  Chemical bond analysis for the topological insulator (TI) Bi2Se3 following
valence bond theory and VSEPR rule is proposed and compared to that of
graphene. The existence of pi-bond trimers on surface Se-layer is revealed to
be responsible for the unusual 2D surface conduction found in the prototype Z2
topological insulator Bi2Se3, which shows great similarity to the ideal
"surface-only" 2D material graphene with surface band in Dirac cone shape. The
pi-bond trimers on the surface of TI form a dynamic pi-bond conjugated system
of implicit chirality. Local pi-bond energy exchange in cross-bridge model is
proposed responsible for the unique 2D surface conduction for topological
insulator Bi2Se3, and the similar low dimensional conduction mechanism can also
be identified in graphene and conductive polymers. Preliminary supporting
evidence for the existence of pi-bonds is provided by the commonly assigned pi
plasmon for the spectral feature near 7 eV in graphite and Bi2Se3 using
electron energy-loss spectroscopy (EELS).
