Promising 2D Carbon Allotropes with Intrinsic Bandgaps Based on Bilayer α-Graphyne

2D carbon allotropes with moderated electronic bandgaps are highly desirable for next-generation carbon-based nanoelectronics beyond graphene. Herein, the structural and electronic properties of bilayer alpha-graphyne have been systematically investigated by using first-principles calculations. Consequently, in addition to typical van der Waals (vdW) stacks of single-layer alpha-graphyne, new 2D carbon allotropes with purely sp2 or sp3 hybridizations can be achieved, arising from the absence of intralayer acetylene linkages during structural relaxation. Compared to the Dirac semimetallic behavior of monolayer alpha-graphyne, the electronic structure of vdW bilayer alpha-graphyne can be further modulated by stacking patterns, yet retains metallic characteristics. In contrast, intrinsic semiconducting characteristics can be observed in the proposed new 2D carbon allotropes with tunable bandgaps dependent on the in-plane biaxial strain. Correspondingly, a pristine 2D carbon sheet with only sp2 hybridization exhibits a promising direct bandgap of 1.062 eV, while the sp3-hybridized carbon network possesses an indirect bandgap of 1.708 eV, which can be further converted to the direct type under small compressive strains, implying great potential in future carbon-based nanoelectronic applications beyond graphene. Also, this work provides a new approach for developing novel carbon allotropes via the vertical stacking of graphyne with acetylenic linkages.