564 research outputs found
Confinement of electrons in size modulated silicon nanowires
Based on first-principles calculations we showed that superlattices of
periodically repeated junctions of hydrogen saturated silicon nanowire segments
having different lengths and diameters form multiple quantum well structures.
The band gap of the superlattice is modulated in real space as its diameter
does and results in a band gap in momentum space which is different from
constituent nanowires. Specific electronic states can be confined in either
narrow or wide regions of superlattice. The type of the band lineup and hence
the offsets of valence and conduction bands depend on the orientation of the
superlattice as well as on the diameters of the constituent segments. Effects
of the SiH vacancy and substitutional impurities on the electronic and magnetic
properties have been investigated by carrying out spin-polarized calculations.
Substitutional impurities with localized states near band edges can make
modulation doping possible. Stability of the superlattice structure was
examined by ab initio molecular dynamics calculations at high temperatures
The response of mechanical and electronic properties of graphane to the elastic strain
Based on first-principles calculations, we resent a method to reveal the
elastic properties of recently synthesized monolayer hydrocarbon, graphane. The
in-plane stiffness and Poisson's ratio values are found to be smaller than
those of graphene, and its yielding strain decreases in the presence of various
vacancy defects and also at high ambient temperature. We also found that the
band gap can be strongly modified by applied strain in the elastic range.Comment: accepted version at: http://link.aip.org/link/?APL/96/09191
Stable single-layer honeycomb like structure of silica
Silica or SiO, the main constituent of earth's rocks has several 3D
complex crystalline and amorphous phases, but it does not have a graphite like
layered structure in 3D. Our theoretical analysis and numerical calculations
from the first-principles predict a single-layer honeycomb like allotrope,
h-silica, which can be viewed to be derived from the oxidation of
silicene and it has intriguing atomic structure with re-entrant bond angles in
hexagons. It is a wide band gap semiconductor, which attains remarkable
electromechanical properties showing geometrical changes under external
electric field. In particular, it is an auxetic metamaterial with negative
Poisson's ratio and has a high piezoelectric coefficient. While it can form
stable bilayer and multilayer structures, its nanoribbons can show metallic or
semiconducting behavior depending on their chirality. Coverage of dangling Si
orbitals by foreign adatoms can attribute new functionalities to
h-silica. In particular, SiO, where Si atoms are saturated by
oxygen atoms from top and bottom sides alternatingly can undergo a structural
transformation to make silicatene, another stable, single layer structure of
silica.Comment: Accepted for publication in Physical Review Letter
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
Superlubricity through graphene multilayers between Ni(111) surfaces
A single graphene layer placed between two parallel Ni(111) surfaces screens
the strong attractive force and results in a significant reduction of adhesion
and sliding friction. When two graphene layers are inserted, each graphene is
attached to one of the metal surfaces with a significant binding and reduces
the adhesion further. In the sliding motion of these surfaces the transition
from stick-slip to continuous sliding is attained, whereby non-equilibrium
phonon generation through sudden processes is suppressed. The adhesion and
corrugation strength continues to decrease upon insertion of the third graphene
layer and eventually saturates at a constant value with increasing number of
graphene layers. In the absence of Ni surfaces, the corrugation strength of
multilayered graphene is relatively higher and practically independent of the
number of layers. Present first-principles calculations reveal the
superlubricant feature of graphene layers placed between pseudomorphic Ni(111)
surfaces, which is achieved through the coupling of Ni-3d and graphene-
orbitals. The effect of graphene layers inserted between a pair of parallel
Cu(111) and Al(111) surfaces are also discussed. The treatment of sliding
friction under the constant loading force, by taking into account the
deformations corresponding to any relative positions of sliding slabs, is the
unique feature of our study.Comment: Accepted paper for Physical Review
Silicite: the layered allotrope of silicon
Based on first-principles calculation we predict two new thermodynamically
stable layered-phases of silicon, named as silicites, which exhibit strong
directionality in the electronic and structural properties. As compared to
silicon crystal, they have wider indirect band gaps but also increased
absorption in the visible range making them more interesting for photovoltaic
applications. These stable phases consist of intriguing stacking of dumbbell
patterned silicene layers having trigonal structure with periodicity of silicene and have cohesive energies smaller but
comparable to that of the cubic diamond silicon. Our findings also provide
atomic scale mechanisms for the growth of multilayer silicene as well as
silicites
Two and one-dimensional honeycomb structures of silicon and germanium
Based on first-principles calculations of structure optimization, phonon
modes and finite temperature molecular dynamics, we predict that silicon and
germanium have stable, two-dimensional, low-buckled, honeycomb structures.
Similar to graphene, they are ambipolar and their charge carriers can behave
like a massless Dirac fermions due to their pi- and pi*-bands which are crossed
linearly at the Fermi level. In addition to these fundamental properties, bare
and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and
magnetic properties, which are size and orientation dependent. These properties
offer interesting alternatives for the engineering of diverse nanodevices.Comment: 4 pages, 3 figures and 1 table. (published in Physical Review
Letters
Two and one-dimensional honeycomb structures of silicon and germanium
Based on first-principles calculations of structure optimization, phonon
modes and finite temperature molecular dynamics, we predict that silicon and
germanium have stable, two-dimensional, low-buckled, honeycomb structures.
Similar to graphene, they are ambipolar and their charge carriers can behave
like a massless Dirac fermions due to their pi- and pi*-bands which are crossed
linearly at the Fermi level. In addition to these fundamental properties, bare
and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and
magnetic properties, which are size and orientation dependent. These properties
offer interesting alternatives for the engineering of diverse nanodevices.Comment: 4 pages, 3 figures and 1 table. (published in Physical Review
Letters
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