Layered narrow band gap semiconductor Bi2Se3 is composed of heavy elements with strong spin-orbital coupling (SOC), which has been identified both as a good candidate of thermoelectric material of high thermoelectric figure-of-merit (ZT) and a topological insulator of Z2-type with a gapless surface band in Dirac cone shape. The existence of a conjugated pi-bond system on the surface of each Bi2Se3 quintuple layer is proposed based on an extended valence bond model having valence electrons distributed in the hybridized orbitals. Supporting experimental evidences of a 2D conjugated pi-bond system on each quintuple layer of Bi2Se3 are provided by electron energy-loss spectroscopy (EELS) and electron density (ED) mapping through inverse Fourier transform of X-ray diffraction data. Quantum chemistry calculations support the pi-bond existence between partially filled 4pz orbitals of Se via side-to-side orbital overlap positively. The conjugated pi-bond system on the surface of each quintuple Bi2Se3 layer is proposed being similar to that found in graphite (graphene) and responsible for the unique 2D conduction mechanism. The van der Waals (vdW) attractive force between quintuple layers is interpreted being coming from the anti-ferroelectrically ordered effective electric dipoles which are constructed with pi-bond trimer pairs on Se-layers across the vdW gap of minimized Coulomb repulsion.
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