pi-bonds in graphene and surface of topological insulator Bi2Se3

TL;DR

This paper proposes a chemical bond analysis revealing pi-bond trimers on Bi2Se3's surface, explaining its 2D surface conduction similar to graphene, supported by spectral evidence and a cross-bridge energy exchange model.

Contribution

It introduces a valence bond theory-based analysis identifying pi-bond trimers on Bi2Se3 surface as key to its topological surface conduction, drawing parallels with graphene.

Findings

Pi-bond trimers exist on Bi2Se3 surface.

Surface conduction in Bi2Se3 is due to dynamic pi-bond conjugation.

Spectral evidence supports pi-bond existence near 7 eV.

Abstract

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).