B40 cluster stability, reactivity, and its planar structural precursor.

We report a comprehensive first-principles study of the structural and chemical properties of the recently discovered B40 cage. It is found to be highly reactive and can exothermically dimerize, regardless of the orientation, by overcoming a small energy barrier ≃0.06 eV. The energy gap of the system varies widely with the aggregation of the increasing number of B40 cages, from 3.14 eV in a single B40, to 1.54 eV in the dimer, to 1.25 eV in the trimer. We also explore a recipe for protecting the B40 cage by sheathing it within a carbon shell and identify carbon nanotubes with a radius of ∼6 Å as optimal hosts for an isolated cage. It is demonstrated that B40 can be unfolded into a planar 'molecule' that tessellates the plane. The corresponding 2D boron sheet constitutes a structural precursor foldable into this unique boron cage structure of current interest.