Effect of the armchair and zigzag edge terminations on the properties of nanohelicenes: First-principles study

The work presents a comparison of the structural, mechanical, electronic and magnetic properties of armchair and zigzag terminated nanohelicenes, using the first principles density functional calculations. It was found that these nanohelicenes are metals in their most symmetric state, but undergo a spontaneous symmetry breaking which results in a metal-insulator transition (MIT). It was shown that the edge termination (armchair or zigzag) determines the type of the MIT in the global energy minima. The armchair terminated nanohelicene undergoes a Peierls MIT and it is a diamagnetic semiconductor. The zigzag terminated nanohelicenes is an antiferromagnetic semiconductor due to a Mott-Hubbard MIT. It was shown that the edge terminations and MIT type influence on helical electronic bands. Analysis of non-covalent interactions demonstrates that the van der Waals forces between coils are stronger in the case of zigzag termination.