Reducing the 3D-2D crossover thickness in Bi2Se3 by heterostructure engineering*

The 3D topological insulator Bi2Se3 crosses over to the 2D limit usually as layer thickness is reduced past 6 Bi2Se3 quintuple layers (QL), as can be measured by the opening of a hybridization gap from its two surface states. In common Bi2Se3 heterostructures with a band-bending-induced potential gradient, the separation of the two surface states in real space and in energy both decreases as layer thickness is reduced, yielding the typical 6QL crossover thickness. Here, a combined first-principles theory and experimental study shows that an energy separation of the two surface states can be sustained to prevent surface state hybridization until 3 QLs. This is achieved by allowing the lower surface state to interact with a BiSe substrate (on top of NbSe2), which relieves the upper surface state from hybridizing with it. We further show that the 3QL Bi2Se3/BiSe system behaves similar to an asymmetric 4QL Bi2Se3 system, where Rashba-type quantum well bands are induced by a compositional gradient perpendicular to the layers.