Study Of Phase Engineering And Oxidation Dynamics Of Few-layer MoTe₂ Films Grown By Chemical Vapor Deposition

Phase engineering of two-dimensional?2D?transition metal dichalcogenides?TMDs?such as MoTe₂ offers tremendous opportunities in various device preparation and applications.However,most of the existing methods so far only address the small-area local phase change or the growth of certain kinds of phases of MoTe₂ film by laser irradiation,mechanical strain,or precursor.Obtaining facile,tunable,reversible,and continuous-phase transition and evolution between different phases in direct growth of large-area,few-layer MoTe₂ still remains challenging.This thesis mainly focuses on the controllable preparation of large-area,high quality,phase-controlled,few-layer MoTe₂ films by chemical vapor deposition?CVD?.Four different phases of highly uniform MoTe₂ films were fabricated,including 1T?-dominant coexistence of both 2H and 1T?phases,100%2H phase,another 2H-dominant coexistence of both 2H and 1T?phases,and 100%1T?phase.We studied how to achieve a complete metallic-semiconducting-metallic phase evolution by regulating tellurization velocity of the precursor during the growth process,and we analyzed and explained the mechanism of this reversible phase transition and evolution.We also studied the relationship between the phase transition of MoTe₂ film and the growth condition,and further investigated their thickness,structure,composition,crystallization,and electrical properties.Moreover,on the basis of the controllable synthesis,we also investigated the oxidation dynamic behavior of our grown few-layer 1T?-MoTe2 films,and studied the quantum transport properties of samples with different degree of oxidation.The specific contents are as follows:?1?We studied both carrier gas flow rate and tellurization temperature can affect the tellurization velocity of the precursor,thus to regulate the synthesis and phase transition of MoTe₂ films.The obtained different phases MoTe₂ samples were characterized by optical microscopy,Raman spectroscopy,atomic force microscopy,transmission electron microscopy and X-ray photoelectron spectroscopy.Importantly,the tellurization velocity should be extremely controlled to obtain 100%2H phase MoTe₂,while 100%1T?phase requiresafasttellurizationvelocity.Wefurtherfoundthatsuch metallic-semiconducting-metallic phase evolution took place with a homogeneous spatial distribution,which signfificantly differs from previous reports in which obvious phase separations are usually found during the phase transition.?2?On the basis of the controllable synthesis,we focused on few-layer 1T?-MoTe₂ to investigate the effect of oxidation on the lattice structure and defects,and its quantum transport properties upon annealing under the different atmospheres?pure oxygen atmosphere,pure nitrogen atmosphere and different concentrations of nitrogen/oxygen mixture?at different temperatures.According to the Raman analysis and XPS characterization,the oxidation dynamic behavior of CVD-grown few-layer 1T?-MoTe₂ film by annealing under oxygen-rich conditions with different temperature was analyzed in detail.