Investigation On Several Low Dimensional Materials By Chemical-Bond-Resolved Non-contact Atomic Force Microscopy

The investigation on the structures and physical properties of low dimensional functional materials is one of the most attractive fields in condensed matter physics.Recently,many kinds of new low dimensional functional materials have been successfully synthesized,which usually have exotic physical properties,such as superconductivity,topology,and low-dimensional magnetism.To deeply understand the properties of these materials,their atomic and electronic structures must be studied thoroughly.Non-contact atomic force microscopy（nc-AFM）based on a q Plus sensor has chemical-bond resolutions,which can give more information than scanning tunneling microscopy（STM）for the surface atomic structures and localized electronic states,thus is an important tool for the investigation of low dimensional functional materials.In this thesis,nc-AFM has been used to achieve the following original research results:1.A combined system of ultra-high vacuum molecular beam epitaxy and cryogenic atomic force microscope was optimized and upgraded.By optimizing the electrical and mechanical stability of the system,the noise level has been improved by an order of magnitude.We designed and added a new fast sample loading device with liquid nitrogen as the refrigerator that can shorten the sampling time to 1/4 of the original ones.In addition,a Raman optical system based on the preparing chamber was bulit and can realize in-situ Raman measurements.The design of the dual-optical Raman optical path is also compatible for tip-enhanced Raman measurements in the future.2.The cleaved surfaces of Co₃Sn₂S₂ were unambiguously identified.The hexagonal appeared Sn surface,not the previously proposed kagome Co₃Sn surface,was found to explicitly exhibit a kagome-like electronic feature.Combined nc-AFM and density functional theory（DFT）calculations,it is found that the electronic hybridization between the surface Sn or the second-layer S atoms with the Co₃ clusters underneath vertically imposes the triangularly shaped electronic states of Co₃ to the topmost Sn and S atoms.These vertical hybridizations,together with subsequently lateral hybridization of triangular Sn and S states,transfer the kagome symmetry from the Co₃Sn layer to the Sn surface.Bond-resolved nc-AFM images,experimental and theoretical vertical short-range force spectra,and DFT calculations verify the surface assessment and the electronic hybridization.Furthermore,scanning tunneling spectrum clearly reveal the topological electronic states on the Sn surface.This surface identification enables further exploring properties of Co₃Sn₂S₂ and also uncovers a novel route to artificially generate surface kagome electronic states.3.Dihydropentalene linkage was built in poly-naphthalocyanine on Ag（110）surface.Scanning tunneling microscopy（STM）and non-contact atomic force microscopy（nc-AFM）measurements confirm the dihydropentalene linkage structure and a possible formation path with reconstruction steps have been proposed.The controlled experiment on Ag（100）surface shows no dihydropentalene structures are formed,which indicates the grooved substrate is necessary for the reconstruction.This work provides insights into the surface restricted reactions that can yield special structures in organic polymers.4.A facile phase conversion method that can locally convert n-type SnSe₂ into p-type SnSe by direct laser irradiation was proposed.Raman spectra of SnSe₂ flakes before and after laser irradiation confirm the phase conversion of SnSe₂ to SnSe.By performing the laser irradiation on SnSe₂ flakes at different temperatures,it is found that laser heating effect induces the removal of Se atoms from SnSe₂ and results in the phase conversion of SnSe₂ to SnSe.By selective laser irradiation on SnSe₂ flakes,a pattern with SnSe₂/SnSe heteostructures is created.This indicates that the laser induced phase conversion technique has relatively high spatial resolution and enables the creation of micron-sized in-plane p-n junction at predefined region.The CVD synthesized SnSe nanoplates exhibit an outstanding Seebeck coefficient,which is comparable to the intrinsic bulk SnSe.Using monochromatic infrared near-field imaging and numerical simulation,the propagated mid-infrared TE₀ waveguide mode in the tin diselenide two-dimensional material was observed in the laser wavelength ranging from 5.13 to 6.57μm.To sum up,this thesis utilize nc-AFM as a key approach to investigate the structures and physical properties of several low dimensional functional materials,demonstrate the ultra-high real-space resolution and multiple measurements of nc-AFM,such as chemical-bond resolved imaging,short-range force measurement,and Kelvin probe force microscopy.Our work therefore provide important references and new ideas for relevant studies on low dimensional functional materials.