Study On Electronic And Valleytronic Properties Of Two-dimensional Transition Metal Chalcogenides From First-principles

Since the first isolation of graphene in experiments,a large family of two-dimensional(2D)materials(such as transition metal chalcogenides,hexagonal boron nitride,silicene,phosphorene,and germanene)have been developed rapidly.The emergence of these new materials brings some novel properties which are usually not present in their three-dimensional counterparts.Considering that more and more 2D materials are theoretically predicted and experimentally synthesized,we can flexibly assemble them to construct van der Waals heterostructures(the layers combined by van der Waals forces),which can further enrich their properties.The properties of these 2D materials can also be reasonably controlled by some external methods,which means that 2D materials and their heterostructures have potential applications in new electronic devices and multifunctional devices It is worth mentioning that the valley degree of freedom in 2D materials has also received widespread attention.As a new degree of freedom for electrons,it expands 2D electronic devices to the field of valleytronic devices.It can be easily controlled by certain ways to explore new applications.In this thesis,we systematically study the structures,electronic properties,magnetic properties and valleytronic properties of 2D materials and their heterostructures as well as the regulations of the relevant properties of these materials through reasonable methods(stack configuration,strain,electric field,doping,and polarized direction).The potential applications in tunnel field-effect transistors,valleytronic devices,and non-volatile memory devices are revealed.And the underlying physical mechanisms are also analyzed,which provides the guidance for the further development of 2D materials and their heterostructures.This thesis is divided into six chapters:Chapter 1 outlines the research status and related applications of 2D materials and their heterostructures;Chapter 2 briefly introduces the basic theoretical knowledge,approximation methods and software used in this thesis;Chapter 3 studies the 2D WTe2/HfS2 heterostructure with type-Ⅲ band alignment and its application in tunnel field-effect transistors and multifunctional devices;Chapter 4 studies valley polarization in monolayer CrX2(X=S,Se)with magnetically doping and proximity coupling;Chapter 5 theoretically verifies that nonvolatile controlling valleytronics by ferroelectricity in VSe2/Sc2CO2 van der Waals heterostructure;Chapter 6 briefly summarizes the main research contents and innovations of this thesis,and provides the outlook.The main research contents and conclusions of the thesis are as follows:(1)van der Waals heterostructures(vdWHs)are attracting a lot of interest for fundamental studies and fabricating novel devices.Currently,most vdWHs exhibit type-I or type-II band alignment,and few systems have been shown to be in the type-III class.Herein,we show first-principle evidence that WTe2/HfS2 vdWH possesses the long-sought type-III band alignment with a broken gap,providing a promising platform for developing tunnel field-effect transistor.Moreover,the electronic features of WTe2/HfS2 vdWH can be effectively modulated via external strain and electric field.Particularly,the interesting transition from type-III to type-Ⅱ band alignment can be observed in WTe2/HfS2 vdWH upon applying strain or electric field,which holds great potential for designing multifunctional devices.Our study not only predicts an extraordinary vdWH with type-III band alignment but also provides an outstanding candidate for realizing multiple-band-alignment transformation.(2)Manipulating the valley degree of freedom as an information carrier has been a focused topic for both fundamental and applied research.Here,using first-principles calculations,we report the identification of monolayer CrX2(X=S,Se)as a novel 2D valleytronic crystal.It shows large valley spin splitting in the valence band,attractive for the integration of valleytronics and spintronics.More importantly,through proximity coupling with monolayer CrCl3,the valley polarization in monolayer CrX2 is achieved,which can be further engineered by stacking patterns.Also,the valley polarization in monolayer CrX2 can be obtained via magnetically doping V and Mn.Specially for V-doped monolayer CrSe2,there are no impurity states in the band gap,beneficial for its practical applications.Our works thus provide not only exceptional 2D valleytronic crystals but also promising ways for realizing valley polarizations in them.(3)Controlling 2D valleytronics by external means is a major challenge to better information technology.Here,we demonstrate that introducing ferroelectric aided layer is an extraordinary approach for realizing the nonvolatile control of 2D valleytronics.When stacking valleytronic monolayer VSe2 with ferroelectric monolayer SC2CO2 and the ferroelectric polarization pointing along-z direction,the system exhibits a ferromagnetic semiconducting nature and harbors rare intrinsic valley polarization,and its spontaneous valley polarization reaches 234 meV.Upon reversing its ferroelectric polarization via a short-term voltage,the ferromagnetism is preserved,but a semiconductor-to-half-metal transition occurs,erasing any possibility for valleytronics.This allows the electrical reversable control of valleytronics in VSe2/Sc2CO2 van der Waals heterostructure through the application of a short-term voltage.Our work thus provides a promising strategy for achieving nonvolatile control of valleytronics at nanoscale and helps the design of novel controllable devices.