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

Abstract

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.

Topics & Concepts