Electrical And Optoelectronic Properties Of MoS₂, WSe₂ And Related Heterostructures

Two-dimensional materials have attracted much attention due to their unique chemical and physical properties. As an example of graphene, it presents ultrahigh carrier mobility, high mechanical stiffness and strength, and chemical stability. Transition metal dichalcogenides（TMDs） has great potential on electronic and optoelectronic applications due to their intrinsic bandgap, strong light-matter interaction and coupled spin and valley degree. Besides, TMDs van der Waals（vd Ws） heterostructures composed of separated TMDs and other two-dimensional materials have been emerging. Compared with conventional semiconductor heterostructures, the relaxed lattice-matching requirement and flexible fabrication approaches make vd Ws heterostructures an ideal platform to study the electrical and optical behavior. In this thesis, on the one hand, from the viewpoint of electrical contacts between two-dimensional materials and metals, the carrier density of MoS₂ is modulated by physical and chemical doping methods to decrease the contact barriers, and then improves the device performance, such as carrier mobility and ON/OFF current ratio. On the other hand, we construct vertical and lateral heterostructures, and then study the optoelectronic and photovoltaic performance. Moreover, we manipulate the photoluminescence properties and excitonic states of TMDs/graphene heterostructures, and reveal the effects on excitonic behavior, which may be beneficial to the exploit optoelectronic applications with new-concept, and understanding of new physics in vd Ws heterostructures.The interlayer screening effect in MoS₂ is investigated for the first time by Kelvin probe microscopy（KFM）, which is attributed to the different screening ability of few-layer MoS₂ from the built-in electric field induced at MoS₂/Si O2 interface. The surface potential of MoS₂ decreases by thickness, and the screening length is about 5 nm. Besides, the electrical contact behavior between Pt and MoS₂ with different thickness is studied by conductive atomic force microscopy（C-AFM）, where the tunneling barrier at Pt/MoS₂ increases by thickness.The carrier density of bilayer MoS₂ is modulated by back-gate electric field. It is observed that when the gate bias is larger than the threshold voltage, the surface potential of MoS₂ increases and the corresponding work function of MoS₂ decreases by about 115 me V at gate bias of 13 V, which is consistent with the calculated results by field effect. The asymmetric contact barriers between Pt and MoS₂ are directly measured by KFM, consistent with two-probe measurements. The charge injection at the electrical contacts obeys to thermonic field emission mechanism. Carrier doping of MoS₂ nanoflakes was achieved by the charge injection from the functional self-assembled monolayers（SAMs） with different dipole moments. The effect of SAMs on the charge transfer between the substrates and MoS₂ nanoflakes was studied by Raman spectroscopy, field-effect transistor（FET） measurements, and Kelvin probe microscope（KFM）. Fluoroalkyltrichlorosilane-SAM（FOTS） with a large positive dipole moment, acting as hole donors, significantly reduced the intrinsic n-doping characteristic of MoS₂ nanoflakes, while 3-（trimethoxysilyl）-1-propanamine-SAMs（APTMS）, acting as electron donors, enhanced the n-doping characteristic. KFM results clearly demonstrated the Fermi level of monolayer MoS₂ can be tuned in a range of 0.47 e V.Photodiode behavior in（both n- and p- type） silicon/monolayer MoS₂ vertical heterostructures was observed. The photocurrent and photoresponsivity of heterostructures photodiodes were dependent both on the incident light wavelength and power density, and the highest photoresponsivity of 7.2 A/W was achieved in nSi/ monolayer MoS₂ vertical heterostructures photodiodes. Compared with nSi/MoS₂ heterostructures, the photoresponsivity of p-Si/MoS₂ heterostructure was much lower. Kelvin probe microscope（KFM） results demonstrated the more efficient separation of photogenerated excitons in n-Si/MoS₂ than that in p-Si/MoS₂. Coupling KFM results with band alignments of（p-, n-） Si/MoS₂ heterostructures, the origins of photodiode-like phenomena of p-Si/MoS₂ and n-Si/MoS₂ have been unveiled, that is intrinsic built-in electric field in p-n junction, and modulated barrier height and width at the interface in n-n junction. On the other hand, it is demonstrated that the TMDs based lateral heterostructure composed of few-layer WSe₂ and WO₃ has superior performance for photodetection and power generation in a wide range of wavelengths from ultraviolet to infrared. Upon light excitation, the laterial junctions showed a maximum photodetection responsivity of 2.5 A/W and photovoltaic power generation with a peak external quantum efficiency of 656%, promising values in a lateral device geometry. Besides, by controlling the partial oxidization of WSe₂ to WO₃, monolayer –fewlayer WSe₂ Schottky photodiode with excellent diode characteristics, high responsivity（R, 10 A/W）, external quantum efficiency（EQE, 2632%）, and power conversion efficiency（PCE, 1.3%）, has been realized.The photoluminescence properties of monolayer MoS₂/graphene heterostructures are systematically via electrochemical gating. It is manifested that PL intensities of excitons in MoS₂ and trion/exciton intensity ratio can be tuned by more than two-order of magnitude and 30 time, respectively. The blueshift of exciton peaks up to 40 me V is also observed. By extracting the carrier density of MoS₂ by electric potential distribution model, and Schottky barrier by first-principle calculations, we found that the carrier density in MoS₂ played a dominant role on PL tuning at positive gate bias, while interlayer relaxation of excitons induced by Schottky barrier had a major contribution at negative gate bias. This was further verified by controlling the tunneling barrier and screening field across MoS₂ by insertion of self-assembled monolayers（SAMs） at the interface.Upon tuning the photoluminescence properties of WSe₂/graphene and WSe₂/MoS₂/graphene heterostructures by virtue of electric field, it is demonstrated that the interlayer relaxation of excitons at the hetero-interface in WSe₂/graphene, that was even stronger than that in MoS₂/graphene and WSe₂/MoS₂ heterostructures, plays a dominant role in PL tuning in WSe₂/graphene heterostructures, while the carrier population in WSe₂ induced by electric field has minor contributions. In addition, we discovered that the interlayer coupling between monolayer WSe₂ and graphene was enhanced under high electric field, which breaks the momentum conservation of first order Raman allowed phonons in graphene, and then induces the enhanced Raman scattering of defects in graphene.