From planar boron clusters to borophenes and borospherenes

Abstract

Photoelectron spectroscopy in combination with computational chemistry has been used over the past decade to systematically elucidate the structures and chemical bonding of sizeselected boron clusters. Small boron clusters have been found to be planar or quasi-planar with both delocalized σ and π bonding. A particularly interesting cluster is B 36 , which has been found to possess a planar structure with a central hexagonal vacancy. The hexagonal B 36 can be viewed as a repeating unit to assemble atom-thin boron monolayers (dubbed borophenes). This finding provides the first indirect experimental evidence that borophenes with hexagonal vacancies are potentially viable. Another exciting discovery has been the observation and characterization of the first all-boron fullerenes. Photoelectron spectroscopy revealed that the B 40 − cluster consisted of two isomers with very different electron binding energies. Global minimum searches led to two nearly degenerate isomers competing for the global minimum: a quasi-planar isomer and an unprecedented cage isomer. In the neutral, the B 40 cage is overwhelmingly the global minimum, which is the first all-boron fullerene to be observed and is named “borospherene”. There is evidence that there exists a family of borospherenes with B 28 being the smallest borospherene. It is expected that the pace of discovery will continue to accelerate in boron clusters, and more interesting structures and chemical bonds will be uncovered with heightened research interests and more sophisticated experimental and computational methods.