Site-dependent stability and electronic structure of single vacancy point defects in hexagonal graphene nano-flakes

Graphene nano-flakes and quantum dots have considerable potential as components for nanodevices, since the finite in-plane dimension and additional edge and corner states provide potential for band gap engineering. However, like semi-infinite graphene membranes, they may contain different configurations of vacancy point defects that may be difficult to predict or control. In this paper we use density functional tight binding simulations to explore the impact of different geometric configurations of vacancies in unterminated (radical), mono-hydride and di-hydride terminated nano-flakes with zigzag or armchair edges. The results reveal that the planar structure is more uniformly preserved (with less distortion) when vacancies are located near the edges and corners, due to the combined effect of vacancy-edge-corner reconstructions, and passivating the circumference reduces the scattering of the band gap, but not the scattering of the energy of the Fermi level. In general, and regardless of the possible application, the use of zigzag-edged nano-flakes with stable edge/corner passivation is desirable to ensure reliability, and reduce the impact of an unknown number and configurations of vacancies.