Boron and Nitrogen Doped Single walled Carbon Nanotubes as Possible Dilute Magnetic Semiconductors
© to the authors 2007
Received: 31 May 2007
Accepted: 19 July 2007
Published: 10 August 2007
The structure of single walled armchair and zig-zag carbon nanotubes having 70 atoms and two carbons replaced by boron or nitrogen is obtained at minium energy using HF/6-31G* molecular orbital theory. The calculations show that the ground state of the zig-zag tubes is a triplet state while for the armchair tubes it is a singlet. In the zig-zag tubes the density of states at the Fermi level is greater for the spin down states compared to the spin up state indicating that the doped tubes could be ferromagnetic.
The switching elements in computers, metal oxide silicon field effect transistors, involve turning of and on the flow of current in the form of holes and electrons. On the other hand the storage of data employs a separate material consisting of nanosized magnetic materials. There is considerable interest in combining storage of information and switching into one material by developing magnetic semiconductors referred to as dilute magnetic semiconductors. There have been a number of reports presenting evidence that semiconductors such as GaP, GaN, GaAs and ZnO when doped with Cu2+ and Mn2+ are ferromagnetic [1–5]. An alternate approach to developing magnetic semiconductors might be to explore the possibility of making single walled carbon nanotubes (SWNTs) ferromagnetic by appropriate doping of other atoms for carbon on the walls of the tube. SWNTs can be viewed as sheets of graphene rolled into tubes which can be microns in length and have diameters from 5 A to 20 A. The tubes may be metal, insulating or semiconducting depending on their chirality. There is much interest in developing electronic devices based on SWNTs and in fact a field effect transistor has been demonstrate using SWNTs . The tubes may be N or P doped by substituting nitrogen or boron for carbon in the walls of the tubes. However there is little experimental data on the electronic properties of such doped SWNTs. One particularly interesting report is the observation of ferromagnetism in nitrogen doped SWNTs opening the possibility of their use as DMS materials . This result motivates a further investigation of the electronic and magnetic properties of boron and nitrogen doped SWNTs using molecular orbital theory.
There have been a number of theoretical treatments of carbon nanotubes employing molecular orbital theory ranging from the semiempirical AM1 method to density functional theory (DFT) [8–11]. The size of the tube needed depends on the property that is to be calculated. The prediction of properties of side wall functionalized tubes generally requires at least a tube having 70 atoms. On the other hand the properties of tubes functionalized at the open ends can be predicted with much shorter tubes. To the best of our knowledge there have been no investigations of the effect of boron and nitrogen doping on the electronic or magnetic structure of SWNTs.
Methods and Results
Calculated total energy of singlet and triplet state for boron and nitrogen doped single wall zig-zag and armchair carbon nanotubes
Total energy au (S = 0)
Total energy au (S = 1)
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