Atomic and electronic structures of strained bulk and thin film Bi₂Se₃ and Bi₂Te₃ from density functional theory with van der Waals interactions

Xin Luo, Michael B. Sullivan, and Su Ying Quek

Received 10 July 2012; revised 7 November 2012; accepted 9 November 2012; published 27 November 2012

Phys. Rev. B 2012, 86, 184111.

ABSTRACT:

Bi₂Se₃ and Bi₂Te₃ are layered compounds of technological importance, being excellent thermoelectric materials as well as topological insulators. We report density functional theory (DFT) calculations of the atomic, electronic and thermoelectric properties of strained bulk and thin film Bi₂Se₃ and Bi₂Te₃, focusing on an appropriate description of van der Waals (vdW) interactions. The calculations show that the van der Waals Density functional (vdW-DF) with Cooper's exchange (vdW-DF^C09_X) can reproduce closely the experimental interlayer distances in unstrained Bi₂Se₃ and Bi₂Te₃. Interestingly, we predict atomic structures that are in much better agreement with the experimentally determined structure from Nakajima than that obtained from Wyckoff, especially for Bi₂Se₃ where the difference in atomic structures qualitatively changes the electronic band structure. The band structure obtained using the Nakajima structure, and the vdW-DF^C09_X optimized structure, are in much better agreement with previous reports of photoemission measurements, than that obtained using the Wyckoff structure. Using vdW-DF^C09_X to fully optimize atomic structures of bulk and thin film Bi₂Se₃ and Bi₂Te₃ under different in-plane and uniaxial strains, we predict that the electronic band gap of both the bulk materials and thin films decreases with tensile in-plane strain and increases with compressive inplane strain. We also predict, using the semiclassical Boltzmann approach, that the magnitude of the n-type Seebeck coefficient of Bi₂Te₃ can be increased by the compressive in-plane strain, while that of Bi₂Se₃ can be increased with tensile in-plane strain. Further, the in-plane power factor of n-doped Bi₂Se₃ can be increased with compressive uniaxial strain, while that of n-doped Bi₂Te₃ can be increased by compressive in-plane strain. Strain engineering thus provides a direct method to control the electronic and thermoelectric properties in these thermoelectric topological insulator materials.

DOI: 10.1103/PhysRevB.86.184111