Theoretical insights into the stability of buckled tetragonal graphene and the prediction of novel carbon allotropes

Buckled tetragonal graphene (BTG), a novel allotrope of graphene, has been reported to possess Dirac-like fermions and high Fermi velocities. However, the stability of BTG is still controversial. Here, first principles calculations and ab initio molecular dynamics (AIMD) were performed to study the stability of three kinds of tetragonal graphenes (TGs), including planar tetragonal graphene (PTG), BTG reported by Liu et al. [Phys. Rev. Lett., 2012, 108, 225505] and the novel BTG constructed by us. For the two BTGs, phonon dispersions predict that they are stable, but this conclusion is contradictory with the results of energy analysis, vibrational mode analysis and AIMD simulations. Our electronic structure analysis shows that the delocalized pi bonds formed by unbonded p(z) electrons drive the stability of PTG and may induce the transformation of the two BTGs into PTG. Our further study of phonon dispersions on planar hexagonal graphene (PHG) and buckled hexagonal graphene (BHG) indicates that the phonon dispersion at 0 K may have some limitations in predicting the stability of 2D carbon materials and thus cannot accurately describe the stability of BTGs. In addition, we have predicted several hydrogenated and fluorinated TGs, and theoretically demonstrated that chemical modification can make metallic PTG become a semiconductor with a certain bandgap. Moreover, the bandgaps of these new materials can be further regulated by increasing the thickness of the carbon atomic layer, which makes them promising for semiconductor devices and energy storage.