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

Silicene has a two-dimensional buckled honeycomb lattice and is chemically reactive because of its mixed sp²–sp³ 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.