Quantum Theory of Advanced Materials

Electron-phonon coupling (EPC) in Bernal stacked bilayer graphene (BLG) at different doping levels is studied by first-principles calculations. The phonons considered are long-wavelength high-energy symmetric and antisymmetric optical modes. Both are shown to have distinct EPC-induced phonon linewidths and frequency shifts as a function of the Fermi level E-F. We find that the antisymmetric mode has a strong coupling with the lowest two conduction bands when the Fermi level E-F is nearly 0.5 eV above the neutrality point, giving rise to a giant linewidth (more than 100 cm(-1)) and a significant frequency softening (similar to 60 cm(-1)). Our ab initio calculations show that the origin of the dramatic change arises from the unusual band structure in BLG. The results highlight the band structure effects on the EPC in BLG in the high-carrier-density regime.

We presented a detailed study of the oxidation functional groups (epoxide and hydroxyl) on graphene based on density-functional calculations. Effects of single functional groups and their various combinations on the electronic and structural properties are investigated. It is found that single functional groups can induce interesting electronic bound states in graphene. Detailed energetics analysis shows that epoxy and hydroxyl groups tend to aggregate on the graphene plane. Investigations of possible ordered structures with different compositions of epoxy and hydroxyl groups show that the hydroxyl groups could form chainlike structures stabilized by the hydrogen bonding between these groups, in close proximity of the epoxy groups. Our calculations indicate that the energy gap of graphene oxide can be tuned in a large range of 0-4.0 eV, suggesting that functionalization of graphene by oxidation will significantly alter the electronic properties of graphene.