Observation of an all-boron fullerene

After the discovery of fullerene-C-60, it took almost two decades for the possibility of boron-based fullerene structures to be considered. So far, there has been no experimental evidence for these nanostructures, in spite of the progress made in theoretical investigations of their structure and bonding. Here we report the observation, by photoelectron spectroscopy, of an all-boron fullerene-like cage cluster at B-40(-) with an extremely low electron-binding energy. Theoretical calculations show that this arises from a cage structure with a large energy gap, but that a quasi-planar isomer of B-40(-) with two adjacent hexagonal holes is slightly more stable than the fullerene structure. In contrast, for neutral B-40 the fullerene-like cage is calculated to be the most stable structure. The surface of the all-boron fullerene, bonded uniformly via delocalized sigma and pi bonds, is not perfectly smooth and exhibits unusual heptagonal faces, in contrast to C-60 fullerene.