The Planar CoB₁₈⁻Cluster as a Motif for Metallo-Borophenes

Monolayer-boron (borophene) has been predicted with various atomic arrangements consisting of a triangular boron lattice with hexagonal vacancies. Its viability was confirmed by the observation of a planar hexagonal B₃₆cluster with a central six-membered ring. Here we report a planar boron cluster doped with a transition-metal atom in the boron network (CoB₁₈⁻), suggesting the prospect of forming stable hetero-borophenes. The CoB₁₈⁻cluster was characterized by photoelectron spectroscopy and quantum chemistry calculations, showing that its most stable structure is planar with the Co atom as an integral part of a triangular boron lattice. Chemical bonding analyses show that the planar CoB₁₈⁻is aromatic with ten π-electrons and the Co atom has strong covalent interactions with the surrounding boron atoms. The current result suggests that transition metals can be doped into the planes of borophenes to create metallo-borophenes, opening vast opportunities to design hetero-borophenes with tunable chemical, magnetic, and optical properties.