Field Emission from a Carbon Nanofiber/Carbon Nanocone Composite Structure Fabricated by a Two-Step Growth Process

A composite structure of carbon nanofiber (CNF) and carbon nanocone (CNC) has been successfully fabricated on the Co ∕ Al ∕ Si ( 100 ) substrates by using microwave plasma-enhanced chemical vapor deposition. A hydrogen pretreatment is required to form Co–Al nanoparticles on the substrate at 450 ° C , which is essential for the subsequent CNF/CNC growth. The CNF/CNC composite structure is fabricated by a two-step process in which bundles of CNFs are formed on the as-pretreated substrate at 450 ° C and a CNC structure is subsequently formed at 650 ° C by a coalescence mechanism. Raman spectra indicate that sp 2 bonds in C = C chains play a role in connecting CNFs in each bundle during the formation of CNCs. An upward growth of CNFs on the top of CNCs occurs during coalescing CNFs. The CNF/CNC composite structure exhibits very good field emission characteristics, better than CNFs. The optimum turn-on field for the CNF/CNC structure is 2.5 V ∕ μ m , which is defined as the electric field at a current density of 10 μ A ∕ cm 2 . The current density is 4.5 mA ∕ cm 2 at 6 V ∕ μ m .