New Structural Materials With Different Dimensions: A First-principles Study

As we all know, structure determines the properties of material. In the modern material science, we cannot rely on nature to directly give us materials with superior properties. While we need to discover it by design. To design a new material with new properties, there are basically two approaches. One is structure simplification and the other one is complication. In the present thesis,2D graphene and amorphous structural material were taken as examples for simple and complicate structure, respectively. The geometric structure, electronic structure, magnetism properties and optical properties were studied by performing the first-principle calculation. The contents of this thesis were shown below:In the first chapter, the background knowledge on first-principle calculation method which is based on density functional theory was introduced. It includes the choice on exchange-correlation functions and some extensions. For instance, local density approximation （LDA）, generalized gradient approximation （GGA）, non-local function, hybrid function, LDA+U, self-consistent U calculation, dynamic mean field theory （DMFT）, GW approximation, etc. Besides, different atomic potentials, molecular dynamic methods and some common first-principle codes were introduced.In the second chapter, geometric structure, electronic structure and magnetism properties were studied on atom doped graphene structure （including absorption and embedding）. Very different electronic structures were found in graphene doped with different atoms. Graphene embedded with B or N atom both maintain the planar structure of pure grephene, and show a gap at the dirac point. This finding make them have a potential to be used in semiconductor electronic devices. While on Cu/Mn doped calculation, the effect of on-site coulomb repulsion interaction （U parameter）on electronic structure was considered in the calculation.In the third chapter, fluorination induced unzipping of graphene was studied. In experiment, the technic to fabricate large area graphene with good quality is already available. However, the reliable and effective technic to manipulate the form of the graphene sample was needed to be developed as well. From our calculation, it’s proved that fluorination on graphene is energetically favorable. The severe structure distortion induced by hybridization of C atoms from sp2to sp3and repulsion between the F atoms after fluorination weaken the C=C double bond. The high temperature MD simulation showed that fluorinated graphene was unzipped at1500K.Last two chapters are both about2D graphene. Starting from chapter4, we will focus on the amorphous structures. In the fourth chapter, X-ray absorption spectra （XAS） was calculated in first-principle calculations, combining with the experimental synchrotron radiation measurement, the structure transitions in amorphous silica and carbon dioxide under pressure were studied. It was found that the second peak in O K-edge XAS of silica indicates increased coordination of Si atoms, while it’s not necessary6coordinations. This second peak was induced by the hybridization between O₂p orbital and Si₃d orbital. For carbon dioxide, the phase transition from CO2-Ⅰ to CO2-Ⅲ starts at around7GPa. At higher pressure （37GPa）, carbon oxide transfers from a molecular crystalline solid with2coordination of carbon atoms, to a non-molecular amorphous state with3and4coordination of carbon atoms.In chapter5, phase transition of amorphous alloys under high pressure were studied, including the amorphous-to-crystalline phase transition in amorphous Ce75Al25alloy, and amorphous-to-amorphous phase transition in amorphous CaxAl（100-x） alloy. The first-principles molecular dynamic simulations indeed reproduce these phase transitions. In amorphous Ce75Al25alloy, it was found4f delocalization in Ce atoms is not the direct reason inducing the disorder-to-order transition. While the dominant and efficient packing of Ce atom in the FCC lattice plays a key role during the transition. For amorphous-to-amorphous phase transition in amorphous CaxAl（100-x） alloy, it’s induced by the discontinuous charge transfer between Al and Ca atoms under high pressure.In chapter6, we summarized the projects I have done during the Ph. D study, and made a prospect for the future research.