ABSTRACT
Rolled graphene sheets result in cylindrical carbon nanostructures (CNSs) of different morphologies like single-and multiwalled nanotubes, nanobers, or tapes. In catalytic chemical vapor deposition (CCVD) method one may obtain all these morphologies of cylindrical nanostructures by decomposition of carbon bearing precursor gas on the surface of the catalyst nanoparticles, followed by the formation of nanostructures. Depending on the size and composition of the catalyst particles as well as the prevailing growth conditions in the deposition chamber including the ow rate of the carbon bearing gas, the growth of a specic morphology of cylindrical nanostructure is favored. There are a large number of investigations using catalysts of nanoparticles of transition metals such as cobalt, nickel, iron, and their oxides. A number of other metals have also been used. Recent in situ investigations inside environmental transmission electron microscope (ETEM) and the molecular dynamic (MD) simulations have revealed the possible steps leading to the formation of CNSs. But still there are unresolved issues pertaining to the necessity of surface melting of catalyst nanoparticles claimed to be responsible for substantial change in shape of the catalyst nanoparticles inside growing nanotubes and to the necessity of carbide formation as an intermediate step as well as the possibility of survival of oxide nanoparticles in the reducing environment during growth of nanostructures. There are diverse claims with regard to these issues based on experimental ndings by different research workers. The studies carried out so far indicate that one may produce cylindrical CNSs of specic morphology by CCVD method if one tailors the catalyst nanoparticles and controls the growth condition appropriately. However, more research is necessary
to attain the capability of controlled production of a specic morphology, which is extremely important from technological standpoint.
