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.     

An STM investigation of the interaction and ordering of pentacene molecules on the Ag/Si(1 1 1)-(√3×√3)R30° surface

Scanning tunneling microscopy has been used to study the ordering of pentacene (C₂₂H₁₄) molecules on the Ag/Si(1 1 1)-(√3×√3)R30° surface at room temperature. Two solid phases, S1 and S2, are observed at coverages of ∼0.35 monolayer (ML) and ∼1.0 ML respectively. It is shown that the solid phase S1 has a high-order commensurate lattice, Ag/Si(1 1 1)-(25 × 25)-pentacene, containing 75 molecules. The structure of this phase is determined from STM measurements at very low coverages where it is possible to image both the pentacene molecules and the structure of the Ag/Si(1 1 1) substrate. Two adsorption sites are identified, a three-fold hollow site at the centre of a Ag-trimer (CA-site) and a six-fold hollow site at the centre of the hexagonal arrangement of silver atoms (CB-site). A higher pentacene coverage of ∼1 ML lead to a molecular reorganization and forms a new commensurate structure Ag/Si(1 1 1)-(2 × 3)-pentacene, containing two molecules per unit cell. Because low energy electron diffraction patterns were not obtainable for this system, the structure of this second phase is determined by using the bias voltage as a tunable parameter to “focus” on either the molecular film or on the substrate. In this phase adsorption takes place exclusively on the Ag-trimer (CA) site and the CB-site is lost because of strong lateral molecule-molecule interactions. The role of competition between intermolecular and molecule-substrate interactions and the nature of the adsorption sites in determining the structure of the pentacene layers is discussed.     

Etching reaction of methylchloride molecule on the GaAs (0 0 1)-2 × 4 surface

Adsorption process of methylchloride (CH₃Cl) on the GaAs (0 0 1)-2 × 4 surface was studied by a scanning tunnelling microscopy (STM) measurement. The arsenic rich 2 × 4 surface, which was prepared by molecular beam epitaxy (MBE), was exposed to a supersonic molecular beam of CH₃Cl with a kinetic energy of 0.06 eV. New bright spots appeared on the CH₃Cl exposed surface. They were largely observed at the “B-type” step edge and divided into two types according to their locations. It was suggested that new spots were due to weakly adsorbed CH₃Cl molecules without any dissociation. The adsorption mechanism of CH₃Cl molecule was also studied by an ab initio Hartree-Fock calculation, which explained the experimental results well.     

Methanol adsorption on silicon (0 0 1)

Using a first-principles pseudopotential technique, we have investigated the adsorption of CH₃OH on the Si(0 0 1) surface. We have found that, in agreement with the overall experimental picture, the most probable chemisorption path for methanol adsorption on silicon (0 0 1) is as follows: the gas phase CH₃OH adsorbs molecularly to the electrophilic surface Si atom via the oxygen atom and then dissociates into Si-OCH₃ and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. Other possible adsorption models and dissociation paths are also discussed. Our calculations also suggest that the most probable methanol coverage is 0.5 ML, i.e., one molecule per Si-Si dimer, in agreement with experimental evidences. The surface atomic and electronic structures are discussed and compared to available theoretical and experimental data. In addition, we propose that a comparison of our theoretical STM images and calculated vibrational modes for the adsorbed systems with detailed experimental investigations could possibly confirm the presented adsorption picture.     

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).     

Simulations of iodine adsorbed Ge(0 0 1) surface and its STM images

A layer of iodine at Ge(0 0 1) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(0 0 1) surface covered by 0.25 monolayer of iodine. The p(2×4), p(2×2), c(2×4) and p(1×4) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I₂ molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data.     

Empty-state scanning tunneling microscopy image of C2H2 on Si(0 0 1)--new evidence for paired end-bridge di-σ configuration

New evidence of the paired end-bridge configuration in the room-temperature adsorption geometries of C₂H₂ molecules on Si(0 0 1) is presented by scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. The distinct four-leaved feature occupying two adjacent Si dimer sites in the experimental empty-state STM images are well reproduced by simulations of the paired end-bridge adsorption configuration. Calculated energetics suggests that the Si(0 0 1) surface is covered by paired end-bridge structures at the saturation coverage of 1 ML, in agreement with the existing experiments.     

Photoemission, NEXAFS and STM studies of pentacene thin films on Au(1 0 0)

We have investigated the initial stages of the growth of pentacene thin films on the Au(1 0 0) substrate using synchrotron radiation photoelectron spectroscopy (PES), near edge X-ray absorption fine structure (NEXAFS) and scanning tunnelling microscopy (STM). Results indicate a well-ordered structure with the pentacene molecules adopting a predominantly flat orientation with respect to the substrate for coverages of less than three monolayers. NEXAFS and photoemission data indicates the presence of a second molecular orientation for thicker films, with the introduction of a slight tilting away from planar bonding geometry at higher pentacene coverages. STM images of coverages less than three monolayers indicate a well-ordered pentacene structure allowing for the calculation of pentacene unit cell parameters. The pentacene molecular rows adopt a side-by-side bonding arrangement on the surface. For pentacene deposited at room temperature, step edges were observed to act as nucleation centres for film growth. Annealing of the substrate to 373 K was found to remove excess molecules and improve film quality, but did not otherwise change the bonding geometry of the pentacene with respect to the surface.     

STM study of the initial stages of C60 adsorption on the Pt(1 1 0)-(1 × 2) surface

We have studied the initial stages of adsorption of C₆₀ on the Pt (1 1 0)-(1 × 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5-10 molecules) C₆₀ aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates.     

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).     

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.     

Low reactivity of molecular oxygen with Si(1 1 1)-c(2 × 8)

The adsorption of molecular oxygen on the c(2 × 8) reconstruction of quenched Si(1 1 1) surfaces has been studied at the atomic scale using scanning tunneling microscopy (STM) at room temperature (RT). It has been found that clean well reconstructed c(2 × 8) adatoms do not react with O₂ molecules but that a limited oxidation can start where adatom sites arranged in reconstructed structures are present. Comparison between O₂ adsorption on Si(1 1 1)-c(2 × 8) and Si(1 1 1)-7 × 7 reconstructions coexisting on the same quenched silicon surface has been carried out in detail. For each atomic site present on the surface the variation of reacted sites with exposure has been measured. For low O₂ exposures, bright and dark oxygen induced sites appear on the Si(1 1 1)-7 × 7, while Si(1 1 1)-c(2 × 8) does not oxidized at all. At high O₂ exposures, large oxidation areas have spread on the 7 × 7 reconstruction, preferentially on the faulted halves of the unit cell, and smaller oxidation areas induced by topological defects have grown all around clean un-reacted c(2 × 8) regions.     

Morphology of pentacene films deposited on Cu(1 1 9) vicinal surface

We investigate the morphology of a pentacene (C₂₂H₁₄) film adsorbed on the Cu(1 1 9) vicinal surface by scanning tunnelling microscopy (STM). Thermal treatment of a thick film of molecules generates a long-range ordered structure. Series of molecular rows are alternated with areas where the molecules assume two equivalent orientations. STM data analysis suggests that the ordered structure can be described by a rippled morphology. The behaviour of the film at different annealing temperatures suggests a possible explanation of the film structure as due to an adsorbate-induced modification of the substrate.     

Dimerization of three xanthene dyes on Au(1 1 1) surface

The adsorption structures of three xanthene dyes (rhodamine B (Rh B), fluorescein and eosin) on Au(1 1 1) in HClO₄ solution, have been investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry. High-resolution STM images reveal the molecular orientation and packing arrangement in the ordered adlayers. A (5 × 10) structure is found on Rh B adlayer. (5 × 8) structures are observed on fluorescein and eosin adlayers, respectively. An intriguing aspect of this work is that three xanthene molecules form dimeric structures on Au(1 1 1) surface. The electrostatic interaction and van der Waals force are responsible to the dimeric formation of Rh B, while the interaction between Br atoms and hydrogen bond correspond to the dimerization of eosin and fluorescein, respectively. The structural models are tentatively proposed for the three ordered adlayers. The results obtained will be helpful to understand the interaction mechanism of dimerization and the degradation mechanism of dye pollutant.     

Comparison of O adsorption on Ni3Al (0 0 1), (0 1 1), and (1 1 1) surfaces through first-principles calculations

First-principles calculations were performed to study the properties of O adsorption on Ni₃Al (0 0 1), (0 1 1), and (1 1 1) surfaces using the Cambridge serial total package (CASTEP) code. Stable adsorption sites are identified. The atomic and electronic structures and adsorption energies are predicted. The adsorption sites for O on the Ni₃Al (0 0 1) surface are at the 2Ni–2Al fourfold hollow site, whereas O prefers to adsorb at the Ni–Al bridge site on (0 1 1) surface and 2Ni–Al threefold hollow site on (1 1 1) surface. It is found that O shows the strongest affinity for Al and the state of O is the most stabilized when O adsorbs on (0 0 1) surface, while the affinity of O for Al on (0 1 1) surface is weaker than (0 0 1) surface, and (1 1 1) surface is the weakest. The stronger O and Al affinity indicates more stable Al₂O₃ when oxidized. The experiment has shown that the oxidation resistance of single crystal superalloy in different orientations improves in the order of (1 1 1), (0 1 1), and (0 0 1) surface, suggesting that the oxidation in different crystallographic orientations may be related to the affinity of O for Al in the surface.     

Pd adsorption on Si(1 1 3) surface: STM and XPS study

Pd-induced surface structures on Si(1 1 3) have been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In the initial process of the Pd adsorption below 0.10 ML, Pd silicide (Pd₂Si) clusters are observed to form randomly on the surface. By increasing the Pd coverage to 0.10 ML, the clusters cover the entire surface, and an amorphous layer is formed. After annealing the Si(1 1 3)-Pd surface at 600 °C, various types of islands and chain protrusions appears. The agglomeration, coalescence and crystallization of these islands are observed by using high temperature (HT-) STM. It is also found by XPS that the islands correspond to Pd₂Si structure. On the basis of these results, evolution of Pd-induced structures at high temperatures is in detail discussed.     

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.     

Adsorption states of NO on the Pt(1 1 1) step surface

Using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), we investigated the adsorption states of NO on the Pt(9 9 7) step surface. At 90 K, we observe three N-O stretching modes at 1490 cm⁻¹, 1631 cm⁻¹ and 1700 cm⁻¹ at 0.2 ML. The 1490 cm⁻¹ and 1700 cm⁻¹ peaks are assigned to NO molecules at fcc-hollow and on-top sites of the terrace, respectively. The 1631 cm⁻¹ peak is assigned to the step NO species. In the present STM results, we observed that NO molecules were adsorbed at the bridge sites of the step as well as fcc-hollow and on-top sites of the terrace. To help with our assignments, density functional theory calculations were also performed. The calculated results indicate that a bridge site of the step is the most stable adsorption site for NO, and its stretching frequency is 1607 cm⁻¹. The interactions between NO species at different sites on Pt(9 9 7) are also discussed.