Thorsten Schmitt

System for in situ studies of atmospheric corrosion of metal films using soft x-ray spectroscopy and quartz crystal microbalance

We present a versatile chamber (atmospheric corrosion cell) for soft x-ray absorption/emission spectroscopy of metal surfaces in a corrosive atmosphere allowing novel in situ electronic structure studies. Synchrotron x rays passing through a thin window separating the corrosion cell interior from a beamline vacuum chamber probe a metal film deposited on a quartz crystal microbalance (QCM) or on the inside of the window. We present some initial results on chloride induced corrosion of iron surfaces in humidified synthetic air. By simultaneous recording of QCM signal and soft x-ray emission from the corroding sample, correlation between mass changes and variations in spectral features is facilitated.

X-ray absorption and emission spectroscopy of ZnO nanoparticle and highly oriented ZnO microrod arrays

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.

Resonant soft X-ray emission spectroscopy of doped and undoped vanadium oxides

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

Resonant soft X-ray emission spectroscopy of V2O3, VO2 and NaV2O5

Resonant soft X-ray emission (RSXE) spectra of V2O3, VO2 and NaV2O5 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 V2O3, which we explain as being due to its metallic character at room temperature. Vanadium dioxide (VO2), 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 NaV2O5. 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.

Resonant soft X-ray emission of solids and liquids

Abstract 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.

The electronic structure and lithiation of electrodes based on vanadium-oxide nanotubes

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 Vu200a3d states in the RSXES spectra, indicating that the intercalated charges occupy bands of Vu200a3d. 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...

Momentum selectivity and anisotropy effects in the nitrogen K-edge resonant inelastic x-ray scattering from GaN

High-resolution soft x-ray emission and absorption spectra near the N K -edge of wurtzite GaN are presented. The experimental data are interpreted in terms of full-potential electronic structure ca ...

Li insertion into V6O13 battery cathodes studied by soft x-ray spectroscopies

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.

Observation of multiple Zhang-Rice excitations in a correlated solid: Resonant inelastic X-ray scattering study of Li2CuO2

Multiple Zhang-Rice type spectral features have been observed in resonant inelastic X-ray scattering (RIXS) from the quasi–one-dimensional cuprate charge transfer insulator Li2CuO2. The first feature appears at constant emission energy, and is associated with a Zhang-Rice singlet final state. The second is an interplaquette charge transfer excitation that results in a novel triplet Zhang-Rice–type final state. It is accompanied by the presence of a O 2p nonbonding to upper Hubbard band excitation at an energy close to that of a calculated triplet charge transfer Zhang-Rice–type excitation. The site selectivity and polarization rules associated with RIXS allows these two excitations to be distinguished.

Internal symmetry and selection rules in resonant inelastic soft x-ray scattering

Resonant inelastic soft x-ray scattering spectra excited at the dissociative 1 sigma(g) -> 3 sigma(u) resonance in gas-phase O(2) are presented and discussed in terms of state-of-the-art molecul ...