Anders Flodström

Dissociative NH3 adsorption on the Si(100)2 × 1 surface at 300 K

High resolution electron energy loss spectroscopy has been used to determine how NH3 adsorbs on the Si(100)2 × 1 surface at 300 K. We find that the NH3 molecules dissociate into NH2 and H on adsorption. Combination bands, overtones and double losses for the Si---NH2 group are observed. An anneal to 800 K is sufficient to dissociate the adsorbed NH2 groups while the majority of the Si---H bonds remain intact. An anneal to 1100 K is enough to break the Si-H bonds and start the formation of silicon nitride.

Adsorption of H2O on planar and stepped Si(100): Structural aspects

The adsorption of water on two silicon surfaces [reconstructed planar (100)2×1 and single domain stepped (100)2×1 cut 5° towards (011)] was studied using low-energy electron diffraction and digital imaging electron stimulated desorption ion angular distributions (ESDIAD) as a function of temperature (145–700 K) and coverage. Water has been shown previously to chemisorb dissociatively to form surface OH groups. At 300 K the H + ESDIAD pattern for the planar surface is a nearly symmetric halo, indicating that OH is oriented with its bond vector inclined away from the surface normal, while at low temperatures (200 K) a four-lobed pattern that preserves substrate symmetry is observed. This reversible temperature dependence is related to librations and rotations of the OH complexes. ESDIAD from the stepped surface exhibits a two-lobed pattern, with enhanced emission towards the steps, consistent with bonding of OH to single-domain terrace sites. An interpretation is presented based on the dimer model of the Si(100) reconstruction in which the OH bond axis azimuths are oriented nearly perpendicular to the dimer azimuths. (Less)

A photoelectron spectroscopy and photon stimulated desorption study of H2O on Si(100) 2×1

We have studied H2O adsorption on the Si(100) surface using photoelectron spectroscopy to record Si valence bands and Si 2p core level spectra; and photon stimulated desorption to record Si 2p edge total electron yield and H+‐ion yield spectra. We assign the valence‐band H2O induced peaks at EB=6.3 and 11.2 eV to the Si–OH and to the O–H bonds, respectively. The H2O dosed Si 2p core level spectrum exhibits two H2O induced equal intensity surface peaks with surface core level shifts of +0.25 and +1.00 eV that we assign to surface Si atoms in the Si–H and the Si–OH bonds, respectively. We interpret the features in the Si 2p edge H+‐ion yield spectrum as ion desorption from SiO2 at surface defect minority sites. We conclude that H2O adsorbs dissociatively as H and OH radicals on the Si(100) 2×1 surface dimers and that there are defect minority sites on the surface where H2O adsorption causes SiO2 formation.

Influence of oxygen and nitrogen on the growth of hot-filament chemical vapor deposited diamond films

The effect of incorporating oxygen and nitrogen into the feed gases on the texture and surface morphology of diamond film synthesized by hot filament chemical vapor deposition (HFCVD) is investigated. The reactant gas composition is determined by the gas flow rates. At a constant flow rate of hydrogen (33 sccm) and methane (0.68 sccm), the oxygen and nitrogen were varied in the O/(O 1 C) ratio from 0.05 to 0.43 and in the N/(N 1 C) ratio from 0.15 to 0.60. The films were grown under a constant pressure (20 Torr) and a constant substrate temperature (8008C). Clearly nitrogen in the reactant gases has a distinct tendency to promote the k100l texture and the corresponding {100} morphology, whereas oxygen promotes the development of k111l texture and {111} morphology. According to the Wulff theorem (Ga d100=d111a g100=g111) and the evolutionary selection of crystallites and the surface configurations of diamond, the results reveal that during growth nitrogen plays a critical role in activating the CD‐H surface site and consequently increases the surface free energy g 111, of the {111} surface. In contrast, oxygen activates the CDvH2 surface site and increases the surface free energy g 100, of the{100} surface. These results indicate that the texture and the surface morphology of polycrystalline diamond film can be completely controlled by the reactant gas composition. q 1999 Elsevier Science S.A. All rights reserved.

Si(100)2 × 1: the clean and ammonia exposed surface studied with high resolution core-level spectroscopy

High resolution core-level spectroscopy was utilized to study the clean and NH3 exposed Si(100)2×1 surface. The clean surface exhibits two approximately equal intensity surface core-level components at −0.48 and 0.28 eV binding energy referred to the bulk component. NH3 exposure at 300 K induces two surface core-level components at 0.31 and 0.72 eV relative binding energy that can be assigned to surface Si atoms bonded to H and NH2, respectively. Alternative interpretations for the adsorption based on different interpretations of the clean surface core-level spectra are discussed. The steps between adsorption at 300 K and the beginning of subsurface silicon nitride formation by annealing the surface up to 1000 K are investigated.

Catalytic reduction of nitric oxide on copper. Part I

Abstract The mechanism of NO reduction on copper in the presence of oxygen and isobutene was studied at T = 770 K, under reducing conditions. Mass spectroscopy was used to identify gas-phase intermediates and reaction products. One intermediate species resulting from the partial oxidation of isobutene was shown to be active in the NO conversion. X-ray photoelectron spectroscopy was used to analyze the copper surface at different stages of the reaction. Our conclusion is that oxygen first activates the surface, by forming coexisting phases of Cu 2 O and CuO. The copper surface is then reduced by the hydrocarbon, leading to Cu + in majority, and to the formation of an active intermediate. At last, the reduction of NO proceeds rapidly on a zero-valent copper surface together with consumption of the previously formed intermediate. At the end of the reaction, the surface is poisoned by carbon.

Core-level shifts on the H2O exposed Ge(100)2×1 surface

Core‐level spectroscopy and valence‐band photoelectron spectroscopy were used to study the Ge(100)2×1 surface dosed with 0.5–100 L H2O at 160 K. It is determined that H2O adsorbs molecularly at 160 K and forms ice. The H2O molecules dissociate into H and OH radicals on the Ge(100)2×1 surface when the sample is heated to 300 K. A simple adsorption model that accounts for the calculated H and OH coverages is proposed.

Catalytic reduction of nitric oxide over copper. Part II: Influence of sulfur dioxide

Abstract The influence of sulfur dioxide on the reduction of nitric oxide over copper, in the presence of isobutene and oxygen, has been examined under reducing conditions in the 720 to 820 K temperature range. The catalytic activity of NO conversion was drastically decreased by the presence of a few ppm of SO 2 in the gas phase, and the toxicity of sulfur was reduced when the temperature was increased. The Cu sample was characterized by X-ray photoelectron spectroscopy, at different stages of the reaction. It was demonstrated that sulfur dioxide poisoned NO conversion by hindering the partial oxidation of the hydrocarbon and by occupying the active sites, both factors contributing to a decrease in the surface concentration of the active intermediate (oxygenated product). Two kinds of adsorbed sulfided species, SO 2− 4 and S 2− , were identified on the Cu surface. At the end of the reaction, the catalyst surface was poisoned both by carbon and sulfur species.

Dissociative H2O adsorption on the Si (100) 2× 1 and Ge (100) 2× 1 surfaces

Core-level spectroscopy and valence band photoelectron spectroscopy were used to study H2O adsorption on the Si(100) and Ge(100) surfaces. We find that H2O dissociates into H and OH on both surfaces at 300 K. The H and OH are adsorbed in on top positions on the surface. The OH group is tilted with respect to the surface normal on the Si(100) surface. We consider two possible interpretations for the results from the H2O dosed Ge(100) surface at 300 K. Either a similar model as for the Si(100) surface or a model based on two adsorption sites.

Orientation of (1×1)-surface free energies of crystals

Abstract The free energy of a (1×1)-surface, with no relaxation and no adsorption, is calculated using a bond-breaking mode in which the potential energy of the crystal is treated as the sum of the energy of the bonds connecting pair-wise atoms. Based on a purely geometrical model, the number of broken bonds or dangling bonds per atom is calculated on the surface of the crystal when an atomically flat plane h ( hkl ) is created. The results provide a general expression of the surface free energy in terms of Miller indices hkl . The anisotropy of the surface free energy is completely described in the expression. Considering nearest-neighboring bonding only, the orientation dependence of the surface free energy is discussed for simple cubic (sc) and cubic tetrahedral (cth) crystals, respectively. Wulff plots and the equilibrium forms for the sc and the cth crystals are obtained on the basis of their expressions of the surface free energy, implying the cube and the octahedron are the equilibrium forms for the sc and the cth crystals, respectively. Furthermore a predicted anisotropy of fcc metals is discussed.