Samir Farhat

Diameter control of single-walled carbon nanotubes using argon–helium mixture gases

A method is reported for controlling the diameter of single-walled carbon nanotubes (SWCNTs) during the electric-arc-discharge process. Using argon as inert atmosphere provides smaller diameters as compared with those when pure helium is used. Varying the gas mixture from argon to helium changes the diameter distribution to higher values. A linear fit of the average diameter shows a 0.2 A diam decrease per 10% increase in the argon–helium ratio.

Effect of temperature on carbon nanotube diameter and bundle arrangement: Microscopic and macroscopic analysis

The diameter distribution of the nanotubes produced by electric-arc discharge are measured using Raman spectroscopy at various wavelengths. These measurements agree with the results provided by two other techniques: high-resolution transmission electron microscopy and x-ray diffraction. The mean tube diameter shifts more than 0.1 nm with the increase of argon in the inert atmosphere. Some argon concentrations favored the synthesis of metallic tubes with specific diameters. Furthermore, the background gas influences the macroscopic characteristics of nanotube yield and bundle size, as determined by Brunauer–Emmett–Teller surface area measurements and x-ray diffraction. The information collected on nanotube diameter and arrangement is correlated with temperatures calculated using a numerical model of the plasma generated between the two electrodes. Indeed, plasma temperature control during the production process is achieved using argon–helium mixtures as buffer gases. The variation of the gas mixture from p...

Thermodynamics of Nanoparticles: Experimental Protocol Based on a Comprehensive Ginzburg-Landau Interpretation

The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. On the basis of experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Ginzburg-Landau theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on the discrepancies in the literature as it identifies several possible parameters that should be taken into account to properly describe the transformations. For the sake of clarity and standardization, we propose an experimental protocol that must be followed during high-pressure investigations of nanoparticles in order to obtain coherent, reliable data that can be used by the scientific community.

Synthesis of graphene by cobalt-catalyzed decomposition of methane in plasma-enhanced CVD: Optimization of experimental parameters with Taguchi method

This article describes the significant roles of process parameters in the deposition of graphene films via cobalt-catalyzed decomposition of methane diluted in hydrogen using plasma-enhanced chemical vapor deposition (PECVD). The influence of growth temperature (700–850 °C), molar concentration of methane (2%–20%), growth time (30–90 s), and microwave power (300–400 W) on graphene thickness and defect density is investigated using Taguchi method which enables reaching the optimal parameter settings by performing reduced number of experiments. Growth temperature is found to be the most influential parameter in minimizing the number of graphene layers, whereas microwave power has the second largest effect on crystalline quality and minor role on thickness of graphene films. The structural properties of PECVD graphene obtained with optimized synthesis conditions are investigated with Raman spectroscopy and corroborated with atomic-scale characterization performed by high-resolution transmission electron micr...

Determining electron temperature and density in a hydrogen microwaveplasma

A three-temperature thermo-chemical model is developed for analyzing the chemical composition and energy states of a hydrogen microwave plasma used for studying diamond deposition. The chemical and energy exchange rate coefficients are determined from cross section data, assuming Maxwellian velocity distributions for electrons. The model is reduced to a zero-dimensional problem to solve for the electron temperature and ion mole fraction, using measured vibrational and rotational temperatures. The calculations indicate that the electron temperature may be determined to within a few percent error even though the uncertainty in dissociation fraction is many times larger.