First-Principles Studies Of Structural, Mechanic And Electronic Properties Of Graphene Grain Boundaries

Graphene, a two dimensional of carbon atoms arranged in a honeycomb lattice, have fascinating properties and attracted many interesting from physics, chemical, materials and biological, etc. Graphene is the thinnest and strongest material ever measured, and have the excellent optical, thermal properties and ultrafast mobility. Thus, graphene have great potential in many applications, such as supercapacity, sensor, solar cell, etc. However, in the experimental synthesis using the chemical vapor deposition (CVD) methods, it is inevitable to get the polycrystalline graphene composed of many grains and grain boundaries (GBs). Then, it is important to clarify the gain boundaries effects in graphene for future applications.In this study, both the symmetric and nonsymmetric graphene grain boundaries with periodic length up to18A have been studied. By using the density functional theory, atomic structures, thermodynamic stabilities, mechanical, electronic and transport properties of graphene grain boundaries were investigated. According to the arrangements of pentagons and heptagons on the boundary, grain boundaries were cataloged into four classes. Some nonsymmetric grain boundaries constructed in this work have identical misorientation angles with the experimentally observed ones. The formation energies of grain boundaries can be correlated with the misorientation angle and inflection angle. Some nonsymmetric grain boundaries can possess comparable formation energies to their symmetric counterparts when the periodic length along the defect line is larger than1nm.Electronic density of states analysis shows that the existence of grain boundary usually increases the density of states near the Fermi level, whereas some symmetric grain boundaries can open a small band gap up to0.5eV due to local sp2-to-sp3rehybridization. Nonequilibrium calculations of transmission coefficients showed that the conduction channels can be totally blocked or reduced due to existence of GBs, which may take responssible for the higher resistance of the inter-domains. Moreover, the detailed defect arrangements have influence on the transport behavior of graphene GBs.The mechanical properties of graphene grain boundaries were explored using density functional theory and molecular dynamics. With different arrangements of the pentagonal and heptagonal rings, the graphene boundary may remain flat or become inflected up to72°. For the flat GBs, the intrinsic tensile strength is proportional to the formation energy with a maximum value of93GPa, close to that of perfect graphene. The intrinsic tensile strength of the inflected GBs were found to generally decrease with increase of inflection angle, and the relationship remains with varies temperatures. Different from that at low temperatures, Stone-Wales transformation was identified as the major failure mechanism of graphene GBs at high temperatures, whereas the initial fracture site can be either on the boundary line or inside the domain. We found that the intrinsic strength and critical failure strain of a bilayer graphene are governed by the twist angle. If a bilayer graphene is formed by one perfect graphene sheet and another graphene monolayer with grain boundaries, its overall mechanical properties are dominated by the layer with grain boundaries. The larger grain boundary angle in bilayer graphene, the higher intrinsic strength it would possess because of the lower energy and shorter C-C bond lengths on the boundary.Heterostructures composed of two kinds of materials with distinct physics properties can be applied in many areas. Based on the studies of graphene grain boundaies, structural and electric properties of monolayer graphene and hexagonal boron nitrid (graphene/h-BN) interfaces have been explored. We found that the formation energies of different interfaces are lower than edge energies of both graphene and h-BN ribbons. The Clar’s rule plays important role in thereodynamic stablilities, electric properties and band offsets. All the heterostructures shows a semiconducting properties with the interface under Clar’s rule. The band offsets changed silightly with the changed misorientation angle of graphene/h-BN heterostructuresThese theoretical results constitute a useful picture for the grain boundary effect on the mechanical behaviors of polycrystalline graphene.