140 research outputs found
Electron Interference Effects on the Conductance of Doped Carbon Nanotubes
We investigate the effects of impurity scattering on the conductance of
metallic carbon nanotubes as a function of the relative separation of the
impurities. First we compute the conductance of a clean (6,6) tube, and the
effect of model gold contacts on this conductance. Then, we compute the effect
of introducing a single, two, and three oxygen atom impurities. We find that
the conductance of a single-oxygen-doped (6,6) nanotube decreases by about 30 %
with respect to that of the perfect nanotube. The presence of a second doping
atom induces strong changes of the conductance which, however, depend very
strongly on the relative position of the two oxygen atoms. We observe regular
oscillations of the conductance that repeat over an O-O distance that
corresponds to an integral number of half Fermi-wavelengths ().
These fluctuations reflect strong electron interference phenomena produced by
electron scattering from the oxygen defects whose contribution to the
resistance of the tube cannot be obtained by simply summing up their individual
contributions.Comment: 13 pages, 5 figures (eps and gif), to appear in J.Phys.Che
Switching behavior of semiconducting carbon nanotubes under an external electric field
We investigate theoretically the switching characteristics of semiconducting
carbon nanotubes connected to gold electrodes under an external (gate) electric
field. We find that the external introduction of holes is necessary to account
for the experimental observations. We identify metal-induced-gap states (MIGS)
at the contacts and find that the MIGS of an undoped tube would not
significantly affect the switching behavior, even for very short tube lengths.
We also explore the miniaturization limits of nanotube transistors, and, on the
basis of their switching ratio, we conclude that transistors with channels as
short as 50\AA would have adequate switching characteristics.Comment: 4 pages, 3 figures, ReVTe
Strongly reshaped organic-metal interfaces: Tetracyanoethylene on Cu(100)
The interaction of the strong electron-acceptor tetracyanoethylene (TCNE)
with the Cu(100) surface has been studied with scanning tunneling microscopy
experiments and first-principles density functional theory calculations. We
compare two different adsorption models with the experimental results and show
that the molecular self-assembly is caused by a strong structural modification
of the Cu(100) surface rather than the formation of a coordination network by
diffusing Cu adatoms. Surface atoms become highly buckled and the chemisorption
of TCNE is accompanied by a partial charge-transfer.Comment: 4 pages, 3 figures, to appear in Physical Review Letter
The Effect of Structural Distortions on the Electronic Structure of Carbon Nanotubes
We calculated the effects of structural distortions on the electronic
structure of carbon nanotubes. The key modification of the electronic structure
brought about by bending a nanotube involves an increased mixing of
and -states. This mixing leads to an enhanced density-of-states in the
valence band near the Fermi energy region. While in a straight tube the states
accessible for electrical conduction are essentially pure C()-states,
they acquire significant C() character upon bending. Bending also
leads to a charge polarization of the C-C bonds in the deformed region
reminiscent of interface dipole formation. Scattering of conduction electrons
at the distorted regions may lead to electron localization at low temperatures.Comment: 11 pages and 4 figures, (figure 4 corrected
Anisotropic growth of the thiophene-based layer on Si(111)-B
International audienceThe formation of large assemblies on the Si(111)-B surface is discussed with the help of STM simulations and DFT calculations. Although highly regular assemblies of DTB10B along the Si row direction are observed, the existence of two herringbone isomers introduces a lower periodicity within the 2D molecular network. The formation of herringbone units is explained by weak intermolecular interactions while the 1D assembling depends mainly on the interactions of the C10 side chains with the Si(111)-B surface
Conductance of Distorted Carbon Nanotubes
We have calculated the effects of structural distortions of armchair carbon
nanotubes on their electrical transport properties. We found that the bending
of the nanotubes decreases their transmission function in certain energy ranges
and leads to an increased electrical resistance. Electronic structure
calculations show that these energy ranges contain localized states with
significant - hybridization resulting from the increased curvature
produced by bending. Our calculations of the contact resistance show that the
large contact resistances observed for SWNTs are likely due to the weak
coupling of the NT to the metal in side bonded NT-metal configurations.Comment: 5 pages RevTeX including 4 figures, submitted to PR
Atomically precise incorporation of BN doped rubicene into graphene nanoribbons
Substituting heteroatoms and non-benzenoid carbons into nanographene
structure offers an unique opportunity for atomic engineering of electronic
properties. Here we show the bottom-up synthesis of graphene nanoribbons (GNRs)
with embedded fused BN-doped rubicene components on a Au(111) surface using
on-surface chemistry. Structural and electronic properties of the BN-GNRs are
characterized by scanning tunneling microscopy (STM) and atomic force
microscopy (AFM) with CO-terminated tips supported by numerical calculations.
The periodic incorporation of BN heteroatoms in the GNR leads to an increase of
the electronic band gap as compared to its undoped counterpart. This opens
avenues for the rational design of semiconducting GNRs with optoelectronic
properties.Comment: 18 pages, 4 figure
Evaluation of lung recovery after static administration of three different perfluorocarbons in pigs.
International audienceBackground: The respiratory properties of perfluorocarbons (PFC) have been widely studied for liquid ventilation inhumans and animals. Several PFC were tested but their tolerance may depend on the species. Here, the effects of asingle administration of liquid PFC into pig lungs were assessed and compared. Three different PFC having distinctevaporative and spreading coefficient properties were evaluated (Perfluorooctyl bromide [PFOB], perfluorodecalin[PFD] and perfluoro-N-octane [PFOC]).Methods: Pigs were anesthetized and submitted to mechanical ventilation. They randomly received an intra-trachealadministration of 15 ml/kg of either PFOB, PFD or PFOC with 12 h of mechanical ventilation before awakening andweaning from ventilation. A Control group was submitted to mechanical ventilation with no PFC administration. Allanimals were followed during 4 days after the initial PFC administration to investigate gas exchanges and clinicalrecovery. They were ultimately euthanized for histological analyses and assessment of PFC residual concentrationswithin the lungs using dual nuclei fluorine and hydrogen Magnetic Resonance Imaging (MRI). Sixteen animals wereincluded (4/group).Results: In the PFD group, animals tended to be hypoxemic after awakening. In PFOB and PFOC groups, blood gaseswere not significantly different from the Control group after awakening. The poor tolerance of PFD was likely related toa large amount of residual PFC, as observed using MRI in all lung samples (â10% of lung volume). This percentage waslower in the PFOB group (â1%) but remained significantly greater than in the Control group. In the PFOC group, thepercentage of residual PFC was not significantly different from that of the Control group (â0.1%). Histologically, themost striking feature was an alveolar infiltration with foam macrophages, especially in the groups treated by PFD orPFOB.Conclusions: Of the three tested perfluorocarbons, PFOC offered the best tolerance in terms of lung function, gasexchanges and residuum in the lung. PFOC was rapidly cleared from the lungs and virtually disappeared after 4 dayswhereas PFOB persisted at significant levels and led to foam macrophage infiltration. PFOC could be relevant for shortterm total liquid ventilation with a rapid weaning
Olfactory Enrichment Influences Adult Neurogenesis Modulating GAD67 and Plasticity-Related Molecules Expression in Newborn Cells of the Olfactory Bulb
The olfactory bulb (OB) is a highly plastic region of the adult mammalian brain characterized by continuous integration of inhibitory interneurons of the granule (GC) and periglomerular cell (PGC) types. Adult-generated OB interneurons are selected to survive in an experience-dependent way but the mechanisms that mediate the effects of experience on OB neurogenesis are unknown. Here we focus on the new-generated PGC population which is composed by multiple subtypes. Using paradigms of olfactory enrichment and/or deprivation combined to BrdU injections and quantitative confocal immunohistochemical analyses, we studied the effects of olfactory experience on adult-generated PGCs at different survival time and compared PGC to GC modulation. We show that olfactory enrichment similarly influences PGCs and GCs, increasing survival of newborn cells and transiently modulating GAD67 and plasticity-related molecules expression. However, PGC maturation appears to be delayed compared to GCs, reflecting a different temporal dynamic of adult generated olfactory interneuron integration. Moreover, olfactory enrichment or deprivation do not selectively modulate the survival of specific PGC phenotypes, supporting the idea that the integration rate of distinct PGC subtypes is independent from olfactory experience
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