Study On The Optoelectronic Devices Based On Anisotropic Two-dimensional Materials And Heterostructures

With the deepening of research on low-dimensional materials,low-dimensional nanomaterials have shown broad application prospects in the field of optoelectronics.Their unique crystal structure characteristics and excellent optoelectronic properties conferred by their electronic structure make these low-dimensional nanomaterial optoelectronic devices exhibit many novel characteristics.The anisotropy of crystal structure and electronic structure extends the application potential of low-dimensional nanomaterials to photoelectric devices based on polarized light.At present,optoelectronic devices based on highly anisotropic nanomaterials are still in the basic research stage,and the performance of some anisotropic nanomaterials based polarization-sensitive photodetectors reported so far needs to be further improved.The development of a new type of low-dimensional nanomaterials with strong anisotropy is a very effective solution to improve the performance of polarized light detection.This paper mainly adopts experimental research methods to better understand and optimize the device performance,so as to further promote the application of low-dimensional nanomaterials in optoelectronic devices,especially polarized light detectors.Along with the research route of material growth—structure characterization—device performance testing,we study systematically the optoelectronic properties of several new low-dimensional materials,reveal the anisotropic physical properties,and achieve the excellent response to external fields.We have confirmed in experiments that these nanomaterials have excellent anisotropic physical properties and excellent polarized light detection performance.The main content and results of this thesis are as follows:1.The tellurium nano-flakes and wires were successfully prepared using the facile and scalable physical vapor deposition(PVD)method,and the transistors based on them were fabricated.We achieved the high hole mobility up to 1485 cm²/Vs in Te flakes at room temperature,outperforming the most of 2D materials such as transition metal dichalcogenides,Bi,black phosphorus,black arsenic as well as the Te prepared by previous solution method.The Te nano-wires also exhibit high hole mobility of 833 cm²/Vs and offer a new possibility for mixed-dimensional optoelectronics.This work reports the PVD-grown p-type tellurium with high carrier mobility,and can pave the way towards applications in unique optoelectronics including the FETs,photodetectors and photovoltaic cells.2.Large-scale ultrathin circular Sn S_0.5Se_0.5 nanosheets with an orthorhombic structure and highly crystalline quality has been successfully synthesized using the improved low-pressure PVD(LPPVD)method with computational fluid dynamics(CFD)simulation.Firstly,the thickness of the measured nanosheets is in the range of 9 to 12 nm.The optical bandgap is about 1.09 e V.It is confirmed that the chosen ultrathin circular Sn S_0.5Se_0.5 with a high crystalline degree has a similar anisotropic Raman intensity toward Sn S and Sn Se nanosheets.,the AC or ZZ direction can be determined according to the strongest Raman intensity of A_g (77 cm^-1).The two-terminal circular Sn S_0.5Se_0.5-based phototransistor delivered a p-type behavior and good photo-response under 532 nm illumination.In particular,the I_on/I_off ratio,conductivity and photoconductivity are enhanced significantly along the ZZ direction(such as maximum I_on/I_off ratio of 78,hole mobility of 0.384 cm²/Vs).The measured anisotropic conductance ratio (?_zigzag/?_armchair)of 19 and the anisotropic mobility(?_zigzag/?_armchair)of 6.87 are also obtained.Moreover,the corresponding photodetector displayed good photo- response behavior under 532-1064 nm along the ZZ direction.As a result,the maximum responsivity,external quantum efficiency and specific detectivity are 142.7 A/W,2.78×10⁴%and 1.55×10¹² Jones under 635 nm illumination.Besides,the anisotropic photocurrent ratio is enhanced with the increment of the incident wavelength.The photocurrent reach its maximum under 808 and 1064 nm across the AC direction,which is as the same as the direction of the alloyed-like Raman peak(77 cm^-1).The polarization-sensitive PD showed the photocurrent anisotropic ratio of 2.5 and 3 under 808 nm and 1064 nm,respectively.These properties make 2D circular Sn S_0.5Se_0.5 a promising material for the application in flexible polarization-light PDs,near-infrared imagers and polarization-perception neural morphology transistors.3.A multilayer ambipolar Mo Te₂/Bi₂Se₃ heterojunction device with type-III band alignment was sucessfully fabricated.Firstly,we report Au-assisted exfoliation and non-destructive transfer method to fabricate large-scale Bi₂Se₃ thin nanosheets at the thickness range of 5-30 nm.The problem of the mechanical exfoliation for the thinner Bi₂Se₃ sample is already resolved.Meanwhile,a novel broken-gap tunneling van der Waals heterostructure is designed by combing 2H-Mo Te₂(19-22 nm)and Bi₂Se₃ via the dry-transfer method.As a result,the polarity of 2H-Mo Te₂ is changed from ambipolar to p-type domination under various forward bias,while the polarity of Bi₂Se₃ shows heavily n-type conductive behavior.Moreover,the I_ds-V_ds characteristics of the heterostructure clearly exhibited various backward rectification ratio in the V_ds range of-1 to 1 V(The maximum rectification ratio reach to about 100).The transfer curve belongs to a P-type dominated ambipolar behavior.Unlike the traditional p-n junction working via the diffusion of minority carriers,the transport mechanism in this type-III band alignment is composed of thermionic emission(backward bias),direct tunneling(small forward bias)and Fowler-Nordheim tunneling(large forward bias).At last,the device have the capability of detecting the broadband spectrum (405 nm-1550 nm)with or without small bias under the direct tunneling mechanism,which indicates the potential application in optical imaging and imaging sensor,etc.At last,the device shows ultralow dark current of 0.2 p A,superior photo-switching ratio of 10⁶and fast response time of 21 ms for a visible light of 405 nm under zero-bias.Our work demonstrates a new universal method to fabricate a topological insulator and paves a new strategy for the topological insulator/transition metal dichalcogenide family in the novel synthesis of two-dimensional materials,multi-value logical circuit and broadband photodetector to be used in future high performance,high integration and the chips with low-power consumption.4.We discovered the strong anisotropy of the phonon mode through angle-resolved polarized Raman(ARPR)spectroscopy.By constructing a heterostructure with multilayer Bi₂O₂Se as the polarization photosensitizer and two-dimensional WSe₂ as the optical carrier transmission channel,a high-performance polarization-sensitive photodetector is realized.The photodetector exhibits a broadband response spectrum from 405 nm to 1064 nm,as well as high responsivity,fast response speed and high sensitivity.More importantly,the photocurrent shows a strong light polarization dependence,with a maximum anisotropic ratio of 4.9.This work provides new insights into the anisotropic physical properties of Bi₂O₂Se and expands its applications in angle-resolved electronics and optoelectronics.