Showing papers by "Pulickel M. Ajayan published in 2000"

New metallic allotropes of planar and tubular carbon

Abstract: We propose a new family of layered sp(2)-like carbon crystals, incorporating five-, six-, and seven-membered rings in 2D Bravais lattices. These periodic sheets can be rolled so as to generate nanotubes of different diameter and chirality. We demonstrate that these sheets and tubes are metastable and more favorable than C-60, and it is also shown that their mechanical properties are similar to those of graphene. Density of states calculations of all structures revealed an intrinsic metallic behavior, independent of orientation, tube diameter, and chirality.

Coalescence of Single-Walled Carbon Nanotubes

Abstract: The coalescence of single-walled nanotubes is studied in situ under electron irradiation at high temperature in a transmission electron microscope. The merging process is investigated at the atomic level, using tight-binding molecular dynamics and Monte Carlo simulations. Vacancies induce coalescence via a zipper-like mechanism, imposing a continuous reorganization of atoms on individual tube lattices along adjacent tubes. Other topological defects induce the polymerization of tubes. Coalescence seems to be restricted to tubes with the same chirality, explaining the low frequency of occurrence of this event.

Growth of carbon micro-trees

Abstract: Carbon deposition under extreme conditions causes tree-like structures to spring up. The complex deposition and growth of graphitic carbon has come up with several surprises in recent years, including the growth of fullerenes and carbon nanotubes1. Here we describe the spectacular growth of micron-sized tree-like carbon structures produced by chemical vapour deposition of methane without the use of any catalysts. Our results show that carbon surfaces can be tailored while they are growing simply by controlling the rate of deposition from the vapour phase.

Dynamic behavior of nickel atoms in graphitic networks

Abstract: The dynamic behavior of nickel atoms in graphitic carbon onions, observed by in situ atomic-resolution electron microscopy, shows the formation of stable new C-Ni phases. Nickel is observed to take substitutional in-plane positions in graphene layers, forming a planar graphitelike C-Ni lattice. Evidence is furthermore seen for the formation of a cubic C-NI phase, suggesting a possible phase transformation in C-Ni from a graphitelike to a diamondlike structure. The stability of the planar phases is shown by first-principles calculations which also indicate that the C-NI planes are metallic.

Carbon Nanotubes. From Macromolecules to Nanotechnology

Abstract: The discovery of fullerenes (1) provided exciting insights into how architectures built from pure carbon units can result in new symmetries and structures with remarkable physical properties. Carbon nanotubes represent the most striking example (2). A carbon nanotube can be considered as the ultimate fiber, reflecting highly organized, near ideal sp2 bonded carbon structure. The organization of the hexagonal honeycomb carbon lattice into cylinders with helical arrangement of hexagonal arrays has created a very unusual macromolecular structure that is by far the most superior carbon fiber ever made.