Epitaxial Growth And Physical Properties Of Several Layered Chalcogenides Thin Films

Since the discovery of graphene in 2004,two-dimensional?2D?materials have attracted much more attention.From bulk graphite to single-layer graphene,graphene possesses many unique and outstanding properties,such as high charge carrier mobility(200,000 cm²V^-1S^-1),remarkable mechanical properties,high optical transparency and thermal conductivity,and room temperature quantum hall effect,showing the potential applications in various fields.The success of graphene inspired people to explore and discover the other 2D materials beyond graphene,resulting in a huge family of 2D material.Among of them,transition metal dichalcogenides?TMDs?and other layered chalcogenides have emerged as an important class of 2D materials with fascinating physical properties and potential applications,attracting increasing attention and intensive research in the last decade.In this thesis,we mainly focused on three 2D materials,named PdSe₂,PdTe₂ and CeTe₃.High quality thin films have been fabricated by molecular beam epitaxy?MBE?method.The structural and physical properties have been investigated by scanning tunneling microscopy?STM?,low energy electron diffraction?LEED?and angle-resolved photoemission spectroscopy?ARPES?combined with first-principles calculations.?1?PdSe₂ is a member of layered TMDs known for its unique pentagonal structure and remarkable layer-dependent electronic structure as well as notable FETs performance.Here bilayer PdSe₂ islands have been fabricated on bilayer or monolayer graphene on SiC?0001?by MBE.A bandgap of 1.15±0.07 eV was revealed by STS measurement.Moreover,a 0.2 eV bandgap shift was observed in PdSe₂ layers grown on monolayer graphene as compared to those grown on bilayer graphene.The realization of nanoscale and atomically sharp electronic junctions in PdSe₂ films suggests the possibility of tuning the electronic or optoelectronic properties for potential technological applications.In addition,structurally well-defined PdSe₂nanoribbons have been achieved on PdSe₂ islands and the nanoribbons exhibit stacking growth with a fixed orientation.The bottom-up fabrication of 1D PdSe₂nanoribbons is expected to enable substantial exploration of 1D TMDs.?2?PdTe₂,another member of layered TMDs,has received significant research interests due to the coexistence of superconductivity and type-II Dirac fermions.Moreover,PdTe₂ is also a substrate applicable for epitaxial growth of monolayer antimonene.In this part,PdTe₂ films have been fabricated on both a bilayer graphene-SiC?0001?and SrTiO₃?100?substrate by MBE method.In situ LEED,STM,and X-ray photoelectron spectroscopy?XPS?measurements demonstrate high quality and single crystalline of the epitaxial PdTe₂ films.Raman-active modes in PdTe₂,in-plane?E_g?and out-of plane(A_1g)motions,have been analyzed as compared to that in bulk PdTe₂.ARPES measurement of 6-layer PdTe₂ films reveals its metallicity and a type I Dirac cone contributing from Z₂ topological surface states.?3?Charge density waves?CDW?have been a subject of great interest in solid state physics.The rare-earth tritellurides,like CeTe₃,are excellent model systems to study the CDW phase.In this part,single-layer CeTe₃ films have been grown on bilayer graphene on SiC?0001?.LEED patterns confirm three rotational domains due to different symmetry between CeTe₃ and graphene substrate.Atomically resolved STM topographies on islands confirm the existence of CDW phase in monolayer CeTe₃ and its CDW modulation has no difference with bulk.Furthermore,a detailed investigation of the electronic structure by ARPES combined with first-principles calculations reveals the nature of CDW order in monolayer CeTe₃ originating from Fermi surface nesting.?4?Besides the above-mentioned works,I design and build one mini MBE system on the base of the original ultrahigh vacuum system.This mini MBE system is compatible with original system and makes us evaporate some polluting or deleterious material such as Se and Te without contaminating other part of entire system.