Two-Dimensional Borane with 'Banana' Bonds and Dirac-Like Ring

Designing new two-dimensional (2D) semiconductors with novel topological characters is highly desirable for further material innovation. We propose a theoretical design of a stable 2D inorganic material, namely, borane, which is jointly stabilized by traditional B–B localized and unique B–H–B delocalized chemical bonds. In borane, the bonding natures along different directions are distinguishing, which lead to huge differences in mechanical strengths of 142.73 and 97.47 N/m for a and b directions, respectively. In a unit cell, each hydrogen atom binds to two boron atoms forming a three-center-two-electron (3c-2e) bridge bond B–H–B. This can be considered as an extension of diborane molecules from 0D to 2D. The collaboration of localized and delocalized chemical bonds endows borane with high structural stability, as indicated by its favorable cohesive energy, high mechanical strength, absence of imaginary modes in the phonon spectrum, and moderate melting point. Remarkably, borane has a fascinating electronic property featured with a Dirac-like ring in the electronic band structure. The unique bonding nature and electronic property in borane would attract intensive interests in both theory and experiment.