Tuning The Optoelectronic Properties Of Molybdenum Disulphide And Related Device Applications

The discovery of graphene has inspired tremendous studies on 2D materials over the past decade.These materials display unique layered architectures and band structures,which result in remarkable electrical,optical,thermal and mechanical properties.On the basis of the 2D materials,a variety of optoelectronic devices with novel concepts,principles and functions are proposed,giving rise to new possibilities and innovations in the field of nanoelectronics and optoelectronics.Molybdenum disulfide?MoS₂?,belonging to the family of transition metal dichalcogenides,is one of the most famous 2D materials.Owing to its sizeable bandgap,heavy carrier effective mass,strong light-matter interaction and optical nonlinearity,MoS₂ shows promising potential in the application directions ranging from logic circuit and photodetection to optical sensing.Moreover,by tuning the optoelectronic properties of MoS₂ via physical and chemical strategies elaborately,it could not only boost the functions of devices,but also has important scientific significance for designing and realizing novel optoelectronic devices,providing insight into the novel physical mechanisms based on MoS₂.In this thesis,we aimed to explore the novel physical phenomena and mechanisms in terms of carrier transport,interface characteristics and band structures of MoS₂ resulting from the tuning effect of gate electrostatic field and chemical doping.On the basis of the research of the above physical process,several electrical and optoelectronic devices based on MoS₂ were designed and fabricated.The mechanisms of the devices were further revealed and the performances were improved.Moreover,the novel photoluminescence property of 0D MoS₂ nanostructures as well as its application was explored.The thesis began with the introduction of the basic characteristics of MoS₂ as well as the intrinsic factors and tuning methods that affect its optoelectronic properties.The research status and development tendency of MoS₂ based field-effect transistor in the field of nanoelectronics and photodetection was then reviewed.After the introduction,we showed the optoelectronic properties of MoS₂ and tuning mechanisms both in theory and experiment.Then we demonstrated several functional devices,including electrical rectifier and photodetectors.Finally,we showed our research work on the fabrication and photoluminescence of 0D MoS₂nanostructures.The major achievements of the thesis are listed as follows,1.The interface characteristics of MoS₂ and metals were studied and a novel rectifying device was constructed.The electrical contact theory model was built considering the Fermi level pinning effect.Besides,the electrical contact behavior and interface barrier between MoS₂ and different metals were characterized.Then a Schottky diode with novel symmetric electrodes structure was proposed.The current rectifying ratio reached up to 2000 at a certain gate bias and the ideal factor of the diode was 1.5.The electron injection and transport behavior at different gate biases were analyzed based on the theoretical model of band structure.A dynamic rectifying circuit was further made consisting of the Schottky diode with the operating frequency at 100 Hz.2.The broad-band photoresponse properties and working mechanisms of MoS₂ phototransistor were studied.Negative infrared photoresponse phenomena were first observed and the response wavelength reached 1550 nm with the corresponding photon energy smaller than the bandgap of MoS₂.By studying the realtionship between the photocurrent and the incident optical power at different gate biases,the ultrasensitive photoresponse mechanism was revealed and further verified based on the photoresponse decay process.The device exhibited ultra-sensitive photoresponse with the responsivity reaching as high as 10⁵ A/W and the effective optical power on the order of picowatts upon the visible-light illumination.In addition,the device with asymmetric electrode showed reduced dark current and improved signal-to-noise ratio for the visible light detection.3.A hybrid photodetector structure consisting of MoS₂ and CdSe quantum dot was proposed and fabricated.Then the effect of different functional groups of CdSe quantum dots?QDs?ligand on the electrical properties of the transistors were studied.Besides,the layer-by-layer assembly method was explored to replace the ligand of the QDs.The interaction mechanisms of MoS₂ and CdSe QDs with different ligands were studied in terms of the in-situ photoluminescence intensity mapping and the photoresponse speed measurement.Taking advantage of the p-doping effect and efficient charge transfer arose from the short-ligand QDs,the detectivity of the hybrid photodetector reached 3×10¹³ Jones and the response speed was improved by two orders compared with the pristine MoS₂ device.4.The 0D MoS₂ nanostructures with strong quantum confinement effect in three dimensions were fabricated.The change of bandgap resulting from the reduced lateral size of MoS₂ and quantum confinement effect was quantitatively analyzed.Aiming at the problem of the low quantum yield,a sonication combined with ion intercalation method in the alkaline environment was proposed,leading to an increase of the quantum efficiency by five times.The prepared nanostructures showed unique excitation-dependent photoluminescence?PL?characteristic with the multicolor PL emission almost covering the visible spectrum.Finally,the 0D MoS₂nanostructures were employed to image and label different kinds of biologic cells and bacteria.