Crystal Structures Design And Physical Properties Of Two-Dimensional Carbon-Based Spintronics And Dirac Materials

The development of human society depends on the discovery and application of new materials that are closely related to people's daily lives.From the stone age to the information era,every major economic and social change in history comes along with the development of new materials.However,the traditional way of discovering new material is constantly trying and testing in experiments.This is an arduous task,which requires a great deal of human and material resources,and inevitably leads to the waste of experimental resources.With the development of computational physics,a way to solve this problem was proposed.First,the structure and properties of materials are theoretically designed and predicted.According to the theoretical results,the materials are initially screened and then verified by experiments.This will greatly improve the work efficiency of experimentalists and shorten the cycle of material preparation.This is also the the significance of our four research work included in this thesis.That is,the work about structural design and physical properties of these two-dimensional?2D?carbon-based spintronics and Dirac materials provides a solid theoretical support and guidance for their experimental synthesis.Spintronics is based on the electronic spin freedom of materials for information transfer,processing and storage.It has advantages such as faster running speed,higher integration and lower energy consumption,which are unmatched by the traditional semiconductor electronic devices.Therefore,it has become a hot spot for people's research since being proposed.Currently,spintronics still faces three major challenges:spin generation and injection,long-range spin transport,and spin detection and regulation.The core and most fundamental task in solving these problems is to find suitable spintronic materials.In addition,with the increasing integration and miniaturization of electronic devices,the search for 2D spintronic materials has gradually become an important research field in the field of spintronics.It has very important theoretical and practical significance for the development of low-dimensional spintronics.The important developments of graphene and topological insulators have led to the emerging field of "Dirac physics," in which the quantum relativistic properties of a class of special materials with Dirac cones,namely Dirac materials,are investigated.Such materials exhibit linear electronic band dispersion at the Fermi level,i.e.,the Dirac band,and thus have charge carriers behaved like massless Dirac fermions.The unique Dirac band endows materials with many novel phenomena in electronic transport,including ballistic charge transport and high carrier mobility,Klein tunneling,various quantum Hall effects,etc.Because of these specific transport properties,Dirac materials have a wide range of promising applications in high-speed low-dissipation devices.In addition,the continuous reduction in the size of these devices necessitates the development of low-dimensional materials.Thus,2D Dirac materials are much more desirable for applications in nanoscale integrated circuits.Theoretically,based on the generalized von Neumann-Wigner theorem,at least three conditions are required to achieve Dirac bands in 2D materials:1)specific symmetries,2)proper parameters,and 3)appropriate Fermi level and band overlap.Due to these rigorous conditions,2D Dirac materials,espectively 2D organic Dirac materials,are found to be very rare.Only two types of 2D organic Dirac material have been discovered so far and have very low Fermi velocities.Therefore,designing 2D organic Dirac materials with high Fermi velocities is of great practical significance.This dissertation focuses on the structural design and physical properties prediction of 2D carbon-based spintronics materials and Dirac materials.The main contents are as follows:1.By using the particle swarm optimization structure searching method combined with density functional calculations,two kinds of boron carbonitride monolayer structures?B₄CN₃ and B₃CN₄?were proposed and confirmed to be dynamically and kinetically stable.The results show that the magnetic ground states of the two B_xC_yN_z systems are all ferromagnetic ordering with a high Curie temperature of respectively 337 K for B₄CN₃ and 309 K for B₃CN₄.Furthermore,based on their respective band structures,the B₄CN₃ monolayer is found to be a bipolar magnetic semiconductor?BMS?,while the B₃CN₄ monolayer is identified to be a type of spin gapless semiconductor?SGS?,both of which are potential spintronic materials.In particular,carrier doping in the B₄CN₃ monolayer can induce a transition from BMS to half-metal,and its spin polarization direction is switchable depending on the doped carrier type.The BMS property of the B₄CN₃ monolayer is very robust under an external strain or even a strong electric field.By contrast,as a SGS,the electronic structure of the B₃CN₄ monolayer is relatively sensitive to external influences.Our findings successfully disclose two promising materials toward 2D metal-free spintronic applications.2.In the second work,we further investigated the structural and electronic properties of two metal-free heterobilayers constructed by vertically stacking 2D spintronic materials?B₄CN₃ and B₃CN₄?on h-BN monolayer using density functional calculations.It is found that both the B₄CN₃ and B₃CN₄ monolayers can be stably adsorbed on the h-BN monolayer due to the van der Waals interactions.We demonstrate that the BMS behavior of the B₄CN₃ layer and the SGS property of the B₃CN₄ layer can be well preserved in the B₄CN₃/BN and B₃CN₄/BN heterobilayers,respectively.The magnetic moments and spintronic properties of the two systems originate mainly from the 2p_,electrons of the carbon atoms in the B₄CN₃ and B₃CN₄ layers.Furthermore,the BMS behavior of the B₄CN₃/BN bilayer is very robust while the electronic property of the B₃CN₄/BN bilayer is sensitive to the interlayer couplings.These theoretical results are helpful both in understanding the interlayer coupling between the B₄CN₃ or B₃CN₄ and h-BN monolayers and in providing a possibility of fabricating the 2D composite B₄CN₃/BN and B₃CN₄/BN metal-free spintronic materials theoretically.3.Based on the first-principles calculations,we proposed a stable 2D material with a honeycomb-kagome lattice,i.e.,the Mg₃C₂ monolayer.This monolayer is an anti-ferromagnetic?AFM?semiconductor at its ground state.A transition from AFM semiconductor to ferromagnetic half-metal in this 2D material can be induced by carrier?electron or hole?doping.In addition,the half-metallicity arises from the 2p_<sub>z orbitals of the carbon?C?atoms for the electron-doped system,but from the C 2p_x and 2p_y orbitals for the case of hole doping.Our findings highlight a new promising material with controllable magnetic and electronic properties toward 2D spintronic applications.4.Based on the framework of pyrrole molecules,we designed a 2D organic material with C₄N₃H stoichiometry that possesses fascinating structure and good stability in its free-standing state.More importantly,we demonstrated that this monolayer is a semimetal with anisotropic Dirac cones and very high Fermi velocity(1.1×10⁶ m s^-1).This Fermi velocity is roughly one order of magnitude larger than the largest velocity ever reported in 2D organic Dirac materials.In the band structure of this system,the Dirac point is located between the ? and K points.The Dirac states in this monolayer arise from the extended ?-electron conjugation system formed by the overlapping 2p_<sub>z orbitals of carbon and nitrogen atoms.Our finding paves the way to a search for more 2D organic Dirac materials with high Fermi velocity.To sum up,we performed the structural search among some 2D carbon-based materials and obtained some stable structures,including the B₄CN₃ and B₃CN₄ monolayers,the B₄CN₃/BN and B₃CN₄/BN heterobilayers,the Mg₃C₂ monolayer,and the organic C₄N₃H monolayer.Based on the first principles calculations,we further systematically studied their physical properties?including electronic structure,magnetic and mechanical properties?,which provides the theoretical supports and guidances for the further synthesis of these materials.