Marian W. Radny

A first-principles density functional study of chlorophenol adsorption on Cu2O(110):CuO

First-principles density functional theory and a periodic-slab model have been employed to explore the adsorption of a two-chlorophenol molecule on a Cu(2)O(110) surface containing surface Cu-O bonds, namely, the Cu(2)O(110):CuO surface. The two-chlorophenol molecule is found to interact very weakly with the Cu(2)O(110):CuO surface, forming several vertical and flat orientations. These weakly bound states tend to result from interaction between the phenolic hydrogen and an oxygen surface atom. The formation of a two-chlorophenoxy moiety and an isolated hydrogen on the Cu(2)O(110):CuO surface from a vacuum two-chlorophenol molecule is determined to have an endothermicity of 8.2 kcal/mol (0.37 eV). The energy required to form a two-chlorophenoxy radical in the gas phase is also found to be much smaller when assisted by the Cu(2)O(110):CuO surface than direct breaking of the hydroxyl bond of a free two-chlorophenol molecule. The calculated binding energy of a two-chlorophenoxy radical adsorbed directly onto the Cu(2)O(110):CuO surface is -12.5 kcal/mol (0.54 eV). The Cu(2)O(110):CuO and Cu(100) surfaces are found to have similar energy barriers for forming a surface-bound two-chlorophenoxy moiety from the adsorption of a two-chlorophenol molecule.

A modified Stillinger-Weber potential for modelling silicon surfaces

Abstract The widely used Stillinger-Weber potential for silicon interactions has been modified to provide an accurate description of the Si(111)-(7 × 7) surface including the highly reactive adatom and rest-atom sites. This modified potential also provides a good representation of bulk silicon, and the Si(001)-(1 × 1), Si(001)-(2 × 1), Si(111)-(1 × 1) and Si(111)-(2 × 1) surfaces. Above the melting temperature of 1683 K, however, the original Stillinger-Weber potential is probably superior.

Formation of Environmentally Persistent Free Radicals on α-Al2O3

Metal oxides exhibit catalytic activity for the formation of environmentally persistent free radicals (EPFRs). Here, we investigate, via first-principles calculations, the activity of alumina α-Al2O3(0001) surface toward formation of phenolic EPFRs, under conditions relevant to cooling down zones of combustion systems. We show that, molecular adsorption of phenol on α-Al2O3(0001) entails binding energies in the range of -202 kJ/mol to -127 kJ/mol. The dehydroxylated alumina catalyzes the conversion of phenol into its phenolate moiety with a modest activation energy of 48 kJ/mol. Kinetic rate parameters, established over the temperature range of 300 to 1000 K, confirm the formation of the phenolate as the preferred pathways for the adsorption of phenol on alumina surfaces, corroborating the role of particulate matter in the cooling down zone of combustion systems in the generation of EFPRs.

Interface and nanostructure evolution of cobalt germanides on Ge(001)

Cobalt germanide (CoxGey) is a candidate system for low resistance contact modules in future Ge devices in Si-based micro and nanoelectronics. In this paper, we present a detailed structural, morphological, and compositional study on CoxGey formation on Ge(001) at room temperature metal deposition and subsequent annealing. Scanning tunneling microscopy and low energy electron diffraction clearly demonstrate that room temperature deposition of approximately four monolayers of Co on Ge(001) results in the Volmer Weber growth mode, while subsequent thermal annealing leads to the formation of a Co-germanide continuous wetting layer which evolves gradually towards the growth of elongated CoxGey nanostructures. Two types of CoxGey nanostructures, namely, flattop- and ridge-type, were observed and a systematic study on their evolution as a function of temperature is presented. Additional transmission electron microscopy and x-ray photoemission spectroscopy measurements allowed us to monitor the reaction between ...

Application of the extended Brenner potential to the Si(111)7×7:H system I: cluster calculations

Abstract Previous work has shown that the extended Brenner potential provides a good description of the Si(111)7×7 surface and Si–H molecules. In this paper, we employ this empirical potential to study the interaction of atomic hydrogen with the Si(111)7×7 surface. Energy curves for a single hydrogen atom chemisorbed above the adatom and rest atom sites of the Si(111)7×7 surface, and the various equilibrium structures and binding energies of the hydrogen chemisorbed Si(111)7×7 surface, have been determined. These empirical potential calculations have been performed using both small and large clusters of atoms. Comparison of the results with previous ab initio Hartree–Fock/density functional theory calculations shows that the extended Brenner potential provides a good representation of the Si(111)7×7:H system.

Ab initio cluster calculations of the chemisorption of hydrogen on the Si(111)√3 × √3R30°-B surface

Abstract First principles all-electron Hartree-Fock and density functional theory cluster calculations have been performed to investigate the chemisorption of atomic hydrogen on the Si(111)√3 × √3R30°-B surface. Both the equilibrium geometries corresponding to n hydrogen atoms (1 ≤ n ≤ 5) chemisorbing on the B-S5 and B-T4 reconstructed structures, and the desorption energies for a silicon or boron adatom bonded to x hydrogen atoms (0 ≤ x ≤ 3), have been obtained. As successively more hydrogen is chemisorbed, a silicon or boron adatom is found to move from its original threefold site to an adjacent bridge site, and then to a neighbouring on-top site. It is also found that boron will most likely occupy a subsurface substitutional S5 site at low hydrogen coverages (≤ 0.67ML), but appear as an adatom at an on-top site directly above one of the first-layer silicon atoms for hydrogen exposures higher than 0.67 ML. This boron segregation at high hydrogen exposures prevents the formation of SiH2 and SiH3 complexes and leads to the prediction that only SiH and BH2 will be observed on an Si(111):B hydrogenated surface. It also provides an explanation for the lack of Si surface etching at high hydrogen exposure. All of these results are in good agreement with the available experimental data on hydrogenated B/Si(111) surfaces.

An ab initio Hartree-Fock/density functional study of the cluster simulated Si(111)7 × 7:Cl adsorption system

Abstract The first-principles all-electron Hartree-Fock/density functional method contained within the Gaussian92/DFT package has been employed to study the chemisorption of atomic chlorine onto small clusters modelling the adatom and restatom sites of the Si(111)7 × 7 surface. Both the equilibrium configurations and the binding energies of the various SiClx complexes (1 ≤ x ≤ 3) produced by the adsorption of up to six chlorine atoms in the vicinity of an adatom and restatom site, have been determined. As progressively more chlorine atoms are adsorbed near these chemisorption sites, the surface topology is found to reconstruct to yield stable configurations for SiCl, SiCl2 and SiCl3. A chlorine bonded adatom is observed to move from its original three-fold T4 site (one chlorine atom) to an adjacent bridge site (two and three chlorine atoms), and then on-top of a neighbouring first layer silicon atom (more than three chlorine atoms). Similar behaviour is exhibited by the restatom except that movement towards the on-top position occurs when three chlorine atoms are chemisorbed, rather than four as in the adatom case. The lowest binding energies are determined to be 0.74 eV for SiCl3 desorption from a restatom site and 0.80 eV for the removal of SiCl2 from an adatom site. Our results also suggest that a relatively low fluence of chlorine should produce a stable chlorosylil layer consisting of SiCl on the restatom sites and SiCl3 on the adatom sites, and yielding a saturation coverage of 1.35 mL. Increased chlorine exposure should result in further uptake at the restatom sites to produce SiCl3 complexes at both chemisorption sites and a maximum saturation coverage of 1.84 ML. Both of these predictions appear consistent with experiment.

Ab initio HF/DFT studies of the chemisorption of hydrogen on the cluster simulated Si(111)-(√3 × √3)R30°-Al and -Ga surfaces

Abstract The chemisorption of atomic hydrogen on the Si(111)-(√3 × √3)R30°-Al and -Ga surfaces has been studied by first-principles all-electron Hartree-Fock/density functional theory (DFT) cluster calculations. It has been found that the hydrogen chemisorption behaviour on the Ga terminated cluster modelled surface is similar to that for the Al terminated cluster. Hydrogen atoms chemisorbing near an adatom site are found to cause the breaking of SiAl (or SiGa) bonds and the formation of HSi bonds. The aluminium (or gallium) adatoms move from their original threefold sites to the adjacent bridge sites (one hydrogen atom), and then to the neighbouring on-top sites (two hydrogen atoms). Calculations of the binding energies have shown that aluminium or gallium adatoms occupying the bridge and on-top sites may be able to migrate on the real surface. This suggests the possible formation of Al (or Ga) islands, together with a local (1 × 1)H surface reconstruction. Further exposure to hydrogen does not cause any further structural transformations and supports the (√3 × √3)-(1 × 1) phase transition.

Boron, hydrogen and silicon adatoms on the Si(111) surface : an ab initio Hartree-Fock/density functional cluster study

Silicon and boron adatoms on the Si(111) surface have been studied using atomic clusters and Hartree - Fock/density functional theory. The equilibrium geometries of the , and structures, and those formed by the chemisorption of a single hydrogen atom onto these structures, have been obtained. The binding energies of the B and Si adatoms, and the resultant SiH and BH complexes, have also been determined. The electronic structures of the clusters have been investigated via a Mulliken population analysis. The calculated results are shown to be in good agreement with both the available LEED and synchrotron x-ray diffraction data, and other simulations. It is found that the binding energy of the Si adatom of the structure is greater than that of the B adatom of the structure by 1.93 eV, and 1.15 eV larger than the binding energy of the Si adatom of the structure. The chemisorption of atomic hydrogen stabilizes the geometry through the formation of a silicon adatom - hydrogen (SiH) complex located at the threefold position. In contrast to this, the Si adatom in the structure, and the B adatom bonded with a chemisorbed hydrogen atom in the structure, are observed to move away from their positions to the adjacent bridge sites.

Image induced surface resonance of clean (001) and (111) surface of aluminium

Abstract The effect of an image potential on the local surface density of states is investigated in the framework of a nearly free electron model by employing the matching Green function method. The influence of the image plane position on the shift and broadening of image induced surface resonance is discussed. Finally, results are compared to experimental data and other theoretical calculations.