Modeling the photoelectron spectra of the valence O2p-band of zinc oxide by the Xα-scattered wave method

The photoelectron spectra (PES) of the valence O2p-band of zinc oxide are modeled by Xα-scattered wave cluster calculations in a wide range of incident quantum energies hv (from 30 to 150 eV and 1253.6 eV). For the Zn₁₀O₁₀ cluster, the calculated intensities of PES reproduce well the specific features of the experimental spectra. It is shown that Zn3d-electrons participate in covalent binding of zinc and oxygen. The admixture of the Zn3d-states in the hybrid orbitals of the valence band is ≈7%. Deceased. Translated fromZhurnal Struktumoi Khimii, Vol. 38, No. 5, pp. 877–886, September-October, 1997.     

Structure of X-ray photoelectron, emission, and conversion spectra and molecular orbitals of uranium compounds

The fine structure of X-ray photoelectron spectra of uranium compounds in the range of electron binding energies from 0 to ∼50 eV is largely determined by the electrons of the outer and inner valence molecular orbitals arising from the valence atomic shells, including the U6p and Lns low-energy occupied atomic shells. This result is in agreement with the data of the electronic structure calculations of these compounds and confirmed by the nuclear electron (conversion) and X-ray emission spectroscopic investigations. It is shown that the fine structure of X-ray photoelectron spectra associated with the electrons of inner valence molecular orbitals makes it possible to judge the participation of the electrons of the occupied atomic shells in chemical bonding, the structure of the nearest environments of the atom, and the bond lengths in the compounds. The overall contribution of the electrons of these molecular orbitals to the absolute value of binding energy may prove to be comparable to the contribution of the electrons of the outer valence molecular orbitals to atomic bonding. This is a new and important fact in chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1037–1046, November–December, 1998.     

Structure of X-ray photoelectron and X-ray emission spectra of ThO2 and ThF4 due to electrons of molecular orbitals

The fine structure of the X-ray photoelectron and O_4,5(Th) X-ray emission spectra of the low-energy (0 …) ∼50eV) electrons of thorium in ThO₂ and ThF₄ is studied. It is established that both outer (0 … ∼15 eV) and inner (15… ∼50 eV) valence molecular orbitals, which are mostly due to the Th6p and O(F)2s shells of the neighboring thorium atoms and ligands, are formed in these compounds. Translated fromZhumal Struktumoi Khimii, Vol. 39, No. 6, pp. 1052–1058, November–December, 1998.     

Spectroelectrochemical characteristics of poly(3,4-ethylenedioxythiophene)/iron hexacyanoferrate film-modified electrodes

Spectroelectrochemical measurements of poly(3,4-ethylenedioxythiophene) (pEDOT) modified by iron hexacyanoferrate (Fehcf) network, chloride (Cl), polystyrenesulfonate (PSS), and hexacyanoferrate (FeCN) ions were shown. Depending on the electrode potential, three main maxima absorbance were recorded. The first related to, the π–π* transition in undoped state of pEDOT, the second and the third are ascribed to transitions between the valence band and the polaron and bipolaron bands, respectively. There is also identified spectrophotometric response from the ligand–metal charge transfer of hexacyanoferrates from pEDOT modified with Fehcf and FeCN. The energy band gap (E _g) was evaluated from the spectroelectrochemical curves of the undoped pEDOT films. The pEDOT/Fehcf material exhibits the band gap of 1.40 eV which is the lowest among measured E _g values equal 1.55, 1.53, and 1.58 eV for pEDOT/FeCN, pEDOT/Cl, and pEDOT/PSS, respectively. Thus, synergetic effect of polymer and Prussian blue is proved as a significant decrease of the E _g value.     

Oxygen Functionalization of Hexagonal Boron Nitride on Ni(111)

The interaction of single-layer hexagonal boron nitride (h-BN) on Ni(111) with molecular oxygen from a supersonic molecular beam led to a covalently bonded molecular oxygen species, which was identified as being between a superoxide and a peroxide. This is a rare example of an activated adsorption process leading to a molecular adsorbate. The amount of oxygen functionalization depended on the kinetic energy of the molecular beam. For a kinetic energy of 0.7 eV, an oxygen coverage of 0.4 ML was found. Near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy revealed a stronger bond of h-BN to the Ni(111) substrate in the presence of the covalently bound oxygen species. Oxygen adsorption also led to a shift of the valence bands to lower binding energies. Subsequent temperature-programmed X-ray photoelectron spectroscopy revealed that the oxygen boron bonds are stable up to approximately 580 K, when desorption, and simultaneously, etching of h-BN set in. The experimental results were substantiated by density functional theory calculations, which provided insight to the adsorption geometry, the adsorption energy and the reaction pathway.     

Structure of associates in tetramethylurea aqueous solutions from the data of Raman scattering spectroscopy

The results from investigating interactions between tetramethylurea (TMU) and water molecules by means of Raman light scattering spectroscopy (RS) are presented. It is established that spectroscopic manifestations of association of TMU and H₂O molecules are observed for the TMU molecule valence vibration band ν(CO) 1638 cm⁻¹. By means of quantum-chemical calculations, it is shown that the band sensitivity to molecular environment is defined by the number of 5–6 H₂O molecules. It is found that in a pure TMU medium, chain dimers are formed due to the interactions of molecular dipoles arranged in parallel and hydrogen bonds of the C-H…O type. Ground state geometries of TMU, TMU-TMU, H₂O molecules and TMU · (H₂O) _n complexes (n = 1−14) are optimized by the density functional B3LYP procedure, using the 6–31++G(d, p) basis set. The main vibration frequencies are calculated in the harmonic approximation. Associate formation energies are calculated with allowance for the basis set superposition error (BSSE).     

Determination of the interface band alignment of Mg2Si/4H-SiC heterojuction for potential photodetector application

The Mg₂Si/4H-SiC heterojunction was prepared by radio frequency (RF) magnetron sputtering technique. The binding energies of Mg 2p, Si 2p, and C 1s core levels and the maxima of valence band were measured by X-ray photoelectron spectroscopy (XPS). Using the optical bandgap of Mg₂Si (0.78 eV) and 4H-SiC (3.25 eV), the band offsets of valence band (VBO) and conduction band (CBO) at Mg₂Si/4H-SiC interface were identified as 1.47 and 1.00 eV, respectively. The band alignment was evaluated to be type-I band alignment. The Mg₂Si/4H-SiC heterojunction could be a promising candidate for the infrared (IR) photodetector.     

Correcting electronic spectra for distortions caused by inelastic collisions of photoelectrons

A procedure is developed for correcting X-ray photoelectron spectra for distortions caused by inelastic collisions of photoelectrons, leading to an intense tail on the side of higher bond energies. The procedure was used to correct the form of the valence band X-ray photoelectron spectra of pure copper and a Ti-Ni-Cu alloy. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1145–1146, November–December, 1998.     

Rare earth element doping effect on the bonding and the transport property of δ-MoN

Combining non-equilibrium Green’s function technique with density functional theory, the rare earth element doping effect on the bonding and the transport property of δ-MoN were theoretically investigated. The Mo–N bond lengths become more uneven after dopings. Some Mo–N bonds were heavily lengthened by the La- and Gd-dopings, resulting in obvious damages of their bonding. Evident covalent-like La–N and Gd–N bonds were formed in La–MoN and Gd–MoN, respectively, while the Yb atom underwent an ionic-like interaction with its neighboring N atoms in Yb–MoN. A clear drop of the conductivity was found after La- and Gd-dopings. On the contrary, the conductivity was improved upon the Yb-doping. This case was rationalized from the carrier density and the scattering of the carriers. The backscattering effect was evident at the impurities. The La- and Gd-dopings could not effectively increase the carrier density near the Fermi level, while the Yb atom could offer f-carriers to transfer from the valence band to the conduction band.     

SCF-Xα MO studies of the x-ray spectra of CuO and ZnO

SCF-Xα scattered wave cluster MO calculations for the oxyanions CuO^?6₄ (D_4h symmetry) and ZnO^?6₄ (Td symmetry) yield results in good agreement with the X-ray photoelectron and X-ray emission spectra of CuO and ZnO, respectively. Agreement of the calculations with optical data is fair. Calculations of the valence electron and core electron hole states of these oxyanions support the assignment of photoelectron shakeup satellites to valence band to conduction band transitions. Calculated shakeup energies for the Cu2p core spectrum in CuO are 7.4 and 9.9 eV (cf. experimental values of 7.5 and 10.0 eV) while shakeup peaks in the valence region spectrum are predicted at 6.1 and 8.0 eV. (Cf. a broad peak with maximum at 8.1 eV observed experimentally.) The absence of intense low energy satellites in the spectra of ZnO is explained by the small amount of electron reorganization in the outer valence levels attendant upon hole formation.     

Valence band structure of cellulose and lignin studied by XPS and DFT

In this report, X-ray induced photoelectron spectroscopy (XPS) measurements of the valence band structure of cellulose and lignin are combined with a theoretical reconstruction of the spectra based on density functional theory (DFT) calculations. These calculations involve an analysis of the valence band structures and their respective orbitals in which basic units of cellulose and lignin are considered. In addition, photoionization cross sections are incorporated for reconstruction of the XPS spectra. This combination of theoretical calculations and experimental measurements revealed that an emission present up to 10 eV in the valence band structure is dominated by oxygen rather than by carbon, as reported in literature. Furthermore, a quantitative elemental analysis shows significant carbon contributions at binding energies above 13 eV. The valence band analysis supported by DFT provides a powerful basis for a detailed interpretation of spectroscopic data and enables a profound insight into application relevant processes in future.     

X-ray spectra and electronic structure of boron nitride in different crystallographic modifications

The electronic energy structure of boron nitride with ZnS (c-BN) and wurtzite (w-BN) type crystal lattices is calculated by the local coherent potential (LCPA) method in a multiple scattering approximation. The local partial 2p states of boron with c-BN and w-BN are compared with the boron K emission spectra in the corresponding compounds. Fine structure is first obtained in the region of the top of the valence band. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 6, pp. 1083–1087, November–December, 1998.     

Resonant emission of UO<Subscript>2</Subscript>, U<Subscript>3</Subscript>O<Subscript>8</Subscript>, and UO<Subscript>2+<Emphasis Type="Italic">x</Emphasis></Subscript> valence electrons under SR excitation near the O<Subscript>4,5</Subscript>(U) absorption edge

The structure of the resonant electron emission (REE) spectra of UO₂ (REE appears under the excitation with synchrotron radiation near the O_4,5(U) absorption edge at ∼100 eV and ∼110 eV) is studied with regard to the X-ray O_4,5(U) absorption spectrum of UO₂ and a quantitative scheme of molecular orbitals based on the X-ray electron spectroscopy data and the results of a relativistic calculation of the electronic structure of UO₂. The structure of the REE spectra of U₃O₈ and UO_2+x is studied for comparison, and the effect of the uranium chemical environment in oxides on it is found. The appearance of such a structure reflects the processes of excitation and decay involving the U5d and electrons of the outer valence MOs (OVMOs, from 0 to ∼13 eV) and inner valence MOs (IVMOs, from ∼13 eV to ∼35 eV) of the studied oxides. It is noted that REE spectra show the partial density of states of U6p and U5f electrons. Based on the structure of REE spectra, it is revealed that U5f electrons directly participate (without losing the f nature) in the chemical bonding of uranium oxides and are delocalized within CMOs (in the middle of the band), which results in the enhancement of the intensity of the REE spectra of CMO electrons during resonances. The U6d electrons are found to be localized near the bottom of the outer valence band and are observed in the REE spectra of the studied oxides as a characteristic maximum at 10.8 eV. It is confirmed that U6p electrons are effectively involved in the formation of IVMOs, which leads to the appearance of the structure in the region of IVMO electron energies during resonances. This structure depends on the chemical environment of uranium in the considered oxides.     

CASSCF and multireference CI with singles and doubles study of low-lying valence and Rydberg states of 2H-tetrazole

Complete active space self-consistent field (CASSCF) and multireference CI with singles and doubles (MR-CISD) calculations [including extensivity corrections, at MR-CISD+Q and multireference averaged quadratic coupled cluster (MR-AQCC) levels] have been performed to characterize the low-lying valence and the Rydberg states of 2H-tetrazole. The highest level results (MR-AQCC/d'-aug'-cc-pVDZ) indicate the following ordering of the valence singlet excited states: S(1) (n-pi*), 6.06 eV; S(2) (n-pi*), 6.55 eV; S(3) (pi-pi*), 6.55 eV. The MR-CISD+Q/d'-aug'-cc-pVDZ results indicate the same ordering, but at slight higher energies: 6.16, 6.68, and 6.69 eV, respectively. According to our MR-CISD+Q/d'-aug'-cc-pVDZ results, the next two states are Rydberg states, at 7.69 eV (pi-3s) and 7.89 eV (n-3s). The calculated energies of these two states, as well as their proximity, are consistent with the conclusion reached by Palmer and Beveridge (Chem Phys 1987, 111, 249) that the first band of the photoelectron spectrum of 2H-tetrazole is likely to be associated to the first two ionizations processes (of pi and N lone pair electrons), at energies close to 11.3 eV.     

LnCu2O4(Ln=Gd,Nd)电子结构的XPS研究

用XPS测定了LnCu2O4(Ln=Gd, Nd)的内层和价层电子能谱,观察到LnCu2O4中稀土金属的3d电子结合能比相应的稀土金属简单氧化物的3d结合能低0.8～0.9 eV,而Cu的2p电子结合能比CuO的高0.4～0.5 eV,因此推断在LnCu2O4的Ln－O－Cu链中存在Cu→O→Ln电荷转移.XPS分析还表明LnCu2O4的Cu原子上有较低的电荷密度,但不存在混合价态.此外,通过比较价电子能谱,发现NdCu2O4的Ln 4f Cu 3d O 2p价带中心比GdCu2O4的价带中心向Fermi能级移近了3.4 eV,而且NdCu2O4的价带谱更窄.     

LnCu_2O_4(Ln=Gd,Nd)电子结构的XPS研究

用 XPS测定了 LnCu2O4(Ln=Gd, Nd)的内层和价层电子能谱,观察到 LnCu2O4中稀土金属的 3d电子结合能比相应的稀土金属简单氧化物的 3d结合能低 0.8～ 0.9 eV,而 Cu的 2p电子结合能比 CuO的高 0.4～ 0.5 eV,因此推断在 LnCu2O4的 Ln－ O－ Cu链中存在 Cu→ O→ Ln电荷转移 .XPS分析还表明 LnCu2O4的 Cu原子上有较低的电荷密度,但不存在混合价态 .此外,通过比较价电子能谱,发现 NdCu2O4的 Ln 4f Cu 3d O 2p价带中心比 GdCu2O4的价带中心向 Fermi能级移近了 3.4 eV,而且 NdCu2O4的价带谱更窄 .     

Physical and photoelectrochemical characterization of CuCrO2 single crystal

CuCrO₂ single crystal, elaborated by the flux method, is a narrow-band-gap semiconductor crystallizing in the delafossite structure with an indirect optical transition at 2.12 eV. The relatively longer Cu–Cu is consistent with the semi-conducting behavior. The conductivity in the (001) plans is thermally activated and occurs predominantly by small polaron hopping through mixed-valence states Cu^+/2+ in conformity with a classical dielectric behavior. The activation energy (0.05 eV) gave an effective mass of 9 m _o, indicating that the levels in the vicinity of the Fermi level E _f are strongly localized. The oxide shows an excellent chemical stability over the whole pH range; the semi-logarithmic plot gave an exchange current density of 0.7 mA cm⁻² and a corrosion potential of 0.18 V_/SCE in KOH (0.5 M) electrolyte. The electrochemical study is confined in (001) plans, and reversible oxygen intercalation is evidenced from the cyclic voltammetry. The Mott–Schottky plot (C⁻²-V) is characteristic of p type conduction and exhibits a linear plot from which a flat band potential of +0.21 V_/SCE and a holes density N _A of 5.06 × 10¹⁴ cm⁻³ were obtained. The photocurrent is due to Cu⁺: d → d transition and the valence band is positioned at 5.34 eV below vacuum.     

Improved relativistic energy-consistent pseudopotentials for 3d-transition metals

Energy-consistent relativistic pseudopotentials for 3d-transition metals Sc to Ni based on modified valence energies are proposed. The pseudopotentials are adjusted at the finite difference level within the intermediate coupling scheme with respect to multi-configuration Dirac–Hartree–Fock data based on the Dirac–Coulomb Hamiltonian with an estimate of the Breit contributions in quasidegenerate perturbation theory. Typically a few hundred to thousand J levels arising from about 35 to 40 configurations ranging from the anion down to the highly charged cation are considered as references. It is shown that introducing a small common energetic shift of all valence energies reduces the errors in the parameter adjustment considerably. Results of highly correlated atomic and molecular test calculations using large basis sets and basis set extrapolation techniques are presented. To be submitted to Theoretical Chemistry Accounts (special volume on the occasion of Prof. Dr. H. Stoll's 60th birthday)     

Electronic structure of crystalline FeSi and FeGe

X-Ray photoelectron valence band spectra were recorded for stoichiometric compounds FeSi and FeGe; their electronic structure was calculated by the ab initio full-potential linear muffintin orbital method. For nonmagnetic FeSi, good agreement was obtained between the calculated densities of states and the X-ray photoelectron spectrum in both peak positions and forbidden gap. For FeGe, which is an antiferromagnet, the nonmagnetic calculation yields worse agreement with experiment and explicitly indicates that the paramagnetic phase is unstable. In both compounds, the calculation gives a high degree of d—p hybridization and covalence, which is estimated quantitatively. In FeGe, the degree of covalence of the Fe-Fe bond is higher than that of the Fe-Ge bond. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1098–1102, November–December, 1998.