From planar boron clusters to borophenes and metalloborophenes

Elemental boron and its compounds exhibit unusual structures and chemical bonding owing to the electron deficiency of boron. Joint photoelectron spectroscopy and theoretical studies over the past decade have revealed that boron clusters possess planar or quasi-planar (2D) structures up to relatively large sizes, laying the foundations for the discovery of boron-based nanostructures. The observation of the 2D B-36 cluster provided the first experimental evidence that extended boron monolayers with hexagonal vacancies were potentially viable and led to the proposition of 'borophenes' - boron analogues of 2D carbon structures such as graphene. Metal-doping can expand the range of potential nanostructures based on boron. Recent studies have shown that the CoB18- and RhB18-clusters possess unprecedented 2D structures, in which the dopant metal atom is part of the 2D boron network. These doped 2D clusters suggest the possibilities of creating metal-doped borophenes with potentially tunable electronic, optical and magnetic properties. Here, we discuss the recent experimental and theoretical advances in 2D boron and doped boron clusters, as well as their implications for metalloborophenes.