High-resolution Core-level Spectroscopy Research Articles

CO dissociation on Mo(1 1 0) studied by high-resolution core-level spectroscopy

We have studied the CO/Mo(1 1 0) adsorption system at different sample temperatures by high-resolution core-level spectroscopy and near-edge X-ray-absorption fine-structure measurements. We demonstrate that CO molecules having different tilt angles with respect to the surface normal may be assigned to different components of the C 1s core-level spectra. The tilt of the CO molecules is shown to depend on the CO coverage. The thermally induced dissociation of CO is studied and is shown to differ at low and high initial CO coverages. At low coverage CO dissociation starts already at temperatures as low as 125 K whereas at saturation CO coverage no dissociation occurs below 280 K. The reasons for this difference in dissociation behavior are discussed and it is concluded that steric effects play a large role.

Spectroscopic characterization of catalytically active surface sites of a metallic oxide

High-resolution core-level photoelectron spectroscopy in combination with density functional theory calculations was used to identify the spectroscopic signatures of the catalytically active sites on the RuO 2(1 1 0) surface, i.e., the coordinatively unsaturated Ru sites (cus-Ru) and the bridging oxygen atoms. These assignments provide the necessary spectroscopic information for in situ studies of catalytical reactions over the RuO 2(1 1 0) surface.

High resolution photoemission core level spectroscopy study and TEM analysis of the Ge/As/Si(0 0 1) growth

The deposition of 1 ML of As on Si(001)-(2×1) surface and the heteroepitaxial Ge/As/Si(001)-(2×1) growth were studied by using high resolution core-level spectroscopy and transmission electron microscopy (TEM). From the spectral decomposition of the Si2p core levels, collected on the Ge/Si interfaces obtained by codepositing As and Ge atoms at different Ge thickness, we identify the contribution of different Si layers to the Si2p lineshape, in agreement with recent theoretical final-state pseudopotential calculations. The bulk-like Ge3d core levels reflected the occurrence of a complete Ge–As site-exchange process, while the pseudomorphic layer-by-layer Ge/Si growth was confirmed by TEM investigation.

Initial oxidation states on Si(0 0 1) surface induced by translational kinetic energy of O2 at room temperature studied by Si-2p core-level spectroscopy using synchrotron radiation

Initial oxidation states on Si(001) surfaces caused by the translational kinetic energy (Et) of O2 at room temperature have been studied by the Si-2p core-level spectroscopy using synchrotron radiation. From the results of oxygen coverages as a function of the translational kinetic energy of O2 studied by the O-1s photoemission spectroscopy, it was found that there were potential energy barriers at Et=1.0eV corresponding to the backbond oxidation of the top-layer Si atoms and at Et=2.6eV associated with the oxidation between the second and the third Si layer. The relation between oxidation states (Si1+, Si2+, Si3+ and Si4+) and the translational kinetic energy of O2 were investigated by the high-resolution Si-2p core-level spectroscopy.

Sharp high-resolution Si2pcore level on the Sb-terminated Si(111) surface: Evidence for charge transfer

An Sb/Si(lll)root 3x root3 surface has been studied by high-resolution core-level spectroscopy. All the components of the Si 2p core level related to the clean surface have disappeared, and one strong Sb-induced component could be identified at a binding energy +0.13 eV with respect to the bulk peak. Such a component is mainly due to charge transfer between Si atoms at the top layer and Sb trimers. Two more small components are observed: one at +0.29 eV, due to Sb atoms residing at a different position, and one at -0.14 eV, due to local disorder. On the Si 2p core level taken in bulk sensitive mode, we found a very narrow bulk component with a total full width at half maximum of 130 meV at T=100 K.

Temperature effect on the reconstruction of Sb/Si(001) interface studied by high resolution core level spectroscopy and RHEED analysis

The adsorption of three monolayers (3 MLs) of Sb at −120°C on the Si(001)c(4×2) surface and the subsequent annealing up to 650°C were followed by high resolution core level spectroscopy and RHEED analysis. By relating the electron diffraction patterns to the Si2p and Sb4d core level spectra measured after each annealing cycle, it was possible to monitor the evolution of the Sb/Si interface structure up to the achievement of a diffuse (2×1) reconstruction. Even after the annealing to 650°C, the quality of the surface reconstruction remained poor, as attested by the RHEED pattern, by the intensity of the Si2p surface component and by the broadening of the Sb4d lineshape.

Sb-terminated Si(110), Si(100) and Si(111) surfaces studied with high resolution core-level spectroscopy

We report on synchrotron radiation photoelectron spectroscopy studies of the Si 2p core levels obtained with very high resolution from Sb overlayers onto the low-index silicon surfaces Si(110), Si(100) and Si(111). The improved energy resolution of the third generation synchrotron radiation facilities has shown the presence of a strong interfacial component shifted 0.24, 0.2 and 0.13 eV, respectively, on the high binding energy side respect to the bulk peak. Such Si–Sb interface contribution is ascribed to charge transfer between the metal and the topmost silicon atoms and to the latest layers of silicon atoms distorted by the reconstruction from the ideal bulk position. A small component is always present on the high kinetic energy side and is presumably due to contribution from defects.

Coadsorption of Li and Na on Al(100)

The coadsorption of Li and Na on Al(100) has been studied by high-resolution core-level spectroscopy and low energy electron diffraction. Coadsorption of 0.25 ML Li and 0.25 ML Na at room temperature leads to the formation of a c(2×2) structure, which is shown to be a substitutional surface ternary alloy, in which the alkali atoms randomly occupy the vacancies in a c(2×2)-Al sublattice. Adsorption of 0.5 ML Li followed by 0.5 ML Na at room temperature leads again to the formation of a c(2×2) structure. This is shown to be a multilayer surface ternary alloy, consisting of a mixed c(2×2)-Al/Na layer, followed by a (1×1)-Al layer, followed by a mixed c(2×2)-Al/Li layer. This structure is also obtained by reversing the adsorption sequence. Adsorption of 0.5 ML Li at room temperature followed by 0.5 ML Na at 120 K leads again to the formation of a c(2×2) structure, which consists of a chemisorbed Na overlayer on a mixed c(2×2)-Al/Li layer. Surprisingly, reversing the adsorption sequence leads again to the same structure. The mechanisms of formation of the different phases are discussed.

Ge/Si(001)c(4×2)interface formation studied by high-resolution Ge3dand Si2pcore-level spectroscopy

Ge epitaxy on $\mathrm{Si}(001)c(4\ifmmode\times\else\texttimes\fi{}2)$ was monitored by high-resolution core-level spectroscopy up to the deposition of 20 ML. Ge $3d$ and Si $2p$ core-level spectra were acquired at normal- and grazing-emission angles. Taking into account the whole evolution of the Ge $3d$ core-level line shape during growth, in addition to the three surface components previously reported for thin Ge layers, a fourth component was identified and attributed to bulklike Ge atoms. For submonolayer coverage, the difference between the number of up- and down-dimer sites occupied by Ge unequivocally indicated the formation of mixed Si-Ge dimers. The presence in the Ge $3d$ core level of the component related to second-layer atoms demonstrated that Ge starts to diffuse below the surface before all the dimer sites are occupied. During deposition of the second ML, the behavior of the bulklike component indicated that Ge diffuses to layers deeper than the second, probably in order to occupy sites under tensile stress. At a Ge coverage of 20 ML, the persistence of surface components in the Si $2p$ core level is related to segregation of Si in the Ge layer or, more appropriately, to Stranski-Krastanov growth, which determines an inhomogeneous Ge coverage, locally as low as 1\char21{}2 ML.

Identification of a laterally mobile state during CO adsorption

We have investigated CO adsorption at 300 K on ~1.5 atomic layer thick Pd films on a Mo(110) surface by high-resolution core level photoemission. We describe how high-resolution core level spectroscopy may be utilized to study the influence of laterally mobile states on the sticking probability of molecules on such a laterally heterogeneous surface. The present Pd films are laterally heterogeneous in the sense that the additional ~0.5 atomic Pd layer forms mesoscopic one-layer thick islands on top of the first Pd layer. At 300 K, CO chemisorbs on these two-layer thick islands but not on the one-layer parts of the film. The rate at which these two-layer islands are filled by CO molecules as the surface is exposed to CO is found to be consistent with a picture where CO molecules that initially impinge on the one-layer parts of the surface enter a laterally mobile state and diffuse to the two-layer islands and adsorb there. This mobile state is in many respects similar to a classical precursor state.

Electron-Spectroscopy Study of Amorphous CN:Ti Films

Amorphous CN:Ti films deposited by using the reactive magnetron sputtering method were annealed in vacuum under 200 to 600° C. Incorporation of Ti (less than 3 at.%) has brought some new perspectives to the material. Electron-energy-loss spectrum indicates improved conductivity in CN:Ti films as in the insulating CN films. The high-resolution core-level x-ray photoelectron spectroscopy (XPS) studies show that most carbon atoms form homeopolar bonding in as-deposited CN:Ti film. While the N 1s line always shrinks with increasing annealing temperature, the total full width at half maximum of C 1s line decreases only at annealing temperature over 300° C when the broadening due to heterobonding formation is outdone by the effect of increasing order. An enhanced π-band feature in the valence-XPS spectra at high annealing temperatures confirms the graphitisation tendency of this material.

Surface alloy formation by adsorption of Li on Al(100): studied by high-resolution core-level spectroscopy and low energy electron diffraction

The adsorption of Li on Al(100) has been studied by high-resolution core-level spectroscopy and low energy electron diffraction (LEED) at different Li coverages and temperatures. Adsorption at room temperature leads to the formation of a c(2×2)-Li phase which grows as islands in the coverage range 0.15–0.5 monolayer (ML). Analysis of Li1s and Al2p core-level spectra leads to the conclusion that the c(2×2)-Li phase contains Li atoms in a single environment and Al atoms in three different local environments. This is consistent with the results of a recent LEED analysis, which reported that the c(2×2)-Li phase is a surface binary alloy in which every second Al atom in the first layer is substituted by Li. Al2p core-level spectra taken after adsorption at 120K followed by annealing indicate that reconstruction of the Al substrate begins to occur at temperatures as low as 140K. At coverages between 0.5 and 1.0ML, Li1s spectra indicate the growth of a second layer of adsorbed Li. It is shown that the c(2×2)-2Li phase formed by adsorption of 1ML Li contains two mixed Al/Li layers separated by an Al layer, in a geometry similar to that of the first three layers of the (100) surface of the metastable Al3Li bulk alloy.

Ge/Bi/Si(001)-c(4×2) interface studied by high-resolution core-level spectroscopy

The adsorption of germanium on the Bi/Si(001)-(2×5) surface [bismuth saturation coverage of 0.7 ML (monolayers)] was investigated by core-level photoemission spectroscopy. At submonolayer germanium coverage, the presence of a bulk component largely dominating the Ge 3d core level demonstrates that the favourite adsorption sites are the bismuth-terminated terraces, where Ge–Bi site exchange takes place. At larger germanium coverage, in addition to the previous process, the occurrence of a pure germanium epitaxy is indicated by the presence of the (2×1) surface components in the Ge 3d core level.

Surface reconstructions of 3C-SiC(001) studied by high-resolution core-level photoemission

The different surface reconstructions of the 3C-SiC(001) surface have been systematically studied by high-resolution core-level photoelectron spectroscopy. For the Si-rich 3×2 surface, three different Si 2p surface components are clearly identified supporting the structure model with 2 3 ML of Si ad-dimers. It is confirmed that the c(2×2) and 2×1 surfaces are respectively C- and Si-terminated by observing a single dominant surface component in the C 1s or Si 2p spectra with binding energy shifts of 1.04 eV and −0.50 eV respectively relative to the bulk component. The atomic origins of these surface core levels are assigned and the core-level shifts are compared with previous measurements and recent theoretical calculations.

Adsorption sites in O and CO coadsorption phases on Rh(111) investigated by high-resolution core-level photoemission

High-resolution core-level spectroscopy is used in combination with low-energy electron diffraction (LEED) and photoelectron diffraction to identify the adsorption sites for three different coadsorbed phases consisting of ordered overlayers of oxygen coadsorbed with CO on the Rh(111) single-crystal surface. The three ordered overlayer structures, which may be denoted as 2O+CO/Rh(111), O+CO/Rh(111) and O+2CO/Rh(111), all show (2×2) LEED patterns. In the 2O+CO and O+CO phases the CO molecules are found to occupy only on-top sites while the O+2CO phase shows CO molecules in both on-top and three-fold hollow sites. In all cases the oxygen atoms are found in three-fold hollow sites. For the O+CO and O+2CO phases our results confirm previous determinations by LEED, while the 2O+CO phase has not been observed before on Rh(111). The core-level binding energies of the C 1s and O 1s core levels for both adsorbates are characteristic of the adsorption site and are very close to the binding energies found for the pure cases of only oxygen or CO adsorbed on Rh(111). In the coadsorption phases we find that the interaction between the adsorbates has only a minor influence on the core-level binding energies. For the O+2CO/Rh(111) coadsorption phase we find that a full CO coverage is not obtained; less than 80% of the unit cells contain two CO molecules, in line with previous findings.

Thermal reactions of phosphine with Si(100): a combined photoemission and scanning-tunneling-microscopy study

This study investigates the adsorption and thermal decomposition of phosphine (PH 3) on the Si(100)-(2×1) surface. The adsorption species, dissociation reactions, atomic ordering, and surface morphology of the phosphine/Si(100) surface at temperatures between 300 and 1060 K are examined by scanning tunneling microscopy (STM) and high-resolution core-level photoemission spectroscopy employing synchrotron radiation. The P 2p core level spectra clearly indicate that phosphine molecularly adsorbs at room temperature and partially dissociates into PH 2 and H on a time scale of minutes at low (<0.2 ML) coverages. An exposure of >15 Langmuirs (L, 1 Langmuir=10 −6 Torr s −1) of phosphine on the Si(100)-(2×1) surface at room temperature produces a saturated and disordered surface. The total amount of P on the saturated surface is ∼0.37 ML as calibrated by the P 2p photoemission intensity. Successive annealing of the saturated surface at higher temperatures converts PH 3 into PH 2, converts PH 2 to P–P dimers, and causes the desorption of PH 3. These processes become complete at ∼700 K, and the resulting surface is a H/Si(100)-(2×1) surface interspersed with one-dimensional P–P islands. Desorption of hydrogen from that surface occurs at ∼800 K, and is accompanied by partial displacement of P with Si atoms on the substrate. At 850 K, the Si(100) surface, interspersed with 0.22 ML of two-dimensional islands, is a random alloy of nominal 0.5 ML Si–P heterodimers and 0.5 ML Si–Si dimers.

Chemical bonding and electronic properties of SeS2-treated GaAs(100)

SeS 2 -passivated n-type GaAs (100) surfaces, formed by treatment of GaAs in SeS2:CS2 solution at room temperature, were studied with high-resolution core-level photoemission spectroscopy excited with synchrotron radiation source. The SeS2-treated surface consists of a chemically stratified structure of several atomic layers thickness. Arsenic-based sulfides and selenides reside in the outermost surface layer while gallium-based selenides are adjacent to the bulk GaAs substrate. The shift of the surface Fermi level within the band gap was monitored during controlled thermal annealing, allowing for the identification of the specific chemical entities responsible for the reduction in surface band bending. Arsenic-based species are removed at low annealing temperature with little shift of the Fermi level. Gallium-based selenides are shown to be associated with the unpinning of the surface Fermi level.

Core-level and valence band photoemission study of perovskite oxide powders synthesized by mechanically and thermally activated solid-state reaction

High-resolution core-level and valence band x-ray photoemission spectroscopy measurements were performed on perovskite oxide powders synthesized for applications in solid-oxide fuel cells by high-temperature solid-state reaction (x = 0.3 and 0.19) and by room-temperature mechanical activation of the precursors (x = 0.3). A structure in the valence band at about 1 eV below the Fermi level was clearly observed and assigned to the emission from the Mn 3d-derived states, thereby allowing the extraction of information about correlation effects in this type of material. Both the core-level and valence band spectral features were found to be independent of the choice of synthesis route. This finding indicates that mechanical activation, due to its lower synthesis temperature, can represent a valid alternative method of synthesis allowing a better control of the microstructure.

Adsorption of acetylene and hydrogen on Pd(111): formation of a well-ordered ethylidyne overlayer

The C 2H 2(+H)/Pd(111) system has been studied using high resolution core level spectroscopy and low energy electron diffraction (LEED). Two ordered phases were found when adsorbing C 2H 2 at 125 K, giving rise to a (2×2) and a ( 3 × 3 ) R30° LEED pattern, respectively. When preadsorbing H at 110–150 K, the sticking probability of the C 2H 2 molecules is strongly reduced and only the ( 3 × 3 ) R30° overlayer is formed. In all three of the ordered structures the C 2H 2 molecules are suggested to occupy the hollow sites. Heating of the ( 3 × 3 ) R30° C 2H 2+H layer to 350 K yields a well ordered ( 3 × 3 ) R30° overlayer consisting exclusively of ethylidyne (CCH 3) positioned in the hollow sites. The formation of ethylidyne is furthermore found to be preceded by the formation of an intermediate species (“ β”) which probably is vinylidene.

Sb/Si(110)2×3surface studied by high-resolution Si2pcore-level photoemission spectroscopy

A $\mathrm{S}\mathrm{b}/\mathrm{S}\mathrm{i}(110)2\ifmmode\times\else\texttimes\fi{}3$ surface has been studied by high-resolution core-level spectroscopy. All the components of the Si $2p$ core level related to the clean surface have disappeared and one strong Sb-induced component could be identified at a binding energy +0.24 eV with respect to the bulk peak. Such component is suggested to be mainly due to the charge transfer between Si and Sb atoms at the top layer. A small component, probably due to local disorder, is present on the high kinetic energy side at -0.12 eV. The Sb $4d$ core-level spectrum is well represented by one doublet, characteristic of Sb atoms adsorbed in a unique environment.