Theoretical Studies on the Si（001）-SiO2 Interface Structure

1 INTRODUCTION Silicon and its alloy have been widely applied in such fields as electronic industry, high-temperature structural ceramics, etc. In addition, the researches on silicon and its relevant materials greatly promote the rapid development of modern optics and infor- mation technology. Therefore, more and more at- tention is focused on the structure of silicon, oxide of silicon and the interfaces between silicon and metal or nonmetal. As an ideal passive film on the Si surface, S…     

Theoretical prediction of heterogeneous molecular wires on the Si(001) surface

Using first-principles density-functional calculations, we propose a self-assembly technique for fabrication of the heterogeneous molecular wire on the dangling-bond wire generated on a H-passivated Si(001) surface. Here, we choose pyridine and borine as Lewis base and acid molecules, respectively, to demonstrate different behaviors in the chemical reactivity and selectivity on the dangling-bond wire, leading to formation of the heterogeneous pyridine-borine wire.     

Theoretical study on the gas phase reaction of acrylonitrile with atomic hydrogen

The complex potential energy surface of the H + CH₂=CHCN reaction has been investigated at the BMC-CCSD level based on the geometric parameters optimized at the BHandHLYP/6-311++G(d,p) level. This reaction is revealed to be one of the significant loss processes of acrylonitrile. The BHandHLYP and M05-2X methods are employed to obtain initial geometries. The reaction mechanism confirms that H can attack on the C=C double bond or C and N atom of –CN group to form the chemically activated adducts IM1 (CH₃CHCN), IM2 (CH₂CH₂CN), IM3′ (CH₂=CHCHN) and IM5 (CH₂=CHCNH), and direct H-abstraction paths may also occur. Temperature- and pressure-dependent rate constants have been carried out using Rice–Ramsperger–Kassel–Marcus theory with tunneling correction. IM1 (CH₃CHCN) formed by collisional stabilization is the major product at the 760 Torr pressure of H₂ and in the temperature range (200–1,600 K); whereas the production of IM2 (CH₂CH₂CN) is the main channel at 1,600–3,000 K. The calculated rate constants are in good agreement with the experimental data.     

Density-functional study of the cycloaddition of acrylonitrile on the Si(100) surface

Using a density functional approach, we have explored the cycloaddition of acrylonitrile on the Si(100) surface. The buckling of the surface dimers characteristic for the (2x1) reconstructed surface is shown to favor structures with a dipolar moment such as the resonant form of acrylonitrile with cumulative double bonds. The bond of acrylonitrile via a single C atom is a possible intermediate leading to the nitrile structure of the adsorbed molecule.     

Theoretical study on the gas phase reaction of acrylonitrile with a hydroxyl radical

The mechanism and kinetics of the reaction of acrylonitrile (CH(2)=CHCN) with hydroxyl (OH) has been investigated theoretically. This reaction is revealed to be one of the most significant loss processes of acrylonitrile. BHandHLYP and M05-2X methods are employed to obtain initial geometries. The reaction mechanism conforms that OH addition to C[double bond, length as m-dash]C double bond or C atom of -CN group to form the chemically activated adducts, 1-IM1(HOCH(2)=CHCN), 2-IM1(CH(2)=HOCHCN), and 3-IM1(CH(2)=CHCOHN) via low barriers, and direct hydrogen abstraction paths may also occur. Temperature- and pressure-dependent rate constants have been evaluated using the Rice-Ramsperger-Kassel-Marcus theory. The calculated rate constants are in good agreement with the experimental data. At atmospheric pressure with N(2) as bath gas, 1-IM1(OHCH(2)=CHCN) formed by collisional stabilization is the major product in the temperature range of 200-1200 K. The production of CH(2)CCN and CHCHCN via hydrogen abstractions becomes dominant at high temperatures (1200-3000 K).     

A new Si tetramer structure on Si (001)

Interactions between Si ad-dimers on Si (001) have been studied by molecular dynamics simulations using the Stillinger–Weber potential. The interactions determine the formation of clusters from diffusing dimers. We show different pathways for the formation of multiple-dimer clusters and propose a new tetramer (T_CC) structure formation by two diffusing dimers interacting. This tetramer structure has been found to be energetically stable with respect to isolated ad-dimers. Moreover, their local bonding configuration is very similar to the B-type step edge which is known to be the favoured adsorption site for epitaxial growth. The proposed tetramer may play a crucial role as the nucleus of the new epitaxial layer on Si (001).     

Theoretical simulations of the tip-induced configuration changes of the 4,4(')-diacetyl-p-terphenyl molecule chemisorbed on Si(001)

We have investigated from a theoretical point of view modifications of the 4,4(')-diacetyl-p-terphenyl molecule chemisorbed on Si(001) induced by the scanning tunneling microscope (STM). In previous experiments, these modifications were observed to occur preferentially at the end of the molecule after a +4.0 V voltage pulse and at the center after a +4.5 V voltage pulse. In the framework of ab initio simulations, we have realized a systematic energetic study of the dissociative chemisorption of one, two, or three phenyl rings of the substituted p-terphenyl molecule. Charge densities were then calculated for the investigated configurations and compared to the STM topographies. Before manipulation with the STM tip, the substituted p-terphenyl molecule is preferentially adsorbed without phenyl ring dissociation, allowing a partial rotation of the central phenyl ring. Our results show that the STM induced modifications observed at the end of the molecule might originate from the dissociation of two phenyl rings (one central and one external ring), while the modifications occurring at the central part of the molecule can be interpreted as a dissociation of the two external rings.     

Different adsorption structures of pyridine on Si(001) and Ge(001) surfaces

The adsorption and reaction of pyridine on the Si(001) and Ge(001) surfaces are investigated by first-principles density-functional calculations within the generalized gradient approximation. On both surfaces the N atom of pyridine initially reacts with the down atom of the dimer, forming a single bond between the N atom and the down atom. On Ge(001) such an adsorption configuration is most favorable, but on Si(001) a further reaction with a neighboring dimer occurs, resulting in formation of a bridge-type configuration. Especially we find that on Ge(001) the bridge-type configuration is less stable than the gas phase. Our results provide an explanation for a subtle difference in the adsorption structures of pyridine on Si(001) and Ge(001), which was observed from recent scanning tunneling microscopy experiments.     

Adsorption dynamics of ethylene on Si(001)

The dynamics of ethylene adsorption on the Si(001) surface was investigated by means of molecular beam techniques. A constant decrease of initial sticking probability s(0) was observed with increasing kinetic energy indicating a non-activated adsorption channel. With increasing surface temperature, s(0) decreases as well, pointing towards adsorption via a precursor state. Quantitative evaluation of the temperature dependence of s(0) via the Kisliuk model was possible for surface temperatures above 250 K; below that value, the temperature dependence is dominated by the adsorption dynamics into the precursor state. Maximum surface coverage was found to be reduced with increasing surface temperature, which is discussed on the basis of a long lifetime of the precursor state at low temperatures.     

Ab initio density-functional study of the bridging addition of acrylonitrile on the Si(100) surface

Acrylonitrile may react with the Si(100) surface in a number of ways. Fifteen different configurations have been identified. This study which complements an earlier study devoted to cycloadditions and bonds involving a single atom deals essentially with configurations involving cumulative double bonds. Factors affecting the relative stability of various mesomeric forms are shown to be the proximity to tetrahedral geometry and the compatibility of the dipole form with buckled structures. Tripods are shown to be a way to accommodate a priori unfavorable structures.     

Structure and binding energies of unsaturated hydrocarbons on Si(001) and Ge(001)

The adsorption of acetylene, ethylene, and benzene on the Si(001) and Ge(001) surfaces is investigated by first-principles density-functional calculations within the generalized-gradient approximation. We find that the adsorption energies of the three hydrocarbons containing a triple bond, a double bond, and a pi-conjugated aromatic ring decrease as the sequence of C2H2>C2H4>C6H6. We also find that the bondings of acetylene, ethylene, and benzene to Ge(001) are much weaker than those to Si(001). As a result, benzene is weakly bound to Ge(001) while it is chemisorbed on Si(001), consistent with temperature-programmed desorption data.     

Ab initio surface reaction energetics of SiH4 and Si2H6 on Si(001)-(2 x 2)

First-principles pseudopotential calculations, within a simple dynamically constrained scheme, have been performed to investigate the reaction of 0.25 ML coverage of SiH4 and Si2H6 with the Si(001)-(2 x 2) surface. The silane molecule (SiH4) is adsorbed on to the surface at a number of different sites (on dimer, interrow, or intrarow) with varying barrier heights. Two distinct structures, which are similar in energy, arise from the initial dissociative reaction SiH4-->SiH3(silyl) + H, where the dissociated species are adsorbed either on the same dimer components or on adjacent dimer components. Several further decays of silyl from SiH4 are presented in two separate regimes of high and low ambient hydrogen coverages. The decomposition of silyl can form two different bridging structures: an on top or an intrarow bridging structure in both of the two hydrogen coverage regimes. The disilane molecule (Si2H6) is also adsorbed upon this surface with varying energy barriers, resulting in a dissociation reaction where two SiH3 species are adsorbed on one dimer or in an adjacent dimer configuration. Plausible energy reaction paths for the above models are presented. The stability of the SiH2 species is also discussed.     

Molecular beam investigation of hydrogen dissociation on Si(001) and Si(111) surfaces

The influence of molecular vibrations on the reaction dynamics of H2 on Si(001) as well as isotopic effects have been investigated by means of optical second-harmonic generation and molecular beam techniques. Enhanced dissociation of vibrationally excited H2 on Si(001)2 x 1 has been found corresponding to a reduction of the mean adsorption barrier to 390 meV and 180 meV for nu=1 and nu=2, respectively. The adsorption dynamics of the isotopes H2 and D2 show only small differences in the accessible range of beam energies between 50 meV and 350 meV. They are traced back to different degrees of vibrational excitation and do not point to an important influence of quantum tunneling in crossing the adsorption barrier. The sticking probability of H2 on the 7 x 7-reconstructed Si(111) surface was found to be activated both by H2 kinetic energy and surface temperature in a qualitatively similar fashion as H2/Si(001)2 x 1. Quantitatively, the overall sticking probabilities of H2 on the Si(111) surface are about one order of magnitude lower than on Si(001), the influence of surface temperature is generally stronger.     

Density functional theory investigation of product distribution following reaction of acrylonitrile on diamond (001)-2 x 1 surface

The reaction of acrylonitrile with the C(001)-2 x 1 surface has been investigated by employing density functional cluster model calculations. The calculations revealed eight possible reaction pathways for acrylonitrile with the surface dimer. Full geometry optimized structures were obtained for all adducts, including intra- and interdimer reaction products. These results were analyzed in terms of both the total energy values and the detailed optimized geometries. We find that the reaction of acrylonitrile with the diamond (001) surface occurs primarily through its nonpolar C=C group and the intradimer [2+2](cc) product is the dominant product. All these results are in good agreement with the experimental work by Schwartz. It is noteworthy that the isomerization process plays an important role in the chemisorption process. Both intradimer [4+2] product and interdimer [2+2](cc) product can isomerize to the intradimer [2+2](cc) product. The present study shows that the isomerization between intradimer [4+2] product and intradimer [2+2](cc) product is slightly favorable over the direct path to formation of the intradimer [2+2](cc) product.     

First-principles study of the adsorption of cesium on Si(001)(2 x 1) surface

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed on cesium adsorption on Si(001)(2 x 1) surface. The optimized geometries and adsorption energies have been obtained and the preferred binding sites have been determined for the coverage (Theta) of one monolayer and half a monolayer. At Theta = 0.5 ML the most stable adsorption site is shown to be T3 site. At Theta = 1 ML two Cs atoms are adsorbed at HH and T3 sites, respectively. It was found that the saturation coverage of Cs for the Si(001)(2 x 1)-Cs surface is one monolayer instead of half a monolayer. This finding supports the majority of experimental observations but does not support recent coaxial impact collision ion scattering spectroscopy investigations [Surf. Sci. 531, L340 (2003)] and He(+) Rutherford backscattering spectroscopy studies [Phys. Rev. B 62, 4545 (2000)]. Mulliken charge and overlap population analysis showed that the Cs-Si bond is indeed ionic rather than polarized covalent as generally assumed for alkali metal (AM) on Si(001)(2 x 1) surface. Geometrical structure analysis seems to have limitations in determining the nature of AM-substrate bond. We also found that the silicon surface is metallic and semiconducting for the coverages of 0.5 and 1 ML, respectively.     

Theoretical modeling of oxygen adsorption on the PbTe(001) surface

For the semiconducting compound PbTe, the initial stages of oxidation, which are important for technology of IR-and thermoelectric devices, have been theoretically studied. The structure, stability, and changes in the electrostatic potentials at the oxidized sites in lead telluride have been calculated in the framework of the cluster approach by the hybrid density functional theory B3LYP method. Different variants of attachment of one to six oxygen atoms to the atoms of the surface and subsurface layers have been considered. The most stable oxidation products have been found. The calculation results are quantitatively consistent with experimental XPS data on chemical shifts.     

Structural analysis of the reconstructed Si(001)-C surface

The atomic structure of reconstructed Si(001)c(4 x 4)-C surface has been studied by coaxial impact collision ion scattering spectroscopy. When the 100L of ethylene (C(2)H(4)) molecules have been exposed on Si(001)-(2 x 1) surface at 700 degrees C, it is found that C atoms cause the ordering of missing Si dimer defects and occupy the fourth layer of Si(001) directly below the bridge site. Our results provide the support for the previous model in which a missing dimer structure is accompanied by C incorporation into the subsurface.     

Peierls instability in one-dimensional borine wire on Si(001)

We present a theoretical study of the Peierls instability in the 1D borine wire formed on the Si(001) surface. Our first-principles density-functional theory calculations show that this molecular wire is stabilized by formation of a 1D-CDW, accompanying a structural distortion with a double periodicity and a band-gap opening at the Fermi level. Such a 1D molecular wire on the 2D substrate provides a surface CDW system to allow a real-space observation of CDWs, their fluctuations or critical behaviors at atomic scale.     

Adsorption of acrylonitrile on diamond and silicon (001)-(2 x 1) surfaces: effects of dimer structure on reaction pathways and product distributions

Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) are used to compare the reaction of acrylonitrile with Si(001) and C(001) (diamond) surfaces. Our results show that reaction with Si(001) and C(001) yield very different product distributions that result from fundamental differences in the ionic character of these surfaces. While acrylonitrile reacts with the C(001) surface via a [2 + 2] cycloaddition reaction in a manner similar to nonpolar molecules such as alkenes and disilenes, reaction with the Si(001) surface occurs largely through the nitrile group. This work represents the first experimental example of how differences in dimer structure lead to very different chemistry for C(001) compared to that for Si(001). The fact that Si(001) reacts with the strongly polar nitrile group of acrylonitrile indicates that the zwitterionic character of this surface controls its reactivity. C(001) dimers, on the other hand, behave more like a true molecular double bond, albeit a highly strained one. Consequently, while alternative strategies will be necessary for chemical modification of Si(001), traditional schemes from organic chemistry for functionalization of alkenes and disilenes may be available for building molecular layers on C(001).