Surface conduction and pi-bonds in graphene and topological insulator   Bi2Se3
  A hybrid orbital model for the topological insulator Bi2Se3 is proposed and compared to that of graphene. The existence of a pi-bond trimer on the Se1 layer at the van der Waals gap and on the surface is thought to be responsible for the unusual surface conduction mechanism found in Bi2Se3 as a topological insulator. The three pi-bonds are locked as permanent electric dipoles between the Se1 layers in the van der Waals gap as an attractive force for the bulk as a gapped semiconductor. In contrast, the pi-bond trimers on the surface are unlocked and exhibit two degenerate quantum states, creating a local charge transfer mechanism in the cross-bridge model that is responsible for the surface conduction. The role of pi-bonds on the Bi2Se3 surface is compared with that in graphene for a similar 2D band structure containing Dirac cones. 
G. J. Shu and F. C. Chou
cond-mat.mes-hall cond-mat.mtrl-sci