Theoretical Study Of Electronic Structures Modulation And Devices Design Of Several Novel Two-dimensional Materials

With the increasing demand for miniaturization of electronic devices in recent years,existing silicon-based microelectronic processes are approaching the theoretical limits predicted by Moore’s Law.In this context,two-dimensional（2D）materials such as graphene,with their extreme size effect and excellent physicochemical properties,hold the promise for the development and exploration of new nanodevices in the post-Moore era.Nevertheless,despite the high electron mobility of graphene,its zero-bandgap nature means that it can only stay in the conduction state and cannot be used directly in the construction of high-performance logic circuits.Similar to the problems encountered with graphene,electronic nanodevices based on 2D semiconductors also face various challenges,such as silicene,germanene and black phosphorus,which are unstable in air and difficult to achieve large-scale practical applications.With the field of basic materials and 2D material microelectronics constantly hitting a wall,many researchers have set their sights on the field of novel 2D materials.Recently,novel 2D materials have been widely used in fundamental nanodevices based on their unique advantages,which not only to improve device performance but also facilitate the application of devices in integrated circuits,optoelectronic detection,nonvolatile storage,gas sensing,etc.To this end,this thesis presents a systematic work on novel 2D materials from three aspects:structures and physical properties prediction,electronic structures modulation,electronic transport and related devices design.In Chapter 1,we first introduce the basic theory and research progress of several novel types of2D materials,mainly focusing on their electronic structures,experimental exploration and applications in electronic devices.Moreover,we present the superior performance of several new classes of 2D nanodevices,including photodetectors,gas sensors and spintronic devices.In Chapter 2,we first summarize the theoretical research tools used in this paper,and review the development of density functional theory and non-equilibrium Green’s function method and related essential concepts.Then,we introduce the quantum transport theory model and the computation of basic properties in first-principles calculations.Finally,we give a brief overview of the two commercial software packages for materials simulation used in this paper.In Chapter 3,we investigate the structural,electrical and optical properties of novel 2D WSi2N4nanosheets（monolayer,bilayer and trilayer）with uniaxial and biaxial strain,and explore their potential applications in the field of photodetection.The results show that WSi2N4 nanosheets are dynamically stable indirect bandgap semiconductors in the strain-free condition and have a strong absorption coefficient in the visible light region(～10⁵ cm^-1).Uniaxial and biaxial strains can flexibly modulate the band gap and absorption coefficient of WSi2N4 nanosheets,and biaxial tensile strains can also achieve their semiconductor-metal transition.Furthermore,WSi2N4-based p-i-n junction photodiodes can generate considerable photocurrents under circularly polarized light irradiation and show a strong photoelectronic response to purple light.Importantly,biaxial tensile strain not only significantly enhances the photoelectric performance of this photodiode,but also effectively tunes its detection range in the visible light region.These findings provide a useful reference for further design and modification of new MA2Z4-based electronic and optoelectronic devices.In Chapter 4,we investigate the adsorption behavior of inorganic（CO,NH3,NO,NO2,and SO2）and organic（CH4,C2H2,C2H4,HCHO,and CH3OH）gas molecules on BC3N2 monolayer and explored the gas-sensitive performance of BC3N2-based gas sensor.The results reveal that the BC3N2monolayer remains metallic with thermodynamic stability.Also,the sensing performance analysis shows that the inorganic molecules CO,NO,and NO2 as well as the organic molecules C2H2 and HCHO have strong chemical interactions with BC3N2 and are chemically adsorbed onto BC3N2.On the contrary,the interactions between NH3,SO2,CH4,C2H4 and CH3OH and BC3N2 are very weak and these molecules adopt physical adsorption.In the case of chemisorption,the electronic transport behaviors of the 2D BC3N2 gas sensor is molecularly sensitive and the gas sensitivity of BC3N2 is highly anisotropic,especially for organic C2H2 with the gas sensitivity ratios as high as 10.43 and2.79 along the zigzag and armchair direction,respectively,and the anisotropy ratio is also reach up to 6.22.Therefore,BC3N2 could be used in 2D gas sensing devices,particularly for sensing organic molecule C2H2.In Chapter 5,we design a variety of BPN-based conceptual 2D nanodevices and reveal the electronic transport properties of the relevant nanodevices.First,the pristine BPN-based devices exhibit obvious electronic transport anisotropy along the zigzag and armchair directions.Moreover,a significant negative differential resistance（NDR）effect along the armchair direction is observed as well.Second,the gas sensitivities of the BPN-based gas sensor to NO and NO2 along the armchair direction are as high as 1.63 and 2.75,respectively.Finally,the BPN monolayer undergoes a metal–semiconductor transition in the case of N13 or N14 doping strategy.Therefore,Schottky junction device formed by the pristine and N14 doped BPN monolayer is constructed and its maximum rectification ratio reaches～10⁴ at low bias voltage.These results demonstrate that the BPN monolayer is a multifunctional material and may find various potential applications in electronic anisotropy,NDR,gas sensor,and even Schottky junction devices.In Chapter 6,we design a novel 2D multiferroic heterostructure consisting of ferromagnetic Fe I2 monolayer and ferroelectric In2S3 monolayer,and explore its electronic structures and device transport properties.The results reveal that the Fe I2 monolayer can be reversibly switched between semiconducting and half-metallic properties by nonvolatile control of the In2S3 ferroelectric polarization states.Correspondingly,the proof-of-concept two-probe nanodevice based on the Fe I2/In2S3 vd W heterostructure by modulating the ferroelectric switching exhibits a significant valving effect with switching ratios up to 10³.Moreover,it is also found that the adsorption of nitrogen-containing gases（NH3,NO and NO2）on the surface of Fe I2/In2S3 vd W heterostructures strongly depends on the polarization direction of the ferroelectric layer.In particular,the Fe I2/In2S3heterostructure shows a reversible capture behavior for NH3.As a result,the Fe I2/In2S3 vd W heterostructure-based gas sensor demonstrates high selectivity and sensitivity.These findings may open up a new route for the application of multiferroic heterostructures to spintronics,nonvolatile memories and gas sensors.In Chapter 7,we provide a brief summary of the work in this thesis and a brief outlook on the remaining problems in this research area and our next work plan.