First-Principles Study On Valley Polarization And Modulation Of Two-Dimensional Janus-Type Materials

Since graphene was successfully exfoliated from graphite in 2004,the development and application of 2D materials have been of great interest to researchers.At the same time,facing the continuous miniaturization of electronic products,the design of nanoelectronic devices and functional materials has posed a huge technical challenge,and the development of 2D materials has provided an opportunity to solve this problem.With the successful preparation of more and more monolayers,the study of the properties of 2D materials has been intensified.Valleytronics-a hot research field based on valley degrees of freedom emerged,which relies on the valley degrees of freedom of carriers to process information and perform logic operations instead of the traditional degrees of freedom spin and charge of electrons.This field has enriched the family of Hall effect while giving new ideas for the development of next-generation microelectronic devices.However,the binary manipulation of valley degrees of freedom is the key to its application,which requires breaking the valley degeneration to achieve and is currently the core topic hindering the application of valleytronics.Recently,theoretical and experimental advances in 2D materials with intrinsic magnetic properties have been obtained,and monolayers such as Cr I₃ and Cr₂Ge₂Te₆ have been synthesized successively,which have injected new vitality into the application of 2D layered materials.In valleytronics,2D intrinsically magnetic materials possessing simultaneously broken time and spatial reversal symmetries are decisive conditions for generating spontaneous valley polarization.Such valley degrees of freedom obtained on the basis of ferromagnets exhibit the advantages of spontaneity,non-volatility and easy tunability,which will facilitate the further development of valleytronic devices.Meanwhile,2D ferroelectric materials have received much attention in recent years,including In₂Se₃,1T-Mo S₂,and MX（M=Ge,Sn;X=S,Se,Te）,which exhibit a variety of applications of 2D ferroelectric materials in physics and devices and promise to provide new possibilities in the manipulation of the valley degrees of freedom.Finally,recent advances in heterostructures constructed by van der Waals interactions provide an exciting platform for the realization of higher performance nanoelectronics,spintronics,and also new techniques for the generation and modulation of valley polarization in valleytronics using magnetic or electrical proximity effects.By stacking 2D materials with different properties to obtain van der Waals heterostructures,the design flexibility and functionality of 2D material-based devices for nanoelectronics,spintronics,and valleytronics is further expanded.Based on the above research background,this thesis presents structural predictions of novel Janus 2D valleytronics materials using first-principles calculations,and initiates an in-depth study of the generation and modulation methods of valley polarization.Through systematic studies,we propose several 2D Janus-type materials with the presence of valley and achieve valley polarization by two more desirable means:constructing 2D heterostructures and intrinsic magnetic exchange fields.In addition,we also use the modulation of the heterostructure layer spacing,the in-plane biaxial stress of the system,the magnetization direction of the magnetic substrate,and the strong correlation effect of transition metals,etc.to effectively modulate the valley properties such as valley spin splitting and valley polarization,and the magnetic properties such as magnetic ground state and Curie temperature,and to realize the electrically reversible switching of valley signals in multiferroic heterostructures by using the nearest neighbor effect.The specific research content of this thesis is as follows.1.Robust valley polarization in two-dimensional ferrovalley heterostructures Hf NX/Fe Cl₂The exploration of methods to achieve non-volatile valley polarization has gradually become a hot research topic in the field of valleytronics and has received much attention.Though the first-principles calculations,a two-dimensional ferromagnetic half-metal 1T-Fe Cl₂ with super-exchange interactions and high Curie temperature is used to construct ferrovalley heterostructures under the means of magnetic proximity effects.The results show that monolayer Hf NX（X=Cl,Br,I）is an indirect bandgap semiconductor with a pair of valleys in the conduction band.After considering the spin-orbit coupling,the spin degeneracy is broken and the valley spin splitting is generated in the conduction band,especially in the conduction band with a considerable valley spin splitting（310-334 me V）.In the Hf NX（X=Cl,Br,I）/Fe Cl₂,the magnetic proximity effect of the Fe Cl₂ breaks the time reversal symmetry in Hf NX,and the energy level degeneracy between the two valleys is broken after considering the spin-orbit coupling effect,and the valley polarization up to 47.6 me V can be achieved.Meanwhile,we also effectively modulate the valley polarization by changing the interlayer spacing,applying biaxial strain,and changing the magnetization directions.Surprisingly,we find that the super-exchange interactions of the 1T-Fe Cl₂ lead to particular robustness of the valley polarization,so the valley signals in this structure are nonvolatile.In addition,we found that there is a charge transfer phenomenon between Hf NX and Fe Cl₂,and the more charge transfer,the stronger the interlayer coupling,which is more favorable for the stable existence of heterostructures and the generation of valley polarization.Finally,the Berry curvature demonstrates that the Hf NI/Fe Cl₂ is capable of achieving the anomalous valley Hall effect（AVHE）.The controllability and the robustness of the valley polarization in Hf NX/Fe Cl₂ heterostructure are expected to overcome the volatility of the valley signals,which will be of great help for future applications in spintronics and valleytronics.2.Reversible switching of AVHE in Janus 1T-Cr OX and multiferroic heterostructures Cr OX/In₂Se₃The central issue for practical applications of the AVHE is the tunable and nonvolatile nature of the valley polarization.Exploring intrinsic two-dimensional valleytronic materials is a key issue in this field,in addition to how to flexibly modulate the valley polarization is extremely challenging to study.Through the first principal calculation,we predict a type of ferrovalley material,Janus 1T-Cr OX（X=F,Cl,Br,I）,in which the switching effect of the AVHE can be achieved by adjusting the biaxial strain or building a multiferroic heterostructure Cr OX/In₂Se₃.Stable out of plane magnetization in Cr OX induces the valley polarization which can reach to 112me V in the Cr OBr monolayer.Interestingly,we find that the valley polarization of Cr OCl is robust against the biaxial strain both in the conduction band and the valence band.In contrast,the valley polarization of the Cr OBr at the conduction band can be linearly modulated,while it has a switching response at the valence band due to the strong orbital hybridization induced by compressive strain,so a reversible switch of the AVHE can be achieved.Furthermore,through van der Waals interaction,we use the electric proximity effect to build a multiferroic heterostructure Cr OCl/In₂Se₃ with ferrovalley,ferromagnetic and ferroelectric properties at the same time.We find that the multiferroic heterostructure Cr OCl/In₂Se₃ can obtain an electrically reversible valley polarization switch,and the reversible switch of the AVHE can also be manipulated by controlling the polarization states of the ferroelectric layer.The AVHE in Cr OX can be readily switched on or off by either applying biaxial strain or reversing the ferroelectric polarization of the substrate In₂Se₃,which may be a promising application in the field of valleytronics.3.Tunable AVHE and magnetic phase transition in MHf N₂Cl₂ bimetallic nitrogen halide monolayersTransition metal nitrogen halides（TMNHs）are environmentally friendly and widely studied van der Waals two-dimension materials.However,there are still few 2D TMNHs with magnetic properties that have been synthesized,which greatly limits the further applications of them in spintronics and valleytronics.Here,we substitute Hf atoms by magnetic elements V and Cr inβ-type TMNHs Hf₂N₂Cl₂,which evolved into ferromagnetic half-metals and ferrovalley semiconductors MHf N₂Cl₂（M=V,Cr）.Meanwhile,the valley polarization and magnetic properties can be effectively regulated by modulating the strong correlation effect of transition metals and applying biaxial strain.Valley polarization in VHf N₂Cl₂ increases linearly with the increasing strain and the U value,however,valley polarization is robust to the strain and decreases linearly with the increasing U value in Cr Hf N₂Cl₂,and the maximum valley polarization values reaches to 175 and 62 me V for the two monolayers,respectively.Cr Hf N₂Cl₂ remains robustly ferromagnetic under different strain and U values,while there is magnetic phase transition in VHf N₂Cl₂ from ferromagnetic to antiferromagnetic,accompanied by a half-metal-to-semiconductor transition.The Curie temperatures of VHf N₂Cl₂ and Cr Hf N₂Cl₂ can be increased by a maximum of 2.5 and 2.36 times under modulation.This opens ideas for the study of bimetallic TMNHs in spintronics and valleytronics.