Search

This study used O K-, Zn L3-, Zn K-, and Al K-edges x-ray absorption near-edge structure (XANES) and O K-edge x-ray emission spectroscopy (XES) measurements to investigate the electronic structure of transparent Al-doped ZnO (AZO) thin film conductors. The samples were prepared on glass substrates at a low temperature near 77 K by using a standard RF sputtering method. High-purity Ne (5N) was used as the sputtering gas. The crystallography of AZO thin films gradually transformed from the ZnO wurtize structure to an amorphous structure during sample deposition, which suggests the suitability to grow on flexible substrates, eliminating the severe degradation due to fragmentation by repeated bending. The O K- and Zn L3-edges XANES spectra of AZO thin films revealed a decrease in the number of both O 2p and Zn 3d unoccupied states when the pressure of Ne was increased from 5 to 100 mTorr. In contrast, Al K-edges XANES spectra showed that the number of unoccupied states of Al 3p increased in conjunction with the pressure of Ne, indicating an electron transfer from Al to O atoms, and suggesting that Al doping increases the negative effective charge of oxygen ions. XES and XANES spectra of O 2p states at the O K-edge also revealed that Al doping not only raised the conduction-band-minimum, but also increased the valence-band-maximum and the band-gap. The results indicate that the reduction in conductivity of AZO thin films is due to the generation of ionic characters, the increase in band-gap, and the decrease in density of unoccupied states of oxygen. [ABSTRACT FROM AUTHOR]

Owing to the current energy crisis and extreme changes in the global climate, there is great interest in finding renewable energy resources. Vast progress has been made in the development of new materials related to renewable energy, and their physical/chemical properties can be tailored by nanostructuring and other advanced synthetic approaches. In many important energy systems, such as solar hydrogen systems, the atomic/electronic structures of materials and fundamental interfacial phenomena of systems critically determine the energy conversion efficiency of materials [1, 2]. Without knowledge of the fundamental electronic structures of the materials during conversion reactions, better engineering of the material for practical use is difficult. Understanding and controlling the interfaces in energy generation/conversion/storage materials requires in-situ/operando approaches [3, 4]. The Taiwan Photon Source (TPS) Soft X-ray Spectroscopic beamline provides the capabilities for X-ray absorption (XAS) and X-ray emission (XES) spectroscopies, which can be utilized to investigate unoccupied (conduction-band) and occupied (valence-band) electronic states, respectively. Moreover, resonant inelastic X-ray scattering (RIXS) can be used to study intra-band (includingd-dorf-fexcitations) and inter-band (charge transfer) transitions [5, 6]. The former provides details about electronic energy splitting in various crystal fields and the latter involves electron transfer between a metal and a ligand, which determines chemical activity [7, 8]. [ABSTRACT FROM AUTHOR]

We have investigated the electronic structure of GdBaCo2O5.5 across the metal--insulator transition (MIT) using soft x-ray absorption and photoelectron spectroscopy. For the low-temperature insulating phase, we find that half of the Co3+ ions at the octahedral sites are in the low spin (LS) and the other half in the high spin (HS) state, while the Co3+ ions at the pyramidal sites are in the HS configuration. Upon increasing the temperature across the MIT, part of the LS octahedral Co3+ undergoes a spin-state transition into the HS configuration. We infer that this destroys the spin-state ordering and thus explains the decrease in resistivity. We observed that the band gap is reduced but not closed in the high-temperature phase. [ABSTRACT FROM AUTHOR]

We have performed x-ray absorption spectroscopy (XAS) measurements on Zn1-2xMnxCoxO nanoparticles. From the XAS results, it seems that the Mn and Co ions are in a mixed-valence (2+, 3+, and 4+) state and the relative concentrations of the high-valence (3+ and 4+) Mn and Co ions are higher in the surface region than in the deep core region. We suggest that this is a distinct trend of nanoparticle diluted magnetic semiconductor (DMS) unlike the case of DMS in film and bulk forms, where the transition-metal ions are expected to be 2+. [ABSTRACT FROM AUTHOR]

Nitrogen (N) and metal (Al, Ga, and In) K-edge x-ray absorption near-edge structure (XANES), x-ray emission spectroscopy (XES), and Raman scattering measurements were performed to elucidate the electronic structures of group-III–nitride nanorods and thin films of AlN, GaN, and InN. XANES spectra show slight increase of the numbers of unoccupied N p states in GaN and AlN nanorods, which may be attributed to a slight increase of the degree of localization of conduction band states. The band gaps of AlN, GaN, and InN nanorods are determined by an overlay of XES and XANES spectra to be 6.2, 3.5, and 1.9 eV, respectively, which are close to those of AlN and GaN bulk/films and InN polycrystals. [ABSTRACT FROM AUTHOR]

The electronic structure and magnetic properties of Al-doped Fe3O4 films at various Al concentrations were investigated by Al and O K-edge x-ray absorption near-edge structure (XANES) and Fe L3,2-edge magnetic circular dichroism (MCD) measurements. The O K-edge XANES spectra reveal that the intensity of O 2p–Fe 3d hybridized states decreases linearly as the Al concentration increases, suggesting that nonmagnetic Al substitution reduces couplings of the O 2p with Fe 3d orbitals. The Fe L3-edge MCD spectra show that the intensities of MCD features decrease as the Al concentration increases, which suggests that Al substitution reduces the magnetic moments of Fe ions. The Fe L3,2 XANES spectra suggest the absence of mixed valence states in agreement with previous x-ray resonance scattering measurements. [ABSTRACT FROM AUTHOR]

X-ray absorption spectroscopy has been used to investigate the electronic structure of as-deposited and oxidized Ni/Au contacts on p-GaN. The Ni K-, L[sub 2,3]-, and O K-edges x-ray absorption spectra clearly show the formation of NiO in the annealed contacts. Annealing in air increases Ni-site hole concentration and slightly shortens the nearest-neighbor Ni-O bond length, which enhances p-d hybridization and charge transfer from Ni to O. The observed very low specific contact resistance in the oxidized contacts is found to be due to the enhanced hole concentration at the Ni site. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The correlation between the p-type hole conduction and the electronic structures of Cr-deficient CuCr1-xO2 (x = 0-0.1) compounds was investigated using O K-, Cu, and Cr L3.2-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy, and x-ray emission spectroscopy measurements. XANES spectra reveal a gradual increase in the Cu valence from Cu1+ to Cu2+ with increasing Cr deficiency x, whereas, the valence of Cr remains constant as Cr3+. These results indicate that the p-type conductivity in the CuCr1-xO2 samples is enhanced by a Cu1+-O-Cu2+ rather than a Cr3+Cr4+ or direct Cu1+-Cu2+ hole mechanism. Remarkable Cr-deficiency-induced changes in the densities of Cu 3d, Cu 3d-O 2p, and O 2p states at or near the valence-band maximum or the Fermi level were also observed. In addition, a crossover of conduction mechanism from thermally activated (TA) hopping to a combination of TA and Mott's three-dimensional variable range hopping occurs around 250 K. [ABSTRACT FROM AUTHOR]

We have determined the ground-state wave functions and crystal-field-level schemes of CeRh2Si2 and CeRu2Si2 using linear polarized soft x-ray-absorption spectroscopy (XAS) and inelastic neutron scattering, We find large crystal-field splittings and ground-state wave functions which are made of mainly Jz = | ±5/2) with some amount of |∓3/2) in both the compounds. The 4f° contribution to the ground state of several members of the CeM2Si2 family with M = (Cu, Ru. Rh. Pd, and Au) has been determined with XAS, and the comparison reveals a trend concerning the derealization of the f electrons. Absolute numbers are extracted from scaling to results from hard x-ray photoelectron spectroscopy on CeRu2Si2 by Yano et al. [Phys. Rev. B 77. 0351IX (2008)]. [ABSTRACT FROM AUTHOR]

We investigated the magnetic nature of Fe1/4TaS2 using x-ray absorption spectroscopy, photoemission spectroscopy, and first principles band calculations. The results show a large unquenched orbital magnetic moment (~ 1.0 μB/Fe) at intercalated Fe sites, resulting in a gigantic magnetic anisotropy (HA ≃ 60 T). The magnetic coupling is well understood in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, suggesting a novel RKKY ferromagnet with Ising-type spin states. We also found that this indirect exchange coupling between the neighboring Fe spins is ferromagnetic and maximized at the Fe-Fe distance of 2 x 2 superstructure. [ABSTRACT FROM AUTHOR]

We addressed the so-far unresolved issue concerning the Co valence in the superconducting bilayer hydrated NaxCoO2 yH2O (x ~ 0.35, y ~ 1.3) using soft x-ray absorption spectroscopy at the Co-L2,3 and O-K edges. We find that the valence state of the Co lies in a narrow range from +3.3 to +3.4 for all studied NaxCoO2 yH2O samples and their deuterated analogue with Tc's ranging from 3.8 to 4.7 K. These valence values are far from the often claimed +3.7, the number based on the Na content only. We propose to modify the phase diagram accordingly, where the basic electronic structure of the super- conducting phase is very close to that of the Na0.7CoO2 system, suggesting that the presence of in-plane spin fluctuations could play an important role for the superconductivity. [ABSTRACT FROM AUTHOR]

We report O K-, Ca K- and L3,2-, and Ti L3,2-edge X-ray absorption near edge structure (XANES) spectra of Ba1-xCaxTiO3 (x=0, 0.01, 0.08, and 1) and the electronic properties inferred from these XANES spectra. The spectra of O K-, Ca L3,2- and Ti L3,2-edges show characteristic spectral features attributable to the t2g and eg bands. The Ti and Ca L3,2-edge spectra contain two sets of L3 and L2 features with a L3–L2 separation of about 5.5 and 3.4 eV, respectively. We also observe a pre-edge feature in the Ca K-edge spectra and drastically reduced t2g features in the Ca L3,2-edge spectra. Our XANES spectra reveal that the Ca 3d bands are low-lying and the Ca 3d t2g bands are partially occupied. [Copyright &y& Elsevier]

Abstract: We report spectroscopic evidence for orbital-mediated phases in strained thin films by combining soft X-ray spectroscopy and synthesis of manganite thin films. Measurements of polarization-dependent soft X-ray absorption reveal that electronic states responsible for the lowest-energy excitations in C-type antiferromagnetic films have an orbital symmetry of , while those in A-type AFM films have an orbital symmetry of . Such orbital polarizations in strained films of manganite result from a combined effect of the Jahn–Teller distortion and the electron correlations of Mn 3d electrons. [Copyright &y& Elsevier]