| Since the successful exfoliation of graphene in 2004,two-dimensional(2D)materials have attracted the attention of researchers for their novel physical properties.For instance,with their ultra-thin thickness,high carrier mobility,tunable band structure and excellent optical absorption,2D materials have shown great potential for applications in new ultra-thin and flexible devices,nanoelectronics and optoelectronic devices.In recent years,a large number of 2D materials have been predicted and even successfully synthesised,such as silicene,germanene,hexagonal boron nitride,transition metal dichalcogenides and transition metal silicon-nitrogen compounds.With the deepening research on 2D materials,researchers have found that the properties of a single 2D material always have lim itations and cannot fully satisfy multiple needs.Therefore,attempts have been made to stack different 2D materials vertically together to form van der Waals heterostructures,thus realising the advantages of complementing each other.Based on van der Waals heterostructures,several types of band alignments can be realised between the 2D materials,resulting in an extremely rich physical and chemical property.In particular,the electronic properties of 2D van der Waals heterostructures can be effectively regulated by changing the stacking method,applying electric field and strain,which makes them promising in the field of new multifunctional electronic devices.Therefore,the exploration and design of 2D van der Waals heterostructures with excellent properties are important for the research of multifunctional electronic and optoelectronic devices.In this dissertation,we systematically investigate the geometric structure,electronic structure,transport properties and optoelectronic properties of several 2D materials and their van der Waals heterostructures by using the density functional theory and the non-equilibrium Green’s function method.We also modulate the abovementioned properties by changing the stacking configuration,applying external electric field and applying biaxial strain.These studies provide theoretical guidance for expanding the application of 2D materials and their van der Waals heterostructures in the fields of novel electronic and optoelectronic devices.The main research contents and conclusions of this dissertation are as follows:(1)We reveal the mechanism by which the intrinsic electric field of MoSSe regulates the electronic properties of MoSSe/MoS2 heterostructure,and explore its tunable electronic properties under external electric field and vertical strain.It is shown that by changing the direction of the intrinsic electric field in the MoSSe layer,MoSSe/MoS2 heterostructure can be transformed from the type-Ⅰ to type-II band alignment,which makes it extremely promising for both light-emitting devices and solar cell applications.Meanwhile,by analysing the physical mechanisms of interfacial charge transfer and band alignment,it is found that the construction of heterostructure effectively promotes the charge transfer between MoSSe and MoS2 layers,resulting in enhanced optoelectronic properties in MoSSe/MoS2 heterostructure.In addition,the electronic properties of MoSSe/MoS2 heterostructure under external electric field and vertical strain are further discussed,and interesting transitions between the heterostructure from type-I and type-Ⅱ band alignments can be observed.The results reveal the feasibility of controlling the interfacial properties of heterostructures by modulating the intrinsic electric field of the monolayer MoSSe,and provide theoretical guidance for the design of new multifunctional optoelectronic devices based on MoSSe materials.(2)We explore the enhanced carrier concentration and charge transport properties in MoSSe/graphene/WSSe heterostructure.It is found that the insertion of graphene does not disrupt the type-II band alignment of the MoSSe/WSSe heterostructure and the Dirac cone of graphene is well preserved.At the same time,the net effective electric field in the MoSSe/graphene/WSSe heterostructure promotes interlayer charge transfer,leading to hole doping of graphene.And the hole doping concentrations in graphene up to 9.2 × 1012 cm-2,which is two orders of magnitude higher compared to the intrinsic carrier concentration at room temperature.Next,by comparing the transport currents in MoSSe/WSSe and MoSSe/graphene/WSSe heterostructures,it is demonstrated that the insertion of graphene can effectively improve the charge transport performance of the devices.Finally,the effect of biaxial strain on the carrier concentration of graphene in MoSSe/graphene/WSSe heterostructure is further discussed.It is found that the hole doping concentration of graphene is further increased to the order of 1013 when the compressive strain is-6%.This study provides a new idea for improving the performance of nanoelectronic devices based on 2D semiconductor materials.(3)We predict a MoSi2N4/MoS2 heterostructure with type-Ⅱ energy band alignment and explore its potential application in optoelectronics.The results show that the MoSi2N4/MoS2 heterostructure is an indirect bandgap semiconductor with a bandgap value of 1.12 eV.The type-Ⅱ band alignment of MoSi2N4/MoS2 heterostructure can effectively promote the separation of photogenerated carriers at the heterostructure interface.As a result,the MoSi2N4/MoS2 heterostructure exhibits enhanced optical absorption properties from the near-infrared to the ultraviolet region,exhibiting optical absorption coefficients up to 105 cm-1,while the photocurrent density can be as high as 1.6 mA/cm2.In addition,applying electric field and biaxial strain can effectively modulate the band gap and band alignment of the MoSi2N4/MoS2 heterostructure.Notably,the tensile strain significantly enhanced the optoelectronic properties of MoSi2N4/MoS2 heterostructure.These findings reveal the great potential of MoSi2N4/MoS2 heterostructure for optoelectronic device applications.which is important for the development of strain-tunable optoelectronic devices.This dissertation consists of the following chapters:Chapter 1 introduces the development and applications of 2D materials and their van der Waals heterostructures,as well as several methods to modulate the properties of 2D materials and their heterostructures.Chapter 2 introduces the first-principles calculations method and the non-equilibrium Green’s function method used in this dissertation.Chapter 3 investigates the effect of the intrinsic electric field of monolayer MoSSe on the band alignment of MoSSe/MoS2 heterostructure and analyzes the modulation mechanism.Chapter 4 studies the electronic and transport properties of MoSSe/graphene/WSSe heterostructure and proposes a strategy for biaxial strain regulation of carrier concentration in the heterostructure.Chapter 5 investigates the enhanced interfacial carrier separation in MoSi2N4/MoS2 heterostructure and explores its potential for application in optoelectronic devices.Chapter 6 provides a brief summary and outlook of the research in this dissertation. |
