Theoretical Study On The Crystal Structures And Physicochemical Properties Of Novel Two-Dimentional MB₇(M=Co,Fe) Materials

As unique structure and excellent properties,two-dimensional materials employed in various fields that include electron transmission,energy storage,and surface catalysis.Because of the electron deficiency,boron elements show its diversity in the structure of two-dimensional that arouse grant interest among experimental and theoretical researchers.Based on the density functional theory,the thesis performs comprehensive calculation and simulation for two-dimensional CoB₇ and FeB₇system.A series of new two-dimensional structure with anomalous proportions were successfully predicted and their physical and chemical properties were studied in detail.The first part in this thesis is CoB₇ system.We proposed eight stable structures represented by the global minimum energy structure CoB₇-1.CoB₇-1 is formed by the orderly stacking of B-Co-B atomic layers to form an inverted double sandwich configuration.Moreover,it has great energy,dynamics and thermodynamic stability through various calculations.When a proper biaxial strain is applied,we will find the energy ground state of the deformed structure to change a ferromagnetic state from a non-magnetic state.We believe this new type of two-dimensional metal boride that cause magnetic transformation by applying a gentle strain without changing other external conditions,the example of temperature,will carry out an extensive application in nanomagnetic devices.The second part in this thesis is FeB₇ system,we also found several representatives stable structures,and compared the global minimum structure FeB₇-1 and its twin structures as a whole.The configuration of FeB₇-1 and its twin structures are all connected by Fe atoms in the center to the upper and lower boron layers.Besides,its inside will form a symmetrical sandwich.By the material stability calculations,all three are stable boron alloys with energy,dynamics and thermodynamics.The calculation of electronic properties shows that the above structures are all ferromagnetic metallic materials.In result,their particular band structure,the FeB₇-4 and FeB₇-8,the twin structures of FeB₇-1,maybe become spin voltage filtering devices used in practice.After the application of biaxial strain,a new"band-gap"energy band appears in FeB₇-4,which realizes a large exent the spin filtering effect.As a new two-dimensional alloy material,this characteristic shows its wild potential in the electron transmission field.Furthermore,the unique mechanical properties of FeB₇-1 also make us believe that it can fully be applied in new flexible two-dimensional materials as well.We have proposed a variety of novel two-dimensional metal borides through comprehensive theoretical calculations.Moreover,we also revealed the nature of their chemical bonds and the stabilization mechanism and verified its feasibility of experimental preparation.It hopes to establish a foundation for subsequent research and provide theoretical guidance for future experimental synthesis.