56 research outputs found
Graphene to Graphane: A Theoretical Study
Graphane is a two-dimensional system consisting of a single layer of fully
saturated (sp hybridization) carbon atoms. In an ideal graphane structure
C-H bonds exhibit an alternating pattern (up and down with relation to the
plane defined by the carbon atoms). In this work we have investigated using
\textit{ab initio} and reactive molecular dynamics simulations the role of H
frustration (breaking the H atoms up and down alternating pattern) in
graphane-like structures. Our results show that significant percentage of
uncorrelated H frustrated domains are formed in the early stages of the
hydrogenation process leading to membrane shrinkage and extensive membrane
corrugations. These results also suggest that large domains of perfect
graphane-like structures are unlikely to be formed, H frustrated domains are
always present.Comment: 15 pages, 6 figure
Structure and Dynamics of Boron Nitride Nanoscrolls
Carbon nanoscrolls (CNSs) are structures formed by rolling up graphene layers
into a papyruslike shape. CNNs have been experimentally produced by different
groups. Boron nitride nanoscrolls (BNNSs) are similar structures using boron
nitride instead of graphene layers. In this work we report molecular mechanics
and molecular dynamics results for the structural and dynamical aspects of BNNS
formation. Similarly to CNS, BNNS formation is dominated by two major energy
contributions, the increase in the elastic energy and the energetic gain due to
van der Waals interactions of the overlapping surface of the rolled layers. The
armchair scrolls are the most stable configuration while zigzag scrolls are
metastable structures which can be thermally converted to armchair. Chiral
scrolls are unstable and tend to evolve to zigzag or armchair configurations
depending on their initial geometries. The possible experimental routes to
produce BNNSs are also addressed
Dynamics Of The Formation Of Carbon Nanotube Serpentines.
Recently, Geblinger et al. [Nat. Nanotechnol. 3, 195 (2008)] reported the experimental realization of carbon nanotube S-like shaped nanostructures, the so-called carbon nanotube serpentines. We report here results from multimillion fully atomistic molecular dynamics simulations of their formation. We consider one-μm-long carbon nanotubes placed on stepped substrates with and without a catalyst nanoparticle on the top free end of the tube. A force is applied to the upper part of the tube during a short period of time and turned off; then the system is set free to evolve in time. Our results show that these conditions are sufficient to form robust serpentines and validates the general features of the falling spaghetti model proposed to explain their formation.11010550
Self-consistent electronic structure of Mo(001) and W(001) surfaces
We report results for the surface band structures of molybdenum and tungsten (001) surfaces by employing the surface version of the first-principles, self-consistent real-space linear muffin-tin orbital method in the atomic sphere approximation. The surface state dispersions as well as the spectral density of states were obtained employing the transfer matrix scheme. The resulting surface band structures are compared with recent experimental measurements at temperatures above the transition temperature, as well as theoretical self-consistent calculations.6115104171042
Molecular Dynamics Simulations of Carbon Nanotubes as Gigahertz Oscillators
Recently Zheng and Jiang [PRL 88, 045503 (2002)], based on static models,
have proposed that multiwalled carbon nanotubes could be the basis for a new
generation of nanooscilators in the several gigahertz range. In this work we
present the first molecular dynamics simulation for these systems. Different
nanotube types were considered in order to verify the reliability of such
devices as gigahertz oscillators. Our results show that these nanooscillators
are dynamically stables when the radii difference values between inner and
outer tubes are of ~ 3.4 A. Frequencies as large as 38 GHz were observed, and
the calculated force values are in good agreement with recent experimental
investigations. Moreover, our results contradict some predictions made by Zheng
and Jiang.Comment: 4 pages, 6 figure
Transition from tunneling to direct contact in tungsten nanojunctions
We apply the mechanically controllable break junctions technique to
investigate the transition from tunneling to direct contact in tungsten. This
transition is quite different from that of other metals and is determined by
the local electronic properties of the tungsten surface and the relief of the
electrodes at the point of their closest proximity. The conductance traces show
a rich variety of patterns from the avalanche-like jump to a mesoscopic contact
to the completely smooth transition between direct contact and tunneling. Due
to the occasional absence of an adhesive jump the conductance of the contact
can be continuously monitored at ultra-small electrode separations. The
conductance histograms of tungsten are either featureless or show two distinct
peaks related to the sequential opening of spatially separated groups of
conductance channels. The role of surface states of tungsten and their
contribution to the junction conductance at sub-Angstrom electrode separations
are discussed.Comment: 6 pages, 6 figure
Origin of anomalously long interatomic distances in suspended gold chains
The discovery of long bonds in gold atom chains has represented a challenge
for physical interpretation. In fact, interatomic distances frequently attain
3.0-3.6 A values and, distances as large as 5.0 A may be seldom observed. Here,
we studied gold chains by transmission electron microscopy and performed
theoretical calculations using cluster ab initio density functional formalism.
We show that the insertion of two carbon atoms is required to account for the
longest bonds, while distances above 3 A may be due to a mixture of clean and
one C atom contaminated bonds.Comment: 4 pages, 4 Postscript figures, to be published in Physical Review
Letter
- …