Study On Defect Control And Optical Properties Of Transition Metal Dichalcogenides By Organic Superacid

Transition metal dichalcogenides(TMDs)represented by molybdenum disulfide(MoS₂)are the most widely studied layered two-dimensional semiconductor materials systems.They have emerged as potential candidates for next-generation optoelectronic devices due to atomic-level thickness,good flexibility,extremely high optical transparency and unique layer-dependent band gap characteristics in the visible and near-infrared spectral regions.Different kinds of TMDs can be stacked to assemble van der Waals Heterostructures under the weak van der Waals force and band difference may be as a driving force to facilitate charge transfer at the interface behavior which control the fluorescence of TMDs.In addition,compared with mechanical exfoliation method,chemical vapor deposition(CVD)method can realize the synthesis of large-area and large-size monolayer TMDs(1L-TMDs),which makes it possible for highly integrated optoelectronic devices based on those materials.However,the main challenge of the CVD-grown 1L-TMDs for practical applications is their ultra-low photoluminescence quantum yield(PL QY,0.01%?6%),which could be primarily attributed to the Structural defects introduced to thermal stress and discontinuous supply of precursors during the growth process.Therefore,it is a significant research to effectively improve the luminescence efficiency of 1L-TMDs.Based on the above analysis,we have carried out a series of studies on controlling the luminescence efficiency of 1L-TMDs and the specific work and main contents are as follows:(1)Monolayer TMDs films were prepared by chemical vapor deposition.The morphology and optical properties of the samples were characterized by optical microscopy(OM),scanning electron microscopy(SEM),atomic force microscopy(AFM),raman spectroscopy and photoluminescence spectroscopy.In addition,the prepared samples were transferred to fabricate a MoS₂/MoSe₂ heterostructures and the effect of the charge transfer behavior of the two interfaces on the fluorescence of 1L-MoS₂ was studied.(2)In this work,we demonstrate an organic superacid——trifluoromethanesulfonic(CF₃SO₃H,TFMS)solution to chemically modify the surface of 1L-MoS₂can significantly improve the PL emission of CVD-grown 1L-MoS₂ with a two orders of magnitude enhancement.The specific method is to spin the trifluoromethanesulfonic acid solution on the surface of MoS₂ and control the fluorescence intensity of MoS₂ by controlling the volume of trifluoromethanesulfonic acid solution.raman spectroscopy,atomic force microscopy(AFM),scanning transmission electron microscopy(STEM),X-ray photoelectron spectroscopy(XPS),ultraviolet photoelectron spectroscopy(UPS),fluorescence lifetime characterization results show that there are two main reasons for the enhancement of fluorescence intensity of 1L-MoS₂ samples treated by TFMS:p-type doping and defect repairation.(3)In order to further explore these two effects,scanning transmission electron microscopy(STEM)and density functional theory(DFT)calculations were performed to confirme that the S atom on the TFMS molecule can effectively repair the sulfur vacancy in 1L-MoS₂ on the atomic scale;Furthermore,electrons are transferred from1L-MoS₂ to the TFMS molecule due to the high electronegativity of the O and F atoms of the TFMS molecule to control its optical properties.Similar fluorescence enhancement was also obtained in TFMS-treated monolayer WS₂,verifying the generality of this new superacid treatment strategy for 1L-TMDs and providing an effective strategy to improve the quantum efficiency of TMDs.