First-principles Study On The Electronic Structure Modulation Of Novel Twodimensional Materials And Their Heterojunctions

With the vigorous development of nanoscience and nanotechnology,research on the two-dimensional?2D?materials and the related assembled van der Waals?vdW?heterojunctions are rapidly emerging.Moreover,the urgent need for high-speed computing performance has caused electronic devices to become smaller and more integrated.In order to meet the requirements of large-scale integration of circuits,the design and preparation of low-dimensional materials have become the forefront of scientific research in recent years.Evidently,the research on 2D materials and their heterojunctions will provide an effective way to break through size bottleneck for nanoelectronic devices.The successful preparation of graphene has opened a door to the study of 2D materials.Because the bandgap of graphene is zero,its application in semiconductor devices is limited.The researchers were inspired to synthesize a variety of 2D materials by various methods,such as mechanical peeling or chemical vapor deposition,etc.Simultaneously,these 2D systems can be used to explore the marvelous low-dimensional physical properties and applications in nanoelectronic devices.In addition,as the properties of a single material are always inadequate or defective,the physical properties of the 2D system can be modulated by assembling different 2D materials to construct heterojunctions or external field effects.In fact,there are two important factors to consider when packaging 2D materials into electronic devices.One is whether the material itself has excellent optoelectronic properties,and the other is whether the optoelectronic properties of the material can be regulated by external field effects.The optoelectronic properties of a material are largely determined by its electronic band structure.Therefore,in our work,we concentrate on the electronic structure properties of 2D materials and their assembled vdW heterojunctions.The properties of band structure evolution,electron transport,and interface charge transfer of 2D materials and heterojunctions under various conditions are studied based on the first-principles calculations and atomic-bond-relaxation theory.The research achievement we have acquired are as follows:?1?We investigate the evolution of band structure of monolayer black phosphorus under uniaxial strain is studied.It is found that the stress responses of black phosphorus show evident anisotropy due to different edge type structures.We propose a theoretical model for the variation of bandgap induced by the uniaxial strain from the perspective of atomistic origin.Our results show that the physical mechanism on the strain-dependent band offset can be attributed to the variation of crystal potential induced by the changes of bond length,strength and angle.?2?We research the band structure evolution and band alignment of black phosphorus?BP?/molybdenum disulfide?Mo S₂?vdW heterojunctions under the conditions of interfacial rotation and external electric field.It is found that the positions and orbital contributions of conduction band minimum and valence band maximum in BP/MoS₂heterojunctions will alter as the composition twists relative to each other,while the major orbital contributions that ared_z2 states of Mo atoms and p_z states of P atoms do not change.When applying an electric field,the BP/MoS₂ heterojunctions experiences transformations of type-?/?/?band alignment,indirect/direct/indirect bandgap type,and semiconductor/metal characteristics,respectively.?3?We explore the band structure evolution and electron transport properties of twisted bilayer WSe₂ under the approach of vertical electric fields.We find that the bandgap type of bilayer WSe₂ can be transformed from indirect to direct by twisting two monolayers.The external electric field can enable bilayer WSe₂ under six twist angles to achieve a semiconductor to metal transition.Especially,the stacking structures of 0°and 60°show transport anisotropy,and the transport performance in the zigzag direction is slightly better than that in the armchair direction.Moreover,the transport performance of the case with twisted 60°is better than that of 0°regardless of the edge direction when an external electric field is applied.?4?We study the interfacial properties of heterojunctions formed by Janus MoSSe and graphene.We find that due to the difference in atomic electronegativity and surface work function,the Gr/J-SMoSe heterojunction formed by the contact of S atoms with Gr exhibits an n-type Schottky barrier,whereas the Gr/J-SeMoS heterojunction formed by the contact of the Se atoms with Gr reveals a p-type Schottky barrier.Increasing the number of layers of J-MoSSe allows the Gr/J-MoSSe heterojunction to achieve the transition from Schottky-to-Ohmic contact.Moreover,under the control of external electric field,the Gr/J-MoSSe heterojunction can realize the transitions among n-type Schottky barrier,p-type Schottky barrier,and Ohmic contact.