Preparation And Ultrafast Transient Spectroscopic Studies Of Substrates Modulated Monolayer MoS₂

Two dimensional transition metal sulfides（2D-TMDs）have great potential applications for electronic devices,photocatalysis,energy storage,and other fields due to their excellent physical and chemical properties.Chemical vapor transport（CVT）is a commonly used technique for preparing high-quality and large-sized single crystals.By controlling the thermodynamic and kinetic conditions during the growth,high-quality single-layer Mo S₂ was grown with controllable morphologies.Furthermore,controllable preparation was achieved on various substrates.Then,the modulation effects of different substrates were explored on the as-grown monolayers.Finally,the carrier dynamics were further studied through ultrafast pump-probe spectroscopy.The main work and innovations of this thesis are as follows:（1）In this thesis,samples were prepared by CVT method.Firstly,by controlling the thermodynamic and kinetics conditions,single-layer molybdenum sulfide was successfully prepared on sapphire substrates,with the size from independent triangular individuals of tens of micrometers to millimeters-scale single-layer films.Then,we have successfully achieved controllable growth of single-layer Mo S₂ on various substrates.（2）Morphologies and coverages of the grown monolayer molybdenum sulfide were characterized using optical microscopy and scanning electron microscopy（SEM）.Their chemical compositions were identified by using X-ray energy dispersive spectroscopy（EDS）.The uniformity and crystal quality of monolayer molybdenum sulfide synthesized by CVT method were checked through Raman spectroscopy.The modulation effect of substrates on the properties of monolayer Mo S₂ was revealed using various methods,such as X-ray photoelectron spectroscopy（XPS）,UV-Vis absorption spectra,photoluminescence spectroscopy,ultrafast pump-probe spectroscopy,etc.These results confirmed that several substrates have modulation effects on the epitaxial growth of monolayer molybdenum sulfide on their surfaces,and the modulation effects also vary due to different substrate properties.Through the analysis of carrier dynamics using ultrafast transient spectroscopy,it was found that the modulation effect of semiconductive functional substrate of titanium dioxide is the most prominent,which has the most prominent impact on exciton lifetimes under the same conditions.It was also found that its modulation effect can be well explained by the type-II band alignment.Due to different substrate polarities,molybdenum sulfide growing on insulating substrates（sapphire,mica and silicon dioxide）are different in the crystallization quality,which will also have varying degrees of impact on their carrier lifetime.This provides new ideas for the modification of 2D-TMDs and functional materials,as well as new insights for the modification of TMDs devices.