Burmaka G. Dusty plasma in afterglow regime and formation of carbon nanotubes in plasma

The thesis is devoted to the theoretical study of the effect of metastable atoms and electron secondary emission at ion-electrode collisions on a dusty plasma afterglow, and the theoretical study of the growth of vertically-aligned single-walled carbon nanotubes in plasma. The properties of dusty plasma in the afterglow regime and the growth rates of forest of single-walled carbon nanotubes in plasma are studied. A model of constant density for an argon dusty plasma afterglow is developed. In particular, physical processes taking place in argon/dusty plasma afterglow, when charge density of dust particles is larger than the density of electrons, are investigated. The influence of electron generation in metastable-metastable collisions on electron density is studied. It is shown that the electron generation in metastable-metastable collisions may significantly affect the electron density in a dusty plasma afterglow due to higher metastable densities in dusty plasmas comparing with those in the dust-free case. This process provides an increase of electron density at the beginning of the dusty plasma afterglow. It is found that the electron temperature decreases faster in the dusty plasma afterglow comparing with that in the dust-free case because of the electron energy loss on dust particles. To analyze the effect of secondary emission on the argon plasma afterglow with large density, a zero-dimensional, space-averaged model is developed. In the model, three groups of electrons in the plasma afterglow are assumed: (i) thermal electrons with Maxwellian distribution, (ii) energetic electrons generated by metastable-metastable collisions (metastable pooling), and (iii) secondary electrons generated at collisions of ions with the electrodes, which have sufficiently large negative voltages in the afterglow. The model calculates the time-dependencies for electron densities in plasma afterglow based on experimental decay times for metastable density and electrode bias. The effect of secondary emission on electron density in the afterglow is estimated by varying secondary emission yields. It is found that this effect is less important than metastable pooling. The case of dust-free plasma afterglow is considered also, and it is found that in the afterglow the effect of secondary emission may be more important than metastable pooling. The secondary emission may increase thermal electron density in dust-free and dusty plasma afterglows on a few ten percentages. The calculated time dependencies for electron density in dust-free and dusty plasma afterglows describe well the experimental results. The diffusion model and numerical simulations are used to clarify the growth mechanism and the differences between the plasma- and neutral gas-grown forest of single-walled carbon nanotubes, and to reveal the underlying physics and the key growth parameters. The model accounts for nonuniformity in deposition of neutral particles on surface of nanotubes from discharge chamber, interactions of hydrocarbon molecules and carbon atoms with etching gas, thermal and ion-induced dissociation of hydrocarbon molecules adsorbed on surface of nanotubes. Using the model, the growth rate of forest of nanotubes, growth rates of the carbon film between nanotubes on the substrate, diffusion length and residence time of carbon atoms on nanotubes surfaces are determined, as functions of external parameters. The results show that the nanotubes grown by plasma-enhanced chemical vapor deposition can be longer than those in the case without plasma due to the effects of hydrocarbon ions with velocities aligned with the nanotubes. It is also found that for long nanotubes, their growth rates at low surface temperatures may be even higher than at high temperatures. That is due to the longer carbon residence times at small temperatures compared to high surface temperatures.