  Molecular adsorption at the surface of a 2D 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-principle density
functional theory is employed on the 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. The induced
strain upon ammonia-adsorption opens the bandgap (Eg) of graphene due to the
breaking of translational symmetry and shifts the equilibrium Fermi energy
(EF). The Eg and EF 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 EF. NH3 forms an interfacial occupied state originating from
N2p below the EF within 1.6 - 2.2 eV for all configurations. These findings
enhance the fundamental understanding of the graphene/NH3 system.
