Cone-shaped nanostructures of mixed composition (nanocones) and largely graphitic nanofibers were synthesized on silicon substrates using iron/platinum alloy nanoparticles as the catalyst in a direct-current plasma enhanced chemical vapor deposition reactor. The catalyst nanoparticles were monodisperse in size with an average diameter of 3 (±1) nm. The nanocones were produced on laterally widely dispersed catalyst particles and were oriented perpendicular to the substrate surface with an amorphous internal structure. The nanocones were produced by gas phase mixing and deposition of plasma-sputtered silicon, nitrogen, carbon, and oxygen species on a central backbone nucleated by the Fe–Pt catalyst particle. Field emission measurements showed that a very high turn-on electric field was required for electron emission from the nanocones. In contrast, the graphitic nanofibers that were produced when silicon sputtering and redeposition were minimized had the “stacked-cup” structure, and well-defined voids could be observed within nanofibers nucleated from larger catalyst particles.