Design And Opto-electrical Properties Study Of Two Dimensional Multifunctional Materials

Since the graphene was first proposed and discovered,more and more researchers have paid attention to the research and development of two-dimensional（2D）materials.With the development of nanotechnology,monolayer MoS₂,GaSe,Tellurene and other novel 2D materials have been obtained from corresponding bulk materials.At the same time,more and more multifunctional two-dimensional materials that subvert traditional cognition have been found or synthesized.For example,two-dimensional ferroelectric materials（SnTe,GeTe,In₂Se₃）,two-dimensional chalcogenide polarized materials MXY（X≠Y,such as Janus MoSSe,WSSe）,two-dimensional magnetic materials（VSe₂,CrI₃,Cr₂Ge₂Te₆）have been discovered and synthesized.Due to their novel and distinctive physical electronic properties,these two-dimensional materials have attracted extensive attention in the development of opto-electronic devices,information storage and chips.In this thesis,we design and investigate the optoelectronic and device properties of several novel multifunctional two-dimensional materials by using the density functional theory and discuss the effects of polarization electric field,external stress,gate voltage and bias voltage on their electronic structure and transport properties.The main results are as follows:（1）The effects of the ferroelectric polarization of In₂Se₃ on the band alignment of In₂Se₃/InTe and In₂Se₃/MoS₂ van der Waals heterostructures as well as the mechanism of ferroelectric enhanced carriers separation are revealed.Appropriate band alignment are of great significance for the development of new multifunctional optoelectronic devices.The results demonstrate that,by changing the ferroelectric polarization direction of In₂Se₃,the band alignment of the heterostructure can be changed from staggered（type-Ⅱ）to straddle（type-Ⅰ）type.It is found that the intrinsic polarization electric field can effectively enhance the spatial separation effects of electrons and holes in the heterojunction.In addition,the interlayer coupling and external electric field can also effectively induce the transition of band alignment from type-Ⅰ to type-Ⅱ.（2）The excellent optoelectronic properties,strain enhanced photovoltaic conversion efficiency and external electric field control mechanism of GaSe/GeS van der Waals heterostructure are revealed.Constructing two-dimensional van der Waals heterostructures and understanding their electronic structure properties is the key to develop new electronic devices.It is theoretically revealed that the two-dimensional GaSe/GeS vd W heterostructure is a stable type-Ⅱ band alignment semiconductor with a direct band gap of 1.8eV.It has a significant absorption coefficient of 10⁵cm^-1 in the range of ultraviolet to visible light,and has a high anisotropic carrier mobility.The photoelectric conversion efficiency of the heterojunction is greatly improved under external strain,which is increased from 6.1% to 16.8% at 2%biaxial tensile strain.In addition,we also found that the applied electric field can effectively adjust the band gap and band offsets.（3）The enhanced orbital hybridization at the van der Waals interface of MoSSe/Te heterostructure as well as the efficient exciton solar cells and their photocatalytic effects are revealed.We find that the two components of MoSSe/Te heterostructure are overlap at the bottom of conduction band,which makes MoSSe/Te have excellent exciton solar energy conversion efficiency of 22.6%.In addition,the absorption coefficient of MoSSe/Te heterostructure reaches as high as 7×10⁵cm^-1 from the ultraviolet to visible region,the maximum photocurrent density is up to 3.2mAcm^-2,which is much higher than that of thin-film silicon and double-layer Janus MoSSe devices.Finally,the excellent photocatalytic activity of the catalyst was discussed.（4）We simulate the single layer hexagonal boron phosphorus（BP）MOSFETs and the performance limits of transistor devices with sub -10nm channel length are predicted.In the post Moore era,two-dimensional（2D）materials with natural passivation,gate electrostatics and high mobility have attracted extensive attention in the competition of scale miniaturization of next-generation integrated circuits.We first studied the performance limit of double gate metal oxide semiconductor field effect transistor（MOSFET）with sub-10nm single-layer boron phosphorus（BP）as channel material.We predict that the on current,delay time and power delay of the optimized sub-10nm n-type and p-type single-layer BP MOSFET can reach the standard of International SemiconductorTechnology Roadmap（ITRS）.（5）The feasibility of single-layer ferromagnetic vanadium selenide（2H-VSe₂）as channel material for high performance spin field effect transistor devices is revealed,and the pure spin polarization current of the devices is achieved.In recent years,two-dimensional van der Waals magnetic materials have broad application prospects in data storage,information processing and other fields,which has attracted extensive research interests.By theoretical simulation,we find that the dual gate FET based on single-layer 2H-VSe₂ has the characteristics of electric-field-induced half metal properties.The spin filtering efficiency of the designed dual gated spin filter based on single-layer 2H-VSe₂ reaches 99.78%and 99.97%,respectively,when the gating voltage is 1.5V and-2V.The conduction current reaches 10³μA/μm.This work shows the high performance of monolayer 2H-VSe₂ in spintronic devices.