First Principles Study On The Properties Of Electron Spin Polarization And Regulation In Graphene Nanoflakes

With the fast development of information technology and the continuous improvement of integrated circuit manufacturing technology,traditional silicon-based electronics devices are about to become a significant obstacle to the development of integrated circuits in the post-Moore era,so it is urgent to research and develop devices based on new materials,new structures and new processes.Spintronics is a combination of microelectronics,magnetism and material science in recent years,it is designed to use the spin degree of freedom to achieve information storage,transmission and processing.The research on the new spintronic devices based on spin properties of electrons has become the most active scientific frontier.Compared to conventional semiconductor electronic devices,it has high integration,fast running speed,low energy consumption.In addition to controlling the spin and charge properties of electrons,spintronic devices require materials with high electron polarizability and long electron spin relaxation time.Two-dimensional thin film materials-graphene,composed of carbon atoms with hybrid orbitals hexagonal honeycomb lattice,has peculiar physical and chemical properties,such as ultrahigh electron mobility,high mechanical strength,klein tunneling effect and quantum hall effect.It has huge potential in the field of spintronics for its relatively weak spin-orbit coupling effect with a longer spin relaxation time and the room temperature spin diffusion length.However,carbon atoms do not contain f and d electrons,and graphene's intrinsic nonmagnetism makes it lack of ordered spin magnetic moment,which greatly limits its application in spintronic devices.Therefore,how to induce magnetism,especially long-range ferromagnetism into graphene materials is of great significance for the application of graphene into spintronics.Compared with the two-dimensional graphene materials,low dimensional graphene nanomaterials expose many boundary atoms whose bonding characteristics are different from internal carbon atoms,so that the low dimensional graphene nanomaterials present different electronic and magnetic properties.Therefore,inducing the quantum confined effect on graphene materials is considered to be an effective method of spin polarization.In this dissertation,the zigzag boundary zero-dimensional graphene nanoflakes for various shapes are taken as the research object,since the appearance of non-zero magnetic moment of graphene strongly depends on its geometric topology.The electronic structure and intrinsic electron spin properties of zero-dimensional graphene nanoflakes are taken as the concentration and the first principle is adopted for simulation calculation based on quantum size effect and boundary effect.The main research contents are as follows:(1)The ground state electronic structure and magnetic characteristics of zigzag zero-dimensional graphene nanoflakes with different shapes and sizes were systematically studied.The results show that the ground state magnetism of the zigzag graphene nanoflakes with rectangular,rhombus and bowtie shapes is anti-ferromagnetic coupling orderings,while the ground state magnetism of the triangular graphene nanoflakes is ferromagnetic coupling magnetic orderings.The bowtie structure is composed of two triangular nanoflakes,so the anti-ferromagnetic coupling strength is the strongest.The spin of the electrons in the zero-dimensional graphene nanoflakes in all geometric configurations was degenerate,and the spin polarization effect of the electrons was not generated.An "all graphene" spintronic device was designed based on bowtie-shaped zero-dimensional graphene nanoflakes unit,which can realize the functional operation of the logic gate in digital circuits.(2)The electric field effect on the spin polarization properties of single-layer zigzag zero-dimensional graphene nanoflakes was systematically studied.The research results show that with the increase of electric field intensity,zero-dimensional graphene nanoflake can still show a good magnetic coupling characteristics,but the distribution of symmetry is broken,different spin orientation of electrons no longer degenerate.Electronic spin polarization effect can be realized under a certain critical electric field intensity,namely the half-metallicity.This half-metallic property makes it possible to regulate the effect of electron spin polarization by the electric field,and it also provides a theoretical basis for the application of this material to the emerging spintronic devices.(3)The electronic structure and magnetic characteristics of symmetrical and asymmetric zigzag edge bilayer graphene nanoflakes were systematically studied,and the regulation effect of inter-layer angle effect on graphene electrons was also studied.Calculation results show that under the external vertical electric field,the system of electronic distribution of symmetry is broken,different spin orientation of electronic no longer degenerate,namely original degenerate spin band gaps divided.The electron spin polarization effect of bilayer graphene nanoflakes can be realized under a certain critical electric field intensity,namely the half-metallicity.In addition,the spin polarization effect of electrons in the system can be regulated by the relative rotation angle between the two layers of graphene nanoflakes.(4)The modulation of electron spin polarization properties of a planar hetero-structured system of boron nitride doped zero-dimensional graphene nanoflakes was systematically studied.The results show that the planar heterojunction of graphene-boron nitride can break the electron spin degenerate state of the original zero-dimensional graphene nanoflakes and cause the electron spin polarization.In particular,when the number of atoms of boron nitride and graphene fragment is equal,the most significant spin splitting reaches the maximum,that is,the antiferromagnetic coupling strength of the system is the strongest.In particular,graphene nanoflakes embedded in boron nitride materials can still achieve the spin properties of graphene electrons.In addition,the instability of zigzag boundary can be greatly improved through graphene-boron nitride boundary.