Density Of Valence States Research Articles

Understanding the Impact of Film Disorder and Local Surface Potential in Ultraviolet Photoelectron Spectroscopy of PEDOT.

The spectra of conducting polymers obtained using ultraviolet photoelectron spectroscopy (UPS) exhibit a typical broadening of the tail σUPS ≈ 1 eV, which by an order of magnitude exceeds a commonly accepted value of the broadening of the tail of the density of states σDOS ≈ 0.1 eV obtained using transport measurements. In this work, an origin of this anomalous broadening of the tail of the UPS spectra in a doped conducting polymer, PEDOT (poly(3,4-ethylenedioxythiophene)), is discussed. Based on the semiempirical approach and using a realistic morphological model, the density of valence states in PEDOT doped with molecular counterions is computed. It is shown that due to a disordered character of the material with randomly distributed counterions, the localized charge carriers in PEDOT crystallites experience spatially varying electrostatic potential. This leads to spatially varying local vacuum levels and binding energies. Taking this variation into account the UPS spectrum is obtained with the broadening of the tail comparable to the experimentally observed one. The results imply that the observed broadening of the tail of the UPS spectra in PEDOT provides information about a disordered spatially varying potential in the material rather than the broadening of the DOS itself.

Electronic structure and X-ray magnetic circular dichroism in the Ni-Mn-Ga Heusler alloys

The electronic structure and X-ray magnetic circular dichroism (XMCD) spectra of the Ni2MnGa and Cu doped Ni2MnGa Heusler alloys were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments are analyzed and discussed. The electronic and magnetic structure of Ni-Mn-Ga Heusler alloys were investigated for the cubic austenitic and modulated 7M-like incommensurate martensitic phases. The X-ray absorption spectra (XAS) and XMCD at the Mn, Ni, Ga, and Cu L2,3, and Mn, Ni, and Ga K edges were investigated theoretically from first principles. The origin of the XMCD spectra in the Ni2MnGa compound is examined. The ab initio calculations reproduce well experimental XAS and XMCD spectra. The XAS at the Mn L2,3 edges remains mostly unchanged through martensitic phase transition. A fingerprint of the martensitic phase transition has been found in the Ni L2,3 XAS spectra. The experimental Ni L3 XAS has a pronounced shoulder at the L3 peak at around 853 eV. This peak is nearly suppressed in the martensitic state in comparison with the austenitic phase due to the lifting of the degeneracy in the Ni 3d related unoccupied electronic states. The XMCD of the martensitic phase is increased compared to the XMCD of the austenite phase at the Ni and Mn L2,3 edges.

The mechanism of slow hot-hole cooling in lead-iodide perovskite: first-principles calculation on carrier lifetime from electron-phonon interaction.

We report on an analysis of hot-carrier lifetimes from electron-phonon interaction in lead iodide perovskites using first-principles calculations. Our calculations show that the holes in CsPbI3 have very long lifetimes in the valence band region situated 0.6 eV below the top of the valence band. On the other hand, no long lifetime is predicted in PbI3(-). These different results reflect the different electronic density of states (DOSs) in the valence bands, that is, a small DOS for the former structure while a sharp DOS peak for the latter structure. We propose a reduction of the relaxation paths in the small valence DOS as being the origin of the slow hot-hole cooling. Analyzing the generalized Eliashberg functions, we predict that different perovskite A-site cations do not have an impact on the carrier decay mechanism. The similarity between the DOS structures of CsPbI3 and CH3NH3PbI3 enables us to extend the description of the decay mechanism of fully inorganic CsPbI3 to its organic-inorganic counterpart, CH3NH3PbI3.

X-ray magnetic circular dichroism in Co2FeGa: First-principles calculations

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co2FeGa were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the Co2FeGa compound is examined. The calculated results are compared with available experimental data.

X-ray magnetic circular dichroism in iron chalcogenides Fe1−xS: First-principles calculations

The electronic structure of iron chalcogenides Fe1−xS (x=0 and 0.125) were investigated theoretically from first principles, using the fully relativistic Dirac linear-muffin-tin-orbital band structure method within the local spin-density approximation. At ambient conditions, stoichiometric FeS in the troilite crystal structure orders antiferromagnetically below the Neel temperature TN=598 K in an insulating ground state. The different models of antiferromagnetic ordering in FeS are analyzed and discussed. Densities of valence states, orbital and spin magnetic moments as well as x-ray magnetic circular dichroism (XMCD) spectra of Fe7S8 at the Fe K, L2,3, and S K edges are investigated. The origin of the structure in the XMCD spectra is examined. The calculated results are compared with available experimental data.

Electronic structure, optical and X‐ray emission spectra in FeS2

AbstractThe electronic structure, optical spectra, X‐ray emission (XE) and X‐ray absorption (XA) spectra of FeS2 were investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method in local density approximation (LDA). Theory produces correct semiconductor ground state in FeS2 with the energy gap of Eg = 0.9 eV in good agreement with the experimental data. Densities of valence states are analyzed and discussed. The origin of the optical, XA and XE spectra in the FeS2 compound is examined. The calculated results are compared with available experimental data. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure, optical spectra, and x-ray magnetic circular dichroism inCoS2

The electronic structure, optical spectra, and x-ray magnetic circular dichroism (XMCD) of ${\text{CoS}}_{2}$ were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. Densities of valence states, and orbital and spin magnetic moments are analyzed and discussed. The origin of the optical and XMCD spectra in the ${\text{CoS}}_{2}$ compound is examined. The band-by-band decomposition of the optical conductivity spectrum is presented and the interband transitions responsible for the prominent structures in the spectra are identified. The calculated results are compared with available experimental data.

An analysis of 1.55μm InAs∕InP quantum dash lasers

Calculations show that electron states are not confined in the dashes in 1.55μm InAs∕InP quantum dash-in-a-well laser structures. The combination of strain and three-dimensional confinement reduces the calculated density of states (DOS) near the valence band maximum, with the conduction and valence DOS then almost equal close to the band edges. Calculations and photoabsorption measurements show strongly polarized spontaneous emission and gain spectra. Experimental analysis shows the room temperature threshold current is dominated by nonradiative current paths.

Detection of the valence band in buried Co2MnSi–MgO tunnel junctions by means of photoemission spectroscopy

This work reports on the detection of the valence band of buried Heusler compounds by means of hard x-ray photoemission spectroscopy. The measurements have been performed on the so-called “half” tunnel junctions that are thin films of Co2MnSi underneath MgO. Starting from the substrate, the structure of the samples is MgO(buffer)–Co2MnSi–MgO(tMgO)–AlOx with a thickness tMgO of the upper MgO layer of 2 and 20nm. The valence band x-ray photoemission spectra have been excited by hard x rays of about 6keV energy. The valence band spectra have been used to estimate the mean free path of the electrons through the MgO layer to be 17nm at kinetic energies of about 6keV. In particular, it is shown that the buried Co2MnSi films exhibit the same valence density of states as in bulk samples.

Electronic structure and x-ray magnetic circular dichroism in Heusler-typeFe2−xV1+xAl: First-principles calculations

Soft x-ray absorption spectra and their x-ray magnetic circular dichroism (XMCD) have been theoretically studied at the transition-metal ${L}_{2,3}$ thresholds of the Heusler-type ${\text{Fe}}_{2\ensuremath{-}x}{\text{V}}_{1+x}\text{Al}$. The electronic structure of ${\text{Fe}}_{2\ensuremath{-}x}{\text{V}}_{1+x}\text{Al}$ upon changing the content $x$ was theoretically investigated from first principles by using the fully relativistic Dirac linear-muffin-tin-orbital band structure method and the scalar relativistic Korringa--Kohn--Rostoker method within the coherent potential approximation generalized to treat disorder in multicomponent complex alloys. Densities of valence states and spin and orbital magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the ${\text{Fe}}_{2\ensuremath{-}x}{\text{V}}_{1+x}\text{Al}$ compound is examined. The calculated results are compared to the experimental data.

Electronic structure, noncollinear magnetism, and x-ray magnetic circular dichroism in theMn3ZnCperovskite

${\mathrm{Mn}}_{3}\mathrm{Zn}\mathrm{C}$ possesses a magnetic phase transition at ${T}_{t}=233\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from a ferromagnetic phase to a ferrimagnetic one with a noncollinear magnetic structure. The transition is accompanied by a structural change from cubic to tetragonal. The experimentally measured x-ray magnetic circular dichroism (XMCD) at the $\mathrm{Mn}\phantom{\rule{0.2em}{0ex}}K$ edge shows a drastic change at the magnetic phase transition, which is associated with the appearance of the noncollinear magnetic structure. The electronic structure and XMCD spectra of the ${\mathrm{Mn}}_{3}\mathrm{Zn}\mathrm{C}$ were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method for both the high-temperature cubic and low-temperature tetragonal noncollinear phases. Densities of valence states, spin, and orbital magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the ${\mathrm{Mn}}_{3}\mathrm{Zn}\mathrm{C}$ compound is examined. The calculated results are compared with the experimental data.

Electronic structure and x-ray magnetic circular dichroism in the Heusler alloy Co2MnGe

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co2MnGe were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. Densities of valence states, orbital and spin magnetic moments, as well as polarization of the electronic states at the Fermi level are analyzed and discussed. The origin of the XMCD spectra in the Co2MnGe compound is examined. The calculated results are compared with available experimental data.

Unoccupied electronic states in quaterphenyl oligomer films and at the film-gold and film-oxidized silicon interfaces

Thin films of 4-quaterphenyl (4-QP) are thermally deposited in an ultrahigh vacuum on polycrystalline gold and oxidized silicon substrates. In the process of deposition, the structure of unoccupied electron states 5–20 eV above the Fermi level (EF) and the surface potential are monitored by the method of total current spectroscopy (TCS) using an incident beam of low-energy electrons. During the deposition, the electron work function of the surface changes because of a change in the surface layer composition, reaching a steady-state value of 4.3 ± 0.1 eV at a 4-QP film thickness of 8–10 nm. The density of valence states (DOVS) and the density of unoccupied states (DOUS) in model 4-QP films are calculated using the linearized augmented plane wave method in the generalized gradient approximation of the density functional theory. In the model 4-QP structure, the minimal spacing between carbon atoms of neighboring 4-QP molecules is taken to be 0.4 nm in order that intermolecular interaction can be assumed to be relatively weak, which is observed in disordered 4-QP films. The TCS-measured DOUS and the DOUS predicted theoretically are in good agreement.

Oxidation Electronics: Bond—Band—Barrier Correlation and Its Applications

AbstractFor Abstract see ChemInform Abstract in Full Text.

Site-specific valence-band photoemission study ofα−Fe2O3

We have measured the site-specific valence electronic structure of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ by using a spatially modulated x-ray standing wave as the excitation source for photoemission. Contributions to the valence-band density of states from oxygen and iron ions are separated by this method. Both a bonding and nonbonding state originating from oxygen ions are obtained. The valence densities of states from iron agree well with predictions based on configuration-interaction cluster calculations by Fujimori et al. [Phys. Rev. B 34, 1318 (1986)] which considered charge transfer from ligand to metal. The effects of strong hybridization between Fe and O valence states to x-ray emission and resonance photoemission are also evident.

Geometric and electronic structure of vanadium pentoxide: A density functional bulk and surface study

Density-functional theory (DFT) studies are performed to examine geometric and electronic properties of orthorhombic bulk ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ as well as of its (010) oriented surface. Electronic states, total energies, as well as atom forces (used to obtain equilibrium geometries) are computed with the ab initio full-potential linear augmented plane wave method. The ${\mathrm{V}}_{2}{\mathrm{O}}_{5}(010)$ surface is modeled by periodic single layers in a repeated slab geometry, which is justified by the weak electronic interlayer coupling found in the bulk calculations. The electronic structure of the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}(010)$ single-layer slabs, represented by their valence densities of states (DOS) and its atom contributions, is compared with results of bulk ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ and with previous results obtained by DFT surface cluster studies. The comparison yields good qualitative agreement between the different approaches, which confirms the local nature of interatomic binding in ${\mathrm{V}}_{2}{\mathrm{O}}_{5}.$ Further, the computed valence DOS is used to interpret recent experimental results from photoemission on ${\mathrm{V}}_{2}{\mathrm{O}}_{5}(010),$ which suggests that differently coordinated oxygen sites at the surface can be identified in the spectrum. Thus, ${\mathrm{V}}_{2}{\mathrm{O}}_{5}(010)$ photoemission spectra may be used to monitor the participation of oxygen ions in respective surface reactions.

Electronic properties of the two rutiles TiO2 and VF2: A comparative study

AbstractResults of first‐principles, density‐functional LMTO‐ASA calculations on the two title compounds and of unrestricted Hartree‐Fock (UHF) calculations on VF2 are reported. The calculations on the crystalline materials were performed with special emphasis on optimising all (three) structural parameters, on the valence density of states and the valence electron density, and on the possible existence of spin polarisation. In addition, full‐potential LMTO calculations on isolated monomers were carried through in order to examine the nearest‐neighbour (TiO and VF) chemical bonds in detail and to understand the consequences of various approximations in describing the potential for the crystal. The LMTO‐ASA calculations reproduce the correct lattice constants with, however, some overestimate for VF2. Also the nearest‐neighbour TiO and VF bond lengths are reproduced correctly by the LMTO calculations, whereas the c/a ratios are significantly overestimated. By analysing the charge densities of the isolated monomers we conclude that the overestimates in the c/a ratios are due to the ASA (Atomic‐Sphere Approximation), which does not describe the quadrupole‐quadrupole interactions of the compounds sufficiently accurately. The Hartree‐Fock calculations yield lattice constants for VF2 very close to experiment. Also in agreement with experiment, TiO2 is found to be non‐magnetic and insulating/semiconducting. In contrast, VF2 is found to be metallic within the LMTO‐ASA method, although the density‐functional calculations indicate that magnetic effects will change the compound into an insulator as it is according to experiment. On the other hand, in agreement with experimental evidence, UHF yields a non‐metallic structure. It further shows the stability of a number of patterns of magnetic ordering.

Magnetic dichroism in threshold photoemission

For polarized light obliquely incident on Ni(110) with magnetization in the surface plane, threshold photoemission spectra have been calculated using a relativistic Green function method. Individual intensity peaks can be understood from the layer-resolved density of valence states by explicit consideration of spin-orbit coupling and dipole selection rules. For circular and for two types of linear polarized light, magnetic dichroism results are presented.

Electron distribution in MgO probed by x-ray emission

The relative intensity of the Mg $K\ensuremath{\beta}$ emission band with respect to the $K\ensuremath{\alpha}$ atomic line, obtained by electron-induced x-ray emission spectroscopy, is used to investigate the variation of $p$-like valence electron number located around magnesium in metal and MgO. Calculated spectral intensities of Mg $K$ emissions are obtained within the augmented plane-wave method for the perfect crystal. From experimental and theoretical data, the number of $3p$ electrons present around Mg and involved in the $K\ensuremath{\beta}$ transition does not vary much with the metallic or ionic character of the bonds. This result shows that the charge transfer is weak in the oxide. These experiments can be generalized to other compounds. Spectral densities of states (DOS) around Mg and O are also probed and agreement is found between experimental and theoretical spectral densities of $3p$ Mg and $2p$ O states. Changes of MgO valence DOS with annealing, etching, and aging are discussed.

Microstructural evolution in melt-quenched amorphous Se during mechanical attrition

Microstructural evolution of an as-quenched amorphous Se sample during ball milling was investigated by use of x-ray diffraction, a differential scanning calorimeter, infrared and Raman spectroscopy, as well as x-ray photoemission spectroscopy. It was found that the as-quenched amorphous Se is crystallized completely into a nanocrystalline trigonal Se phase after milling for 30 min. Further milling of the as-milled nanocrystalline trigonal Se resulted in a transformation into another amorphous Se phase, which exhibits a lower crystallization temperature and a smaller crystallization enthalpy compared to those for the as-quenched amorphous Se according to thermal analysis. Infrared and Raman spectroscopy measurements revealed that the as-milled amorphous Se is mainly composed of ${\mathrm{Se}}_{n}$ polymeric chains that are fundamentally different from the as-quenched amorphous Se in which the ${\mathrm{Se}}_{8}$ ring structure is predominant. However, despite the phase evolution and changes in molecular structure in Se, x-ray photoemission measurements indicated that no detectable changes in electronic properties (e.g. the density of valence states, the binding energies of core levels, and the characteristic energy loss functions) were observed among different states of Se samples.