Local electronic structure of phosphorus-doped ZnO films investigated by X-ray absorption near-edge spectroscopy

Using X-ray absorption near-edge structure spectroscopy (XANES), we investigate the local electronic structure of phosphorus (P) and its chemical valence in laser-ablated n-type (as-grown), and p-type (annealed) P-doped ZnO thin films. Both the P L ₁- and P L _2,3-edge XANES spectra reveal that the valence state of P is 3− (P³⁻) in the p-type as well as in the n-type P-doped ZnO. However, the peak intensity is stronger in the former than that in the latter, suggesting that P replaces O (O²⁻ sites with the P³⁻) after rapid thermal annealing. The Zn and O K-edges XANES spectra consistently demonstrate that, in the p-type state, P ions substitutionally occupy O sites in the ZnO lattice.     

Electronic structure of warm dense copper studied by ultrafast x-ray absorption spectroscopy

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10?000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is necessary.     

Landscape of s-triazine molecule on Si(100) by a theoretical x-ray photoelectron spectroscopy and x-ray absorption near-edge structure spectra study

The chemisorbed structure for an aromatic molecule on a silicon surface plays an important part in promoting the development of organic semiconductor material science. The carbon K-shell x-ray photoelectron spectroscopy(XPS) and the x-ray absorption near-edge structure(XANES) spectra of the interfacial structure of an s-triazine molecule adsorbed on Si(100) surface have been performed by the first principles, and the landscape of the s-triazine molecule on Si(100) surface has been described in detail. Both the XPS and XANES spectra have shown their dependence on different structures for the pristine s-triazine molecule and its several possible adsorbed configurations. By comparison with the XPS spectra, the XANES spectra display the strongest structural dependency of all of the studied systems and thus could be well applied to identify the chemisorbed s-triazine derivatives. The exploration of spectral components originated from non-equivalent carbons in disparate local environments has also been implemented for both the XPS and XANES spectra of s-triazine adsorbed configurations.     

Local structure of LiCoO(2) nanoparticles studied by Co K-edge x-ray absorption spectroscopy

We have studied the local structure of LiCoO(2) nanoparticles by Co K-edge x-ray absorption spectroscopy as a function of particle size. Extended x-ray absorption fine structure data reveal substantial changes in the near neighbor distances and the associated mean square relative displacements with decreasing particle size. X-ray absorption near edge structure spectra show clear local geometrical changes with decreasing particle size, similar to those that appear in the charging (delithiation) process. The results suggest that the LiCoO(2) nanoparticles are characterized by a large atomic disorder confined to the Co-O octahedra, similar to the distortions generated during the delithiation, and this disorder should be the primary limiting factor for a reversible diffusion of Li ions when nanoparticles of LiCoO(2) are used as cathode material in rechargeable Li ion batteries.     

Nitrogen-doped carbon spheres: an X-ray absorption near-edge structure spectroscopy study

Carbon spheres (CSs) and low content nitrogen (2.5 and 3.5 at%) doped carbon spheres (NCSs) were synthesized by a pyrolysis process at 900 °C using $\mathrm{C}_{2}\mathrm{H}_{2}$ and CH₃CN. The electronic structure and chemical bonding of CSs and NCSs were studied using Raman spectroscopy, valence-band photoemission spectroscopy (VBPES) and X-ray absorption near-edge structure spectroscopy (XANES). XANES spectroscopy showed the effect of carbon and nitrogen bonds and graphitic as well as pyridinic structures on the NCS structure. For the spheres VBPES showed the effect of nitrogen on the spectral shape of the density of states and confirmed the role played by the $\pi$ bonds in controlling the electronic and structural properties of the NCSs. VBPES also showed that both $\pi$ and $\sigma$ bands shift towards higher binding energies after N-doping which is consistent with the XANES data. The consistency of the spherical geometry, chemical purity, suitable electronic/bonding structure and the availability of macroscopic quantities of the N–CSs makes these spheres promising new materials for different applications.     

Bonding in liquid carbon studied by time-resolved x-ray absorption spectroscopy

Even the most basic properties of liquid carbon have long been debated due to the challenge of studying the material at the required high temperature and pressure. Liquid carbon is volatile and thus inherently transient in an unconstrained environment. In this paper we use a new technique of picosecond time-resolved x-ray absorption spectroscopy to study the bonding of liquid carbon at densities near that of the solid. As the density of the liquid increases, we see a change from predominantly sp-bonded atomic sites to a mixture of sp, sp2, and sp3 sites and compare these observations with molecular dynamics simulations.     

Band-selective measurements of electron dynamics in VO2 using femtosecond near-edge x-ray absorption

We report on the first demonstration of femtosecond x-ray absorption spectroscopy, made uniquely possible by the use of broadly tunable bending-magnet radiation from "laser-sliced" electron bunches within a synchrotron storage ring. We measure the femtosecond electronic rearrangements that occur during the photoinduced insulator-metal phase transition in VO2. Symmetry- and element-specific x-ray absorption from V2p and O1s core levels (near 500 eV) separately measures the filling dynamics of differently hybridized V3d-O2p electronic bands near the Fermi level.     

Sulfur speciation in two typical soil samples using X-ray absorption near-edge structure spectroscopy and chemical fractionation

Sulfur K-edge X-ray absorption near-edge structure spectroscopy and chemical sequential extraction was respectively used to study the speciation of sulfur in two sulfur-rich soils samples. Sulfur K-edge X-ray absorption near-edge structure spectroscopy analysis obtained a variety of spectra. Spectral fitting of the X-ray absorption near-edge structure spectra utilizing a large set of model compounds showed great differences between these two sulfur-rich soil samples. It was found that both of the soil samples had high sulfur content (8.40 and 11.57?g?kg^?1, respectively). Chemical extraction results suggested that sulfur mainly existed as organic in the ancient paddy soil (7.37?g?kg^?1) and more reduced sulfur was identified in it. X-ray absorption near-edge structure spectroscopy also got similar results. These organic forms of sulfur existed in organic matter across a range of oxidation states. There was high proportion of oxidized sulfur in the sulfuric acid plant that mainly existed as sulfate.     

(Fe1-xNix)2P电子结构与磁学特性的X射线近边吸收谱研究

利用X射线近边吸收谱对Fe2P,Ni2P及其掺杂物(Fe1-xNix)2P(x=0.1,0.25,0.5)中Fe,Ni,P的K边进行了研究.结合多重散射理论近边计算,讨论了金属原子不同位置格点3f,3g对近边谱特征的贡献,得出当Ni原子取代Fe原子时将优先占据Fe(3f)格点位置;根据第一性原理对能态的计算发现,不考虑磁性时不同格点P的pDOS未占据态电子结构与P-K近边吸收谱实验相符合;与考虑铁磁性Fe2P的DOS相比较后结果显示Fe2P的磁性主要来源于Fe(3g)格点,铁磁性Ni2P计算的Ni不同格点原子磁矩均接近于0,与它一般显顺磁性结论相一致.     

Unraveling the solid-liquid-vapor phase transition dynamics at the atomic level with ultrafast x-ray absorption near-edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is a powerful probe of electronic and atomic structures in various media, ranging from molecules to condensed matter. We show how ultrafast time resolution opens new possibilities to investigate highly nonequilibrium states of matter including phase transitions. Based on a tabletop laser-plasma ultrafast x-ray source, we have performed a time-resolved (～3 ps) XANES experiment that reveals the evolution of an aluminum foil at the atomic level, when undergoing ultrafast laser heating and ablation. X-ray absorption spectra highlight an ultrafast transition from the crystalline solid to the disordered liquid followed by a progressive transition of the delocalized valence electronic structure (metal) down to localized atomic orbitals (nonmetal-vapor), as the average distance between atoms increases.     

Spin state transition in LaCoO3 studied using soft x-ray absorption spectroscopy and magnetic circular dichroism

Using soft x-ray absorption spectroscopy and magnetic circular dichroism at the Co-L(2,3) edge, we reveal that the spin state transition in LaCoO3 can be well described by a low-spin ground state and a triply degenerate high-spin first excited state. From the temperature dependence of the spectral line shapes, we find that LaCoO3 at finite temperatures is an inhomogeneous mixed-spin state system. It is crucial that the magnetic circular dichroism signal in the paramagnetic state carries a large orbital momentum. This directly shows that the currently accepted low- or intermediate-spin picture is at variance. Parameters derived from these spectroscopies fully explain existing magnetic susceptibility, electron spin resonance, and inelastic neutron data.     

Theoretical calculation of electronic structure and X-ray absorption near-edge structure of cathode materials for Li ion batteries

First principles calculations were performed on the electronic structure, chemical bonding and X-ray absorption near edge structure (XANES) of various lithium transition metal oxides. In the electronic structure using the discrete variational Xα method (DV-Xα), chemical bonding is changed by Li deintercalation. Li is found to be nearly ionized in LiMO₂ and strong covalent bonding between M and O is noted. The larger the difference of covalency between cation and nearest neighbor anion when Li intercalated/deintercalated is, the lower the voltage calculated by Vienna Ab Initio Simulation Package (VASP) is. By calculations of transition state, we reproduce the characteristics of the spectra as well as the chemical shifts and the origin of peaks appearing in the experimental XANES spectra is interpreted in terms of orbital interactions using bond overlap population diagrams.     

Co dimers observed by extended x-ray absorption fine structure spectroscopy

Cobalt was introduced into a silver matrix by ion implantation and observed by extended x-ray absorption fine structure (EXAFS) spectroscopy. In the range 0.10--0.70 at. % evidence of Co dimers dispersed in the matrix was found. The dimers are somewhat contracted with respect to the bulk Co nearest neighbor distance and distributed in a chainlike configuration with each dimer at 90 degrees from each other along opposite square faces of the Ag fcc lattice.     

An electronic structure study of europium chalcogenides by soft x-ray spectroscopy

The M_V, M_IV and M_III X-ray emission and absorption spectra of EuO and EuS have been studied. The relative positions and the widths of 4f states and valence or conduction distributions have been deduced from these spectra. These data are compared with those obtained by u.v. photoemission and the results of band structure calculations.     

Chemical states of nitrogen in ZnO studied by near-edge X-ray absorption fine structure and core-level photoemission spectroscopies

Interaction of low-energy nitrogen ions with ZnO surface has been studied by photoemission spectroscopy (PES) around N 1s core-level and near-edge X-ray absorption fine structure (NEXAFS) around N K-edge. Nitrogen can break Zn-O bonds at the surface and form N-O, Zn-N or Zn-N-O bonds, characterised by specific chemical shifts in PES or absorption peaks in NEXAFS. A distinctive signal from molecular nitrogen has also been observed in ion-bombarded samples in both NEXAFS and PES.