Nanoelectromechanical systems (NEMS) hold promise for a number of scientific and technological applications. In particular, NEMS oscillators have been proposed for use in ultrasensitive mass detection, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems. Perhaps the ultimate material for these applications is a carbon nanotube. They are the stiffest material known, have low density, ultrasmall cross-sections and can be defect-free. Equally important, a nanotube can act as a transistor and thus may be able to sense its own motion. In spite of this great promise, a room-temperature, self-detecting nanotube oscillator has not been realized, although some progress has been made. Here we report the electrical actuation and detection of the guitar-string-like oscillation modes of doubly clamped nanotube oscillators. We show that the resonance frequency can be widely tuned and that the devices can be used to transduce very small forces.

Electron scattering rates in metallic single-walled carbon nanotubes are studied using an atomic force microscope as an electrical probe. From the scaling of the resistance of the same nanotube with length in the low and high bias regimes, the mean free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic phonon scattering at low biases and zone boundary/optical phonon scattering at high biases.

First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a uniform and stable coverage on both sides. A significant part of the electronic charge of the Li2s orbital is donated to graphene and is accommodated by its distorted π * -bands. As a result, semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium with each adsorbed Li absorbing up to four H2 molecules amounting to a gravimetric density of 12.8 wt %.Developing safe and efficient hydrogen storage is essential for hydrogen economy.1 Recently, much effort has been devoted to engineer carbon based nanostructures 2,3,4,5 which can absorb H 2 molecules with high storage capacity, but can release them easily in the course of consumption in fuel cells. Insufficient storage capacity, slow kinetics, poor reversibility and high dehydrogenation temperatures have been the main difficulties towards acceptable media for hydrogen storage.Recently, graphene, a single atomic plane of graphite, has been produced 6 showing unusual electronic and magnetic properties. In this letter, we predict that metallized graphene can be a potential high-capacity hydrogen storage medium. The process is achieved in two steps: Initially, graphene is metallized through charge donation by adsorbed Li atoms to its π * -bands. Subsequently, each positively charged Li ion can absorb up to four H 2 by polarizing these molecules. At the end, the storage capacity up to the gravimetric density of g d =12.8 wt % is attained. These results are important not only because graphene is found to be a high capacity hydrogen storage medium, but also because of its metallization through Li coverage is predicted.Our results have been obtained by performing first-principles plane wave calculations using ultra-soft pseudopotentials.7 We used Local Density Approximation (LDA), since the van der Waals contribution to the Li-graphene interaction has been shown 8 to be better accounted by LDA. Numerical results have been obtained by using VASP, 9 which were confirmed by using the PWSCF code.10 A plane-wave basis set with kinetic energy cutoffh 2 |k + G| 2 /2m = 380 eV has been used. In the self-consistent potential and total energy calculations the Brillouin zone has been sampled by (19x19x1) and (9x9x1) special mesh points in k-space for (2×2) and (4×4) graphene cells, respectively. Atomic positions in all structures are optimized using the conjugate gradient method. Convergence is achieved when the difference of the total energies of last two consecutive steps is less than 10 −6 eV and the maximum force allowed on each atom is less than 10 −2 eV/Å. All configurations studied in this work have also been calculated by using spin-polarized LDA, which were resulted in non-magnetic ground state.Adsorption of a single (isolated) Li atom on the hollow site of graphene (i.e. H1-site above the center of hexagon) is modelled by using (4×4) cell of graphene w...

We present a general study of oscillations in suspended one-dimensional elastic systems clamped at each end, exploring a wide range of slack (excess length) and downward external forces. Our results apply directly to recent experiments in nanotube and silicon nanowire oscillators. We find the behavior to simplify in three well-defined regimes which we present in a dimensionless phase diagram. The frequencies of vibration of such systems are found to be extremely sensitive to slack.Vibrations of one-dimensional systems (i.e. systems with cross-sectional dimension much smaller than their length) suspended under the influence of a downward force have long been of interest in the context of such applications as beams, cables supporting suspension bridges and ship moorings [1,2,3]. Such systems display a wide range of behavior, depending on the amount of slack present in the system, the downward force, and the aspect ratio. Previous studies, which have largely been analytical, have been restricted to certain limiting cases of these parameters. Recently work on oscillating nanoscale systems, such as carbon nanotubes [4] and silicon nanowires [5], has opened the possibility of experimentally exploring such vibrations in entirely new regimes. In the present work, we study numerically the oscillations of a one dimensional elastic system over an extensive range of both the slack and force parameters, providing insight into the physics which separates this parameter space into three distinct regimes of behavior.The treatment below is entirely general with illustrative examples taken from parameters relevant to carbon nanotubes. Since their discovery in 1991 [6], nanotubes have found many applications in device technology due to their small size, robust structure and superior elastic properties [7,8,9,10]. Many of these applications involve the use of nanotubes as mechanical oscillators, making theoretical understanding of the vibrational properties of nanotubes in various geometries of current interest [4,11]. Recent experiments [12] have studied the behavior of the transverse vibrations of a suspended nanotube clamped at both ends, under the action of a downward force, as sketched in Figure 1. These suspended nanotubes generally have around 1% slack, denoted in this work by s, which we define to be the ratio of the excess length of the tube to the distance between clamping points.Analytic model and computational techniques -The potential energy of a one-dimensional elastic continuum under a uniform downward force isHere, l represents distance along the system of unstretched length L; u(l), R(l) and z(l) represent the local strain, radius of curvature and vertical displacement, 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000111 111 respectively; E and F are the extensional and flexural rigidities; and f is the downward force per unit length. For single-walled nanotubes with diamet...

The numerous accurate structural data of cobalamins now available allows us to optimize the geometry of these systems, based on a simplified model by using density functional theory (DFT) calculations. This approach, which reproduces the trend of the experimental distances derived from EXAFS and X-ray crystal structures in the corrin macrocycle, permit us to interpret the electronic properties in the NB3−Co−X axial system. In particular, the results are analyzed for cobalamins containing a sulfur ligand which exhibits a ''regular'' trans influence, i.e., when the Co−S bond shortens, the trans Co−NB3 bond lengthens. This feature appears in contrast with an anomalous effect (''inverse'' trans [a]