Formation of buried-layer CNTs in porous SiO₂ templates

In this work, we have investigated the sub-surface-layers of porous SiO₂ templates, in which emerge the possibility of carbon nanotube (CNTs) inter-connections. CNTs were grown in Co-catalyst doped, cubic, porous SiO₂ templates using thermal - as well as dc-PECVD. The as-grown templates were sonicated to remove the over-layer CNTs, and then underwent chemical etching. The etched templates were characterized by atomic force microscopy (AFM) which showed patterned structures on the surface. Raman spectrum of these structures was found to be identical to that of the CNTs. The analysis revealed that horizontally patterned CNTs were grown in the pores of SiO₂ templates in sub-surface region of the templates. The CNTs grown in sub-layers of template were termed as buried-layer CNTs. In dc-PECVD treatment, under excess arcing conditions, the porous template became separated from the Pt/Si substrate and lifted up. As a result, vertically-aligned CNTs were generated over, as well as under, the lifted template layers. This lifting-up phenomenon was attributed to the dissociation of plasma-energy preferentially at the interface sites of SiO₂-Pt/Si. Possible mechanisms for this phenomenon are discussed. We speculate that buried-layer CNTs can simultaneously be inter-linked to the over-layer CNTs, the lifted template and the substrate, which could be useful for generating three-dimensional CNT-networks in the skeleton of cubic porous SiO₂ templates.