Characterizing Multi-Walled Carbon Nanotube Synthesis for Field Emission Applications

Abstract

The first evidence of carbon nanotubes comes from transmission electron microscope (TEM) micrographs published by Radushkevich in 1952 in the Russian Journal of Physical Chemistry (Radushkevich, 1952). However, without the capability to reliably produce, characterize, or use these carbon nanotubes little was done besides document their existence until 1991. Iijima effectively rediscovered or introduced carbon nanotubes to the scientific community as a by-product of an electric arc discharge method of synthesizing C60 fullerenes (Iijima, 1991). Since Iijima published his seminal article in Nature identifying multi-walled carbon nanotubes (MWCNT) in 1991, followed by the more significant discovery of single-walled carbon nanotubes (SWCNT) in 1993, research into the properties and applications of carbon nanotubes has flourished. In 1995, only four years after carbon nanotubes were introduced to the scientific community by Iijima, de Heer et al demonstrated the field emission capabilities of carbon nanotubes with the fabrication of a small electron gun using multi-walled carbon nanotubes (de Heer, 1995). CNTs have many unique physical and electrical properties making them ideal candidates for field emission sources. There are many potential applications for CNT based field emission devices ranging from flat panel displays to charge neutralization for electric propulsion on satellites. This research effort focuses on the growth of multi-walled carbon nanotubes for field emission. The potential applications being considered require that the CNT synthesis method be compatible with conventional substrate materials, chiefly silicon, and micro-frabrication processes to allow integration with conventional electronic devices. Of the many documented CNT synthesis methods, Chemical vapor deposition (CVD) synthesis occurs at low enough temperatures to facilitate silicon substrates. For this particular study two CVD processes, thermal chemical vapor deposition (T-CVD) and microwave plasma enhanced chemical vapor deposition (MPE-CVD), are used to produce MWCNT films or carpets. The CNT growth process begins with substrate preparation. Each silicon substrate is first coated with a barrier layer of titanium or chrome. This thin film acts as an adhesion layer and as a diffusion barrier. It also facilitates the cathode connection to the CNTs during field emission tests. CNT growth via any CVD method requires a catalyst for the carbon atoms to nucleate around. The transition metals Fe, Ni, Co, and alloys composed of one or more of these elements have proven effective as catalyst materials. For this effort, only