Binding Energy Region Research Articles

Electronic structure of cluster assembled nanostructured TiO2 by resonant photoemission at the Ti L2,3 edge

The electronic structure of cluster assembled nanostructured TiO(2) thin films has been investigated by resonant photoemission experiments with photon energies across the Ti L(2,3) edge. The samples were produced by supersonic cluster beam deposition with a pulsed microplasma cluster source. The valence band shows resonance enhancements in the binding energy region between 4 and 8 eV, populated by O 2p and hybridized Ti 3d states, and in the region about 1 eV below the Fermi level associated with defects related Ti 3d states. The data show that in as-deposited films Ti atoms are mainly fully (sixfolds) coordinated to oxygen atoms in octahedral symmetry and only a small fraction is in a broken symmetry environment. Since resonant photoemission is closely linked to the local electronic and structural configurations around the Ti atom, it is possible to correlate the resonant photoemission intensity and lineshape with the presence of defects of the films and with the degree of hybridization between the titanium and oxygen atoms.

Barrier-free proton transfer in the valence anion of 2′-deoxyadenosine-5′-monophosphate. II. A computational study

The propensity of four representative conformations of 2(')-deoxyadenosine-5(')-monophosphate (5(')-dAMPH) to bind an excess electron has been studied at the B3LYP6-31++G(d,p) level. While isolated canonical adenine does not support stable valence anions in the gas phase, all considered neutral conformations of 5(')-dAMPH form adiabatically stable anions. The type of an anionic 5(')-dAMPH state, i.e., the valence, dipole bound, or mixed (valence/dipole bound), depends on the internal hydrogen bond(s) pattern exhibited by a particular tautomer. The most stable anion results from an electron attachment to the neutral syn-south conformer. The formation of this anion is associated with a barrier-free proton transfer triggered by electron attachment and the internal rotation around the C4(')-C5(') bond. The adiabatic electron affinity of the a_south-syn anion is 1.19 eV, while its vertical detachment energy is 1.89 eV. Our results are compared with the photoelectron spectrum (PES) of 5(')-dAMPH(-) measured recently by Stokes et al., [J. Chem. Phys. 128, 044314 (2008)]. The computational VDE obtained for the most stable anionic structure matches well with the experimental electron binding energy region of maximum intensity. A further understanding of DNA damage might require experimental and computational studies on the systems in which purine nucleotides are engaged in hydrogen bonding.

Study of the dissociation of nitrous oxide following resonant excitation of the nitrogen and oxygen K-shells

A photochemistry study on nitrous oxide making use of site-selective excitation of terminal nitrogen, central nitrogen, and oxygen 1s-->3pi excitations is presented. The resonant Auger decay which takes place following excitation can lead to dissociation of the N2O+ ion. To elucidate the nuclear dynamics, energy-resolved Auger electrons were detected in coincidence with the ionic dissociation products, and a strong dependence of the fragmentation pathways on the core-hole site was observed in the binding energy region of the first satellite states. A description based on the molecular orbitals as well as the correlation between the thermodynamical thresholds of ion formation and the first electronic states of N2O+ has been used to qualitatively explain the observed fragmentation patterns.

Electronic Structure and Field Emission Properties of Double-Walled Carbon Nanotubes Synthesized by Hydrogen Arc Discharge

We have synthesized high-purity double-walled carbon nanotubes (DWCNTs) by an arc-discharge method in hydrogen ambient. The DWCNTs were synthesized using a mixture of Fe catalyst and FeS promoter. Without FeS promoter, we only obtained single-walled carbon nanotubes (SWCNTs). The synthesized DWCNTs had outer diameters in the range of 3.0−3.4 nm and an average interlayer distance of 0.38 nm between graphene layers. The FeS promoter played a key role for the DWCNT growth. The DWCNTs indicated high electronic density of states in the binding energy region between 3.88 and 13.23 eV below Fermi energy, indicating that the DWCNTs had alot of delocalized graphite σ and σ/π electrons. For field emission properties, the typical turn-on field of DWCNTs was about 3.0 V/μm at the emission current density of 0.1 μA/cm2, and the emission current density of DWCNTs was about 10 mA/cm2 at the applied field of 6.5 V/μm. It is considered that the higher current densities of DWCNTs were mainly attributed to the emitted deloc...

X-ray photoemission spectroscopy and Fourier transform infrared studies of electrochemical doping of copper phthalocyanine molecule in conducting polymer

A conducting polymer polythiophene (PT) film incorporated with copper phthalocyanine (CuPc), which was a highly functional molecule, was synthesized and characterized by Fourier transform infrared (FTIR) spectroscopy and x-ray photoemission spectroscopy (XPS). Electrochemical and casting processes were developed as a doping method of CuPc molecules in a PT film. In the XPS measurements of the CuPc doped PT, CuPc-origin peaks on the higher energy side appeared in the C 1s spectrum. The C 1s spectral profiles between electrochemically doped and cast samples were different in the higher binding energy region. S 2p and N 1s core-level spectra of CuPc doped compounds also showed the difference of the spectral profile between electrochemically doped and cast samples, which suggested the dopant-polymer interaction. A split of Cu 2p spectra indicates the charge transfer between CuPc and the polymer chains. In the FTIR measurements, characteristic vibrational modes to CuPc, in-plane, out-of-plane, and ring stretching modes were observed. A central metal-origin peak was also observed. New peaks at 1545 and 1680cm−1 suggest the polymer-PcCu linkage.

Etching characteristics and plasma-induced damage of Ba 0.65Sr 0.35TiO 3 thin films etched in CF 4/Ar/O 2 plasma

Sol–gel-derived Ba 0.65Sr 0.35TiO 3 (BST) thin films were etched in CF 4/Ar/O 2 plasma using magnetically enhanced reactive ion etching technology. The maximum etch rate of BST film is 8.47 nm/min when CF 4/Ar/O 2 gas mixing ratio is equal to 9/36/5. X-ray photoelectron spectroscopy analysis indicates the accumulation of fluorine-containing by-products on the etched surface due to their poor volatility, resulting in (Ba,Sr)-rich and (Ti,O)-deficient etched surface. Compared to the unetched counterparts, the etched Ba 3d 5/2, Ba 3d 3/2, Sr 3d 5/2, Sr 3d 3/2, Ti 2p 3/2, Ti 2p 1/2 and O 1s photoelectron peaks shift towards higher binding energy regions by amounts of 1.31, 1.30, 0.60, 0.79, 0.09, 0.46 and 0.50 eV, respectively. X-ray diffraction (XRD) analysis reveals that intensities of the etched BST (1 0 0), (1 1 0), (2 0 0) and (2 1 1) peaks are lowered and broadened. Raman spectra confirm that the Raman peaks of the etched film shift towards lower wave number regions with the values of 7, 6, 4 and 4 cm −1, and the corresponding phonon lifetimes are longer than those of the unetched film because of the plasma-induced damage. When the etched films are postannealed at 650 °C for 20 min under an O 2 ambience, the chemical shifts of Ba 3d, Sr 3d, Ti 2p and O 1s peaks, the variations for atomic concentrations of Ba, Sr, Ti and O, and the Raman redshifts are reduced, while the corresponding XRD peak intensities increase. It is conceivable that the plasma-induced damage of the etched film could be partially recovered during the postannealing process.

Effect of oxygen gas and annealing treatment for magnetically enhanced reactive ion etched (Ba0.65,Sr0.35)TiO3 thin films

Sol-gel-derived (Ba0.65,Sr0.35)TiO3 (BST) thin films were etched in CF4∕Ar and CF4∕Ar∕O2 plasmas using magnetically enhanced reactive ion etching technology. Experimental results show that adding appropriate O2 to CF4∕Ar can better the etching effects of BST films for the increase of etching rate and decrease of etched residues. The maximum etching rate is 8.47nm∕min when CF4∕Ar∕O2 gas-mixing ratio is equal to 9∕36∕5. X-ray photoelectron spectroscopy (XPS) data confirm accumulation of reaction products on the etched surface due to low volatility of reaction products such as Ba and Sr fluorides, and these residues could be removed by annealing treatment. The exact peak positions and chemical shifts of the interested elements were deduced by fitting XPS narrow-scan spectra with symmetrical Gaussian-Lorentzian product function for Ba 3d, Sr 3d, and O 1s peaks, meanwhile asymmetrical Gaussian-Lorentzian sum function was used to fit Ti 2p doublet to adjust the multiple splitting and/or shake-up process of transition-metal Ti cations. Compared to the unetched counterparts, the etched Ba 3d5∕2, Ba 3d3∕2, Sr 3d5∕2, Sr 3d3∕2, Ti 2p3∕2, Ti 2p1∕2, and O 1s peaks shift towards higher binding energy regions by amounts of 1.31, 1.30, 0.60, 0.79, 0.09, 0.46, and 0.50eV, respectively. While the etched Ti 2p3∕2 and Ti 2p1∕2 peaks have small chemical shifts for two reasons. One is that Ti fluoride (TiFz) is mostly removed from the etched surface because of its higher volatility in the process of thermal desorption. The other is that there is a shift compensation between TiFz and the etched BST matrix in which Ti4+ cations are partially reduced to form Tix+ (0<x<4) cations in the presence of adequate oxygen vacancies. The simulated formula of each BST surface is obtained to be (Ba0.65,Sr0.35)Ti0.97O2.86, (Ba0.70,Sr0.30)Ti0.24O1.39, and (Ba0.68,Sr0.32)Ti0.95O2.74, and then the average valence of Ti cations is estimated to be +3.84, +3.25, and +3.66 with respect to the electroneutrality principle, respectively. It is inferred that electrical properties of the etched BST film may be partially recovered by postannealed after etched.

Resonant photoemission spectroscopy study of electronic structure of V 2O 5

The electronic structure of V 2O 5 has been studied by resonant photoemission spectroscopy (RPES) and band structure calculation. The RPES data showed that the valence band (VB) of V 2O 5 is formed by strong hybridization of O2p and V3d states, the contribution from V3d state is predominant in the high binding energy (BE) region of the VB. The approximate charge distributions of O and V ions in V 2O 5 are calculated for the first time as V 2 3 + O 5 1.2 - from the RPES data.

Photofragmentation study of hexamethyldisiloxane following core ionization and direct double ionization

X-ray photoelectron spectroscopy and Auger spectroscopy studies of gas-phase hexamethyldisiloxane (HMDSO) are presented. The photodissociation of this molecule is studied using various experimental coincidence techniques. We compare the fragmentation pathways observed after core ionization followed by Auger decay and after valence double photoionization of the molecule. A strongly selective production of the doubly charged tetramethyldisiloxane ion is observed in the low binding-energy regions. Theoretical calculations are carried out to tentatively explain the stability of the produced dication.

Temperature-Dependent Photoelectron Spectroscopy of Methyl Benzoate Anions: Observation of Steric Effect in o-Methyl Benzoate

Temperature-dependent photoelectron spectra of benzoate anion (C6H5CO2(-)) and its three methyl-substituted isomers (o-, m-, p-CH3C6H4CO2(-)) have been obtained using a newly developed low-temperature photoelectron spectroscopy apparatus that features an electrospray source and a cryogenically controlled ion trap. Detachment channels due to removing electrons from the carboxylate group and benzene ring pi electrons were distinctly observed. Well-resolved vibrational structures were obtained in the lower binding energy region due to the OCO bending modes, except for o-CH3C6H4CO2(-), which yielded broad spectra even at the lowest ion trap temperature (18 K). Theoretical calculations revealed a large geometry change in the OCO angles between the anion and neutral ground states, consistent with the broad ground-state bands observed for all species. A strong steric effect was observed between the carboxylate and the methyl group in o-CH3C6H4CO2(-), such that the -CO2(-) group is pushed out of the plane of the benzene ring by approximately 25 degrees and its internal rotational barrier is significantly reduced. The low rotational barrier in o-CH3C6H4CO2(-), which makes it very difficult to be cooled vibrationally, and the strong coupling between the OCO bending and CO2 torsional modes yielded the broad PES spectra for this isomer. It is shown that there is no C-H...O hydrogen bond in o-CH3C6H4CO2(-), and the interaction between the carboxylate and methyl groups in this anion is found to be repulsive in nature.

An X-ray Spectral Study of the Electronic Structure of Uranyl Fluoride UO2F2

The fine structure of the X-ray photoelectron spectrum of low-energy electrons and O 4,5(U) X-ray emission spectrum of uranyl fluoride UO2F2 (at binding energies from 0 to 40 eV) was identified on the basis of the electronic structure calculations for the [(UO2)F6]4− cluster with D 6h point symmetry, simulating the nearest surrounding of uranium in UO2F2, by the relativistic X α discrete variation method. It was shown that the U5f electrons can directly participate in chemical bonding, and U6p electrons, in formation of not only inner valence but also outer valence molecular orbitals. The sequence of the inner valence molecular orbitals in the binding energy region from 12 to 40 eV was established, which is essential for development of a procedure for determining the uranium-ligand bond length in the axial direction and in the equatorial plane of the uranyl compounds, based on the characteristics of their X-ray photoelectron spectra.

UPS study of VUV-photodegradation of polytetrafluoroethylene (PTFE) ultrathin film by using synchrotron radiation

The VUV-photodegradation of polytetrafluoroethylene (PTFE) ultra-thin film was studied by ultraviolet photoelectron spectroscopy and quadrupole mass spectrometry. These results were compared with the previous photodegradation studies of the polyvinylidenefluoride (PVDF) and polyethylene (PE). Generation of new peak, π-band originated from the CC bond, was observed in the low binding energy region of the UPS spectra in both PVDF and PE during the photodegradation. In contrast, no new peak generation was observed in the UPS of the photodegraded PTFE. Mass spectrometry analysis also suggested that the CC bond generation is not a major mechanism in the VUV photodegradation of PTFE.

Chemical Composition of the Copper/Benzotriazole Interface in the Presence of Acetic Acid Vapour at Low Humidities

Studies were made of the chemical composition of the copper products formed at 43, 58 and 78% relative humidity (RH) in contaminated (10-ppm acetic acid vapour) and uncontaminated environments in the presence and absence of the volatile corrosion inhibitor benzotriazole (BTA) for 7 d at 30°C. Such studies were conducted using X-ray photoelectron spectroscopy (XPS). General XPS spectra over a wide binding energy region showed C and O peaks in addition to those corresponding to the different copper compounds. Copper specimens exposed to acetic acid vapour in the presence of BTA also exhibited N emissions. The main compounds formed were cuprite (Cu2O), copper hydroxide [Cu(OH)2], copper acetate and copper-benzotriazole (Cu-BTA) complexes.

Study of the oxidation for Si nanostructures using synchrotron radiation photoemission spectroscopy

We report the oxidation characteristics for Si nanostructures. Synchrotron radiation Si 1s X-ray photoemission spectroscopy (XPS) is used to explain the oxidation process and the modification of the electronic structures of Si. Si films of different thicknesses were grown on a graphite surface. Before oxidation, several photoemission components are observed in thin Si films, such as 2 or 4 Å films. The components observed at the higher binding energy regions are identified as Si nanostructures. After oxidation, photoemission results show that the oxidation process is thickness dependent. Quantitatively, in 2 and 4 Å films, the photoemission signals from oxide phase are 18 and 30%, respectively of total intensity. The present result suggests that the higher binding energy components, i.e., Si nanostructures play some significant role during the oxidation, where the reactivity of nanostructures towards oxygen is relatively low or negligible, and nanostructures impose some restrictions on the oxidation of the film.

Surface chemistry of Pu oxides

X-ray photoelectron spectroscopy was used to examine the surface chemistry of a variety of Pu(IV) compounds, including PuO 2 and Pu(OH) 4. The Pu 4f, O 1s and C 1s binding energy regions were line shape fit to unequivocally demonstrate that multiple species are present. Surface hydroxyls were ubiquitous in all PuO 2 samples exposed to H 2O vapor or ambient air, and persisted with heating to 590 °C. Active surface sites for the reaction of H 2O and other small molecules and spectral features consistent with pure stoichiometric PuO 2 can be regenerated by thermal energy or by effects of a radiation field. Evidence of higher valence Pu species was observed in some treated samples with the formula PuO 2+ x .

Electron energy loss spectroscopic investigation of Co metal, CoO, and Co 3O 4 before and after Ar + bombardment

X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (ELS) have been used to study a sputter-cleaned polycrystalline Co metal foil and CoO plus Co 3O 4 powders before and after ion bombardment. The ELS data have been collected using primary beam energies ranging from 100 to 1150 eV in order to distinguish between surface and bulk features and to facilitate determination of peak locations. It is shown that ELS spectra from these reference materials are considerably different and should, consequently, allow species identification in future catalyst characterization studies. The main surface features obtained from Co metal are observed at 3.7 and 7.0 eV, while the major bulk energy losses are located at 8.2, 9.8, 23.1, 25.7, and 28.1 eV. Despite the collected data on Co ELS and the electronic structure of Co metal, unambiguous peak assignments are difficult. Interband transitions in the energy loss region under investigation originate from the relatively localized Co 3d filled states, but the limited information about the empty states in the conduction band make specific assignments of the energy loss features in the Co ELS spectra challenging. XPS analysis of as-entered CoO and Co 3O 4 reveals that both samples consist mainly of Co 3O 4, in agreement with a Co 3O 4-terminated CoO surface. After prolonged Ar + sputtering both oxides are CoO-like according to high-resolution spectra of the Co 2p binding energy region. In agreement with the XPS data the ELS spectra obtained from as-entered CoO and Co 3O 4 are very similar. In contrast, ELS spectra obtained from CoO and Co 3O 4 after prolonged Ar + sputtering are considerably different. The ELS spectra obtained from CoO are not altered significantly with sputtering suggesting that the Co 3O 4 layer cannot be removed using Ar + bombardment. The sputtered Co 3O 4 instead gives rise to ELS spectra that appear CoO-like. The major ELS features in the spectra obtained from Co 3O 4 are located at 2.5, 6.7, 22.1, and 25.0 eV, while those obtained from Ar +-sputtered Co 3O 4 (CoO) are observed at 7.0, 8.6, 18.3, 21.9, and 36.1 eV. The relative intensities of the features at 6–8 eV and 22–25 eV are considerably different in the two sets of spectra. The intensity ratio of the peaks at 6.7 and 25.0 eV in Co 3O 4 is significantly higher than the 7.0 to 21.9 eV peak intensity ratio of CoO. Although several of the energy losses in the two spectra overlap and make identification difficult despite the large intensity difference, there are peaks that are specific for each compound, i.e. energy losses at 2.5 and 28.5 eV in Co 3O 4 and at 36.1 eV in CoO. Consequently, ELS can be used to differentiate the two cobalt oxides and information complementary to XPS data can be obtained from the ELS data.

XPS study of the Ln 5p,4f-electronic states of lanthanides in Ln 2O 3

The present work analyses the fine structure of the low binding energy ( E b, 0–50 eV) X-ray photoelectron spectra XPS of lanthanide (La through Lu excepted for Pm) oxides, and compares it with the non-relativistic X α-discrete variation calculation results for the clusters reflecting the close environment of lanthanides in oxides. The obtained results show that the Ln 4f n -electrons of lanthanides in oxides by their spectral parameters have much in common with the M 3d-electrons in oxides of the 3d-transition metals. According to these data, the Ln 4f shell of lanthanides is rather outer and can participate in the formation of molecular orbitals in compounds. The XPS data at least do not contradict the theoretical suggestion about the significant participation of the Ln 4f-electrons in formation of the molecular orbitals in the studied materials. The spectra in the Ln 5p–O 2s binding energy region of the studied lanthanide oxides were found to exhibit the complicated structure instead of separated peaks due to the electrons of the Ln 5p 3/2,5/2 and O 2s atomic shells. Taking into account the energy differences between the inner (Ln 3d) and outer (Ln 5p) electronic shells for some metallic lanthanides and their oxides, the Ln 5p atomic shells were shown to participate in the formation of the inner valence molecular orbitals (IVMO). That agrees qualitatively with the calculation results.

A study of the inner-valence ionization region in HCl and DCl

An in-depth photoionization study of the inner-valence electrons in HCl and DCl has been performed using synchrotron radiation. A series of photoelectron spectra of HCl were obtained at a resolution of 23 meV over the binding energy range 25–30.5 eV at various excitation energies and at two different electron collection angles relative to the plane of polarization of the undulator radiation. In addition, photoelectron spectra of DCl were recorded at two different excitation energies. These spectra were compared directly with the threshold photoelectron spectra of HCl and DCl that were recorded previously under similar resolution conditions (∼30 meV). This comparative study reveals new information on the nature of the numerous band systems observed in this binding energy region. In addition, we present the experimental confirmation of the theoretical prediction given by Andersson et al (2001 Phys. Rev. A 65 012705) that a vibrational progression showing interference structure would appear in the main inner-valence ionization band in the photoelectron spectrum of DCl at a resolution of 10 meV.

Electronic structure of a bismuth bilayer

Using angle-resolved photoemission spectroscopy, we identified four two-dimensional bands within a 1 eV binding-energy region below the Fermi level. The top two bands that are part of the complex Fermi surface of Bi (111) are located in the projected bulk band gap and exhibit sixfold rotational symmetry of the top Bi bilayer. The two lower-lying bands reside inside the projected bulk band structure. The threefold rotational symmetry of these bands indicates a weak interaction with the underlying bulk bands. Our data can be explained with a tight-binding calculation of a bilayer by eliminating the second nearest-neighbor interactions that determine the coupling between bilayers in bulk Bi.

Interaction of Oxygen with the Polar HfC(111) Surface: Angle-Resolved Photoemission Study

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the adsorption state of oxygen on a HfC(111) surface. Oxygen adsorbs dissociatively on the HfC(111) surface at room temperature forming a (1×1) overlayer at the saturation coverage. The two-dimensional band structure of the chemisorption bonding states formed through the hybridization of O 2p and Hf 5d orbitals (O 2p-derived states) is measured for the HfC(111)(1×1)-O surface. It is found that the 2pz-derived band is formed in slightly higher binding energy region relative to the 2px- and 2py-derived bands, indicating that the vertical interaction involving the 2pz orbital is slightly more important in the chemisorption bond formation relative to the lateral interaction involving the 2px and 2py orbitals. Hf 4f spectra are measured for the clean and oxygen-covered HfC(111) surfaces, and it is found that the oxidation of the surface Hf atoms to form a Hf oxide does not proceed at room temperature.