Research On CVD Growth,photoluminscence And Magnetism Of WS₂ And WSe₂ Nanosheets

The two-dimensional TMDCs are novel two-dimensional layered materials discovered after graphene,which have outstanding optical,electrical and catalytic.Due to their tunable bandgaps as well as the atomic thickness,2D TMDCs are resistant to short-channel effect,which are promising building blocks for next-generation nano-devices and optoelectronics devices.Althoug,progresses have been made in the research on physical property and prototype device,there are still many challenges in establishing a complete physical model and realizing the wide applications of 2D TMDCs.In the aspect of physical research,the phtoluminescence of 2D TMDCs is urgently needed further study to discover mechanism how luminescence responds to external factors and to discover the origin of ferromagnetism of 2D TMDCs.Moreover,in the growth of 2D TMDCs,controllable growth methods are desirable to achieve tunable thickness and morphology.We have studies on chemical vapor deposition of two-dimensional TMDCs and their photoluminescence and ferromagnetism properties.We optimize the chemical vapor deposition(CVD)growth parameters of WSe₂,WS₂ nanosheets and study their vapor-solid growth model.In addition,the physical properties of WSe₂ and WS₂nanosheets,such as photoluminescence and ferromagnetism,are also investigated.We have proposed a method to tune excitons in WS₂ monolayers and discover the origin of ferromagnetism in WS₂ and WSe₂ nanosheets.Firstly,we adopt CVD method to grow high quality nanosheets of WSe₂ and WS₂ and study their growth model.The first step,we choose the proper precursors and design the layout of substrate and precursors.The WSe₂ and WS₂ nanosheets have been successfully obtained using CVD method.Impact of the two key growth parameters,pressure and temperature,on the growth of WSe₂ and WS₂ nanosheets have been investigated.The shape controllable growth of WS₂ nanosheet and hybrid three-dimensional nanostructures have been also explored.Moreover,we also study the growth model of vapor-solid deposition of two-dimensional WSe₂ and WS₂.Our studies on the growth of WSe₂,WS₂nanostructures and their growth model provide new insight for the growth of complex nanostructures based on TMDCs nanosheet and also provide theoretical model for vapor-solid deposition of TMDCs nanosheet.Secondly,the photoluminescence measurement is adopted to investigate the CVD-grown WSe₂ and WS₂ nanosheets.We discover the high quantum effiency,I_LUM/I_Raman,in WSe₂ and WS₂ monolayers which is exceed 100%.The different excitons states in CVD-grown WSe₂ and WS₂ nanosheets and the thickness effect impacts on their photoluminescence are also discussed.As the exciton states play a dominant role in photoluminescence of two-dimensional TMDCs,we explore the method to tune the excitons in WS₂ monolayer.By the passivation of large band gap semiconductor WO₃,we obtain the hybrid WO₃/WS₂ nanoparticle/nanosheet structure and realize the control of exciton states in WS₂ monolayer.This passivation method is very useful,which can not only tune the luminescence of TMDCs nanosheets,but also can tune the magnetic behavior of TMDCs nanosheets.Finally,we study the ferromagnetism of WSe₂ and WS₂ nanosheet with transition temperature higher than room temperature,which is distinctly different from their diamagnetic bulk form.We first adopt the density functional theory calculations on WSe₂ clusters and nanoribbons,and the results show the possibility of ferromagnetic terminations existing in WSe₂ clusters and nanoribbons.The VSM method were adopted to investigate the ferromagnetic behavior of WSe ₂ and WS₂nanosheets at room temperature.The MFM method was also used to tell the ferromagnetic region in single WSe₂ and WS₂ nanosheet to give a clearly microscope information on ferromagnetic WSe₂ and WS₂ nanosheets.Combining the results of theoretical calculations and the results of macroscopic and microscopic magnetic measurements,we confirm the origin of ferromagnetic WSe₂ and WS₂nanosheets is from the unsaturated edges of nanosheets.Our results show the two-dimensional TMDCs nanosheets have great potential in spintronic application and the two-dimensional TMDCs are great flatforms for the study the magnetism in two-dimensional materials.