The Electronic Structure Modulation,thermoelectric And Optoelectronic Properties Of Two-dimensional Heterostructures

The two-dimensional（2D）materials have received considerable attention by reason of their unique properties and a wide range of applications since the successful production of graphene in 2004.With the emergence of abundant 2D materials,the rational combination of distinct monolayer nanosheets becomes a versatile strategy to break through the application limitation of single material systems in multiple fields and produce new materials with thoroughly innovative properties.Being dependent on the different composite patterns,2D materials can be stacked vertically in layers to form van der Waals heterostructures（vd WHs）or seamlessly stitched side-by-side to form lateral heterostructure（LHS）.Vd WHs do not rely on one-to-one chemical bonding and are physically assembled together by interlayer van der Waals interactions,have more relaxed requirements and wider selectivity for components,which is of great importance for the experimental preparation of materials and the broadening of the application range of devices.Compared with vertical heterostructures,LHS has more stringent requirements and lower selectivity,but its smaller contact area and the better covalent bond can guarantee its epitaxial quality and stability,which is very important in device manufacturing.Currently,both in-plane and vertical heterostructures have been prepared in a mature way.Heterostructures provide a rich scope for the fundamental research of materials and the design of novel devices,yet there are still a large number of unknown systems waiting to be explored.Based on this,the following work has been carried out in this paper with different 2D lateral and vertical heterostructures as the object of study.1.Band engineering of graphene/hexagonal boron nitride in-plane heterostructure by interfacial connecting angles.Results show that the interface topography plays a crucial role in determining the band structure of systems.The bandgap can be modified efficiently（from semiconductor to metal）by changing the connecting angle between graphene and h-BN in LHS,on account of the mixture of different interfacial atomic configurations.For LHS with an armchair interface,the strong quantum confinement results in the charge densities of CMB and VBM covering the full region of the graphene side.The edge states of the zigzag interface can be understood by the deformation-induced gauge field and the accompanying effective pseudo spin orbit coupling.These findings provide a theoretical basis for elucidating the relationship between the atomic construction and electronic properties of planar G/h-BN heterostructures,which could pave a way for further controllable and tunable 2D electronic devices.2.Modulation of interfacial electronic properties in N-GY/MoS₂vd WHs via external electric field and strain.We have calculated the thermal stability and electronic structure of the heterostructure using first principles.The system has a negative binding energy,implying the possibility of its successful experimental synthesis,and simulations of molecular dynamics demonstrate the stability of the studied system.The applied electric field and biaxial strain can effectively adjust the band gap and band edge positions of the heterostructures.Under the influence of the positive electric field,the heterostructures transitions from type II to type I and then to type II semiconductor with different charge distribution at the interface,while the charge transfer increases with increasing electric field strength.In addition,the heterostructures have a wide optical absorption range,with absorption peaks from the near infrared to the ultraviolet,and an optical absorption intensity of 10⁵orders of magnitude.3.Electronic,thermoelectric,transport and optical properties of BAs/MoSe₂vd WHs.The AC’-type configuration is the most stable one with excellent dynamical,thermal and mechanical stability,which has a direct bandgap of 1.04 e V and type-I band alignment.The Seebeck coefficient and phononic thermal conductivity of BAs/MoSe₂vd WHs is consistently higher than individual BAs layer and MoSe₂layer at a temperature range from 200 to 1200 K,which suggests the possibility of BAs/MoSe₂vd WHs for thermoelectric applications.For the transport properties of the heterojunction,there is a tendency for the current to increase after the bias voltage exceeds the threshold voltage（0.6 V）and is symmetrical with the forward and reverse bias voltages.The transition from type-I to type-II band alignment can be achieved by applying the in-plane strains and external electric field,but the direct band gap feature remains preserved.For optical properties,the BAs/MoSe₂vd WHs can break through the restrictions of two isolating layer and combine their optical absorption strength together.There has a remarkably high optical absorption coefficient(～10⁵cm^-1)in the visible-ultraviolet region,and the red shift in the optical absorption spectrum can been observed under the in-plane strains and external electric field.The energy conversion efficiency exceeding 20%of the vd WHs under strain and electric field making the heterostructure extremely potential in solar energy harvesting in low-dimensional excitonic solar cells.4.Type-II Ge C/MoSe₂vd W heterostructures:Excellent performance in thermoelectrics and overall photocatalytic water splitting.Our results demonstrate that the Ge C/MoSe₂vd WHs with different stacking configurations all have excellent chemical,dynamical,thermal and mechanical stability.The large Seebeck coefficient and low thermal conductivity giving the AB3 stacking vd WHs a large figure of merit（ZT）of 2.63 for n-type doping at 900 K,which exceeds the values reported so of around 2.6.All heterostructures have remarkably high optical absorption coefficient(～10⁵cm^-1)in the visible light region.The appropriate band edge position and kinetic overpotentials make the system susceptible to reactions with water and drive photocatalytic hydrogen-evolution reaction（HER）and oxygen evolution-reaction（OER）respectively on the surfaces of the different components,the AB3 configuration has the best catalytic activity of the OER.Large photocurrents can be generated in systems under vertical irradiation with linearly polarized light,and the magnitude of the photocurrent is dependent on the configuration of the system and the energy of the incident photons.