First-principles Study Of Low-Dimensional Magnetic Materials

Low-dimensional magnetic materials exhibit excellent physical and chemical properties due to size effects and unique spin properties,which have attracted the interest of researchers in spin catalysis,spin electronic devices and other fields.The traditional trial-and-error method in experiments consumes a lot of time and energy,and hinderes the development of low-dimensional magnetic materials.With the rapid development of first-principle calculations,researchers are now able to efficiently screen and design multifunctional magnetic materials.And this has allowed them to gain a deeper understanding of the underlying structure-property relationships and mechanisms involved,ultimately accelerating and expanding the practical applications of low-dimensional magnetic materials.At the same time,in order to achieve high-performance magnetic materials,effective external regulation of the spin properties in materials is a key challenge that needs to be addressed.Based on first-principles calculations,in this thesis,we have conducted two works focusing on spin regulation of low dimensional materials:Firstly,we design a two-dimensional(2D)metal organic framework(MOF)and explore spin crossover(SCO)phenomenon under compressive strain.Furthermore,the SCO phenomenon is employed to achieve reversible control of catalytic activity in the oxygen evolution reaction(OER),which provides a new approach for spin-controlled catalytic reactions.Secondly,based on the experimentally synthesized porphyrinoid-based diradical organic molecule,two metal organic frameworks(MOFs)structures are constructed,and their electronic and magnetic properties are regulated via intramolecular proton transfer.This provides a new pathway for regulating spintronics properties through chemical isomerization methods.This article is divided into five chapters.The first chapter provides a brief overview of research on low-dimensional magnetic materials,including methods for synthesizing low-dimensional materials,techniques for controlling materials,and the applications and research of low-dimensional magnetic materials.Chapter 2 introduces density functional theory to calculate and reveal the electronic structure of low-dimensional magnetic materials,and the computational softwares used in the thesis.In Chapter 3,we propose to utilize spin crossover(SCO)in 2D MOFs to achieve reversible control of OER catalytic activity.The theoretical design of 2D square lattice MOFs with Co as nodes and tetrakis-substituted cyanimino squaric acid(TCSA)as ligands,which transforms between high spin(HS)and low spin(LS)state by applying an external strain(～2%),confirms our proposal.In particular,the HS-LS spin state transition of Co(TCSA)regulates considerably the adsorption strength of key intermediate HO*in OER process,resulting in a significant reduction of overpotential from 0.62 V in HS state to 0.32 V in LS state,thus realizing a reversible switch for the activity of OER reaction.Moreover,the LS state is shown to possess good OER catalytic performance in both acidic and alkaline conditions.In Chapter 4,intramolecular proton transfer is proposed as a simple approach for regulating the electronic and magnetic structures of porphyrinoid based 2D MOFs.Based on the experimentally synthesized porphyrinoid diradical organic molecule,two MOFs structures are designed:Fe2(cyan-PB)and Fe2(PB).The former is constructed by connecting the Fe atoms and the porphyrinoid molecule via cyano groups while the later is constructed by connecting the Fe atoms and the N atoms of the porphyrinoid molecule.For Fe2(cyan-PB),the magnetic ground state of the system can change from ferromagnetic to ferrimagnetic by migration of the hydrogen atoms in the center of the porphyrinoid molecule,and one of the isomers exhibits characteristics of a bipolar magnetic semiconductor.For Fe2(PB),the system exhibits half-metallic property when two hydrogen atoms are in the cis position,while it is a magnetic semiconductor when they are in the trans position.This suggests that intramolecular proton transfer is an effective way to switch Fe2(PB)between a half-metallic state and a semiconducting state.These results suggest that that intramolecular proton transfer is an effective way to induce magnetic and electronic phase transitions in the porphyrinoid based 2D materials.Chapter 5 provides a brief summary of the work in this thesis and provides a perspective on future research on low-dimensional magnetic materials.