Theoretical Studies Of Novel Two-Dimensional Ferroic Materials And Related Devices

When looking back on the long history of human beings,it is not difficult to find that every major breakthrough in science and technology is achieved with the progress of the new advanced materials.The rapid development of social civilization has been closely associated with the emergence of new materials.Ferroic materials,which are referred to a class of materials with spontaneous polarization states,including traditional ferromagnetism and ferroelectricity,play an important role in the modern information industry and data storage applications.In order to meet the increasing demand for electronic products,it is highly desired to realize the non-volatile devices with the advantages of lower power consumption,higher data density and faster operation speed.However,traditional ferroic materials are not suitable for the applications of the next-generation information storage devices at nanoscale.In the last decade,the rise of graphene has been leading to explosion research of two-dimensional(2D)materials.In particular,the emerging of intrinsic ferromagnetism and ferroelectricity in the low-dimensional systems have attracted immense attention,providing a potential platform for the realization of electronic devices with superior performance.In this regard,this doctorate dissertation takes the intrinsic ferroic properties in 2D materials as the research objects.Combining the first-principles calculations and theoretical analysis methods,this study focuses on the traditional ferroelectricity in the atomic-thick materials and the novel ferrovalley rooted in the valley degree of freedom,designing related information storage devices and valleytronic applications at nanoscale.In addition to the above studies,by using the high-throughput computing method,and in combination with the materials database,lots of potential 2D ferroelectric materials have been successfully identified.This dissertation is aiming to provide a new idea for the research of 2D ferroic materials,and to accelerate the materials development continuum from a laboratory concept into wide commercial applications.As the introduction of this dissertation,Chapter 1 briefly reviews the theoretical limitations and restrictions for the pursuit of ferromagnetism and ferroelectricity,which can exist at the nanoscale.This chapter puts emphasis on the recent process in the discovery of robust ferromagnetism and ferroelectricity in 2D systems.In addition to these two basic ferroic properties mentioned above,we also focus on a new member of ferroic family,which is called ferrovalley rooted in the valleytronics,and reveal the potential mechanism of the spontaneous valley polarization.At the end of this chapter,the research objects and contents are summarized as well.Chapter 2 introduces the theoretical research methodology adopted in this dissertation,reviewing the evolution from the many-body problem in the earlier approximation to the single-electron approximation in the density functional theory.In addition,the non-equilibrium Green's function and the modern theory of ferroelectric polarization are discussed.The rise of 2D materials trigger the research of valleytronics,and furthermore,bring the concept of ferrovalley materials.The robust valley polarization resulting from ferromagnetism has been proposed in hexagonal 2D systems,making nonvolatile valleytronic applications realizable.In Chapter 3,we reveal that monolayer group-IV monochalcogenides with orthorhombic lattice are a new member of ferrovalley family,where the spontaneous valley polarization is induced by the intrinsic ferroelectricity.Different from the valley-selective circular dichroism,combining the first-principles calculations and group theory analysis,we demonstrate that the valley dependent optical selection rule existing in this new orthorhombic ferrovalley materials is coupled with the linearly polarized light.According to these novel properties of such ferrovalley materials,we propose a prototype of an electrically tunable polarizer,by which a laser beam can be dynamically polarized in x-or y-direction as the reversal of valley polarization.This ferrovalley-based device can be further optimized to acquire the leftor right-handed radiation,and to realize the adjustable operating wavelength.As a result,such ferrovalley materials provide a potential platform for realizing the electrically tunable polarizer,which is of great significance in motivating the valleytronic applications in the future.Note that the monolayer group-IV monochalcogenides actually are a kind of multiferroic materials,in which the ferroelectricity and ferrovalley coexist.In Chapter 4,we propose doping engineering in these 2D ferroelectric semiconductors with inplane polarization as an effective strategy to design a 2D ferroelectric tunnel junction composed of homostructural p-type semiconductor/ferroelectric/n-type semiconductor.Owing to the robust in-plane electric polarizations of monolayered ferroelectric barrier,such a 2D ferroelectric tunnel junction is free from the ferroelectric size effect.We show that the In:SnSe/SnSe/Sb:SnSe junction provides an embodiment of this strategy.Combining density functional theory calculations with non-equilibrium Green's function formalism,we investigate the electron transport properties of In:SnSe/SnSe/Sb:SnSe and reveal a giant TER effect of 1460%.The dynamical modulation of both barrier width and barrier height during the ferroelectric switching are responsible for this giant TER effect.These findings provide important insight towards the understanding of the quantum behaviors of electrons in materials at the 2D limit,and enable new possibilities for next-generation non-volatile memory devices based on 2D lateral ferroelectric tunnel junctions.The above chapters focus on intrinsic ferroic properties in 2D materials,and theoretically design potential functional application devices at the nanoscale.In Chapter 5,we introduce a screening strategy to predict the potential 2D ferroelectric materials.Based on the large materials database,this strategy uses a combination of high-throughput computing and first-principles density functional theory,to enhance the cooperation between theoretical calculations and experimental works,then achieving faster materials development.In this chapter,we use a symmetry analysis method to firstly look for the polar 2D structures.According to the symmetry search algorithm,the nonpolar structures with higher symmetry that are related to the polar structures can be generated.If the energy difference between the polar and nonpolar phases is within a reasonable range,which is switchable through an external electric field,then such material is likely to be a candidate of 2D ferroelectric materials.After the whole screening process,we find 20 potential 2D ferroelectrics,showing various mechanisms of the ferroelectric phase transition.This work not only extends the family members of 2D ferroelectric materials that are of great importance in the nextgeneration non-volatile electric devices,but also provides a more expeditious and economical way for the research of advanced materials in the future.In the last chapter,we have summarized the main conclusions of this dissertation,and the potential research directions that can be further explored are prospected,aiming to promote the practical applications of functional devices based on 2D ferroic materials and providing new ideas for materials development.