Study On The Preparation And Transfer Method Of Thin-layer Niobium Disulfide And Large-area Graphene

Since its discovery in 2004,graphene has become a hot topic in the field of research of condensed matter physics due to its superior physical properities.However,the lack of energy band gap hiners their applications in electronics.Comparatively,the two-dimensional transition metal dichalcogenides（TMDCs）with an energy band gap attracts increasing attention.TMDCs have a stacked layer structure similar to graphene,which contain covalent bonds in layers and van der Waals interaction between layers.When their number of layers is reduced to a few or a single atomic layer,TMDCs exhibit rich physical properties,becoming another promising two-dimensional materials.For some new-type TMDCs,to efficiently fabricate and transfer high-quality thin layer samples free from surface contaminants is still a challenge.Using niobium disulfide（NbS₂）as a model material,this thesis carries out a series of research studies on preparation and precise point transfer of ultra-thin flakes.In this thesis,we prepare NbS₂ nanosheets by micromechanical exfoliation method in anhydrous and oxygen-free environment,develop a dry transfer technique with assistance of PDMS/PVA,which allows non-destructive transfer the NbS₂ nanosheets between two silicon substrates without contamination.At the same time,the influence of using different substrates on the thickness of the prepared sheet is studied,and a 30 nm alumina/silicon substrate is finally selected for exfoliation of ultra-thin（as low as two atomic layers）NbS₂nanosheets.Finally,we fabricate electronic devices with the prepared NbS₂ nanosheets,and obtain the dependence of superconducting transition temperature on the thickness of the NbS₂ nanosheets by using four-probe method.When the thickness of the sheet is below 7.0nm（～10 layers）,the superconducting transition temperature decreases as the thickness decreases,which is important for the mechanism study of low-dimensional superconductivity.In addition,we have improved the traditional PMMA transfer technique for large-area,high-quality graphene films prepared by chemical vapor deposition.On one hand,the organic small molecule MMA is used to replace the macromolecule PMMA as the protective layer of the graphene film.On the other hand,the graphene film grown on the back of the copper substrate is removed by using oxygen plasma or deionized water on the basis of original wet transfer process.Subsequently,we evaluate the quality of the transferred film by characterizing their surface morphology and structure.Subsequently,thin-film devices are fabricated using the prepared film to study the electrical properties.The measured mobility reached 3000 cm²V^-1s^-1,which is twice higher than the mobility of the film obtained by original wet transfer process.