Elasticity, Flexibility and Ideal Strength of Borophenes

TL;DR

This study reveals that borophenes, a form of 2D boron, exhibit exceptional flexibility, elasticity, and strength, with properties tunable by structural modifications, making them promising for flexible material applications.

Contribution

First-principles calculations demonstrate the high flexibility, strength, and tunability of borophenes, highlighting their potential for flexible and composite material design.

Findings

Borophenes have in-plane modulus up to 210 N/m and bending stiffness as low as 0.39 eV.

The Foppl-von Karman number per area exceeds graphene's, indicating high flexibility.

Borophenes exhibit ideal strengths of 16 N/m, comparable to graphene.

Abstract

We study the mechanical properties of two-dimensional (2D) boron, borophenes, by first-principles calculations. The recently synthesized borophene with 1/6 concentration of hollow hexagons (HH) is shown to have in-plane modulus C up to 210 N/m and bending stiffness as low as D = 0.39 eV. Thus, its Foppl-von Karman number per unit area, defined as C/D, reaches 568 nm-2, over twofold higher than graphene's value, establishing the borophene as one of the most flexible materials. Yet, the borophene has a specific modulus of 346 m2/s2 and ideal strengths of 16 N/m, rivaling those (453 m2/s2 and 34 N/m) of graphene. In particular, its structural fluxionality enabled by delocalized multi-center chemical bonding favors structural phase transitions under tension, which result in exceptionally small breaking strains yet highly ductile breaking behavior. These mechanical properties can be further tailored by varying the HH concentration, and the boron sheet without HHs can even be stiffer than graphene against tension. The record high flexibility combined with excellent elasticity in boron sheets can be utilized for designing composites and flexible systems.