Indirect adsorbate-adsorbate interactions mediated through the surface electronic structure of the Si(100) surface

Indirect adsorbate-adsorbate interactions between adsorbed ammonia (NH3) molecules on the Si(100) surface are investigated using density functional theory. Two different nonlocal effects mediated through the surface electronic structure are observed: "poisoning" and hydrogen bonding. We find that adsorbed NH3 "poisons" adsorption of NH3 on neighboring Si dimers on the same side of the dimer row whereas neighboring NH2(a) groups favor this configuration. Adsorption of NH3 involves charge transfer to the surface that localizes on neighboring Si dimer atoms, preventing adsorption of NH3 at these sites. These indirect interactions are similar to Friedel-type interactions observed on metal surfaces with an estimated range of less than 7.8 A on the Si(100) surface. These interactions may be manipulated to construct local ordering of the adsorbates on the surface.     

Orthogonal self-assembly of interconnected one-dimensional inorganic and organic nanostructures on the Si(100) surface

Orthogonal, interconnected inorganic and organic one-dimensional nanostructures have been fabricated by parallel self-assembly on the Si(100) surface and investigated using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, bismuth nanowires were self-assembled on the clean Si(100)-2 x 1 surface perpendicular to the Si dimer rows, followed by hydrogen passivation of the surrounding Si surface. Styrene molecular chains were then self-assembled on the H-passivated Si(100)-2 x 1 surface to intersect perpendicularly with the Bi nanowires. This general approach can likely be applied to the wide range of inorganic and organic species that spontaneously form one-dimensional nanostructures on the Si(100) surface.     

In situ x-ray photoelectron spectroscopic and density-functional studies of Si atoms adsorbed on a C60 film

The interaction between C(60) and Si atoms has been investigated for Si atoms adsorbed on a C(60) film using in situ x-ray photoelectron spectroscopy (XPS) and density-functional (DFT) calculations. Analysis of the Si 2p core peak identified three kinds of Si atoms adsorbed on the film: silicon suboxides (SiO(x)), bulk Si crystal, and silicon atoms bound to C(60). Based on the atomic percent ratio of silicon to carbon, we estimated that there was approximately one Si atom bound to each C(60) molecule. The Si 2p peak due to the Si-C(60) interaction demonstrated that a charge transfer from the Si atom to the C(60) molecule takes place at room temperature, which is much lower than the temperature of 670 K at which the charge transfer was observed for C(60) adsorbed on Si(001) and (111) clean surfaces [Sakamoto et al., Phys. Rev. B 60, 2579 (1999)]. The number of electrons transferred between the C(60) molecule and Si atom was estimated to be 0.59 based on XPS results, which is in good agreement with the DFT result of 0.63 for a C(60)Si with C(2v) symmetry used as a model cluster. Furthermore, the shift in binding energy of both the Si 2p and C 1s core peaks before and after Si-atom deposition was experimentally obtained to be +2.0 and -0.4 eV, respectively. The C(60)Si model cluster provides the shift of +2.13 eV for the Si 2p core peak and of -0.28 eV for the C 1s core peak, which are well corresponding to those experimental results. The covalency of the Si-C(60) interaction was also discussed in terms of Mulliken overlap population between them.     

A hybrid approach combining energy density analysis with the interaction energy decomposition method

We propose a new analysis technique for characterizing molecular interactions that combines an energy decomposition scheme, such as the Kitaura-Morokuma decomposition method, with energy density analysis, which partitions the total energy of the system into atomic contributions. The combined scheme, termed Interaction-EDA, enables us to estimate the local contribution of interaction energy components, such as electrostatic, exchange, polarization, and charge transfer. The evaluation of the local interaction energy is rather important in large systems. For a numerical assessment, the Interaction-EDA method is applied to the process of CO adsorption on Si(100) - (2 x 1) surface.     

Origin of nonlocal interactions in adsorption of polar molecules on Si(001)-2 x 1

Using density functional theory slab calculations, we have investigated (i) the origin of nonlocal interactions occurring in the adsorption of small polar molecules (H2O,NH3,CH3OH,CH3NH2) on the clean Si(001)-2 x 1 surface and (ii) the nonlocal effects on two-dimensional arrangement of adsorbates. Our results show the adsorption properties are significantly altered in the presence of adsorbates on an adjacent dimer along a row. We have identified that the coverage dependent behavior arises from a combination of (i) surface polarization change, (ii) adsorbate-induced charge delocalization, (iii) adsorbate-adsorbate repulsion, and (iv) hydrogen bonding. The nucleophilic-electrophilic molecular adsorption involves charge delocalization to neighboring dimers along a row, which in turn undermines molecular adsorption on the neighboring dimers. Nonlocal effects associated with polar interactions with neighboring dimers and adsorbates vary with adsorption system. While such polar interactions are unimportant in CH3OH adsorption, hydrogen bonding and adsorbate-adsorbate repulsion play an important role in determining the adsorption structures of H2O and NH3CH3NH2, respectively. In addition, the electrostatic attraction with the buckled-up Si atoms of adjacent dimers contributes to stabilization of H2O, NH3, and CH3NH2 adsorption. We also discuss kinetic effects on two-dimensional ordering of adsorbates, in conjunction with surface phase transition and adsorption-dissociation rates.     

Thermal chemistry of styrene on Si(100)2x1 and modified surfaces: electron-mediated condensation oligomerization and posthydrogenation reactions

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2x1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2x1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2x1 reconstructed surface. The chemisorption of styrene on the 2x1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2x1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2x1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2x1 upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.     

Mechanisms for NH3 decomposition on Si(100)-(2 x 1) surface: a quantum chemical cluster model study

In this paper, we present a detailed mechanism for the complete decomposition of NH3 to NHx(a) (x = 0-2). Our calculations show that the initial decomposition of NH3 to NH2(a) and H(a) is facile, with a transition-state energy 7.4 kcal mol-1 below the vacuum level. Further decomposition to N(a) or recombination-desorption to NH3(g) is hindered by a large barrier of approximately 46 kcal mol-1. There are two plausible NH2 decomposition pathways: 1) NH2(a) insertion into the surface Si-Si dimer bond, and 2) NH2(a) insertion into the Si-Si backbond. We find that pathway (1) leads to the formation of a surface Si = N unit, similar to a terminal Si = Nt pair in silicon nitride, Si3N4, while pathway (2) leads to the formation of a near-planar, subsurface Si3N unit, in analogy to a central nitrogen atom (Nc) bounded to three silicon atoms in the Si3N4 environment. Based on these results, a plausible microscopic mechanism for the nitridation of the Si(100)-(2 x 1) surface by NH3 is proposed.     

Surface and subsurface hydrogen: adsorption properties on transition metals and near-surface alloys

Periodic, self-consistent DFT-GGA calculations are used to study the thermochemical properties of both surface and subsurface atomic hydrogen on a variety of pure metals and near-surface alloys (NSAs). For surface hydrogen on pure metals, calculated site preferences, adsorption geometries, vibrational frequencies, and binding energies are reported and are found to be in good agreement with available experimental data. On NSAs, defined as alloys wherein a solute is present near the surface of a host metal in a composition different from the bulk composition, surface hydrogen generally binds more weakly than it binds to the pure-metal components composing the alloys. Some of the NSAs even possess the unusual property of binding hydrogen as weakly as the noble metals while, at the same time, dissociating H(2) much more easily. On both NSAs and pure metals, formation of surface hydrogen is generally exothermic with respect to H(2)(g). In contrast, formation of subsurface hydrogen is typically endothermic with respect to gas-phase H(2) (the only exception to this general statement is found for pure Pd). As with surface H, subsurface H typically binds more weakly to NSAs than to the corresponding pure-metal components of the alloys. The diffusion barrier for hydrogen from surface to subsurface sites, however, is usually lower on NSAs compared to the pure-metal components, suggesting that population of subsurface sites may occur more rapidly on NSAs.     

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.     

Hydrogen bond in 3-acetyl-4-hydroxycoumarin: X-ray diffraction study and quantum-chemical calculations

The proton transfer and the character of the strong intramolecular O--H...O hydrogen bond (O...O 2.442 ) in 3-acetyl-4-hydroxycoumarin were analyzed based on the results of X-ray diffraction study in the temperature range from 100 to 353 K and quantum-chemical B3LYP/6-31G(d,p) calculations. The barrier to proton transfer along the H-bond line is low (2 kcal mol^–1). However, no proton transfer was observed in the crystal at 100 K. Bader's topological analysis of the electron density distribution both in the crystal and in the isolated molecule demonstrated that the hydrogen bond corresponds to an intermediate type of interatomic interactions (E(r) < 0, ²(r) > 0 at the critical point (3, –1)).     

Oxidation of nitrided si(100) by gaseous atomic and molecular oxygen

The nitridation of Si(100) by ammonia and the subsequent oxidation of the nitrided surface by both gaseous atomic and molecular oxygen was investigated under ultrahigh vacuum (UHV) conditions using X-ray photoelectron spectroscopy (XPS). Nitridation of Si(100) by the thermal decomposition of NH3 results in the formation of a subsurface nitride and a decrease in the concentration of surface dangling bond sites. On the basis of changes in the N1s spectra obtained after NH3 adsorption and decomposition, we estimate that the nitride resides about four to five layers below the vacuum-solid interface and that the concentration of surface dangling bonds after nitridation is only 59% of its value on Si(100)-(2 x 1). Oxidation of the nitrided surface is found to produce an oxide phase that remains in the outer layers of the solid and interacts only weakly with the underlying nitride for oxygen coverages up to 2.5 ML. Slight changes in the N1s spectra observed after oxidizing at 300 K are suggested to arise primarily from the introduction of strain within the nitride, and by the formation of a small amount of Si2=N-O species near the nitride-oxide interface. The nitrogen bonding environment changes negligibly after oxidizing at 800 K, which is indicative of greater phase separation at elevated surface temperature. Nitridation is also found to significantly reduce the reactivity of the Si(100) surface toward both atomic and molecular oxygen. A comparison of the oxygen uptake on the clean and nitrided surfaces shows quantitatively that the decrease in dangling bond concentration is responsible for the reduced activity of the nitrided surface toward oxidation, and therefore that dangling bonds are the initial adsorption site for both gaseous oxygen atoms and molecules. Increasing the surface temperature is found to promote the uptake of oxygen when O2 is used as the oxidant, but brings about only a small enhancement in the uptake of gaseous O-atoms. The different effects of surface temperature on the uptake of O versus O2 are interpreted in terms of the efficiency at which dangling bond pairs are regenerated on the surface at elevated temperature and the different site requirements for the adsorption of O and O2.     

Competition and selectivity of organic reactions on semiconductor surfaces: reaction of unsaturated ketones on Si(100)-2 x 1 and Ge(100)-2 x 1

A combined experimental and theoretical study of a model system of multifunctional unsaturated ketones, including ethyl vinyl ketone (EVK), 2-cyclohexen-1-one, and 5-hexen-2-one, on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces was performed in order to probe the factors controlling the competition and selectivity of organic reactions on clean semiconductor surfaces. Multiple internal reflection infrared spectroscopy data and density functional theory calculations indicate that EVK and 2-cyclohexen-1-one undergo selective [4 + 2] hetero-Diels-Alder and [4 + 2] trans cycloaddition reactions on the Ge(100)-2 x 1 surface at room temperature. In contrast, on the Si(100)-2 x 1 surface, evidence is seen for significant ene and possibly [2 + 2] C=O cycloaddition side products. The greater selectivity of these compounds on Ge(100) versus Si(100) is explained by differences between the two surfaces in both thermodynamic factors and kinetic factors. With 5-hexen-2-one, for which [4 + 2] cycloaddition is not possible, a small [2 + 2] C=C cycloaddition product is observed on Ge(100) and possibly Si(100), even though the [2 + 2] C=C transition state is calculated to be the highest barrier reaction by several kilocalories per mole. The results suggest that, due to the high reactivity of clean semiconductor surfaces, thermodynamic selectivity and control will play important roles in their selective functionalization, favoring the use of Ge for selective attachment of multifunctional organics.     

Theoretical Investigation on the Adsorption of Ag^＋ and Hydrated Ag^＋ Cations on Clean Si（111） Surface

In this paper, the adsorption of Ag^＋ and hydrated Ag^＋ cations on clean Si（111） surface were investigated by using cluster （Gaussian 03） and periodic （DMol^3） ab initio calculations. Si（111） surface was described with cluster models （Si14H17 and Si22H21） and a four-silicon layer slab with periodic boundary conditions. The effect of basis set superposition error （BSSE） was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Ag^＋ cations and clean Si（111） surface are large, suggesting a strong interaction between hydrated Ag^＋ cations and the semiconductor surface. With the increase of number, water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Ag^＋ cation. The Ag^＋ cation in aqueous solution will safely attach to the clean Si（111） surface.     

Possible gas-phase reactions of H2/CH4/tetramethylsilane in diamond/beta-SiC nanocomposite film deposition: an ab-initio study

The Si-C bond breakings in tetramethylsilane (TMS) when interacting with H/H2 and the successive H abstractions from SiH4/CH4 in the gas mixture of H2/ CH4/TMS were studied at the CCSD(T)/6-311+G**//MP2/6-31+G** level of theory. Their rate constants between 1500 and 2500 K were estimated using a conventional transition state theory. The results indicate that (i) it is mainly the H radical that causes the Si-C bond breaking in TMS, and (ii) the successive H abstractions from SiH4 are much easier and faster than those from CH4. At low temperatures the differences of rate constants among the four types of the reactions are large, but generally reduced at high temperatures. The reaction rates show no selectivity over the pressure as verified at P = 0.00025, 0.025, 1, and 100 atm, respectively. Our results could provide the following microscopic level understanding of reactions in the synthesis of diamond/beta-SiC nanocomposite films. Although the Si content is smaller than that of C in the precursor gases, the gas mixture activated by microwave plasma technique could provide Si sources with a higher rate. The produced Si sources with excellent rigidity in sp3 hybridization competitively occupy the space on the substrate together with C sources, resulting in the deposition of diamond/beta-SiC nanocomposite films.     

Electronic and crystalline structure of Si/SiO2 interface modified by ArF excimer laser

The native oxide layers on Si(100) surface were irradiated under UHV conditions by an ArF excimer laser pulses with energy density varied between melting and evaporating thresholds. The resulting changes were studied by LEED, AES and UPS. The increase of the energy density up to evaporation threshold results in the recrystallisation of native oxide layer. The pulses with energy densities just above the evaporation threshold ablate the top layer leaving an ordered and atomicaly clean surface. The observed (1x1) surface reconstruction is probably stabilised by strains introduced during rapid melting and quenching of the topmost layers. The surface electronic structure is dominated by random satisfaction of dangling bonds swearing a well defined surface states observed on (2x1)Si(100) surface.     

Study by scanning tunneling microscopy of hydrogen adsorption and desorption on Si111 7 x 7 at room temperature and at high temperature

An overview is given on the use of scanning tunneling microscopy (STM) for investigation of the adsorption of hydrogen on Si(111)7 x 7 both at room temperature and at elevated temperature to finally obtain a hydrogen-saturated surface of Si(111). The initial stages are characterized by high reactivity of Si adatoms of the 7 x 7 structure. After adsorption of hydrogen on the more reactive sites in the beginning of the adsorption experiments a regular pattern, which is different for room and elevated temperature, is observed for the less reactive sites. In agreement with previous work, local 1 x 1 periodicity of the rest atom layer and the presence of di- and trihydride clusters is observed for hydrogen-saturated surface. STM has also been used to characterize surfaces from which the hydrogen atoms have been removed by thermal desorption. Finally, tip-induced desorption by large positive sample-bias voltages and by increasing the tunneling current will be described.     

Initial oxidation and hydroxylation of the Ge(100)-2 x 1 surface by water and hydrogen peroxide

We use density functional theory to investigate the surface chemistry of initial oxidation and hydroxylation of the Ge(100)-2 x 1 surface by water and hydrogen peroxide. Comparison of the reaction of water on the Si(100)-2 x 1 and Ge(100)-2 x 1 surfaces shows that the kinetics of oxidation of the Ge(100)-2 x 1 surface with water is slower. Our calculations also show that oxidation products on the Ge(100)-2 x 1 surface are less thermodynamically stable than on Si. We also investigate two competing dissociation reactions of H2O2 on the Ge(100)-2 x 1 surface. We find that dissociative adsorption via cleavage of the OH bond is less exothermic than OO dissociation. Furthermore, interdimer OO dissociation has a lower activation barrier than interdimer or intradimer OH dissociation, although interdimer dissociation products are found to be less stable compared than those formed from intradimer dissociation reactions. Finally, we find that the oxidation products formed from hydrogen peroxide are more stable than those formed from water.     

Ultrafast charge transfer and atomic orbital polarization

The role of orbital polarization for ultrafast charge transfer between an atomic adsorbate and a substrate is explored. Core hole clock spectroscopy with linearly polarized x-ray radiation allows to selectively excite adsorbate resonance states with defined spatial orientation relative to the substrate surface. For c(4 x 2)S/Ru(0001) the charge transfer times between the sulfur 2s(-1)3p*+1 antibonding resonance and the ruthenium substrate have been studied, with the 2s electron excited into the 3p perpendicular* state along the surface normal and the 3p parallel* state in the surface plane. The charge transfer times are determined as 0.18+/-0.07 and 0.84+/-0.23 fs, respectively. This variation is the direct consequence of the different adsorbate-substrate orbital overlap.     

A theoretical investigation on photocatalytic oxidation on the TiO2 surface

The TiO(2) photocatalytic oxidation mechanism was theoretically investigated by using long-range corrected time-dependent density functional theory (LC-TDDFT) with a cluster model of the anatase TiO(2)(001) surface. We found that LC-TDDFT with the cluster model quantitatively reproduces the photoexcitations of the TiO(2) surface by calculating the electronic spectra of a clean TiO(2) surface and one with oxygen defects. We calculated the electronic spectra of a molecularly adsorbed TiO(2) surface for the adsorptions of phenol, methanol, and methane molecules as typical organic molecules. We obtained the surprising result that the main peak of the phenol-adsorbed TiO(2) surface, which overlaps with the main peak of the clean TiO(2) surface, corresponds to charge transfers from the phenol molecule to the TiO(2) surface. This indicates that the TiO(2) photocatalytic oxidation proceeds through direct charge transfer excitation from the substrate molecules to the TiO(2) surface. In contrast, we found slight and no charge transfer for methanol and methane adsorption, respectively, in agreement with the experimental findings for their reactivities. In light of these results, we propose a new mechanism for heterogeneous TiO(2) photocatalytic oxidations.     

Hydrogen desorption kinetics from the Si(1-x)Gex(100)-(2x1) surface

We study the influence of germanium atoms upon molecular hydrogen desorption energetics using density functional cluster calculations. A three-dimer cluster is used to model the Si((1-x))Ge(x)(100)-(2x1) surface. The relative stabilities of the various monohydride and clean surface configurations are computed. We also compute the energy barriers for desorption from silicon, germanium, and mixed dimers with various neighboring configurations of silicon and germanium atoms. Our results indicate that there are two desorption channels from mixed dimers, one with an energy barrier close to that for desorption from germanium dimers and one with an energy barrier close to that for desorption from silicon dimers. Coupled with the preferential formation of mixed dimers over silicon or germanium dimers on the surface, our results suggest that the low barrier mixed dimer channel plays an important role in hydrogen desorption from silicon-germanium surfaces. A simple kinetics model is used to show that reasonable thermal desorption spectra result from incorporating this channel into the mechanism for hydrogen desorption. Our results help to resolve the discrepancy between the surface germanium coverage found from thermal desorption spectra analysis, and the results of composition measurements using photoemission experiments. We also find from our cluster calculations that germanium dimers exert little influence upon the hydrogen desorption barriers of neighboring silicon or germanium dimers. However, a relatively larger effect upon the desorption barrier is observed in our calculations when germanium atoms are present in the second layer.