Self-Encapsulation of Silicene in Cubic Diamond Si: Topological Semimetal in Covalent Bonding Networks

Silicene has a two-dimensional buckled honey-comb lattice and is chemically reactive because of its mixed sp(2)-sp(3) bonding character unlike graphene. Despite recent advances in epitaxial growth, it remains a great challenge to synthesize a stable silicene layer. Here, we propose an encapsulation method, in which silicene is self-encapsulated between Si(110) layers in the cubic diamond lattice and effectively protected from reaction with environmental gases. Although Si atoms are all fourfold coordinated, self encapsulated silicene exhibits the band topology of Dirac semimetals. In a superlattice structure, in which silicene is periodically encapsulated between Si(110) layers, we also find a topological transition from a normal semiconductor to a topological nodal line semimetal as the number of Si(110) layers increases. Our results provide insights into the design of a stable silicene layer that retains the nontrivial band topology and is useful for applications of Si-based devices.