Polar edges and their consequences for the structure and shape of hBN islands

The ionic component of the strong bond in hexagonal boron nitride (hBN) has been grossly disregarded in literature. Precisely this quantity is demonstrated to govern the shape of monolayer hBN islands grown at high temperatures. HBN zigzag edges are charged and energetically less favorable than the neutral armchair edges, in contrast to those of the purely covalent graphene. Nucleation of hBN islands occurs exclusively on either the inner or the outer corners of substrate steps. Taking into account the charge at edges of hBN islands offers a powerful framework to understand the nucleation of the islands and their orientation with respect the founding steps, as well as various equilibrium shapes, including prominently a right-angled trapezoid. BN dimers are identified as basic building blocks for hBN. A surprisingly strong interaction between hBN and the pre-existing steps on the moderately reactive Ir(1 1 1) substrate is uncovered. Localized charges are probably relevant for all 2D-materials lacking inversion symmetry.