NO adsorption on Cu(110) and O(2 × 1)/Cu(110) surfaces from density functional theory calculations.

In a recent study [M. Feng, et al., ACS Nano, 2011, 5, 8877], it was shown that CO molecules adsorbed on the quasi-one-dimensional O(2 × 1)/Cu(110) surface reconstruction tend to form highly-ordered single-molecule-wide rows along the direction perpendicular to the Cu-O chains. This stems from the peculiar tilted adsorption configuration of CO on this substrate, which gives rise to short-range attractive dipole-dipole interactions. Motivated by this observation, here we study the adsorption of nitric oxide (NO) on O(2 × 1)/Cu(110) and Cu(110) using density functional theory, with the aim of elucidating whether a similar behaviour can be expected for this molecule. We first study NO adsorption on a clean Cu(110) surface, where the role of short-range attractions between molecules has already been pointed out by the observation of the formation of NO dimers by scanning tunnelling microscopy [A. Shiotari, et al., Phys. Rev. Lett., 2011, 106, 156104]. On the clean Cu(110), the formation of dimers along the [110̄] direction is favourable, in agreement with published experimental results. However, the formation of extended NO rows is found to be unstable. Regarding the O(2 × 1)/Cu(110) substrate, we observe that NO molecules adsorb in between the Cu-O chains, causing a substantial disruption of the surface structure. Although individual molecules can be tilted with negligible energetic cost along the direction of the Cu-O chains, the interaction among neighbouring molecules was found to be repulsive along all directions and, consequently, the formation of dimers unfavourable.

X-ray photoemission analysis of clean and carbon monoxide-chemisorbed platinum(111) stepped surfaces using a curved crystal.

Surface chemistry and catalysis studies could significantly gain from the systematic variation of surface active sites, tested under the very same conditions. Curved crystals are excellent platforms to perform such systematics, which may in turn allow to better resolve fundamental properties and reveal new phenomena. This is demonstrated here for the carbon monoxide/platinum system. We curve a platinum crystal around the high-symmetry (111) direction and carry out photoemission scans on top. This renders the spatial core-level imaging of carbon monoxide adsorbed on a 'tunable' vicinal surface, allowing a straightforward visualization of the rich chemisorption phenomenology at steps and terraces. Through such photoemission images we probe a characteristic elastic strain variation at stepped surfaces, and unveil subtle stress-release effects on clean and covered vicinal surfaces. These results offer the prospect of applying the curved surface approach to rationally investigate the chemical activity of surfaces under real pressure conditions.

Consecutive Mechanism in the Diffusion of D2O on a NaCl(100) Bilayer.

The motion of D2O monomers is investigated on a NaCl(100) bilayer on Ag(111) between 42.3 and 52.3 K by scanning tunneling microscopy. The diffusion distance histogram reveals a squared diffusion lattice that agrees with the primitive unit cell of the (100) surface. From the Arrhenius dependence, we derive the diffusion energy, the pre-exponential factor, and the attempt frequency. The mechanism of the motion is identified by comparison of the experimental results to theoretical calculations. Via low temperature adsorption site determination in connection with density functional theory, we reveal an influence of the metallic support onto the intermediate state of the diffusive motion.

Hybridization between Cu-O chain and Cu(110) surface states in the O(2×1)/Cu(110) surface from first principles.

The O(2×1)/Cu(110) surface reconstruction of the Cu(110) surface is induced by 0.5 ML of oxygen adsorption and is formed by Cu-O chains running along the [001] direction. Here, we show that hybridization between surface states of the Cu(110) substrate and one-dimensional states of the Cu-O chains is crucial in understanding the electronic structure of this surface. Specifically, the interaction between one occupied antibonding band of the Cu-O chain with O(p(y)) character (y-axis taken along the Cu-O chain direction) and the partially occupied surface state at the Y point of the clean Cu(110) surface with Cu(p(y)) character causes major changes in the electronic structure close to the Fermi energy (E(F)). This surface state decays very slowly into the bulk and a thick slab is needed to properly describe it, which might explain why the importance of this hybridization has not been recognized so far. In our calculations we obtain two hybrid bands: (i) a fully occupied band that strongly hybridizes with the bulk Cu sp states nearby E(F), becoming a very broad resonance, thus explaining why it is not observed in photoemission experiments; (ii) an empty band that acquires surface state character, including its dispersion close to the zone boundary at the Y point. This splitting induces a partial population of the p(y) antibonding band that is necessary to reconcile the calculated charge transfer from the Cu(110) substrate to the Cu-O chain (~0.5 electrons/f.u.) with the apparently fully occupied band structure of the adsorbed Cu-O chain (consistent with 1 electron transferred per formula unit).

SAM-like arrangement of thiolated graphene nanoribbons: decoupling the edge state from the metal substrate.

Density functional theory calculations have been used to analyze the electronic and magnetic properties of ultrathin zigzag graphene nanoribbons (ZGNRs) with different edge saturations. We have compared a symmetric hydrogen saturation of both edges with an asymmetric saturation in which one of the edges is saturated with sulphur atoms or thiol groups, while the other one is kept hydrogen saturated. The adsorption of such partially thiolated ZGNRs on Au(111) has also been explored. We have considered vertical and tilted adsorption configurations of the ribbons, reminiscent of those found for thiolated organic molecules in self-assembled monolayers (SAM) on gold substrates. We have found that saturation with sulphur atoms or thiol groups removes the corresponding edge state from the Fermi energy and kills the accompanying spin polarization. However, this effect is so local that the electronic and magnetic properties of the mono-hydrogenated edge (H-edge) remain unaffected. Thus, the system develops a spin moment mainly localized at the H-edge. This property is not modified when the partially thiolated ribbon is attached to the gold substrate, and is quite independent of the width of the ribbon. Therefore, the upright adsorption of partially thiolated ZGNRs can be an effective way to decouple the spin-polarized channel provided by the H-edge from an underlying metal substrate. These observations might open a novel route to build spin-filter devices using ZGNRs on gold substrates.

Orthogonal interactions of CO molecules on a one-dimensional substrate.

We investigate the chemisorption structure of CO molecules on the quasi-one-dimensional Cu(110)-(2 × 1)-O surface by low-temperature scanning tunneling microscopy and density functional theory. Contrary to flat metal surfaces, where CO molecules adsorb in an upright geometry and interact through repulsive intermolecular interactions, we find the most stable adsorption structure of single CO molecules to be at Cu atoms of substrate Cu-O- chains with the Cu-CO unit bent by ~±45° in two equivalent structures at low coverages. At higher coverages, CO molecules combine in the same structure into highly ordered single-molecule-wide rows perpendicular to the substrate chains in an approximately 8 × 1 full monolayer structure. First-principles calculations attribute the unprecedented chemisorption behavior of CO molecules to lifting of the host Cu atoms by 1 Å from the surface Cu-O- chains, in order to optimize the bonding and reduce the repulsive interactions with the substrate. This structural distortion enables short-range intermolecular dipole-dipole attraction and creates orthogonal long-range surface-mediated repulsion leading to unusual self-assembly of CO molecules into coherent nanometer scale molecular grating structures.

First-principles investigation of electron-induced cross-linking of aromatic self-assembled monolayers on Au(111).

We have performed a density functional theory study of the possible layered geometries occurring after dehydrogenation of a self-assembled monolayer (SAM) of biphenyl-thiol molecules (BPTs) adsorbed on Au(111), as it has been experimentally observed for low energy electron irradiated SAMs of 4'-nitro-1,1'-biphenyl-thiol adsorbed on a Au(111) surface. [Eck et al., Advanced Materials 2000, 12, 805] Cross-link formation between the BPT molecules has been analyzed using different models with different degrees of complexity. We start by analyzing the bonding between biphenyl (BP) molecules in a lineal dimer and their characteristic vibration frequencies. Next, we consider the most stable cross-linked structures formed in an extended free-standing monolayer of fully dehydrogenated BP molecules. Finally, we analyze a more realistic model where the role of the Au(111) substrate and sulfur head groups is explicitly taken into account. In this more complex model, the dehydrogenated BPT molecules are found to interact covalently to spontaneously form "graphene-like" nanoflakes. We propose that these nanographenes provide plausible building-blocks for the structure of the carbon layers formed by electron irradiation of BPT-SAMs. In particular, it is quite tempting to visualize those structures as the result of the cross-link and entanglement of such graphene nanoflakes.

Adsorption and reactivity of CO(2) on defective graphene sheets.

Density-functional calculations have been performed to investigate the adsorption of CO(2) on defected graphite (0001) represented by a single graphene sheet. The interaction with a vacancy defect gives a computed molecular binding energy of approximately 136 meV in a strong physisorbed state. Subsequently, chemisorption by lactone group formation will occur after overcoming a barrier of approximately 1 eV relative to the gas phase, with an exothermicity of about 1.4 eV. Further reaction paths from this chemisorbed state lead to dissociation of the CO(2) through the formation of epoxy groups followed by oxygen recombination and desorption of O(2), after overcoming successive energy barriers of approximately 0.9 and approximately 1.0 eV. The global minimum ("O(2) desorbed + graphene sheet") entails an energy release of about 3.4 eV with respect to the initial state.

On the structure of the first hydration layer on NaCl(100): role of hydrogen bonding.

The authors have investigated the structure and energetics of the first hydration layer on NaCl(100) by means of density functional calculations. They have analyzed in detail the role of the hydrogen bond between the adsorbed molecules for the determination of the most favorable structures. They have shown that, using the water dimers as basic building blocks, very stable structures can be constructed. They discuss here two important examples: (i) a model with (1x1) periodicity at 2 ML coverage, and (ii) icelike bilayers with a c(4x2) unit cell at 1.5 ML. Both structures present high adsorption energies per water molecule of approximately 570 meV, in comparison to the 350 meV adsorption energy obtained for the previously studied (1x1) structures composed of weakly interacting monomers. Based on these findings, they propose an interpretation for the experimental observations of Toennies et al. [J. Chem. Phys. 120, 11347 (2004)], who found a transition of the periodicity of the first hydration layer on NaCl(100) from (1x1) to c(4x2) upon electron irradiation. According to the model, the transition would be driven by the partial desorption of (1x1) bilayer structures corresponding to a local coverage of 2 ML and the further rearrangement of the remaining water molecules to form a quasihexagonal structure with c(4x2) periodicity at coverage close to 1.5 ML.

Water adsorption and diffusion on NaCl(100).

At low coverage and temperature the water-surface interaction determines the adsorption geometry of the water molecule on the NaCl(100) surface. However, at room temperature the molecules are also able to move on the surface and form islands where the water molecules are held together by hydrogen bonds. As a step toward the description of such complex phenomenology, in this work we have used density functional theory calculations to study the most favorable adsorption geometry of an isolated water molecule and the energy barriers associated with different hopping mechanisms between equivalent adsorption configurations on this surface. We propose different hopping processes that can be classified as translations, if the molecule moves from one adsorption site to the adjacent one, or reorientations, if the molecule only changes its orientation on the surface and remains in the same adsorption site. The straightforward parallel translation of the water molecule along the surface exhibits the highest barrier. All other processes, either translations or reorientations, involve the rotation of the water molecule around certain axes and present much smaller barriers (at least 50% smaller). To obtain a net movement of the molecule along the surface it is always necessary to combine one of these translational and reorientational processes. Such combinations provide favorable and plausible pathways for the diffusion of the water molecule on the NaCl(100) substrate.