Yun-Mui Yiu

Modification of surface band bending of diamond by low energy argon and carbon ion bombardment

Argon and carbon ion bombardment of p‐diamond at 500–5000 eV in ultrahigh vacuum were studied by in situ x‐ray photoelectron spectroscopy (XPS) and low energy electron diffraction analysis. Both argon and carbon ion bombardment at room temperature in the present energy range created a defective surface layer. The radiation damage was manifested by the introduction of a distinct C 1s peak (referred to as the ‘‘defect’’ peak later) with a binding energy about 1 eV less than that of the bulklike diamond peak, and by the introduction of some additional filled states (referred to as the ‘‘filled states’’) near the valence band edge of diamond. It was found that in comparison to argon bombardment, carbon bombardment was more efficient in producing the filled states but less efficient in raising the C 1s defect peak. While the filled states disappeared by annealing at about 500 °C, the C 1s defect peak did not change much even with a 1000 °C anneal. These results suggest that the C 1s defect peak, which has also...

Calcium L-edge XANES study of some calcium compounds

The Ca L3,2-edge XANES spectra of six calcium salts have been measured in both total electron and fluorescence yields using a high-resolution spherical grating monochromator. The compounds investigated were; CaF2, CaCO3, CaCl2 x 2H2O, calcium phosphate, calcium glycerophosphate and calcium gluconate. We find that the fine structure of the Ca L3,2-edges for each compound is unique and relates to the local structure of the Ca atom. The implications of these results to the study of the structure of calcium binding sites in systems of biologically interest will be discussed.

Ionic nature of Ge(II)-centered dications: a germanium K-edge X-ray absorption near edge structures study

Measurement of the ionic nature of [Ge(cryptand[2.2.2])](2+) by XANES has provided direct experimental evidence that the germanium center is best described as a nearly-naked dication encased within an electron rich cryptand cage.

X-ray excited optical luminescence (XEOL) studies of CaF2 at the Ca L3,2-edge

We report a study of the optical response of CaF2 specimens (crystal and powder) with excitation photon energy tuned across the Ca L3,2-edge. The luminescence was in turn used to monitor the absorption across the Ca L-edge (optical XAFS). It is found that the photoluminescence yield produces an inverted spectrum. This observation is interpreted in terms of total absorption (thickness effect), the change in decay dynamics below and above the edge and the atomic origin of the luminescence. By comparing crystal and powder results, we find that the surface component of CaF2 exhibits a positive shift (∼0.15 eV) relative to the bulk at the p–d resonance at the Ca L3,2-edge. A DFT calculation has been carried out and it is in good accord with the experiment. The implications of these results are discussed.

Electronic structure of nanopolycrystalline pulsed laser deposited LaB6 films and single crystals: The boron perspective

We report an investigation of the electronic structure of LaB6 nanopolycrystalline pulsed laser deposited (PLD) film and single crystal with x-ray absorption near edge structures (XANES) spectroscopy at the B K-edge. The experimental results are compared with theoretical calculations using density functional theory, and real space multiple scattering. It is found that (i) the LaB6 PLD film is of high quality and metallic albeit it is a polycrystalline phase of nanocrystallites and (ii) the B K-edge XANES for the single crystal specimens exhibit well defined spectral features corresponding to the theoretical partial densities of states of B p character and the threshold energy is relatively low and free electronlike, revealing its metallic character. The implications of these observations to the enhancement in electron emission from the PLD films are discussed.

X-ray absorption and luminescence studies of Ba2Ca(BO3)2: Ce3+/Na+ phosphors.

X-ray excited optical luminescence (XEOL) spectroscopy has been used to investigate the optical emission properties of Ce(3+) activated Ba(2)Ca(BO(3))(2) with a charge-compensating Na(+) and the results are compared with the optical emission properties from UV excitation. Further, x-ray absorption near-edge structure (XANES) has been employed to study the chemical environment and energy transfer efficiency to optical emission upon x-ray excitation. XEOL results agree well with optical emission with UV excitation. XANES results across various absorption edges show that while the chemical environment of host materials does not change significantly with doping, luminescence yield decreases significantly at absorption edges due to an abrupt change in the de-excitation mechanism.

Tracking Drug Loading Capacities of Calcium Silicate Hydrate Carrier: A Comparative X-ray Absorption Near Edge Structures Study.

Mesoporous spheres of calcium silicate hydrate (MS-CSH) have been prepared by an ultrasonic method. Following an earlier work in which we have revealed the interactions between ibuprofen (IBU) and CSH carriers with different morphologies by X-ray absorption near edge structures (XANES) analysis. In the present investigation, two new drug molecules, alendronate sodium (ALN) and gentamicin sulfate (GS), were incorporated into MS-CSH, and their drug loading capacities (DLCs) were measured using thermogravimetric analysis to establish the relationship between drug-carrier interactions and DLCs. The XANES spectra clearly indicate that acidic functional groups of the drug molecules linked to the active sites (Ca-OH and Si-OH groups) of MS-CSH on the surface by electrostatic interactions. In addition, it is found that the stoichiometric ratio of Ca(2+) ions of CSH carriers and the functional groups of drug molecules may significantly influence the DLCs.

The local structure and ferromagnetism in Fe-implanted SrTiO3 single crystals

We report a connection between the local structure of low-level Fe impurities and vacancies as the cause of ferromagnetic behavior observed in strontium titanate single crystals (STO), which were implanted with Fe and Si ions at different doses then annealed in oxygen. The effects of Fe doping and post-implantation annealing of STO were studied by X-ray Absorption Near Edge Structure (XANES) spectroscopy and Superconducting Quantum Interference Device magnetometry. XANES spectra for Fe and Ti K- and L-edge reveal the changes in the local environment of Fe and Ti following the implantation and annealing steps. The annealing in oxygen atmosphere partially healed implantation damages and changed the oxidation state of the implanted iron from metallic Fe0 to Fe2+/Fe3+ oxide. The STO single crystals were weak ferromagnets prior to implantation. The maximum saturation moment was obtained after our highest implantation dose of 2 × 1016 Fe atom/cm2, which could be correlated with the metallic Fe0 phases in addition to the presence of O/Ti vacancies. After recrystallization annealing, the ferromagnetic response disappears. Iron oxide phases with Fe2+ and Fe3+ corresponding to this regime were identified and confirmed by calculations using Real Space Multiple Scattering program (FEFF9).We report a connection between the local structure of low-level Fe impurities and vacancies as the cause of ferromagnetic behavior observed in strontium titanate single crystals (STO), which were implanted with Fe and Si ions at different doses then annealed in oxygen. The effects of Fe doping and post-implantation annealing of STO were studied by X-ray Absorption Near Edge Structure (XANES) spectroscopy and Superconducting Quantum Interference Device magnetometry. XANES spectra for Fe and Ti K- and L-edge reveal the changes in the local environment of Fe and Ti following the implantation and annealing steps. The annealing in oxygen atmosphere partially healed implantation damages and changed the oxidation state of the implanted iron from metallic Fe0 to Fe2+/Fe3+ oxide. The STO single crystals were weak ferromagnets prior to implantation. The maximum saturation moment was obtained after our highest implantation dose of 2 × 1016 Fe atom/cm2, which could be correlated with the metallic Fe0 phases in additi...

Experimental and theoretical XANES of CdSxSe1−x nanostructures

The morphology and electronic properties of the CdSxSe1−x nanostructures with varying alloy compositions have been acquired experimentally by X-ray Absorption Near-Edge Structures (XANES) at the Cd, Se and S K-edge and L3,2-edges. The theoretical XANES spectra have been calculated using the density functional approach. It is found that the optical band-gap emission of these CdSxSe1−x nano-ribbons can be tuned to the range between that of pure CdS (2.43 eV) and CdSe (1.74 eV) by changing the S and Se ratio. This gradual shift in (optical and structural) properties from CdS character to CdSe character is also seen in the electronic structures. The densities of states and band structures show that with the addition of Se replacing S in CdS, the band gap shrinks. The K and L3,2 edges of Cd, Se, and S of the XANES structures of both the CdS and CdSe in B4 (wurtzite) and B3 (cubic zinc-blende) structures have been calculated and compared.

Ab-initio Calculation of the XANES of Lithium Phosphates and LiFePO4

Lithium iron phosphate has been regarded as a promising cathode material for the next generation lithium ion batteries due to its high specific capacity, superior thermal and cyclic stability [1]. In this study, the XANES (X-ray Absorption Near Edge Structure) spectra of lithium iron phosphate and lithium phosphates of various compositions at the Li K, P L3,2, Fe M3,2 and O K-edges have been simulated self-consistently using ab-initio calculations based on multiple scattering theory (the FEFF9 code) and DFT (Density Functional Theory, the Wien2k code). The lithium phosphates under investigation include LiFePO4, γ-Li3PO4, Li4P2O7 and LiPO3. The calculated spectra are compared to the experimental XANES recorded in total electron yield (TEY) and fluorescence yield (FLY). This work was carried out to assess the XANES of possible phases presented in LiFePO4 based Li ion battery applications [2].