Interfacial properties and characterization of Sc/Si multilayers

We investigate the intermixing of layers in Sc/Si and Sc/B₄C/Si/B₄C multilayers using electron and synchrotron excited soft X-ray emission and absorption spectroscopy. The multilayers are annealed at 100, 200, 300, 400 and 500 °C after preparation by magnetron sputtering. Silicon K_β emission and reflectivity measurements verify that the non-annealed multilayer systems are composed of distinct layers with only a minor interdiffusion in Sc/Si samples whereas annealing Sc/Si multilayers at 400 °C leads to a degradation of the multilayer structure and the formation of intermittent scandium silicide, ScSi. The presence of B₄C barriers in Sc/B₄C/Si/B₄C hinders this degradation from developing for the entire temperature range considered. The barrier layers continue to be effective for the entire temperature range even after an extended shelf-life.     

Structure and bandgap determination of InN grown by RP-MOCVD

InN thin films are grown on sapphire substrates by remote plasma-assisted metal organic chemical vapor deposition while varying the indium pulse length and substrate temperature. The effects of the indium pulse length and temperature on the structural, morphological, electronic, and optical properties of the thin films are studied. The structural parameters are determined by X-ray diffraction and X-ray photoelectron spectroscopy and the effects of incorporating oxygen atoms in the structure is described. The N K-edge X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) measurements are used to determine the band gap and it is found to be 1.80?±?0.25 eV for all samples. A complementary measurement namely, X-ray excited optical luminescence measurement is performed to confirm the band gap value obtained from XAS and XES measurements. O K-edge XAS measurements are performed to determine the presence of oxygen impurities in the samples. Meanwhile, we carry out the density functional theory calculations for Wurtzite InN, hypothetical Wurtzite-type InO_0.5N_0.5, and InO_0.0625N_0.9375 structures. We find that the measured N-edge spectra agree well with our Wurtzite InN calculations and the measured O K-edge spectra agree better with hypothetical Wurtzite-type InO_0.0625N_0.9375 than Wurtzite-type InO_0.5N_0.5.

     

The Metallic Nature of Epitaxial Silicene Monolayers on Ag(111)

Silicene is a two‐dimensional structure composed of a buckled hexagonal honeycomb lattice of silicon atoms. Freestanding silicene is yet to be synthesized, but epitaxial silicene monolayers have been directly observed or predicted to exist on a number of supporting substrates. Herein the atomic and electronic structures of five distinct epitaxial silicene morphologies on Ag(111) are examined through the complementary techniques of density functional theory and soft X‐ray spectroscopy at the Si L_2,3 edge. Hybridization with the Ag(111) substrate is shown to cause these silicene monolayers to become strongly metallic, and the specific electronic interactions that are responsible for this metallic nature are determined. The results imply that epitaxial silicene on Ag(111) does not possess the Dirac cone electronic structure that is characteristic of freestanding silicene and graphene sheets.     

Polymer conversion into amorphous ceramics by ion irradiation

We present results of X-ray fluorescence measurements (carbon K and Si L_2,3) of methylhydroxylsiloxane and phenylmethylhydroxylsiloxane films. The films are converted into amorphous ceramics by irradiation with 500 keV C and 3 MeV Au ions. Changes in the composition (O/Si ratio) of some polysiloxane films are found when treated with ion irradiation. It is shown that ion irradiation induces a segregation of carbon atoms from methyl groups into diamond-like clusters whereas carbon atoms from phenyl groups maintain their sp² configuration.     

Element-specific electronic structure of Mn dopants and ferromagnetism of (Zn,Mn)O thin films

Element-specific electronic structure of (Zn,Mn)O thin films with various Mn concentrations has been investigated using X-ray absorption and emission spectroscopy. According to comparison between the experimental spectra and the density functional theory calculations (partial density of states and exchange interactions for various Mn defect configurations), the substitutional Mn impurities do not induce ferromagnetism in (Zn,Mn)O samples. The ferromagnetic properties can be obtained when defect configurations consisting of both substitutional and interstitial Mn atoms are present. The ferromagnetism in ZnO-based magnetic semiconductors is favored to be Ruderman-Kittel-Kasuya-Yoshida type and the established theoretical model is in a good agreement with the X-ray spectroscopic measurements.     

Thermodynamic and kinetic factors effecting hydrogen absorption on metal hydrides

The work of Guvendiren et al. on the effects of additives on mechanical milling and hydrogenation of Magnesium Powder which is published in this journal [Guvendiren M, Bayboru E, Ozturk T. International Journal of Hydrogen Energy 2004; 29: 491–496] shows excellent experimental work which agrees with previous published work. However they did not explain the right phenomenon which is undergoing during hydrogen absorption on magnesium hydride's system. In this communication it is the objective to distinguish between thermodynamic and kinetics factor effecting hydrogen absorption. It will be shown that the phase rule of thermodynamics will determine the variation of Pressure–composition isotherms at constant temperature during hydrogen absorption in Magnesium or Titanium. This is because the Pressure–composition isotherms at constant temperature is different for a single-phase (e.g. beta-Titanium) than two phases (e.g. delta and epsilon phases). Thus the data of Guvendiren et al. published in this journal (Fig. 5) can be explained by this effect.     

Excited states in yttrium orthovanadate YVO4 measured by soft X-ray absorption spectroscopy

We present the electronic structure of vanadium in YVO₄ compound obtained by X-ray absorption (XAS) at the L_2,3 edge. We also performed a charge transfer multiplet calculation (CTM) to analyze the experimental results. YVO₄ compound was synthesized by the polyacrylamide sol–gel method. For a comparative evaluation, YVO₄ compound was prepared by solid-state reaction. XAS results show that the increase of heat treatment favored the presence of one oxidation state V⁵⁺. We observed a distinctive broadening at L₂ edge of vanadium due to the L₂L₃v Coster–Kronig process. We identify the excitations of 3d bonding orbitals by CTM on the XAS results. Tetragonal parameters (D _s and D _t) in D _4h symmetry reveal a compression of the 3d orbitals in the z direction.     

Effect of N, C and B interstitial atoms on local bonding structure in mechanically activated TiH2/h-BN, TiH2/C, and TiH2/B mixtures

Mechanically activated TiH₂/h-BN, TiH₂/C and TiH₂/B mixtures was studied by temperature-programmed desorption, X-ray diffraction, transmission electron microscopy and X-ray emission spectroscopy. Ball milling in the presence of additives results in a modification of hydrogen occupation sites. Additional Ti-N, Ti-C or Ti-B bonds from chemical bonding of Ti with interstitial N, C and B atoms, are formed in TiH₂ due to contact of TiH₂ nanoparticles with the respective additive matrix materials. Mixed configurations around Ti atoms with proportional combination of local Ti-H and Ti-N, Ti-C or Ti-B bonds significantly decrease the thermal stability of TiH₂. The effect is most pronounced when boron is the additive.     

Characterization of chemically treated titanium using soft X-ray fluorescence

High energy resolution soft X-ray fluorescence measurements of commercially pure (cp-Ti) and chemically treated Ti (Ti + HCl and Ti + HCl + NaOH) are presented. They show the formation of thick hydrogen-(TiH₂) and oxygen-passivated (TiO₂) layers on Ti + HCl and Ti + HCl + NaOH, respectively; it is shown that the oxygen-passivated layer has primarily anatase structure. A bone-like carbonated apatite layer is found to be formed on the surface of chemically treated Ti after exposure to simulated body fluid (SBF). The spectroscopic results indicate that (CO₃)^2? substitutes both OH^? (type A substitution) and (PO₄)^3? (type B substitution) sites in the apatite structure.