First-Principles Design And Characterization Of Low Dimensional Magnetic Systems

In quantum mechanics,spin is an intrinsic form of angular momentum carried by elementary particles,which is the intrinsic property of microscopic particles.Since it was discovered in 1924,the development and application of spin have been a hot topic in the research field.As a new branch,spintronics is an emerging field,which aims to utilize the spin degree freedom of electrons for novel information storage and logic devices.It has the advantages of high integration and low energy consumption.How-ever,the spintronic materials with excellent properties are still limited,it is urgent for us to find more new materials.In addition,we need to accurately detect and control the spin state from the atomic-and molecular-scale during the development and appli-cation of spin.Here,we use the Kondo effect,a typical strong correlation phenomenon in condensed matter physics,to characterize and analyze the influence of environments on the spin state,and deepen the understanding of spin-spin correlation.At the same time,we use the thermochemical effect of Kondo screening of a local atomic spin by free electrons in the metal support to reduce the energy barrier in a thermal catalytic reaction,thus increasing the reaction speed.Therefore,this thesis focuses on the de-sign of functional materials,and the characterization and application of Kondo effect.Specifically,it is divided into four chapters.In Chapter 1,it is mainly divided into two parts.In the first part,we briefly summa-rize the development history of spintronics and put forward some key challenges.For spintronic devices,although they have the advantages of high integration,fast operation,low energy consumption,they still face many challenges,including spin generation and injection,long distance spin transport,and manipulation and detection of spin orienta-tion.In order to solve these questions,we have summarized some possible development directions for spintronics.In the second part,we briefly introduce the research progress of Kondo effect and put forward some key challenges.With the invention of scanning tunneling microscope(STM),the Kondo effect has been extensively developed for the adsorption of magnetic impurities on the surface,showing various fascinating phenom-ena.However,the Kondo effect belongs to a many-body phenomenon.It is not only difficult to accurately simulate from theoretical calculations,but also difficult to clearly explore the relationship with other strongly related phenomena.Faced with these diffi-culties,we have summarized some potential development directions of Kondo effect.In Chapter 2,we introduce the development of density functional theory(DFT)in computational quantum chemistry.Although,the commonly used LDA+U or GGA+U method can be used to predict the electronic structure properties of general magnetic systems.However,it often contains d electrons or f electrons for some strongly corre-lated electronic systems,such as transition metal oxides,rare earth metal compounds,etc.As a result,they cannot accurately describe the corresponding strong correlation characteristics,and even the qualitative description is wrong,such as the exploration of Kondo effect.In order to accurately describe the Kondo effect and other characteris-tics of a strong correlation system theoretically,our group have proposed a combined DFT and hierarchical equations of motion(HEOM)approach.The results calculated by this method are quantitatively accurate.The corresponding differential conductance spectrum can be directly compared with the results obtained in STM experiment.In ad-dition,it can also be used to predict some new systems and provide theoretical guidance for experimental preparation.In Chapter 3,we predict a series of two-dimensional(2D)high-temperature mag-netic half metal and semiconductor materials with excellent properties through theoret-ical simulations.First of all,we propose a general strategy for synthesizing spintronics materials,in which 2D room temperature ferromagnetic materials are fabricated by em-bedding transition metal(M)atoms into borophene nanosheets and thereby introducing magnetism into the non-magnetic system.During the embedding,the system may un-dergo a series of phase transitions.In different phases,these structures exhibit diverse magnetic properties,which expand the application scope of borophene.Take the ?3 phase borophene as an example,it have been predicted several M embedded borophene nanosheets with high temperature ferromagnetism and large perpendicular magnetic anisotropy.Among them,the global minimum structure,P4/MBM CrB4,is identified to be a ferromagnetic quasi-half metal.Next,considering the serious drawbacks of in-organic materials,including energetically expensive fabrication,limited availability of certain constituent elements,and poor scope for chemical tunability,we turn our at-tention to the low-cost,easily tunable systems of magnetic metal-organic frameworks(MOFs).For MOFs,their properties and functions can be easily tuned by changing the metal centers,organic ligands,or framework geometries,thus enabling their di-verse potential applications in fuel storage,catalysis,sensing,and so on.However,the development of magnetic MOFs is still in the early stage,most of them have a rel-atively low magnetic order temperature.Here,by replacing the type of M centers,a series of 2D K3M2[PcMO8](M=Cr-Co)MOFs with multifunctional characteristics are predicted.All of them are multiferroic materials,and have the characteristics of ferro-magnetism/ferrimagnetism and vertical electric polarization.The Curie temperature Tc can be adjusted from 66 K to 150K.For 2D K3M2[PcMO8](M=Mn,Fe,Co)MOFs,they belong to ferrimagnetic semiconductors.Furthermore,we introduce an intermedi-ate exchange interaction medium(such as:magnetic organic linkers)between M atoms and thus establish a d-p hybrid coupling mechanism to significantly enhance the Tc in MOFs.we take a Cr(pyz)2 monolayer as an example.Based on the experimentally synthesized layered Li0.7[Cr(pyz)2]Cl0.7·0.25(THF)MOFs,a 2D room temperature fer-rimagnetic semiconductors is obtained by the mechanical exfoliation method.The rea-son for its room temperature magnetic order is that a strong antiparallel hybridization exists between the d orbitals of Cr ion(S=2)and the p orbitals of N and C atoms in the pyz group(S=1/2).In Chapter 4,in order to solve the problem of spin sensitivity to the surrounding environment,we propose a molecular mold strategy to regularize the Kondo states at the atom/metal interfaces by combining theories and experiment.This strategy open a new horizon for the construction of next-generation spintronic devices using the spin freedom of electrons.Specifically,we first use the STM tip to push a CoPc molecule on the Au(111)surface to capture the discrete Co adatoms,and finally synthesize a series of CoPc@nCo complexes.Due to the strong d?-? bonding between the Co adatoms and conjugated isoindole units of CoPc,the spin of the Co adatom is no longer affected by other environments.A highly robust and uniform Kondo state is obtained at the Co/Au(111)interfaces.Even more remarkably,the CoPc further enables a fine tuning of Kondo states through the molecular-mold-mediated superexchange interactions be-tween Co adatoms separated by more than 12 A.Since the strategy of molecular mold we proposed can not only stabilize the adsorption position of magnetic atoms,but also protect the spin of magnetic atoms from the external environments.Thus,it provides a good method for preparing single-atom catalysts,and simultaneously provides a plat-form for studying the influence of the spin degree of freedom of the electron on the catalytic reactions.For years various methods have been attempted to improve the cat-alytic activity of single-atom catalysts.However,it remains largely unknown whether the atomic spin could be utilized to enhance the catalytic activity.We explore such a pos-sibility by investigating the thermochemical effect of Kondo screening of a local atomic spin by free electrons in the metal support.Inspired by the exothermicity of Kondo spin-screening,a novel approach to heterogeneous catalysis-reaction on a rink(ROAR)-is proposed.In contrast to the conventional notion of thermal catalytic reaction,lowering the temperature of metal support is predicted to result in a reduced reaction barrier.As a proof of concept,CO oxidation catalyzed by the Co@CoPc/Au(111)composite is scru-tinized and the efficacy of ROAR is demonstrated.This work accentuates the potential usefulness of the spin degrees of freedom to heterogeneous single-atom catalysis.