First-principles Calculations Of Two-dimensional Transition Metal Compounds Based Interfaces And Device Applications

Two-dimensional(2D)transition metal compounds have attracted much attentions due to their atomic thickness,large specific surface area,and rich physicochemical properties.2D transition metal compounds have potential applications in the fields of electronic,energy and novel spintronic devices.Many interfaces existed in these 2D devices(such as semiconductor/metal and semiconductor/semiconductor)may have important effects on the device performance.Based on the first-principles calculations,the interfacial properties of layered transition metal compounds such as MoN and MX2(M=Mo,W,V;X=S,Se,Te)were studied,and their potential applications in lithium-ion battery electrode,field-effect transistor(FET)and magnetic tunneling junction(MTJ)were evaluated.Our main research works are as follows:1.We calculated the interface electric properties of monolayer MoN contacted with Li atoms,and evaluated the potential of MoN as anode of Li-ion battery.In recent years,monolayer MoN has been synthesized successfully in experiment,but its intrinsic characteristics still need to be explored in theory.We researched the unique electric properties caused by the two hetero outmost layers of planar MoN.The work functions of two sides of monolayer MoN are different.The work function of the exposed nitrogen atom surface is up to 6.3 eV.We further explored the potential of monolayer MoN performing as an anode material for lithium-ion batteries.The small diffusion barrier(0.41 eV)and large theoretical capacity(406 mAh·g-1)render 2D MoN a suitable anode material for lithium-ion batteries.2.We calculated the interface electrical properties of the vertically stacked 2H/1T MX2 heterostructure.Transition metal dichalcogenides not only have the common 2H phase,but also have other stable phases,such as 1T phase and 1T' phase.Different phases stacked heterostructures of the same material can be constructed by phase engineering.Compared with the heterostructures made of different materials,the heterostructures made of the same material has fewer impurities,smaller lattice mismatch and lower contact resistance.In order to explore the intrinsic MX2 heterostructure with different stacking phases,we studied the electrical properties of the vertically stacked 2H/1T van der Waals heterostructures of the six transition metal chalcogenides(MoS2,MoSe2,MoTe2,WS2,WSe2 and WTe2)based on density functional theory(DFT).We found the different relative positions of 2H phase and 1T phase in the heterostructure lead to six unequal stackings.We further studied the projected energy band structures of the 2H/1T heterostructures in order to analyze the influence of stacking 1T-MX2 on the electronic properties of 2H-MX2.3.We proposed the suitable 2D electrode materials for single-layer sub-10 nm WSe2 FETs.It is very important to find suitable electrode materials for small size FETs with 2D channel(such as WSe2).In order to analyze the internal mechanism of the electrical contact between the 2D layers on the device performance and find out the suitable 2D electrode materials for sub-10 nm WSe2 FETs,we applied density functional theory coupled with the nonequilibrium Green's function method(DFT-NEGF)to compare device performance of sub-10 nm WSe2 FETs with six different 2D electrodes:NbSe2,Mo2CF2,Mo2CO2,Ti2C,Ti2C(OH)2,and borophene.We calculated the electrical properties and simulated the transport properties of the devices.Two kinds of 2D electrode materials(Ti2C(OH)2 and NbSe2)are proposed by comparing the key factors affecting the device performance,such as on-state current,delay time and power delay product of sub-10 nm WSe2 FETs with different electrodes.4.We calculated the device performance of the 2D graphene/1T-VSe2/h-BN/1T-VSe2/graphene magnetic tunnel junction(MTJ)with rotating h-BN barrier layer.Some three-dimensional(3D)materials,such as CrI3,1T-VSe2 and Fe3GeTe2,can still retain the magnetism when they are stripped into 2D materials.These 2D materials and other 2D materials can be stacked into heterojunction to form a new type of magnetic tunnel junction.Based on the DFT-NEGF,we studied the electromagnetic properties of the vertically stacked 2D graphene/1T-VSe2/h-BN/1T-VSe2/graphene magnetic tunnel junction.In this 2D magnetic tunnel junction,1T-VSe2 is the ferromagnetic layer,the single h-BN layer is the barrier layer,and graphene is the metal electrode.We studied the effect of the rotating insulating h-BN layer within 0°-172.4°relative to VSe2 ferromagnetic layer on the performance of the devices.We calculated the transmission spectra of the MTJs with different rotation angles of h-BN layer,and compared the changes of the tunneling magnetoresistance of the devices with different h-BN rotation angles.