Computational Studies of Defects in Nanoscale Carbon Materials
Energetics and Electronic Properties of Doped Carbon Nanotubes
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Doping into semiconducting nanotubes should be an important process to make p-type and n-type semiconductor nanotubes. Experimentally, however, the atomically controlled substitutional doping into nanoscale materials including carbon nanotubes remains to be realized in the future.
In the present work, we study the energetics and geometries of B and N-doped carbon nanotubes in the framework of the density functional theory. Interestingly, it is found from the energetics study that it is easier to dope B atoms into thin nanotubes than into thick nanobues. This preference can be understand from the geometrical strain energy by substitutional B doping which causes the outward displacement of the B atom. It is also found that the energetics of the B doping depends systematically on the metallic versus semiconducting electronic properties of the host nanotube.
We have also studied electronic properties of impurity-induced states ("impurity levels") in B and N doped semiconducting carbon nanotubes in detail. Their spatial distribution is found to correlate well with the depth of the state from the top (bottom) of the valence (conduction) band. Deep states show rather narrow spatial distribution, while shallow states show wider distribution. Finally, the limit of the density-functional theory to predict the depth of the impurity levels and the necessary corrections will be discussed.
T. Koretsune and S. Saito, Phys. Rev. B 77 (2008) 165417.