SiO adsorption on a p(2 × 2) reconstructed Si(1 0 0) surface

We have investigated the adsorption mechanism of SiO molecule incident on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. Stable adsorption geometries of SiO on Si surface, as well as their corresponding activation and adsorption energies are identified. We found that the SiO molecule is adsorbed on the Si(1 0 0) surface with almost no activation energy. An adsorption configuration where the SiO binds on the channel separating the dimer rows, forming a Si-O-Si bridge on the surface, is the energetically most favourable geometry found. A substantial red-shift in the calculated vibrational frequencies of the adsorbed SiO molecule in the bridging configurations is observed. Comparison of adsorption energies shows that SiO adsorption on a Si(1 0 0) surface is energetically less favourable than the comparable O₂ adsorption. However, the role of SiO in the growth of silicon sub-oxides during reactive magnetron plasma deposition is expected to be significant due to the relatively large amount of SiO molecules incident on the deposition surface and its considerable sticking probability. The stable adsorption geometries found here exhibit structural properties similar to the Si/SiO₂ interface and may be used for studying SiO_x growth.     

DFT study of NH3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH₃ molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH₃ molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH₃ incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH₃ molecule an energy barrier of ∼0.3 eV was calculated, yielding NH₂ on the adatom and H on the restatom. When two molecules are adsorbed, the NH₃-NH₃ interaction yields the weakening of a N-H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH₃ molecule at the adatom-restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).     

Adsorption of CO, CO2 and NO molecules on a BaTiO3 (0 0 1) surface

Since the development of Scanning Tunnelling Microscopy (STM) technique, considerable attention has been devoted to various molecules adsorbed on various surfaces. Also, a new concept emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale are thus particularly invaluable. The present work describes the first Density Functional Theory (DFT) study of adsorption of CO, CO₂ and NO molecules on a BaTiO₃ surface following a first preliminary calculation of O and O₂ adsorption on the same surface. In the previously considered work, we found that a (0 0 1) surface with BaO termination is more stable than the one with TiO₂-termination. Consequently, we extended our study to CO, CO₂ and NO molecules adsorbed on a (0 0 1) surface with BaO termination. The present calculation was performed on a (1 × 1) cell with one monolayer of adsorbed molecules. Especially, a series of cases implying CO molecules adsorbed in various geometrical configurations has been examined. The corresponding adsorption energy varies in the range of −0.17 to −0.10 eV. The adsorption energy of a CO₂ molecule directly located above an O surface atom (called O_s) is of the order of −0.18 eV. The O-C distance length is then 1.24 Å and the O-C-O and O-C-O_s angles are 134.0° and 113.0°, respectively. For NO adsorption, the most important induced structural changes are the followings: (i) the N-O bond is broken when a NO molecule is absorbed on a Ba-O_s bridge site. In that case, N and O atoms are located above an O and a Ba surface atom, respectively, whereas the O-Ba-O_s and N-O_s-Ba angles are 106.5° and 63.0°, respectively. The N-O distance is as large as 2.58 Å and the adsorption energy is as much as −2.28 eV. (ii) In the second stable position, the NO molecule has its N atom adsorbed above an O_s atom, the N-O axis being tilted toward the Ba atom. The N-O_s-Ba angle is then 41.1° while the adsorption energy is only −0.10 eV. At last, the local densities of states around C, O as well as N atoms of the considered adsorbed molecules have also been discussed.     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.     

Dissociation of water molecule at three-fold oxygen coordinated V site on the InVO4 (0 0 1) surface

Water molecule adsorption properties at the surface of InVO₄ have been investigated using an ab initio molecular dynamics approach. It was found that the water molecules were adsorbed dissociatively to the three-fold oxygen coordinated V sites on the (0 0 1) surface. The dissociative adsorption energy was estimated to be 0.8-0.9 eV per molecule. The equilibrium distance between V and O of the hydroxyl -OH was almost the same as the V-O distance of tetrahedra VO₄ in the InVO₄ bulk crystal (1.7-1.8 Å).     

An effective Hamiltonian for sulfur adsorption at Au(1 0 0) surface

Adsorption of sulfur at the (1 0 0) surface of gold is analyzed with the help of the density functional theory (DFT). Potential energy surface for a single S atom at the Au(1 0 0) surface is computed and a simple analytical formula was found to reproduce the ab initio results to a good accuracy. Vibration frequencies of the adsorbed S atom are computed using the harmonic approximation and the contribution of zero-point motion to the adsorption energy is evaluated. The effects of surface Au atoms relaxation in the sulfur adsorption is analyzed. The interactions between S atoms adsorbed at the nearest and the next nearest equivalent adsorption sites are computed and used to define the effective Hamiltonian describing the interactions between the adsorbed sulfur atoms.     

Theoretical study of hydrogen adsorption on the GaN(0 0 0 1) surface

Ab initio density functional theory, using the B3LYP hybrid functional with all-electron basis sets, has been applied to the adsorption of H on the (0 0 0 1) surface of wurtzite GaN. For bulk GaN, good agreement is obtained with photoemission and X-ray emission data for the valence band and for the Ga 3d and N 2s shallow core levels. A band gap of E_g = 4.14 eV is computed vs the experimental value (at 0 K) of 3.50 eV. A simple model, consisting of a (2 × 2) structure with 3/4-monolayer (ML) of adsorbed H, is found to yield a density of states in poor agreement with photoemission data for H adsorbed on surfaces prepared by ion bombardment and annealing. A new model, consisting of co-adsorbed Ga (1/4 ML) and H (1/2 ML), is proposed to account for these data.     

Coverage effects on phenol adsorption on Si(1 0 0)2 × 1 as: A first principle calculation

We investigated the adsorption of a 6-dimers Si(1 0 0)2 × 1 surface as a function of coverage and adsorption type (molecular/dissociative) by first principle calculations. In particular, we performed calculations on models with 2, 3, 4 and 6 phenol molecules, corresponding to coverage Θ = 0.34, 0.5, 0.67 and 1. We found that total adsorption energy, when at least one phenol is in a molecular state is lower than the sum of the corresponding singly adsorbed molecules. The dissociative adsorption of multiple molecules, both in parallel and switched configuration is most favoured for a coverage Θ = 0.34 (2.6 eV per adsorbed molecule). This values decreases to 2.0 eV and remains constant till the coverage 1 is reached.The energy barrier for the molecular-to-dissociated transition of a phenol molecule, in presence of another dissociatively adsorbed molecule is ∼0.008 eV and it is similar to the value in case of single adsorption. Possible hydrogen displacements were also considered.     

Theoretical simulation of butane isomers adsorption on a Pt(1 0 0) surface

The adsorption of the two butane isomers on Pt(1 0 0) has been characterised with use of density functional simulations. The adsorption energies corresponding to various adsorption configurations were evaluated in good agreement with experimental values. Limited changes of the molecular structure were evidenced. The C-H bond length increases at a degree depending on the surface-hydrogen distance, while the C-C bond length remains similar to that of the free molecule. The surface on-top Pt sites exert a preferential attraction on the molecule, probably through the interaction with the H atoms. The local density of states curves around H as well as C of the adsorbed molecules show dispersed states below the metal Fermi level indicating a molecule-Pt mixing demonstrating a chemical interaction.     

Vibration of small molecules on Pt(1 1 1) surface

The adsorption of methanol and other small molecules onto transition metal surfaces is an important issue in electrochemistry, fuel cells, etc. Despite the overwhelming interest there are still unresolved issues beginning from the geometry of the adsorbed species to the correct assignments of different vibrational modes of the adsorbed molecules on the surface.In order to understand the adsorption processes, we have performed density functional theory (DFT) calculations for small molecules (methanol, formaldehyde, formic acid) on Pt(1 1 1) surfaces. We investigated the nature of the metal-ligand bonding in these adsorption processes using electron density difference and PDOS (partial density of states) methods. Ab initio vibration spectra have been calculated for these systems.     

First-principle study of NO adsorption on the LaFeO3 (0 1 0) surface

The adsorption of NO molecule on the LaFeO₃ (0 1 0) surface was studied using first-principle calculations based on density functional theory. The calculated results indicate that the Fe-top site is the most favorable for NO adsorption. The N-O bond length, Mulliken charge, and the N-O vibration frequency of the NO molecule are discussed after adsorption. The analysis results of the density of the states show that when NO is adsorbed with the Fe-NO configuration, the bonding mechanism is mainly from the interaction between the NO and the Fe d orbit.     

Study of ammonia molecule adsorbing on diamond (1 0 0) surface

The diamond (1 0 0) surface with amino terminations is investigated based on density function theory within the generalized gradient approximation. Our calculated negative electron affinity of diamond (1 0 0) surface with hydrogen termination provides a necessary condition for initiating radical reaction. The results display that the ammonia molecule can form stable C-N covalent bonds on the diamond surface. In addition, due to the lower adsorption energy of one amino group binding on diamond surface, single amino group (SAG) model is easy to be realized in experiment with the comparison of double amino group (DAG) model. The adsorbed ammonia molecule will induce acceptor-like gap states with little change of the valence and conduction band of diamond in SAG model. The adsorption mechanism in the formation of ammonia monolayer on H-terminated diamond (1 0 0) surface, and two possible adsorption structures (SAG and DAG) were especially studied.     

First-principles calculations of hydrogen molecule adsorption on Ti (0 0 0 1)-(2 × 1) surface

Adsorption of H₂ molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H₂ molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H₂ molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H₂ molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H₂ dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects.     

Molecular assembly of tetracene on the (2 × 1)O reconstructed Cu(1 1 0) surface

Using a combination of scanning tunneling microscopy (STM) and density functional theory calculations, we have studied the adsorption of tetracene on the Cu(1 1 0) (2 × 1)O substrate. At monolayer coverage the adsorbed molecules are in the flat-laying geometry with their long axis along the close-packed [0 0 1] direction of the substrate and a long-range ordered structure on the length scale up to 100 nm has been observed. DFT calculation results indicate a stronger interaction between tetracene molecules and Cu(1 1 0) substrate than Cu(1 1 0) (2 × 1)O substrate. The preferential adsorption sites have also been pointed out on both substrates. The observed wavelike structure is explained by the interdigitation of C-H bonds of adjacent molecules.     

The reaction of vinyl acetate with Pd(1 1 0) studied with TPD and molecular beams

The adsorption and reaction of vinyl acetate with the clean Pd(1 1 0) surface has been investigated using temperature programmed desorption and molecular beam reaction measurements. These show that, under low pressure conditions, the main reaction pathway above 400 K is total dehydrogenation to yield hydrogen and carbon dioxide in the gas phase, and surface carbon. This occurs at a steady state, notwithstanding the fact that carbon is being deposited continuously onto the surface. The reaction continues because the vast majority of this carbon is lost from the surface to the bulk of the sample. Between about 320-380 K the reaction profile is somewhat different; the molecule dissociates at the CH₃COOCHCH₂ bond, producing the most stable intermediate, the acetate, and the reaction stops after the build-up of adsorbed acetate and surface carbonaceous species. At ∼300 K, the products are very similar to those for acetaldehyde adsorption (namely, methane, CO and some surface carbon), and they evolve in a non-steady state manner due to the build up of adsorbed CO on the surface. Thus the mechanism is dominated here by dissociation at the CH₃COOCHCH₂ bond, and formation of the acetyl intermediate. Consideration is given to the connection between these data and vinyl acetate synthesis.     

A semi-empirical study of polyacene molecules adsorbed on a Cu(1 1 0) surface

To describe the adsorption of large organic molecules on metal surfaces, to calculate the corresponding diffusion and rotation barriers, the semi-empirical mono-electronic Hamiltonian of the ASED molecular orbital method have been completed to take into account three body interaction terms. The full re-parametrization of this ASED+ version of ASED was determined on the specific case of benzene adsorbed on Cu(1 1 0) and a full transferability assumed for the member of the polyacene series also adsorbed on Cu(1 1 0). The adsorption energies, geometries, diffusion and rotation barriers are very well described by this new semi-empirical technique of calculation opening the way of optimizing larger conjugated molecule on surface for uni-molecular mechanics or electronics.     

Investigation of the interaction of some astrobiological molecules with the surface of a graphite (0 0 0 1) substrate. Application to the CO, HCN, H2O and H2CO molecules

Detailed interaction potential energy calculations are performed to determine the potential energy surface experienced by the molecules CO, HCN, H₂O and H₂CO, when adsorbed on the basal plane (0 0 0 1) of graphite at low temperatures. The potential energy surface is used to find the equilibrium site and configuration of a molecule on the surface and its corresponding adsorption energy. The diffusion constant associated with molecular surface diffusion is calculated for each molecule.     

Comparative study of H2 adsorption on W(1 0 0)-c(2 × 2)Cu and W(1 0 0): Surface alloying effects

The interactions of H and H₂ with W(1 0 0)-c(2 × 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H₂ molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H₂ dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 × 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.