Two-Dimensional Carbon Allotropes and Nanoribbons based on 2,6-Polyazulene Chains: Stacking Stabilities and Electronic Properties.

The previously predicted phagraphene [Wang et al., Nano Lett. 15, 6182 (2015)] and a recently proposed TPH-graphene have been synthesized from fusion of 2,6-polyazulene chain (5-7 chain) in a recent experiment [Fan et al., J. Am. Chem. Soc., 141, 17713 (2019)]. Theoretically, phagraphene and TPH-graphene can be considered as the combinations of the 5-7 chains with distinct 6-6-6 and 4-7-7 interfacial stacking manners, respectively. In this work, we propose another new graphene allotrope, named as penta-hex-hepta-graphene (PHH-graphene), which can be constructed by coupling the synthesized 5-7 chains with a new type of 5-7-6 stacking interface. It is found that the PHH-graphene is dynamically and thermally stable, and especially notable, the total energy of PHH-graphene is lower than that of synthesized TPH-graphene. Thus, it is highly possible that PHH-graphene can be realized through assembly of 5-7 chains. We have systematically investigated the electronic properties of these three graphene allotropes and their nanoribbons. The results show that PHH-graphene is a type-I semimetal with a highly anisotropic Dirac cone similar to phagraphene, while TPH-graphene is a metal. Their nanoribbons exhibit different electronic band structures as the number (n) of 5-7 chains increases. For TPH-graphene nanoribbons, they become metal rapidly as n ≥ 2. The nanoribbons of the semimetallic phagraphene and PHH-graphene are narrow band gap semiconductors with gaps decreasing as n increases, which are similar to the graphene nanoribbons. We also find that the band gaps of PHH-graphene nanoribbons exhibit two distinct families with n = 2i and n = 2i + 1, which can be understood by the width-dependent symmetries of the system.