First-principles Researches On Novel Two-dimensional Silicon-germanium Alloys

With the rapid development of the two-dimensional material graphene and the science and technology, the quest and improvement of new materials have become ex-tremely urgent. Advances in nano-scale technology also make high performance mate-rials realized in experiment and industry. Nowadays, people do not wait for the product that the nature entrusts to any longer, on the contrary, to be the transformer of nature. Scientific advances and human development offer us technical support to transform the nature. With the complexity and miniaturization of materials, first-principles cal-culations can help us predict the structures and properties of new materials very well. Conversely it provides a theoretical basis for the success of the experiment. It becomes a pioneer and strong background in experiments and industry. Theoretical calculations has been accepted and recognized by human gradually and applied to expose the nature of social and science widely.This thesis is about the first-principles studies in structures and electronic prop-erties of novel two-dimensional silicon-germanium alloys. Start from the researches on structures and properties of two-dimensional materials constituted by Group14el-ements like graphene, silicene, germanene, we predict the geometric structures and electronic properties of the silicon-germanium alloys via first-principles theory and regulate their electronic properties through different means.In the first chapter, we introduce the development and basic concepts of the den-sity functional theory. Density functional theory is based on quantum mechanics. So in the beginning of this chapter, we briefly introduce some basic concepts of quan-tum mechanics, which is around the Hartree-Fock approximation and introduce Born-Oppenheimer approximation principle. Next, We mainly comment on the very early contribution of Thomas, Fermi and Slater. These people put forward that use density as basic variable in the time. Then we continue to introduce the Hohenberg-Kohn theory, Kohn-Sham equation and the several types of approximate exchange-correlation func-tionals currently and the calculation steps of density functional theory. Finally, there is a brief description of some common softwares based on density functional theory.In the second chapter, we will start by introducing the history of graphene, and then its geometry and properties such as:electronic properties, thermal properties, me-chanical properties. We also cite a number of preparation of graphene. Along with the graphene, we list several common allotropes of carbon element and their applica-tions. Since there are many types of technology in regulating the electronic properties of graphene, here we only discuss the hydrogenation. Finally, a brief description of the structures and the properties of the two types of two-dimensional material is men-tioned.In the final chapter, according to the properties of two-dimensional materials like graphene, silicene and germanene, we construct two-dimensional alloys mixed sili-con and germanium elements and study their electronic properties. We find that the electronic properties of such two-dimensional alloys can also be adjusted by hydro-genation, and by changing the composition of the component the band structures can be changed in a certain range as well. Through analyzing the results, we can estimate the type of band and the band gap in any composition. Finally, the results show that not only the concentration of the component can adjust the electronic properties, but also the stress can change their electronic properties. Through changing the concentration of the component combined with the appropriate application of stress, we can predict the structures and the properties of the silicon-germanium alloys theoretically.