First-Principles Study Of Functional Graphene Molecules And Their Self-Assembly Molecules

Graphene, a monolayer of graphite, is the first truly two dimensional material and a promising candidate for silicon replacement in semiconductor industry or gas sensing applications. Since graphene was discovered in 2004, continuous effort in chemical synthesis of graphene type molecules has produced various graphene molecules with structures and properties beyond simple graphene. Such as polycyclic aromatic hydrocarbons (PAHs) compounds which belong to a class of important functional graphene molecules. Their derivatives FTBC-Cn molecule self-assemble on highly oriented pyrolytic graphite (HOPG) formed the self-assembly supramolecules, which showed significant advantages, such as good stability of structural, electronic property, and semiconductor property, so it could be used in logical circuit and transistor devices rather than the zero-band graphene.In our work, FTBC-Cn (n=4,6,8,12) (hexafluorotribenzo [a, g, m] coronene with n-carbon alkyl chains) and their supramolecule self-assembly on HOPG surface are studied. Molecular dynamics simulations using COMPASS force field have been used to optimize the geometry of FTBC-Cn single molecule and FTBC-Cn supramolecule self-assembly. Frontier (molecular) orbitals, STM experimental images and band gaps have been simulated by the density functional theory (DFT) and the first principle methods based on DFT. The used simulation results of stable structures and STM images of FTBC-Cn supramolecule self-assembly (n=4,6 and 8) are validated by the corresponding experimental results at room temperature, and then the same items are predicted at OK,333K and 353K.The geometry and electronic properties of above FTBC-Cn molecules and FTBC-Cn supramolecule self-assembly have been analyzed, and their thermal stabilities at different temperatures are predicted by simulation. The main results are as follows:1. the geometry of FTBC-Cn supramolecule self-assembly can be tunable and controlled by changing the length of alkyl chain, which can obtain new kind of two-dimensional surface material; 2. when triangular graphene molecule joined with six alkyl chains, the band gap increased, which made it present broad band semiconducting feature, then after FTBC-Cn molecules assembling on HOPG substrate, the band gap reduced, which presents narrow band semiconducting property. And the band gap became slightly larger when the alkyl chain is longer; 3. The electronic density of state distribution of FTBC-Cn molecules changed obviously after they orderly self-assembly on HOPG substrate; 4. The geometry and electronic density of state distribution of FTBC-Cn supramolecule self-assembly (n=4,6 and 8) are stable from 0 K to 353 K (except that the electronic density of state distribution of FTBC-C4 graphene supramolecule self-assembly changed at 353 K).In short, a kind of new two-dimensional surface material with narrow band semiconducting feature can be obtained via changing the topological configurations of graphene, adding alkyl chains and self-assembly on HOPG, which overcome the problem of zero band gap of graphene, furthermore its geometry and electronic density of state distribution can be tunable and controlled by changing the length of alkyl chain.