PCF-Graphene: A 2D sp2-Hybridized Carbon Allotrope with a Direct Band Gap

Motivated by the recent progress in synthesizing graphene by using aromatic benzene molecules as precursors, herein, we propose a new two-dimensional (2D) carbon allotrope by using nonaromatic molecule cyclo-octatetraene as the precursor from a bottom-up approach. In this structure, all the carbon atoms are threefold coordinated, similar to the sp(2)-hybridized atoms in graphene; thus, we name the poly-cyclooctatetraene framework as PCF-graphene. First-principles calculations reveal that although PCF-graphene is metastable compared with graphene, it is thermally, mechanically, and dynamically stable. Different from graphene with a single-atomic thickness and zero band gap, PCF-graphene has a finite thickness of 2.45 angstrom and is a semiconductor with a direct band gap of 0.77 eV. It exhibits anisotropies in mechanical properties, carrier mobility, and optical adsorption because of its anisotropic lattice. This study not only sheds insights into the design of 2D carbon materials from some simple organic molecules but also expands the 2D purely three-coordinated carbon family from a single-atomic layer to a finite thickness. PCF-graphene can be used as a rung in the ladder to stabilize three-dimensional three-coordinated carbon system that remains elusive currently.