Synthesis and properties of carbon nanotube arrays grown from sandwich catalyst stacks

Carbon nanotubes (CNTs) are ideal field emission sources because of combining a high aspect ratio with chemical inertness, good electron and thermal conductivity and mechanical strength. A stable field emission current of 1 A from one multi-walled CNT and a high emission current density of 1 A/cm/sup 2/ from CNTs emitter arrays have been reported. Vertically aligned, sparse CNTs are proposed as being the ideal field emitters. Well-aligned CNTs have been made with hot filament chemical vapor deposition (HFCVD), plasma enhanced chemical vapor deposition (PECVD) and microwave plasma chemical vapor deposition (MWPCVD). However, up to now it is difficult to obtain CNT arrays with uniform and large current density, because of the strong field shielding effect due to the high density of these well-aligned CNTs. We report in this paper a method to grow CNT arrays, which are freestanding, well-aligned, and vertically-oriented. Furthermore, these CNTs have a uniform length and diameter. Growth has been realized at lithographically defined sites on metallic, semi-conducting and glass substrates. A sandwiched catalyst structure and MWPCVD is utilized to form multi-walled and also single-walled carbon nanotubes. Figure 1a) and 1b) is a scanning electron microscope (SEM) image of our as-grown CNTs using sandwiched catalyst structure. It is feasible to grow CNTs between two terminals directly, as shown in figure 1a), and grow straight vertical CNT bundles with small diameter of less than 2 m, as shown in figure 1b). The transmission electron microscope (TEM) image of the as-grown CNTs shows typical multi-walled CNTs lattice structure with few defects, as shown in figure 1c). After treating the as-grown CNTs in a oxygen radio-frequency plasma, a highly stable field emission current density of more than 6A/cm/sup 2/ at a electric field of 7.4V/m with a total field emission current of >600 A was obtained, as shown in figure 3. The work functions of the CNTs before treatment and after treatment were 4.56 eV and 4.0 eV respectively, measured by XPS. Our experiments indicate a fabrication route for largely improving the field emission characteristics of CNT-based field emitters. Further more, this new technology offers a simple and scalable pathway to create nano-sized electronic circuits and devices with a 3D-structure.