Work function

Nanomaterials

Computational chemistry

Density functional theory

Chemical physics

Nanotechnology

Organic chemistry

Covalent bond

Chemistry

Materials science

Borophene

Layer (electronics)

Graphene

Phase (matter)

Optoelectronics

Boron

Doping

Scanning tunneling microscope

Crystallography

Nature Materials

Synthetic two-dimensional (2D) materials have no bulk counterparts and typically exist as single atomic layers due to substrate-stabilized growth. Multilayer formation, although broadly sought for structure and property tuning, has not yet been achieved in the case of synthetic 2D boron: that is, borophene1,2. Here, we experimentally demonstrate the synthesis of an atomically well-defined borophene polymorph beyond the single-atomic-layer (SL) limit. The structure of this bilayer (BL) borophene is consistent with two covalently bonded α-phase layers (termed BL-α borophene) as evidenced from bond-resolved scanning tunnelling microscopy, non-contact atomic force microscopy and density functional theory calculations. While the electronic density of states near the Fermi level of BL-α borophene is similar to SL borophene polymorphs, field-emission resonance spectroscopy reveals distinct interfacial charge transfer doping and a heightened local work function exceeding 5 eV. The extension of borophene polymorphs beyond the SL limit significantly expands the phase space for boron-based nanomaterials. A borophene polymorph with two covalently bonded boron monolayers was synthesized, expanding the physical properties of borophene and filling the gap between monolayer borophene and icosahedron-based bulk boron.

Borophene synthesis beyond the single-atomic-layer limit