First-Principles Studies Of Graphene Oxide And Chemical Functionalization Of Two-Dimensional Materials

With the rapid progress in the enhancement of computational ability and relative methods, density functional theory (DFT) based first-principles calculations has been widely used in materials science, and become one of the powerful tools for material characterization and design. In experiment, many efficient techniques have been de-veloped to synthesize materials and determine their structures. Theoretical calculations can not only explain experimental phenomena, helping us understand the experiments better, but also predict new materials or new properties for existing materials. Due to their novelty in mechanic, electronic, optical, magnetic, and catalysis properties, nano-materials have become an important field in recent study. In this dissertation, based on first principles calculations, we mainly investigate an important low dimensional material, graphene oxide, and also functionalization of two-dimensional materials.In the first chapter, we briefly introduce DFT, nuclear magnetic resonance (NMR) simulation, and free energy calculation. The first part includes fundamental theorems of DFT, exchange correlation functional, basis set, pseudopotential potential, and some commonly used package in DFT calculation, etc. Then a brief introduction has been done to NMR simulation, especially the gauge including projector augmented waves (GIPAW) method suitable for periodic systems. Finally, first principles free energy calculation has been introduced.Due to its unique properties and broad application potential, graphene has become a rinsing start in materials science. A method for high quality sample preparation at large scale is thus very desirable. Chemical oxidation and reduction is a promising way for this purpose, and graphene oxide (GO) is an important intermediate. At the same time, graphene oxide has many potential applications. Therefore, it is an important material. In the second chapter, we review the background and some recent progresses of graphene and graphene oxide.The atomic structure of GO is still unclear, and NMR is a widely used method in GO structure studies. In Chapter3, based on first principles13C chemical shift calculations using the GIPAW method, we provide a spectrum-structure connection. The13C chemical shift in GO is found to be very sensitive to the atomic environment, even for the same type of oxidation groups. Factors determining the chemical shifts of epoxy and hydroxy groups have been discussed. GO structures previously reported in the literature have been checked from the NMR point of view. The energetically favorable hydroxyl chain structure is not expected to be widely existed in real GO samples according to our NMR simulations. The epoxy pair proposed previously is also supported by our chemical shift calculations.By calculating vibrational frequencies and comparing with infrared experiment, we confirmed that hydroxyl chain structure is not available in experiment. To resolve this controversy, in chapter4, we check both thermodynamic and kinetic aspects of GO structure. First principles thermodynamics gives a free-energy based stability ordering similar to that solely based on energy, and hydroxyl chain is indeed thermodynami-cally very favorable. Therefore, kinetics during GO synthesis is expected to make an important role in GO structure. Transition state calculations predict large energy bar-riers between local minima, which suggests that experimentally obtained GO samples have kinetically constrained meta-stable structures.Besides computational characterization, we also perform theoretical design of new materials. For two-dimensional systems, it is an important means to tune their electronic structures by on-plane chemical functionalization. In Chapter5, on the basis of DFT, we study the electronic structures of fluorine-substituted planar polysilane and graphane. We find that carbon and silicon present very different surface chemistries. The indirect energy gap of planar polysilane becomes direct upon fluorine decoration, and its gap width is mainly determined by fluorine coverage regardless of its distribu-tion on the surface. However, the electronic structure of fluorine doped graphane is very sensitive to the doping configuration, due to the competition between antibonding states and nearly free electron (NFE) states. With specific fluorine distribution pat-terns, zero-dimensional and one-dimensional NFE states can be obtained. Our results demonstrate the advantages of two-dimensional silicon based materials compared with carbon based materials, in the viewpoint of practical electronic structure engineering by surface chemical functionalization.