Gold nanoparticles supported on magnesium oxide for CO oxidation

Au was loaded (1 wt%) on a commercial MgO support by three different methods: double impregnation, liquid-phase reductive deposition and ultrasonication. Samples were characterised by adsorption of N2 at -96°C, temperature-programmed reduction, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Upon loading with Au, MgO changed into Mg(OH)2 (the hydroxide was most likely formed by reaction with water, in which the gold precursor was dissolved). The size range for gold nanoparticles was 2-12 nm for the DIM method and 3-15 nm for LPRD and US. The average size of gold particles was 5.4 nm for DIM and larger than 6.5 for the other methods. CO oxidation was used as a test reaction to compare the catalytic activity. The best results were obtained with the DIM method, followed by LPRD and US. This can be explained in terms of the nanoparticle size, well known to determine the catalytic activity of gold catalysts

Amorphous hafnium silicates: structural, electronic and dielectric properties

Models of amorphous (HfO2)x(SiO2)(1), for varying hafnium, content x are generated by ab initio molecular dynamics. The structural properties are analyzed in terms of pair distribution functions from which typical bond lengths and coordination numbers are extracted. Electronic band gaps are calculated with a hybrid density functional and are found to decrease in a nonlinear way with increasing x, in accord with experimental observations. Static dielectric constants are evaluated through the application of a finite electric field. The calculated values show a linear dependence on x, supporting recent experimental data

Band gaps and dielectric constants of amorphous hafnium silicates: A first-principles investigation

Electronic band gaps and dielectric constants are obtained for amorphous hafnium silicates using first-principles methods. Models of amorphous (HfO2)(x)(SiO2)(1-x) for varying x are generated by ab initio molecular dynamics. The calculations show that the presence of Hf gives rise to low-lying conduction states which explain the experimentally observed nonlinear dependence of the band gap on hafnium content. Static dielectric constants are found to depend linearly on x, supporting recent experimental data. (c) 2007 American Institute of Physics

Oxygen vacancy in monoclinic HfO2: A consistent interpretation of trap assisted conduction, direct electron injection, and optical absorption experiments

The authors calculate energy levels associated with the oxygen vacancy in monoclinic HfO2 using a hybrid density functional which accurately reproduces the experimental band gap. The most stable charge states are obtained for varying Fermi level in the HfO2 band gap. To compare with measured defect levels, they determine total energy differences specific to the considered experiment. Their results show that the oxygen vacancy can consistently account for the defect levels observed in (Poole-Frenkel-type) trap assisted conduction, direct electron injection, and optical absorption experiments. (c) 2006 American Institute of Physics

Defect levels of dangling bonds in silicon and germanium through hybrid functionals

Defect levels of dangling bonds in silicon and germanium are determined within their respective band gaps through the use of hybrid density functionals. To validate our approach, we first considered the dangling bond in silicon finding two well-separated defect levels in excellent correspondence with their experimental location. Application to the dangling bond in germanium then yields two very close defect levels lying just above the valence band, which is consistent with the experimental location of the charge neutrality level. The occurrence of negative-U behavior leads to a reduced fraction of neutral dangling bonds, thereby suppressing the electron-spin-resonance activity

Hydrogen in Si(100)-SiO2-HfO2 gate stacks: Relevant charge states and their location

Using a density functional approach, we study the energetics of various charged hydrogen states in the Si(100)-SiO2-HfO2 gate stack. We describe the SiO2-HfO2 transition region through model structures of amorphous hafnium silicate HfxSi1-xO2 with different Hf contents x. Hydrogen is found to be amphoteric with a +/- charge transition level lying close to the Si conduction band minimum. This implies that protons are the most stable form of hydrogen for most electron chemical potentials in the Si band gap. Formation energies of the positively charged state across the Si(100)-SiO2-HfO2 stack indicate that protons mainly locate in the Si-SiO2 or SiO2-HfO2 transition regions. (c) 2007 American Institute of Physics

Band alignments and defect levels in Si-HfO2 gate stacks: Oxygen vacancy and Fermi-level pinning

The determination of band alignments and defect levels is demonstrated for the technologically relevant Si-SiO2-HfO2 gate stack. The proposed scheme, which combines first-principles molecular dynamics for model generation and hybrid density functionals for electronic-structure calculations, yields band offsets in close agreement with experiment. Charge transition and pinning levels associated with oxygen vacancies are aligned with respect to the silicon band edges. The vacancies are shown to preferentially reside in the amorphous transition layer, consistent with experimental observations of Fermi-level pinning

Migration of oxygen vacancy in HfO2 and across the HfO2/SiO2 interface: A first-principles investigation

Oxygen vacancy migration is studied in monoclinic HfO2 and across its interface with SiO2 through density functional calculations. In HfO2, long-range diffusion shows activation barriers of 2.4 and 0.7 eV for the neutral and doubly positively charged vacancy, respectively. In the latter case, the migration preferentially occurs along one-dimensional pathways. A HfO2/SiO2 interface model is constructed to address O vacancy migration across high-kappa gate stacks. The vacancy is shown to stabilize in its neutral charge state upon entering the SiO2 layer