Because of its electron deficiency, elemental boron forms unusual crystals dominated by three-dimensional cage-like structural units. However, early computational studies suggested that three-dimensional icosahedral cage structures were not stable for small and medium sized boron clusters. The Wang group and collaborators have been investigating the structures and chemical bonding of size-selected boron clusters over the past decade through combined photoelectron spectroscopy and ab initio calculation studies and found that anionic boron clusters are planar up to very large sizes. Essential to these studies is the task of searching relevant areas of the potential energy landscape for low energy candidate structures.  We have systematically studied the structural evolution and chemical bonding of anionic boron clusters Bn-  (n = 3-25, 30, 36) aided by a host of potential energy surface searching algorithms which employ density functional theory for energy evaluation and local minimization (modified basin hopping, coalescence kick, Cartesian walking). It was found that all of these clusters possess planar or quasi-planar geometries stabilized by 2-Dimensional electron delocalization. Our most exciting recent discovery in boron cluster is that of the highly stable, hexagonally symmetric, quasi-planar B36 which contains a central hexagonal hole. The photoelectron spectrum of B36-  yields a relatively simple spectral pattern, which suggests a highly symmetric cluster. Global minimum searches for B36-  revealed a quasiplanar structure with a central hexagonal hole as the most stable. The B36 cluster is the smallest boron cluster to have sixfold symmetry, contains a perfect hexagonal vacancy in the center, and it can be viewed as a potential basis for the theoretically proposed extended two-dimensional boron sheets, for which we coined a word 'borophene'.

Planar hexagonal B<sub>36</sub>: A potential basis for borophenes

248th National Meeting (2014)

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Planar hexagonal B36: A potential basis for borophenes

Planar hexagonal B₃₆: A potential basis for borophenes