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The electronic structures of ZnO nanoparticles and microrod arrays are studied by O 1s X-ray absorption spectroscopy (XAS) and O Ka X-ray emission spectroscopy (XES). We show that the present LDACU SIC calculation approach is suitable to correct the LDA self-interaction error of the cation d-states. The atomic eigenstates of 3d in zinc and 2p in oxygen are energetically close, which induces strong Zn-3d-O-2p hybridization. This anomalous valence band cation-d-anion-p hybridization is affected when the localization of the Zn 3d-states is taken into account. Experimentally, the XES spectra show energy dependence in the spectral shape revealing selected excitations to the Zn 3d, 4s and 4p states, hybridized with O 2p states. Strong anisotropic effects are observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. The nanostructured ZnO has primarily bulk XAS and XES properties. q 2006 Elsevier Ltd. All rights reserved.

We studied the size dependence of the electronic structure of nanocrystalline copper embedded in silicon carbide by means of soft X-ray absorption spectroscopy. Changes in the local electronic states of copper nanoclusters, including the shift in binding energy and the reduction of s–p–d hybridization, occurred. The experimental result was compared with the ab initio self-consistent, real-space, multiple-scattering calculation. The calculation was in good agreement with the trend found in our experimental results. We concluded that the reduction of d–d interaction and the concomitant changes in s–p–d hybridization in copper nanoclusters arise due to the surface effect.

Highly aligned CuS nanorods have been studied by polarization dependent X-ray absorption spectroscopy. In contrast to bulk Cu2S, strong s, p, and d hybridization is found in the nanorods. The polar ...

Soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have been performed on CeAl{sub 2} bulk and nanoparticles of 8 nm to study the quantum size effect of this heavy-fermion compound. XAS and RIXS spectra recorded at selected photon energies, reflect the local unoccupied and occupied electronic states. These spectral shape changes are attributed to surface effects originated from the reducing particle size, which gives rise to different hybridization between the 4f orbitals and conduction band. The Ce in nanoparticles exhibits mixed valencies with a small portion of tetravalent Ce in contrast to the mostly trivalent Ce observed in the bulk CeAl{sub 2}. RIXS spectra show a strong dependence on the excitation energies. The experimental results suggest the different f-electron configurations for bulk and nanoparticle CeAl{sub 2}.

The electronic structure of pure poly(ethylene oxide) (PEO) for four different polymeric chain conformations has been studied by Hartree-Fock (HF) and density functional theory (DFT) through the analysis of their valence band photoelectron spectroscopy (VB-PES), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). It is shown that the valence band of PEO presents specific conformation dependence, which can be used as a fingerprint of the polymeric structures. The calculated spectra have been compared with experimental results for PEO powder.

Nanocrystalline copper clusters embedded in silicon carbide were made by island growth during sputter deposition. The distribution and morphology of metal clusters were observed by high-resolution transmission electron microscopy. To investigate chemical bonding at the copper–silicon carbide interface, we studied the electronic states of copper and silicon using X-ray photoelectron spectroscopy (XPS). It was found that the formation of copper silicide was suppressed in this system and that small shifts in binding energy were observed for different sizes of clusters, which was different from the chemical shift for copper silicide formation.

The molecular structure of alcohol-water mixtures determined by soft-X-ray absorption and emission spectroscopy

Changes in the electronic structure of V6O13 on lithium-ion insertion into battery cathodes were studied by soft x-ray absorption (SXA) spectroscopy and resonant soft x-ray emission (SXE) spectroscopy. SXA and resonant SXE spectra were recorded ex situ for cycled battery cathodes discharged to different potentials corresponding closely to distinct lithiated stages (LixV6O13,x=0,1,…,6). Large systematic changes were observed in the vanadium and oxygen x-ray spectra, reflecting the effects of electrochemical reduction associated with the Li-ion insertion. Spectral shape analysis indicates that a large fraction of the vanadium ions have been reduced to V3+ ions for the highest degree of lithiation, x=6. Nevertheless, further lithiation may be possible, in view of the linear development of the vanadium and oxygen bands on charge uptake.

We have measured X-ray absorption and emission near the C 1s edge of graphite electrodes cycled in lithium-ion battery cells. Resonantly excited emission spectra of graphite electrodes exhibit features characteristic of both highly oriented pyrolytic graphite as well as polycrystalline graphite. Spectra of three electrodes cycled in two different electrolytes are presented and compared with spectra of the pristine electrode. A solid electrolyte interphase (SEI) was detected on the electrochemically cycled electrodes. By the use of selective excitation, resonant X-ray emission spectra of the SEI-species were obtained and compared to spectra of reference compounds. The SEI on the cycled graphite anode was shown to comprise lithium oxalate (Li2C2O4), lithium succinate (LiO2CCH2CH2CO2Li) and lithium methoxide (LiOCH3).

Resonant soft X-ray emission (RSXE) spectra of NaV2O5, MoxV1-xO2 and V2O3 have been recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. Resonant excitation allows us, firstly, to separate V 3d and O 2p projected density-of-states of the valence band and, secondly, to study charge-neutral low-energy excitations due to resonant inelastic X-ray scattering (RIXS). We found that both the V L- and the O K-emission spectra clearly show components originating from O 2p- and V 3d-states, reflecting the high degree of hybridization of the valence band in all compounds. At threshold excitation we observed that NaV2O5 spectra are dominated by RIXS whereas MoxV1-xO2 and V2O3 spectra show bandlike features, which may be due to differences in the correlation effects of the compounds. We compared the RSXE spectra with cluster model calculations, which gives a good account for NaV2O5 whereas the RSXE spectra of the other compounds show RIXS only at certain energies well above the threshold. In fact, we interpret the trend in the RSXE spectra of the MoxV1-xO2 compound system as a successive filling of the (rigid) V 3d band with increasing Mo content. (C) 2003 Elsevier B.V. All rights reserved. (Less)

Resonant soft X-ray emission spectroscopy (SXES) has become a powerful tool for investigating the electronic structure of a large variety of materials and systems. This is due to the vastly improved performance of third generation high brightness synchrotron radiation sources and soft X-ray emission spectroscopy dedicated beamlines. The experiments presented here were performed at two synchrotron facilities: beamline 7.0.1 at the Advanced Light Source (Berkeley, CA, USA) and beamline I5-11c at MAXLAB (Lund, Sweden). We show that resonant soft X-ray emission spectroscopy can give valuable information about the electronic structure of highly correlated materials such as cuprates and vanadates. Recently, we have succeeded in constructing soft X-ray window-sealed liquid cell containers for the study of the electronic structure of liquid state samples. Here we present some first results from salt solutions showing how information on the water–ion interaction can be extracted.

Lithium insertion into nanoporous anatase TiO2 electrodes has been studied using resonant inelastic soft-x-ray scattering spectroscopy (RIXS). We have measured Ti 2p absorption, L2,3-emission and resonant inelastic scattering spectra of pristine anatase TiO2 and Li inserted TiO2. Inelastic scattering features in RIXS spectra of pristine TiO2 are assigned to charge-transfer excitations. Low energy-loss features for Li inserted TiO2 electrodes, appearing within t2g bands, due to d–d excitations, indicate the presence of Ti3+ states. The experimental observations suggest strong electron correlation in anatase TiO2 introduced by the Li-insertion.

Ex situ soft X-ray absorption spectroscopy (SXAS) and X-ray photoelectron spectroscopy (XPS) were used to study the redox behavior of sodium-containing vanadium oxide (Na−VOx) nanotubes in a rechar...

Resonant soft X-ray emission (RSXE) spectra of V 2 O 3, VO 2 and NaV 2 O 5 were recorded for a series of excitation energies at resonances of the V L- and O K-absorption band. The V L- and O K-emission in these vanadium oxide bands possess considerable overlap. By resonant excitation we can tune the energy to the absorption thresholds, thereby eliminating this overlap. Hereby we obtain the V 3d and O 2p projected density-of-states of the valence band. Resonant inelastic X-ray scattering (RIXS) is found to be weak in V 2 O 3, which we explain as being due to its metallic character at room temperature. Vanadium dioxide ( VO 2), semiconducting at room temperature, shows considerable RIXS features only at the O K-emission band. Distinct RIXS structures are visible in the RSXE spectra of the insulator NaV 2 O 5. In the emission spectra excited at the V L-thresholds of this ternary vanadium oxide, dd-excitations of the V dxy subband at an energy loss of -1.7 eV are observed. Our observation, that RIXS is stronger for insulators than for metals, should be taken advantage of for studying insulator-to-metal transitions in vanadium compounds in the future.

The electronic structure of LiC6 has been investigated by soft X-ray absorption and emission spectroscopies. The results reveal that upon full lithiation of graphite, the Li 2s electrons are transferred into the carbon π* states in a near rigid-band behavior, resulting in the increased density of states near EF and the shift of σ* states to lower energies. In addition, the resonant inelastic X-ray scattering spectra of LiC6 do not show strong dispersive features as that of graphite, indicating that the crystal momentum is not conserved during the scattering process due to the delocalization of electrons in the intermediate state.

Polarized resonant x-ray fluorescence spectroscopy at O $1s$ resonances was used to study ${\mathrm{CuGeO}}_{3}.$ Two distinct spectral contributions are identified, i.e., an intense ``main band'' of soft-x-ray fluorescence, reflecting primarily bandlike $\mathrm{O} 2p$ states, and resonant inelastic x-ray scattering (RIXS) characterized by spectral features that disperse linearly with excitation energy and resonate at the $\mathrm{O} 1s$ pre-edge peak. RIXS reflects excitoniclike (``optical'') final states and has previously only been observed for metal-ion core edges. Moreover, analysis of the excitation energy and polarization dependence of the main band allows the identification of features deriving from the site-specific partial density of states as well as strong evidence for the observation of a two-hole state (Zhang-Rice singlet).

A pre-study in a project aimed at increasing the understanding of drug solubility by applying X-ray spectroscopy to substances in solid phases, in aqueous solution, and in gas-phase is presented. Influence of the molecular surrounding on the local electronic structure is reflected in X-ray yield fine structure, and in site-selectively excited X-ray emission spectra. Results for atenolol and nadolol in solid form are discussed.

We report the results of Oxygen K-edge soft X-ray absorption and emission spectroscopy that was performed on an infinite-layer insulating cuprate thin film CaCuO2. Experimentally obtained spectra are consistent with local density approximation calculations. X-ray absorption spectra show a close resemblance to spectra obtained from homologous single crystal cuprates. In addition to d–d excitations, X-ray emission spectra reveal the presence of Zhang-Rice singlet states in the infinite-layer CuO2 planes. The question of whether the Zhang-Rice singlet features are masked by the O 2p main-band is addressed: it is possible to quantify the position of the Zhang-Rice singlet using emission intensity profiles. X-ray emission is demonstrated as a tool for understanding CuO2 planar electronic correlation in the prototypical infinite-layer. The energy difference, 2.0 eV, between the oxygen main-band and the Zhang-Rice singlet band is found to match values obtained theoretically using established planar electronic correlation parameters.

The electronic structure of pol(ethylene oxide) (PEO) in a thin (

We use x-ray spectroscopy to examine the electronic structure of high-temperature superconducting superlattices $[({\mathrm{Ba}}_{0.9}{\mathrm{Nd}}_{0.10}){\mathrm{CuO}}_{2+\ensuremath{\delta}}{]}_{2}/[{\mathrm{CaCuO}}_{2}{]}_{2}$. The O $2p$ density of states reveals the insulating character of the individual component layers and the metallic character of the superlattices. We report the first direct observation of Zhang-Rice singlets in artificial high-temperature superconducting heteroepitaxial structures. The experimental findings in the superlattices and its component layers offer evidence of charge transport from the so-called charge reservoir layer to the superconducting infinite layer. This suggests a strong link between superconductivity and both electron correlation and charge transfer within the superlattices.

Resonant soft x-ray emission (SXE) spectroscopy was used to study the electronic structure of LixV6O13 battery cathodes. We observe that the V 3d-bands of V6O13 exhibit a rather rigid behavior. Upon lithiation, electrons enter the top of the valence band and add intensity to the corresponding part of the V L-emission spectrum without significantly distorting the lower lying bands. We perform ab initio calculations which are in good agreement with the experimental results. Moreover, we find that lithiation leads to an overall decrease of the V 3d–O 2p hybridization. In contrast to x-ray diffraction, it is possible to study charge transfer effects in Li-batteries with SXE spectroscopy over the entire lithiation range.

$\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ absorption spectroscopy (XAS) and $\mathrm{O}\phantom{\rule{0.3em}{0ex}}K\ensuremath{\alpha}$ emission spectroscopy (XES) were performed to study the electronic structure of nanostructured ZnO. The band gap is determined by the combined absorption-emission spectrum. Resonantly excited XES spectra showing an energy dependence in the spectral shape reveal the selected excitations to the different $\mathrm{Zn}\phantom{\rule{0.3em}{0ex}}3d$, $4s$, and $4p$ states in hybridization with $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ states. The partial density of state obtained from local density approximation (LDA) and $\mathrm{LDA}+U$ calculations are compared with the experimental results. The $\mathrm{LDA}+U$ approach is suitable to correct LDA self-interaction error of the cation $d$ states. The atomic eigenstates of $3d$ in zinc and $2p$ in oxygen are energetically close, which induces the strong interaction between $\mathrm{Zn}\phantom{\rule{0.3em}{0ex}}3d$ and $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ states. This anomalous valence band cation-$d$--anion-$p$ hybridization is verified by taking into account the strong localization of the $\mathrm{Zn}\phantom{\rule{0.3em}{0ex}}3d$ states.

Lithium ion insertion into electrode materials is commonly used in rechargeable battery technology. The insertion implies changes in both the crystal structure and the electronic structure of the e ...

This thesis addresses the investigation of doubly excited 2l´nl states in helium atoms and double core excitations in solid lithium compounds.Measurements on He are made in field free environments and under the influence of electric and magnetic fields, using synchrotron based inelastic photon scattering. Cross sections for scattering to singly excited final states are directly determined and compared to theoretical results and are found to be in excellent agreement. Radiative and spin-orbit effects are quantified and are shown to play an important role in the overall characterization of highly excited He states below the N =2 threshold. A dramatic electric field dependence is also observed in the flourecence yield already for relatively weak fields. This signal increase, induced by electric as well as magnetic fields, is interpreted in terms of mixing with states of higher fluorescence branching ratios.Double core excitations at the lithium site in solid lithium compounds are investigated using resonant inelastic x-ray scattering (RIXS). The lithium halides LiF, LiCl, LiBr and LiI are studied as well as the molecular compounds Li2O, Li2CO3 and LiBF4. States with one, as well as both, of the excited electrons localized at the site of the bare lithium nucleus are identified, and transitions which involve additional band excitations are observed. A strong influence of the chemical surrounding is found, and it is discussed in terms of the ionic character of the chemical bond.

Resonant soft x-ray emission (SXE) spectra of the mixed valence vanadium oxide ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ have been recorded for a series of excitation energies across the V $L$-absorption band. Resonant excitation allows one to distinguish between charge neutral low-energy excitations and continuum-excited, more band-like V $3d$ valence band states in the spectra. We find that the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ consist of two distinct components that can be assigned to nearly pure V $3d$ states, and to V $3d$ states that are strongly hybridized with O $2p$ states, respectively. Band structure calculations of the density functional theory support the assignment of these features. At threshold excitation the V $L$-emission spectra of ${\mathrm{V}}_{6}{\mathrm{O}}_{13}$ show strong signatures from excitonic states, the energy dependence of which shows Raman-like behavior. We compare these spectral features in the resonant SXE spectra with cluster model calculations and assign them to $\mathrm{dd}$ excitations and charge-transfer excitations, respectively. Finally, we discuss changes in the V $L$-absorption and emission spectra that take place when changing the sample temperature from 295 K to 120 K. We relate the changes to redistributions in the V $3d$ partial density of states, which occur at the transition temperature for the metal-semiconductor-transition ${T}_{\mathrm{MST}}=150\mathrm{K}$ and find support in our temperature dependent band structure calculations.

The O Kalpha x-ray emission spectra of water clusters with different sizes and conformations embedded in a continuum medium are simulated. The calculations have successfully explained the experimen ...

We present now a possible way to carry out soft‐x‐ray fluorescence spectroscopy of liquids. The liquid cell has a window to attain compatibility with UHV conditions of the spectrometer and beamline, The synchrotron radiation enters the liquid cell through a 100 nm‐thick silicon nitride window and the emitted x‐rays exit through the same window. This allows in particular liquid‐solid interfaces to be studied. Such a liquid cell has been used to study the electronic structure of a variety of systems ranging from water solutions of inorganic salts and inertial drugs to nano materials and actinide compounds in their wet conditions.

We use x-ray emission spectroscopy to elucidate the molecular structure of liquid methanol, water, and methanol-water solutions. We find that molecules in the pure liquid methanol predominantly persist as hydrogen-bonded chains and rings with six and/or eight molecules of equal abundance. For water-methanol solutions we find evidence of incomplete mixing at the microscopic level. Our results provide a new explanation for a smaller entropy increase in the solution due to water molecules bridging methanol chains to form rings.

The effect of lithium insertion (lithiation) in electrodes of vanadium oxide nanotubes has been studied using resonant soft-x-ray emission spectroscopy (RSXES) and soft x-ray absorption spectroscopy (SXAS). Spectra at the V L edges were recorded ex-situ on a series of cycled electrodes that had been discharged to different potentials (3.0, 2.5, 2.0, and 1.8 V). Different chemical states upon discharge effectively correspond to different doping levels: the lower the cell potential, the higher the lithiation. The RSXES and SXAS spectra reflect the occupied and unoccupied electronic states, respectively. Spectral changes are observed as a function of the reduction process of vanadium by the electrochemical process. Lithiation induces a gradual enhancement of features attributed to occupied V 3d states in the RSXES spectra, indicating that the intercalated charges occupy bands of V 3d. Both SXAS and RSXES results suggest a partial reduction to V3+ at potentials below 2.0 V and the presence of a mixture of thr...

We use x-ray emission spectroscopy to examine the influence of the intermolecular interaction on the local electronic structure of liquid water. By comparing x-ray emission spectra of the water molecule and liquid water, we find a strong involvement of the a(1)-symmetry valence-orbital in the hydrogen bonding. The local electronic structure of water molecules, where one hydrogen bond is broken at the hydrogen site, is separately determined. Our results provide an illustration of the important potential of x-ray emission spectroscopy for elucidating basic features of liquids.