The electron field emission properties of different graphitic and diamond-like nanostructures films are compared. They are prepared in the same CVD chamber on SiO2/Si(100) and Si(100) flat surfaces, respectively. These nanostructures are thoroughly characterized by scanning electron emission (SEM), transmission electron microscopy (TEM), Raman
spectroscopy, X-ray photoelectron spectroscopy (XPS) and Auger electron
spectroscopy (AES). Films of dense aligned carbon nanotubes by far display
the lowest threshold fields around few V/$\mu $m and the largest emission
currents. Carbon nanofibers, with platelet arrangement of the graphitic
planes parallel to the substrate, exhibit higher emission thresholds around
10 V/$\mu $m. Diamond nanostructures, either modified through ammonia
incorporation within the gas phase or not, exhibit the largest emission
threshold around 25 V/$\mu $m. The high enhancement factors, deduced from
the Fowler-Nordheim plots, can explain the low emission thresholds whereas
limitations to the electron transport ever occur through different processes
(i) surface modifications of the surface, as the transformation of the
SiO2 barrier layer into SiNx in the presence of ammonia
evidenced by XPS; (ii) different orientation of the graphitic basal planes
relative to the direction of electron transport (carbon nanofiber) and (iii)
presence of a graphitic nest at the interface of the carbon nanostructure
and the substrate, observed when catalyst is deposited through mild
evaporation.