Theoretical Studies On Molecular Design,Synthesis Mechanism And Properties Regulation Of Low-dimensional Spintronics Materials

With the reduction of complementary metal-oxide-semiconductor(CMOS)feature size in chips,heat dissipation problems and quantum effects limit the further improvement of chip integration density.But at the same time,the demand for data storage and information processing is increasing day by day in the development of modern industry.In order to solve this contradiction,it’s necessary to explore alternative technologies of CMOS.As a non-charge-based technology,lowdimensional spintronics can realize information storage and logic operations by utilizing the spin of electrons in low-dimensional materials,which theoretically has the advantages of low power consumption,high speed and zero heat generation,providing a technical solution for replacing CMOS.The development of low-dimensional magnetic materials is the key to realize the application of low-dimensional spintronics.However,experimentally synthesized low-dimensional spintronics materials suffer from the problems of low magnetic ordering temperature and no pure spin-polarized electronic structure,which limits their practical applications in low-dimensional spintronics devices.In recent years,a series of organic open-shell molecules have been experimentally synthesized on metal surfaces.By assembling these organic open-shell molecules,metal-free low-dimensional covalent organic magnetic materials can be designed.In addition,the magnetism of low-dimensional inorganic nonmagnetic materials can be induced by surface functionalization,which also aims for the application in low-dimensional spintronics.With the development of computational chemistry methods,first-principles calculations have become an important tool to design and study novel low-dimensional spintronics materials.In this paper,by density functional theory calculations,a series of low-dimensional covalent organic spintronics semiconductor materials with good stability,high magnetic order temperatures and pure spin polarization band structures are designed from the perspective of organic open-shell molecule assembly.And we systematically study their magnetic properties,electronic structures,and synthetic routes.Furthermore,this paper also studies the mechanism of magnetism regulation of two-dimensional tellurium nanosheets by oxygen adsorption.The main research content and results of this paper are as follows:1.The theoretical design and study of graphene anti-point lattices(GALs)semiconductor materials with room temperature magnetism.This chapter contains three parts of work.1)From the perspective of topological frustration,the magnetic properties of two open-shell graphene nanoflakes(GNFs),i.e.,cis/trans-triangulene dimers are studied,and we demonstrate that their multiradical characters originate from Coulomb repulsion of π electrons.Subsequently,the magnetisms of the π-extended triangulene dimers are studied,and the results reveal that the intramolecular magnetic coupling strength after π-extension depends on the topology of GNFs.2)Assemble open-shell GNFs to obtain a series of GALs semiconductors with room temperature magnetism.Finally,seven magnetic GALs semiconductors with thermodynamic and kinetic stability were designed.The triangular antidot GALs are bipolar magnetic semiconductors with Curie temperatures between 287 K and 528 K,while hexagonal antidot GALs are antiferromagnetic semiconductors with Neel temperatures change from 210 K to 391 K.3)To study the interlayer interaction of GALs.The results reveal that the interlayer magnetism of bilayer GALs depends on their stacking order.In addition,the contact between GALs and Au(111)surface will weaken the ferromagnetism of GALs,while GALs can maintain strong ferromagnetism after being encapsulated by inert h-BN.This research provides the candidate materials for lowdimensional covalent organic spintronics devices working at room temperature.2.Design two-dimensional ferromagnetic covalent organic radical frameworks(2D CORFs)through the donor-acceptor mechanism between antiaromatic linkers and radicals.This chapter contains two parts of work.1)The magnetic properties of dehydrotruxene monomers and polymers are studied,and the results reveal that the ferromagnetic coupling between monomers cannot be achieved by directly covalent bonding dehydrotruxene monomers.However,the ferromagnetic coupling between dehydrotruxene can be realized by linking with antiaromatic hydrocarbons,and forming donor-acceptor molecular chains.We find that antiaromatic linkers with low singlettriplet excitation energies and higher frontier orbitals facilitate the ferromagnetic coupling of dehydrotruxene.2)To study the 2D CORFs with antiaromatic linkers,and we finally find that by linking dehydrotruxene monomers with pentalene,a kinetically and thermodynamically stable ferromagnetic 2D CORF is obtained with Curie temperature of 92 K.The results show that this 2D dehydrotruxene-pentalene CORF is a bipolar magnetic semiconductor.This study provides a theoretical strategy of designing high-spin ferromagnetic 2D CORFs for low-dimensional covalent organic spintronics.3.To study the synthesis mechanism and physical property regulation of lowdimensional nanomaterials.This chapter includes four parts.1)The self-assembly of organic radicals on the surface of Ag(100)to form chiral supramolecular structures is studied,and the intermolecular hydrogen bonds are found to play a significant role in the formation of organic radical self-assembled structures.2)Design a synthetic route for the synthesis of carbon five-membered ring molecules on the surface of Ag(110)through[4+1]annulation reaction.It’s shown that the template effect of Ag(110)surface alters the reaction pathways of the intermediates and finally improves the selectivity of the carbon five-membered ring main products.3)By using different surface heat treatments,disordered dendritic structures and ordered porous graphene nanoribbons are synthesized on Cu(111)surface.The study indicates that the negative effects of precursor self-assembly can be avoided by surface preheating treatment,thereby synthesizing ordered low-dimensional organic nanostructures.4)Twodimensional tellurium nanosheets are synthesized by topo-chemical reactions.The study finds that the two-dimensional tellurium nanosheets with oxygen adsorption and a thickness of 2.34 nm possess the spin-polarized electronic structure and negative photoresponse.These studies provide an experimental and theoretical basis for the design of synthetic routes and physical properties regulation strategy of lowdimensional spintronics materials.