Theoretical Modulation Of The Electronic Structure Of Bi₂O₂Se-Based Heterostructures

The increasing discovery of graphene and various two-dimensional(2D)materials has established the material basics for the researches of 2D van der Waals heterostructures(vdWHs).The vertical integration of two-dimensional materials with different properties into van der Waals heterostructures provides a new research approach for breaking the application limits of a single material,designing excellent and multi-functional physical properties.Band alignment is crucial for the stability and transmission of electrons and holes in van der Waals heterostructures and is one of the key properties that determine the performance of devices such as optoelectronics.It is an effective way to obtain excellent photoelectric properties and realize multi-functional regulation of heterostructures by the types of the component heterostructure materials,number of layers,stacking patterns,ferroelectric polarization,and electric field,which is of great significance to improve the application performance of heterostructure materials,broaden their application scope and realize their multi-functional integration.2D Bi2O2Se material has recently received increasing attention for its ultrahigh carrier mobility,moderate bandgap,excellent air stability,and responsiveness to various outside signals.In addition,ultrathin Bi2O2Se nanosheets exhibit room temperature ferroelectric property due to spontaneous lattice distortion resulting in reduced symmetry;the band structure of 2D Bi2O2Se is also more sensitive to strain and number of layers,and the bandgap and band edge positions display high tunability.Numerous reports have shown that these unique properties make 2D Bi2O2Se material have great potential for the construction of multi-functional heterostructure materials and optoelectronic devices such as photodetectors.However,the regulation law and mechanism of the band structure of 2D Bi2O2Se-based heterostructure materials are still unclear,which is closely related to the fact that ferroelectric property of 2D Bi2O2Se materials are not considered in most mechanism studies.Therefore,it is urgent to systematically study the micro-mechanism of ferroelectric properties on the regulation of the electronic structure and band alignment evolution of the Bi2O2Se-based heterostructures.In this thesis,we investigated the ferroelectric structure of Bi2O2Se monolayer and constructed two Bi2O2Se-based vdWHs(i.e.,MoSe2/Bi2O2Se and PdSe2/Bi2O2Se)based on the first-principles calculations.Furthermore,we studied the regulation law and mechanism of multiple ways(i.e.,ferroelectric polarization direction of Bi2O2Se,in-plane biaxial tensile and compressive strain,number of layers in two-dimensional materials)on electronic structures such as bandgap,band alignments and interface charge transfer.We realized the multi-functional properties regulation of the heterostructure materials and explored their potential applications in the field of optoelectronics.The main research contents and conclusions are as follows:(1)We developed the asymmetric-zipper and zigzag-zipper models based on the zipper model proposed by predecessors to induce ferroelectric polarization and obtained ferroelectric Bi2O2Se monolayer structure that matches well with MoSe2.(2)The electron transfer between the two materials can be well regulated by changing the ferroelectric direction of the Bi2O2Se monolayer,and then changed the position of the band edge.When the polarization direction of Bi2O2Se deviates from MoSe2 to form the MoSe2/Bi2O2Se(↓).The heterostructure exhibits a typical unipolar barrier structure,which is conducive to achieving electron migration being blocked and holes migration being unimpeded.It has promising application prospects in the unipolar barrier photodetectors.(3)When the MoSe2/Bi2O2Se(T)is subjected to compressive strain,its band alignment changes from type-Ⅱ to type-I.The bandgap type undergoes the semiconductor-metal transition by further increasing the compressive strain.For MoSe2/Bi2O2Se(↓),it can obtain a good type-Ⅱband alignment and realize the transition from type II to type I under tensile strain.And its bandgap type can achieve the transition from semiconductor to metal under compressive strain.Therefore,in-plane biaxial strain can effectively modulate the electronic structure and band alignment of MoSe2/Bi2O2Se heterostructures,which provides a good theoretical basis for the design and fabrication of high-performance and multi-functional devices.(4)When the polarization direction of the Bi2O2Se monolayer was shifted from up to down,the band alignment of PdSe2/Bi2O2Se heterostructure changes from type-Ⅰ to type-Ⅱ.The synergistic effect of ferroelectric polarization and built-in electric field enhances the transfer of electrons from PdSe2 to Bi2O2Se and promotes the effective separation of electrons and holes,which is beneficial for the PdSe2/Bi2O2Se heterostructure in optoelectronic devices.(5)The band offset of type-Ⅱ PdSe2/Bi2O2Se heterostructure can be adjusted with the increase in the number of layers of Bi2O2Se.The PdSe2/Bi2O2Se heterostructure can achieve the transition from type-Ⅱto type-Ⅰ and a good unipolar barrier band structure by changing the number of layers of PdSe2.The above research has a far-reaching significance for realizing the multi-functional properties of heterostructure materials and exploring their potential applications in nano-optoelectronic devices.