| Graphene has attracted extensive attention due to its unique electronic structure,excellent physical properties as well as potential applications in electronics,optics,energy,flexible display and sensors,and so on.Epitaxial graphene on transition metal substrates has advantages of large-scale,high-quality,and controllable number of layers.Based on such system,the developed intercalation technique can in-situ place largescale graphene onto other substrates,such as metals,semiconductors or insulators,without degrading its quality.Such technique offers new opportunities for scientific research and potential applications of graphene,and provides a new way to build graphene-based,stable 2D van der Waals heterostructures.The thesis mainly focuses on the intercalation of semiconductors(Si,Ge)and their oxides at the interfaces between single-layer(SLG)or bilayer(BLG)graphene and metal substrates.The structure and mechanism of oxides intercalation are systematically studied.Using intercalation technique,graphene/semiconductor/metal,graphene/2D oxides/metal heterostructures are successfully constructed.The thesis contains three parts as following:In the first part,silicon and its oxide intercalation at the interface between SLG and Ru(0001)were systematically studied.It is found that two different structures,silicene and RuSi alloy,form at the interface with the increase of the intercalated silicon atoms.The silicon oxide layer is obtained by oxidation of the intercalated silicon through a subsequent intercalation of oxygen.After oxidation of the silicene,a 2D silicon oxide layer in amorphous,polycrystalline or single-crystalline forms can be obtained.Vertical transport characteristics show that the intercalated silicon oxide is a good 2D insulator.But the thickness(< 2 nm)is not enough to insulate graphene from the metal substrate.The electrons transport through silicon oxide barrier by quantum tunneling.Oxidation of the RuSi film results in a thicker oxide layer(> 10 nm),a mixture of ruthenium oxides and silicon oxides,which can insulate graphene from Ru.In the second part,intercalation of germanium and its oxide at the interface between SLG and Ir(111)was discussed.Firstly,the single-crystalline SLG was epitaxially grown on Ir(111).Then,germanium and germanium oxide were intercalated below graphene.The interfacial germanium forms a crystalline 2D structure with a 2 × 2 superstructure with respect to Ir(111),while the germanium oxide is amorphous.Both intercalated germanium and germanium oxide films effectively decouple the graphene from metal substrate and induce a p-type doping in graphene.Vertical transport measurements demonstrate that the barrier provided by germanium oxide differs from that provided by silicon oxide.The electrons transport through germanium oxide by thermally activation at the high-temperature regime and by quantum tunneling in the low temperature regime.In the third part,silicon intercalation of AB-stacking,single-crystalline BLG on Ru(0001)was studied.By silicon intercalation technique,van der Waals heterostructures of BLG/silicene were constructed successfully.Electronic structure characterizations show that a large band gap of about 0.22 eV is opened in the BLG.Together with DFT calculations,it is clear that the charge rearrangement at the interface of BLG/silicene and the strain existing in BLG itself contribute to the band gap opening.This part of work is of great significance to the future applications of BLG/silicene heterostructure in devices as well as the study of graphene band gap engineering.In addition,intercalation of heteroatoms can effectively decouple the BLG from Ru,making BLG nearly freestanding.Assisted by oxygen intercalation,centimeter-scale,AB-stacked,single-crystalline BLG was successfully transferred from Ru to insulating substrates.The results pave a way to the further scalable fabrication of functional devices based on large-scale and high-quality BLG. |
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