Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (Stone- Wales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen is physisorbed between the graphite planes, but cannot diffuse across a graphitic plane.

Ab-initio electronic structure calculations have been used to evaluate the binding energy of atomic and molecular hydrogen to graphite lattice defects. Results show that graphite defects (Stone- Wales, vacancy) are preferred binding sites with respect to regular lattice sites. We find that molecular hydrogen can be physisorbed between the graphite planes, but cannot diffuse across a graphitic plane.

We studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.

Tight Binding molecular dynamics simulations have been performed on single wall carbon nanotubes, in order to evaluate thermal stability and the effect of the most relevant defects (the single vacancy and a Stone-Wales -SW- defect). The nanotubes are stable up to the graphite instability temperature. Both the considered defects have a large formation energy (EF(vac)=6.10 eV, EF(SW)= 5.55 eV).