First-principles approach to monitoring the band gap and magnetic state of a graphene nanoribbon via its vacancies

Graphene based materials are expected to be very important in future technology. A wide range of magnetic and electronic properties of graphene nanoribbons holds the promise of novel nanoscale devices for future applications. The design and fabrication of these devices based on graphene nanoribbons require a proper understanding all the factors that influence its electronic and magnetic properties. Formation of vacancies and defects in graphene nanoribbons is not a rare situation and we have shown that they play crucial roles influencing the electronic and magnetic properties of graphene nanoribbons. Using first-principles plane-wave calculations we predict that electronic and magnetic properties of graphene nanoribbons can be modified by the defect-induced itinerant states. Structure optimization gives rise to significant reconstruction of atomic structure, which is in good agreement with transmission electron microscope images. The band gaps of armchair nanoribbons can be modified by hydrogen-saturated holes. The band-gap changes depend on the width of the ribbon as well as on the position of the hole relative to the edges of the ribbon. Defects due to periodically repeating vacancy or divacancies induce metallization as well as magnetization in nonmagnetic semiconducting nanoribbons due to the spin polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to the edges. It is also shown that these effects can, in fact, be realized without really creating defects. we propose a method to achieve the formation of periodic local defects like holes or vacancies. The sharp electrodes like STM tips are situated at desired locations on the graphene ribbon with a repeat periodicity. A potential difference,  common to all electrodes (tips) is applied between the tip and underlying insulator through the ribbon. This way the electronic potential of graphene atoms just below the tip is locally lowered or raised depending on the polarity of applied voltage. Here, the effect of locally and periodically applied potential difference has been modeled by empirical tight binding methods (ETB).

M. Topsakal, E. Akturk, H. Sevinçli, and S. Ciraci
PHYSICAL REVIEW B 78,  235435 (2008)
Editorially selected  for Virtual Journal of Nanoscale Science and Technology.