4,146 research outputs found
Engineering molecular aggregate spectra
We show that optical properties of linear molecular aggregates undergo
drastic changes when aggregates are deposited on a metal surface. The
dipole-dipole interactions of monomers with their images can result in strong
{re-structuring of both the exciton band and the absorption spectrum, depending
on the arrangement of the monomer transition dipoles with respect to the
surface.Comment: 4 two-column pages, Proceedings of the 8th International Conference
on Excitonic Processes in Condensed Matter, June 22-27, Kyoto, Japa
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
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
Local Reconstructions of Silicene Induced by Adatoms
The interaction of silicene with Si, C, H, O, Ti atoms along with H,
HO and O molecules are investigated and the induced functionalities
thereof are analyzed using first principles density functional theory. Si
adatom initially adsorbed at the top site of silicene pushes down the Si atom
underneath to form a dumbbell like structure with 3+1 coordination. This
prediction is important for silicene research and reveal new physical phenomena
related with the formation of multilayer Si, which is apparently the precursor
state for missing layered structure of silicon. We found that dumbbell
structure attributes coverage dependent electronic and magnetic properties to
nonmagnetic bare silicene. Even more interesting is that silicene with
dumbbells is energetically more favorable than the pristine silicene: The more
dense the dumbbell coverage, the stronger is the cohesion. Incidentally, these
structures appear to be intermediate between between silicene and silicon.
Carbon adatom, which is initially adsorbed to the bridge position, substitutes
one Si atom, if it overcomes a small energy barrier. Oxygen molecule can
dissociate on silicene surface, whereby constituent oxygen atoms oxidize
silicene by forming strong bonds. By varying the concentration and decoration
of carbon, hydrogen and oxygen atoms one can tune the band gap of silicene.
Through the adsorption of hydrogen or titanium adatom, silicene acquires spin
polarized state. A half metallic ferromagnetic behavior is attained at specific
uniform coverage of Ti adatom, which may function as a spin valve.Comment: Accepted for publication in The Journal of Physical Chemistry
http://pubs.acs.org/doi/abs/10.1021/jp408647
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
Does Innovativeness Matter for International Competitiveness in Developing Countries? The Case of Turkish Manufacturing Industries
I. Introduction. - II. Evolution of the Theory of International Trade: Attitudes towards the Technology Factor and the Schumpeterian Viewpoint as a Rationale for Studies at the Firm-Level. - III. A Survey of Firm-Level Studies on the Determinants of Export Performance with an Emphasis on Technology Factor. - IV. Technological and International Competitveness of the Turkish Manufacturing Industry. - V. Determinants of International Competitveness: Estimation Results. - VI. Concluding Remarks.Innovation, R&D, International Competitiveness, Exports
The Blume-Emery-Griffiths Spin Glass and Inverted Tricritical Points
The Blume-Emery-Griffiths spin glass is studied by renormalization-group
theory in d=3. The boundary between the ferromagnetic and paramagnetic phases
has first-order and two types of second-order segments. This topology includes
an inverted tricritical point, first-order transitions replacing second-order
transitions as temperature is lowered. The phase diagrams show disconnected
spin-glass regions, spin-glass and paramagnetic reentrances, and complete
reentrance, where the spin-glass phase replaces the ferromagnet as temperature
is lowered for all chemical potentials.Comment: Added discussion, references, and figure insets. 5 pages, 6 figures.
Published versio
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