97 research outputs found
Formation and Stability of Cellular Carbon Foam Structures:An {\em Ab Initio} Study
We use ab initio density functional calculations to study the formation and
structural as well as thermal stability of cellular foam-like carbon
nanostructures. These systems with a mixed bonding character may be
viewed as bundles of carbon nanotubes fused to a rigid contiguous 3D honeycomb
structure that can be compressed more easily by reducing the symmetry of the
honeycombs. The foam may accommodate the same type of defects as graphene, and
its surface may be be stabilized by terminating caps. We postulate that the
foam may form under non-equilibrium conditions near grain boundaries of a
carbon-saturated metal surface
Effect of electron and hole doping on the structure of C, Si, and S nanowires
We use ab initio density functional calculations to study the effect of
electron and hole doping on the equilibrium geometry and electronic structure
of C, Si, and S monatomic wires. Independent of doping, all these nanowires are
found to be metallic. In absence of doping, C wires are straight, whereas Si
and S wires display a zigzag structure. Besides two preferred bond angles of 60
deg and 120 deg in Si wires, we find an additional metastable bond angle of 90
deg in S wires. The equilibrium geometry and electronic structure of these
nanowires is shown to change drastically upon electron and hole doping.Comment: 5 pages including 5 figure
Transforming carbon nanotubes by silylation: An ab initio study
We use ab initio density functional calculations to study the chemical
functionalization of single-wall carbon nanotubes and graphene monolayers by
silyl (SiH3) radicals and hydrogen. We find that silyl radicals form strong
covalent bonds with graphene and nanotube walls, causing local structural
relaxations that enhance the sp3 character of these graphitic nanostructures.
Silylation transforms all carbon nanotubes into semiconductors, independent of
their chirality. Calculated vibrational spectra suggest that specific frequency
shifts can be used as a signature of successful silylation.Comment: 4 pages, 3 figure
Computational study of the thermal conductivity in defective carbon nanostructures
We use non-equilibrium molecular dynamics simulations to study the adverse
role of defects including isotopic impurities on the thermal conductivity of
carbon nanotubes, graphene and graphene nanoribbons. We find that even in
structurally perfect nanotubes and graphene, isotopic impurities reduce thermal
conductivity by up to one half by decreasing the phonon mean free path. An even
larger thermal conductivity reduction, with the same physical origin, occurs in
presence of structural defects including vacancies and edges in narrow graphene
nanoribbons. Our calculations reconcile results of former studies, which
differed by up to an order of magnitude, by identifying limitations of various
computational approaches
Interplay between structure and magnetism in nanowires
We investigate the equilibrium geometry and electronic structure of
MoSI nanowires using ab initio Density Functional
calculations. The skeleton of these unusually stable nanowires consists of
rigid, functionalized Mo octahedra, connected by flexible, bi-stable sulphur
bridges. This structural flexibility translates into a capability to stretch up
to approximate 20% at almost no energy cost. The nanowires change from
conductors to narrow-gap magnetic semiconductors in one of their structural
isomers.Comment: 4 pages with PRL standards and 3 figure
Designing rigid carbon foams
We use ab initio density functional calculations to study the stability,
elastic properties and electronic structure of sp2 carbon minimal surfaces with
negative Gaussian curvature, called schwarzites. We focus on two systems with
cubic unit cells containing 152 and 200 carbon atoms, which are metallic and
very rigid. The porous schwarzite structure allows for efficient and reversible
doping by electron donors and acceptors, making it a promising candidate for
the next generation of alkali ion batteries. We identify schwarzite structures
that act as arrays of interconnected quantum spin dots or become magnetic when
doped. We introduce two interpenetrating schwarzite structures that may find
their use as the ultimate super-capacitor.Comment: 6 pages, 5 figure
Coherent Control of Photocurrents in Graphene and Carbon Nanotubes
Coherent one photon () and two photon () electronic
excitations are studied for graphene sheets and for carbon nanotubes using a
long wavelength theory for the low energy electronic states. For graphene
sheets we find that coherent superposition of these excitations produces a
polar asymmetry in the momentum space distribution of the excited carriers with
an angular dependence which depends on the relative polarization and phases of
the incident fields. For semiconducting nanotubes we find a similar effect
which depends on the square of the semiconducting gap, and we calculate its
frequency dependence.
We find that the third order nonlinearity which controls the direction of the
photocurrent is robust for semiconducting t ubes and vanishes in the continuum
theory for conducting tubes. We calculate corrections to these results arising
from higher order crystal field effects on the band structure and briefly
discuss some applications of the theory.Comment: 12 pages in RevTex, 6 epsf figure
Prismane C_8: A New Form of Carbon?
Our numerical calculations on small carbon clusters point to the existence of
a metastable three-dimensional eight-atom cluster C which has a shape of a
six-atom triangular prism with two excess atoms above and below its bases. We
gave this cluster the name "prismane". The binding energy of the prismane
equals to 5.1 eV/atom, i.e., is 0.45 eV/atom lower than the binding energy of
the stable one-dimensional eight-atom cluster and 2.3 eV/atom lower than the
binding energy of the bulk graphite or diamond. Molecular dynamics simulations
give evidence for a rather high stability of the prismane, the activation
energy for a prismane decay being about 0.8 eV. The prismane lifetime increases
rapidly as the temperature decreases indicating a possibility of experimental
observation of this cluster.Comment: 5 pages (revtex), 3 figures (eps
Orientational Melting in Carbon Nanotube Ropes
Using Monte Carlo simulations, we investigate the possibility of an
orientational melting transition within a "rope" of (10,10) carbon nanotubes.
When twisting nanotubes bundle up during the synthesis, orientational
dislocations or twistons arise from the competition between the anisotropic
inter-tube interactions, which tend to align neighboring tubes, and the torsion
rigidity that tends to keep individual tubes straight. We map the energetics of
a rope containing twistons onto a lattice gas model and find that the onset of
a free "diffusion" of twistons, corresponding to orientational melting, occurs
at T_OM > 160 K.Comment: 4 page LaTeX file with 3 figures (10 PostScript files
Beans with bugs: Covert carnivory and infested seed selection by the red-nosed cuxiú monkey
Members of the Neotropical primate genus Chiropotes eat large volumes of immature seeds. However, such items are often low in available proteins, and digestion of seeds is further inhibited by tannins. This suggests that overall plant-derived protein intake is relatively low. We examined the presence of insect larvae in partially eaten fruits, compared with intact fruit on trees, and examined fecal pellets for the presence of larvae. We found that red-nosed cuxiú (Chiropotes albinasus) individuals may supplement their limited seed-derived protein intake by ingesting seed-inhabiting insects. Comparison of fruits partially eaten for their seeds with those sampled directly from trees showed that fruits with insect-containing seeds were positively selected in 20 of the 41 C. albinasus diet items tested, suggesting that fruits with infested seeds are actively selected by foraging animals. We found no differences in accessibility to seeds, that is, no differences in husk penetrability between fruits with infested and uninfested seeds excluding the likelihood that insect-infestation results in easier access to the seeds in such fruits. Additionally, none of the C. albinasus fecal samples showed any evidence of living pupae or larvae, indicating that infesting larvae are digested. Our findings raise the possibility that these seed-predating primates might provide net benefits to the plant species they feed on, since they feed from many species of plants and their actions may reduce the populations of seed-infesting insects
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