B: a bilayer boron cluster.

Size-selected negatively-charged boron clusters (B) have been found to be planar or quasi-planar in a wide size range. Even though cage structures emerged as the global minimum at B, the global minimum of B was in fact planar. Only in the neutral form did the B borospherene become the global minimum. How the structures of larger boron clusters evolve is of immense interest. Here we report the observation of a bilayer B cluster using photoelectron spectroscopy and first-principles calculations. The photoelectron spectra of B exhibit two well-resolved features at low binding energies, which are used as electronic signatures to compare with theoretical calculations. Global minimum searches and theoretical calculations indicate that both the B anion and the B neutral possess a bilayer-type structure with D symmetry. The simulated spectrum of the D B agrees well with the experimental spectral features, confirming the bilayer global minimum structure. The bilayer B clusters are found to be highly stable with strong interlayer covalent bonding, revealing a new structural type for size-selected boron clusters. The current study shows the structural diversity of boron nanoclusters and provides experimental evidence for the viability of bilayer borophenes.