J. W. Chiou

Diameter dependence of the electronic structure of ZnO nanorods determined by x-ray absorption spectroscopy and scanning photoelectron microscopy

OK-, ZnL3, and K-edges x-ray absorption near-edge structure (XANES) spectra and scanning photoelectron microscopy (SPEM) spectra were obtained for ZnO nanorods with various diameters. The analysis of the XANES spectra revealed increased numbers of O2p and Zn4p unoccupied states with the downsizing of the nanorods, which reflects the enhancement of surface states when the diameter is decreased. Valence-band photoemission spectra show a significant narrowing of the valence band for the 45nm diameter nanorod. The Zn3d intensities in the Zn3d SPEM spectra are drastically diminished for all nanorods as compared to the ZnO reference film, which can be interpreted as a reduction in density of itinerant final states or in transition probability.

Electronic structure of ZnO nanorods studied by angle-dependent x-ray absorption spectroscopy and scanning photoelectron microscopy

Angle-dependent x-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy measurements were performed to differentiate local electronic structures at the tips and sidewalls of highly aligned ZnO nanorods. The overall intensity of the O K-edge XANES spectra is greatly enhanced for small photon incident angles. In contrast, the overall intensity of the Zn K-edge XANES is much less sensitive to the photon incident angle. Both valence-band photoemission and O K-edge XANES spectra show substantial enhancement of O 2p derived states near the valence band maximum and conduction band minimum, respectively. The spatially resolved Zn 3d core level spectra from tip and sidewall regions show the lack of chemical shift. All the results consistently suggest that the tip surfaces of the highly aligned ZnO nanorods are terminated by O ions and the nanorods are oriented in the [0001] direction.

Electronic structure of the carbon nanotube tips studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

Angle-dependent x-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been performed to differentiate local electronic structures of the tips and sidewalls of highly aligned carbon nanotubes. The intensities of both π*- and σ*-band C K-edge XANES features are found to be significantly enhanced at the tip. SPEM results also show that the tips have a larger density of states and a higher C 1s binding energy than those of sidewalls. The increase of the tip XANES and SPEM intensities are quite uniform over an energy range wider than 10 eV in contrast to earlier finding that the enhancement is only near the Fermi level.

Structural, electrical transport and x-ray absorption spectroscopy studies of LaFe1−xNixO3 (x⩽0.6)

Electronic structures of LaFe1−xNixO3 (x⩽0.6) have been studied by x-ray absorption near edge structure spectra of OK, FeL2,3 and LaM4,5 edges. Upon substitution of Ni at Fe site in LaFeO3, the OK-edge spectra show a feature about 2.0eV lower than that of LaFeO3. This feature is growing as the concentration of Ni is increasing. This is consistent with our resistivity data which show that the resistivity decreases very fast with Ni substitution from GΩcm for LaFeO3 to a few mΩcm for the sample with 60% Ni substitution. The resistivity data have been fitted with a variable-range hopping model and it is found that the gap parameter reduces from 2eV to 2.1meV with the Ni substitution. This gap parameter decreases very systematically with the increase in Ni concentration. The structural analysis of these samples shows that they have single-phase orthorhombic structure with space-group Pnma in the studied range (0⩽x⩽0.6). The study of FeL2,3-edge structures confirm the trivalent state of Fe. The observed featur...

Electronic properties of a‐CNx thin films: An x-ray-absorption and photoemission spectroscopy study

The electronic properties of amorphous carbon nitride were studied by x-ray-absorption near-edge structure (XANES) and valence-band photoelectron spectroscopy (PES). The nitrogen incorporation was found to induce graphitization, as evidenced by an increase of the sp2 cluster in C and N K-edge XANES spectra. The structure is found to be similar to pyridine. Hybridized C–N bond lengths were determined from the position of the σ* resonance of XANES spectra and the obtained results suggest sp2 hybridization. A valence-band PES spectrum showed that the p‐π band became more intense than the p‐σ band upon higher at. % nitrogen addition, which confirmed the role played by the π bonds in controlling the electronic structure of a‐CNx films.

Electronic structures of Ba1−xCaxTiO3 studied by x-ray absorption spectroscopy and theoretical calculation

We report O and Ca K-edges x-ray absorption near edge structure (XANES) spectra of Ba1-xCaxTiO3 (x = 0.01 and 0.08), BaTiO3 and CaTiO3 and the electronic structure of Ba0.875Ca0.125TiO3 obtained by first-principles calculation. The characteristic features in the O K-edge XANES spectra of these ferroelectric perovskites are influenced by the Ca concentration. They differ substantially from those of the reference TiO2. The O K-edge spectra suggest that the combination of the alkaline-earth-metal oxides, CaO and/or BaO, with TiO2 enhance the effective charge of the O ions. Thus, a large dipole moment may result from the displacement of the Ti ion from the centre of the TiO6 octahedron leading to collective displacement of Ti ions through attractive dipole-dipole couplings and may give rise to ferroelectricity. In the Ca K-edge XANES spectra there is a pre-edge feature similar to those found in other 3d transition-metal perovskites, which may provide information about hole doping.

X-ray absorption spectroscopy (XAS) study of dip deposited a-C:H(OH) thin films

This work measures the C and O K-edge x-ray absorption near-edge structure (XANES) spectra of hydrogenated amorphous carbon (a-C:H) films deposited at various baking temperatures Tb (Tb = 300–500 °C at 50 °C). The C–H σ* peak related to the content of the sp2 graphite-like bonding in the C K-edge spectra was found to yield to the C–H π* peak related to the sp3 diamond-like bonding at high temperature (500 °C). We find that the intensities of both the sp2 and sp3 features in the C K-edge XANES spectra decrease with increase of Tb, which suggests an increase of the defect concentration with Tb. The intensities of the O K-edge XANES spectra are found to decrease with increase of Tb, which suggests thermally induced decomposition of carbonyl contaminants on the surface. The elemental analysis C/O (or O/C) ratio was obtained from XPS spectra and indicates that films are not hydrogenated amorphous carbon but rather oxyhydrogenated amorphous carbon thin films.

Electron- and hole-doping effects on the electronic structure of manganite studied by x-ray absorption spectroscopy

The electronic structures of hole-doped La0.7Ca0.3MnO3 and electron-doped La0.7Ce0.3MnO3 manganites are investigated by x-ray absorption near-edge structure spectroscopy at the O and Mn K-, and Mn L3,2-edges. The Mn K- and L3,2-edge results show that Ce dopants increase the occupation of the Mn 4p and majority-spin eg orbitals and reduce the positive effective charge of some Mn ions. However, Ce doping also induces holes in O 2p derived states. As for La0.7Ca0.3MnO3, in contrast to previous understanding that Ca doping converts some Mn ions into the Mn4+ state, we find that Ca dopants actually increase the number of majority-spin eg electrons. We find instead that the holes created by Ca dopants are in the O 2p derived states.

Electronic structure of GaN nanowire studied by x-ray-absorption spectroscopy and scanning photoelectron microscopy

X-ray absorption near edge structure (XANES) and scanning photoelectron microscopy (SPEM) measurements have been employed to obtain information on the electronic structures of the GaN nanowires and thin film. The comparison of the XANES spectra revealed that the nanowires have a smaller (larger) N (Ga) K edge XANES intensity than that of the thin film, which suggests an increase (decrease) of the occupation of N 2p (Ga 4p) orbitals and an increase of the N (Ga) negative (positive) effective charge in the nanowires. The SPEM spectra showed that the Ga 3d band for the nanowires lies about 20.8 eV below the Fermi level and has a chemical shift of about −0.9 eV relative to that of the thin film.

Effect of the Ca content on the electronic structure of Pb1−xCaxTiO3 perovskites

This study performs O K- and Ti L3,2-edge x-ray absorption near-edge structure (XANES) measurements and first-principles pseudopotential calculations for the electronic structures of ABO3-type Pb1−xCaxTiO3 (x=0–1) perovskites. The features in the O K-edge XANES spectra are found to be contributed primarily by hybridization between O 2p and Ti 3d, Pb 6p, and Ca 3d orbitals. The O K-edge XANES spectra reveal that partial substitution of A cations, Pb, by Ca not only decreases O 2p–Pb 6p but also O 2p–Ti 3d hybridization. The Ti L3,2-edge measurements find that the off-center displacement of Ti, and hence, ferroelectricity persist up to a Ca concentration between 0.3 and 0.4.