Anthony Ayari

Raman spectra of misoriented bilayer graphene

We compare the main feature of the measured Raman scattering spectra from single layer graphene with a bilayer in which the two layers are arbitrarily misoriented. The profiles of the 2D bands are very similar having only one component, contrary to the four found for commensurate Bernal bilayers. These results agree with recent theoretical calculations and point to the similarity of the electronic structures of single layer graphene and misoriented bilayer graphene. Another new aspect is that the dependance of the 2D frequency on the laser excitation energy is different in these two latter systems.

Digital and FM Demodulation of a Doubly Clamped Single-Walled Carbon-Nanotube Oscillator: Towards a Nanotube Cell Phone

Electromechanical resonators are a key element in radio-frequency telecommunication devices and thus new resonator concepts from nanotechnology can readily find important industrial opportunities. Here, the successful experimental realization of AM, FM, and digital demodulation with suspended single-walled carbon-nanotube resonators in a field-effect transistor configuration is reported. The crucial role played by the electromechanical resonance in demodulation is clearly demonstrated. The FM technique is shown to lead to the suppression of unwanted background signals and the reduction of noise for a better detection of the mechanical motion of nanotubes. The digital data-transfer rate of standard cell-phone technology is within the reach of these devices.

High Q factor for mechanical resonances of batch-fabricated SiC nanowires

The authors present here the measurements of high mechanical Q factors for singly clamped, batch-fabricated SiC nanowires measured by field emission (FE) in ultrahigh vacuum. The resonances of two nanowires, glued to the ends of tungsten support tips, were electrostatically excited and detected by the variation in the FE microscopy (FEM) images. Low amplitude oscillations were measured by numerical analysis of the FEM image blurring during frequency scans through the resonances. This avoided the artificial broadening of the resonances by nonlinear effects. A room temperature Q factor of 159 000 was achieved after high temperature in situ cleaning.

Evidence for Poole-Frenkel conduction in individual SiC nanowires by field emission transport measurements

In this paper we examine carrier transport mechanisms in individual Silicon Carbide nanowires (NWs) by an original use of field emission (FE). Total energy distributions were measured as a function of temperature and extraction voltage allowing us to determine the voltage drops along the NWs and thus the temperature-dependent current-voltage (I-V-T) characteristics. The measurements were analyzed using different transport mechanisms of which only the Poole–Frenkel model gives an excellent fit. The dielectric constant was estimated for several samples at ɛ~10 in excellent agreement with the bulk value. The characteristic trap energies, Ea, were determined from the I-V-T data to be ∼0.3 eV. In general this work shows how FE can be used for transport measurements on individual semiconducting NWs.

Current Saturation in Field Emission from H-Passivated Si Nanowires

This paper explores the field emission (FE) properties of highly crystalline Si nanowires (NWs) with controlled surface passivation. The NWs were batch-grown by the vapor-liquid-solid process using Au catalysts with no intentional doping. The FE current-voltage characteristics showed quasi-ideal current saturation that resembles those predicted by the basic theory for emission from semiconductors, even at room temperature. In the saturation region, the currents were extremely sensitive to temperature and also increased linearly with voltage drop along the nanowire. The latter permits the estimation of the doping concentration and the carrier lifetime, which is limited by surface recombination. The conductivity could be tuned over 2 orders of magnitude by in situ hydrogen passivation/desorption cycles. This work highlights the role of dangling bonds in surface leakage currents and demonstrates the use of hydrogen passivation for optimizing the FE characteristics of Si NWs.

Gold contact to individual metallic carbon nanotubes: A sensitive nanosensor for high-pressure

A strong and universal piezoresistive effect is evidenced for individual metallic carbon nanotubes contacted to gold electrodes through high contact resistances. The effect is well explained through a pressure modulation of the tunnel barrier width at the contact. The pressure dependence (−16%/GPa) is much stronger than for standard resistive high pressure gauges, and it depends neither on the initial resistance nor on the pressure transmitting medium.

Simple modeling of self-oscillations in nanoelectromechanical systems

We present here a simple analytical model for self-oscillations in nanoelectromechanical systems. We show that a field emission self-oscillator can be described by a lumped electrical circuit and that this approach is generalizable to other electromechanical oscillator devices. The analytical model is supported by dynamical simulations where the electrostatic parameters are obtained by finite element computations.

Adhesion energy of single wall carbon nanotube loops on various substrates

The physics of adhesion of one-dimensional nano structures such as nanotubes, nano wires, and biopolymers on different material substrates is of great interest for the study of biological adhesion and the development of nano electronics and nano mechanics. In this paper, we present force spectroscopy experiments of a single wall carbon nanotube loop using our home-made interferometric atomic force microscope. Characteristic force plateaux during the peeling process allows us to access to quantitative values of the adhesion energy per unit length on various substrates: graphite, mica, platinum, gold and silicon. By combining a time-frequency analysis of the deflexion of the cantilever, we access to the dynamic stiffness of the contact, providing more information on the nanotube configurations and its intrinsic mechanical properties.

Frequency modulated self-oscillation and phase inertia in a synchronized nanowire mechanical resonator

Synchronization has been reported for a wide range of self-oscillating systems. However, even though it has been predicted theoretically for several decades, the experimental realization of phase self-oscillation, sometimes called phase trapping, in the high driving regime has been studied only recently. We explored in detail the phase dynamics in a synchronized field emission SiC nanoelectromechanical system with intrinsic feedback. A richer variety of phase behavior has been unambiguously identified, implying phase modulation and inertia. This synchronization regime is expected to have implications for the comprehension of the dynamics of interacting self-oscillating networks and for the generation of frequency modulated signals at the nanoscale.

Role of fluctuations and nonlinearities on field emission nanomechanical self-oscillators

A theoretical and experimental description of the threshold, amplitude, and stability of a self-oscillating nanowire in a field emission configuration is presented. Two thresholds for the onset of self-oscillation are identified, one induced by fluctuations of the electromagnetic environment and a second revealed by these fluctuations by measuring the probability density function of the current. The ac and dc components of the current and the phase stability are quantified. An ac to dc ratio above 100% and an Allan deviation of