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 degrees 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 degrees C and a CNC structure is subsequently formed at 650 degrees 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/mu m, which is defined as the electric field at a current density of 10 mu A/cm(2). The current density is 4.5 mA/cm(2) at 6 V/mu m. (c) 2005 The Electrochemical Society. [DOI: 10.1149/ 1.2131823] All rights reserved.