Spontaneous Dynamical Disordering of Borophenes in MgB2 and Related Metal Borides

Abstract

Layered boron compounds have attracted significant interest in applications from energy storage to electronic materials to device applications, owing in part to a diversity of surface properties tied to specific arrangements of boron atoms. Here, first-principles calculations coupled with global optimization are performed to explore the energy land scape for surface atomic configurations of MgB2, a prototypical layered metal diboride. We demonstrate, that contrary to previous assumptions, multiple reconstructions are thermodynamically preferred and kinetically accessible within the exposed B surfaces in MgB2, and other layered metal diborides. Such a dynamic environment and intrinsic disordering of the B surface atoms in metal borides presents new opportunities to realize a diverse set of 2D boron structures. We validated the predicted dynamic surface disordering by characterizing exfoliated boron-terminated MgB2 nanosheets. Application-relevant implications are discussed, with a particular view towards understanding the impact of boron surface heterogeneity on hydrogen storage performance.