The
interest in carbon nanotubes (CNTs), which are seamlessly rolled up sheets of
graphene (see
below for a picture), originates
from their unique properties such as electronic and thermal transport and the
fact that they are strong and light weight.
Depending on their structure (chirality), these tubes are either metallic
or semiconducting. This, together
with their nanometer diameters, makes them ideal for producing nanoscale
electronic devices. Since they are light
and strong they are also used to reinforce structures such as plastics
(polymers).
Two
of the challenges that remain in carbon nanotechnology are:
1)
Development of methods to produce CNTs having a desired
property. Present methods produce a mixture of metallic and semiconducting
tubes.
2) Controlled processing of polymer nanocomposites to produce products with desired properties (such as strong textiles that conduct electricity and heat).
Use of carbon nanotubes in electronic circuitry requires that one knows the electrical properties of the nanotubes. Since mixtures of carbon nanotubes that have a range of diameters and properties are grown in all known production methods, it is necessary to separate out the nanotubes with the desired property from all other nanotubes. Ultimate control of these separation techniques will allow us to separate out nanotubes based on their chiral (n,m) numbers.
The scheme below illustrates a possible method of separating armchair from zigzag nanotubes (two structures or chiralities). In this scheme both types of nanotubes are functionalised at the end of the nanotube. If the bond between the zigzag nanotube and the polymer is weaker than between the armchair nanotube and the polymer, then the zigzag nanotube-functional group bond breaks at lower temperatures and the zigzag nanotubes can be separated from the armchaor tubes.
Our electronic structure theory calculations show that this type of separation is possible. The figure below shows the nanotube-functional group bond energies for a variety of (n,n) armchair and (n,0) zigzag nanotubes. It is clear that the armchair bonds are 0.4 eV stronger that the zigzag bonds for nanotubes with the same diameter. This is sufficient to allow for separation.
Separation of nanotubes based on their chirality can be avoided if one could grow the desired nanotube (i.e., having the desired structure) in the absence of other nanotubes. A deep understanding of the carbon nanotube growth mechanisms will allow us to achieve this goal. The figure below shows results of our simulated growth of carbon nanotubes from an iron-carbide particle. These simulations allow us to understand many details of the growth procedure, such as the supersaturation of the iron particle in carbon before nanotubes can be nucleated, and the role of the particle in maintaining an open end of the carbon nanotube (which is critical for continued nanotube growth).
Collisions between ambient gas molecules and nanotubes
are important during nanotube growth and are are important when using CNTs as sensor devices. It is thus important to understand scattering dynamics between gas molecules and tubes. The movies below (click on the figures) show collisions between a Xe atom (in red) and a carbon nanotube. In the first movie the nanotube is at 0 K so that the impact induced indentation of the nanotube by the Xe is clearly seen. The second movie shows a single-encounter collision with a tube at 1300 K. The frames are more frequent in this movie (one frame per 10 fs) so that the large amplitude thermal motion of the tube atoms are clearly seen. Two encounters of a multiple-encounter collision are shown in the third movie. Expeirmental results have also ahown that these large amplitude collisions change the electrical (scattering) properties of the CNTs, hence making them very accurate sensors of inter gases.
More information can be found in our publications.
This research, which is done in collaboration with experimental groups, has been or is funded by the Swedish Foundation for Strategic Research, Honda Research Institute Inc, and The Swedish Research Council.