Epitaxial Growth And Scanning Tunneling Microscopy Studies Of Low-dimensional Molybdenum Telluride

As a component of transition metal dichalcogenides/chalcogenides,molybdenum telluride includes both two-dimentional(2D)MoTe2 ultrathin films and one-dimensional(1D)Mo6Te6 nanowires.Low-dimensional molybdenum-telluride materials have recently attracted particular research interests for their exotic properties and potential applications.Controllable preparing and precisely tuning their atomic and electronic properties are therefore crucial for further researches.In this thesis,low-dimensional MoTe binary compounds on graphene substrates are synthesized and researched,utilizing an ultrahigh vacuum system integrated of molecular beam epitaxy,X-ray photoemission spectroscopy and scanning tunneling microscopy(UHV MBE-XPS-STM).The main conclusions are listed as follows.(1)The MBE growth of Mo-Te binary compounds is temperature-dependent,that is,2D MoTe2 ultrathin films are synthesized at low temperature range,while 1D Mo6Te6 nanowires gradually arise and dominate as temperature increasing.(2)Both semiconducting 2H phase and semi-metallic 1T’ phase are obtained simultaneously in 2D MoTe2 ultrathin films,fabricating atomically-coherent lateral heterophase homojunctions.Moreover,two typical structures co-exist in regions of 2HMoTe2.One is the dense triangle network of mirror twin boundaries(MTBs).Another is the modulation structure with a periodic length of 1.2 nm,which could not be perfectly explained with the 2H-MoTe2/graphene Moiré patterns.2D charge density wave(CDW)states are therefore supposed to exist in this periodic structure.(3)1D Mo6Te6-nanowire-phase networks and ultrathin films are prepared with onestep and two-step methods,respectively.Density functional theory(DFT)calculations and scanning tunneling spectroscopy(STS)experiments reveal the semi-metallic property of the grown lateral Mo6Te6 nanowire bundles.Moreover,the 1D Mo6Te6-2D MoTe2 lateral heterostructures could be fabricated at atomic level,near the contact area of which an upward band bending of 2H-MoTe2 is observed via STS mapping.Such a phenomenon could be explained with the metal-semiconductor junction model.To sum up,our results suggest a new rount to controllably grow low-dimensional molybdenum telluride and fabricate related heterostructures,thus paving the way for their future applications in high-performance electronic and optoelectronic devices.