W. F. Pong

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.

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.

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.

Extended x-ray-absorption fine-structure studies of Zn sub 1 minus x Mn sub x Se alloy structure

Bond lengths, Debye-Waller factors, and site occupancy in the diluted magnetic semiconductor Zn{sub 1{minus}{ital x}}Mn{sub {ital x}}Se have been measured using extended x-ray-absorption fine structure. The nearest-neighbor bond lengths at both room temperature and low temperature (77 K) are found to be constant as a function of alloy composition within the experimental uncertainty of 0.01 A. Because the average cation-cation distance changes with Mn content, these results necessarily imply distortion of the tetrahedral bond angles. The anion sublattice is shown to suffer the largest distortion, but the cation sublattice also exhibits some relaxation. The repercussions of these results are discussed, in terms of the amount of cation and anion sublattice distortion at low temperature and its connection to the superexchange mechanism occurring between the Mn{sup 2+} ions and mediated by the intervening anion in Zn{sub 1{minus}{ital x}}Mn{sub {ital x}}Se.

Field emission enhancement in nitrogen-ion-implanted ultrananocrystalline diamond films

Enhanced electron field emission (EFE) properties for ultrananocrystalline diamond (UNCD) films grown on silicon substrate were achieved, especially due to the high dose N ion implantation. Secondary ion mass spectroscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy measurements indicated that the N ion implantation first expelled H−, induced the formation of disordered carbon (or defect complex), and then induced the amorphous phase, as the ion implantation dose increased. The postimplantation annealing process healed the atomic defects, but converted the disordered carbon to a stable defect complex, and amorphous carbon into a more stable graphitic phase. The EFE characteristics of the high dose (>1015ions∕cm2) ion-implanted UNCD were maintained at an enhanced level, whereas those of the low dose (<1014ions∕cm2) ion-implanted ones were reverted to the original values after the annealing process. Ion implantation over a critical dose (1×1015ions∕cm2) was required to improve the EFE propertie...

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...

Graphene Supported Graphone/Graphane Bilayer Nanostructure Material for Spintronics

We report an investigation into the magnetic and electronic properties of partially hydrogenated vertically aligned few layers graphene (FLG) synthesized by microwave plasma enhanced chemical vapor deposition. The FLG samples are hydrogenated at different substrate temperatures to alter the degree of hydrogenation and their depth profile. The unique morphology of the structure gives rise to a unique geometry in which graphane/graphone is supported by graphene layers in the bulk, which is very different from other widely studied structures such as one-dimensional nanoribbons. Synchrotron based x-ray absorption fine structure spectroscopy measurements have been used to investigate the electronic structure and the underlying hydrogenation mechanism responsible for the magnetic properties. While ferromagnetic interactions seem to be predominant, the presence of antiferromagnetic interaction was also observed. Free spins available via the conversion of sp2 to sp3 hybridized structures, and the possibility of unpaired electrons from defects induced upon hydrogenation are thought to be likely mechanisms for the observed ferromagnetic orders.

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.