From borophene polymorphs towards a single honeycomb borophane phase: reduction of hexagonal boron layers on Al(111).

The recent interest in characterizing 2D boron polymorphs has led to claims of the first stabilization of a honeycomb phase with conical Dirac-like electron dispersion. However, the synthesis of chemically stable, single, and homogeneous 2D boron phases still represents a significant experimental challenge. This is ascribed to the intrinsic boron electronic configuration that, at variance with carbon, leads to the formation of multi-center covalent bonds. External charge compensation by substrate-induced doping can steer the geometry of the layer, both in the buckling and in the density of B vacancies, like in the case of the recently achieved stabilization of honeycomb boron layers on Al(111). The price to pay is however a strong boron-support interaction, resulting in general in a limiting kinetic hindrance with respect to the synthesis of homogenous single phases. In the specific case of Al(111) an AlB 2 layer is known to form at the surface, quite far from a desirable quasi-freestanding borophene monolayer and at variance with graphene, which can be easily synthesized in an almost freestanding configuration e.g. on Ir(111). We provide here evidence for the (reversible) formation of well-ordered honeycomb borophane upon hydrogenation of the honeycomb boron phase on Al(111).