The effect of mechanical strain on the Dirac surface states in the (0001) surface and the cohesive energy of the topological insulator Bi2Se3

Abstract

The band gap (E_g) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick Bi₂Se₃ slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the Bi₂Se₃ slab primarily varies the bandwidth, modifies the p_z – orbital population of Bi and moves the Dirac point of the (0001) surface of Bi₂Se₃. The Dirac cone feature of the (0001) surface of Bi₂Se₃ is preserved for the entire range of the biaxial strain. However, around 5% tensile uniaxial strain and even lower value of volume conservation strain annihilate the Dirac cone, which causes the loss of topological (0001) surface states of Bi₂Se₃. The biaxial strain provides ease in achieving the Dirac cone at the Fermi energy (E_F) than the uniaxial and volume conservation strains. Interestingly, the transition from direct E_g to indirect E_g state of the (0001) surface of Bi₂Se₃ is observed in the volume conservation strain-dependent E_g. The strain on Bi₂Se₃, significantly modifies the conduction band of Se2 atoms near E_F compared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of Bi₂Se₃. The atomic cohesive energy of the Bi₂Se₃ slab is very close to that of (0001) oriented nanocrystals extracted from the Raman spectra. The strain-dependent cohesive energy indicates that at a higher value of strain, the uniaxial and volume conservation strain provides better stability than that of the biaxial strain (0001) oriented growth of the Bi₂Se₃ nanocrystals. Our study establishes the relationship between the strained lattice and electronic structures of Bi₂Se₃, and more generally demonstrates the tuning of the Dirac point with the mechanical strain.