Precursor Mediated Cycloaddition Reaction of Ethylene to the Si(100)c(4 × 2) Surface

Bond Insertion at Distorted Si(001) Subsurface Atoms

Using density functional theory (DFT) methods, we analyze the adsorption of acetylene and ethylene on the Si(001) surface in an unusual bond insertion mode. The insertion takes place at a saturated tetravalent silicon atom and the insight gained can thus be transferred to other saturated silicon compounds in molecular and surface chemistry. Molecular orbital analysis reveals that the distorted and symmetry-reduced coordination of the silicon atoms involved due to surface reconstruction raises the electrophilicity and, additionally, makes certain σ bond orbitals more accessible. The affinity towards bond insertion is, therefore, caused by the structural constraints of the surface. Additionally, periodic energy decomposition analysis (pEDA) is used to explain why the bond insertion structure is much more stable for acetylene than for ethylene. The increased acceptor abilities of acetylene due to the presence of two π*-orbitals (instead of one π*-orbital and a set of σ*(C–H) orbitals for ethylene), as well as the lower number of hydrogen atoms, which leads to reduced Pauli repulsion with the surface, are identified as the main causes. While our findings imply that this structure might be an intermediate in the adsorption of acetylene on Si(001), the predicted product distributions are in contradiction to the experimental findings. This is critically discussed and suggestions to resolve this issue are given.

Adsorption of ethylene on Sn and In terminated Si(001) surface studied by photoelectron spectroscopy and scanning tunneling microscopy

Interaction of ethylene (C2H4) with Si(001)-Sn-2 × 2 and Si(001)-In-2 × 2 at room temperature has been studied using core level (C 1s) X-ray photoelectron spectroscopy with synchrotron radiation and scanning tunneling microscopy. Sn and In form similar dimer chains on Si(001)2 × 1, but exhibit different interaction with ethylene. While ethylene adsorbs on top of Sn dimers of the Si(001)-Sn-2 × 2 surface, the Si(001)-In-2 × 2 surface turned out to be inert. Furthermore, the reactivity of the Sn terminated surface is found to be considerably decreased in comparison with Si(001)2 × 1. According to the proposed adsorption model ethylene bonds to Sn dimers via [2 + 2] cycloaddition by interacting with their π dimer bonds. In contrast, indium dimers do not contain π bonds, which renders the In terminated Si(001) surface inert for ethylene adsorption.

Repulsion-Induced Surface-Migration by Ballistics and Bounce

The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Recent Scanning Tunneling Microscopy studies have demonstrated the electron-induced long-range surface-migration of ethylene, benzene, and related molecules, moving tens of Angstroms across Si(100). We present a model of the previously unexplained long-range recoil of chemisorbed ethylene across the surface of silicon. The molecular dynamics reveal two key elements for directed long-range migration: first 'ballistic' motion that causes the molecule to leave the ab initio slab of the surface traveling 3-8 Å above it out of range of its roughness, and thereafter skipping-stone 'bounces' that transport it further to the observed long distances. Using a previously tested Impulsive Two-State model, we predict comparable long-range recoil of atomic chlorine following electron-induced dissociation of chlorophenyl chemisorbed at Cu(110).

Unusual two-stage kinetics of ethylene adsorption on Si(100) unraveled by surface optical spectroscopy and Monte Carlo simulation

The adsorption of ethylene on a Si(100)-2×1 surface in an ultrahigh vacuum has been monitored at room temperature by use of real-time surface differential reflectance spectroscopy, which clearly demonstrated that the adsorption follows a two-stage process. About half a monolayer is obtained for 1 L, while the second stage is much slower, yielding the complete monolayer for an exposure of ∼400  L. The kinetics over the full range has been successfully reproduced by a Monte Carlo calculation. The key point of this two-stage adsorption kinetic lies in the reduced adsorption probability (by a factor of several hundreds) on the Si dimers, neighbors of dimers which have already reacted, with respect to the adsorption probability on isolated dimers. This new kind of adsorption kinetics, due to a repulsion between already adsorbed molecules and additional molecules impinging on the surface, makes it a textbook case for a "cooperative" adsorption process.

CYCLOADDITION REACTION BETWEEN ORGANIC MOLECULES AND Si(100) AND ELECTRONIC PROPERTIES OF ADSORBED MOLECULES

The mechanism of cycloaddition reaction between ethylene and the Si dimer on Si (100) is discussed from our recent experimental results using high-resolution electron energy loss spectroscopy and valence photoelectron spectroscopy at ~50 K. The weakly adsorbed π-complex state is identified as a precursor for di-σ bond formation and the activation barrier is roughly estimated to be ~0.2 eV. A different reaction route may exist since a di-σ species is also observed at low temperature. Using cycloaddition reaction, unsaturated organic molecules are chemically attached to the dimer on Si (100), and the adsorbed species are relatively stable. Thus, scanning tunneling spectroscopy (STS) measurements of a single adsorbed molecule can be carried out at room temperature. We report the STS results of 2-butyne adsorbed on Si (100).

Regioselective Cycloaddition Reaction of Alkene Molecules with the Asymmetric Dimer on Si(100)c(4×2)

We investigated the adsorption states of 2-methylpropene and propene on Si(100)c(4 x 2) using low-temperature scanning tunneling microscopy. We have found that regioselective cycloaddition reactions (di-sigma bond formation) occur between the asymmetric alkene molecules and the asymmetric dimers on Si(100)c(4 x 2). First-principles calculations have elucidated that the regioselectivity is closely related to the structures of precursor species and these precursor species have carbocation-like features. Thus, we conclude that Markovnikov's rule is applicable for the cycloaddition of asymmetric alkene with the asymmetric dimer on Si(100)c(4 x 2).

Two bonding configurations of acetylene on Si(001)-(2×1): A combined high-resolution electron energy loss spectroscopy and density functional theory study

Two coexisting adsorption states of molecularly adsorbed acetylene on the Si(001)-(2 x 1) surface have been identified by a combined study based on the high-resolution electron energy loss spectroscopy and density functional computations. Seven possible adsorbate-substrate structures are considered theoretically including their full vibrational analysis. Based on a significantly enhanced experimental resolution, the assignment of 15 C2H2- and C2D2-derived vibrational modes identifies a dominant di-sigma bonded molecule adsorbed on top of a single Si-Si dimer. Additionally there is clear evidence for a second minority species which is di-sigma bonded between two Si-Si dimers within the same dimer row (end-bridge geometry). The possible symmetries of the adsorbate complexes are discussed based on the specular and off-specular vibrational measurements. They suggest lower than ideal C(2v) and C(s) symmetries for on-top and end-bridge species, respectively. At low coverages the symmetry reductions might be lifted.

Trapping-mediated chemisorption of ethylene on Si(001)-c(4 x 2)

Adsorption of ethylene molecules on Si(001)-c(4 x 2) was studied using scanning tunneling microscopy at low temperatures. Ethylene molecules trapped at the surface at 50 K were imaged only after decay to chemisorption, each bonding to a Si dimer. Atomic-scale observations of temperature-dependent kinetics show that the decay exhibited Arrhenius behavior with the reaction barrier of 128 meV in clear evidence of the trapping-mediated chemisorption, however, with an anomalously small preexponential factor of 300 Hz. Such a small prefactor is attributed to the entropic bottleneck at the transition state caused by the free-molecule-like trap state.

Chemical Structure and Orientation of Ethylene on Si(114)−(2×1)/c(2×2)

The basic chemical structure and orientation of ethylene chemisorbed on Si(114)-(2 x 1) at submonolayer coverage is characterized in ultrahigh vacuum using transmission Fourier transform infrared (FTIR) spectroscopy. The spectra are consistent with di-sigma bonding of ethylene to the surface with a preferential molecular orientation over macroscopic lengths. These results are supported by density functional theory (DFT) calculations of vibrational frequencies for optimized ethylene-Si(114) structures occupying the dimer and rebonded atom surface sites. A detailed analysis of the strong angular and polarization dependence of the C-H stretching mode intensities is also consistent with the adsorption structures identified by DFT, indicating that ethylene chemisorbs with the C-C bond axis parallel to the structural rows oriented along the [10] direction on the Si(114)-(2 x 1) surface. The results indicate that the unique structure of this surface makes it an excellent template for elucidating relationships between surface structure and organic reaction mechanisms on silicon.

Dynamical steric effect in the decomposition of methyl chloride on a silicon surface

We report results of a study on the incident energy and the surface-temperature dependence of the steric effects in the dissociative adsorption of CH3Cl on a Si{100} surface. Data presented here show that the initial sticking probability for the Cl-end collision is larger at an incident energy of 120 meV than that in the CH3-end collision. Furthermore, this steric preference is quite sensitive to the kinetic energy and the rotational state of CH3Cl and the surface temperature. This study shows that the nonequilibrium surface trapping plays a key role in the initial step of the decomposition of CH3Cl on Si{100}.