Investigating the CVD Growth Kinetics of 2D Crystals Through In Situ Optical Microscopy and Automated Image Processing

Two-dimensional (2D) materials have gained significant attention for their unique physical and chemical properties with promising applications. Numerous efforts have been dedicated to synthesizing large-area, highly crystalline 2D materials using various techniques. Among these methods, chemical vapor deposition (CVD) stands out for its ability to enable wafer-scale growth with precise thickness control. However, comprehending the intricate relationship between the intrinsic thermodynamic parameters and the nucleation and growth of 2D crystals to achieve precise control over crystal orientation and grain size during CVD synthesis remains a formidable challenge.<br/>This work uses a custom-built miniature CVD system with in-situ optical microscopy to investigate the real-time formation of individual molybdenum disulfide (MoS2) crystals during CVD growth. An automated image processing pipeline, which extensively employs machine learning algorithms, is developed to quantitatively analyze the microscopic video footage. Using this pipeline, we were able to extract the growth trajectory of each and every MoS₂ crystal before and after coalescence. Such information offers valuable insight into the growth behavior of 2D crystals. We observe that MoS₂ crystals that nucleated earlier also have larger growth rates. The crystal growth speed decreases monotonically with the crystal size but is independent of the crystal orientation. Interestingly, the in-situ observation reveals that neighboring MoS₂ crystals do not interfere with each other’s growth and exhibit weak pair correlation when the reactor temperature is above 800 °C. This supports a ground-feeding growth mechanism in which nanoparticulate Mo precursors are deposited onto the sapphire substrate and supply Mo feedstocks to the surrounding MoS₂ crystals via surface diffusion.