Near-field Characterization Of Thin-layer Transition Metal Dichalcogenides:Nanometer Defects And Interlayer Interaction

Two-dimensional transition metal dichalcogenides(TMDC)have been widely used in the areas of photo-electronics and catalysis,due to their excellent optical,electronic,and magnetic properties.These unique properties strongly depend on the special structure of TMDC.However,the properties of TMDC are tuned by defects as well as the real application environment.There are two core problems in TMDC research.The first is that most preparation methods cannot balance the sample quality and size.To address to this problem,new fabrication principles need to be developed to precisely and efficiently control the fabrication process,achieving on-demand preparation of TMDC samples.The second is that defects are so small that their signals are easily vanished in the surrounding perfect lattice background,and most high-resolution characterization techniques cannot obtain the defect information in a room-temperature and atmospheric environment.To address this problem,it is necessary to develop a technique that can characterize various properties of defects with multi-modes,at nanometer scale,in the real application environment,to establish the structureproperties relationship.This can provide guidance for future applications of TMDC materials.In this thesis,we mainly focused on developing new methods for the fabrication of high-quality/large-size TMDC and revealing the special electronic and lattice properties of defects.The main results and conclusions are listed as follows:1.It is still challenging to fabricate high-quality and large-size TMDC monolayers in an efficient,easy and controllable way.We developed a new method based on photoinduced liquid exfoliation to fabricate high-quality and large-size monolayers under a very mild light irradiance,in pure water,and on conductive substrates.From in-situ dynamic observation of the exfoliation process,we found that the exfoliation started from the defect sites,and continued downwards and inwards.The thickness of the peeled TMDC was 1-3 layers per time.The exfoliation could automatically terminate when the multilayer was exfoliated to a monolayer on the substrate.The quality of the as-fabricated TMDC monolayer is comparable to that of the mechanical exfoliated one.We further studied the exfoliation mechanism by introducing electrons and holes scavengers.It was found that the photo-induced holes of multi-layers oxidized the upper layer materials resulting in the exfoliation while the photo-induced electrons flew to the substrate leading to continuance of the reaction under the light irradiation.The excitons in the monolayer are so stable that the holes cannot participate in the reaction,resulting in the final single layer,which can be stably retained on the substrate.We realized the efficient and precise rate control by tuning the substrate potentials.Based on above,we obtained a high exfoliation rate that is 108 time faster than that of the literature and a sub-millimeter scale large-size monolayer.We utilized the laser scan to selectively exfoliate materials to fabricate patterned TMDC monolayers.2.Defects significantly influence the electronic,optical and chemical properties of materials.We utilized the tip-enhanced photoluminescence(TEPL)spectroscopy with a high spatial resolution to obtain the PL spectra of defects(wrinkles and edges)in thinlayer TMDC under normal temperature and pressure conditions.We systematically analyzed the changes of band gap and electron density induced by intrinsic defects.We discovered that the wrinkle had a narrow band gap adjusted by the local strain,which promotes the flow of electrons from the basal plane to defects leading to an increase of electron density.We obtained the nano-TEPL image of the monolayer MoS2 wrinkles and found the heterogeneity in electronic band structure along the individual wrinkle due to the different local strain.Edges also had an increased electron density also because of the presence of local strain,but their dangling bonds are susceptible to oxygen doping at the same time which ultimately results in a decreased electron density.From TEPL images of mono-and bi-layer MoS2 edges,we found that the monolayer was more effective in modifying the electronic properties than the bilayer.We also observed that the maximum point of electron density shifted to the basal plane which indicates the strong impact of oxygen doping.We further compared the different influence of monolayer WSe2 and MoS2 edges on the change of PL spectra induced by the same type of the defect.3.The special properties of electrons,phonons and electron-phonon coupling of defects vest its unique optical properties.We explored the defect properties by tip-enhanced Raman spectroscopy(TERS)technique.For the first time,we discovered a new peak at～302 cm-1 that is related to defects and assigned this peak as a combination mode of E++S,which is sensitive to local strain.The new peak can be a nano-strain sensor to characterize the local strain.Combining E++S and A1g modes,we can quantitatively measure the influence range of in-plane lattice and electron density of the same defect.We found the monolayer WSe2 edge would induce a～1.5 nm structure reconstruction and a 13.9 nm electron density redistribution around the defect.This result can rationally guide the defect engineering.4.Heterostructures(HS)lead to a variety of novel physical phenomena due to their special interlayer interaction.Combination of the interlayer interaction with the plasmon nanocavity can realize the stronger light and matter interaction.We performed the near-field optical study on 1L WSe2/hBN heterostructure by TERS,and for the first time,observed an unusual near-field enhancement effect due to the hBN encapsulation.We revealed that the near-field enhancement on HS was different from that of classical electromagnetic field enhancement.The enhancement on HS does not decrease with the increase of hBN thickness and TERS signal is always stronger than that of direct detection of WSe2.We performed a series of control experiments by comparing the enhancement with/without hBN and different HS,and discovered that the unusual nearfield enhancement can only occur on 1L WSe2/hBN HS.We found that the unusual near-field enhancement is sensitive to the energy of nanocavity by changing the substrate and ratio of tip apexes which tunes the energy of nanocavity.This result proved this phenomenon comes from a coupling of near-field cavity and interlayer interaction.