Theoretical Prediction Of New Silicon Phases And Two-Dimensional Planer Pentagonal Structures

Owing to its peculiar physicochemical properties and potential application,graphene has attracted many researchers'attention to two-dimensional materials.Atomically and few layers thick sheets of 2D layered inorganic materials exist many types of two-dimensional materials,such as the familiar hexagonal boron nitride（h-BN）,transition metal disulfide and some metal oxide,etc.In recent years,the reduction of natural resources and environmental pollution have drawn more and more attention to the development and application of new renewable energy,especially the research in solar cell device.Conversing solar energy into electricity is considered to be the most promising technology.In comprehensive comparison,silicon material is undoubtedly to be one of the best choices in optoelectronic applications.In this paper,based on the density functional theory,on the one hand,we predict two types of two-dimensional pentagonal planar Al₂C and AlC₂ structures,and on the other hand,few new bulk silicon materials are also proposed based on the known ice phases.And then,GGA/PBE exchange correlation functions are used to calculate the geometric optimization and the related properties of these target structures.The final results are as follows:In chapter three,based on pentagonal graphene model for calligraphy,first of all the Al₂C structure can be obtained by replacing the three-coordination carbon atoms with aluminum atoms.That is,the stoichiometric ratio of the Al atom and the C atom is 2:1.The optimized structure presents a quasi-two-dimensional monolayer planar structure.The molecular dynamics shows that its geometrical frame is basically remain complete at the effective temperature of 1372K and can withstand a certain high temperature,so it can be used as resistant materials in high temperature.Besides,the calculation of the electronic properties present that the Al₂C monolayer is an indirect bandgap semiconductor with no any magnetic.The density of state shows that the main contribution at Fermi surface comes from the hybridization of the C-p orbits and the Al-s orbits.In another case,if the four-coordination of carbon atoms are exchanged for aluminum atoms,the AlC₂ structure will be acquired with the stoichiometric ratio of the Al atom and the C atom is 1:2.From the side view,it looks like a core sandwich with presence of double-wrinkles atomic arrangement.The electronic properties show that the structure is metallic and non-magnetic,but it can withstand actual high temperature in 1856K,indicating its favorable thermal stability and it is very promising to become a class of resistant materials.In chapter four,to obtain the new silicon phases,the first step we need to do is to replace all the oxygen atoms in the known ice network with silicon atoms and then remove all the hydrogen atoms.Our calculation of optimization structures and correlation electronic properties are carried out by using first-principles calculation based on the density functional theory.The final calculated results show that silicon atoms exhibit sp³ hybridization.The energy results indicate that Si _IIIII and Si_Ih structures have relatively the lowest energy and Si _III exhibits metallic properties.In addition,Si_Ih and Si_XII are semiconductors with indirect band gap of 0.68 eV and 0.16 eV,respectively.While the Si _III structure has a direct bandgap of 1.39eV closely to the Shockley-Queisser limit,which may greatly improve the efficiency of energy conversion and become one of the best options for potential solar cell devices.Simultaneously,in order to provide more theoretical reference for experimental research,the X-ray diffraction spectra of these structures are also simulated under zero pressure.