Carbon nanostructures (nanotubes, nanofibers and nanosponges) were synthesized onto Si (001) substrates using a microwave assisted plasma enhanced chemical vapor deposition (PECVD) from C2H2-Ar mixtures at low substrate temperatures (120 °C). Catalytic films (Ni and Cu) 3 nm thick were used. Different structures were formed, depending on the C2H2 partial pressure. Atomic force microscopy (AFM) and scanning electron spectroscopy (SEM) were employed for the morphological characterization of the catalytic films and the carbon nanostructures, respectively. Raman spectroscopy was used to identify carbon hybridization states. AFM was used to observe the morphology of the catalytic films. At low C2H2 partial pressures, nanotubes with nanospheres in their tips, growing from nanoholes were seen. With increasing C2H2 pressures, longer nanotubes were observed, reaching lengths from 300 to 500 nm. In their growth, the nanotubes laterally touch each other, forming nanotube bundles, or nanofibers. For the higher C2H2 partial pressures, dense sponge-like (nanosponge) structures, consisting of a large number of bundles, are formed. From the Raman spectra, a mixture of sp2 and sp3 hybridizations were identified. Furthermore, the low substrate temperature at which depositions can be carried out, makes possible growth of carbon nanostructures on materials to which high temperatures would be deleterious.