Pyridine intercalated Bi₂Se₃heterostructures: controlling the topologically protected states

Abstract

We use ab initio simulations to investigate the incorporation of pyridine molecules (C 5 H 5 N) in the van der Waals gaps of Bi 2 Se 3 . The intercalated pyridine molecules increase the separation distance between the Bi 2 Se 3 quintuple layers (QLs), suppressing the parity inversion of the electronic states at the Γ-point. We find that the intercalated region becomes a trivial insulator. By combining the pristine Bi 2 Se 3 region with the one intercalated by the molecules, we have a non-trivial/trivial heterojunction characterized by the presence of (topologically protected) metallic states at the interfacial region. Next we apply an external compressive pressure to the system, and the results are (i) a decrease on the separation distance between the QLs intercalated by pyridine molecules, and (ii) the metallic states are shifted toward the bulk region, turning the system back to insulator. That is, through a suitable tuning of the external pressure in Bi 2 Se 3 , intercalated by pyridine molecules, we can control its topological properties; turning-on and -off the topologically protected metallic states lying at the non-trivial/trivial interface.Three dimensional topological insulators (TIs) are insulator materials in the bulk phase, but they present metallic topological surface states (TSSs), which are protected by time-reversal symmetry; as a result backscattering processes by time-reversal invariant impurities or defects are forbidden 1 . These materials are of great promise for spintronics applications, due to the formation of nearly dissipationless spin-polarized surface current 2 . Currently, Bi 2 Se 3 is one of the most investigated TI due to its large band gap (∼ 0.3 eV), nearly idealized single Dirac cone and for being a pure compound 3-5 . Bi 2 Se 3 presents a rhombohedral structure composed by quintuple layers (QLs) of Se and Bi atoms, forming a sequence of Se-Bi-Se-Bi-Se atoms covalently bonded; these QLs are stacked along the c-axis of a hexagonal structure by van der Waals (vdW) interactions.Recent experiments have been exploring the possibility of inserting guest species in the vdW gaps of Bi 2 Se 3 , this process is known as intercalation. Koski et al. 6,7 have added various zerovalent metals in the vdW gaps of Bi 2 Se 3 nanoribbons, it is expected that new properties and/or tuning of the Bi 2 Se 3 properties can be achieved by intercalation, e.g. superconductivity 8 . Beside metals, molecules have been intercalated in Bi 2 Se 3 , for instance, it has been shown that pyridine molecules in Bi 2 Se 3 present interesting properties for optoelectronic applications 9 . In this work we aim to investigate the geometry and electronic structure of Bi 2 Se 3 intercalated by pyridine molecules (py-Bi 2 Se 3 ). Upon the presence of the pyridine molecules between the QLs, we find that the pyBi 2 Se 3 system becomes a trivial insulator. By considering a heterojunction composed by py-Bi 2 Se 3 and pristine Bi 2 Se 3 (py-Bi 2 Se 3 /Bi 2 Se 3 ), we analyze (i) the occurrence a) Electronic mail: igor.oliveira@...

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