Jinghua Guo

Nitrogen bonding structure in carbon nitride thin films studied by soft x-ray spectroscopy

Soft x-ray absorption (SXAS) and emission (SXES) spectroscopies were applied to study the nitrogen bonding structure in magnetron sputtered CNx thin films. By comparing with calculated spectra of N in different model systems, N in three main bonding environments can be identified: (i) C≡N bonds, with a sharp SXAS peak at 399.5 eV, (ii) pyridine-like N (i.e., N bonded to two C atoms), with an x-ray absorption resonance at ∼398.5 eV, and (iii) N substituted in graphite, possibly with one sp3 carbon as a neighbor (SXAS energy ∼401 eV). These bondings are present in all CNx films analyzed; however, as shown earlier, the relative intensities between the peaks may vary with the growth conditions. Differences in the coordination of the nearest or second nearest C neighbors only cause slight changes in the peak positions and spectrum shape.

Structural and electronic properties of low dielectric constant fluorinated amorphous carbon films

Fluorinated amorphous carbon (a-CFx) films were studied by high-resolution x-ray absorption, emission, and photoelectron spectroscopy. The composition and local bonding information were obtained and correlated with substrate temperature during deposition. The data suggest that the structure of the a-CFx is mostly of carbon rings connected by CF2 groups. The cross linking increases with substrate temperature.

Polarization-dependent soft-x-ray absorption of highly oriented ZnO microrod arrays

Polarization-dependent x-ray absorption measurements were performed on crystalline ZnO three-dimensional arrays consisting of highly oriented microrods as well as on particulate thin films consisting of monodisperse spherical nanoparticles. Strong anisotropic effects have been observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. Full-potential calculations of the orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including Zn 3d among the valence states, show very good agreement with the experimental findings. Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II–VI semiconductor.

Instrumentation for soft X-ray emission spectroscopy

Abstract An account is presented of developments in instrumentation for soft X-ray emission spectroscopy (SXES) based on synchrotron radiation. An account for grating spectrometers for soft X-ray emission spectroscopy is given, and some considerations regarding synchrotron radiation applications of the spectroscopy are presented. A few points that relate to the new features of resonant SXES and polarization dependent studies are discussed in some detail. A brief discussion on future developments in SXES instrumentation is included.

The Effect of Thermal Reduction on the Photoluminescence and Electronic Structures of Graphene Oxides

Electronic structures of graphene oxide (GO) and hydro-thermally reduced graphene oxides (rGOs) processed at low temperatures (120–180°C) were studied using X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). C K-edge XANES spectra of rGOs reveal that thermal reduction restores C = C sp2 bonds and removes some of the oxygen and hydroxyl groups of GO, which initiates the evolution of carbonaceous species. The combination of C K-edge XANES and Kα XES spectra shows that the overlapping π and π* orbitals in rGOs and GO are similar to that of highly ordered pyrolytic graphite (HOPG), which has no band-gap. C Kα RIXS spectra provide evidence that thermal reduction changes the density of states (DOSs) that is generated in the π-region and/or in the gap between the π and π* levels of the GO and rGOs. Two-dimensional C Kα RIXS mapping of the heavy reduction of rGOs further confirms that the residual oxygen and/or oxygen-containing functional groups modify the π and σ features, which are dispersed by the photon excitation energy. The dispersion behavior near the K point is approximately linear and differs from the parabolic-like dispersion observed in HOPG.

First results from the SpectroMicroscopy Beamline at the Advanced Light Source

The SpectroMicroscopy Facility at the Advanced Light Source is based on a high brightness, high‐resolution beamline, and includes a collection of projects designed to exploit the unique characteristics of the soft x‐ray beam. The beamline itself is comprised of a 5‐m long, 5‐cm‐period undulator, a spherical‐grating monochromator with water‐cooled gratings. Adaptive optics refocus the monochromatic beam to two ‘‘microfocus’’ experimental stations with spot sizes less than 50 μm diameter and a third ‘‘nanofocus’’ station uses a zone‐plate lens to further demagnify the microfocus spot. Experimental stations include an ‘‘ultraESCA’’ spectrometer for small‐area spectroscopy and photoelectron diffraction, a scanning transmission x‐ray microscope, and photoelectron microscopes. Commissioning experiments of microscopic actinide photoemission, surface‐core‐level photoelectron diffraction, and high‐resolution soft x‐ray fluorescence demonstrate dramatic improvements in sensitivity due to the high brightness and sma...

Electronic structure of VO2 studied by x-ray photoelectron and x-ray emission spectroscopies

A single crystal has been the subject of a combined investigation by high-resolution x-ray photoelectron spectroscopy (XPS), x-ray emission spectroscopy (XES) with both electron and energy-selective x-ray excitation (, and emission) and x-ray absorption spectroscopy (XAS) . We performed first-principles tight-binding LMTO band-structure calculations for in both the monoclinic and tetragonal rutile phases and compared the densities of states (DOS) with the experimental data. From this we conclude that the electronic structure of is more bandlike than correlated.

Resonant x-ray scattering beyond the Born-Oppenheimer approximation: Symmetry breaking in the oxygen resonant x-ray emission spectrum of carbon dioxide

Although resonant x-ray scattering of molecules fulfills strict electronic symmetry selection rules, as now firmly proven by spectra of diatomic molecules, the accumulated body of data for polyatomic molecules indicates that an apparent breaking of these rules represents the common situation rather than the exception. The CO2 molecule provides a good example of symmetry breaking, with the oxygen x-ray emission spectra showing strong intensity for transitions that are forbidden by the parity selection rule. We present time-independent and time-dependent theories for frequency-dependent resonant x-ray scattering beyond the Born–Oppenheimer approximation in order to explore under what circumstances one can anticipate symmetry breaking in the spectra of polyatomic molecules. The theory starts out from the Kramers–Heisenberg dispersion relation and is generalized for vibrational degrees of freedom and for nonadiabatic coupling of the electronic (vibronic) states, including the frequency dependency of the scatt...

Resonant and nonresonant x-ray scattering spectra of some poly(phenylenevinylene)s

The electronic structure of some poly(phenylenevinylene)s have been investigated by resonant and nonresonant x-ray inelastic scattering spectroscopies. The nonresonant as well as all resonant spect ...

Selectively excited X-ray emission spectra of N2

Abstract Resonant and non-resonant X-ray emission spectra of N2 are presented. The role of the parity selection rule in the case of homonuclear diatomic molecules is discussed thoroughly and illustrated by the X-ray emission spectra. We also discuss how the parity selection rule and the angular anisotropy of the resonant X-ray emission may affect intensities in a fluorescence yield measurement of N2. Simulated vibrational band profiles calculated using the lifetime–vibrational interference formula are found to be in excellent agreement with the experimental emission bands.