First-Principles Investigation On Field Emission Mechanism Of Graphene Composite

Graphene is the first kind of two-dimensional(2D) crystal structures stability exist in nature, its found in the laboratory by people since 2004, graphene-based composite materials have attracted people`s interest in solid physics, materials and micro/nano electronic disciplines, and other fields. Graphene is a new carbonaceous material formed by close packing monolayer of carbon atoms into a 2D honeycomb structure.It is composed of carbon atoms with sp2 hybridization state, and its thickness merely one carbon atom layer. Graphene exhibits unique and excellent properties in force,light, heat, electricity, magnetism, mechanical properties and other disciplines because of its stable and unique lattice structure. Graphene has potential and important applications in the fields of micro/nano electronic devices, solar cells, gas sensors,super capacitors, field emission and hydrogen storage.By virtue of its unique electronic properties, high surface areas, unique boundary conditions and continuous current density, graphene has been regarded as an excellent field emission material. Currently, specific to the field emission properties of graphene nanosheets and graphene oxide has made a breakthrough progress. However,commercialization applications of graphene still face many technical problems yet to be solved. Considering the graphene has many unique properties, but so far there are not many conventional materials to combine with it, makes much of its excellent performance is not fully play out. Meanwhile, a high carrier concentration graphene,low resistivity, but its semiconductor properties is not significant, it also limits the development and applications of graphene to some extent. Based on the above analysis, further investigation on graphene-based composite material becomes very necessary. In this paper, we have investigated the field emission properties of graphene-CNTs, N-doped graphene-CNTs, graphene-C60, and N-doped graphene-C60 compound by using first-principles method.In order to explore the field emission properties of graphene compounds, we established the graphene-CNTs and graphene-C60 two kinds of composite structure.Under different electric fields, we calculated the binding energy, ionization energyand work function of the composite structure, also analyzed the local electron density distribution, HOMO-LUMO energy levels and the band gap, Mulliken charge distribution correspondingly. Studies have found that: the ionization energy and work function decreased continuously with the increase of the electric field, indicating that the field emission performance has been further improved. Under the action of applied electric field, HOMO and LUMO levels are moving towards the direction of the vacuum level, the Fermi level while also moving to the vacuum level and the HOMO-LUMO band gap remained small value and decreased gradually. The local electron states are appeared on the two kinds of composite structure under the action of electric field; however, these local electron states have a positive impact on the field emission properties. The effective move of Mulliken charge, further verify that the composites we constructed are indeed have excellent field emission characteristics.In order to further enhance the field emission properties of graphene composites,we constructed the N-doped graphene-CNTs and N-doped graphene-C60 two kinds of composite structure respectively. To find the optimum doping point in the composite structure, we have chosen a number of different doping positions to compare.Researches showed that doped with N atoms make HOMO-LUMO band gap further reduced, ionization energy and work function are also further decreased. In addition,with the introduction of N atoms, the LUMO states of two kinds of composite structures are transferred from the initial graphene nanodisk to the CNTs and C60 respectively, and formed the coupling states and local states respectively, and also more negative charge concentrated on the N atoms largely. The above phenomena will improve the electronic structure of the whole composite structure, thereby improving the field emission characteristics of the composite.