Distinguishing strain, charge and molecular orbital induced effects on the electronic structure: graphene/ammonia system

Molecular adsorption at the surface of a two-dimensional material poses numerous questions regarding the modification to the band structure and interfacial states, which of course deserve full attention. In line with this, first principles density functional theory is employed on a graphene/ammonia system. We identify the effects on the band structure due to strain, charge transfer and presence of molecular orbitals (MOs) of NH₃ for six adsorption configurations. Induced-strain upon ammonia-adsorption opens the band gap (E _g) of graphene due to the breaking of translational symmetry. The charge transfer/MOs of NH₃ shifts the equilibrium Fermi energy (E _F). The E _g and E _F values and charge density distribution are dependent on the adsorption configuration, where the MO structure of NH₃ plays a crucial role. The presence of MOs of N or H-originated pushes the unoccupied states of graphene towards E _F. NH₃ forms an interfacial occupied state originating from N2p below the E _F within ∼1.6 to 2.2 eV for all configurations. These findings enhance fundamental understanding of graphene/NH₃ system. (paper)