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Author: search.filters.author.Ramamoorthy, HariharaSubject: search.filters.subject.Raman

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    CVD Synthesis of MoS2 Using a Direct MoO2 Precursor: A Study on the Effects of Growth Temperature on Precursor Diffusion and Morphology Evolutions
    (2023-07-01)
    Somphonsane, Ratchanok
    ;
    Chiawchan, Tinna
    ;
    Bootsa-ard, Waraporn
    ;
    Ramamoorthy, Harihara
    In this study, the influence of growth temperature variation on the synthesis of MoS<inf>2</inf> using a direct MoO<inf>2</inf> precursor was investigated. The research showed that the growth temperature had a strong impact on the resulting morphologies. Below 650 °C, no nucleation or growth of MoS<inf>2</inf> occurred. The optimal growth temperature for producing continuous MoS<inf>2</inf> films without intermediate-state formation was approximately 760 °C. However, when the growth temperatures exceeded 800 °C, a transition from pure MoS<inf>2</inf> to predominantly intermediate states was observed. This was attributed to enhanced diffusion of the precursor at higher temperatures, which reduced the local S:Mo ratio. The diffusion equation was analyzed, showing how the diffusion coefficient, diffusion length, and concentration gradients varied with temperature, consistent with the experimental observations. This study also investigated the impact of increasing the MoO<inf>2</inf> precursor amount, resulting in the formation of multilayer MoS<inf>2</inf> domains at the outermost growth zones. These findings provide valuable insights into the growth criteria for the effective synthesis of clean and large-area MoS<inf>2</inf>, thereby facilitating its application in semiconductors and related industries.
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    Cvd synthesis of intermediate state-free, large-area and continuous mos2 via single-step vapor-phase sulfurization of moo2 precursor
    (2021-10-01)
    Chiawchan, Tinna
    ;
    Ramamoorthy, Harihara
    ;
    Buapan, Kanokwan
    ;
    Somphonsane, Ratchanok
    The low evaporation temperature and carcinogen classification of commonly used molybdenum trioxide (MoO3) precursor render it unsuitable for the safe and practical synthesis of molybdenum disulfide (MoS2). Furthermore, as evidenced by several experimental findings, the associated reaction constitutes a multistep process prone to the formation of uncontrolled amounts of intermediate MoS2−yOy phase mixed with the MoS2 crystals. Here, molybdenum dioxide (MoO2), a chemically more stable and safer oxide than MoO3, was utilized to successfully grow cm-scale continuous films of monolayer MoS2. A high-resolution optical image stitching approach and Raman line mapping were used to confirm the composition and homogeneity of the material grown across the substrate. A detailed examination of the surface morphology of the continuous film revealed that, as the gas flow rate increased by an order of magnitude, the grain-boundary separation dramatically reduced, implying a transition from a kinetically to thermodynamically controlled growth. Importantly, the single-step vapor-phase sulfurization (VPS) reaction of MoO2 was shown to suppress intermediate state formations for a wide range of experimental parameters investigated and is completely absent, provided that the global S:Mo loading ratio is set higher than the stoichiometric ratio of 3:1 required by the VPS reaction.