(666a) Electronic Structure of Electron-Irradiated Graphene and Effects of Hydrogen Passivation

Authors: 
Weerasinghe, A. - Presenter, University of Massachusetts Amherst
Ramasubramaniam, A., University of Massachusetts Amherst
Maroudas, D., University of Massachusetts, Amherst
Defect engineering through irradiation processes and chemical functionalization of graphene are promising routes for fabrication of carbon nanostructures and two-dimensional metamaterials with unique properties and function. In previous computational studies, we reproduced experimentally observed structures of electron-irradiated graphene sheets through introduction of random distributions of vacancies in the graphene lattice and proper structural relaxation. We found that a vacancy-induced crystalline-to-amorphous transition in graphene occurs for an inserted vacancy concentration between 5% and 10%. This amorphization transition is accompanied by changes in graphene’s thermomechanical response, including a brittle-to-ductile transition in the fracture mechanism upon uniaxial tensile straining as well as a transition in the lattice thermal transport mechanism in these irradiated graphene sheets.

Here, based on molecular-dynamics (MD) simulations in conjunction with first-principles density functional theory (DFT) calculations, we report results for the electronic structure of irradiated and irradiation-induced amorphized graphene. We find that localized states appear at the Fermi level upon irradiation and the corresponding local density of states increases with increasing inserted vacancy concentration. Furthermore, electronic band structure calculations show that band flattening occurs due to electron localization in the vicinity of irradiation-induced defects and reduces the charge carrier mobility. This band flattening effect becomes stronger with increasing vacancy concentration inducing an increasing number of flat bands near the Fermi level. Moreover, we present electron wave functions (as frontier orbitals) and local charge density distributions, which provide clear evidence of carrier localization near the irradiation-induced carbon dangling bonds. Passivating these bonds with hydrogen atoms leads to delocalization of the charge density, hence increasing the carrier mobility, which also is seen in the reduced density of states observed at the Fermi level and the increased band dispersion with increasing inserted vacancy concentration. Importantly, we find these spatially localized states to be spin polarized, which gives rise to a net local magnetic moment. Passivation of these states can cause the complete removal of these induced local magnetic moments. Our studies set the stage for understanding and designing electronic two-dimensional materials for specific applications using irradiated graphene and passivated irradiated graphene as a well understood template.