First-Principles Study of Field Emission Properties of Graphene-ZnO Nanocomposite

The electronic structures and field emission properties of hybrid graphene-ZnO are investigated by density functional theory. The binding energies, energy levels, and the corresponding local electron density distributions of neutral and charged graphene-ZnO in electric fields are analyzed, which show that the electronic structures of graphene-ZnO are modified significantly by the applied electric fields. The reduction of the energy gaps and the change of the electron density distributions of the states in the vicinity of the Fermi level are observed. The work functions of graphene-ZnO decrease linearly with increasing electric fields, which indicate the enhancement of field emission properties. In addition, the ionization potentials decrease drastically with increasing electric fields, which further indicate the improvement of field emission properties of graphene-ZnO. The Hirshfeld charges and the electrostatic potential derived charges are also explored, and it is found that the electron accumulation becomes obvious on the topmost six-membered ring with increasing electric field. Our results show that the graphene-ZnO is a promising candidate for a field emission electron source.