Theoretical Studies Of Band Alignment And Carrier Dynamics For Low Dimensional Heterostructures

Two-dimentianal(2D)materials have attracted extensive attention duo to their superior physical and chemical properties,however their pracrical application is limited duo to their intrinsic shortcomings.Constructing 2D lateral heterostreuictures is a teasible and effective strategy to solve above problems,and significant progresses have been made at lateral graphene/h-BN and heterostructures based-on transitional metal dichalcogenides.Besides,combining 2D materials with other nanosturectures can effectively improve their performance,such as composite electrodes of graphene/metal oxidation exhibit much more excellent cycle stability in lithium ion batteries compared with metal oxidation electrode,and catalytic performance can be significantly improved by loading MoS2 at semiconductor photocatalyst.Therefore,it is pretty important to study these low dimensional heterostructures.In this theise,we design a sirous of low dimensional heterostructures,and explore the band alignment and ultrafast photoinduced carrier dynamics based on density functional theory(DFT)and time-dependent density functional theory(TDDFT),the mainly contents of this theise are listed as below:1.We proposed four 2D lateral heterostructures constructed by phosphorene-like monolayer group-IV monochalcogenides,including GeS/GeSe,SnS/GeSe,SnSe/GeS and GeS/SnS.Based on DFT calculations,we investigated the energetics and electronic properties of these lateral heterostructures.The band structures and formation energies from supercell calculations demonstrate that these heterostructures retain semiconducting behavior and can be easily synthesized in the laboratory.The band offsets of monolayer,bilayer and trilayer heterojunctions at the Anderson limit are calculated from the valence/conduction band edges with respect to the vacuum energy level for each individual component.Among them,some heterostructures belong to type ? band alignment and are promising in optoelectronics.2.We proposed five kinds of 2D lateral heterostructures constructed by monolayer group-III monochalcogenides,including GaS/GaSe,GaSe/InS,GaTe/InS,GaTe/InSe,and InS/InSe,and each kind of heterostrutcure has two kinds of interface,which is along zigzag or arm-chaire direction.First-principles calculations show there is also covalent bond at these interfaces,and it is much easier to synthesize these heterostructures compared with 2D lateral transitional metal dichalcogenides or group-IV monochalcogenide heterosteutcures.According to the computed band offsets,most of the heterojunctions belong to type II band alignment,which can prevent the recombination of electron-hole pairs.Besides,the electronic properties of these lateral heterostructures can be effectively tailored by the number of layers,leading to a high theoretical powerconversion efficiency over 20%.3.We performed DFT calculations to explore the interfacial interaction between graphene and metal oxidation.Our results reveal generally weak physical interactions between graphene and several metal oxidations(including Cu2O and NiO).The Schottky barrier height in these metal/semiconductor heterostructures are computed using the macroscopically averaged electrostatic potential method,and the role of interfacial dipole is discussed.The calculated Schottky barrier heights below 1 eV suggest low contact resistance;thus these graphene/metal oxidation composites are favorable anode materials for better lithium ion batteries.4.Based on DFT,we calculated the band alignment at S-CdS(001)/MoS2 and Cd-CdS(001)/MoS2,and reveraled how and which kind of carrier will transfer across interface.Besides,the exsitance of intrinsic electric field at CdS(001)polar surface produce a energy shift at two sides of polar surface,making band alignment completely different at Cd-CdS(001)/MoS2 and S-CdS(001)/MoS2.Based on TDDFT,photoinduced carrier dynamics were explored.Some excited electrons at Cd-CdS(001)/CdS are scattered into higher energy state through electron-electron scatter or auger process.These results not only demenstrate the high photo-catalytic efficiency in experiment,but also provide a new physical explanation for carrier transfer at semiconductor heterojunctions.