen as substrate, being the usual refractory material for catalytic mufflers. The multilayers were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). In particular, the combination of SIMS and XPS allowed us to investigate both surface and in-depth chemical composition, studying also film-intermixing phenomena induced by annealing processes.     

High-resolution X-ray photoelectron spectroscopy of chlorine-terminated GaAs(111)A surfaces

Oxide-terminated and Cl-terminated GaAs(111)A surfaces have been characterized in the As and Ga 3d regions by high-resolution, soft X-ray photoelectron spectroscopy. The Cl-terminated surface, formed by treatment with 6 M HCl(aq), showed no detectable As oxides or As(0) in the As 3d region. The Ga 3d spectrum of the Cl-terminated surface showed a broad, intense signal at 19.4 eV and a smaller signal at 21.7 eV. The Ga 3d peaks were fitted using three species, one representing bulk GaAs and the others representing two chemical species on the surface. The large peak was well-fitted by the bulk GaAs emission and by a second doublet, assigned to surface Ga atoms bonded to Cl, that was shifted by 0.34 eV from the bulk GaAs 3d emission. The smaller peak, shifted by 2.3 eV in binding energy relative to the bulk GaAs Ga 3d signal, is assigned to Ga(OH)3. The data confirm that wet chemical etching allows for the formation of well-defined, Cl-terminated GaAs(111)A surfaces free of detectable elemental As, that can provide a starting point for further functionalization of GaAs.     

Molecular beam photoelectron spectroscopy of Ni(CO)4

The nickel carbonyl HeI (21.2175 eV) photoelectron spectrum has been reinvestigated with improved resolution and molecular beam sampling. The 9T₂ and 2E photoelectron bands are shown to be intrinsically diffuse, which is attributed to D_2d geometries. The ordering of the next seven outer valence electronic states is proposed from a linear least-squares fit of the spectrum.     

A X-ray photoelectron spectroscopy study of HDTMAB distribution within organoclays

X-ray photoelectron spectroscopy (XPS) in combination with X-ray diffraction (XRD) and high-resolution thermogravimetric analysis (HRTG) has been used to investigate the surfactant distribution within the organoclays prepared at different surfactant concentrations. This study demonstrates that the surfactant distribution within the organoclays depends strongly on the surfactant loadings. In the organoclays prepared at relative low surfactant concentrations, the surfactant cations mainly locate in the clay interlayer, whereas the surfactants occupy both the clay interlayer space and the interparticle pores in the organoclays prepared at high surfactant concentrations. This is in accordance with the dramatic pore volume decrease of organoclays compared to those of starting clays. XPS survey scans show that, at low surfactant concentration (<1.0 CEC), the ion exchange between Na⁺ and HDTMA⁺ is dominant, whereas both cations and ion pairs occur in the organoclays prepared at high concentrations (>1.0 CEC). High-resolution XPS spectra show that the modification of clay with surfactants has prominent influences on the binding energies of the atoms in both clays and surfactants, and nitrogen is the most sensitive to the surfactant distribution within the organoclays.     

X-ray photoelectron spectroscopy of some Co(II) dithiocarbamates

The X-ray photoelectron spectra of a series of Co(II) dithiocarbamate complexes have been studied. Data confirmed the substantial identity of the electronic structure of the chromophores throughout the series, and Co2P_3/2 BE values of 780.1 eV are in good agreement with the expectation on the base of Feltham-Brant scale. Shake-up satellites associated with the Co2P_3/2 photoelectron line have been investigated and compared with magnetic moments.     

Platinum colloid supported on graphite: X-ray photoelectron spectroscopy study

Two states of platinum, Pt⁰ and Pt^+δ, with the binding energy of the Pt(4f_7/2) line at 71.4±0.1 and 72.2±0.3 eV, are revealed by XPS in colloidal platinum deposited onto graphite. The latter state is assigned to surface Pt atoms partially oxidized due to the interaction with water and/or oxygen-terminated sulfonate group in an organic shell. The colloid deposition is found to be temperature dependent with the apparent activation energy of ∼40 kJ/mol.     

D-alaninol adsorption on Cu(100): photoelectron spectroscopy and first-principles calculations

The adsorption of a single molecule of the D-enantiomer of alaninol (2-amino-1-propanol) on the surface of Cu(100) is investigated through density functional theory calculations. Different possible adsorption sites for D-alaninol are tested, and it is found that the most stable configuration presents both amino and hydroxyl group covalently interacting with "on top" copper atoms. The electronic structure is analyzed in detail and compared with experimental photoelectron spectra. Another adsorption structure in which a dehydrogenation process is assumed to occur on the amino group is analyzed and provides a possible explanation of the valence band electronic structure and of the experimentally observed N 1s core-level shift at full coverage, where a self-assembled ordered chiral monolayer is formed on the copper surface.     

Bonding structure analysis of carbon nitride films by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS)

Carbon nitride films were prepared by an opposed-target DC reactive sputtering system and the bonding behaviors were investigated according to the nitrogen content and annealing temperature. Annealing leads to a loss of nitrogen from the films and the CN is totally removed at 600 °C. Due to annealing at 600 °C, the C---C out-of-plane vibrational band intensity at 700 cm⁻¹ becomes very low and also the CN band at 2200 cm⁻¹ disappears completely. The sp² bonding in a CN compound is the most stable phase. Due to carbon's atypical nature in having its p orbital more compact and tightly bound compared to s states, the C=N sp² phase is more stable than the C---C sp³ phase. As the C=N sp² phase dominates the structure, the film is mainly graphite-like with some proportion of C---C, CN and N=N bonds.     

Characterization of lithium alkyl carbonates by X-ray photoelectron spectroscopy: experimental and theoretical study

Lithium alkyl carbonates ROCO2Li result from the reductive decomposition of dialkyl carbonates, which are the organic solvents used in the electrolytes of common lithium-ion batteries. They play a crucial role in the formation of surface layers at the electrode/electrolyte interfaces. In this work, we report on the X-ray photoelectron spectroscopy (XPS) characterization of synthesized lithium methyl and ethyl carbonates. Using Hartree-Fock ab initio calculations, we interpret and simulate the valence spectra of both samples, as well as several other Li alkyl carbonates involved in Li-ion batteries. We show that Li alkyl carbonates can be identified at the electrode's surface by a combined analysis of XPS core peaks and valence spectra.     

A study of the 1s core region of atomic magnesium by X-ray photoelectron spectroscopy

The 1s core region of atomic magnesium has been measured by X-ray photoelectron spectroscopy using AlKα (1486.6 eV) radiation in order to determine the energy and intensity of the main satellite corresponding to 3_s → 4_s excitation accompanying core ionization. It was found to lie at 11.56 eV higher bonding energy than the main 1s?1 peak, with a relative intensity of 17%. The accent ab initio Cl calculation of Kosugi and Kuroda gives a separation which is in good agreement with experiment but somewhat overestimates the relative intensity.     

Site-sensitive X-ray photoelectron spectroscopy of Fe3O4 by photoelectron diffraction

Fe 2p core-level photoelectron spectra of magnetite were measured using soft X-ray and hard X-ray, and its emission angle dependence was investigated. The photoelectron diffraction pattern from different atomic sites differs because the atomic arrangement surrounding each site is different. By selecting the forward-focusing-peak (FFP) directions characteristic to each atomic site and measuring the kinetic energy dependence of the FFP intensities at the Fe 2p core-level range, we succeed in detecting the variation of the peak intensity of Fe 2p core-level spectra at different emission directions. This result, consistent with recent results, suggests that the lower-binding-energy peak of the Fe 2p core-level spectrum may be assigned as the B site component.     

Au-Ni and Au-Fe heterometallic systems: an X-ray photoelectron spectroscopy study

Results of X-ray photoelectron spectroscopy (XPS) studies of Au-Ni and Au-Fe bimetallic nanoparticles (in the form of black and supported on SiO₂) are considered and systematized. The nanoparticles studied were prepared by metal-vapor synthesis. XPS studies on the chemical compositions, structures, and electronic states of heterometallic nanocomposites carried out both in the traditional and differential charging mode with a bias voltage applied to the sample holder are analyzed. The effect of the composition and electronic state of the Au-Ni heterometallic system on its catalytic activity is studied. It was shown that the variable differential charging mode allows one to identify and characterize species of different nature in multi-component heterometallic nanostructured materials. A state which correlates with the catalytic activity of the Au-Ni/SiO₂ system was identified in the Au 4f photoelectron spectra.     

Chemisorption of carbon monoxide on the platinum(100) surface: A definition of electronic states by photoelectron spectroscopy

UPS studies are reported for carbon monoxide chemisorbed, at varying coverages, on a clean Pt(100) surface. A new assignment of surface states is obvious from the broadening and resolution of the emission peak, positioned at ca. 8 eV below the Fermi level, following pre-adsorption of propene and pyridine. The valence band density of states, determined by He(I) and He(II) emission spectroscopy, is compared with that reported earlier by X-ray electron emission spectroscopy.     

Identification of localised and delocalised adsorbate orbitals on W(100) by photoelectron spectroscopy

The comparison of the photoelectron spectra of chemisorbed layers taken with glancing photon incidence and those with normal photon incidence permits a qualitative determination of the degree of delocalisation of the various orbitals at the surface. For glancing photon incidence, both localised and delocalised orbitals can be photoionised. However, for normally incident photons, those orbitals localised around particular atomic sites have a much higher photoionisation cross section than those orbitals extensively delocalised in the plane of the surface. Examples of this behaviour in the spectra of hydrogen and carbon monoxide on W(100) are presented.     

A study of the SiO<Subscript>2</Subscript>-aqueous electrolyte (NaCl,CsCl) interface by X-ray photoelectron spectroscopy

The surface of amorphous silicon dioxide in NaCl and CsCl solutions with various values of ionic strength and pH is studied by X-ray photoelectron spectroscopy. Samples are prepared through the fast freezing of wet pastes prepared by the centrifugation of suspensions. It is shown that the X-ray photoelectron spectra of such samples provide experimental information on the composition of the SiO₂-solution interface, the number of electrolyte ions near the solid phase surface, and its chemical modification. The sign of the charge of silicon dioxide particles can be judged from the dependence of a measured Na(Cs)/Cl atomic ratio; the controlled removal of water from the samples in the chamber of an electronic spectrometer makes it possible to directly estimate the surface potential and study the energy effects of counterion hydration using the shifts in the binding energies of the corresponding photoelectron levels. An analysis of the binding energies of Cs 3d^5/2 and Cl 2p_3/2 lines in the X-ray photoelectron spectra for the SiO₂-CsCl solution interface yields additional information on the structure of the formed electrical double layer, thus making it possible to distinguish between the ions adsorbed at the surface and the counterions compensating for its charge.     

A flotation related X-ray photoelectron spectroscopy study of the oxidation of galena surface

The initial steps of the oxidation of galena (PbS) surface with special attention to the oxidation in an aqueous environment were investigated by the X-ray photoelectron spectroscopy method (XPS). Galena monocrystals from two different sources were cleaved and exposed to either air or an aqueous environment. The results show that the presence of organic contamination at the galena surface may exert a dramatic effect on the course of its surface oxidation. In the case of aerial oxidation, the presence of impurities changes the reaction path, suppressing the oxidation of sulfide sulfur to sulfate like species selectively. In the case of aqueous oxidation, the presence of organic contamination at the surface (this contaminants are supposed to be mainly the carboxylic acids and their salts) inhibits the oxidation. Very little oxidation products were found on the galena surface contacted with aqueous solutions at the spontaneously established potential, in accordance with the congruent dissolution-oxidation mechanism.     

Quantitative characterization of DNA films by X-ray photoelectron spectroscopy

We describe the use of self-assembled films of thiolated (dT)25 single-stranded DNA (ssDNA) on gold as a model system for quantitative characterization of DNA films by X-ray photoelectron spectroscopy (XPS). We evaluate the applicability of a uniform and homogeneous overlayer-substrate model for data analysis, examine model parameters used to describe DNA films (e.g., density and electron attenuation length), and validate the results. The model is used to obtain quantitative composition and coverage information as a function of immobilization time. We find that when the electron attenuation effects are properly included in the XPS data analysis, excellent agreement is obtained with Fourier transform infrared (FTIR) measurements for relative values of the DNA coverage, and the calculated absolute coverage is consistent with a previous radiolabeling study. Based on the effectiveness of the analysis procedure for model (dT)25 ssDNA films, it should be generally valid for direct quantitative comparison of DNA films prepared under widely varying conditions.     

Characterization of sputter-deposited YBaCuO films by X-ray photoelectron spectroscopy

Summary Thin films of YBaCuO have been deposited onto SrTiO₃ (100) single crystals by different sputter techniques. At substrate temperatures around 700°C critical current densities as high as 5.9×10⁶ A/cm² at 77 K could be achieved. X-ray photoelectron spectroscopy (XPS) has been used to characterize the surface composition and the surface homogeneity of the films. The existence of the superconducting perovskite structure beside other surface species can be detected for example by observing the Ba 4d_5/2 emission at its characteristic binding energy of 87.3 eV. Using XPS as a quantitative analytical technique it has been shown that it is possible to detect metallic contamination (Pt, Cr) in the films at a detection limit of 0.1 atomic percent of the cations. The occurrence of the metallic contamination has been traced back to formation of volatile metal-oxides of the substrate heater wires during the deposition process.