A new two-dimensional (2D) material, borophene (2D boron sheet), has been grown successfully recently on single crystal Ag substrates by two parallel experiments [Mannix \textit{ et al., Science}, 2015, \textbf{ 350}, 1513] [Feng \textit{ et al., Nature Chemistry}, 2016, \textbf{ advance online publication}]. 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. This work provides a benchmark for examining the relative stability of different structures of borophene.
Categories: cond-mat.mtrl-sci cond-mat.mes-hall physics.comp-ph