Surface Functionalization And Physical Property Modification Of Molybdenum Disulfide

In the past decade,two-dimensional transition metal chalcogenides have attracted tremendous attention because of its unique physical and chemical properties and promising application prospects.As one of the outstanding representatives of transition metal chalcogenides family,two-dimensional MoS₂ has electronic band structures related to the number of layers,excellent chemical stability and good carrier mobility,which make it a very promising candidate for channel materials for new generation integrated circuit components,optoelectronic devices,photovoltaic devices,sensors and so on.However,it was soon found that there were sulfur vacancy（SV）defects as high as 10¹³ cm^-2 in MoS₂crystal.These defects occurred in the process of crystal growth and device preparation,which were difficult to avoid.These defects become the scattering centers during carrier transport in the device,which directly leads to mobility far lower than theoretical predictions in MoS₂ devices.Researchers have tried various methods to reduce SV and try their best to overcome the above disadvantages.It is of great scientific significance to study repairing surface SV of MoS₂using small organic molecules containing sulfhydryl groups,which reduces Coulomb Impurities and lower carrier transport scattering rate,so as to improve the electrical performance of MoS₂ devices and to reveal the related physical mechanism.In addition,through the functionalization of sulfur vacancy surface,the application range of MoS₂ devices can be extended to broader fields such as sensing and biomedicine,which has important practical significance.This thesis studies the modification of electrical and optical properties of MoS₂ by surface surface functionalization.Firstly,a simple method of functionalizing MoS₂ with lipoic acid is introduced.The surface functionalization enhances the mobility of MoS₂ FET,which shows a strong MoS₂thickness dependence.The physical mechanism of this phenomenon is discussed.On this basis,a scalable surface modification method was demonstrated to generate functionalized MoS₂ flakes by the step-by-step covalent assembly of lipoic acid（LA）and fluorescein isothiocyanate（FITC）molecules.We expect to build a chemical platform with this method,which can conveniently and controllably adjust the sensing and photoelectric characteristics of MoS₂ devices.The major research works and scientific findings of the thesis are listed as follows:1.High quality 2H phase MoS₂ single crystal was grown by chemical vapor transport method.MoS₂ thin layer samples were prepared by mechanical stripping.The number of layers of MoS₂ flakes was accurately and quickly determined by atomic force microscopy,Raman spectroscopy and optical methods.Two dimensional MoS₂ back gate FET devices with different thicknesses were successfully fabricated by"micro nano processing method".The variation of mobility with the number of layers is studied.The test shows the average mobility of FET devices with thickness from single layer to about 15 layers increases from7.8 cm²V^-1s^-1 to 58.4 cm²V^-1s^-1,which can fully meet the requirements of subsequent research and product development.2.Lipoic acid was used to functionalize MoS₂ nanosheets to repair the sulfur vacancy on the sample surface.It was proved that LA was successfully chemically connected to the surface of MoS₂ by AFM,XPS and Raman.We observed that the modification of LA can significantly improve the field effect mobility of FET devices and show an obvious thickness dependence.The mobility enhancement of single-layer MoS₂ devices is the most obvious,close to 100%.With the increase of the number of layers,the improvement effect decreases gradually.We use the Coulomb Impurity scattering theory model to analyze and explain its physical mechanism.3.Based on the above experiments,the MoS₂-LA was further functionalized with FITC molecule containing-NH2 group.It was confirmed that FITC can realize chemical connection on the surface of MoS₂ by condensation reaction between its-NH₂ group and-COOH group of LA molecule.The linked FITC molecule can inject 10¹² cm^-2 electrons into MoS₂ channel.The molecular fluorescence of FITC also significantly changed the luminescence characteristics of MoS₂.In addition,we also applied other organic molecules containing-NH₂ group such as phenylethylamine to verify the scheme,which proved that the scheme has certain universal applicability.