Epitaxial growth and structural properties of silicene and other 2D allotropes of Si

Since the breakthrough of graphene, considerable efforts have been made to search for two-dimensional (2D) materials composed of other group 14 elements, in particular silicon and germanium, due to their valence electronic configuration similar to that of carbon and their widespread use in the semiconductor industry. Silicene, the silicon counterpart of graphene, has been particularly studied, both theoretically and experimentally. Theoretical studies were the first to predict a low-buckled honeycomb structure for free-standing silicene possessing most of the outstanding electronic properties of graphene. From an experimental point of view, as no layered structure analogous to graphite exists for silicon, the synthesis of silicene requires the development of alternative methods to exfoliation. Epitaxial growth of silicon on various substrates has been widely exploited in attempts to form 2D Si honeycomb structures. In this article, we provide a comprehensive state-of-the-art review focusing on the different epitaxial systems reported in the literature, some of which having generated controversy and long debates. In the search for the synthesis of 2D Si honeycomb structures, other 2D allotropes of Si have been discovered and will also be presented in this review. Finally, with a view to applications, we discuss the reactivity and air-stability of silicene as well as the strategy devised to decouple epitaxial silicene from the underlying surface and its transfer to a target substrate.