CPM Seminar

The discovery of carbon nanotubes is now a decade old, yet nanotubes continue to make headlines because of their outstanding mechanical and electronic properties. Indeed, they may well provide a natural structural component for the emerging field of nanotechnology, as well a convenient paradigm for the testing of Nanometer Electro-Mechanical Systems (NEMS). In my talk I will outline some theoretical investigations of nanotube properties, with a focus on growth, their defects and some simple devices.

While the synthesis of carbon nanotubes takes place via a variety of methods, two trends have emerged. First, the type of nanotube that is formed depends upon the presence/absence of catalysts: multiwalled tubes form in the absence of a a catalyst, while single-wall tubes are the dominant product in their presence. Second the nanocatalytic growth of tubes is open-ended. This is surprising, because a closed-tip structure with no dangling bonds is energetically preferred by a wide margin. Our simulation shows that closure of single-wall tubes is controlled by the nucleation of curvature-inducing defects, such as pentagon pairs whose formation is favoured on relatively narrow nanotubes with diameters of less than 3 nm. This effect, combined with a kinetic stabilization allows for open-ended growth of large diamter, single-wall tubes. In addition, the role of "lip-lip" interactions in controling multiwall nanotube growth will also be discussed.

In order to elucidate their mechanical properties, we have studied the defects of nanotubes under tension. Pristine nanotubes give rise to sets of (5-7) pairs, whose evolution determine the plastic versus brittle behavior of the tubes. In the presence of addimers, a new set of extended defects are formed which ultimately wrap themselves about the tube circumference, thereby evolving into segments of nanotubes with altered helicity. In order to aid in the identification of these defects, we have theoretically generated scanning tunneling microscopy images of these defects and their subsequent evolution.

Turning to their electronic properties, it is now well known that nanotubes may be either metals or semiconductors. In principle, nanotubes therefore have the potential of forming the basis of a future, molectronic system. To explore this possibility, we have investigated the electronic properties of a number of hybrid device structures, in which the nanotubes couple to ferromagnetic and/or superconducting leads.

Friday, February 16th 2001, 15:30
Ernest Rutherford Physics Building, room 115