P. L. de Andres

Strong covalent bonding between two graphene layers

Financed by CICYT under contracts MAT-2005-3866, MAT-2006-03741, FIS-2006-12117-C04-03,NAN-2004-09183-C10-08. We acknowledge the use of the Spanish Supercomputing Network and the CTI (CSIC).

Hydrogen on Graphene Under Stress: Molecular Dissociation and Gap Opening

Density-functional calculations are employed to study the molecular dissociation of hydrogen on graphene, the diffusion of chemisorbed atomic species, and the electronic properties of the resulting hydrogen on graphene system. Our results show that applying stress to the graphene substrate can lower the barrier to dissociation of molecular hydrogen by a factor of 6 and change the process from endothermic to exothermic. These values for the barrier and the heat of reaction, unlike the zero stress values, are compatible with the time scales observed in experiments. Diffusion, on the other hand, is not greatly modified by stress. We analyze the electronic structure for configurations relevant to molecular dissociation and adsorption of atomic hydrogen on a graphene single layer. An absolute band gap of 0.5 eV is found for the equilibrium optimum configuration for a narrow range of coverages

Epitaxial growth of cobalt films on Cu(100): a crystallographic LEED determination

(\ensuremath{\theta}\ensuremath{\approx}0.25)

First-principles calculation of the effect of stress on the chemical activity of graphene

. This value is in good agreement with experiment [D. Elias et al., Science 323, 610 (2009)].

Ab initio molecular dynamics simulation of hydrogen diffusion in alpha -iron

Epitaxial thin films of cobalt ranging from 1 ML up to 10 ML have been grown on a Cu(100) substrate and characterized by LEED. The cobalt is found to grow, adopting a slight tetragonal distortion of its high-temperature FCC phase. Further growth of 5 ML Cu on top of a 5 ML Co film results in an almost perfect Cu(100) surface.

Crystal structure and electronic states of tripotassium picene

Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects σ and π bonds and the resulting change in the chemical activity. Stress affects more strongly π bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing 120° and 90°, an intermediate state between sp2 and sp3 bonding. We use ab initio density functional theory to study the adsorption of hydrogen on large clusters and two-dimensional periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed.

Ab initioelectronic and geometrical structures of tripotassium-intercalated phenanthrene

First-principles atomistic molecular-dynamics simulation in the microcanonical and canonical ensembles has been used to study the diffusion of interstitial hydrogen in α-iron. Hydrogen to iron ratios between θ=1/16 and 1/2 have been considered by locating interstitial hydrogen atoms at random positions in a 2×2×2 supercell. We find that the average optimum absorption site and the barrier for diffusion depend on the concentration of interstitials. Iron Debye temperature decreases monotonically for increasing concentration of interstitial hydrogen, proving that iron-iron interatomic potential is significantly weakened in the presence of a large number of diffusing hydrogen atoms

Understanding atomic-resolved STM images on TiO2(110)-(1 × 1) surface by DFT calculations

The crystal structure of potassium doped picene with an exact stoichiometry (K3picene) has been theoretically determined within Density Functional Theory allowing complete variational freedom of the crystal structure parameters and the molecular atomic positions. A modified herringbone lattice is obtained in which potassium atoms are intercalated between two paired picene molecules displaying the two possible orientations in the crystal. Along the c-axis, organic molecules alternate with chains formed by three potassium atoms. The electronic structure of the doped material resembles pristine picene, except that now the bottom of the conduction band is occupied by six electrons coming from the ionized K atoms (six per unit cell). Wavefunctions remain based mainly on picene molecular orbitals getting their dispersion from intralayer edge to face CH/ bonding, while eigenenergies have been modified by the change in the electrostatic potential. The small dispersion along the c axis is assigned to small H-H overlap. From the calculated electronic density of states we expect metallic behavior for potassium doped picene.

Theory of ballistic electron emission microscopy

The geometrical and electronic structure of tripotassium doped phenanthrene, \ce{K3C14H10}, have been studied by first-principles density functional theory. The main effect of potassium doping is to inject charge in the narrow phenanthrene conduction band, rendering the system metallic. The Fermi surface for the experimental X-rays unit cell is composed of two sheets with marked one and two dimensional character respectively.

Crystallography of epitaxial face centered tetragonal Co/Cu(100) by low energy electron diffraction

We present a combination of experimental STM images and DFT calculations to understand the atomic scale contrast of features found in high-resolution STM images. Simulating different plausible structural models for the tip, we have been able to reproduce various characteristics previously reported in experimental images on TiO(2)(110)-(1 x 1) under controlled UHV conditions. Our results allow us to determine the influence of different chemical and morphological tip terminations on the atomic-resolution STM images of the TiO(2)(110)-(1 x 1) surface. The commonest images have been properly explained using standard models for a W tip, either clean or with a single O atom located at the apex. Furthermore, a double transfer of oxygen atoms can account for different types of bizarre atomic-resolution features occasionally seen, and not conclusively interpreted before. Importantly, we discuss how typical point-defects are imaged on this surface by different tips, namely bridging O vacancies and adsorbed OH groups.