Honeycomb Boron on Al(111): From the Concept of Borophene to the Two-Dimensional Boride

A great variety of two-dimensional (2D) boron allotropes (borophenes) were extensively studied in the past decade in the quest for graphene-like materials with potential for advanced technological applications. Among them, the 2D honeycomb boron is of specific interest as a structural analogue of graphene. Recently it has been synthesized on the Al(111) substrate; however it remains unknown to what extent does honeycomb boron behave like graphene. Here we elucidate the structural and electronic properties of this unusual 2D material with a combination of core-level X-ray spectroscopies, scanning tunneling microscopy, and DFT calculations. We demonstrate that in contrast to graphene on lattice-mismatched metal surfaces, honeycomb boron cannot wiggle like a blanket on Al(111), but rather induces reconstruction of the top metal layer, forming a stoichiometric AlB2 sheet on top of Al. Our conclusions from theoretical modeling are fully supported by X-ray absorption spectra showing strong similarity in the electronic structure of honeycomb boron on Al(111) and thick AlB2 films. On the other hand, a clear separation of the electronic states of the honeycomb boron into pi- and sigma-subsystems indicates an essentially 2D nature of the electronic system in both one-layer AlB2 and bulk AlB2.

Automated summary

The structural and electronic properties of two-dimensional honeycomb boron on the Al(111) substrate have been elucidated through a combination of experimental techniques and theoretical calculations. It is found that honeycomb boron induces reconstruction of the top metal layer, forming a stoichiometric AlB2 sheet on top of Al, in contrast to graphene behavior on lattice-mismatched metal surfaces. The electronic states of honeycomb boron exhibit a clear separation into pi- and sigma-subsystems, indicating the 2D nature of the electronic system in both mono-layer AlB2 and bulk AlB2.