Preparation And Spectroscopic Studies Of Rare Earth Doped Monolayer MoS₂ And Its Heterojunctions

Two-dimensional transition metal dichalcogenides(2D-TMDCs)have attracted great attention due to their unique physical properties,such as atomically thin thickness,tunable optical band gap,strong exciton binding energies,and high carrier mobility.It is promising to break through the bottleneck of silicon-based material technologies and become potential materials for future applications.In 2D-TMDCs,neighboring layers are only connected by van der Waals forces.This unique stacking can be employed to design new 2D-heterostructures to fit for the requirement by stacking various layers without considering lattice mismatching.These stacked layered materials exhibit novel electronic,optical,and mechanical properties,which provides a new way to study their physical properties and develop novel optoelectronic devices.During the past years,most of the researches on two-dimensional heterojunctions focused on the preparation by stacking or direct growth,as well as exploring their novel properties.Nevertheless,few studies were reported on the luminescence of rare earth doped 2D-layered heterojunctions,neither did on the exciton dynamics of rare earth ion doped monolayer TMDCs and their heterojunctions.Therefore,in order to further understand the influence of rare earth elements with rich 4f energy levels on 2D-monolayers and their heterostructures,this work mainly focuses on how to prepare high-quality rare earth doped monolayer molybdenum sulfide and its heterostructures,as well as study their luminescence properties and exciton dynamics by ultrafast transient spectroscopy.The main contents of the work are as the following:(1)Various rare earth elements doped molybdenum sulfide monolayers were grown by a liquid-phase precursor-assisted chemical vapor deposition(CVD)method.Using ammonium molybdate and sodium cholate as mother liquor,and rare earth salt solution and ammonium tetramolybdate as dopants,the growth of monolayer molybdenum sulfide was regulated by CVD method,providing essential monolayers for the construction of rare earth doped monolayer molybdenum sulfide heterojunctions.(2)The doped single-layer molybdenum sulfide with high quality crystallinity was transferred by liquid-assisted mechanically stripping method to ensure that the transferred single-layer molybdenum sulfide nanosheet still keeps the original physical and chemical properties,without damage and wrinkles.Then,two-layer molybdenum sulfide heterojunctions with different rare earth doped ions were successfully built by using the 2D-transfer platform.(3)After obtaining the target heterojunction sample,the fundamental characterization,such as Raman spectra and photoluminescence spectra,were characterized,and the energy transfer and exciton dynamics of different rare earth doped monolayer molybdenum sulfide and their stacked heterojunctions were investigated by femtosecond laser transient reflection spectroscopy.This thesis provides a method for growing high-quality rare earth doped singlelayer molybdenum sulfide.The heterojunctions composed of rare earth doped molybdenum sulfide monolayers were successfully constructed by using a safe and non-toxic transfer strategy.The optical properties were preliminarily explored by means of Raman spectroscopy,photoluminescence spectroscopy,and ultrafast transient reflection spectroscopy.The exciton dynamics of heterojunctions were further revealed.This work may do good to the studies and development of 2D-optical and optoelectronic devices based on rear earth doped TMDCs.