Stability and strength of atomically thin borophene from first principles calculations

A new 2D material, borophene, has been grown successfully recently on single crystal Ag substrates. Three main structures have been proposed (beta(12), chi(3) and striped borophene). However, the stability of three structures is still in debate. Using first principles calculations, we examine the dynamical, thermodynamical and mechanical stability of beta(12), chi(3) and striped borophene. Free-standing beta(12) and chi(3) borophene is dynamically, thermodynamically and mechanically stable, while striped borophene is dynamically and thermodynamically unstable due to high stiffness along a direction. The origin of high stiffness and high instability in striped borophene along a direction can both be attributed to strong directional bonding. Our work shows a deep connection between stability and strength, and helps researchers to estimate accurately the mechanical performance of 2D materials. [GRAPHICS] IMPACT STATEMENT A benchmark for examining the relative stability of different structures of borophene is provided. Strong directional bonding in striped borophene leads to high stiffness and high brittleness.