Lower Binding Energy Side Research Articles

Pulsed laser ultrahigh vacuum deposited silicon in the presence of excess cesium and oxygen studied with x-ray photoelectron spectroscopy and atomic force microscopy

Si/Cs/O clusters were prepared in an ultrahigh vacuum system with pulsed laser ablation of Si in the presence of Cs vapor and molecular oxygen. The effects of an O2 ambient, the pressure, and the deposition sequence of Cs and O2 on the oxidation state of cesium oxides and silicon oxides, as well as on the formation of Si/Cs/O clusters, were studied with x-ray photoelectron spectroscopy and atomic force microscopy (AFM). Oxygen 1s spectra were deconvoluted to cesium oxide(s) at around 530.5 eV and silicon oxide(s) at 532.3 eV. From the low binding energy side to the high binding energy side, Si 2p spectra were assigned to negatively charged Si clusters (94 and 96 eV), silicon suboxide (102 eV), SiO2 (104 eV), and Si/Cs/O clusters (106 eV), respectively. The high binding energy species was explained by the formation of Si(CsO1+x)n clusters. Surface morphologies were influenced by the oxidation state of cesium oxides and their amount. The work function decrease as much as 0.5±0.1 eV was measured with ex situ AFM.

Comparison of electronic structures of ScAl 3 and ScRh 3: X-ray photoelectron spectroscopy and ab initio band calculation

The electronic structure of ScAl 3 was compared with that of ScRh 3, where the electronegativity difference between the two elements in ScAl 3 is smaller than that in ScRh 3. The Al 2p and Sc 2p XPS for ScAl 3 have clear plasmon loss peaks as the Al 2p peak for the elemental Al, but Sc 3p spectrum for ScRh 3 has a featureless energy loss profile as the Sc 2p peak for the elemental Sc. The electronic structures in the valence and conduction band for ScAl 3 and ScRh 3 are calculated by ab initio band calculation method. The partial density of states of Sc 3d for ScAl 3 is higher than that for ScRh 3. The center of gravity of Sc 3d for ScAl 3 is located at lower binding energy side than that for ScRh 3. The electron density map for ScAl 3 shows that between Sc and Al atoms there is a ridge with two minimum points. On the other hand, Sc–Rh bond in ScRh 3 has one minimum point. It indicates that the bonds between Sc and Al atoms are highly covalent.

Surface XPS core-level shifts of FeS2

The XPS S 2p core-level spectra of pyrite FeS2 have been measured to study the surface core-level shifts (SCLSs). In addition to the previously found features on the low binding energy (BE) side of the line, evidence for a new structure on the high BE side is given. This feature seems to be surface sensitive. Also it is found that the final state effect may give a significant contribution to the SCLS.

Surface Electronic Structure of Nitric-oxide-treated Indium Tin Oxide

ABSTRACTThe surface electronic properties of the nitric oxide (NO) treated indium tin oxide (ITO) are examined in-situ by a four-point probe and X-ray photoelectron spectroscopy (XPS). The XPS N1s peak emerged at a high binding energy of 404 eV indicating that NO is reactive with ITO. NO adsorption induces an increase of film sheet resistance, arising from an oxygen rich layer near the ITO surface region, with approximately 2.5 nm thick. This implies that the interaction of NO with ITO is occurred around surface region. Valence band maximum measured for NO-absorbed ITO was shifted to the low binding energy side. This is related to the upward surface band bending.

TEMPERATURE EVOLUTION OF THE PHOTOEMISSION SPECTRA FOR THE Si(111) SURFACE USING THE LASER ANNEALING METHOD

We have studied the electronic structures in a wide temperature range for the Si(111) surface using photoemission spectroscopy combined with the laser annealing method. The temperature dependence of the Si 2p surface-sensitive core level photoemission spectra shows some gradual changes along with the thermal broadening above ~1063 K. In addition, the spectral change in the valence band photoemission spectra was also observed across the 7 × 7–1 × 1 transition temperature. These results indicate that the surface band structure is changed along with structural change at the 7 × 7–1 × 1 transition temperature. With increase of the temperature, the shift of the Si 2p core-level photoemission spectra to the lower binding energy side was observed. We discuss the temperature-induced effects such as the thermal broadening and the observed shift.

Photodetachment of F−(H2O)n (n=1–4): Observation of charge-transfer states [F−(H2O)n+] and the transition state of F+H2O hydrogen abstraction reaction

We report a photoelectron spectroscopic study of F−(H2O)n (n=1–4) at 193 and 157 nm. In addition to a detachment feature from F−, we observed a higher binding energy feature in all spectra due to ionization of the solvent molecules leading to F−(H2O)n+ charge-transfer states. Detachment of an electron from F− in F−(H2O) reaches the transition state region of the reaction, F+H2O→HF+OH. The bound to non-bound transition resulted in a long tail at the low binding energy side in the photoelectron spectrum of F−(H2O). We estimated that the neutral F⋯H2O complex at the anion geometry, which is near the transition state region of the H-abstraction reaction, is 6±1 kcal/mol above the F+H2O asymptote, consistent with a previous estimation of a 4 kcal/mol entrance barrier height for this reaction.

Reactivity of surface sites on fractured arsenopyrite (FeAsS) toward oxygen

The reactivity of arsenopyrite (FeAsS) fractured surfaces toward oxygen was studied using synchrotron radiation excited photoelectron spectroscopy (SXPS). The spectra of the pristine surface provide evidence of different As and S surface sites. Signals for As3d with 0.33 eV lower and 0.37 eV higher binding energy than the bulk signal are attributed to arsenic surface sites with filled and unfilled dangling bonds, respectively, caused by the rupture of Fe-As and As-S bonds. Sulfur surface sites with filled dangling bonds, bonded to three iron as well as to two iron and one arsenic atom, give rise to a composite signal in the S2p spectra shifted by 0.79 eV to lower binding energy. Reaction of oxygen with FeAsS surfaces in ultra-high vacuum reveals fast oxidation of As surface sites with filled dangling bonds to As species of increasing oxidation state. The detection of oxidation states As0, As2+, As3+, and As5+ indicate a consecutive reaction scheme for arsenic oxidation involving elementary one-electron transfer steps. The Fe3p spectra have a corresponding intensity increase of a component with a binding energy 1.1 eV higher than the Fe3p signal emitted from the pristine surface. This signal is assigned to Fe bonded to oxidized arsenic. The very small changes in the S2p spectra together with their decreased intensity indicate the formation of an arsenic and iron containing overlayer of oxidation products on top of the FeAsS mineral surface where the S2p signal arises from. In air oxidation of arsenic continues with As5+ being the final oxidation product. An additional Fe3p signal at 56.1 eV binding energy is attributed to Fe bonded to O atoms formed during Fe oxidation. Sulfur oxidation leads to numerous intermediate oxidation products with sulfate being the final product. During air oxidation of up to 30 min, the sulfur signal at the low binding energy side of the S2p spectrum is broadened which is probably caused by S2- formed in layers underneath As and Fe oxidation products. These oxidation products reach the surface by diffusion from the bulk (reaction induced segregation). A model of homogeneous oxidized layers on arsenopyrite indicates that reaction with air has produced a layer containing iron bonded to oxygen on top of the increasingly oxidized arsenic and iron containing layer. The Fe-O overlayer is about 1.8 monolayers thick and is probably formed through interaction of water with iron surface sites

Effectiveness of high energy ion beam techniques for the characterization of mesoporous low dielectric-constant materials

Highly porous silica films can potentially reduce power dissipation, cross talk, and interconnection delay in the deep submicron regime of semiconductor devices. We have synthesized “mesoporous” silica films using both cationic and non-ionic surfactants to template porosity in spin-on sol–gel silica. During this development we have effectively used high-energy ion beam techniques along with optical profilometry to characterize the porosity of these films. Rutherford backscattering spectrometry (RBS), and 16O(d, p 1) 17O nuclear reaction were used to determine the total number of Si and O atoms in the films. Interaction of these films with water was characterized by the 1H( 19F, αγ) 16O resonant nuclear reaction. Combination of these techniques provides fast, accurate, and quantitative methods for characterizing these films. However, the high-energy ion beams appear to cause significant damage in the films. X-ray photoelectron spectroscopy (XPS) measurements from the ion beam interacted region show a tail in the low binding energy side of the Si 2p core level spectrum which is characteristic to metal Si. In addition, craters as deep as 175 nm were left in the films where the ion beams interacted with the material.

Casted titanium for dental applications: an XPS and SEM study

Casted titanium for dental crowns has been investigated by means of X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The samples were prepared according to the Ohara process. XRD analysis revealed that the investment consisted essentially of SiO 2, Mg 2P 2O 7, SiO 2 · H 2O and Mg 2SiO 4. SEM experiments combined with EDX analysis revealed the following atomic concentrations Si (62.6%), Mg (19.7%), P (17.1%), and Ca (0.6%). After casting, the titanium samples exhibit a rough and irregular surface, and XRD analysis indicated that titanium contains essentially the α phase and traces of the β phase. After demoulding titanium samples, the investment side which was in contact with titanium was black colored, proving the reaction between titanium and the investment. XPS analysis performed on this side of the investment indicated that the Si 2p peak presents a shoulder on the low binding energy side and thus suggest that SiO 2 has been transformed into metallic Si.

Adsorption of Rb on Si(100)2 × 1 at room temperature studied with photoelectron spectroscopy

Photoelectron spectra of the Si and Rb core-levels have been analyzed for increasing Rb coverage. At the initial stage of adsorption, an abrupt decrease of the band bending at the surface of the n-type sample by ∼0.3 eV was observed, which is due to a partial occupation of an empty surface band existing already for the clean surface. A 2 × 3 low-energy electron diffraction (LEED) pattern was observed for coverages of about 1/3 of the saturation coverage. The Rb 4p and 3d spectra show two components for further Rb adsorption, which provide evidence for two Rb adsorption sites supporting the so-called double-layer model. At saturation coverage, the surface shows a 2 × 1 periodicity in LEED due to the underlying Si dimer reconstruction. In the Si 2p spectra a well resolved structure on the lower binding energy side with an energy shift of ∼0.48 eV is identified as due to Si dimers bonding to the Rb atoms. In the Rb core-level spectra two components, separated by ∼0.55 eV with approximately the same emission intensity, are well resolved in both 4p and 3d core-level spectra. A comparison to other alkali-metal-saturated 2 × 1 surfaces (Na, K and Cs) is also presented.

Temperature Characteristics Studies of Nanocrystallite SrTiO3 Grain Boundaries by X-Ray Photoelectron Spectroscopy

Strontium titanate (SrTiO3) nanocrystallite thin films have been studied by x-ray photoelectron spectroscopy at different temperature. The surface oxygen concentration and Ti/Sr ratio increase with temperature rising up to 700 K, and then decrease. New O 1s and Ti 3d peaks appear at the low binding energy side of the corresponding main peaks, these new peaks have some different features from the corresponding main peaks, this phenomenon was not observed in single crystal SrTiO3. A model is proposed to explain this interesting phenomenon.

Comparison of magnetic linear dichroism in 4f photoemission and 4d–4f photoemission from Gd on Y(0001)

Magnetic linear dichroism (MLD) in 4d–4f resonant and 4f nonresonant photoemission (PE) is studied from thin epitaxial gadolinium films. In an angle resolved and high-energy resolution mode, experiments were conducted with the electric-field vector of the incident light perpendicular to the sample magnetization. Our results show a significant difference in behavior of MLD in resonant PE as compared to that in nonresonant PE. Off-resonance, the MLD signal is dominated by a negative feature at the low binding energy side of the peak. Near the 4d–4f resonance maximum, the MLD displays a plus–minus shape, with a negative signal at the low binding energy side of the 4f peak and a positive signal at the high binding energy side. Analysis of MLD in 4d–4f resonant PE may provide insight into interactions of the 4d core hole with the 4f core level in the intermediate state.

Comparison Spectra of (111) Textured CeO2 Thin Films by XPS

CeO2 is a large band gap insulator (Eg = 6 eV) and oxygen ion conductor, which is of particular interest as a promoter oxide in three way catalytic converters for automotive exhaust gas purification and as a solid electrolyte in high temperature solid oxide fuel cells. Theoretical work on the photoemission of CeO2 and its lower oxides (Ce2O3 and intermediate phases) has mainly focused on an understanding of the complicated Ce 3d spectra. Of great practical interest is the evaluation of the oxygen stoichiometries from a detailed Ce 3d peak shape analysis. Except for the Ce 3d signals, only very little work has been published on the photoemission of CeO2. In this work photoemission spectra of in situ prepared, stoichiometric (111)-oriented CeO2 thin films (thickness: 40 nm) on clean, sputtered Pt-foil substrates are presented. Instabilities in the energy scale, spectral broadening due to inhomogeneous charging and reduction induced by electron bombardment are circumvented by the use of the conduction Pt substrates. Spectra are recorded at room temperature and sample charging occurs during XPS analysis. The Ce 3d spectra exhibit the typical Ce4+ signals of CeO2. Ce3+ associated emission is beyond the detection limit. The O 1s spectra show only one sharp peak due to lattice oxygen. Higher binding energy shoulders which might be expected in the presence of hydroxyls, carbonates or defect oxygen are not present. The latter are frequently discussed in work concerning high surface area powders relevant for catalysis. The valence band spectrum presented is consistent with literature data and shows at its low binding energy side the oxygen 2p-dominated valence band. Within the detection limit of valence band XPS Ce 4f associated emission is not detected in the valence band. This corresponds to Ce ions being in the +IV oxidation state. In order to fully characterize the photoemission spectra of CeO2 and to provide a full data set for comparison to more complex cerium-based materials, all minor photoemission peaks (Ce 4d, Ce 4p, Ce 5s, Ce 5p, O 2s) and x-ray excited Auger spectra of the O KLL and the Ce MNN lines have been recorded.

Alkali Rb-doped C60: The movement of the valence band (abstract)

Using synchrotron radiation light source, we have measured the photoemission spectra of undoped C60 and alkali Rb-doped C60 to reveal their electron structure. We observed a complex movement of the top of the valence band through the process of alkali Rb doping. When Rb is doped into C60, at first the spectrum moves to the high binding energy side for about 0.5 eV. As doping continues, the spectrum moves to the low binding energy side again. The spectrum moves back toward the original site by a small amount until the stoichiometry reaches about 1. This phenomenon has not been reported in other alkali doped C60 systems. We suggest the backward movement is due to the instability of the RbC60, Rb3C60, and other phases.

Theory assisted interpretation of copper phthalocyanine core levels XPS spectra

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 Cu3dx2−y2 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.

Chemical interaction at the Cu/PPA interface: an X-ray photoemission investigation

A coverage-dependent XPS investigation of the early stages of interaction between evaporated Cu and polyphenylacetylene (PPA) has been carried out. Copper films have been deposited at 300 K to a final thickness of about 5 monolayers. Overlayer and substrate core-level emission behaviour indicates the layer-by-layer growth of Cu on the PPA surface without interdiffusion. At low copper coverages a chemical interaction between the Cu adatoms and the substrate is observed by monitoring the changes occurring at the π å π∗ satellite feature of the C1s core-level line. We observe a decrease of intensity for this satellite as the copper coverage increases, interpreted in terms of the formation of a Cu-π complex with the phenyl ring. The Cu Auger spectra demonstrated the presence of positively charged metal atoms up to a coverage of ∼ 5 Å. In the submonolayer region the occurence of broadening of the C1s core level on the low binding energy side is observed together with a positive shift for the Cu2p 3 2 emission. Upon completion of the first copper monolayer these phenomena are removed. The observed behaviour can be accounted for in terms of differential charging with injection and trapping of charge into the PPA film from the Cu atoms that have not yet completed the first monolayer.

Aspects of 3p-3dResonant Photoemission in CeNi Single Crystal

In the valence band photoemission spectra of CeNi single crystals, we have observed a new satellite feature which appears at the lower binding energy side of the two-hole bound state satellite and shows resonance enhancement at the Ni 3 p threshold. On the basis of the observed excitation energy dependences of the spectral features, we investigate the origin and the resonance aspect of this new feature. We also investigate the origin of the other weak valence band satellites which appear at 5.3 eV and 10.6 eV.

Photoelectron spectra of a higher fullerene compound C 82 and its potassium complex

Photoelectron spectra of C 82, which has been reported to contain metal atoms inside, are measured with a synchroton radiation light source. The spectra resemble those of C 84. The spectra begin at 1.1 5 eV below the Fermi level (E f). The first band consists of two peaks located at 2.2 eV and 3.3 eV accompanied by a slight shoulder structure at the lower binding energy side. This shoulder is not observed in C 84. The intensity of the two peaks of the first band oscillates with the incident photon energy change, as are observed in C 60, C 70 and C 84. The second band locates between 4 and 6.2 eV with a peak at 5.3 eV and a shoulder at the lower binding energy side. The third band locates between 6.2 and 9 eV and consists of a peak at 7.8 eV and a distinct shoulder at 6.5 eV. Potassium dosing brings a new band between the E f and the first band and its intensity grows as the potassium dosage increases. The spectral onset approaches toward the E f in accordance with increase of potassium dosage but it does not cross the E f, which indicates a semiconductive nature of K xC 82.

X-ray photoelectron studies of oxygen states in metaloxyde ceramics for HTSC

Abstract Oxygen states have been studied in La2CuO4, La1.8Sr0.2CuO4-δ′ YBa2Cu3O7-δ′ EuBa2Cu3O7-δ′ T12Ba2CaCu2O8+Δ′, T12Ba2Ca2Cu3O10+Δ′ Bi2CaCu2O8+Δ′ Bi4Sr3 Ca3 Cu4O16+Δ,. by means of X-ray photoelectron spectroscopy and compared with those in oxides of the metals. It was found that the oxygen ions in the ceramics exist in several chemically nonequivalent states in which Ols electron binding energies differ within 1 eV. Binding energies Eb(Ols) for oxygen ions within Sr-O or Ba-O layers are higher a little than those in the oxides SrO or BaO respectively; Eb (Ols) for oxygen ions within La-O or T1-O or Bi-O layers are of approximately the same as those in the oxides La2O3 or T12O3 or Bij2O3 respectively; Eb(Ols) for oxygen nearby copper ions are lower than that in the oxide CuO. It was assumed that weighted average O2p level position for oxygens nearby copper was shifted to the lower binding energy side in the ceramics relative to the oxide CuO.

STM observation of (NH4)2Sx-treated GaAs(100) surface.

The surface structure of (NH4)2Sx-treated GaAs (100) was observed with a scanning tunneling microscope (STM) both in air and in vacuum. STM images obtained showed line shaped protrusions which run [110] direction and are 220 nm in width. For the sulfur treated n- and p-GaAs surfaces with 400°C-20 min heating, about 1 eV surface band bendings toward higher and lower binding energy side were observed, respectively. No band gap state appeared for either case.