Search

NEXAFS data [S. Rangan et al., Phys. Rev. B 71 (2005) 165319] and FTIR data [M.P. Schwartz, R.J. Hamers, Surf. Sci. 601 (2007) 945] apparently do not converge in the identification of the reaction products of acetonitrile (CH 3 CN) with Si(0 0 1)-2 × 1 at room temperature. Using DFT calculations of core-excited/core-ionized spectra and of IR vibrational frequencies and intensities, we show the consistency of the body of experimental data. Three species are present on the surface in equivalent amounts, a C N moiety, a pendent C N and a C C N ketenimine submitted to a strong twist imposed by the Si bond directionality. More generally, the paper shows the usefulness of spectroscopic data simulations in the elucidation of complex surface chemistry problems.

This paper focuses on the use and value of XPS and NEXAFS spectroscopies to unveil the nature of the chemical bond of various bifunctional nitrile molecules adsorbed on Si (001) 2×1 at 300 K. The adsorption modes are also discussed in the light of recent theoretical publications devoted to optimized geometries and reaction paths of these molecules on Si (001).

Acetonitrile (CH3CN) adsorbs on Si(0 0 1)-2 × 1 at room temperature under two forms, a cycloaddition-like adduct (Si–C N–Si) and a pendent cyano (Si–CH2–C N) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N 1s core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (∼6 fs) as an internal clock. It is shown that the πC N∗ NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the πC N∗ NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent C N to the silicon surface, on a timescale estimated to tens of femtoseconds.

The present work is a comparative study of the adsorption modes of acetonitrile and of the conjugated molecule acrylonitrile on Si(0 0 1)-2×1 at room temperature, using synchrotron radiation photoemission spectroscopy (PES) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Both molecules chemisorb on the surface. Besides dissociated species revealed by PES, NEXAFS spectroscopy shows that a large fraction of acetonitrile molecules are di-σ bonded to the surface, e.g. after a [2+2]-like cycloaddition of the CN group with a silicon dimer. In the case of acrylonitrile, PES indicates that most of the molecules chemisorb non-dissociatively. Nevertheless multiple bonding geometries must be envisaged to account for the NEXAFS spectra: the major reaction products has a flat lying geometry, the two outer atoms of the molecule skeleton being bonded to two silicon atoms. Other geometries, which correspond to the reaction of the CC bond with a silicon dimer or to the formation of a dative bond through the nitrogen lone pair with one silicon atom have also to be envisaged.

The electronic structure of 1,3,5,7-cyclooctatetraene, an antiaromatic molecule, adsorbed on Si(001)-2×1 at 300 K has been investigated by a combination of core-level spectroscopies using synchrotron radiation. Si 2p and C 1s photoemission spectroscopy (PES) are used to follow the reaction of silicon surface states and the formation of Si−C bonds. C 1s near-edge X-ray absorption fine structure (NEXAFS) spectroscopy shows that p bonds survive after chemisorption. The orientational order of the free p bonds is deduced from the angular dependence of the NEXAFS spectra; performed on a single domain (vicinal) surface, this study indicates that multiple adsorption geometries are actually adopted by the chemisorbed molecules. Finally, resonant valence band spectroscopy gives the binding energy position of the occupied p band of the chemisorbates, information that may be useful for the interpretation of scanning tunneling microscopy images.

This work is a resonant Auger spectroscopy study of solid acrylonitrile at the N K-edge. The three π* NEXAFS transitions, denoted 3a″, 13a′ and 4a″ in the Cs symmetry group of the molecule, are polarized in the molecular plane (a′ symmetry) or perpendicular to it (a″ symmetry), the highest energy transition, 4a″, being close to the ionization potential (the term value is about −1 eV). A strong selectivity with the symmetry of the probed unoccupied orbital is observed for both participator and spectator channels intensities. At the 3a″ and 13a′ NEXAFS resonances, intense participator channels and strong spectator shifts are observed, indicative of a localization of the promoted electron around the core-hole. Departures from a Raman behavior are also seen and discussed in terms of possible vibronic effects. At the third NEXAFS resonance (4a″), two spectator branches are observed with two different spectator shifts. We propose that the electron promoted to this orbital remains localized around the core hole, however it has a non-zero probability of being transferred into a more ‘diffuse’ level.

The electronic structure of poly(4-hydroxystyrene) (PHS), which presents strong similarities with that of phenol, has been studied via resonant Auger spectroscopy. Spectator shifts and relative intensities of participator channels are extracted from our data. Symmetry considerations on the occupied and unoccupied molecular orbitals (MO) are helpful in the assignment of the resonating valence band structures of PHS. However they are clearly not sufficient to explain the MO intensities when close-lying intermediate electronic states are involved, as interference effects may play an important role.

Pseudomorphic c-SiC alloys on Si(1 1 1) were grown by exposure to acetylene (C 2 H 2 ) in ultra-high-vacuum conditions. The behavior of C incorporation in the lattice for different substrate growth temperatures was studied by UHV electronic techniques. We found that C atoms occupy tetrahedral substitutional sites in the Si lattice when the substrate temperature is kept below 650°C. With increasing temperature, up to 950°C, we have observed a substantial increase of the C atoms mainly bounded to silicon in nonsubstitutional sites in a nearly planar sp 2 configuration. This is evidenced by the analysis of the line shape of the near-edge energy loss and by extended energy loss fine structure measurements which are particularly sensitive to the bond length around each carbon atom.

The Si 2p soft X-ray photoemission study of various oxidized surfaces (thermal oxidation and room temperature (RT) oxidation) shows the benefit derived from the use of high-energy resolution (70 meV). Interesting structural information can be retrieved from an analysis of the line widths of the oxides, suboxides, and elemental Si peaks. In particular the binding energy (BE) of elemental silicon layers adjacent to the oxidized layer can be distinguished from that of deeper silicon layers (the two prominent `interfacial' elemental Si lines are shifted by about +0.2 and −0.2 eV). These new data show a possible effect of oxygen second neighbors on elemental Si 2p binding energies. Moreover, changes in the oxide/suboxide line widths – as seen in a comparative study of the Si(1 1 1)-7×7 surface thermally oxidized in O2 and exposed at RT to O2 or H2O – may be attributed to static disorder, i.e., variations in Si–O bond lengths.

The adsorption of two nonconjugated linear dienes, 1,7-octadiene and 1,4-pentadiene, on $\mathrm{Si}(001)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ is studied using soft x-ray photoemission spectroscopy (XPS), C $1s$ near-edge x-ray absorption fine structure (NEXAFS) and core-decay spectroscopy. It is found that both dienes attach to the silicon surface via only one of their two $\mathrm{C}=\mathrm{C}$ bonds. This result, expected for the 1,4-pentadiene, is more surprising for 1,7-octadiene, as its length fits the distance separating two adjacent silicon dimer rows. The angular dependence of the C $1s$ absorption spectra indicates some ordering of the chemisorbed layer. C K-edge core-decay spectroscopy gives complementary information on the electronic structure, more specifically on the localization (versus delocalization) to the C $1s$ core hole of the electron promoted to an unoccupied level, which can be useful for the understanding of the transport properties of the grafted films. Independently of the molecule length, it is shown that the excited electron, promoted to a ${\ensuremath{\pi}}^{*}$ level, remains bound to the core hole, while it delocalizes when it is lifted to the ${\ensuremath{\sigma}}_{\mathrm{CH}}^{*}$ level (although this level lies below the ionization potential) and to higher-energy levels. The energy positions of NEXAFS resonances, in the chemisorbed case (characterized by the presence of a high-dielectric constant substrate) and in the condensed case (molecular solid) have been compared. We propose that resonance energies could be sensitive to final-state relaxation shifts when the promoted electron delocalizes before core-hole decay.

The room temperature oxidation of H-terminated Si(111) surfaces by activated oxygen has been investigated using high resolution (70 meV) Si 2p core-level spectroscopy with synchrotron radiation. The data are compared with previous studies using photoemission spectroscopy and high resolution electron energy loss spectroscopy. The metastability of the oxidized layer formed at room temperature is also examined.

We report a study using X-ray Photoemission Spectroscopy (XPS) of a nanocomposite which is a mixture of silicon nitride (Si3N4) micrometric grains (mean diameter of 0.2 μm) and silicon carbide (SiC) nanoparticles (from 0.01 μm to 0.06 μm). A differential charge effect was detected in this compound for the C1s and Si2p core levels. We concluded that the existence of such an effect was because the first raw acquired spectra display selective partial reductions of the charge for 6–7 h. Contributions to the XPS core levels from silicon carbide (Si–C4 and C–Si4) are not affected by charge shifts, in contrast to other contributions. This differential charge effect was not observed for SiCN nanopowders which display a rigid charge shift of all the contributions in the same core levels. This is a good fingerprint for the structural organization in the compound. We concluded the existence of SiC nanoparticles dispersed in a vitreous YSiAlON[C] phase located at the grain boundaries in the material which made up a conducting cement/skeleton surrounding large silicon nitride grains (0.2 μm).

Chemical bonding in SiCNAlY(O) nanometric powders was investigated by x-ray photoemission spectroscopy. The powders were prepared by laser pyrolysis of binary mixtures of liquid hexamethyldisilazane and disolved solid isopropoxydes. As-prepared powders are amorphous and the atomic proportion of aluminum and yttrium do not exceed 1 at. %. They were annealed at 1600 °C under N2 atmosphere and we followed the evolution of the bonding configuration with annealing treatment for the composition with a C/N atomic ratio equal to 0.78. Two other compositions were studied (powders annealed at 1600 °C): a carbon rich powder (C/N=5.05) and a nitrogen rich powder (C/N=0.09). The strong increase of Al 2p and Y 3d peak intensities relatively to Si 2p peak intensity with annealing underlines an increase of the Al and Y content in superficial layers after annealing. The analysis of the chemical shift of Al 2p core level after annealing underline the appearance of Al–N bonds at 1600 °C. The chemical environment of Y atoms ...

The different carbonization processes of a Si(111) 7 x 7 reconstructed surface exposed to acetylene (C 2 H 2 ) were studied by ultrahigh-vacuum electronic and luminescence techniques. We found that the molecule breaks at about 450°C and for temperature up to 600°C a Si 1-x C x alloy is formed in the near-surface region of the sample. For higher temperatures, up to 850°C, crystalline SiC crystallite precipitation epitaxially grown on the Si surface is observed. The luminescence spectra obtained at each stage of carbonization revealed the formation of well defined Si 1-x C x phases (with x equal to a few per cent) and it clearly demonstrates that the exposure to C 2 H 2 is highly efficient procedure to change in a continuous way the near-infrared optical properties of Si surfaces in the spectral range from 0.9 to 1.1 eV. The observation at 650°C exposure of an intense and broadened photoluminescence emission in the range 1.2-1.6eV is particularly interesting. This indicates the possibility that in the near-surface region a local concentration of the first-nearest-neighbour Si-C bond of between 15-20% is present, indicating the formation of a high C-rich layer.

The carbonization process of a Si(111) 7×7 reconstructed surface exposed to acetylene (C 2 H 2 ) has been investigated by combining several structural, electronic, and luminescence techniques. The aim was to give a comprehensive scenario of the different carbonaceous phases formed on the clean silicon surface starting from the C 2 H 2 molecule chemisorbed at room temperature up to the formation of a crystalline c-SiC compound. We found that the molecule breaks at about 450°C and for temperatures up to 600°C a Si 1− x C x alloy is formed inside the sample. For higher temperatures, up to 850°C, c-SiC crystallite precipitation epitaxially grown on the Si surface is observed. The near-edge-energy loss spectra at the carbon K edge ensures that for C 2 H 2 deposition temperatures higher than 600°C the carbon bonds are sp 3 -like, thus demonstrating the absence of non-substitutional carbon concentration. The luminescence spectra performed at each stage of carbonization revealed the formation of well-defined Si 1− x C x phases (with x of a few percent) and clearly demonstrate that the exposure to C 2 H 2 is a highly efficient method to change in a continuous way the near-infrared optical properties of silicon surfaces in the spectral range from 0.9 to 1.1 eV.

Temperature effects on the Si/SiO2 (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 Pb1 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.

The electronic structure of ultrafine laser synthesized powders with controlled composition have been investigated by using x-ray photoelectron spectroscopy. The Si 2p, C 1s, and N 1s core levels as well as the valence band distributions have been analyzed. The experimental analysis is focused on intermediate composition samples with C/N equal to 0.58 and 0.67; in this range the C/N value remains approximately the same under heat treatments. In as-formed samples, the nanopowders exhibit a high degree of chemical disorder; mixed tetrahedra with both C and N atoms around each Si atom are likely to occur. After a 1500 °C treatment, the powders remain amorphous and the presence of N–C and C–C bonds is clearly shown. After a 1600 °C annealing the samples are crystallized; the measured core levels and valence spectra reveal the presence of C–Si and N–Si bonds in Si–C4 and Si–N4 groups. It is noteworthy that in this case the C 2s and N 2s valence band states appear as two well separated peaks in the valence band...

The adsorption of ethylene on $\mathrm{Si}(001)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ and $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ has been studied at 300 K using core-level (Si $2p,$ C $1s)$ and valence-band x-ray photoemission with synchrotron radiation. Dangling bonds play a very different role in the chemisorption process on these two surfaces. While for $\mathrm{Si}(001)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ the quenching of the surface states ``monitors'' the molecule uptake, this is not the case for $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7:$ the adsorption process continues well after all surface states have disappeared, and carbon coverages greater than that deduced from the rest-atom/adatom bridge model are obtained. An alternative model is proposed, in which molecules are bonded directly to the rest plane. The thermal stability of both surfaces covered by ethylene is also studied. We relate the formation of carbon-rich layers at 820\char21{}860 K to the strong changes in the electronic structure of the silicon surface.

The structure of thin strained-layer (Ge n Si n ) p heterostructures on Si(001) is investigated by a multiple scattering approach to X-ray absorption spectroscopy (XAS) at Ge L 3 edge. Our results confirm the absence of sharp interfaces for those Ge–Si multilayers grown without Sb and, in the case of very thin Ge 2 Si 2 specimens, indicate the formation of a single phase, chemically ordered, Si–Ge alloy where the widely spaced (111) planes are occupied alternatively by the same atom type.

Intermixing in Ge/Si(001) systems has been recently observed and has emerged as a crucial and controversial topic in the study of Ge first stages of interface formation. In this paper we investigated the structure of Ge overlayers epitaxially grown on Si(001) substrates by a refined approach to x-ray absorption spectroscopy (XAS) at the Ge ${L}_{3}$ edge. XAS experimental spectra, recorded in situ using plane polarized synchrotron radiation, have been directly compared to several Ge ${L}_{3}$ near-edge spectral features calculated for different Ge/Si(001) growth models making use of a multiple-scattering approach. This analysis clearly excludes a layer-by-layer Ge laminar growth and confirms the occurrence of intermixing processes even in the case of Ge room temperature deposition. Moreover, information on the interface structure has been evidenced and the occurrence of a preferential double-layer ordering mechanism along the $〈111〉$ crystallographic directions has been singled out.

We report here on the determination of depth profiles of oxygen and nitrogen with nanometer resolution in silicon oxynitride films with thicknesses below 3 nm. We have studied oxynitride films grown by rapid thermal processing of Si (0 0 1) wafers in various Nitric Oxide (NO) atmospheres. Angle Resolved X-Ray Photoelectron Spectroscopy (ARXPS) analyses were carried out to determine the chemical composition of the films as well as the spatial distribution of the chemical bonds. The nuclear depth profiling method uses narrow, isolated resonances found in the cross section of the nuclear reactions 18 O(p,α) 15 N at 152 keV and 15 N(p,αγ) 12 C at 429 keV. The excitation curves of the nuclear reactions in the neighbourhood of the resonance energies can be used to determine the concentration versus depth profiles by means of SPACES, a personal computer implementation of the stochastic theory of energy loss. We discuss aspects of the simulation of excitation curves and the effect of NO pressure on the structure of the oxynitride film.

The carbonization process of a single domain 2{times}1-reconstructed Si(001) vicinal surface (5{degree} off axis from [001] in the [{bar 1}10] direction) in acetylene has been studied by combining in situ surface science techniques (x-ray photoemission spectroscopy, x-ray photoelectron diffraction, reflection-electron energy loss spectroscopy, low-energy electron diffraction) and ex situ analytical techniques ({sup 12}C and {sup 2}H dosing by nuclear reaction analysis, scanning electron microscopy, and reflection high-energy electron diffraction). It is found that at a growth temperature of about 820{degree}C a variety of growth mechanisms can be observed, particularly during the first step of carbonization. An analysis of C1s and Si2p core-level shifts and of the respective intensities of them, combined with the examination of photoelectron diffraction curves, gives evidence for a penetration of C atoms into the silicon substrate, to form a nonstoichiometric compound. Contemporaneously 3C-SiC nuclei form, aligned with respect to the substrate. Then a quasicontinuous 3C-SiC film grows heteroepitaxially (cube on cube unstrained growth) on the substrate up to a thickness of {approximately}40{Angstrom}. C1s and Si2p photoelectron diffraction patterns, compared with calculated ones, show that the single domain initial surface does not necessarily force a preferential alignment of one of the two inequivalent SiC{l_brace}110{r_brace} planes with respectmore » to the (110) Si plane. Consequently, such vicinal Si(001) surfaces are not necessarily templates for the growth of crystalline films free of antiphase boundary domains. Finally, we have observed that an imperfect coalescence of 3C-SiC nuclei leaves easy paths for Si out migration from the substrate and SiC polycrystalline growth, even at a temperature as low as 820{degree}C. The current models of Si(001) carbonization are examined and compared to our experimental findings. (Abstract Truncated)« less

Synchrotron radiation X-ray photoemission spectroscopy (SRXPS), used with an optimized surface and interface sensitivity, is a unique tool to determine the chemical composition and spatial extension of the suboxide layer present at the Si/SiO2 interface. The bonding at ‘thermal interfaces’ appear to be essentially dependent on the Si crystal orientation. For Si(001)/SiO2 a detailed photon energy-dependent and angle-dependent. SRXPS study (in the 120 ÷ 175 eV photon energy range) has been performed. Previous results on higher-oxidation states cross-section resonances around hv = 130 eV are re-examined. The various oxide states do not present photoelectron diffraction peaks, when polar scans are performed in the (110) azimuthal plane. On the other hand, there is evidence that substrate silicon atoms close to the interface retain orientational order. Limits in the vertical distribution of the suboxides are given. The compatibility of our experimental findings with models of the current literature — in particular the so-called ‘dimerized interface’ model — is examined.

Nanometric Si/C/N powders have been obtained from the laser synthesis of a liquid precursor: hexamethydisilazane. The chemical composition of the products is a function of the experimental parameters: the C/N ratio in the powders is controlled in the range 0.34 to 1.34 by varying the ammonia content in the flowing gas (argon). This paper presents some results obtained by X-ray Photoelectron Spectroscopy (XPS) about the local atomic structure of these powders. The evolution of the powders after annealing treatments at 1500 and 1600 °C under nitrogen atmosphere has been studied. The chemical composition deduced from XPS wide spectra are compared with chemical analysis. From the analysis of the Si-2p core level, information about the local atomic bonding around silicon atoms are obtained. The different environments present in the system and their evolution with temperature have been deduced from the comparison with data obtained for stoichiometric compounds (SiC, Si 3 N 4 and SiO 2 ). The existence of local chemical disorder is shown in the as-formed powders. For powders with C/N 0.6, the samples remains amorphous until 1500 °C; crystallization which starts around 1500 °C is total at 1600 °C. The existence of C-N bonds at 1500 and 1600 °C is evidenced. This result is in good agreement with results obtained by neutron diffraction and X-ray absorption (XAS).

In comparison with oxidized bulk crystals, we show that the morphology of oxidized porous silicon can offer a unique opportunity of measuring suboxide distribution by conventional XPS and to characterize thoroughly interfacial defects by EPR.

The growth mode of metal phthalocyanines (MPc, MNi, Cu) on clean Cu(001) and Si(001) (i.e. bonding and adsorption geometry) and their electronic structure are studied by a combination of spectroscopic techniques: x-ray photoemission, x-ray absorption at Metal L 2,3 edges and scanning tunnelling microscopy.

We have studied by soft X-ray photoemission (hv = 145 eV) the modifications of the Si 2p core-level and of the valence band during the adsorption of water on Si(001)-2 × 1 and Si(111)-7 × 7 surfaces, at two substrate temperatures (300 and 90 K). In all cases, water reacts with silicon. We have followed the decay of Si 2p and valence-band surface states, as well as the growth of oxidation states, with increasing exposures to H2O. The breaking of H2O into -H and -OH fragments and their attachment to the triply coordinated surface atoms (silicon dimers on Si(001), adatoms and rest-atoms on Si(111)) is not the only reactional mechanism. Oxygen atoms are also inserted into SiSi bonds, so that oxidation offers an alternative channel to H2O dissociation and chemisorption. For both orientations, surface oxidation is facilitated at low temperature. The relative reactivity of the dangling-bonds of Si(111)-7 × 7 has been investigated: both valence band and Si 2p spectra indicate clearly that the rest-atoms are less reactive than the adatoms.

The resistance of the As-terminated Si(001) surface to oxidation in O 2 is the subject of this study. Photoemission spectra of As3d and Si2p core levels excited with synchrotron radiation reveals that spectral changes are evident, simultaneously for As and Si, only from ∼10 12 L (Langmuir) on, in stark contrast with a previous report indicating a saturation coverage in the 400–2000 L range. Oxidation proceeds slowly, as dimerized As remain intact (∼24%) up to exposures of ∼10 13 L. In the oxidized areas, the four Si oxidation states (indicative of subsurface oxidation) and three As oxidation states plus metallic arsenic show up. This suggests a reduction of the arsenic oxide by silicon. Arsenic losses are also observed, probably via sublimation of As 4 O 6 molecules.

The strong resistance of the As-terminated Si(001) surface to oxidation in molecular oxygen at room temperature is the subject of this study. As 3d and Si 2p photoemission spectra, as well as valence band spectra, excited with synchrotron radiation (135 eV ≤ hv ≤ 165 eV), have been recorded as a function of increasing O2 exposures, over the range 2400–1.4 × 1013 L (langmuir). Spectral changes, simultaneously observed in the As 3d and Si 2p core-levels near ∼ 1012 L are indicative of the onset of oxidation. However a modification of the valence band, due to the contribution of O 2p states, is clearly evident from ∼ 1011 L, pointing to the presence of weakly bound oxygen species at that exposure. These results are confronted with a previous Auger study reporting an oxygen saturation coverage (0.5 monolayer) already in the 400–2000 L range, interpreted as the insertion of an oxygen atom between two dimerized arsenic atoms. On the contrary our data show that surface oxidation is a slow process, spatially inhomogeneous, as some dimerized As remain intact (∼ 24%) up to exposures as large as ∼ 1013 L. Oxidation is not limited to the outermost layer, as three arsenic oxidation states and the four silicon oxidation states (indicative of subsurface oxidation) are detected simultaneously. Moreover metallic arsenic shows up: this suggests a reduction of the arsenic oxide by silicon. Arsenic losses are also observed, probably via sublimation of As4O6 molecules.

Silicon surfaces with different reconstruction, Si(111)-7 × 7 and Si(001)-2 × 1, are exposed to a molecular beam of copper phthalocyanine. On Si(111) the adsorbate-substrate interaction is strong, as suggested by a partial reduction of copper shown in the XPS spectra and by the evidence provided by STM of a chemical reaction involving some of the Si adatoms. As a consequence, the molecules are not imaged at atomic scale by STM. At variance with this behaviour, on the single-domain Si(001)-2 × 1 face, the adsorbed molecules are clearly imaged by STM, suggesting an adsorption state more weakly bonded. This is the first observation of Cu phthalocyanine molecules imaged on a silicon surface, and it further illustrates the potential of STM for surface chemistry.

The chemical bonding and molecular orientation of copper phthalocyanine (CuN 8 C 32 H 16 ), deposited on clean single crystal silicon substrates of (111) and (001) orientation, were studied using X-ray photoemission spectroscopy (XPS) of the Cu 2p, C 1s, and N 1s core-levels and synchrotron radiation X-ray absorption spectroscopy (XAS) at the Cu L 2,3 edges. In the monolayer range, the strong interaction of the molecule with the Si (111) surface is mostly evidenced by changes in the Cu 2p XPS spectra that indicate a partial reduction (up to 30%) of the Cu(II) atoms to Cu 3d 10 . The linear dichroism of the XAS spectra shows that those molecules, whose copper is still Cu(II), lie down on the surface. On the other hand, the bonding of the molecule on Si(001) is weaker, as Cu 2p XPS spectra do not exhibit Cu(II) reduction, while XAS spectroscopy indicates a random orientation of the molecules with respect to the surface. N 1s XPS spectra exhibit, on both surfaces, very intense satellites which present, on Si(111), a sharp dependence on the electron take-off angle. With the growth of thicker films, the XPS line shapes of the molecular solid are recovered, and the average tilt angle of the molecules (around 70°) does not depend on the chosen Si surface. We conclude that the structure of the first deposited layer strongly depends on the specific substrate, but does not affect the successive growth of thick layers.

X-ray photoemission spectroscopy (XPS) and ab initio quantum mechanical calculations (SCF, CI, MCSCF) are used in combination to study the electronic structure of copper phthalocyanine (CuPc). The XPS core level spectra as well as the molecular orbital calculations suggest that in this molecule, the CuN bonds are ionic to a large extent. In the Cu2p spectrum, the final state corresponding to the main line, at the lower binding energy side, appears as a large mixing of two configurations; in one of them the core hole is fully screened by a charge transfer from the ligand 2p orbitals to the Cu3d x 2 − y 2 orbital. For the Cu3s and Cu3p core holes the screening by the ligand to metal (L → M) charge transfer is less important. For the Cu2p and Cu3p core levels, the higher binding energy states (satellites) correspond to shake-up processes exhibiting L → M charge transfer as well as M → M transitions, whereas in the case of Cu3s these are purely L → M.

The role of surface structure in controlling NH3 surface chemistry has been investigated by N 1s and Si 2p core level photoemission on Si(001)-2 × 1 and Si(111)-7 × 7, taking advantage of the surface sensitivity provided by synchrotron radiation tunability. Sequential treatments, i.e. 90 K adsorption/300 K annealing/1193 K nitridation, have been carried out. A common feature of low-temperature NH3 adsorption on Si(001) and Si(111) is the growth of a solid NH3 layer over a decomposed ammonia interlayer where, among other species, atomic nitrogen is present. Increasing the temperature causes this solid NH3 layer to desorb. A 300 K annealing of the Si(001) surface eliminates also the adsorbed atomic nitrogen species to reach a situation in which only H atoms and NH2 fragments decorate the silicon dimer broken bonds. In contrast to the Si(001) case, a 300 K annealing of the rougher Si(111) surface does not lead to a unique adsorption site/NH3 species: in particular atomic N remains. The formation of higher subnitride states (already at 90 K) is also evidenced on Si(111) with respect to Si(001). The situation of greater complexity (due to the 7 × 7 reconstruction) of the Si(111) surface with respect to Si(001), when one considers low-and room-temperature adsorption processes, is strikingly reversed when one deals with thermal nitridation. The Si(111) subnitride distribution is compatible with an ideal abrupt interface, when the Si 3 N 4 Si(001) interface appears as rougher. Using the clean and 300 K annealed surfaces as templates, the amounts (per unit area) of subnitrides at the Si 3 N 4 Si interface are estimated.

X-ray photoelectron spectroscopy (XPS) is used to characterize copper sulphides inserted into Langmuir-Blodgett (LB) films of behenic acid. Comparisons are made between samples exhibiting very different electrical conductivity but synthesized by the same method. Sulphur atoms with two different environments are found: divalent sulphur S 2- and dimer sulphur (S 2 ) 2- , higher concentrations of dimer sulphur (S 2 ) 2- being present in insulating samples. Chemical bonding of copper in LB films is compared with CuS and Cu 2 S powders. The copper to sulphur ratio and the divalent to dimer sulphur ratio are obtained from various samples and lead to the formula: Cu 2 S, x S 2 with x ranging from 0.5 to 0.8.

In this work, we examine the possibility of using core-level spectroscopies, X-ray photoabsorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) to obtain a (semi)quantitative measurement of the distribution of copper oxidation states in copper oxide layers of overall stoichiometry CuOy, otherwise precisely determined by ion beam analysis (IBA), a technique independent of the chemistry. It turns out that the agreement between the analysis of XAS spectra and IBA measurements is excellent as long as y>0.5, that is for mixtures of CuO and Cu2O. The limits of the applicability of XPS are also discussed. In the field of material sciences, the application of XAS (and also of XPS, to some extent) to determine the Cu(II)/Cu(I) ratios is thus promising in those cases which do not allow precise stoichiometry measurements by ion beam techniques.

In this paper, we review the main properties of BiSrCaCuO thin films obtained by laser ablation followed by a post deposition oxygen annealing treatment. These properties were investigated by the complementary use of Rutherford backscattering spectrometry, X-ray diffraction, scanning electron microscopy, Raman microprobe analysis, X-ray photoelectron spectroscopy and resistivity measurements. The high temperature annealing treatment was found to be the important parameter for obtaining superconducting transitions, i.e. growth of the superconducting Bi 2 Sr 2 Ca 1 O 8 phase. However, this treatment also induces large modifications in the films, in general the formation of a multiphased material with appreciable surface roughness of the film. We discuss in this paper the precise influence of the post deposition high temperature oxygen annealing treatment on the atomic composition, surface morphology, structural and chemical nature of the films and on their electrical properties.

Rapid thermal nitridation (RTN) of silicon dioxide films and silicon covered with its native oxide was performed at atmospheric pressure in a lamp heated furnace in NH3 at a temperature of 1000 ° C. Physicochemical characterizations of grown films were carried out by nuclear reaction analysis (NRA) and X-ray photoelectron spectroscopy (XPS). For the films formed by the nitridation of silicon with its native oxide, the XPS measurements, using normal and grazing emergence angles, indicate that a stoichiometric silicon nitride appears between two nitroxide (SiOxNy) films at the silicon/dielectric interface and at the surface of the dielectric. The depth profile of nitrogen in films formed by the rapid thermal nitridation of a 36 A silicon dioxide was determined using chemical etching associated with NRA and estimated through the XPS technique. A homogeneous nitroxide was evidenced for the most part of the dielectric film, and a monolayer (∼ 0.7 × 1015 atoms cm−2) of stoichiometric silicon nitride was observed at the interface between the silicon substrate and the nitroxide film.

The aim of this work is to determine if, after oxidation in pure O2, a vicinal surface Si(001) miscut 5° towards the [1 1 0] direction, exhibits a preferential SiO2 formation on the [110] ledges. In a first stage, combining XPS (X-ray photoelectron spectroscopy) and LEED (low-energy electron diffraction) available in an auxiliary chamber, we examine the cleaning conditions leading to a non-faceted stepped Si surface with dominant (2 × 1) single domains. This surface preparation is reproduced in the synchroton facility chamber where ARPR-single-resolved photoemission spectroscopy) is performed: after cleaning the Si surface and studying the starting situation (san- states, initial contamination), an ultra-thin oxide film (∼ 1.5 × 1015 oxidised Si atoms/cm2) is grown at 785–800°C, under Torr of oxygen. The absence of anisotropy in the ARPES emission of the Si 2p core levels taken at the minimum electron escape depth (hv=130eV), points to no extra-growth of the oxide on the ledges. The in-depth distribution of the oxide states seems to be homogeneous, within the sensitivity of the probe. On the other hand, after partial thermal desorption of the ultra-thin oxide film in vacuum, a strong anisotropy in the emission of electrons from SiO2 is seen in areas where oxide patches coexist with bare silicon. It is shown that this ensues from a significant step bunching and (111) faceting of the surface. A correlation is made with SEM (scanning electron microscopy) images of the surface. The problem of step bunching, inasmuch as it can alter the interpretation of shadowing effects, is emphasized.

The electronic structure of SiC prepared by synthesis from vapour phase reactants heated with a high power CO 2 laser has been investigated. From an analysis of the photoelectron C 1s and Si 2p core levels we show that only heteropolar SiC bonds are present in the materials, without segregation tendency. The total and partial Si 3p valence band distributions are studied by photoelectron and X-ray emission spectroscopy respectively. The general shape of the valence band is that of a chemically ordered sp 3 -bonded network. The positions of the conduction states relative to E f are obtained from the Si K photoabsorption spectrum.

X-ray photoelectron spectroscopy (XPS) is applied to the physico-chemistry study of inserted HgS into multilayers of behenic acid. Comparison is made with precursors used during the in-situ synthesis: behenic acid Langmuir-Blodgett films, mercury behenate Langmuir-Blodgett films and the two allotropic forms α-HgS and δ-HgS. The results obtained show that the mercury sulfide is stoichiometric, close to the δ-HgS form and a correlation has been established between sample conductivity and XPS spectra.

Spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are applied to the chemical characterization of very thin films (200–250 A) of BiSrCaCuO fabricated in a two-step process. Amorphous and insulating films are deposited on (001) MgO single crystals by the laser ablation technique (Nd:YAG laser equipped with a frequency-doubling crystal). Although quasi-congruent for the cations, the ablation process induces an oxygen depletion from the stoichiometric superconducting Bi2Sr2CaCu2O8 target. Indeed, the surface sensitive X-ray photoelectron spectroscopy (XPS) evidences the presence of a Bi°/Bi(III) mixture as well as the occurence of Cu(I) alone in the deposited film, instead of Bi(III) and Cu(II) as in the parent target material. Core level binding energies of Column II elements are characteristic of their ionic compounds (oxides and carbonates). A large amount of Cu(I) present in the film depth is confirmed by the more bulk-sensitive soft X-ray absorption spectroscopy (XAS) at Cu 2p threshold, operated in the secondary electron total yield mode. The films need to be annealed in 1 atm O2 at a temperature of 865°C, so that they are converted to the ordered and highly textured Bi2Sr2CaCu2O8 superconducting phase with Tc(R=0)=82 K. After the O2 annealing, XPS indicates a noticeable decrease of the carbonate content. All the bismuth is reoxidized to Bi(III). Cu(I) is also partly converted to Cu(II). In depth, as shown by XAS, there is a complete conversion to Cu(II), which is the oxidation degree of copper in the superconducting material. Remarkably, a part of the spectral weight of Column II elements and oxygen XPS core levels shifts towards smaller binding energies, which is interpreted in terms of the formation of the metallic lamellar compound. It is shown that an ill-reacted insulating compound remains in the outermost layer.

The system constituted by the $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface exposed to water molecules at room temperature has been chosen to single out the electron structure of the hydroxyl SiOH, a surface species playing an important role in many technologically relevant processes. We confront here original core-electron spectroscopy density functional theory (DFT) calculations to synchrotron radiation O $1s$ x-ray photoelectron spectroscopy (XPS) and (polarization-dependent) near-edge x-ray absorption spectroscopy (NEXAFS) data. Using clusters to mimic the silicon surface, various SiOH environments (unpaired and paired hydroxyls) have been examined. The impact of the hydrogen bond on the calculated core-ionized and/or neutral core-excited states is examined, and compared to the limit case of the water dimer. The theoretical approach enables us to label the main experimental NEXAFS transitions and to interpret their polarization-dependent dichroism. As water dissociation on the surface can go beyond the formation of hydroxyls, the DFT electron structure of bridging oxygens (SiOSi) is calculated. It is predicted that the XPS line associated to the latter species is shifted by $0.5--1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ to lower binding energy with respect to that of the hydroxyls. Then, on this basis, the reconstruction of the experimental O $1s$ XPS spectrum can provide the relative distribution of hydroxyls and of the bridging oxygens (a minority species). The present work paves the way for future spectroscopic works examining the bonding and reactiveness of hydroxyls with other molecular species.

The room temperature adsorption of acetonitrile $({\mathrm{CH}}_{3}--\mathrm{C}\mathrm{N})$ on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ is examined by synchrotron radiation N $1s$ x-ray photoemission and x-ray absorption spectroscopies. The experimental spectroscopic data point to multiple adsorption geometries. Candidate structures are optimized using density functional theory (DFT), the surface being simulated by silicon clusters encompassing one (adjacent) adatom--rest atom pair. This is followed by the DFT calculation of electron transition energies and cross sections. The comparison of theoretical spectra with experimental ones indicates that the molecule is adsorbed on the surface under two forms, a nondissociated geometry (an $s{p}^{2}$-hybridized CN) and a dissociated one (leading to a pendent sp-hybridized CN). In the nondissociative mode, the molecule bridges an adatom--rest atom pair. For bridge-type models, the discussion of the core-excited state calculations is focussed on the so-called silicon-molecule mixed-state transitions that strongly depend on the breaking or not of the adatom backbonds and on the attachment of the nitrogen end either to the adatom or to the rest atom. Concerning the dissociated state, the CH bond cleavage leads to a cyanomethyl $(\mathrm{Si}\text{\ensuremath{-}}{\mathrm{CH}}_{2}\text{\ensuremath{-}}\mathrm{CN})$ plus a silicon monohydride, which accounts for the spectroscopic evidence of a free $\mathrm{C}\mathrm{N}$ group (we do not find at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ any spectroscopic evidence for a $\mathrm{C}\mathrm{N}$ group datively bonded to a silicon atom via its nitrogen lone pair). Therefore the reaction products of acetonitrile on $\mathrm{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ are similar to those detected on the $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface at the same temperature, despite the marked differences in the reconstruction of those two surfaces, especially the distance between adjacent silicon broken bonds. In that respect, we discuss how adatom backbond breaking in the course of adsorption may explain why both surface orientations react the same way with acetonitrile.

Using a combination of local -- scanning tunneling microscopy -- and spatially integrated, but chemically sensitive probes -- X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure spectroscopy -- we have examined how 3-butenenitrile reacts with the Si(001)-2 x 1 surface at room temperature. Electron spectroscopies indicate three different nitrogen chemical bonds: a Si-C=N-Si bond, a C=C=N cumulative double bond, and a CN moiety datively bonded to a silicon atom. All molecular imprints detected by scanning tunneling microscopy (STM) involve two adjacent silicon dimers in the same row. The three geometries we propose -- a double di-sigma bonding via the CN and the C=C, a cumulative double bond formation associated with alphaC-H bond dissociation, and a di-sigma vinyl bonding plus a CN datively bonded to a silicon atom -- are all compatible with electron spectroscopies and data. Real-time Auger yield kinetic measurements show that the double di-sigma bonding geometry is unstable when exposed to a continuous flux of 3-butenenitrile molecules, as the Si-C=N-Si unit transforms into a CN moiety. A model is proposed to explain this observation.

Combining synchrotron radiation N $1s$ x-ray photoemission and x-ray absorption spectroscopy to density functional theory calculations of electron transition energies and cross sections performed on silicon clusters, we have re-examined the issue of acetonitrile adsorption on single- and two-domain $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surfaces, taking into consideration the various adsorption models proposed in the recent theoretical works. It is shown that at 300 K and saturation coverage, the molecule is chemisorbed both under nondissociated and dissociated forms. The nondissociative adsorption mode results from a $\mathrm{di}\text{\ensuremath{-}}\ensuremath{\sigma}$ bonding involving the cyano group. The use of a vicinal surface allows us to show that the side-on (a cycloadditionlike product on the Si dimer) is the majority species of $\mathrm{di}\text{\ensuremath{-}}\ensuremath{\sigma}$ type. The datively bonded molecule (end-on) is not observed at 300 K. The molecule also dissociates on the surface, under the form of a cyanomethyl plus a silicon monohydride, a model which has not been so far proposed by theoretical works.

We have studied the adsorption geometries of benzonitrile (C{sub 6}H{sub 5}-C{identical_to}N) deposited on Si(001)-2x1 at 300 K, using synchrotron radiation x-ray photoemission spectroscopy and x-ray absorption spectroscopy. The experimental data are interpreted through density functional theory calculations of ground and excited states. It is shown that the molecule chemisorbs under multiple forms. While the literature indicated that adsorption is highly selective at 110 K (bonding via the cyano), at 300 K, cyano bonding is observed concomitantly with other geometries for which the molecule interacts with the surface by the phenyl ring. Among the phenyl-bonded species, the cyano group can remain unreacted or can make a dativelike bond with the substrate via its nitrogen end (the latter species is a minority species at surface saturation). Real-time kinetic studies show that all species are present on the surface with the same distribution from the very beginning of the adsorption process.

Using scanning tunnelling microscopy (STM), photoelectron and photoabsorption spectroscopies, we have examined how acrylonitrile $({\mathrm{H}}_{2}\mathrm{C}\mathrm{C}\mathrm{H}\text{\ensuremath{-}}\mathrm{C}\mathrm{N})$ reacts with the $\mathrm{Si}(001)\text{\ensuremath{-}}2\ifmmode\times\else\texttimes\fi{}1$ surface for coverages ranging from $\ensuremath{\sim}{10}^{12}\phantom{\rule{0.3em}{0ex}}\text{molecules}∕{\mathrm{cm}}^{2}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}\ensuremath{\sim}{10}^{14}\phantom{\rule{0.3em}{0ex}}\text{molecules}∕{\mathrm{cm}}^{2}$. At $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, in the very low coverage regime (below ${10}^{13}\phantom{\rule{0.3em}{0ex}}\text{molecules}∕{\mathrm{cm}}^{2}$), filled- and empty-state STM images show that the molecule bridges, via its $\ensuremath{\beta}$ carbon and nitrogen ends, two silicon dangling bonds, across the trench separating two dimer rows. A cumulative-double-bond unit $(\mathrm{C}\mathrm{C}\mathrm{N})$ is formed. The $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ STM image results from the dynamic flipping of the molecule between two equivalent equilibrium positions, which can be seen when the molecular motion is slowed down at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For coverages larger than ${10}^{13}\phantom{\rule{0.3em}{0ex}}\text{molecules}∕{\mathrm{cm}}^{2}$, for which STM does not show ordered adsorption any more, the adsorption kinetics were observed in real-time using valence band photoemission and resonant Auger yield, associated with N $1s$ x-ray absorption spectroscopy (NEXAFS). At $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, these techniques point to a situation more complex than the one explored by STM at very low coverage. Three species (cyano-bonded, vinyl-bonded, and cumulative-double-bond species) are detected. Their distribution does not vary with increasing coverage. All dimerization-related surface states are quenched at saturation. The uptake rates versus coverage relationship points to the presence of a mobile precursor. Finally, the paper discusses a possible mechanism leading to the formation of cross-trench $\mathrm{C}\mathrm{C}\mathrm{N}$ unit at low coverage, and the reasons why the product branching ratio changes with increasing coverage.

This paper presents the results of X-ray photoelectron spectroscopy and secondary ion mass spectrometry experiments performed to obtain information about the sensing mechanism involved in the detection of phosphine by conducting Langmuir-Blodgett (LB) films based on TCNQ and EDT-TTF derivatives. In contrast to the EDT-TTF based LB films, it is shown that with the TCNQ derivative the action of phosphine (PH3) results in the insertion of phosphine subproducts in the LB film. This information is correlated with the general behaviour of each film towards phosphine detection.