Two-Dimensional Boron Hydride Sheets: High Stability, Massless Dirac Fermions, and Excellent Mechanical Properties.

Two-dimensional (2D) boron sheets have been successfully synthesized in recent experiments, however, some important issues remain, including the dynamical instability, high energy, and the active surface of the sheets. In an attempt to stabilize 2D boron layers, we have used density functional theory and global minimum search with the particle-swarm optimization method to predict four stable 2D boron hydride layers, namely the C2/m, Pbcm, Cmmm, and Pmmn sheets. The vibrational normal mode calculations reveal all these structures are dynamically stable, indicating potential for successful experimental synthesis. The calculated Young's modulus indicates a high mechanical strength for the C2/m and Pbcm phases. Most importantly, the C2/m, Pbcm, and Pmmn structures exhibit Dirac cones with massless Dirac fermions and the Fermi velocities for the Pbcm and Cmmm structures are even higher than that of graphene. The Cmmm phase is reported as the first discovery of Dirac ring material among boron-based 2D structures. The unique electronic structure of the 2D boron hydride sheets makes them ideal for nanoelectronics applications.