Ambient contamination of poly-crystalline diamond surfaces studied by high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy

In this work we provide direct evidence of hydrogen, carbon and oxygen contamination of poly-crystalline diamond surfaces from ambient conditions and their thermal stability upon vacuum annealing. Deuterated diamond films were exposed to ambient conditions for ~ 3 months and then studied by high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy. Hydrocarbon contaminations posses at least two different binding states which desorb upon annealing to ~ 300 °C and ~ 600 °C. Oxygen contaminations gradually desorb upon annealing to 700–800 °C. It is shown that thermal desorption of contaminations creates sp² carbon atoms on the diamond film surface.     

Volumetric distribution of Pt nanoparticles supported on mesoporous carbon substrates studied by X-ray photoelectron spectroscopy depth profiling

We present an X-ray photoelectron spectroscopy depth profiling study of the size and volumetric distributions of Pt nanoparticles supported on two mesoporous carbons. Based on the analysis of the chemical shift of the Pt4f peaks, we found the one with a mesopore diameter of ∼5 nm has a uniform distribution of crystalline Pt nanoparticles throughout the porous body. In contrast, the other with a mesopore diameter of ∼3 nm has a distinct distribution of crystalline Pt nanoparticles on the outermost surfaces of the porous material and extremely small (<1 nm) Pt nanoparticles in the interior region. The small Pt particle size in the interior represents the effect of pore entrance blockage of those mesopore channels with diameter less than 3 nm. Such blockage gives rise to the encapsulation of very small amounts of Pt precursors inside the nanospace of the porous matrix, with the result that the particles grown in the interior region of the support are less than 1 nm in size.     

Heavy metal-adsorption on micas and clay minerals studied by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to study the adsorption of Cs-, Ba-, Cu-, Zn-, and Pb-ions on the external surfaces of various, well characterized 2:1 layer silicates (micas and illites).Before studying metal adsorption, it was necessary to determine the charge magnitude of the adsorption surface. This was done for chemically well-characterized micas (margarite, muscovite, sericite). The XPS analyses showed that the depth of analysis is about 15 Å. As a result it was possible to measure the surface- and interlayer ions on both sides of the outermost 2:1 layer. In determining the layer charges, the following strategy was used. The outer surface cations were replaced by Ba²⁺, giving, for ideal margarite an interlayer cation (Ca²⁺)/surface cation (Ba²⁺) ratio of 2:1 and in the case of muscovite a K⁺/Ba²⁺ ratio of 4:1. Deviations from these ratios indicate an asymmetry of layer charge in the outer sheet. Using the margarite, muscovite and sericite as standards, surface charge determination of a number of micas, illites, and I/S clays could be carried out by XPS.The properties of the metal-ions (charge, ionic radius, ionic potential), as well as layer charge characteristics of the clay, including surface charge magnitude and point of origin from tetrahedral or octahedral substitution, are factors which influence adsorption selectivity [Sposito, G., 1989. Surface reactions in natural aqueous colloidal solutions, G. Chimia, 43, 169–176]. The selection of previously well-characterized minerals, margarite, muscovite, celadonite, illite, montmorillonite, and beidellite for XPS study made it possible to relate these factors to heavy metal adsorption by the clay minerals.The results show that Cu²⁺ and Zn²⁺ are adsorbed as monovalent ions, presumably as (CuOH)¹⁺ and (ZnOH)¹⁺ hydroxy surface-complexes, due to their high ionic potential. Saturating the mica series with equimolar pairs of Cu–Zn and Cu–Pb, the ratios of Cu/Zn and Cu/Pb increase systematically with external surface charge. The higher the surface charge, the more selective is the exchange process for Cu with respect to Zn or Pb. Increasing external surface charge parallels increasing tetrahedral charge, which indicates that selectivity takes place at points of tetrahedral negativity on the crystallite surface, whereas octrahedral charge plays little role in the selective adsorption process.     

Investigation of nucleation centres in diamond chemical vapour deposition with spatially resolved X-ray photoelectron spectroscopy

The nucleation of diamond films on silicon substrates deposited by hot filament chemical vapour deposition has been investigated for the first time with an X-ray photoelectron spectroscopy microscope. The distribution of carbon, oxygen, silicon and tungsten on the surface was imaged with a spatial resolution of 10 μm. Significant differences were found between untreated areas of the substrates and scratches created by a diamond tip. Our data reveal that material from the tip abraded in the scratching process is not of key importance for nucleation.     

Application of X-ray photoelectron spectroscopy to surface analysis of carbon fiber

Various applications of X-ray photoelectron spectroscopy (XPS) for carbon fiber are described. Carbonization process on surface of fiber is tracked by XPS as variation of elemental composition and also of chemical species revealed by chemical shift in XPS spectra as shown in Fig. 2. Surface oxidation and also high temperature treatment of carbon fiber are also examined as shown in Table 3. Chemically heterogeneous structure of carbon fiber is studied by XPS depth profiling by utilising an Ar⁺ etching gun as shown in Fig. 7.     

Investigation of gold nanoparticles immobilized on the surface of pyrite by scanning probe microscopy,scanning tunneling spectroscopy,and X-ray photoelectron spectroscopy

The characteristics of gold spontaneously deposited on the surface of pyrite from an HAuCl₄ solution at room temperature are investigated by scanning probe microscopy, scanning tunneling spectroscopy, atomic-force microscopy, and X-ray photoelectron spectroscopy with synchrotron excitation. Within minutes after the onset of the deposition, gold is deposited in the form of metallic particles with a diameter ranging from 8 to 15 nm, which, in turn, subsequently form agglomerates with sizes up to several hundred nanometers. It is revealed that the Au 4f _7/2 lines in the X-ray photoelectron spectra of the gold samples are shifted as compared to those for bulk gold and that tunneling in scanning tunneling spectra is suppressed. The effects caused, apparently, by the Coulomb blockage are unusually pronounced for such relatively large particles and decrease rather slowly upon the aggregation of particles.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

X-ray photoelectron spectroscopy of aluminium-substituted tobermorite

We have synthesised 11-Å tobermorite hydrothermally, both pure and with increasing isomorphic substitution of aluminium for silicon. The samples were analysed by X-ray photoelectron spectroscopy (XPS). Aluminium was found, on the basis of its Al 2p binding energies, to be tetrahedrally coordinated. We observed no changes in Ca/(Si+Al) ratio upon aluminium substitution, implying that charge balancing does not occur via the incorporation of additional calcium into the tobermorite structure. Aluminium substitution into the silicate structure led to a decrease in Si 2p binding energies. This implies one of two alternatives. Firstly, that charge balancing occurs via substitution of OH⁻ for O²⁻ in the tobermorite structure. Secondly, the presence of aluminium in the tobermorite structure may negatively influence the degree of silicate polymerisation. Further work is required to determine which of these possibilities is the case.     

X-ray photoelectron spectroscopy study of Schottky junctions based on oxygen-/fluorine-terminated (100) diamond

In this study, investigation of Schottky junctions based on oxygen-/fluorine-terminated (100) diamond (O-/F-diamond) film has been carried out. Both of the O-/F-diamond surfaces have been formed on different areas of one (100) diamond sample by O₂ and CF₄ plasma. Metals of Au, Pd, and Cu have been evaporated on the diamond surfaces to form Schottky junctions, whose barrier heights on O-/F-diamond have been investigated by X-ray photoelectron spectroscopy technique, the results of which indicate that the barrier heights of the metals on O-diamond are about 1.70 eV, and those on F-diamond are about 2.30 eV, respectively.     

Diamond surface conductivity experiments and photoelectron spectroscopy

A unique feature of diamond surfaces is a highly conductive p-type layer which is usually observed when the surfaces are hydrogen terminated. We present a combination of conductivity and photoelectron yield measurements on a variety of different diamond samples in order to elucidate the role of hydrogen and adsorbates for this phenomenon. The experiments show that hydrogen termination is a necessary but not a sufficient condition for the appearance of the surface conductivity. Additionally, adsorbates from the atmosphere are needed. On the basis of the experiments an electrochemical model is developed which can explain the effect of the hydrogen termination and also shows why hydrogen terminated diamond is the only semiconductor with p-type surface conductivity.     

Thermal degradation of PTFE studied by ultraviolet photoelectron spectroscopy (u.p.s.)

The early stages of the thermal degradation of PTFE were followed by ultraviolet photoelectron spectroscopy in order to evaluate the technique and to establish the effects of changes in molecular weight and sample thickness, also γ-irradiation and copolymerization on the degradation reaction. The rate of decomposition and the concentration—time profile for the evolution of monomer from homopolymers was that predicted for a first order reaction, and depended on sample weight and area. Copolymers and γ-ray irradiated samples showed some differences in products and rates.     

Chemical composition,thermal stability and hydrogen plasma treatment of laser-cut single-crystal diamond surface studied by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

X-ray photoelectron spectroscopy examination shows that after laser cutting under ambient condition, the upper surface of diamond consists of a heavy oxidized layer consisting of a variety of carbon–oxygen chemical states comprising –C═O, –C–O–C– and –C–O–H species. The thickness of the oxide layer was estimated to be ～22 ?. Upon vacuum annealing to 700 °C the thickness of the oxide layer decreases to ～10 A and the upper surface layer becomes more diamond-like through desorption of C–O species. Exposure of the laser cut diamond surface to a microwave hydrogen (MW-H) plasma results in removal of the oxide layer and exposure of the diamond phase. This is evidenced by the appearance of characteristic diamond surface and bulk plasmons which accompanied the C (1s) X-ray photoelectron peak. Our studies show that the surface chemical composition and thermal stability of the laser cut and polished surfaces both after MW-H exposure are nearly similar. The morphology of the laser cut surface shows an ill-defined laminar structure without any characteristic features which is not significantly affected by MW-H plasma exposure. This is in contrast to the polished surfaces for which exposure to the MW-H may result in its planarization.     

The correlation between surface conductivity and adsorbate coverage on diamond as studied by infrared spectroscopy

A high-resolution analysis of CH vibrational modes on a single crystal diamond(100) surface using Fourier-transform infrared (FTIR) spectroscopy in combination with conductivity measurements is reported. On a plasma-hydrogenated diamond(100) surface, the IR spectra measured in the multiple internal reflection mode reveal three absorption lines. Two of them at 2921 and 2854 cm⁻¹ vanish in air at an annealing temperature of 190°C and are assigned to the antisymmetric and symmetric CH₂ stretching modes of a physisorbed hydrocarbon species, respectively. The third band at 2897 cm⁻¹ has a width of 16 cm⁻¹, is stable up to 230°C and is associated with the stretching frequency of C₂H₂ monohydride units on the C(100) 2×1:2H surface. Upon annealing in air at temperatures lower than 200°C, the surface conductivity is reversibly reduced by up to five orders of magnitude. After cooling down to room temperature, it recovers the value of 1×10⁻⁵ Ω⁻¹ measured immediately after the plasma hydrogenation with a time constant of several days. Annealing at 230°C destroys the surface conductivity irreversibly and yields conductance values below the measurement limit of 5×10⁻¹² Ω⁻¹. We show that the chemisorbed hydrogen in the C₂H₂ configuration, together with at least one physisorbed species, is responsible for the surface conductivity of hydrogen-terminated diamond(100).     

Interfacial electronic band alignment of Ta2O5/hydrogen-terminated diamond heterojunction determined by X-ray photoelectron spectroscopy

Ta₂O₅ films have been deposited on hydrogen-terminated diamond (H-diamond) by a radio-frequency sputter-deposition technique at room temperature. Electronic band structure of Ta₂O₅/H-diamond heterojunction has been investigated by X-ray photoelectron spectroscopy. Based on the binding energies of core-levels and valence band maximum values, valence band offset has been found to be 1.5 ± 0.2 eV for the Ta₂O₅/H-diamond heterointerface. It shows a type-II band configuration with conduction band offset of 2.4 ± 0.2 eV. The large ΔE_V value makes the Ta₂O₅/H-diamond heterojunction probably suitable for the application of high power and high frequency field effect transistors.     

Investigation of the surface oxidation of bornite by linear potential sweep voltammetry and X-ray photoelectron spectroscopy

The products of surface oxidation of a natural sample of the copper iron sulphide mineral, bornite, have been determined from analysis of linear potential sweep voltammograms and from X-ray photoelectron spectroscopic examination of the oxidized layer. Anodic oxidation of bornite in alkaline media results initially in the formation of an iron(III) oxide/hydroxide and an iron-free copper sulphide of stoichiometry Cu₅S₄. The latter species is oxidized further at higher potentials to form a copper sulphide of lower copper content and cupric hydroxide. Air oxidation involves the first of these two steps. In acid solution, anodic oxidation yields iron(II) ions rather than an iron oxide in the first stage and copper(II) ions in the second. Sulphate and elemental sulphur are not formed under the experimental conditions investigated.     

Pulsed laser surface modifications of diamond thin films

In view of practical applications requiring diamond films, plates and membranes with very smooth surfaces, ArF excimer laser polishing treatments were applied to thin (30 μm) diamond films grown by CVD on silicon substrates. The as-prepared diamond surfaces and the laser-treated parts of the samples were characterised by SEM analysis, Raman and micro-Raman spectroscopy. The presence on the laser-treated surface of a thin amorphous carbon layer responsible for the higher surface electrical conductivity and for the different optical reflectivity properties was evidenced. Using confocal micro-Raman spectroscopy a comparative depth profile analysis of the phase quality, below the surface in different regions of the films, was carried out. After short (10 min) treatment by H₂ plasma etching in the CVD chamber the graphitic top layer was completely removed from the samples.     

Analysis of the surface of tricalcium silicate during the induction period by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy allows the analysis of surface layers with a thickness of a few nanometers. The method is sensitive to the chemical environment of the atoms since the binding energy of the electrons depends on the chemical bonds to neighboring atoms. It has been applied to the hydration of tricalcium silicate (Ca₃SiO₅, C₃S) by analyzing a sample after 30 min of hydration. Also two references have been investigated namely anhydrous C₃S and intermediate phase in order to enable a quantitative evaluation of the experimental data. In the hydrated C₃S sample, the analyzed volume (0.2 mm² surface by 13 nm depth) contained approximately 44 wt.% of C₃S and 56 wt.% of intermediate phase whereas CSH was not detected. Scanning Electron Microscopy data and geometric considerations indicate that the intermediate phase forms a thin layer having a thickness of approximately 2 nm and covers the complete surface instead of forming isolated clusters.     

Infrared and X-ray photoelectron spectroscopy of aminophenyltrimethoxysilane films on metals

The molecular structures of thin films formed by aminophenyltrimethoxysilane (APTMS) deposited onto mechanically polished aluminum, iron, and copper substrates from dilute aqueous alcohol solutions were determined using reflection-absorption infrared spectroscopy (RAIR) and X-ray photoelectron spectroscopy (XPS). The as-deposited films on all three substrates were similar and consisted of hydrolyzed oligomers that condensed to form siloxane polymers during heating at elevated temperatures. APTMS films on all three substrates reacted readily with epoxy resins at 150°C. Although some loss of nitrogen was observed, the films formed on aluminum were relatively stable during heating at 250°C and inhibited oxidation of the substrate. The films formed on iron and copper depolymerized rapidly during heating at 250°C and failed to inhibit oxidation of the substrates. It was concluded that APTMS films were potentially useful primers for use at high temperatures on aluminum, but not on copper or iron.     

X-ray photoelectron spectroscopy of polybithiophene–polystyrene composites

An investigation of the structure, stability, and charge distributions of conducting polybithiophene–polystyrene composite chemically synthetized using X-ray photoelectron spectroscopy (XPS) and electron microprobe analysis is described. XPS results confirm the reduction of the oxidant (Fe³⁺ is reduced to Fe²⁺ and Cu²⁺ to Cu⁺) during the bithiophene polymerization and indicate that the positive charges of doped polybithiophene are preferentially localized on the carbon atoms. Measurements versus ambient atmosphere exposure support a decreasing of the atomic ratio Cl/Fe or Cl/Cu and an increasing of the atomic ratio O/C, which could be responsible for the observed electrical conductivity instability. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1839–1845, 1997