Two-Dimensional Boron Crystals: Structural Stability, Tunable Properties, Fabrications and Applications

Boron, as a unique element nearest to carbon in the periodic table, has been predicted to form many distinctive two-dimensional (2D) structures that significantly differ from other well-studied 2D materials, owning to its exceptional ability to form strong covalent two-center-two-electron bonds as well as stable electron-deficient multi-center-two-electron bonds. Until recently, the successful syntheses of atomically thin crystalline 2D boron sheets (i.e., borophenes) provoked growing passion in 2D boron crystals. In this feature article, we present a survey of the latest achievements on 2D boron structures, starting from a concise introduction of the structures and properties of the bulk allotropes of boron, boron clusters, and especially potential building blocks for 2D boron crystals. Then we review important achievements and the current status of research on single-layered metallic borophene, and discuss 2D few-layered boron sheets, from their possible structures to tunable properties and potential applications in electronics, spintronics, and photoelectronics. We also systematically investigate the stability and functionalization of 2D icosahedral boron sheets in comparison with borophenes through first-principles studies. Finally, we present an outlook on the advance in fabrications of 2D boron sheets, and the challenges and prospects in the realm of 2D boron crystals.