By first principles calculations based on the density functional theory and nonequilibrium Green's function technique, we have studied the electronic and transport properties of C-doped zigzag-edged boron-nitride nanoribbons (ZBNNRs). Due to the two sub-lattices in boron-nitride nanoribbons (BNNRs), C substitutions at B sites and N sites naturally provide ways for electron doping and hole doping. Different combinations of the C chain substitution schemes are utilized to tune the electron transport of nano junctions constructed with ZBNNRs. It is found that, either substitution for B or N by C, in symmetric doping, the junction always behaves as a good conductor. However, in the asymmetric doping, the performance of the junctions highly depends on the positions of the C chain. When the C atoms are doped at opposite edges on the two sides of the junction, there is no current across the junction although dopings at B site and N site can both transform a BNNR from an insulator into a metal. Interestingly, when the doping sites are moved to the middle of the ribbons, the junctions conduct very well and negative differential resistance (NDR) is observed due to the special alignment of the energy bands of the two leads.
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