Electronic Structure of Large-Scale Graphene Nanoflakes

摘要

With the help of the recently developed SIESTA-PEXSI method [J. Phys.:Condens. Matter \textbf{26}, 305503 (2014)], we perform Kohn-Sham densityfunctional theory (DFT) calculations to study the stability and electronicstructure of hexagonal graphene nanoflakes (GNFs) with up to 11,700 atoms. Wefind the electronic properties of GNFs, including their cohesive energy,HOMO-LUMO energy gap, edge states and aromaticity, depend sensitively on thetype of edges (ACGNFs and ZZGNFs), size and the number of electrons. We observethat, due to the edge-induced strain effect in ACGNFs, large-scale ACGNFs'cohesive energy decreases as their size increases. This trend does not hold forZZGNFs due to the presence of many edge states in ZZGNFs. We find that theenergy gaps $E_g$ of GNFs all decay with respect to $1/L$, where $L$ is thesize of the GNF, in a linear fashion. But as their size increases, ZZGNFsexhibit more localized edge states. We believe the presence of these statesmakes their gap decrease more rapidly. In particular, when $L$ is larger than6.40 $nm$, we find that ZZGNFs exhibit metallic characteristics. Furthermore,we find that the aromatic structures of GNFs appear to depend only on whetherthe system has $4N$ or $4N+2$ electrons, where $N$ is an integer.