Microstructure evolution of diazonium functionalized graphene: A potential approach to change graphene electronic structure

Graphene sheets were successfully functionalized with 4-nitrophenyl diazonium (NPD). Two dimensional Raman analysis demonstrated that the reaction preferred to happen on single-layer graphene rather than bi-layer or multi-layer, and the edges of graphene were more reactive than the central areas. Atomic force microscopy (AFM) indicated the aryl groups were covalently bonded to one side of the graphene basal plane in a perpendicular configuration. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) manifested that the modified graphene maintained the hexagonal symmetry but its microstructure changed. The main change was that the crystal lattice expanded compared with that of pristine graphene. Meanwhile, for the first time, a crystal lattice constant d ≈ 5.30 Å of functionalized graphene was obtained, which was approximately twice that of the pristine graphene's crystal lattice constant. This implied that the modified graphene had a super-lattice microstructure. Furthermore, the fast Fourier transform (FFT) of the modified graphene verified the formation of the super-lattice structures, and density functional theory (DFT) calculations showed the stability of the super-lattice structures. These modifications - elongation of crystal lattice constant and formation of super-lattice structures - may induce different electronic structures in graphene.