Synthesis and characterization of carbon nanotubes using plasma-enhanced chemical vapor deposition

Abstract

This thesis presents the synthesis and characterization of carbon nanotubes using Plasma Enhanced Chemical Vapor Deposition (PECVD). The nature of nucleation of carbon nanotubes was found to be influenced by the preparation methods of the catalyst nanoclusters and the diffusion barrier on silicon substrates while the growth mode of the nanotubes governed by the strength of the interaction between the catalyst and the underlying substrate material. Catalyst nanoclusters were formed after a plasma treatment which provided nucleation sites for the growth of the nanotubes. The methodologies to accomplish controlled growth of nanotubes, i.e., diameter, length, density, alignment, and position, were investigated. In general, as the thickness of the initial catalyst layer was increased, the diameter of nanotubes increased but their length and density decreased. The self-induced bias generated in the plasma, the crowding effect, and the substrate bias govern the alignment growth of the nanotubes. Pure nanotubes were grown despite using nitrogen as a carrier gas during the growth process. Nanotubes with uniform diameter were fabricated by controlling the thickness of the iron catalyst film deposited on the silicon substrates. With iron film thickness of 0.5-5 nm, the standard deviation was 11.4-15.7% of the average diameter of the nanotubes. Patterned growth of nanotubes deposited on TEM grid was also demonstrated. The growth of nanotubes was found to be substrate dependent. The growth mechanisms of the “bamboo-type” nanotubes and cedar tree nanotube assemblies were discussed. The nanotubes were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Raman spectroscopy. SEM revealed that nanotubes up to 12 μm in length were grown. The maximum growth rate was ∼100 nm/s. Atomic Force Microscopy (AFM) was utilized to investigate the relationship between the iron film thickness and the size of the iron clusters formed after the plasma treatment. The structures and the diameters of the nanotubes were revealed by TEM. Almost all nanotubes had the “bamboo-type” or polymerized nanobell structure and were multi-walled with an inter-shell spacing of ~0.34 nm. Field electron emission measurements were utilized to evaluate the ability and stability of the nanotubes to emit electrons, i.e., the field emission I-V (current against voltage) and I-T (current against time) performance. The turn-on field and threshold field are defined as the electric field applied to the sample which can obtain a current density of 10 μA/cm2 and 10 mA/cm2 respectively, where 10 mA/cm2 is the current density required for the application of flat panel field emission displays. The field electron emission properties varied with different growth conditions and recipes were studied. The best field emission performance of nanotubes exhibited a turn-on field of 2.8 V/μm and a threshold field of 3.7 V/μm. For comparison, the field electron emission properties of other materials, such as diamond films, diamond cones, silicon cones, SiC nanotubes, coral-like carbon nanotubes, gallium nitride nanowires, were also studied.

Date of Award	15 Jul 2004
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shuit Tong LEE (Supervisor)