Synthesis And Modulation Of Electronic And Optoelectric Properties Of Molybdenum Disulfide Nano Films

Two-dimension（2D）semiconductor material MoS₂,which owns a similar layered structure to graphene,has attracted great interest due to its fascinating electrical,optical and mechanical properties.The growth of materials is the foundation of research.However,how to control the morphology of MoS₂ films,microcosmic structure of grain boundary（GB）,the growth of p-type MoS₂ films,epitaxial growth of non-layer materials on its surfaces and multi-level structures of MoS₂ films are still challenges.Until now,the morphology control of MoS₂ via chemical vapor deposition（CVD）is difficult as a result of different growth equipments,diverse growth methods and varied growth parpmeters from lab to lab.Especially,the regulation of element composition and the morphology structure on the electrical properties and piezoelectric properties is not comprehensive and need to be further perfected.Herein,we successfully preparation of MoS₂ material and studies the effect of GBs on the electrical transport properties and piezoelectric properties.Effective ways are developed,such as doping,epitaxial growth and composited structure,to improve the electrical,optical and mechanical properties of MoS₂ and make a series of important breakthrough in optical,electrical performance and sensor applicationIn the research of controllable growth of monolayer MoS₂ nanosheets and dependence between electrical properties and morphology,we realize the controllable growth of MoS₂ mono-ctrstalline,poly-crystalline and polygonous MoS₂ nanosheets and study the law of morphology evolution.The electrical transport properties based on poly-crystalline structure are carried out to investigate the effect s of GBs on the performance of FET devices.It’s found that GBs can greatly degrade transport properties.It is known that GBs play an important role as scattering cent ers which can trap charges and degrade charge carrier mobility.By investigating the effects of morphology structure on current output signals of piezoelectric devices,we find that the piezoelectric output signals of MoS₂ mono-ctrstalline structure along armchair orientation is higher than that along zigzag direction.At the same time,the existence of grain boundaries can increase the piezoelectric power output signal as it destroyed the symmetry of MoS₂ crystal.Finally,we fabricate self-powered piezoelectric devices and usedapply in the detection of human movement and mechanics sensors.In the research of epitaxial growth of nonlayered Cd S on monolayer MoS₂ and optoelectrical properties,epitaxial growth of nonlayered Cd S on MoS₂ is achieved using a one-step CVD method for the first time,resulting in vertical heterostructures.It is proposed that the substrate surface energy and reaction kinetics regulation plays an important role in the growth of Cd S.Compared to manual transfer,CVD epitaxial growth of Cd S on the surface of MoS₂ is a powerful approach to fabricating 2D vd W heterostructures with clean interface.Furthermore,this method can the feasability of Cd S/MoS₂ heterostructure-based photodetectors is demonstrated.The photocurrent and photoresponsivity of Cd S/MoS₂ heterostructure-based photo detectors are greatly enhanced compared to their MoS₂ counterparts,and the response speed increases from 10 s to 100 ms which making the presented strategy very attractive for highperformance optelectronic devices.In the research of preparation of high deformation of MoS₂ kirigami structure and the study of mechanical sensor,we develope a method combine traditional paper-cyutting techniques and MoS₂ films to fabricate an ultra-high shape variable mechanical sensor.The results of electrical properties indicate that the complex kirigami structures make the deformation level of flexible MoS₂ electronic devices obtain considerable improvement,increasing from 0.75% to about 15%.Three stages can be identified during the deformation process.The first stage with minor current increase is simply composed of elastic srentching.While the second stage contains both elastic strength and out-of-plane twist distortion and the current swiftly increase at this stage.When the materials are destroyed,the current signal will gradually fall off,which indicates the beginning of the final stage.We use this kirigami MoS₂ electronic device in the detection of robot movement,which shows high sensitivity and good stability.Thus,these devices are attractive candidates for large-deformation wearable and human-machine interface applications.In the research of patternable growth of p-type MoS₂ films and electrical properties,we report a novel approach for one-step and patterned growth of p-type MoS₂ atomic layers doping by WO_x using molybdenum acid peroxide sol–gel as the precursors.A great advantage of this method is its high controllability over sample size and thickness,and easy operability of different kinds of arrays with designed patterns.A small amount of W is introduced during the growth.Tungsten oxide compounds are formed due to the incomplete reduction at the low reaction temperature.Density functional theory calculations（DFT）find that the presence of small tungsten oxide clusters can indeed lead to p-type behavior.Finally,atomically thin p–n junctions are fabricated by stacking the as-prepared p-type MoS₂ with n-type MoS₂ atomic layers.The as-fabricated p–n junction demonstrates significant rectification effect.This thesis has enriched the synthesis methods and electronic a nd optoelectronic properties of MoS₂ nanoflakes,paving a way for MoS₂ nanoflakes applications in high performance electronic devices,optoelectronic devices and strain sensors.