983 research outputs found
Atomic and electronic structure of nitrogen- and boron-doped phosphorene
First principle modeling of nitrogen- and boron-doped phosphorene
demonstrates the tendency toward formation of highly ordered structures.
Nitrogen doping leads to the formation of -N-P-P-P-N- lines. Further
transformation to -P-N-P-N- lines across the chains of phosphorene occurs with
increasing band gap and increasing nitrogen concentration, which coincides with
the decreasing chemical activity of N-doped phosphorene. In contrast to the
case of nitrogen, boron atoms prefer to form -B-B- pairs with the further
formation of -P-P-B-B-P-P- patterns along the phosphorene chains. The low
concentration of boron dopants converts the phosphorene from a semiconductor
into a semimetal with the simultaneous enhancement of its chemical activity.
Co-doping of phosphorene by both boron and nitrogen starts from the formation
of -B-N- pairs, which provide flat bands and the further transformation of
these pairs to hexagonal BN lines and ribbons across the phosphorene chains.Comment: 21 pages, 8 figures, 2 tables, to appear at PCC
Absence of stable atomic structure in fluorinated graphene
Based on the results of first-principles calculations we demonstrate that
significant distortion of graphene sheets caused by adsorption of fluorine
atoms leads to the formation of metastable patterns for which the next step of
fluorination is considerably less energetically favorable. Existence of these
stable patterns oriented along the armchair direction makes possible the
synthesis of various CFx structures. The combination of strong distortion of
the nonfluorinated graphene sheet with the doping caused by the polar nature of
C-F bonds reduces the energy cost of migration and the energy of migration
barriers, making possible the migration of fluorine atoms on the graphene
surface as well as transformation of the shapes of fluorinated areas. The
decreasing energy cost of migration with increasing fluorine content also leads
to increasing numbers of single fluorine adatoms, which could be the source of
magnetic moments.Comment: 16 pages, 6 figures (one figure added), accepted in PCC
Oxidation of graphite surface: the role of water
Based on density functional calculations, we demonstrate a significant
difference in oxidation patterns between graphene and graphite and the
formation of defects after oxidation. Step-by-step modeling demonstrates that
oxidation of 80% of the graphite surface is favorable. Oxidation above half of
the graphite surface significantly decreases the energy costs of vacancy
formation with CO2 production. The presence of water is crucial in the
transformation of epoxy groups to hydroxyl, the intercalation with further
bundle and exfoliation. In water-rich conditions, water intercalates graphite
at the initial stages of oxidation and oxidation, which is similar to the
oxidation process of free-standing graphene; in contrast, in water-free
conditions, large molecules intercalate graphite only after oxidation occurs on
more than half of the surface.Comment: 10 pages, 3 figures, accepted to J. Phys. Chem.
Modelling of epitaxial graphene functionalization
A new model for graphene, epitaxially grown on silicon carbide is proposed.
Density functional theory modelling of epitaxial graphene functionalization by
hydrogen, fluorine and phenyl groups has been performed with hydrogen and
fluorine showing a high probability of cluster formation in high adatom
concentration. It has also been shown that the clusterization of fluorine
adatoms provides midgap states in formation due to significant flat distortion
of graphene. The functionalization of epitaxial graphene using larger species
(methyl and phenyl groups) renders cluster formation impossible, due to the
steric effect and results in uniform coverage with the energy gap opening.Comment: 15 pages, 4 figures, to appear in Nanotechnolog
First-principles modeling of the interactions of iron impurities with graphene and graphite
Results of first principles modelling of interactions graphene and graphite
with iron impurities predict the colossal difference between these two carbon
allotropes. Insertion of the iron atoms between the planes of graphite is much
more energetically favourable than adsorption of the iron adatom at graphite or
graphene surface. High mobility of iron adatom over graphite surface and within
bulk graphite is reported. Calculated values of formation energies for the
substitutional iron impurities suggest that iron is more destructive for
graphite than for graphene. This effect caused formation of uniform carbon
environment of the iron atom inside the multilayer system. In contrast to
graphene segregation of the substitutional iron impurities in graphite at the
ambient conditions is not energetically favourable. Enhancement of interlayer
bonding in contaminated graphite and purity of graphene from iron impurities
are also reported.Comment: 14 pages, 3 figures, to appear in phis. stat. solidi (b
Defect-induced ferromagnetism in fullerenes
Based on the ab initio electronic structure calculations the picture of
ferromagnetism in polimerized C60 is proposed which seems to explain the whole
set of controversial experimental data. We have demonstrated that, in contrast
with cubic fullerene, in rhombohedral C60 the segregation of iron atoms is
energetically unprofitable which is a strong argument in favor of intrinsic
character of carbon ferromagnetism which can be caused by vacancies with
unpaired magnetic electrons. It is shown that: (i) energy formation of the
vacancies in the rhombohedral phase of C60 is essentially smaller than in the
cubic phase, (ii) there is a strong ferromagnetic exchange interactions between
carbon cages containing the vacancies, and (iii) the fusion of the magnetic
vacancies into nonmagnetic bivacancy is energetically profitable. The latter
can explain a fragility of the ferromagnetism.Comment: 11 pages, 7 figures, final version to be published in Eur. Phys. J
A new route towards uniformly functionalized single-layer graphene
It is shown, by DFT calculations, that the uniform functionalization of upper
layer of graphite by hydrogen or fluorine does not change essentially its
bonding energy with the underlying layers, whereas the functionalization by
phenyl groups decreases the bonding energy by a factor of approximately ten.
This means that the functionalized monolayer in the latter case can be easily
separated by mild sonication. According to our computational results, such
layers can be cleaned up to pure graphene, as well as functionalized further up
to 25% coverage, without essential difficulties. The energy gap within the
interval from 0.5 to 3 eV can be obtained by such one-side funtionalization
using different chemical species.Comment: 15 pages, 3 figures, to appear in J. Phys. D: Applied Physic
Chemical functionalization of graphene
Experimental and theoretical results on chemical functionalization of
graphene are reviewed. Using hydrogenated graphene as a model system, general
principles of the chemical functionalization are formulated and discussed. It
is shown that, as a rule, 100% coverage of graphene by complex functional
groups (in contrast with hydrogen and fluorine) is unreachable. A possible
destruction of graphene nanoribbons by fluorine is considered. The
functionalization of infinite graphene and graphene nanoribbons by oxygen and
by hydrofluoric acid is simulated step by step.Comment: 13 pages, 11 figures. Invited paper for J. Phys. Cond. Mater.
"Graphene" special issue. References added, typos correcte
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