arXiv subjects

Shoaib Ahmad

Publications and source records attributed to Shoaib Ahmad.

39 records · Page 3Linked to original sources

arXiv2016

Criteria for the growth of fullerenes and single-walled carbon nanotubes in sooting environments

The spherical curvature induced by pentagons in corannulenes and hexagonal sheets is shown to be the basic constituent that controls the growth of fullerenes and single-walled carbon nanotubes (SWNTs) in soot forming and carbon vapour environments. Formation of the initial ring of five or six atoms is the essential step which with the addition of further pentagons and hexagons determines whether a spinning fullerene is to be formed or the cap that lifts up and leads to the formation of an SWNT. A continuum elastic model is developed to determine the criteria for the growth of these structures. The observed dominance of the growth of 14 Å diameter armchair SWNTs in sooting and carbonaceous environments is explained by using the nanoelastic model of C shells.

cond-mat.mes-hall↗

arXiv2016

Continuum elastic model of fullerenes and the sphericity of the carbon onion shells

A continuum elastic model of fullerenes is presented by utilizing the analogy between the closed carbon cages and elastic shells. We derive expressions for the curvature related strain energies Ep of the pentagonal protrusions. We propose to explain the observed sphericity of the carbon onions shells as opposed to the predicted protrusions around the pentagonal defects on the basis of our continuum elastic model of fullerenes. In our model the energy inherent in the pentagonal protrusions Ep is due to the stretching and bending of the shell and shown to be a function of the structural parameters. It also defines the upper limit on the size of the free-standing fullerenes. Using Ep and the topological arguments, we show that the pentagonal protrusions will be smoothed out, resulting in spherical shells of the carbon onions denoted as C60@C240@C540@C960@C1500 ,... .

cond-mat.mes-hall↗

arXiv2016

The role of the C2 gas in the emergence of C60 from the condensing carbon vapour

A model has been developed that illustrates the emergence of C60 from the condensing carbon vapor. It is shown to depend upon the decreasing heats of formation for larger cages, exponentially increasing number of isomers for fullerenes that are larger than C60, large cages buckling induced by the pentagon-related protrusions that initiate fragmentation, the structural instability induces fragmentation that shrinks large cages and an evolving gas of C2 that is crucial to the whole process. The model describes a mechanism for the provision and presence of plenty of C2 during the formation and fragmentation processes. The bottom-up formations of large cages followed by the top-down cage shrinkage are shown to be stable, dynamical processes that lead to the C60 dominated fullerene ensemble.

cond-mat.mes-hall↗