Low Energy Electron Microscopy On 2D Materials And Ultrafast Microscopy Design

Two-dimensional materials contain novel and rich quantum properties and have giant application value,which attract researchers from physics and materials science.With the increasing research on two-dimensional materials,they have continuously discovered their new physical properties,including Dirac electronics,semiconductors,superconductors,charge density waves,ferromagnets,etc.to provide the basis of high-speed multi-function electronic devices in the future.Recently,two-dimensional material single-layer Cu₂Si has been found topological nodal line semimetal on the surface of copper single crystal and is a new structure of two-dimensional Dirac material.Therefore,directly epitaxially growing a large-area,high-quality single-layer Cu₂Si on the surface of a silicon single crystal has become an important research direction.The two-dimensional single-layer topological semimetal Cu₂Si has wide applications in high-speed and low energy comsuption electronic devices.However,the growing dynamics of single layer Cu₂Si on silicon single crystal and its new physical properties still require in-situ research and exploration.In this thesis,we in-situ real-time study the growth kinetics and intrinsic physical properties of two-dimensional single-layer Cu₂Si on silicon single crystal substrates in the ultra-high vacuum environment using surface sensitive aberration-corrected low energy electron microscope?AC-LEEM?with various quantum imaging mechanisms.We in-situ study the growth dynamics of two-dimensional Cu₂Si fabricated by three different methods:substrate precipitation,in-situ room temperature deposition then high temperature annealing and high temperature epitaxial growth and compare their surface crystal structures using AC-LEEM.We find the high temperature epitaxial growth is the ideal method to controllably fabricate the high quality Cu₂Si.The characteristic peak related to the local Cu atom concentration is found by comparing the experimental IV curves in very low erngy region.Through the spatially resolved IV curves near the characteristic peak,it is found that in the non-equilibrium state during Cu₂Si growing,the surface has a region where the Cu atom concentration change continuously at the?535?and 7×7phase boundaries and there exists a critical Cu coverage driving phase change from 7×7to?535?.In combined with the atom-resolved STM results,it is found that the mixture of the?535?reconstructed precursor and the high quality Cu₂Si is responsible for the region where the Cu concentration continuously changes.The conductivity of high quality Cu₂Si is measured under low temperature and strong magnetic field by four probes and the weak anti-localization effect is found on Cu₂Si.Another kind of two-dimensional materials van der Waals?vdW?layered materials allow different types of atomic layer materials to be assembled with one another to produce vdW heterojunctions with unprecedented properties and functions that can be used to design new multifunctional electronic devices.Among these functional vdW materials,ferroelectric vdW layered materials have potential application in high-speed non-volatile memory,sensors and transistors.However,there are only a few ferroelectric vdW layered materials,especially a two-dimensional vdW layered material with strong ferroelectricity at room temperature remains to be discovered.In this thesis,the imaging method of a roated beam around optical axis at a fixed inclined electron beam is innovatively used to find the ultra-thin layer vdW layered material??-In₂Se₃has in-plane ferroelectricity at room temperature for the first time.?LEED confirms the existence of a one-dimensional superstructure on the surface of the ferroelectric state??-In₂Se₃.In the in-situ temperature dependent LEEM experiment,the reversible phase transition process from??to?phase was observed.The critical temperature of the phase transition is as high as 200°C.The surface ferroelectricity and the surface one-dimensional superstructure disappear as the phase change to the?phase confirming the ferroelectricity is closely related to the one-dimensional superstructure.The existence of ferroelectric domains and direction of electric dipole on the surface of??-In₂Se₃ are directly confirmed by piezoresponse force microscopy.Linear polarization optical microscopy revealed that the ferroelectric domain of??-In₂Se₃ has linear dichroism consisted with PFM results.The low temperature atomic resolved STM and high-resolution transmission electron microscopy find the one-dimensional superstructure is composed of two sets of atomic chains with slightly different widths.First-principles calculations show that the displacement of Se atoms can produce ferroelectric polarization and the system energy become lower,which provides a theoretical evidence in favour of ferroelectricity of??-In₂Se₃ at room temperature.As more and more two-dimensional materials are applied to future miniaturized multifunction electronic devices,the response time of electronic devices become shorter and shorter.These ultrafast response processes have attracted more and more researchers?interest.However,the technique,which is able to perform high-resolution and directly image on the ultra-fast response process of two-dimensional materials in the laboratory,is still lacking.In this thesis,based on AC-LEEM,an ultrafast electron source and a femtosecond laser are used to perform ultrafast low-energy electron microscopy?ULEEM?using pump-probe techniques in order to create a new technique to study ultrafast processes on two-dimensional materials with ultrafast time resolution and high spatial resolution.The flight time of the electrons in the entire ULEEM system is simulated and the double parallel mirror multiple reflection time compensation optical setup is designed to compensate the delay time of electron.The whole ULEEM optical path design is optimized and completed.Some parts of ULEEM equipment are assembled.An ultrafast phase transition experiment on Bi polycrystal film is design.These are the basis of realization of the ULEEM.