Computational Design Of Two-dimensional MA₂Z₄ Family And Studies Of Their Properties

Two-dimensional(2D)materials have attracted extensive studies in the fields of electronics,information and energy for their novel physical and chemical properties.From graphene to MoS2,to Bi2Se3 and to MnBi2Te4,2D van der Waals monolayer materials exhibit the trendency towards coexisting multicomponent compositional and multi-atomic layered structures.Multicomponent multi-atomic layered 2D materials combine different atomic layers and various structural units,providing a platform for coupling or designing the diversity of various structures,physical and chemical properties.For instance,inserting a ferromagnetic MnTe layer into the topological insulator Bi2Te3 can form a new 2D material,MnBi2Te4.It combines together both the magnetic properties of MnTe layer and the topological properties of Bi2Te3,exhibiting layer-dependent coupling of magnetism and topology.The fact leads to the emergence of exotic properties,such as quantum anomalous Hall effect and axion insulator in MnBi2Te4.At present,known multicomponent multi-atomic layered 2D materials have demonstrated excellent performances in magnetic topology,high-temperature quantum anomalous Hall effect,and negative piezoelectricity,which has facilitated the development of 2D materials.It should be noted that multiple elements and multiple atomic layered structures inevitably lead to complex phase structures,which pave challenges for their theoretical design and experimental growth.Currently,the 2D materials known in literature are overwhelmingly stripped from three-dimensional(3D)parent layered materials,while 3D layered materials are only a small proportion of 3D materials.Hence,the design and discovery of new 2D materials,which do not have corresponding counterparts in known 3D parent layered materials,will greatly expand the varieties of 2D materials.Certainly this is providing great opportunities to explore 2D materials with exotic physical and chemical properties as well as their potential applications.In this thesis,by means of first-principles calculations and models in combination with available experimental results,the intercalated architecture by strong chemical bonding was proposed to design septuple-atomic-layer 2D MA2Z4 materials family that do not have corresponding counterparts in 3D parent layered materials known in literature.On basis of this design,we investigated a 2D MA2Z4(MZ)n material system by inserting n-MZ atomic layers into 2D MA2Z4 monolayer and discovered a 2D MA2Z4 family,a so-called obstructed atomic insulators,with 34 total valence electrons.The main results are summarized,as follows.By first-principles calculations,we precisely identified the lattice structure of the experimentally synthesized septuple-atomic-layer 2D MoSi2N4 material.Based on the identified lattice structure,we further theoretically predicted twelve stable 2D materials with rich physical properties,including semiconducting,metallic,and magnetic semiconducting properties,and proposed a family of 2D MA2Z4 materials.Furthermore,to design more 2D MA2Z4 materials consisting of seven atomic layers,we proposed a design method of intercalated architecture by strong chemical bonding.We illustrated this unique strategy by means of first-principles calculations,which not only reproduced the structure of MoSi2N4 and MnBi2Te4 that were already experimentally synthesized,but also predicted 72 compounds that were thermodynamically and dynamically stable.We found that the systems with 32 and 34 total valence electrons are mainly semiconductor,while the systems with 33 total valence electrons can be non-magnetic metals or ferromagnetic semiconductors.Among the predicted compounds,we revealed that SrGa2Te4 is a topological insulator by both the standard density functional theory and hybrid functional calculations.VSi2P4 is a ferromagnetic semiconductor material with Curie temperature of 90 K.TaSi2N4 is a type-I Ising superconductor with a superconducting transition temperature of 12 K at 3%compressive strain.And WSi2P4 is a semiconductor with peculiar spin-valley properties,and its band gap type does not change with layer thickness.The strong chemical bonding of the intercalation architecture method enables the physical and chemical properties of 2D MA2Z4 materials to be greatly reconstructed from their constituent units and enhances the structural stability,which not only provides a platform for yielding more novel properties,but also paves the way for the design of new multicomponent multi-atomic layer 2D materials.Through the strategy of intercalated architecture via strong chemical bonding,we further explored the possibility of creating intercalated structures with more atomic layers and their novel physical properties.On basis of this idea,the unique crystal stacking order of the experimentally synthesized 2D MoSi2N4(MoN)4m(m is non-negtive integer)material was found to be originated from the template effect of multilayered MoSi2N4.Computations demonstrate that 2D MoSi2N4(MoN)4 is superconducting with a superconducting transition temperature(Tc)of 9.4 K,while the 2D Mo5N6 material obtained by removing the surface SiN layer from MoSi2N4(MoN)4 shows a Tc of 19.9 K,indicating that the surface SiN layer has a strong impact on its properties.The computational results also reveal that the Young’s modulus of MoSi2N4(MoN)n(n=0,1,2,3,4)that covers the surface SiN layer almost unchanges as the thickness increases,whereas the Young’s modulus of Mon+1Nn+2 that removes the surface SiN layer increases rapidly with its thickness firstly and then approaches the Young’s modulus value of the bulk phase MoN.For the 2D MA2Z4 semiconductor materials with 34 total valence electrons,we found that a typical band inversion feature appearing in their electronic band structure,but their mirror Chern number is zero,This fact indicates that these 2D materials are topologically trivial insulators,rather than expected topological insulators.However,it is noted that,by means of topological quantum chemical method,we found that these materials are typical obstructed atomic insulators(OAIs).For OAIs,although they are topologically trivial,they have the appearance of metallic edge state due to the presence of charge occupation at the atomic unoccupied Wyckoff positions in their lattice structures.Our calculations reveal that the 2D MA2Z4 insulators with 34 total valence electrons are all 2D OAIs.The analysis of the electronic structure and edge states as well as comer states of MoSi2N4 monolayer reveals that the edge cutting through the Wyckoff positions without real atomic occupation but with localized charge occupation is metallic.In particular,zero-energy corner states are found in C3-symmetric hexagonal nanodisks of MoSi2N4,which are very similar to the zero-energy corner states in 2D second-order topological insulators.