| The development of microelectronics represented by semiconductor integrated circuits has brought a revolution of information technology to mankind.However,with the chip characteristic length reaching 5 nm level,Moore’s law is facing great challenges after half a century.In a sense,the interdisciplinary disciplines such as molecular electronics and spin electronics are all preparing for the post Moore era from their respective perspectives.On the other hand,with the discovery of graphene,the field of two-dimensional materials has been developed rapidly.Because of the high mechanical strength,large specific surface area and high carrier mobility,the two-dimensional materials are considered to be the excellent carrier of the new generation of electronic devices,which has aroused great interest in research.In addition,the two-dimensional materials not only have very interesting intrinsic characteristics,but also have the characteristics of very easy modulation.They can change their physical and chemical properties by doping,defect,modification,surface adsorption,stress,and can also be made into vertical or transverse heterojunction by stacking and preparation,which further improves the application range of 2D materials.Nowadays,the application of 2D materials in nano electronics,spin electronics,optoelectronics and other fields has been widely concerned.In the application of nano devices,quantum effect must be considered when the device size is close to the relaxation length of electrons.The current transport in devices is a core problem of device functionalization,and the non-equilibrium Green’s function method can deal with the elastic scattering and coherent transport problems in the process of transport.In addition,the spin relaxation and light induced current transport can be dealt with in the device by considering spin and introducing the interaction between light and electron by first-order Born approximation.This greatly improves the applicability of our research.Based on the above considerations,in this paper,we use density functional theory(DFT)and non-equilibrium Green’s function(NEGF)method to study the electronic structure,magnetism,spin polarization,carrier mobility,exciton effect,spin transport and photoelectric properties of a variety of two-dimensional materials and their heterojunctions.Some theoretical guidance is provided for the development of new generation electronic devices.The main contents and results of the thesis are as follows:(1)A set of lateral heterostructure(LHS)nanoribbon designs based on the Silicene and Ⅲ-Ⅴ group compounds are proposed.After optimization,the nanoribbon composed of h-AlP and h-GaAs can form stable LHS structure with silicene respectively.Because there are Ⅲ-Si and Ⅴ-Si interfaces in LHS formed by Ⅲ-Ⅴcompounds and silicene,the electronic structure and properties of the two heterojunctions are different.We find that the difference between spin polarization direction and band gap is due to the opposite transverse electric field produced by the connection of group Ⅲ(Al,Ga)or Ⅴ(P,As)elements and Si atoms at the interface of heterojunction.Based on this,we design a set of devices based on the LHS of Ⅲ-Si and V-Si,and calculate the current and voltage curves of the devices by NEGF method.The results show that the two devices have the opposite spin filtration characteristics.Under the condition of low bias voltage(less than 0.2V),the two devices have more than 90%spin filtration efficiency,and the Si-Ⅲ interface hetero-nanoribbon is more suitable for spin filter applications.At the same time,we also prove that the spin transport characteristics of LHS are independent of the width of nanoband and other factors,and only depend on the adjacent atoms on the heterojunction surface,therefore they are robust.(2)With the successful preparation of more and more single atom layer two-dimensional materials(such as graphene,germanene,etc.),many spin electronic devices based on nanoband have been proposed.Most of the schemes are based on the manipulation of the spin polarization state at the boundary of the zigzag nanoribbon.Many of the work is on the research of magnetoresistance single layer devices.However,it is difficult to realize spin antiparallel states(AP)in different parts of nano devices in experiments.Here,we propose a lateral heterojunction(LHS)consisting of B4CN3 and B3CN4 nanoribbons,each of which has opposite spin polarization.Due to the doping effect of the two materials,the heterojunction can show strong spin related charge rectification and giant magnetoresistance effect.In addition,we find that the heterojunction has high spin filtering efficiency in parallel and antiparallel spin configuation.This discovery opens a new way for the establishment of a high efficiency magnetoresistance and spin filter.(3)A family of ternary 2D materials NaCuX(X=S,Se,Te)is predicted.We prove that the materials are stable and easy to be stripped from the layered blocks,so they are easier to be prepared in experiments.The calculation of HSE06 functional shows that NaCuX(X=S,Se and Te)has direct band gap of 1.2 eV to 1.6 eV which are suitable for photoelectric applications.In order to consider the effect of exciton effect,we calculated the optical absorption coefficient by GW+BSE method.It is found that the materials have high absorption rate for both visible and infrared wavelengths.The mobility of NaCuX(X=S,Se,Te)is the same order as that of black phosphorus BP(103～104 cm2 V-1 s-1),so it is suitable for the application of electronic devices.Based on this,we designed a set of photocell based on single layer NaCuX(X=S,Se and Te)and calculated the parameters of photocurrent,among which the device optical response(Rph)based on NaCuTe can reach 0.105 AW-1.Therefore,the two-dimensional ternary materials have potential application value in the field of photoelectric.This paper is divided into the following chapters:In chapter 1,introduction,we summarized the research path from Moore’s law to post Moore era,from the discovery and development of two-dimensional materials to the preparation and control of two-dimensional materials,and its application in spin electronics and optoelectronics,etc.,introduces the application prospects of two-dimensional materials,and gives the research background of this thesis.In chapter 2,the density functional theory(DFT)and the transport theory based on the nonequilibrium Green’s function method(NEGF)are introduced.In chapter 3,the spin polarization of LHS nanoribbons composed of silicene and Ⅲ-Ⅴ group compounds is studied.A scheme for the modulation of spin polarization of silicene is given.In chapter 4,we mainly study the magnetic and transport properties of the heterojunction composed of two boron carbonitrides,and propose a multifunctional device with giant magnetoresistance effect,rectification and spin filter effect.In Chapter 5,we propose a new ternary two-dimensional material NaCuX(X=S,Se,Te),and demonstrate its application prospects in photoelectric field from the aspects of material stability to electronic and optical properties of materials. |
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