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 (Eg) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick Bi2Se3slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the Bi2Se3slab primarily varies the bandwidth, modifies the pz- orbital population of Bi and moves the Dirac point of the (0001) surface of Bi2Se3. The Dirac cone feature of the (0001) surface of Bi2Se3is 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 (EF) than the uniaxial and volume conservation strains. Interestingly, the transition from directEgto indirectEgstate of the (0001) surface of Bi2Se3is observed in the volume conservation strain-dependentEg. The strain on Bi2Se3, significantly modifies the conduction band of Se2 atoms nearEFcompared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of Bi2Se3. The atomic cohesive energy of the Bi2Se3slab 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 Bi2Se3nanocrystals. 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.