The influence of oxygen deficiency and Sb doping on inverse photoemission spectra of SnO2

The changes in the empty electronic states in SnO₂ produced by ion-beam induced oxygen deficiency and by Sb doping have been studied by inverse photoemission spectroscopy. Inverse photoemission in SnO₂ itself is dominated by peaks 4 and 12 eV above the Fermi level, the former associated with empty states of dominant Sn 5p atomic character. Sb doping populates states in the Sn 5s conduction band, shifting the empty state structure closer to the Fermi energy. By contrast oxygen deficiency introduces new states above the main Sn 5p peak. These are tentatively described as 5s-5p hybrids pushed up in energy from the 5p band by mixing between atomic orbitais of different parity in the non-centrosymmetric cation environment of oxygen deficient SnO₂.     

The near edge structure of cubic boron nitride

We compare the near edge structure (NES) of cubic boron nitride (cBN) measured using both electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) with that calculated using three commonly used theoretical approaches. The boron and nitrogen K-edges collected using EELS and XAS from cBN powder were found to be nearly identical. These experimental edges were compared to calculations obtained using an all-electron density functional theory code (WIEN2k), a pseudopotential density functional theory code (CASTEP) and a multiple scattering code (FEFF). All three codes were found to reproduce the major features in the NES for both ionisation edges when a core-hole was included in the calculations. A partial core hole (1/2 of a 1s electron) was found to be essential for correctly reproducing features near the edge threshold in the nitrogen K-edge and to correctly obtain the positions of all main peaks. CASTEP and WIEN2k were found to give almost identical results. These codes were also found to produce NES which most closely matched experiment based on χ² calculations used to qualitatively compare theory and experiment. This work demonstrated that a combined experimental and theoretical approach to the study of NES is a powerful way of investigating bonding and electronic structure in boron nitride and related materials.     

硼烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理的方法计算H修饰边缘不同宽度硼稀纳米带的电荷密度、电子能带结构、总态密度和分波态密度。结果表明,硼烯纳米带的宽度大小影响着材料的导电性能,宽度5的硼烯纳米带是间接带隙简并半导体,带隙值为0.674 eV,而宽度7的硼烯纳米带却具有金属材料的性质。分波态密度表明,宽度5的硼烯纳米带的费米能级附近主要是由B-2s、2p电子态贡献,H-1s主要贡献于下价带且具有局域性,消除了材料边缘的不稳定性。宽度7的B-2p和H-1s电子态贡献的导带和价带处于主导地位,费米能级附近B-2p和H-1s电子态的杂化效应影响材料的整体发光性能。     

Electron energy loss spectroscopy study of the initial stage of lanthanum oxidation

The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E _p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide.     

Core level absorption spectroscopy of perovskites and high temperature superconductors

We present K-edge, or Is, absorption spectra of oxygen in different oxides, and relate the fine structure near the absorption edge to different degrees of charge transfer between the cations and anions, and to features of the electronic structure. We use the experimental technique of electron energy loss spectroscopy (EELS) where incident fast electrons, rather than incident X-rays, excite the core electrons of the atoms. The spectrometer is attached to a transmission electron microscope enabling us to analyze nanometer sized volumes and study bonding and local electronic structure near defects and interfaces. Comparisons are made with the closely related technique of X-ray absorption spectroscopy. By examples we demonstrate that increase in the oxygen content above, or the reduction of the cation content below, the equilibrium level, as well as the interchange of a cation with one of lower formal valency result in a pre-edge peak a few eV below the threshold of the oxygen K-edge. We attribute this feature to transfer ofs valence electrons away from the oxygen anions, thus creating empty states below the conduction band of the originally stoichiometric compound.     

Detection of Fe 3d electronic states in valence band and magnetic properties of Fe-doped ZnO film

Fe-doped ZnO film has been grown by laser molecular beam epitaxy (L-MBE) and structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), all of which reveal the high quality of the film. No secondary phase was detected. Resonant photoemission spectroscopy (RPES) with photon energies around the Fe 2p-3d absorption edge is performed to detect the electronic structure in the valence band. A strong resonant effect at a photon energy of 710 eV is observed. Fe³⁺ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV～ 8 eV. There are no electronic states related to Fe near the Fermi level. Magnetic measurements reveal a typical superparamagnetic property at room temperature. The absence of electronic states related to Fe near the Fermi level and the high quality of the film, with few defects, provide little support to ferromagnetism.     

Energy of metal states in intercalated graphite

X-ray photoemission and electron energy loss spectroscopy of carbon and potassium core levels are used to measure the energy distribution of potassium 4s states in KC₈. Contrary to many earlier theoretical calculations these states form a completely empty band with a minimum energy 2.2 eV above the Fermi level in good agreement with the most recent ab initio KKR calculations. Evidence for hybridized states near the Fermi level is presented.     

Electronic structure of graphite: Single particle and collective excitations studied by EELS,SEE and K edge loss techniques

We present some EELS, SEE and K edge loss measurements on in situ cleaved graphite. Single particle excitations in the EELS measurements have been identified accordingly with some previous results on pure polycrystalline iron and are compared with available band structure calculations of graphite. The core edge loss spectroscopy performed in the reflection mode at low primary electron energy proved to be a powerful and rapid technique to study the partial density of empty states.     

Graphene with the secondary amine-terminated zigzag edge as a line electron emitter

An extraordinary low vacuum barrier height of 2.30?eV has been found on the zigzag-edge of graphene terminated with the secondary amine via the ab?initio calculation. This edge structure has a flat band of edge states attached to the gamma point where the transversal kinetic energy is vanishing. We show that the field electron emission is dominated by the flat band. The edge states pin the Fermi level to a constant, leading to an extremely narrow emission energy width. The graphene with such edge is a promising line field electron emitter that can produce a highly coherent emission current.     

Core-level spectroscopy of point defects in single layer h-BN

Electron energy-loss spectroscopy (EELS) is used to analyze single-layered hexagonal boron-nitride with or without point defects. EELS profiles using a 0.1 nm probe clearly discriminate the chemical species of single atoms but show different delocalization of the boron and nitrogen K edges. A monovacancy at the boron site is unambiguously identified and the electronic state of its nearest neighboring nitrogen atoms is examined by energy-loss near edge fine structure analysis, which demonstrates a prominent defect state. Theoretical calculations suggest that the observed prepeak originates from the 1s to lowest unoccupied molecular orbital excitation of dangling nitrogen bonds, which is substantially lowered in energy with respect to the three coordinated nitrogen atoms.     

Density-functional theory study on the electronic properties of laves phase superconductor CaIr_2

In this work we have used density-functional theory methods such as full-potential local orbital minimum basis(FPLO) and ELK-flapw to study the electronic structure of newly discovered Laves phase superconductor CaIr_2.The calculation of density of states(DOS) indicates that the bands near Fermi level are mostly occupied by the d-electrons of iridium.The simulation of de Haas-van Alphen(dHvA) effect has been performed by using Elk code to check the Fermi surface topology.The results show that there exist four Fermi surfaces in CaIr_2,including two electron-type and two hole-type surfaces.The optical response properties of CaIr_2 have been calculated in the dipole-transition approximations combined with including intra-band Drude-like terms.In the optical spectrum σ(ω) shows that the crossover from intraband to inter-band absorption occur near 1.45 eV.Further analysis on the electron energy loss spectra(EELS) matches the conclusion from that of optical conductivity σ(ω).     

Intrinsic electron accumulation at clean InN surfaces

The electronic structure of clean InN(0001) surfaces has been investigated by high-resolution electron-energy-loss spectroscopy of the conduction band electron plasmon excitations. An intrinsic surface electron accumulation layer is found to exist and is explained in terms of a particularly low Gamma-point conduction band minimum in wurtzite InN. As a result, surface Fermi level pinning high in the conduction band in the vicinity of the Gamma point, but near the average midgap energy, produces charged donor-type surface states with associated downward band bending. Semiclassical dielectric theory simulations of the energy-loss spectra and charge-profile calculations indicate a surface state density of 2.5 (+/-0.2)x10(13) cm(-2) and a surface Fermi level of 1.64+/-0.10 eV above the valence band maximum.     

Momentum and energy dependence of the anomalous high-energy dispersion in the electronic structure of high temperature superconductors

Using high-resolution angle-resolved photoemission spectroscopy we have studied the momentum and photon energy dependence of the anomalous high-energy dispersion, termed waterfalls, between the Fermi level and 1 eV binding energy in several high-T_{c} superconductors. We observe strong changes of the dispersion between different Brillouin zones and a strong dependence on the photon energy around 75 eV, which we associate with the resonant photoemission at the Cu3p-->3d_{x;{2}-y;{2}} edge. We conclude that the high-energy "waterfall" dispersion results from a strong suppression of the photoemission intensity at the center of the Brillouin zone due to matrix element effects and is, therefore, not an intrinsic feature of the spectral function. This indicates that the new high-energy scale in the electronic structure of cuprates derived from the waterfall-like dispersion may be incorrect.     

MgCNi3的电子结构、光学性质与超导电性

用第一性原理的密度泛函能带计算方法研究了新近发现的超导体MgCNi3的电子能带结构.计算结果表明其电子结构的基本特征是：Ni的3d态和C的2p态的杂化组成了MgCNi3的导带，费米面附近的物理性质主要由来源于Ni的3d电子态决定.在费米能级(EF)以下30eV的范围内，Ni 3d态构成了能带色散微弱的密集电子态，EF恰好落在Ni 3dyz+zx和3d3z2-r2电子态密度.C 2p态分布在EF以下40—70eV的区域内，Mg主要是以二价离子Mg2+的形式存在.Mg原子的掺杂导致了Ni原子的3d态基本上全部占据，引起Ni原子磁矩的消失.费米能级EF处的态密度N(ＥＦ)是550(states/eV·cell)，由此得到的Sommerfeld常数γeal～445mJ/mol·K2.基于第一性原理的光学性质的计算结果表明：在0—12eV的范围内光吸收主要是从占据的Ni 3d态向C　2p和Ni4s的跃迁.根据这些结果得出结论：MgCNi3的超导电性基本上是强耦合的BCS电子-声子作用机理. 关键词： MgCNi3 高温超导体 电子结构 光学性质     

Electronic structure of Al2O3 from electron energy loss spectroscopy

The electronic structure of Al₂O₃ has been studied by electron energy loss spectroscopy (ELS), and an energy level model of both filled and empty states has been constructed from the ELS and available optical data. For the high temperature pyrolytic α-polycrystalline Al₂O₃ films, the transitions are assumed to originate at the two principal peaks in the valence band density of states and the O(2s) core state, and to terminate on two peaks within the conduction band density of states. We also report energy loss spectra due to excitations out of the deeper Al(2p), Al(2s), Al(1s), and O(1s) core levels. The excitations originating at the Al(2p), Al(2s), and Al(1s) core levels terminate on levels in the conduction band and on an exciton lying about 1 eV below the conduction-band edge.     

The unoccupied electronic structure characterization of hydrothermally grown ThO2 single crystals

Single crystals of thorium dioxide ThO₂, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L₃, M₃, M₄, and M₅ X‐ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n‐type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunneling electron induced bromine hopping on Si(100)-(2 x 1)

Tunneling electrons from the tip of a scanning tunneling microscope can be used to induce adatom hopping on Br-terminated Si(100)-(2x1) at low current and without voltage pulses. Hopping does not occur when electrons tunnel from a sample to a tip. The threshold energy is +0.8 V, and tunneling spectroscopy shows antibonding Si-Br states 0.8 eV above the Fermi level. Electron capture in these states is a necessary condition for hopping, but repulsive adsorbate interactions that lower the activation barrier are also required. Such interactions are strong near saturation for Br but are insufficient when the coverage is low or when Br is replaced by Cl.     

Surface states on gallium phosphide

Ultraviolet photoelectron spectroscopy shows that the surface Fermi level of clean cleaved GaP (10) is pinned 1.50 eV above the valence band maximum of empty surface states. Synchrotron radiation-excited photo-emission partial yield spectroscopy supports this conclusion.     

Spatial chemical inhomogeneity and local electronic structure of Mn-doped Ge ferromagnetic semiconductors

We have investigated the chemical distributions and the local electronic structure of potential diluted magnetic semiconductor Ge0.94Mn0.06 single crystals using scanning photoelectron microscopy (SPEM), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The SPEM image shows the stripe-shaped microstructures, which arise from the chemical phase separation between the Mn-rich and Mn-depleted phases. The Mn 2p XAS shows that the Mn ions in the Mn-rich region are in the divalent high-spin Mn2+ states but that they do not form metallic Mn clusters. The Mn 3d PES spectrum exhibits a peak centered at approximately 4 eV below E(F) and the negligible spectral weight near E(F). This study suggests that the observed ferromagnetism in Ge1-xMnx arises from the phase-separated Mn-rich phase.     

The electronic and chemical structure of the a-B(3)CO(0.5):H(y)-to-metal interface from photoemission spectroscopy: implications for Schottky barrier heights

The electronic and chemical structure of the metal-to-semiconductor interface was studied by photoemission spectroscopy for evaporated Cr, Ti, Al and Cu overlayers on sputter-cleaned as-deposited and thermally treated thin films of amorphous hydrogenated boron carbide (a-B(x)C:H(y)) grown by plasma-enhanced chemical vapor deposition. The films were found to contain ～10% oxygen in the bulk and to have approximate bulk stoichiometries of a-B(3)CO(0.5):H(y). Measured work functions of 4.7/4.5?eV and valence band maxima to Fermi level energy gaps of 0.80/0.66?eV for the films (as-deposited/thermally treated) led to predicted Schottky barrier heights of 1.0/0.7?eV for Cr, 1.2/0.9?eV for Ti, 1.2/0.9?eV for Al, and 0.9/0.6?eV for Cu. The Cr interface was found to contain a thick partial metal oxide layer, dominated by the wide-bandgap semiconductor Cr(2)O(3), expected to lead to an increased Schottky barrier at the junction and the formation of a space-charge region in the a-B(3)CO(0.5):H (y) layer. Analysis of the Ti interface revealed a thick layer of metal oxide, comprising metallic TiO and Ti (2)O (3), expected to decrease the barrier height. A thinner, insulating Al(2)O(3) layer was observed at the Al-to-a-B(3)CO(0.5):H(y) interface, expected to lead to tunnel junction behavior. Finally, no metal oxides or other new chemical species were evident at the Cu-to-a-B(3)CO(0.5):H(y) interface in either the core level or valence band photoemission spectra, wherein characteristic metallic Cu features were observed at very thin overlayer coverages. These results highlight the importance of thin-film bulk oxygen content on the metal-to-semiconductor junction character as well as the use of Cu as a potential Ohmic contact material for amorphous hydrogenated boron carbide semiconductor devices such as high-efficiency direct-conversion solid-state neutron detectors.