7,512 research outputs found
Role of Rotations on Surface Diffusion of Water Trimers on Pd\{111\}
Diffusion barriers for a cluster of three water molecules on Pd(111) have
been estimated from ab-initio Density Functional Theory. A model for the
diffusion of the trimer based in rotations yields a simple explanation of why
the cluster can diffuse faster than a single water molecule by a factor
. This model is based on the differences between the adsorption
geometry for the three monomers forming the cluster. One member interacts
strongly with the surface and sits closer to the surface (d) while the other
two interact weakly and stay at a larger separation from the surface (u). The
trimer rotates nearly freely around the axis determined by the d monomer.
Translations of the whole trimer imply breaking the strong interaction of the d
monomer with the surface. Alternatively, thermal fluctuations exchange the
actual monomer sitting closer to the surface with a lower energetic cost.
Rotations around different axis introduce a diffusion mechanism where a strong
interaction is kept along the diffusion path between the water molecule
defining the axis of rotation and the Pd underneath.Comment: water ; monomer ; trimer ; water clusters ; diffusion ; rotation
assisted ; Pd\{111\} ; ab-initio ; density functional theor
Crystal structure and electronic states of tripotassium picene
The crystal structure of potassium doped picene with an exact stoichiometry
(K3C22H14, K3picene from here onwards) 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 structureof 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/pi 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.Comment: Published version: 8 twocolumn pages, 7 color figures, 2 structural
.cif files include
Trapping of electrons near chemisorbed hydrogen on graphene
Chemical adsorption of atomic hydrogen on a negatively charged single layer
graphene sheet has been analyzed with ab-initio Density Functional Theory
calculations. We have simulated both finite clusters and infinite periodic
systems to investigate the effect of different ingredients of the theory, e.g.
exchange and correlation potentials, basis sets, etc. Hydrogen's electron
affinity dominates the energetic balance in the charged systems and the extra
electron is predominantly attracted to a region nearby the chemisorbed atom.
The main consequences are: (i) the cancellation of the unpaired spin resulting
in a singlet ground-state, and (ii) a stronger interaction between hydrogen and
the graphene sheet.Comment: 11 pages, 8 figures, to be published in PR
Ab-initio calculation of the effect of stress on the chemical activity of graphene
Graphene layers are stable, hard, and relatively inert. We study how tensile
stress affects and bonds and the resulting change in the
chemical activity. Stress affects more strongly bonds that can become
chemically active and bind to adsorbed species more strongly. Upon stretch,
single C bonds are activated in a geometry mixing and ; an
intermediate state between and bonding. We use ab-initio
density functional theory to study the adsorption of hydrogen on large clusters
and 2D periodic models for graphene. The influence of the exchange-correlation
functional on the adsorption energy is discussed
Water dimer diffusion on Pd{111} assisted by an H-bond donor-acceptor tunneling exchange
Based on the results of density functional theory calculations, a novel mechanism for the diffusion of water dimers on metal surfaces is proposed, which relies on the ability of H bonds to rearrange through quantum tunneling. The mechanism involves quasifree rotation of the dimer and exchange of H-bond donor and acceptor molecules. At appropriate temperatures, water dimers diffuse more rapidly than water monomers, thus providing a physical explanation for the experimentally measured high diffusivity of water dimers on Pd{111} [Mitsui et al., Science 297, 1850 (2002)]
One-dimensional potential for image-potential states on graphene
In the framework of dielectric theory the static non-local self-energy of an
electron near an ultra-thin polarizable layer has been calculated and applied
to study binding energies of image-states near free-standing graphene. The
corresponding series of eigenvalues and eigenfunctions have been obtained by
solving numerically the one-dimensional Schr{\"o}dinger equation.
Image-potential-state wave functions accumulate most of their probability
outside the slab. We find that a Random Phase Approximation (RPA) for the
non-local dielectric function yields a superior description for the potential
inside the slab, but a simple Fermi-Thomas theory can be used to get a
reasonable quasi-analytical approximation to the full RPA result that can be
computed very economically. Binding energies of the image-potential states
follow a pattern close to the Rydberg series for a perfect metal with the
addition of intermediate states due to the added symmetry of the potential. The
formalism only requires a minimal set of free parameters; the slab width and
the electronic density. The theoretical calculations are compared to
experimental results for work function and image-potential states obtained by
two-photon photoemission.Comment: 24 pages; 10 figures. arXiv admin note: text overlap with
arXiv:1301.448
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