Ultra-stable metallic freestanding multilayer borophene with elevated work function

Understanding the structural stability of freestanding multilayer borophene is crucial because the interlayer interaction effect can lead to a new stabilized mechanism that can play a key role in determining the probability of its experimental realization, as evidenced by two recent experimental studies (substrate-supported bilayer α- [Nat. Mater. 2021, 21, 35] and β12-like borophene [Nat. Chem. 2022, 14, 25]). This study examines the structural stability of 1–3-layer structures of five typical borophene classes (α, χ3, β12, δ6, and δ3), and the results obtained indicate that α-borophene (BBP) exhibits the most favorable stability in the 1–3-layer series. Accordingly, the structural stabilities, electronic structures, and work functions of 1–5-layer BBP are investigated systematically. The reduced interlayer bonds, as well as increased binding and interlayer interaction energy with an increasing layer number, demonstrate that both the interlayer interaction and stability are enhanced, as confirmed by ab initio molecular dynamics simulations. Moreover, metallicity and in-plane multi-centered bonding remain in the four multilayer structures. Remarkably, the interlayer bonding shifts from the isolated covalent dominant B–B bonds (2-/3-layer) to 5- or 6-centered localized bonds with mixed covalent and ionic components (4-/5-layer) via electronic localization function and bond population analyses. The interlayer and in-plane multi-centered bonds lead to the formation of an unprecedented interconnected 2D tubular geometry (α-type boron nanotubes), which significantly enhances the interlayer bonding strength, resulting in highly stable 4- and 5-layer BBP. The work functions of 4- and 5-layer BBP, in particular, are similar to those of the frequently used Pt anode material; therefore, they are considered highly attractive anode materials for applications in electronic devices.