A. Maiti

GROWTH OF CARBON NANOTUBES : A MOLECULAR DYNAMICS STUDY

Abstract Molecular dynamics with realistic many-body atomic potentials was used to study the growth of carbon nanotubes. Analysis of the bond switching and ring migration processes has led to an identification of tube growth mechanisms. Wide tubes, initially open, were found to grow straight maintaining an all-hexagonal structure, while narrow tubes were found to develop permanent pentagonal rings that lead to tube closure upon further deposition. Continued deposition on the top of a closed tube yields a disordered cap structure, implying that open tubes are critical for defect-free growth.

Structural defects and the shape of large fullerenes

Abstract Motivated by the recent experimental observation of nearly spherical multi-layer fullerenes, we investigate theoretically the role of thermally generated structural defects on the shape and stability of large fullerenes by equilibrium statistical mechanics methods. Low energy defects are generated via atom insertions and bond rotations and the resulting structures are relaxed using realistic atomic potentials. The lowest defect formation energies are about 1 eV. A free energy analysis shows that large fullerenes will contain a substantial number of such defects at high temperatures. The presence of these defects effectively spreads out the sharp “kinks” associated with the faceted ground state structure, leading to a more spherical cage of a uniform curvature.

Theoretical Investigations of Carbon Nanotube Growth

Abstract The growth of carbon nanotubes was investigated using a variety of complementary simulation techniques. Currently, a number of experimental methods are used to synthesize carbon nanotubes suggesting that different mechanisms play a role in their formation. However, it has been shown that growth of nanotubes takes place primarily at the open-ended tips of nanotubes. Ab initio simulations show that the high electric fields present at the nanotube tips in carbon arc discharges cannot be responsible for keeping the tubes open. Rather, the opening and closing of tubes is controlled by the formation of curvature-inducing defects such as adjacent pentagon pairs. On narrow tubes, the formation of such defects is favored leading to the rapid closure of the tubes. By contrast, the formation of hexagons, which lead to straight open-ended growth is favored on large-diameter tubes, with an estimated crossover radius of about 3 nm. Large-scale molecular dynamics and kinetic Monte Carle simulations have been used to verify these ideas. We have also explored the role of the so-called lip–lip interactions during growth. Such an interaction is important in producing multiwalled nanotubes, where the interaction between two open nanotube tips leads to the formation of a network of bonds. Simulations show that such an interaction is indeed significant, but does not provide the additional stabilization required for straight, open-ended, multiwalled nanotube growth. Finally, we consider the formation of nanotubes in the presence of large and small catalytic particles. In the former case, growth is believed to take place via a root-growth mechanism, while the direct adsorption and extrusion of carbon from the vapor dominates the latter. Both mechanisms lead to the formation of small-diameter, single-wall nanotubes.

ZERO AND FINITE TEMPERATURE STUDY OF SINGLE FULLERENE CAGES AND CARBON “ONIONS” — GEOMETRY AND SHAPE

Scaling arguments are used to show that above a critical size of several thousand atoms, there is a stability crossover from single to multilayer cages. Conjugate gradient minimization using a classical three-body interatomic potential, as well as tight-binding electronic structure calculations yield ground-state configurations for large fullerene shells that are polyhedral with clearly faceted geometry. The structure, energetics and configurational entropy associated with low-energy defects are calculated and the number of defects estimated as a function of temperature. The role of these thermally generated defects on the shape of large fullerenes is investigated in order to explain the nearly spherical shapes of the newly discovered carbon “onions”.

Structural transformations, reactions, and electronic properties of fullerenes, onions, and buckytubes

Abstract We describe the results of extensive quantum molecular dynamics calculations of the properties of fullerenes and microtubules. The topics to be discussed include: (i) stability of C 60 isomers and barriers to isomerization; (ii) reactivity of C 60 and C 58 with C 2 and C 3 , and its implications on the formation and growth of fullerenes; and (iii) atomic and electronic structure and doping of semiconducting microtubules. We also discuss the structures, stabilities and atomic transformations of large multishell fullerenes and offer an explanation for the formation of spheroidal “onions” under high fluence electron irradiation conditions. The last results, which involved calculations for up to ∼ 15 000 atoms, were obtained using classical three-body potentials.