190 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
Native surface oxide turns alloyed silicon membranes into nanophononic metamaterials with ultra-low thermal conductivity
A detailed understanding of the relation between microscopic structure and
phonon propagation at the nan oscale is essential to design materials with
desired phononic and thermal properties.Here we uncover a new mechanism of
phonon interaction in surface oxidized membranes, i.e., native oxide layers
interact with phonons in ultra-thin silicon membranes through local resonances.
The local resonances reduce the low frequency phonon group velocities and
shorten their mean free path. This effect opens up a new strategy for ultralow
thermal conductivity design as it complements the scattering mechanism which
scatters higher frequency modes effectively. The combination of native oxide
layer and alloying with germanium in concentration as small as 5% reduces the
thermal conductivity of silicon membranes to 100 time lower than the bulk. In
addition, the resonance mechanism produced by native oxide surface layers is
particularly effective for thermal condutivity reduction even at very low
temperatures, at which only low frequency modes are populated.Comment: 6 pages, 5 figures, Accepted for publication in Physical Review
Anisotropic In-Plane Phonon Transport in Ultrathin Silicon Membranes Guided by Nano-Surface-Resonators
Anisotropic phonon transport along different lattice directions of
two-dimensional (2D) materials has been observed, however, the effect decreases
with increasing the thickness beyond a few atomic layers. Here we establish a
novel mechanism to induce anisotropic phonon transport in quasi-2D materials
with isotropic symmetry. The phonon propagation is guided by resonance
hybridization with surface nanostructures. We demonstrate that the thermal
conductivity of 3 nm-thick silicon membrane with surface nanofins is greater by
parallel to the fins than that perpendicular to the fins.Comment: 7 pages, 5 figure
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