Fabrication and Field Emission Properties of Carbon Nanotube Cathodes
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215 Mat. Res. Soc. Symp. Proc. Vol. 593 ©2000 Materials Research Society
EXPERIMENT The nanotubes grown using laser ablation [8, 9] and arc discharge [2, 10] are typically collected as a black spongy mat or as loose powders. A spray deposition technique was developed to process bulk nanotubes into films that can be used as cold cathodes. A simple diagram of the spray deposition apparatus is shown in Figure 1(a). Nanotubes, suspended in organic solvent by ultrasonication, are sprayed onto a heated substrate approximately twelve inches from the spray nozzle. A standard glass "atomizer" (Kontes Co.) was used with 10-20 psi of Argon. Any organic solvent that reaches the substrate is quickly evaporated due to the heat. Figure 1(b) is a SEM (scanning electron microscope) micrograph of a spray deposited SWNT film. The filaments are actually bundles of SWNTs typically 20-30 nm in diameter and many microns long. The SWNTs used in this study were purified using an ultrasonic filtration technique [I I]. (a)
(b)
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Figure 1. (a) Experimental set-up for nanotube spray deposition and (b) a SEM micrograph of a spray deposited SWNT film. Each filament is a 30-50 nm bundle of SWNTs. Recent progress in the production of aligned MWNTs using chemical vapor deposition (CVD) techniques [12-14] prompted us to investigate CVD techniques as an alternative means of fabricating nanotube films. CVD techniques are desirable because the nanotubes are grown in the form of a film, and CVD methods can be readily scaled to manufacturing levels. We have successfully grown films consisting of highly aligned MWNTs using microwave plasma enhanced CVD (MPE-CVD) with a C2H2/NH 3 chemistry [15]. The system that was used to grow the nanotubes consists of a 2.45-GHz 5kW microwave power supply with a rectangular waveguide that is coupled to a cylindrical growth cavity, a 6-inch inner-diameter stainless-steel chamber, and a molybdenum substrate stage with a RF heater that allows independent control of the substrate temperature. During the growth, the substrate temperature was maintained at 8250 C, and the chamber pressure was kept at 20 ton'. Total gas flow rates of acetylene (C2H 2) and ammonia (NH3) were maintained at 200 sccm, and the mass flow ratio of C2 H2 over NH 3 was varied in the range of 10-30%. Thin cobalt films applied by
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sputtering were used as catalyst for the nanotube growth. The cobalt coatings could also be pre-patterned by lithography or shadow mask to allow selective growth of nanotubes. The growth typically lasted from 30 seconds to 10 minutes, depending on how long the tubes were desired to grow. We used both SEM and TEM (transmission electron microscopy) to examine the surface morphologies and internal structures and X-ray diffraction to evaluate the alignment of the nanotubes. The aligned MWNTs shown in Figure 2 were grown for two minutes and are approximately 10 k.tm long and 30 nm in diameter. Figure 2 is an
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