As it is well known, carbon nanotubes may be one fold (Single Wall Carbon Nanotubes, SWCNT) or contain several cylinders nested one inside another (Multi Wall Carbon Nanotubes, MWCNT). SWCNT's, in many cases, self-organize into crystalline bundles (set of a few to a few hundred aligned tubes arranged in a two-dimensional triangular lattice in the plane perpendicular to their common axes).
A thorough understanding of carbon nanotubes (CNT's) includes the detailed and comprehensive characterization of their possible deformations due to interactions with a substrate or with other tubes, and their stability under thermal treatment or chemical agents.
In this communication we concentrate on i) the structural characteristics and ii) the structural transformations under thermal treatment of bundles of SWCNT's. i) Deformations of the tubes away from their ideal circular cross sections may have non trivial effects in their properties such as conductivity or phonon spectrum. We have being able to synthesize novel crystalline bundles of 'polygonized' SWCNT's. Our finding opens up the question of what is the equilibrium configuration of a lattice of aligned tubes and the possibility of the existence of several metastable structures which could be obtained depending on the growth conditions. To shed some light on this problem we have performed extensive molecular dynamics simulations of lattices of monodisperse armchair and zigzag SWCNT's as a function of tube diameter. We find several metastable structures of the lattice characterized by different tube cross sections, hexagonal, rounded-hexagonal and circular, and increasing cell volume. The competition between different tube shapes is analyzed and compared to experiments. ii) On the other hand, CNT's are metastable; (the most stable form of carbon is graphite). Due to their metastable character CNT's may transform into more stable structures under the appropriate annealing conditions. We have found that bundles of SWCNT's coalesce forming MWCNT's, containing from two to six nested tubes, under thermal treatment at high temperatures.
This structural transformation is confirmed by extensive Molecular Dynamics (MD) simulations. The simulations suggest a 'patching--and--tearing' mechanism for the SW-- to MWCNT's transformation underlying the 'concerted' coalescence of the tubes that begins with their polymerization. Tubes of different sizes and chiralities are considered.
Density functional theory is used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single wall carbon nanotubes. Molecular physisorption is predicted to be the most stable absorption state, with the molecule at a distance of 5-6 a.u. from the nanotube wall. The physisorption energies outside the nanutobes are about 0.07 eV, and twice as large inside. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state with the molecule dissociated has also been found, with the H atoms much closer to the nanotube wall. However, this dissociative state is separated from the physisorption state by an activation barrier of 2 eV or more. The dissociative chemisorption weakens C-C bonds, and the concerted effect of many incoming molecules with sufficient kinetic energies can lead to the scission of the nanotube.