Abstract
Carbon nanotubes were synthesized in a mixture of methane and hydrogen as precursors by the microwave plasma enhanced chemical vapor deposition (MPECVD) system equipped with a dc bias to the substrate of silicon. By varying carbon nanotubes form, catalyst particles, and electroless plating time, the field emission characteristics were compared. The onset emission field as low as 1.8 Vμm-1 can be achieved (through the form of a “tubes on tube”) in comparison with other carbon-based electron field emitters. This fact is mainly attributed to the high aspect ratio, low tip radius of curvature, well-aligned morphology, and the removal of hydrogen from the carbon nanotubes. Carbon nanotubes catalyzed by electroless plating Pd for 20 minutes have the highest array density, best morphology, and field emission characteristics than other methods or metal catalysts in our MPECVD system. Raman scattering spectra show that all the carbon nanotubes have a graphite structure. SAD and X-ray analysis evidenced that Pd2Si and Ni3Si2 are also the good catalysts for carbon nanotubes synthesis. TEM analysis shows a bamboo-shaped structure of the “tubes on tube” carbon nanotubes. The possible growth mechanisms of the “tubes on tube” form carbon nanotubes will be discussed.