Theoretical Study On The Electronic Structure And Optoelectronic Properties Of Heterojunctions Based On Novel Two-dimensional Material

With the popularization of wearable devices and the widespread use of internet of things（Io T）devices,traditional photodetectors that rely on external power sources for photoelectric detection are challenged by problems such as high dark current and small switching ratios.Therefore,the performance of self-powered photodetectors has become the key to limit the wider application of photodetectors.After the development of more than ten years,the self-powered photodetectors have made important progress in terms of integration,flexibility and stability,but their efficiency is still greatly constrained.In recent years,heterostructures have shown great prospects and value in the rational design of self-powered photodetectors due to the excellent performance exhibited by the synergy between their multiple components.In this thesis,some heterostructures stacked with different dimensional packing are constructed based on new two-dimensional materials.The transfer process of photogenerated carriers in heterostructures is studied by first principles and nonadiabatic molecular dynamics.The influence of interface effect on the electronic structure and the regulation of strain on the photoelectric characteristics for heterostructure is systematically researched,which provides theoretical support for the design and fabrication of high performance and self-powered photodetectors.The primary research contents and results are as follows:（1）Ga Se/Sn S₂ heterostrunction is constructed by using Ga Se monolayer and Sn S₂ monolayer.It is found that the Ga Se/Sn S₂ heterostrunction has a stable structure and type-II energy band arrangement,and the larger conduction band offset（CBO）and valence band offset（VBO）promote the effective separation of photogenerated electrons and holes.The presence of potential drop（E_P）in the heterostrunction drives the built-in electric field between the layers,leading to the ability of the heterostrunction to operate at zero bias voltage and achieve self-powered capability.The absorption coefficient of Ga Se/Sn S₂ heterostrunction is significantly improved in the UV and visible regions compared to a single material.In addition,both vertical strain and biaxial strain can effectively modulate the band gap and optoelectronic properties of Ga Se/Sn S₂ heterostrunction.This work not only verifies the experimentally discovered p-n type heterostructure and explains the high performance of Ga Se/Sn S₂ heterostrunction,but also theoretically analyzes the self-powered capability of heterostrunction and the modulation effect of strain on Ga Se/Sn S₂ heterostrunction.（2）The carrier transfer and recombination dynamics at the interface of In Se/BP heterostructure are studied by nonadiabatic molecular dynamics.It is found that the In Se/BP heterostructure has a type-II band arrangement,and the difference in average electrostatic potential leads to the formation of potential drop at the interface of the heterostructure,which make for In Se/BP heterostructure has a strong self-powered ability.The light detection range of In Se/BP heterostrunction covers the range from ultraviolet light to near-infrared light,and there is ultrafast photogenerated electron transfer at the heterostructure interface,which makes the heterostructure have fast light response ability.The stacking of BP monolayer and In Se monolayer significantly extends the lifetime of photogenerated carriers,thus reducing energy loss,which effectively improves the energy conversion performance of photodetectors.In particular,with the increase of compressive strain in In Se/BP heterostrunction,the self-powered ability and light detection ability in near-infrared light of the heterostructure are significantly enhanced.This work not only explains the self-powered mechanism,fast light response and infrared light detection ability found in the experiment,but also provides an effective way to prepare photodetectors with high performance and self-powered ability.（3）2D/2D BP/Cs₂Sn I₄ heterostructure with Cs-I-BP and Sn-I-BP interfaces is designed and builded by all inorganic two-dimensional perovskite（Cs₂Sn I₄）and monolayer black phosphorus（BP）.It is found that the interface effect enhances the self-powered ability and light trapping ability of the two type heterostructures.The Cs-I-BP heterostructure can be transformed from semiconductors to metals when a large compression force（-8%～-4%）is applied to them.Especially when±10%strain is applied,the E_P value is significantly increased and the self-powered ability is improved.In addition,applying biaxial compressive strain not only significantly broaden the responsiveness of Cs-I-BP and Sn-I-BP heterostructures in the near-infrared light region,but also enhances the optical absorption coefficient of two heterostructures in the full spectrum range,thereby improving the photodetection performance of Cs-I-BP and Sn-I-BP heterostructures.（4）2D/3D heterostructure is builded based on Cs₂Sn I₂Cl₂ and Cs₂Ti I₆.The results show that Cs₂Sn I₂Cl₂/Cs₂Ti I₆ heterostructure exhibits a type-II band arrangement and excellent light trapping ability.Due to the spontaneous transfer of carrier caused by CBO and VBO,the built-in electric field is formed in the heterostructure and the self-powered capability is provided.Through the DISH method,it is found that the recombination time of carriers at the heterostructure interface is 3～4 orders of magnitude slower than the separation time of charges,which reduces the energy loss of heterostructure photoelectric conversion and improves the performance of photodetectors.In addition,biaxial compressive strain can not only broaden the photoresponse of Cs₂Sn I₂Cl₂/Cs₂Ti I₆ heterostructure in the near-infrared region and enhance the optical absorption coefficient of the heterostructure in the full spectrum,but also enhance the self-powered ability of the heterostructure.（5）Ga N/WS₂/Mo S₂ heterostructure is systematically studied by first principles and non-adiabatic molecular dynamics methods.The interfacial effect leads to the stepwise transfer of photogenerated carriers at the interface of Ga N/WS₂/Mo S₂heterostructure,which results in the effective separation of photogenerated carriers,the formation of built-in electric field and the self-powering capability.The heterostructure can enhance its light absorption in the full spectral region due to the reduction of the band gap.In particular,the carrier compounding time at the heterostructure interface reaches subtle magnitude,which greatly extends the lifetime of the photogenerated carriers and effectively enhances the energy conversion performance of the photodetector.In addition,the self-powered capability of the Ga N/WS₂/Mo S₂ heterostructure and its enhanced photodetection capability in the near-infrared light are improved with the increase of tensile strain.