Ultra-stable metallic freestanding multilayer borophene with tunable work function

This study examines the structural stability of 1–3-layer structures of seven typical borophene classes (α, α1, α5, χ3, β12, δ6, and δ3), and the results obtained indicate that α- and α5-borophene exhibits the top two favorable stability in the 1–3-layer series. Accordingly, the structural stabilities, electronic structures, and work functions of 1–5-layer α- and α5-borophene are investigated systematically. The increased binding energies (Eb) with increasing layer number for both α- and α5-borophene demonstrate that their stabilities are enhanced, as confirmed by ab initio molecular dynamics simulations. Remarkably, the interlayer bonding of α-borophene shifts from the isolated covalent dominant B–B bonds (2-layer) to 5- or 6-centered localized bonds with mixed covalent and ionic components (4-/5-layer), leading to the formation of an unprecedented interconnected 2D tubular geometry (α-type boron nanotubes), which significantly enhances the interlayer bonding strength and contributes dominantly to the increasing Eb with increasing layer number. Contrarily, the increased thermodynamic stability for α5-borophene with increasing layer number mainly originates from the increased in-plane bonding interaction. In particular, the highly stable metallic 5-layer α5- (4.54 eV) and α-borophene (5.66 eV) structures with favorable work functions are considered alternative material of graphene and highly attractive anode materials for applications in electronic devices, respectively.