Molecular growth paths and dust‐particles nucleation precursors in Ar/C₂H₂ low pressure discharges

Abstract

A numerical model was used to investigate the effect of acetylene concentration in the feed gas on molecular growth and solid particle nucleation in argon-acetylene CCRF discharges. Results showed that for large acetylene conversion yields and small acetylene concentrations in the discharge, the ionization kinetic is driven by Penning process and the molecular growth is governed by neutral polyynes. The cluster size distribution predicted shows that nucleation is unlikely in this situation. On the opposite, for limited conversion yields and significant acetylene concentrations in the discharge, the ionization kinetic is driven by electron-impact process; the molecular growth is governed by positive ions and leads to significant concentrations of neutral polyynes and enhanced nucleation rate.

Molecular dynamics simulations of reactive neutral chemistry in an argon‐methane plasma

“…Thus, performing simulations without argon has the advantage of reducing the simulation time, while keeping similar results for the formation of new species. Based on these results, argon is excluded from the simulation box, and simulations are run with the same parameters as in box 2 at temperatures of 300, 400, 500, and 1000 K. A total of 2683 molecules with the same composition ratios as in box 2 were introduced into a simulation box of size 10 × 10 × 10nm 3 . This box size and the number of molecules were determined by the relation ( 9) considering a pressure of 4% of the total gas pressure since 4% of methane is dissolved in 96% of Ar (fluid model).…”

“…These plasmas are, therefore almost uniform along the electrode radius and may be described using a one-dimensional model along the gap direction. The details of the model used in this work are given in reference, [3,33] and we will limit ourselves here to describe its main characteristics. The plasma is generated under a pressure above 10 Pa.…”

“…Therefore, the discharge module includes 10 continuity equations, while the reactive transport module involves 12 continuity equations. The boundary conditions required for the continuity equations, the electron energy equation, and Poisson's equations are thoroughly discussed in Tetard et al [3,33] Basically, we assume that the ions recombine totally on the electrodes; that the stable neutral species do not react on the electrode surfaces, while the radical species recombine on the electrode with probability values given by De Bie et al and reference therein. [36] Since the discharge characteristic time is much smaller than the neutral species characteristic times, the plasma composition may evolve over durations corresponding to a huge number of RF periods, that is, much larger than the number of RF periods required to reach a permanent discharge regime.…”

“…A 1D fluid model calculates the initial neutral composition of the plasma. [3,33] The following section briefly describes the 1D fluid model giving the initial neutral species and their mole fractions. Then, the molecular dynamics simulation using the REBO potential is described, and the procedure for setting up the simulation box based on the fluid model is detailed.…”

“…Reactive low-temperature plasmas containing hydrocarbon gases such as methane or acetylene are weakly ionized gases containing atoms, molecules, ions, electrons, and sometimes, solid particles of nanometer or micrometer size. [1][2][3][4] These plasmas are of great scientific and industrial interest because they are used in many applications. Deposition of thin films of amorphous hydrogenated carbon used for tribological materials, [5,6] passivation layers, [7] cold-field emission cathodes for flatscreen displays, [8,9] or for the deposition of other carbonbased structures such as diamond-like carbon (DLC) films, [7,10] carbon nanotubes, nanowalls, and so on [11,12] have been addressed.…”