Bilayer borophene: The effects of substrate and stacking

TL;DR

This study uses first-principles calculations to explore the structure, stability, and mechanical properties of bilayer borophene, revealing the effects of stacking, substrate support, and interlayer interactions on its properties.

Contribution

It introduces a preferred model for bilayer borophene consistent with experimental data and analyzes the stability and mechanical properties of different stacking configurations.

Findings

AA-stacked bilayer has vdW interlayer interaction with 2.5 Å distance.

AB stacking forms covalent bonds and is stable in freestanding form.

Bilayer borophene exhibits higher stiffness than bilayer MoS2.

Abstract

Bilayer borophene has recently attracted much interest due to its outstanding mechanical and electronic properties. The interlayer interactions of these bilayers are reported differently in theoretical and experimental studies. Herein, we design and investigate bilayer beta12-borophene, by first-principles calculations. Our results show that the interlayer distance of the relaxed AA-stacked bilayer is about 2.5 A, suggesting a van der Waals (vdW) interlayer interaction. However, this is not supported by previous experiments, therefore by constraining the interlayer distance, we propose a preferred model which is close to experimental records. This preferred model has one covalent interlayer bond in every unit cell (single-pillar). Further, we argue that the preferred model is nothing but the relaxed model under a 2% compression. Additionally, we designed three substrate-supported bilayers on the Ag, Al, and Au substrates, which lead to double-pillar structures. Afterward, we investigate the AB stacking, which forms covalent bonds in the relaxed form, without the need for compression or substrate. Moreover, phonon dispersion shows that, unlike the AA stacking, the AB stacking is stable in freestanding form. Subsequently, we calculate the mechanical properties of the AA and AB stackings. The ultimate strengths of the AA and the AB stackings are 29.72 N/m at 12% strain and 23.18 N/m at 8% strain, respectively. Moreover, the calculated Young's moduli are 419 N/m and 356 N/m for the AA and the AB stackings, respectively. These results show the superiority of bilayer borophene over bilayer MoS2 in terms of stiffness and compliance. Our results can pave the way for future studies on bilayer borophene structures.