Borophene is a proposed crystalline allotrope of boron. One unit consists of 36 atoms arranged in an 2-dimensional sheet with a hexagonal hole in the middle.
Reference : Z. A. Piazza et al., Nature Communications 5 (2014) Number 3113
Atomically thin, crystalline 2D boron sheets
Reference : A. J. Mannix et al., Science 350 (2015) 1513
