| Two-dimensional magnetic materials are not only the basis of spintronics devices,but also a platform for studying new physical phenomena.Therefore,realizing magnetism in two-dimensional layered materials has become an important goal of researchers.From the discovery of graphene in 2004 until now,many new two-dimensional materials have been synthesized and discovered by researchers,but there are very few magnetic materials among them.Therefore,looking for new two-dimensional magnetic materials that are magnetic and can exist stably at room temperature is particularly critical for manufacturing a new generation of spintronic devices.Two-dimensional transition metal carbides and nitrides(MXenes),which are similar in structure to graphene,are a new member in the world of two-dimensional materials.It is a composite layered structure composed of multiple materials and has the typical characteristics of two-dimensional materials.Such as large surface area,abundant surface physical and chemical properties,etc.Since the successful synthesis of MXene,its performance has been deeply studied.Among them,Cr-based MXene is one of the few magnetic materials.Researchers adjust its magnetic properties by introducing different surface terminations,adjusting the number of layers,doping,and applying electric fields to control its magnetic properties.Magnetic materials and electronics provide direction.In this paper,Cr2C and Cr2N are the main research objects.The first principle method is used to simulate the effects of different surface modification,doping and external strain on the electronic properties and magnetism of Cr2C and Cr2N MXene.Some meaningful results are obtained.The main work and innovations of this paper are as follows:1.Effects of surface functionalization and doping on electronic properties and magnetic properties of Cr-based MXene(Cr2C).The effects of different functional groups and dopants on the magnetic interaction and electronic properties of the two-dimensional transition metal carbide,Cr2C,were investigated by using first-principles calculations.We predict that the two-dimensional transition metal carbide Cr2C is a potentially controllable magnetic material.The introduction of surface functional groups as chemical dopants during the synthesis process would have large effects on the electronic properties and magnetic interactions of the Cr2C based materials.The different functional groups were found to have great influence on the electronic structure of Cr2CX2(X=O,F,OH),including the transition from the metallic state to insulating state.While Cr2CO2 is metallic,Cr2CX2(X=F,OH)are semiconducting.We studied the electronic and magnetic properties of Cr2CX2(X=F,OH)and found that the electron or hole doping and N-doping only changed the electronic properties and did not change the magnetic properties,while the metal-doping(M=Ti,V,Mn,Fe,Co and Ni)could both change the electronic properties and magnetic properties of the system,which would have potential applications in nanoelectronic devices.2.Effect of mechanical strain on electronic properties and magnetic properties of Cr2CX2(X=O,F,OH).Adjusting the electronic and magnetic properties of materials by strain engineering is an effective strategy to improve the performance of electronic and spin electronic devices.In this paper,we study the electronic and magnetic properties of Cr2C and Cr2CX2(X=O,F,OH)under mechanical strain based on density functional theory.In the strain-free state,Cr2C and Cr2CO2 are in the ferromagnetic(FM)metallic ground states,while Cr2CF2 and Cr2C(OH)2 are in the antiferromagnetic(AFM)semiconducting states.Applying tensile strain can enhance the magnetic moments of the Cr atoms of all the Cr2C and Cr2CX2(X=O,F,OH)systems.The electronic structure can also be affected with the increase of strain,resulting in narrowing of the band gap of the AFM semiconducting systems(Cr2CF2 and Cr2C(OH)2)and enhancing the spin-splitting of the FM metallic systems(Cr2C and Cr2CO2).The results indicate a possible broadened applicability of strained Cr2C-based MXenes in future spintronics.3.Effect of different functional groups and strains on the electronic properties of Mo S2/Cr2CX2 van der Waals heterojunction.Two-dimensional transition metal carbide Cr2C is a potential magnetic material with controllable magnetic properties.Introduction of surface functional groups(O,F,OH)as chemical dopants during the synthesis process will have great impacts on the electronic properties of Cr2C-based materials.The structures of Mo S2 and Cr2CX2(X=O,F,OH)are similar,and the lattice constants are about the same.The electronic structure of the van der Waals heterostructures composed of the Mo S2 and Cr2CX2monolayers is studied using first-principles method.Our results show that the electronic structure of the Mo S2/Cr2CX2 heterostructures with different functional groups is very different.While Mo S2/Cr2C and Mo S2/Cr2CO2 are metals,Mo S2/Cr2CF2 and Mo S2/Cr2C(OH)2 are semiconductors.Biaxial strain applied to the heterostructures of Mo S2/Cr2CF2 and Mo S2/Cr2C(OH)2makes their band gap decreasing and even causes a phase transition from semiconductor to metal.The results indicate that the Mo S2/Cr2CX2heterostructures can have tunable electronic and magnetic properties with different functional groups introduced or strain applied,which would have potential applications in designing high-performance electronic devices.4.Electronic properties and magnetic properties of the new ferromagnetic half-metal material Cr2NX2MXene(X=O,F,OH).Among the two-dimensional ferromagnetic materials,ferromagnetic semi-metallic materials can achieve 100%spin polarization,and are expected to become ideal materials for the next generation of nano-spin electronic devices.Despite the extensive research in the field of spintronics,very few materials have been discovered so far.Here,we use the spin-polarized first-principles calculation method to study the properties of the two-dimensional nitride MXenes Cr2NX2(X=O,F,OH).Half-metallic ferromagnetism is found in Cr2NX2 from electronic band structure calculations.The structures are in a stable ferromagnetic ground state with Curie temperatures much higher than room temperature.At the same time,there is a large half-metal band gap that is enough to prevent spin reversal caused by thermal disturbance.Strong magnetic anisotropy is also found in the Cr2NX2 MXenes monolayers.The half-metallic property of the Cr2NX2systems is quite robust which can be maintained with applying tensile strains up to 10%.Therefore,we predicted a new ferromagnetic half-metallic material Cr2NX2 through theoretical calculations,which is of great significance to the studies and development of spintronics. |
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