M. Choueib

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.

Field emission measure of the time response of individual semiconducting nanowires to laser excitation

A simple technique is explored to determine the temporal photo-response, τ, of individual semiconducting SiC and Si nanowires (NWs), with a high time resolution. Laser-assisted field emission (LAFE) from the NWs is first shown to be highly sensitive to continuous laser illumination. Pulsed illumination is then combined with measurements of the total energy distributions to determine τ which were rather large, 4–200 μs. The time response scaled roughly with the square of the NWs length and could be attributed to laser-induced heating. LAFE is thus a new tool for quantifying rapid thermo-optical effects in such nano-objects.

Physical properties of individual anatase TiO2 nanowires investigated by field emission in a transmission electron microscope

The authors present studies on the field emission (FE) mechanism and the FE-induced transformation of individual anatase TiO2 nanowires (NWs). The NWs were synthesized by electrospinning followed by calcination at 500 °C which produces polycrystalline anatase nanofibers as determined by Raman spectroscopy and transmission electron microscopy (TEM) characterization. Nanowires of ∼100 nm in diameter were individually mounted at the apexes of tungsten tips for further physical characterization. The FE experiments were carried out in a TEM which allows the measurement of the FE current while simultaneously observing structural modifications leading to the NW’s destruction. For low currents (below 100 nA), we observe reproducible FE Fowler-Nordheim I/V characteristics. Higher currents (up to 1 μA) can be obtained but sudden destruction of the NW may take place. Our observations show that a thermally-activated transition occurs and leads to rapid re-crystallization phenomena and a variation of the FE characteri...

Field emission as a tool for exploring new phenemena in nanomechanics

Over these past years our team has been pioneering an original orientation which is the use of field emission (FE) as a probe of mechanically resonating nanotubes (NTs), nanowires (NWs) and recently graphene. The resonators and electrical environment can be considered to be a nano-electro-mechanical system (NEMS). My goal here is to promote this subject better within the FE community whereas it has mostly been exposed within the NEMs community.

Electron beam assisted field evaporation of insulating nanowires/tubes

We demonstrate field evaporation of insulating materials, specifically BN nanotubes and undoped Si nanowires, assisted by a convergent electron beam. Electron irradiation leads to positive charging at the nano-objects apex and to an important increase of the local electric field thus inducing field evaporation. Experiments performed both in a transmission electron microscope and in a scanning electron microscope are presented. This technique permits the selective evaporation of individual nanowires in complex materials. Electron assisted field evaporation could be an interesting alternative or complementary to laser induced field desorption used in atom probe tomography of insulating materials.