Quantum chemical modeling of the interaction of hydrogen atoms with the (111) surface of silicon

The interaction of a hydrogen atom with the (111) face of silicon was studied by the MNDO method in a cluster approximation. The energy of adsorption of a hydrogen atom as well as the values of the barriers for its incorporation and desorption from the surface layer of silicon were calculated taking into account its structure relaxation. It was found that the hydrogen atom can penetrate into the crystal through its surface cavities.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 3, pp. 253–257, May–June, 1992.The authors wish to express their gratitude to P. A. Aleksandrova for fostering interest in the investigation and for her helpful suggestions.     

Cluster models of interaction of a fluorine atom with the (111) face of silicon

The AMI method has been used in calculating activation energies of desorption of surface complexes from the (111) face of silicon. For the desorption of a surface Si atom, the desorption of an FSi radical containing this Si atom when a fluorine atom implanted in the surface layer is present, and the desorption of the FSi radical when such a fluorine atom is not present, the values found for the activation energy are 8.0, 0.9, and 6.4 eV, respectively.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 144–147, March–April, 1992.     

Quantum-chemical cluster models for the Si(111)/SiO2 interface

The MPDP method is used to construct cluster models for the Si(111)/SiO₂ interface. The crystallochemical environment of the clusters is included by a method based on the satisfaction of the unusual cluster boundary valence by hydrogen atoms, whose locations, as far as possible, are fixed by the stoichiometry of the charge distribution on the atoms of silicon and oxygen in the model fragment. In these cluster models for the Si(111)/SiO₂ interface, considering only normal atomic displacements relative to their ideal positions, there appears a 7 Å thick transition layer encompassing four surface atomic planes of the silicon substrate and two successive Si-O and O-Si links from the SiO₂ film.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 268–275, May–June, 1990.The authors thank V. A. Chaplanov for stimulating our interest in this problem and for helpful discussions.     

Oxygen chemisorption on the PbS(001) surface: Quantum-chemical modeling

The structure and stability of local centers involving a different number of oxygen atoms on the surface of crystalline lead sulfide (001) were calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. The trends of the formation of such centers and changes in the core electron binding energies for the sulfur and lead atoms constituting these centers were considered.     

Theoretical study of the adsorption of silver atoms on the (111) face of silicon

The adsorption of one or many silver atoms on a (111) silicon face (reduced to 61 dangling atomic orbitals) is investigated by means of a self-consistent Hartree–Fock method parametrized from atomic and thermodynamical data. The valley sites (above three Si atoms) are favored over the top sites (above one Si atom). The extrapolation of the results obtained for several structures corresponding to the adsorption of n = 1, 2, 3, 4, 6, and 7 Ag atoms allows us to conclude that the most stable structures correspond: for \documentclass{article}\pagestyle{empty}\begin{document}$ \theta = \frac{1}{3} $\end{document} to linear Ag chains (3 × 1 phase), for \documentclass{article}\pagestyle{empty}\begin{document}$ \theta = \frac{2}{3} $\end{document} to an honeycomb lattice (\documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt 3 \times \sqrt 3 $\end{document} phase), and for θ = 1 to a centred hexagonal lattice (\documentclass{article}\pagestyle{empty}\begin{document}$ \sqrt 3 \times \sqrt 3 $\end{document} phase), the Ag atoms located at the centers of the hexagons being beneath the plan of the hexagons. The adsorption energies corresponding to the various θ are practically equal (ca. 3 eV/Ag). The net charges of Ag atoms are equal to 0.35.     

Efficient silicon surface and cluster modeling using quantum capping potentials

A one-electron, silicon quantum capping potential for use in capping the dangling bonds formed by artificially limiting silicon clusters or surfaces is developed. The quantum capping potentials are general and can be used directly in any computational package that can handle effective core potentials. For silicon clusters and silicon surface models, we compared the results of traditional hydrogen atom capping with those obtained from capping with quantum capping potentials. The results clearly show that cluster and surface models capped with quantum capping potentials have ionization potentials, electron affinities, and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps that are in very good agreement with those of larger systems. The silicon quantum capping potentials should be applied in cases where one wishes to model processes involving charges or low-energy excitations in silicon clusters and surfaces consisting of more than ca. 150 atoms.     

The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova     

Quantum-chemical modeling of the detachment of hydrogen atoms by the sulfate radical anion

Nonempirical quantum-chemical calculations of the transition states of reactions between the sulfate radical anion and organic compounds of various classes were performed, and the activation energies of the corresponding reactions were calculated. A correlation dependence between the calculated activation energies and the strength of substrate C-H bonds was found. The polarized continuum model COSMO was used to study the influence of solvents on the kinetics of the reactions.     

Photochemical-controlled switching based on azobenzene monolayer modified silicon (111) surface

Azobenzene-containing compounds were covalently attached onto Si(111) surfaces via Si-O linkages using a two-step procedure. The modified Si(111) surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy measurements. The monolayer surface showed preferably chemical stability. Switchable photoisomerizability of azobenzene molecules on these modified surfaces was observed in response to alternating UV and visible light exposure. The measured conductivity showed distinct difference with trans and cis forms of azobenzene compounds on as-modified Si(111) surfaces.     

Reversible hydrogenation of surface N atoms to form NH on Pt(111)

The formation and dissociation chemistry of the NH species on Pt(111) was characterized with reflection absorption infrared spectroscopy and temperature programmed desorption. Irradiation of a chemisorbed bilayer of ammonia with a 100 eV electron beam at 85 K leads to a mixture of NH, N, and H on the surface. Annealing to temperatures in the range of 200-300 K leads to reaction of N and H to form additional NH. The NH species has an intense and narrow NH stretch peak at 3320 cm(-1), while no peak due to the PtNH bend is observed above 800 cm(-1). The NH species is stable up to a temperature of approximately 400 K. The surface N atoms produced from NH dissociation are readily hydrogenated back to NH by exposure of the surface to H2. However, NH cannot be further hydrogenated to generate adsorbed NH2 or to NH3 under the conditions used here. Exposure of the NH/Pt(111) surface to D2 at 380 K produces the ND species. Comparison with the results of density functional theory calculations based on small Pt clusters indicates that NH occupies three-fold hollow sites with the molecular axis perpendicular to the surface.     

Helium atom scattering study of the interaction of water with the BaF2(111) surface

The interaction of water with the BaF2(111) single crystal surface is investigated using the helium atom scattering technique. It is found that H2O forms a long-range ordered two-dimensional (2D) phase with (1 x 1) translational symmetry already after an exposure of 3 L (1 L=10(-6) Torr s) at temperatures below 150 K. The activation energy for desorption of the saturated 2D phase, which is assigned to a bilayer, is estimated to be 46+/-2 kJ mol(-1), corresponding to a desorption temperature of 165 K. The desorption of multilayers was observed at 150 K, consistent with a binding energy of 42+/-2 kJ mol(-1). Before completion and after desorption of the saturated 2D phase, a superstructure consistent with a disordered (square root of 3 x square root of 3)R30 degrees lattice has been observed, which is attributed to the first layer of water with a coverage of one molecule per surface unit cell, in accordance with recent theoretical studies. Desorption of this phase is observed at temperatures above 200 K, consistent with an unexpectedly strong bonding of the molecules to the substrate.     

Azidation of silicon(111) surfaces

A two-step chlorination/azidation process was reported to prepare azide-modified silicon(111) surfaces. XPS and IR analyses show the covalent bonding of azide with silicon. In combination with scanning tunneling microscopy and spectroscopy analyses, different kinetic rates, azide coverages, and surface-area distributions were derived depending on the azidation solvent.     

Formation of surface CN from the coupling of C and N atoms on Pt(111)

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption have been used to study the coupling of C and N atoms on Pt(111) to form surface CN. This reaction underlies the important synthesis of HCN from methane and ammonia over platinum catalysts. Since CH4 and NH3 do not thermally dissociate on Pt(111) under ultrahigh vacuum conditions, we used CH3I and electron bombardment of NH3 to generate reactive surface species. Surface CN is formed at a temperature of 500 K from surface Nads and Cads atoms. The presence of surface CN is detected by HCN desorption and through the reaction of hydrogen with CNads to form a surface >CNH2 (aminocarbyne) species, which has a characteristic RAIR spectrum.     

Quantum-chemical study of interaction of gases with molecular fragment of carbon surface

The atom-atom potential method has been applied in ranking industrial gases in their affinity for carbon surfaces. From an analysis of the Coulombic induction, and dispersion contributions to the total energy, and also an analysis of the repulsion potentials, it has been found that the energy of dispersion interaction is a factor with a substantial influence on the affinity of the molecules for the matrix.Sorption and Fine Inorganic Synthesis Branch, Institute of General and Inorganic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 488–490, July–August, 1991. Original article submitted December 17, 1990.     

Kinetics of the reactions of fluorine and chlorine atoms with ethylene oxide (oxirane)

The kinetics of the reactions of F and C1 atoms with ethylene oxide have been studied using relative rate techniques in 10–700 Torr of either nitrogen or air diluent at 295 ± 2 K; k(F + C₂H₄O) = (9.4 ± 1.6) × 10^?11 and k(C1 + C₂H₄O) = (5.0 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1. The result for k(F + C₂H₄O) is in good agreement with the literature data. The result for k(C1 + C₂H₄O) is a factor of 5.6 lower than that reported previously. It seems likely that in the previous study most of the loss of C₂H₄O attributed to reaction with C1 atoms was actually caused by unwanted secondary reactions leading to an overestimate of k(C1 + C₂H₄O). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 122–125, 2002     

Molecule-substrate interaction channels of metal-phthalocyanines on graphene on Ni(111) surface

Molecule-substrate interaction channels of metal-phthalocyanines (MPcs, including NiPc, CuPc, ZnPc, FePc, and CoPc) on graphene on Ni(111) were investigated by employing high-resolution electron energy loss spectroscopy (HREELS). Except the expected IR-active modes, some Raman-active modes were also observed in all of MPcs, which are considered in this study. From the origination of the Raman-active features, it was deduced that MPcs are coupled with the substrate mainly through their central metal atom. The Raman-active modes appear as symmetric peaks in the HREELS in the case of MPcs with Ni, Cu, and Zn, whereas they are asymmetric and appear as a Fano line shape in the case of MPcs with Fe and Co. This spectroscopic difference indicates that the molecule-substrate coupling is completely different in the two cases mentioned above. The molecule-substrate interaction strength is considerably weak and comparable with the π-π interaction between molecules in the case of MPcs with Ni, Cu, and Zn, whereas it is much stronger in the case of MPcs with Fe and Co. From the HREELS observations, it can be suggested that the whole molecule can be effectively decoupled from the underneath Ni(111) by inserting a single layer of graphene between them in the case of MPcs with Ni, Cu, and Zn, whereas only benzene rings can be completely decoupled in the case of MPcs with Fe and Co.     

Probing the buried C60/Au(111) interface with atoms

To characterize the C(60)/Au(111) interface, we send Au atoms "diving" through the C(60) layer and observe their behavior at the interface. Our observations show that the interfacial diffusion of gold atoms and the nucleation of small Au islands at the interface are strongly dependent on the local C(60)-Au(111) bonding which varies from one domain to another. The contrast-disordered domain consisting of a large fraction of molecules bonded to Au vacancies has a special structure at the interface allowing Au atoms to be inserted beneath the bright-looking molecules while the dim molecules present a much stronger resistance to the diffusing Au atoms. This leads to the formation of isolated Au islands with discrete sizes, with the smallest island just about 1 nm across.