Design Of Novel Van Der Waals Heterojunction Based Infrared Photodetector

Recent years,due to the atomic layer thickness,stacked with van der Waals force and free of surface chemical dangling bonds,the two-dimensional?2D?materials exhibit a series of unique physical properties,which have attracted much attention of researchers.So far,2D materials,include graphene,transition metal chalcogenides?TMDs?,black phosphorus?BP?,boron nitride?BN?,and so on have been discovered.The emergence of 2D materials structure and electronic diversity provide an ideal research platform for exploring novel physical phenomena and internal mechanisms.Among these 2D materials,TMDs are ideal materials for various optoelectronic devices due to their good chemical stability,high carrier mobility and layer-dependent tunable band-gaps.Among them,molybdenum disulfide?MoS₂?is the most widely studied 2D material in TMDs.MoS₂ has an indirect bandgap of 1.2 eV in its bulk form.When the layers of MoS₂ decreased to monolayer,it will transform to a direct-gap with 1.9 eV.Meanwhile,as the newly discovered precious metal TMDs,platinum selenide?PtSe₂?has a wider adjustable band-gap with band-gap of 1.2 eV for monolayer,0.21 eV for double layers,and zero band-gap for bulk.These 2D TMDs with wide adjustable band-gaps are ideal semiconductors for the fabrication of infrared photodetectors.Therefore,researchers have developed various 2D TMDs-based photodetectors with different device structures operating in different band and demonstrating superior detection performance.Although 2D materials and their photodetectors possess many advantages,there are also some shortcomings.For example,the weak absorption of incident light due to the ultra-thin properties of 2D TMDs;the large exciton effect in the 2D TMDs system greatly hinders the separation and collection of photogenerated carriers;It's still remain challenge in large-area synthesis of some 2D materials,which limits its further application in optoelectronic devices.In view of the above problems,we designed of a synthesis route for preparation of large-area 2D TMDs,and constructed two-dimensional/three-dimensional?2D/3D?mixed-dimensional van der Waals heterojunction devices to realize high-performance infrared photodetectors.By designing a mixed-dimensional van der Waals heterojunction,the absorption of incident light can be effectively enhanced,and the built-in electric field of the heterojunction can effectively accelerate the separation of photogenerated electron-hole pairs,leading to higher device performance.In summary,the large-area 2D MoS₂ and PtSe₂ films were prepared by thermal decomposition and metal selenization method,respectively,and the MoS₂/CdTe and PtSe₂/CdTe mixed-dimensional van der Waals heterojunction devices were designed and constructed,and their infrared photodetection performance were studied.The main results are as follows:A two-step thermal decomposition method was used to prepare a large-area,thickness-controlled 2D MoS₂ films.The morphology,structure and composition were investigated by X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,atomic force microscopy?AFM?and Raman spectroscopy.Large-area 2D PtSe₂ films were prepared by a metal selenization method,and their morphology,structure and composition were characterized by XRD,XPS,AFM,transmission electron microscopy?TEM?and Raman spectroscopy.The MoS₂/CdTe mixed-dimensional heterojunction photodetector was designed and constructed.Due to the type-II heterojunction band arrangement,the detector has a wide-band response range beyond the limitation of band-gaps of MoS₂ and CdTe,covering from deep ultraviolet?200 nm?to near infrared?1700 nm?.Further research found that the MoS₂/CdTe heterojunction photodetector has a responsivity of 36.6mA/W at a wavelength of 780 nm,a specific detection rate of 6.1×10¹00 Jones,and response speed of 43.7/82.1?s at 5 kHz.These results demonstrate that the MoS₂/CdTe p–n heterojunction has great potential in room-temperature infrared detection,and provide a way to design high-performance infrared photodetectors for other 2D materials.The PtSe₂/CdTe mixed-dimensional van der Waals heterojunction photodetector was designed and constructed.The heterojunction device exhibits broadband detection from deep ultraviolet?200 nm?to near infrared?2000 nm?.Under light illumination of 780 nm,the PtSe₂/CdTe heterojunction infrared photodetector has a responsivity of 506.5 mA/W,a specific detection ratio of 4.2×10¹¹Jones,and a current on/off ratio of 10⁶ and the response speed of 8.1/43.6?s at 10 kHz.It is worth noting that the heterojunction device exhibits excellent repeatability,stability and weak light detection capability.In addition,the heterojunction device has a response speed of nanoseconds for a pulse signal with a pulse width of 1 ns.The results show that the 2D layered PtSe₂ film has great application in high performance broadband photodetection.