Fermi Edge of Semimetallic Borophane Sheets and its Reduction by a Porous Structure

Theoretically predicted materials are often synthesized in low yields, and unexpected relationships are often encountered between the target materials and byproducts. Recently, two-dimensional boron materials proposed on the basis of model simulations and first principles calculations and possessing abundant atomic structures have attracted considerable interest. Borophane or the hydrogen boride (HB) sheet has been predicted to be the Dirac nodal semimetal when it has a boron network of nonsymmorphic symmetry. Upgrading the standard method, we fabricated freestanding HB sheets possessing either an apparent Fermi edge, reduced spectral weight, or a Fermi-level energy gap, as confirmed by using microbeam photoemission spectroscopy. The gapless electronic structures were correlated with terminal B-H bonds at the sheet edges, indicating the electronic modification of the porous structure as directly microscopically observed. The gapped or insulating sheet was fabricated via oxidation. This research provides methods for regulating the structural morphology and electronic states of HB sheets during synthesis.