Tunable Electronic And Magnetic Properties Of Functionalized ?-? And ? Two-Dimensional Nanostructures: A First-principles Calculations Study

Recent years,with the development of the super computer,ab initio calculation methods based on density functional theory(DFT)have been widely used in chemistry,physics and materials science.The theoretical calculation method not only can supply theoretical explanations and basis for experimental phenomenons,but also can forecast the unique properties of materials and design new material structures,providing innovative techniques and guidance.Using mechanical exfoliation,graphene which is the single sheet graphite was been first isolated by Geim and Novoselov in 2004 and they two won the Nobel prize for this achievement in 2010.Sine then,graphene has been the hot field in chemistry,physics and materials science.Due to the intriguing and unique properties of graphene and its promising application in nanodevice,scientists and researchers are motivated to explore other two-dimensinal materials.Recent researches show that single sheet BN,Ga N and Si C have been successfully synthesized experimentally.Different from bulk structures,two-dimensional nanosheet has unique mechanical,optical,electronic and magnetic properties.And all the atoms in nanosheet are exposed outside environment,therefore it is possible to modify the properties through adsorption and surface modification.In this thesis,the electronic and magnetic properties of functionalized ?-?(Ga N)and IV(Si C?Ge C)two-dimensional nanostructures are investigated by the first-principles calculations.The results are summarized as below:1.Modifying the structures and properties through chemical functinalizations has been a hot field in two-dimensional nanostructures research.Here,the intriguing magnetic and electronic properties of the semihydrogenated,semichlorinated and semihydroxylated Ge C sheet are investigated by means of the first principle calculations.It is found that Ge C sheet behaves a ferromagnetic semiconductor at room temperature when Ge atoms are hydrogenated,while an antiferromagnetic semiconductor when H atoms are absorbed on C atoms.Energy calculation shows the former a more stable configuration.In addition,using chlorine atom and hydroxyl group to substitute the hydrogen atom,it is found that the semihydrogenated Ge C sheet evolves from semiconducting to half-metallic with the increase of the chlorine substituted concentration,while it evolves from ferromagnetic state semiconductor to antiferromagnetic semiconductor with the increase of the hydroxyl substituted concentration.Such a result suggests the potential applications of semihydrogenated Ge C in the magnetic electronics.2.Defect is unavoidable in the process of crystal growth,and the defect can significantly influent the electronic and magnetic properties of structures.Using first principles calculations,the magnetic and electronic properties of vacancies and substitutions in a graphitic Si C sheet are calculated.We investigate the magnetic and electronic properties of the single Si C sheet with the C atom monovacancy,the Si atom monovacancy and the C,Si atoms divacancy.We find that only the Si atom monovacancy can induce magnetism Furthermore,magnetism induced by the nonmagnetic elements doping is also addressed.B,C,N,Si and P elements are used to substitute for either C or Si atoms as dopants.The formation energies and binding energies of the dopants are calculated to evaluate the stability.Especially,N and P atoms prefer to substitute the Si atom and can induce half-metallic electronic property,suggesting the N and P substitutional doping could be promising process to fabricate spintronic devices.The vacancies and substitutions significantly change the band structure near the Fermi level,and can be used for tailoring the magnetic and electronic properties of Si C sheet.3.Nanoribbon has attracted lots of interests because of its varied edge effect.First principle calculations are performed to investigate the electronic and magnetic properties of hydrogenated and fluorinated zigzag Ga N nanoribbons(z Ga NNRs).Five kinds of possible different hydrogenated structures and four kinds of possible fluorinated structures are considered,and they show different electronic and magnetic properties.We find that the Ga-edges with two hydrogen atoms terminated or two fluorine atoms terminated are ferromagnetic while the N-edges with two hydrogen atoms terminated or two fluorine atoms terminated are nonmagnetic.Results show that the structure is half-metal when the Ga-edges are saturated with two fluorine atoms and N-edges are saturated with one fluorine atom.Moreover,the magnetic and electronic properties can be tailored by external electric field.z Ga NNRs transform from half-metal to semiconductor under Ga?N direction electric field;it also can change from half-metal to magnetic metal then to nonmagnetic metal under N?Ga direction electric field.