Preparation And Regulation Of Fluorescence On Molybdenum Disulfide Quantum Dots And Their Heterojunctions

As one of the important members of transition metal dichalcogenides?TMDCs?,Graphene-like molybdenum disulfide?MoS₂?has attracted wide attention due to its unique properties.Compared with MoS₂ nanosheets,MoS₂ quantum dots?QDs?with a size of less than 20 nm have novel properties and application values due to quantum confinement effects and edge effects.So far,MoS₂ QDs have been used in hydrogen evolution reactions?HER?,photocatalysis,bioimaging,optoelectronic devices and so on.However,the reported preparation methods for MoS₂ QDs mostly involves organic solvents,which are toxic,time consuming,and of cumbersome steps.Therefore,it is of great meaning to explore a convenient and novel method to synthesize MoS₂ QDs.In addition,the combination of 0D and 2D materials into a nano heterojunction structure can effectively promote the separation of photo-excited electron-hole pairs,which may be of great benefit to improve the photo response and detection rate of photoelectric devices.In this thesis,we explore a new and convenient method for preparing MoS₂ quantum dots.MoS₂ QDs are synthesized on few-layer MoS₂ by a combination of mild hydrogen plasma treatment and post-annealing,and the Photoluminescence?PL?regulation of MoS₂ is realized due to the quantum confinement effect of quantum dots.In addition,MoS₂ quantum dots and InSe heterojunction structures are also prepared.The charge transfer characteristics and PL regulation effects between these two materials are systematically studied.The main research work and results are summarized as follows:1.Few-layer MoS₂ samples are treated by mild hydrogen plasma,leading to the formation of MoS₂ nanodots on the surface.The effects of both plasma pressure and hydrogen gas flow on the size of nanodots are systematically studied.The size of MoS₂ nanodots is the smallest?40-50 nm?under the optimum process conditions.After 2 hours'thermal annealing,the size of MoS₂ nanodots can be further reduced to less than 10 nm,and MoS₂ quantum dots are finally achieved.After hydrogen plasma treatment,two new PL peaks of MoS₂ at around574 nm and 582 nm emerge due to the quantum confinement effect of the nanodots.In addition,thermal annealing can also repair the defects?S vacancies?of MoS₂,reflecting by the intensity enhancement of both Raman peaks and new PL peaks.Therefore,we can conclude that we have realized the regulation of PL properties of few-layer MoS₂.Finally,the electrochemical hydrogen evolution test of the prepared MoS₂ quantum dots are carried out to systematically investigate the effect of the size of the MoS₂ quantum dots on the hydrogen evolution performance,the smallest size corresponds to the lowest starting potential?176mV?.2.The solution-based MoS₂ quantum dots are synthesized by liquid phase ultrasonic method with ratio mixing of alcohol and deionized water.The effects of ultrasonic time and centrifugation rate on the size of quantum dots are systematically investigated.After 10 hours'ultrasonication,high-speed centrifugation and filtration,the smallest size?of about 20 nm?of MoS₂ quantum dots can be obtained.The MoS₂ quantum dots and InSe perpendicular heterojunctions are prepared by spin coating method,and the PL of InSe is quenched,indicating that the coupling between the two can promote the charge transfer effect.Aiming at different PL blue shift of different layers of InSe,the effect of MoS₂ quantum dots on the PL regulation of different layers of InSe is systematically studied.When the thickness of InSe is less than 8 nm,the blue shift of PL peak will occur at the same time of PL quenching.When the thickness of InSe is larger than 8 nm,the PL peak is not blue-shifted.This may be attributed to the fact that the band gap is affected by strong interlayer interaction when the thickness of InSe is high,and the PL regulation of InSe by MoS₂ quantum dots is not obvious.