Surface Charge Transfer Doping Of Two-dimensional Transition Metal Dichalcogenides And Their Performance On Optoelectronic Properties

Transition metal dichalcogenides（TMDCs）have attracted much attention in recent years as a compound with great potential for development.When changing from three-dimensional（3D）bulk phase materials to two-dimensional（2D）planar materials,their energy band structure also changes,making them possess excellent optoelectronic properties.The field effect devices constructed from 2D-TMDCs represented by Mo S₂and WS₂ have physical properties such as high carrier mobility,large switching ratio and strong spin-orbit coupling effect,which have great potential applications in logic circuits,photodetectors,light-emitting diodes and other optoelectronic devices.Further modulation of the energy band structure of these materials has the potential to realize the construction of a variety of homogeneous or heterogeneous junction complex logic devices.In the research field of modulating the band gap of semiconductor materials,doping is widely used as an effective means to regulate the electrical properties of their devices.Due to the challenges of conventional doping methods such as complex processes,loss of material intrinsic properties and doping instability,surface charge transfer doping（SCTD）has been introduced as a new doping method for doping 2D-TMDCs in recent years,and its advantages such as fast,reversible and non-destructive have been favored by researchers.Although SCTD has a large number of advantages,how to find efficient and controllable dopants is still a key scientific problem in this research.To solve this problem,we designed molecules with aggregation-induced emission（AIE）effect as dopants,which can provide a rich molecular backbone and can carry a variety of functional groups to achieve controllable charge transfer.In this thesis,high quality monolayer Mo S₂ was prepared by chemical vapor deposition（CVD）,then tetraphenylethylene（TPE）derivatives with AIE effect were synthesized,and controlled doping of monolayer Mo S₂ was achieved by means of SCTD.Finally,the doping mechanism was verified by means of theoretical calculations such as density generalization theory.The doping of monolayers of Mo Se₂ and WS₂ was achieved by the same means.The details are as follows.（1）By designing and synthesizing TPE derivative molecules with AIE effect,TPE-4NO₂ molecule with strong electron-absorbing group-NO₂ and TPE-4OCH₃ molecule with strong electron-pushing group-OCH₃,respectively.The monolayer of Mo S₂prepared by CVD was realized by solution immersion method to achieve SCTD,TPE-4NO₂ as doped could lead to a great enhancement of the peak intensity of the PL spectra of the monolayer Mo S₂ with a blue shift of the peak position,the peak positions of the X-ray photoelectron spectra（XPS）are shift to lower binding energy.The PL spectra of TPE-4OCH₃ doped monolayer Mo S₂ showed a burst in peak intensity and a red shift in peak position,the XPS peak positions are shift to higher binding energy.The doping mechanism was further characterized and analyzed by normalized PL spectrum,energy band structure,density of states（DOS）and differential charge density diagrams.Due to the strong electron withdrawing effect of-NO₂,the charge was transferred from Mo S₂to TPE-4NO₂ molecules after doping,which made the Mo S₂ surface the hole density increases,the band gap become wider,and p-type doping is realized;after TPE-4OCH₃molecule is doped with Mo S₂,thanks to the strong electron donating effect of-OCH₃,the charge is injected from the TPE-4OCH₃ molecule to Mo S₂,which made the surface charge density of Mo S₂ increases,the band gap become narrower,n-type doping is achieved.（2）High-quality monolayer Mo Se₂ and monolayer WS₂ were prepared by CVD method,and SCTD was also realized by TPE-4NO₂ and TPE-4OCH₃.The doping trend of monolayer Mo Se₂ is similar to that of Mo S₂,which is due to the similar band structure of monolayer Mo Se₂ and monolayer Mo S₂,which makes the charge transfer trend similar.After doping the monolayer WS₂,the PL spectra of the monolayer WS₂ is quenched to different degrees,and the XPS peak position also moves to the lower binding energy.This result indicates that the energy band structure of the monolayer WS₂ itself makes the charge only transfer from AIE molecules to it results in the accumulation of surface charges on the monolayer WS₂,showing n-type doping characteristics.