Electronic and Structural Properties of Carbon Nano-Horns

We use parametrized linear combination of atomic orbitals calculations to determine the stability, optimum geometry and electronic properties of nanometer-sized capped graphitic cones, called ``nano-horns''. Different nano-horn morphologies are considered, which differ in the relative location of the five terminating pentagons. Simulated scanning tunneling microscopy images of the various structures at different bias voltages reflect a net electron transfer towards the pentagon vertex sites. We find that the local density of states at the tip, observable by scanning tunneling spectroscopy, can be used to discriminate between different tip structures. Our molecular dynamics simulations indicate that disintegration of nano-horns at high temperatures starts in the highest-strain region near the tip.Comment: 5 pages, 3 figures, submitted for publicatio

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

Interplay between structure and magnetism in Mo12S9I9Mo_{12} S_9 I_9 nanowires

We investigate the equilibrium geometry and electronic structure of Mo$_{12}$S$_{9}$I$_{9}$ 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

Spectroscopic characterization of Stone-Wales defects in nanotubes

We combined resonant photoabsorption and vibration spectroscopy with scanning tunneling microscopy (STM) to unambiguously identify the presence of Stone-Wales (SW) defects in carbon and boron nitride nanotubes. Based on extensive time-dependent ab initio density functional calculations, we propose to resonantly photoexcite SW defects in the infrared and ultraviolet regime as a means of their identification. Onset of nonradiative decay to a local defect vibration with a frequency of 1962 cm-1 serves as a fingerprint of such defects in carbon nanotubes. The bias dependence of the STM images shows distinct features associated with the presence of SW defects.Y.M. was supported by NAREGI Nanoscience Project, Ministry of Education, Culture, Sports, Science and Technology, Japan. A.R. acknowledges support from the EC grants (HPRN-CT-2000-00128 and HPRN-CT-2000-00167) and Spain MCyT. M.Y. and D.T. acknowledge partial support by NSF-NIRT grant DMR- 0103587.Peer reviewe

Magnetism in all-carbon nanostructures with negative gaussian curvature

The electronic and magnetic properties of an sp2 bonded all-carbon nanostructure, consisting of a nanotube junction structurally related to schwarzite, were investigated. The spin density functional theory was used during the investigation. The electronic structure of different tetrapods was calculated using the density functional theory within the local spin density approximation (LSDA). It was found that particular systems, which were related to schwarzite and contain no under-coordinated carbon atoms, carry a net magnetic moment in the ground state. The effect of edge termination on the net magnetic moment of the tetrapod was also studied.open13713

Revealing Sub-Surface Vibrational Modes by Atom-Resolved Damping Force Spectroscopy

We propose to use the damping signal of an oscillating cantilever in dynamic atomic force microscopy as a noninvasive tool to study the vibrational structure of the substrate. We present atomically resolved maps of damping in carbon nanotube peapods, capable of identifying the location and packing of enclosed Dy@C82 molecules as well as local excitations of vibrational modes inside nanotubes of different diameter. We elucidate the physical origin of damping in a microscopic model and provide quantitative interpretation of the observations by calculating the vibrational spectrum and damping of Dy@C82 inside nanotubes with different diameters using ab initio total energy and molecular dynamics calculations.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Lett

Unusually High Thermal Conductivity of Carbon Nanotubes

Combining equilibrium and non-equilibrium molecular dynamics simulations with accurate carbon potentials, we determine the thermal conductivity $\lambda$ of carbon nanotubes and its dependence on temperature. Our results suggest an unusually high value ${\lambda}{\approx}6,600$~W/m$\cdot$K for an isolated (10,10) nanotube at room temperature, comparable to the thermal conductivity of a hypothetical isolated graphene monolayer or diamond. Our results suggest that these high values of $\lambda$ are associated with the large phonon mean free paths in these systems; substantially lower values are predicted and observed for the basal plane of bulk graphite.Comment: 4 pages 3 figures (5 postscript files), submitted for publicatio

Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2, lattice vibrations are observed to thermalize on a time scale of ~8ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp^3-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photo-excitation.Comment: 5 pages, 4 figures; to appear in Phys. Rev. Let