471 research outputs found
Titanium-decorated carbon nanotubes: a potential high-capacity hydrogen storage medium
We report a first-principles study, which demonstrates that a single Ti atom
coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules.
The first H adsorption is dissociative with no energy barrier while other
three adsorptions are molecular with significantly elongated H-H bonds. At high
Ti coverage we show that a SWNT can strongly adsorb up to 8-wt% hydrogen. The
system is quite stable and exhibits associative desorption upon heating, a
requirement for reversible storage. These results advance our fundamental
understanding of dissociative adsorption of hydrogen in nanostructures and
suggest new routes to better storage and catalyst materials.Comment: 4.2 pages, 3 figures, submitted PRL on Nov. 2004, accepted in March
200
Effects of static charging and exfoliation of layered crystals
Using first-principle plane wave method we investigate the effects of static
charging on structural, elastic, electronic and magnetic properties of
suspended, single layer graphene, graphane, fluorographene, BN and MoS2 in
honeycomb structures. The limitations of periodic boundary conditions in the
treatment of charged layers are clarified. Upon positive charging the band gaps
between the conduction and valence bands increase, but the single layer
materials become metallic owing to the Fermi level dipping below the maximum of
valence band. Moreover, their bond lengths increase and their in-plane
stiffness decreases. As a result, phonons are softened and frequencies of Raman
active modes are lowered. High level of charging leads to instability. We
showed that wide band gap BN and MoS2 slabs are metallized as a result of
electron removal and their outermost layers are exfoliated once the charging
exceeds a threshold value.Comment: http://link.aps.org/doi/10.1103/PhysRevB.85.04512
Domain formation on oxidized graphene
Using first-principles calculations within density functional theory we
demonstrate that the adsorption of single oxygen atom results in significant
electron transfer from graphene to oxygen. This strongly disturbs the charge
landscape of the C-C bonds at the proximity. Additional oxygen atoms adsorbing
to graphene prefer always the C-C bonds having highest charge density and
consequently they have tendency to form domain structure. While oxygen
adsorption to one side of graphene ends with significant buckling, the
adsorption to both sides with similar domain pattern is favored. The binding
energy displays an oscillatory variation and the band gap widens with
increasing oxygen coverage. While a single oxygen atom migrates over the C-C
bonds on graphene surface, a repulsive interaction prevents two oxygen adatoms
from forming an oxygen molecule. Our first-principles study together with
finite temperature ab-initio molecular dynamics calculations concludes that
oxygen adatoms on graphene cannot desorb easily without influence of external
agents.Comment: under revie
Armchair nanoribbons of silicon and germanium honeycomb structures
We present a first-principles study of bare and hydrogen passivated armchair
nanoribbons of the puckered single layer honeycomb structures of silicon and
germanium. Our study includes optimization of atomic structure, stability
analysis based on the calculation of phonon dispersions, electronic structure
and the variation of band gap with the width of the ribbon. The band gaps of
silicon and germanium nanoribbons exhibit family behavior similar to those of
graphene nanoribbons. The edges of bare nanoribbons are sharply reconstructed,
which can be eliminated by the hydrogen termination of dangling bonds at the
edges. Periodic modulation of the nanoribbon width results in a superlattice
structure which can act as a multiple quantum wells. Specific electronic states
are confined in these wells. Confinement trends are qualitatively explained by
including the effects of the interface. In order to investigate wide and long
superlattice structures we also performed empirical tight binding calculations
with parameters determined from \textit{ab initio} calculations.Comment: please find the published version in
http://link.aps.org/doi/10.1103/PhysRevB.81.19512
Self-assembly mechanisms of short atomic chains on single layer graphene and boron nitride
Nucleation and growth mechanisms of short chains of carbon atoms on
single-layer, hexagonal boron nitride (h-BN), and short BN chains on graphene
are investigated using first-principles plane wave calculations. Our analysis
starts with the adsorption of a single carbon ad-atom and examines its
migrations. Once a C nucleates on h-BN, the insertion of each additional
carbon at its close proximity causes a short segment of carbon atomic chain to
grow by one atom at at a time in a quaint way: The existing chain leaves its
initial position and subsequently is attached from its bottom end to the top of
the carbon ad-atom. The electronic, magnetic and structural properties of these
chains vertically adsorbed to h-BN depend on the number of carbon atoms in the
chain, such that they exhibit an even-odd disparity. An individual carbon chain
can also modify the electronic structure with localized states in the wide band
gap of h-BN. As a reverse situation we examined the growth of short BN atomic
chains on graphene, which attribute diverse properties depending on whether B
or N is the atom bound to the substrate. These results together with ab-initio
molecular dynamics simulations of the growth process reveal the interesting
self-assembly behavior of the grown chains. Furthermore, we find that these
atomic chains enhance the chemical activity of h-BN and graphene sheets by
creating active sites for the bonding of various ad-atoms and can act as
pillars between two and multiple sheets of these honeycomb structures leaving
wider spacing between them to achieve high capacity storage of specific
molecules.Comment: Accepted for Physical Review
Nanoscale Dielectric Capacitors Composed of Graphene and Boron Nitride Layers: A First Principles Study of High-Capacitance at Nanoscale
We investigate a nanoscale dielectric capacitor model consisting of
two-dimensional, hexagonal h-BN layers placed between two commensurate and
metallic graphene layers using self-consistent field density functional theory.
The separation of equal amounts of electric charge of different sign in
different graphene layers is achieved by applying electric field perpendicular
to the layers. The stored charge, energy, and the electric potential difference
generated between the metallic layers are calculated from the first-principles
for the relaxed structures. Predicted high-capacitance values exhibit the
characteristics of supercapacitors. The capacitive behavior of the present
nanoscale model is compared with that of the classical Helmholtz model, which
reveals crucial quantum size effects at small separations, which in turn recede
as the separation between metallic planes increases.Comment: Published version in The Journal of Physical Chemistry:
http://pubs.acs.org/doi/abs/10.1021/jp403706
Size dependence in the stabilities and electronic properties of \alpha -graphyne and its BN analogue
We predict the stabilities of \alpha-graphynes and their boron nitride
analogues(\alpha-BNyne), which are considered as competitors of graphene and
two-dimensional hexagonal BN. Based on first-principles plane wave method, we
investigated the stability and structural transformations of these materials at
different sizes using phonon dispersion calculations and ab-initio finite
temperature, molecular dynamics simulations. Depending on the number of
additional atoms in the edges between the corner atoms of the hexagons, n, both
\alpha-graphyne(n) and \alpha-BNyne(n) are stable for even n, but unstable for
odd n. \alpha-graphyne(3) undergoes a structural transformation, where the
symmetry of hexagons is broken. We present the structure optimized cohesive
energies, electronic, magnetic and mechanical properties of stable structures.
Our calculations reveal the existence of Dirac cones in the electronic
structures of \alpha-graphynes of all sizes, where the Fermi velocities
decrease with increasing n. The electronic and magnetic properties of these
structures are modified by hydrogenation. A single hydrogen vacancy renders a
magnetic moment of one Bohr magneton. We finally present the properties of the
bilayer \alpha-graphyne and \alpha-BNyne structures. We expect that these
layered materials can function as frameworks in various chemical and electronic
applications.Comment: Published version in The Journal of Physical Chemistr
High-performance planar nanoscale dielectric capacitors
We propose a model for planar nanoscale dielectric capacitor consisting of a
single layer, insulating hexagonal boron nitride (BN) stripe placed between two
metallic graphene stripes, all forming commensurately a single atomic plane.
First-principles density functional calculations on these nanoscale capacitors
for different levels of charging and different widths of graphene - BN stripes
mark high gravimetric capacitance values, which are comparable to those of
supercapacitors made from other carbon based materials. Present nanocapacitor
model allows the fabrication of series, parallel and mixed combinations which
offer potential applications in 2D flexible nanoelectronics, energy storage and
heat-pressure sensing systems.Comment: Published version in PR
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