4,651 research outputs found
Selective Adsorption of a Supramolecular Structure on Flat and Stepped Gold Surfaces
Halogenated aromatic molecules assemble on surfaces forming both hydrogen and
halogen bonds. Even though these systems have been intensively studied on flat
metal surfaces, high-index vicinal surfaces remain challenging, as they may
induce complex adsorbate structures. The adsorption of 2,6-dibromoanthraquinone
(2,6-DBAQ) on flat and stepped gold surfaces is studied by means of van der
Waals corrected density functional theory. Equilibrium geometries and
corresponding adsorption energies are systematically investigated for various
different adsorption configurations.~It is shown that bridge sites and step
edges are the preferred adsorption sites for single molecules on flat and
stepped surfaces, respectively. The role of van der Waals interactions, halogen
bonds and hydrogen bonds are explored for a monolayer coverage of 2,6-DBAQ
molecules, revealing that molecular flexibility and intermolecular interactions
stabilize two-dimensional networks on both flat and stepped surfaces. Our
results provide a rationale for experimental observation of molecular carpeting
on high-index vicinal surfaces of transition metals.Comment: Preprint. 24 pages, 5 figure
Atomistic simulations of heat transport in real-scale silicon nanowire devices
Utilizing atomistic lattice dynamics and scattering theory, we study thermal
transport in nanodevices made of 10 nm thick silicon nanowires, from 10 to 100
nm long, sandwiched between two bulk reservoirs. We find that thermal transport
in devices differs significantly from that of suspended extended nanowires, due
to phonon scattering at the contact interfaces. We show that thermal
conductance and the phonon transport regime can be tuned from ballistic to
diffusive by varying the surface roughness of the nanowires and their length.
In devices containing short crystalline wires phonon tunneling occurs and
enhances the conductance beyond that of single contacts.Comment: 5 pages, 5 figure
Atomistic calculation of the thermal conductance of large scale bulk-nanowire junctions
We have developed an efficient scalable kernel method for thermal transport
in open systems, with which we have computed the thermal conductance of a
junction between bulk silicon and silicon nanowires with diameter up to 10 nm.
We have devised scaling laws for transmission and reflection spectra, which
allow us to predict the thermal resistance of bulk-nanowire interfaces with
larger cross sections than those achievable with atomistic simulations. Our
results indicate the characteristic size beyond which atomistic systems can be
treated accurately by mesoscopic theories.Comment: 6 pages, 4 figure
Structure and Dynamics of the Quasi-Liquid Layer at the Surface of Ice from Molecular Simulations
We characterized the structural and dynamical properties of the quasi-liquid
layer (QLL) at the surface of ice by molecular dynamics simulations with a
thermodynamically consistent water model. Our simulations show that for three
low-index ice surfaces only the outermost molecular layer presents short-range
and mid-range disorder and is diffusive. The onset temperature for normal
diffusion is much higher than the glass temperature of supercooled water,
although the diffusivity of the QLL is higher than that of bulk water at the
corresponding temperature. The underlying subsurface layers impose an ordered
template, which produces a regular patterning of the ice/water interface at any
temperature, and is responsible for the major differences between QLL and bulk
water, especially for what concern the dynamics and the mid-range structure of
the hydrogen-bonded network. Our work highlights the need of a holistic
approach to the characterization of QLL, as a single experimental technique may
probe only one specific feature, missing part of the complexity of this
fascinating system.Comment: 6 Figure
Divergence of the Thermal Conductivity in Uniaxially Strained Graphene
We investigate the effect of strain and isotopic disorder on thermal
transport in suspended graphene by equilibrium molecular dynamics simulations.
We show that the thermal conductivity of unstrained graphene, calculated from
the fluctuations of the heat current at equilibrium is finite and converges
with size at finite temperature. In contrast, the thermal conductivity of
strained graphene diverges logarithmically with the size of the models, when
strain exceeds a relatively large threshold value of 2%. An analysis of phonon
populations and lifetimes explains the divergence of the thermal conductivity
as a consequence of changes in the occupation of low-frequency out-of-plane
phonons and an increase in their lifetimes due to strain.Comment: 6 pages, 7 figures. Accepted for publication in Physical Review
Autocatalytic and cooperatively-stabilized dissociation of water on a stepped platinum surface
Water-metal interfaces are ubiquitous and play a key role in many chemical
processes, from catalysis to corrosion. Whereas water adlayers on atomically
flat transition metal surfaces have been investigated in depth, little is known
about the chemistry of water on stepped surfaces, commonly occurring in
realistic situations. Using first-principles simulations we study the
adsorption of water on a stepped platinum surface. We find that water adsorbs
preferentially at the step edge, forming linear clusters or chains, stabilized
by the cooperative effect of chemical bonds with the substrate and hydrogen
bonds. In contrast with flat Pt, at steps water molecules dissociate forming
mixed hydroxyl/water structures, through an autocatalytic mechanism promoted by
hydrogen bonding. Nuclear quantum effects contribute to stabilize partially
dissociated cluster and chains. Together with the recently demonstrated
attitude of water chains adsorbed on stepped Pt surfaces to transfer protons
via thermally activated hopping, these findings candidate these systems as
viable proton wires.Comment: 19 pages, 4 figure
Evolution of the structure of amorphous ice - from low-density amorphous (LDA) through high-density amorphous (HDA) to very high-density amorphous (VHDA) ice
We report results of molecular dynamics simulations of amorphous ice for
pressures up to 22.5 kbar. The high-density amorphous ice (HDA) as prepared by
pressure-induced amorphization of Ih ice at T=80 K is annealed to T=170 K at
various pressures to allow for relaxation. Upon increase of pressure, relaxed
amorphous ice undergoes a pronounced change of structure, ranging from the
low-density amorphous ice (LDA) at p=0, through a continuum of HDA states to
the limiting very high-density amorphous ice (VHDA) regime above 10 kbar. The
main part of the overall structural change takes place within the HDA
megabasin, which includes a variety of structures with quite different local
and medium-range order as well as network topology and spans a broad range of
densities. The VHDA represents the limit to densification by adapting the
hydrogen-bonded network topology, without creating interpenetrating networks.
The connection between structure and metastability of various forms upon
decompression and heating is studied and discussed. We also discuss the analogy
with amorphous and crystalline silica. Finally, some conclusions concerning the
relation between amorphous ice and supercooled water are drawn.Comment: 11 pages, 12 postscript figures. To be published in The Journal of
Chemical Physic
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