Core-level Line Research Articles

Atomic geometry and electronic properties of the Ge(1 1 1)2 × 1-Sb surface studied by scanning tunneling microscopy/spectroscopy and core-level photoemission

By scanning tunneling microscopy and spectroscopy (STM/S) and high-resolution core-level photoemission using synchrotron radiation, we have investigated the atomic structure and electronic properties of Sb-induced 2 × 1 reconstruction on Ge(1 1 1). Our results support well the zigzag-chain model proposed for this phase in the literature; in particular, the STM images visualize the Sb zigzag (Seiwatz) chain in a real space, and the STS I– V spectrum suggests this surface to be semiconducting, in good agreement with the atomic configuration proposed. However, a closer inspection of the STM results does not support the buckling of Sb chains reported in earlier studies. Moreover, the analysis of the Sb 4d core-level line shape of the (2 × 1) reconstruction shows that the bonding state of the Sb atoms is very similar, suggesting an unbuckled Seiwatz chain. In addition, the Ge 3d core-level emission reveals only one component, giving evidence for the ideal bulk-terminated structure of the Ge substrate.

UO2 (111) Single Crystal: Comparison of Stoichiometric and Defective Surfaces by XPS

This work undertakes a photoelectron study of the surface of a uranium dioxide(111) single crystal. The formation of a defected surface with argon ions sputtering can be identified as an n-type semiconductor, UO2-x phase. Annealing to high temperatures under an oxygen pressure of ∼5 × 10−6 Torr for 30 min or more yielded the hyper-stoichiometric p-type semiconductor, UO2+x phase. A shift in the core level line of U4+ ions is observed and attributed to a shift in the Fermi level from hyper-stoichiometric UO2+x to sub-stoichiometric UO2-x.

Bonding of metal-free phthalocyanine to TiO 2(1 1 0) single crystal

The metal-free phthalocyanine interface formation on rutile TiO 2(1 1 0) is investigated using scanning tunneling microscopy and photoelectron spectroscopy. The molecules are adsorbed flat on the surface, centered on the substrate oxygen rows. High-resolution core-level C1s spectroscopy indicates a strong difference between the second layer and the first monolayer bonding to the surface. The C1s core-level from the second layer has a bulk-like line shape whereas the first layer shows a strongly modified line profile. Upon thermal treatment, changes in the N1s core level line profile points to dehydrogenation of the center of the molecule.

Cu Pc : C 60 blend film: A photoemission investigation

The electronic structure of thin blend films composed of copper phthalocyanine (CuPc) and fullerene (C60) have been studied by x ray, soft-x-ray photoemission spectroscopy, and by theoretical simulations. Samples with different C60∕CuPc concentration ratios were prepared by thermal evaporation in ultrahigh vacuum conditions. A strong shift of the highest occupied molecular orbital (HOMO) of CuPc towards the Fermi level and of the HOMO C60 states towards higher binding energy was observed. The comparison between valence band structures and core level line shape indicates that the central part of the CuPc molecule is lightly modified by the presence of the C60 molecule. The theoretical analysis, performed using density functional theory, confirms this small interaction between the two molecules, with a small charge density localized between the two molecules and a structural modification of the CuPc molecule.

A comparative XPS and UPS study of VO x layers on mineral TiO2 (001)-anatase supports

Four vanadium oxide layers on mineral TiO2(001)-anatase supports with different thickness (3–33 Å) were prepared with reactive d.c. magnetron sputtering and were extensively studied with photoelectron spectroscopy. Al Kα radiation and 150 eV synchrotron radiation were used as excitation sources. The evolution of the 2p, 3s and 3p core level line shapes of V and Ti as a function of the vanadium oxide thickness was studied, as well as the O1s and O2s core lines and the valence band. All the V2p spectra of the deposited vanadium oxide layers consist of at least 60% V5+, the rest being V4+. The V3p region is complicated by multiplet splitting, which prevents the determination of the vanadium oxidation state. The V3p multiplet splitting is different for the two excitation energies. No reduction of the titania support surface due to the vanadium oxide deposition was observed. Copyright © 2006 John Wiley & Sons, Ltd.

Spectroscopic study of plasmons in ion-irradiated single-walled carbon nanotubes

The electronic structure of single-walled carbon nanotubes was experimentally investigated using x-ray photoelectron spectroscopy, reflection electron energy-loss spectroscopy, and Auger electron spectroscopy. A shake-up satellite structure observed near the C 1s core-level lines in the x-ray photoelectron spectra at high binding energies in the range 284–330 eV due to excitation of π and π + σ plasmons was studied. The effect of irradiation by 1-keV argon ions on the shape of the spectra was analyzed. The shape of the C 1s satellite spectra was found to be sensitive to Ar+ irradiation in the electron energy loss range 10–40 eV corresponding to excitation of π + σ plasmons. Auger spectroscopy revealed the presence of argon on the surface of ion-irradiated samples. The argon content increased to ∼4 at. % with increasing irradiation dose. An analysis of the results obtained and their comparison with the data available in the literature led to a qualitative conclusion that the bond angles of the carbon atoms making up the walls of single-walled carbon nanotubes are distorted at sites exposed to Ar+ irradiation.

Characterization of Self-Assembled Organic Films Using Differential Charging in X-ray Photoelectron Spectroscopy

Differential charging is often regarded as a problem in X-ray photoelectron spectroscopic studies, especially for insulating or partially conducting samples. Application of a positive bias can reduce the effect of differential charging by attracting stray electrons from the system, thereby compensating for the electron loss. On the other hand, differential charging effect can be enhanced by the application of a negative bias to the sample during spectrum acquisition. The successful use of the differential charging technique to distinguish between multi- and monolayer organophosphonate films on oxide-covered silicon has been reported. A detailed description of this technique is now presented which shows how differential charging can be used as an important tool for the characterization of self-assembled films deposited on various surfaces. The dependence of this technique on the conductivity of the substrate has been investigated by studying the spectral behavior of the deposited films of phosphonic acid on conducting, semiconducting, and insulating samples (stainless steel, silicon, and glass). Application of either positive or negative dc electrical bias affects the carbon core-level (C1s) line shape and intensity, which is dependent on the atom's physical location above the surface.

Evidence of the Interaction of Evaporated Pt Nanoparticles with Variously Treated Surfaces of Highly Oriented Pyrolytic Graphite

The interactions of Pt nanoparticles, deposited by evaporation onto highly oriented pyrolytic graphite surfaces modified by kiloelectronvolt Ar+ beam treatment, have been studied by X-ray photoelectron spectroscopy core-level line shape analysis. The C1s and Pt4f7/2 peaks were each considered to be composed of one asymmetric peak, and changes in their asymmetry parameters were used to study their interfacial interactions. In addition to these changes, strong signal intensity changes with time were found for both the C1s and Pt4f peaks, indicating an initial crystalline orientational instability of the Pt nanoparticles, which is supported by time-dependent high-resolution electron microscopy studies at elevated temperatures.

Electronic structure of AgNbO 3 and NaNbO 3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO 3 and NaNbO 3 single crystals and ceramics allowed estimate their average composition as Ag 1.1Nb 0.9O 3 and Na 1.2Nb 0.9O 2.9. The valence bands of the AgNbO 3 compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO 3 compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO 3 suggests a stronger structural disorder in comparison with NaNbO 3 compound.

Probing structural disorder in (1−x)PbMg1/3Nb2/3O3–xPbTiO3 single crystals by x-ray photoelectron spectroscopy

The (1−x)PbMg1/3Nb2/3O3–xPbTiO3 ((1−x)PMN –xPT) singlecrystals with x equal to 0, 0.22, 0.28, 0.32, 0.37, 0.47, 0.69 and 1 were studied by x-ray diffraction andx-ray photoelectron spectroscopy. X-ray diffraction results reveal that with increase ofTi content the crystal symmetry changes from pseudo-cubic, characteristic forPbMg1/3Nb2/3O3 (PMN), to rhombohedral, then to monoclinic at the vicinity of the morphotropicphase boundary (MPB) region and finally to tetragonal, specific forPbTiO3 (PT).High-resolution core level spectra of the Pb 4f, O 1s, Nb 3d and Ti 2p states show that the bulk of the(1−x)PMN –xPT crystals,except PbTiO3 where Pb2+ and Pb4+ were detected, is characterized by only one chemical state for each element. However, in thecase of the Pb and O spectra, apart from the main lines attributed to the lattice states, thesatellite lines related to the surface states were also observed. The evolution of the latticecomponents of all core level spectra points to regular concentration dependence of the fullwidth at half maximum (FWHM). With the rise of the Ti content the FWHMs increaseand reach maxima for the MPB region, then decrease and achieve the smallest valuesfor the pure PT. This characteristic FWHM behaviour points to disorder as anorigin of the core level line broadening and it is discussed within the frameworkof the random field model. The FWHM of the Bragg’s reflections show similarconcentration behaviour which confirms the existence of the highest structural disorder in(1−x)PMN –xPT system near the MPB.

Splitting of core-level lines in photoemission spectra of transition metal compounds

The multiplet structure of core-electron binding energies in Cr 3s, Cr 3p and Cr 2p levels of CrCl3 and CrBr3 compounds has been investigated by X-ray photoelectron spectroscopy. The Cr 3s levels show a doublet splitting (about 4.3 eV) for the main emission in both halides. Satellites features are observed in Cr 3s, 3p and 2p levels at higher binding energies. In the Cr 3s spectrum, the main emission is assigned to unscreened intra-atomic multiplet splittings with correlation-induced satellites. The Cr 3p and 2p spectra can be better explained by the multiplet splitting arising from the interaction between valence band 3d electrons and core p holes in the crystal field. Final state screening (charge-transfer) effects do not play a major role in Cr 3s main emission nor do they affect the satellite structures in a relevant way. This explains why the Cr 3s exchange splitting in chromium halides is proportional to the local magnetic moment.

Calcium fluoride on Si(001): Adsorption mechanisms and epitaxial growth modes

Growth of $\mathrm{Ca}{\mathrm{F}}_{2}$ on Si(001) is studied as a function of the substrate temperature during deposition for coverages from fraction of a monolayer (ML) up to several monolayers. Structural and morphological studies using atomic force microscopy, low-energy electron diffraction, and reflection high-energy electron diffraction are combined with measurements of core-level photoemission and x-ray absorption. Bonding between $\mathrm{Ca}{\mathrm{F}}_{2}$ molecules and Si(001) substrates is followed by monitoring core-level shifts and x-ray absorption line shape. It is found that a dissociative reaction occurs at high deposition temperatures $(\ensuremath{\sim}750\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, giving rise to a 1-ML-thick uniform wetting layer, which is bonded with the substrate through Ca atoms. This wetting layer changes the surface periodicity from double domain $2\ifmmode\times\else\texttimes\fi{}1+1\ifmmode\times\else\texttimes\fi{}2$ to single domain $3\ifmmode\times\else\texttimes\fi{}1$. Three-dimensional $\mathrm{Ca}{\mathrm{F}}_{2}$ elongated islands develop on top of the wetting layer, with their (110) planes parallel to the Si surface plane. At temperatures below $600\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ no dissociative reaction takes place for $\mathrm{Ca}{\mathrm{F}}_{2}$; nanodimensional islands develop in the form of rectangular-based huts. The crystallographic orientation of these islands is parallel to that of the Si(001) substrate. The data are compared to results obtained on $\mathrm{Ca}{\mathrm{F}}_{2}$ deposited on Si(111).

The origin of the Auger-photoelectron coincidence spectroscopy (APECS) spectral line broadening of AgPd alloy

Recently, Jiang et al. [Jiang et al., J. Electron Spectrosc. Relat. Phenom. 130 (2003) 33] concluded that when the energy fixed photoelectron analyzer is positioned below the center photoelectron kinetic energy, the extra broadening of Ag core-level photoelectron line of disordered Ag X Pd X ( X = 0.5) alloy by the disorder broadening manifests as the Ag coincidence Auger-electron spectroscopy (AES) line broadening on the high kinetic energy side, compared to when the analyzer is positioned on the center kinetic energy. We show that the change in the coincidence AES lineshape by a different positioning of the energy fixed photoelectron analyzer is not due to the disorder broadening. When the electron analyzer is positioned below the center photoelectron kinetic energy, the lineshape change is due to the decay of the core hole left by the photoelectron which suffers from the inelastic scattering before it goes out of a solid. The inelastically scattered photoelectrons are also detected when the electron analyzer is positioned below the center photoelectron kinetic energy. The change in the relative intensity of the inelastic AES line by a different positioning of the analyzer supports the present explanation.

Bulk and surface charge states ofK3C60

We detect a significant angle-dependence in the core level and valence line shapes of photoelectron spectra of single crystal K3C60. This allows the identification of bulk and surface components in the data, and allows us to explain the anomalous line shapes observed for this system. The states near the Fermi level are associated with the bulk of the sample. There is strong evidence of an insulating surface layer, which we ascribe to intermolecular electron correlations. These results simplify the interpretation of previous, apparently conflicting observations.

Photoemission measurements of transition-metal oxides under laser illumination

The effects of photoexcitation on Sr 14−xCa xCu 24O 41 and La 2−2xSr 1+2xMn 2O 7 single crystals were measured via X-ray photoemission spectroscopy. Changes observed in the Sr 14−xCa xCu 24O 41 Cu 2p core-level line shape indicate that the cleaved surface is surprisingly unstable under vacuum and that changes in the surface stoichiometry are easily obtained with laser ablation. In La 2−2xSr 1+2xMn 2O 7, the O 1s core-level peak binding energy irreversibly increases upon optical excitation. The magnitude of the increase depends on the sample hole doping and is observed to be largest in a x = 0.5 sample, where phase competition between electronic ordered states is strongest. It appears that the photoinduced electronic structural change is permanent due to stabilization by strong electron-lattice coupling.

Titanium dioxide surface stoichiometry and ordering studied by resonant photoemission spectroscopy

The electronic structure of titanium dioxide surfaces having undergone different preparations leading to different stoichiometries and crystallinities has been studied using resonant photoemission spectroscopy. Valence band photoemission spectra through the Ti 3p–3d/4s absorption edge between 45 and 55 eV were measured and allowed a characterization of defects present at the surface as well as of the quality of the surface organization. Indeed, from the comparison of the resonance results obtained for each kind of surface with the LEED patterns on the one hand and the corresponding Ti 2p core level lines on the other hand, it was evidenced that the high binding energy part of the valence band seems rather related to the ordering of the surface than to the amount of defects.

Magnetic and XPS studies on TbNi 5–x Al x system

Magnetic measurements were performed on TbNi5–xAlx system, in the temperature range 1.7–300 K. At T = 1.7 K the saturation magnetizations are smaller than gJJ value of Tb. The nickel moments at 0 K, determined by band structure calculations are antiparallely oriented to Tb one and decrease in magnitude, being essentially null for TbNi3Al2 sample. The effective nickel moments vary from 2.10 μB (x = 0) to ≅ 1.10 μB (x = 2). The XPS studies show that the Ni2p core level lines are similar to those of pure Ni, although the intensities of Ni 6 eV satellite decrease. The valence band spectra suggest an increase of the Ni3d–Al3p hybridization, when increasing Al content, in agreement with band structure calculations. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure of UC x films prepared by sputter co-deposition

Thin layers of UC x ( x=0–12) have been prepared by sputter co-deposition of uranium and carbon in an Ar atmosphere. The films were investigated in-situ by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively). Special interest was put on the evolution of the electronic structure with the composition of the films, as deduced from the U-4f, C-1s and valence region spectra. With increasing carbon content, three types of carbon species were detected according to C-1s core level line, at 282, 282.6 and 284.5 eV binding energy (BE). They are attributed to the UC, UC 2 and graphite phases, respectively. The U-4f core levels do not change strongly with increasing carbon content, showing well-itinerant U-5f electrons. Similarly, valence region spectra show three types of carbon species for different UC x films, which are differentiated by their C-2p signals. A strong hybridisation between C-2p and U-5f states is detected in UC, while the C-2p signal in UC 2 appears only weakly hybridised, and for higher carbon contents a π band characteristic of graphite appears.

Photoelectron core levels for enargite, Cu3AsS4

AbstractThe mineral chemistry of enargite surfaces and near‐surfaces prepared by fracture within an ultrahigh vacuum are investigated using XPS and synchrotron radiation XPS (SRXPS). The purpose of the study is to identify surface core‐level line positions in high‐resolution photoelectron spectra. The XPS spectra obtained using monochromatic Al Kα radiation show that there are near‐identical monosulphide line positions in S 2p spectra for the As–S and Cu–S bonds in enargite. The SRXPS spectra for sulphur are remarkably similar to the XPS spectra for sulphur, showing that there is minimal difference in the chemical environment between sulphur atoms at the surface and in the mineral matrix. The Cu 2p XPS data have only cuprous contributions and a minor surface contribution. The surface Cu contribution is observed as a high‐binding‐energy tail in the Cu 2p3/2 spectrum. The As 3d data for both XPS and SRXPS show contributions from arsenic atoms at the surface and in the bulk mineral matrix. The surface contribution is distinct and is found 1 eV below the bulk contribution. The results of the study suggest that, following fracture, the enargite surface is reorganized in such a manner that the surface is characterized by protrusions of individual arsenic atoms. Copyright © 2004 John Wiley & Sons, Ltd.

Minimum XPS core-level line widths of insulators, including silicate minerals

A monochromatic AlKα source and magnetic confinement charge compensation yield an As 3d line width of 0.52 eV for the bulk insulator As 2 S 3 (orpiment) which is as narrow as the As 3d line width obtained for the semiconductor FeAsS (arsenopyrite). These results, and the XPS results for silicates, demonstrate that charge broadening of XPS spectra has been overcome for most insulators provided care is taken with sample preparation. XPS core level spectral investigations of these minerals now may be conducted at the same level of detail as have been conducted on semi-conductors and metals. The Si 2p spectrum of a non-conducting orthosilicate (Mg-rich olivine) displays a total line width of 1.36 eV compared to very high resolution synchrotron XPS line widths of 1.1 eV for an Si 2p spectrum of gaseous Si(OCH 3 ) 4 , and 1.42 eV for the Si 2p peak of non-charging SiO 2 thin-films grown on Si metal. These results demonstrate that Si 2p peak broadening of these silicates does not result from differential charging. Comparison of the Si 2p spectrum of Si in olivine which contains non-polymerized, independent SiO 4 tetrahedra, with that of a gaseous analogue, Si(OCH 3 ) 4 , also composed of non-polymerized Si atoms tetrahedrally bonded to O, provides insight into the cause of Si 2p peak broadening for olivine (and probably for all other silicates). The vibrational contributions to the Si 2p spectrum of Si(OCH 3 ) 4 were used to fit the Si 2p peak of olivine, with each vibrational contribution broadened to reflect the minimum line width of the XPS instrument. Although the Si 2p envelope resulting from this procedure yields a reasonable fit to the olivine Si 2p spectrum, additional minor vibrational contributions may be required to reproduce accurately the Si 2p spectrum of olivine. Continued development of the technique should allow determination of transition metal oxidation states and other chemical state properties of non-conductor Al-silicates