Modulation Of Structures And Properties Of Two-dimensional Materials During Molecular Beam Epitaxial Growth

Since graphene was successfully produced in 2004,the unique properties and applications of it have been receiving extensive attention from researchers.Afterwards,researchers fabricate and characterise the special properties of many 2D materials that rely on van der Waals forces between layers by the help of mechanical exfoliation.Based on theoretical predictions and experiment,hundreds of two-dimensional materials have been created and deeply studied.Although 2D materials show unique properties and industrial application prospectively,the high-quality manufacturing of it has always been one of the main problems faced by the practical applications of it in the future.For example:two-dimensional topological insulator 1T?-WSe₂ it has the topological energy gap of 130 me V,but the unstable phase structure make it to easily undergo phase changes under external disturbances;Although the epitaxial graphene produced has the advantages of fewer defects and high quality,the electrical properties of the silicon carbide substrate still not up to perfect ideal standards due to the strong interface force of the silicon carbide substrate.Therefore,exploring the modulation of structural properties of various new two-dimensional materials during the sample growth is an urgent problem to be solved.In this thesis,we have combined experimental methods such as molecular beam epitaxy(MBE),scanning tunneling microscope(STM)and angle-resolved photoelectron spectroscopy(ARPES),and conducted in-depth research on the two examples cited above,and obtained main results are summarized as follows:1.We successfully grow metastable monolayer 1T?-WSe₂ film on a bilayer graphene substrate use molecular beam epitaxy.At lower substrate temperature(250°C),we can get mixed phase of 1T?and 2H.At a relatively higher substrate temperature,we can obtain a pure phase monolayer 2H-WSe₂.At the same time,the high temperature annealing process can also transform the 1T?phase obtained in the low temperature growth into the 2H phase.2.Based on first-principles calculations,we found the total energy of single-layer WSe₂ in the 1T?and 2H phases depend on interlayer interaction between WSe₂ and substrate.When the interlayer interaction is strong enough,the 1T?phase become stable phase.So we use Sr Ti O₃(100)as the substrate successfully control the strength of interface during the growth process,then achieve a perfect phase selective growth mode.At the same time,1T?-WSe₂ grown on Sr Ti O₃(100)is under in-plane compressive strain along the a and b axes,which will drive 1T?-WSe₂ into topological semimetal phase.Our results provide an effective method for the stably growth of metastable 2D materials through enhancing interfacial interaction.3.We successfully insert hydrogen atoms between graphene and the substrate by performing hydrogen plasma treatment at 500°C.The inserted hydrogen atom does not replace the C-Si bond in the interface,but independently located in the interface and effectively weakens the influence of the substrate on the sample,suspends the epitaxial MLG into the original graphene.In addition,we found the band structure of the double-layer graphene treated with hydrogen is temperature-dependent,and the transition of the double-layer graphene energy band can be effectively adjusted by adjusting the temperature.This method provides a new direction for the future applications of graphene as hydrogen storage and devices.