ANALYTICAL AND NUMERICAL SIMULATION OF NUCLEATION AND GROWTH OF CARBON NANOTUBES IN A COMPLEX PLASMA

Abstract

Carbon nanotubes (CNTs) are allotropes of carbon with sp2 hybridization and could be considered a sheet of graphene (a hexagonal lattice composed of carbon atoms) rolled up in cylindrical geometry. They were first discovered by Sumio Iijima in 1991 while synthesizing fullerene in an arc discharge apparatus. CNTs have significantly contributed in many scientific fields such as physics, chemistry, mathematical modelling, and material sciences since their discovery. CNTs are the most extensively studied allotrope of carbon as it possess a hollow structure which gives astonishing thermal, electrical, and mechanical properties including high tensile strength, enhanced thermal conductivity compared to diamond, high electrical conductivity, etc. hence, they seem as the best fit for several applications. CNTs are used in biosensors, nano-scale electronics, field emission, and hydrogen storage. They can be synthesized using various methods such as laser ablation, arc discharge, thermal chemical vapour deposition (CVD) or plasma-enhanced CVD. However, CNTs made with the assistance of a reactive plasma medium are known to be vertically aligned. The plasma synthesis conditions influencing the CNT growth and optimizing their field emission properties are studied thoroughly in the current thesis. The thesis explores the effects of plasma operating conditions, plasma parameters, etc. on the nucleation and growth of CNTs in a reactive plasma environment. Various analytical models have been considered in the work which accounts for the plasma aided vertical alignment of nanotubes; plasma pre-treatment of metal catalyst thin film for CNT synthesis and effect of different catalyst nanoparticles; growth of an array of vertically aligned CNTs; and effect of varying gas ratio and different carrier gases, respectively. Each model is divided into two parts. The first part encompasses the kinetics of all the plasma species (ions, electrons, and neutrals) which involves processes such as their ionization, dissociation, and excitation; and kinetics of catalyst nanoparticle including all the variations of the operating conditions. The second part comprises of all the processes leading to vi UMANG SHARMA, Delhi Technological University deposition of nanotube. These processes include adsorption and desorption of hydrocarbon ions and neutral atoms onto/from the surface of catalyst, generation of carbon and hydrogen species on the active area of catalyst, surface diffusion of carbon over the surface of the catalyst, bulk diffusion of carbon into the volume of the catalyst, precipitation of carbon from the saturated catalyst nanoparticle which gives rise to graphitic tubular structure (CNT growth initiates), hydrogen etching of amorphous carbon that hinders the CNT growth, and simultaneous vertical alignment of CNTs due to plasma sheath induced electric field that gives rise to an alignment force. All these process are contained within the model equations and are solved for experimentally determined operating conditions and glow-discharge parameters. It was found that plasma sheath induces and electric field that produces an electrostatic force which is responsible for vertical alignment of nanostructures in plasma. Also this electric field and the consequent force are also dependent upon various plasma parameters. The plasma pre-treatment of catalyst thin films and the effect of process parameters, i.e. power, pressure, gas flow and plasma temperature, was deliberated. The effect of the nature and thickness of the catalyst on CNT growth parameters such as diameter, length, number of CNT walls, etc. was studied. The relation between the catalyst diameter and nanotube walls was established. The substrate temperature, plasma concentration, power and pressure greatly influence the CNT array growth. The change in average length and diameter of CNTs of the array with power and pressures is revealed. The effect of gas ratio and substrate temperature on the width of a carbon film deposited on the substrate between CNTs is also discussed. The field enhancement characteristics of the nanotube array are also studied. It is examined that on increasing the hydrogen ion density, CNT radius decreases due to decease in deposition rate. Also, with the increase in hydrogen ion densities, the hydrocarbon density decreases which endorses the decreases in CNT radius. The results indicate the decline in the CNT radius with increasing gas ratio. Also, the height of CNT decreases with the increasing gas ratio. With the increasing gas ratio i.e., as the concentration of hydrocarbon gas increases, CNTs of smaller radius and height were produced as abundance of carbon atoms are produced compared to hydrogen atoms and the amorphous carbon layer which hinders the UMANG SHARMA, Delhi Technological University vii CNT growth generates rapidly. We have compared the effects of several carrier gases on the structure of CNT using acetylene as the hydrocarbon source. As a result of the study, it was concluded that whereas argon promotes the formation of CNTs, ammonia and nitrogen inhibit their growth. The results of the models have been assessed and compared with the existing experimental observations which accredit the proposed mechanisms. Since the field emission properties of the carbon nanotubes depend on their geometrical aspects, i.e., radius and length, the current work of the thesis can be expanded to construct the ideal vertically aligned carbon nanotubes for their potential application as field emitters. The current research work can potentially be expanded to understand the synthesis of other carbon-based nanostructures.