Atmospheric pressure chemical vapor deposition (APCVD) has been used extensively for synthesizing two-dimensional (2D) materials, due to its low cost and promise for high-quality monolayer crystal synthesis. However, the understanding of its reaction mechanism is still in the embryonic stage, hence, leading to poor scalability in achieving large scale continuous film. One of the key reasons behind the uncertainty of the APCVD process is that it involves chemical reactions coupled with complex fluid mixing where the transfer of momentum, heat, and mass significantly affects the reaction process and thereafter the final product.

In this study, we, for the first time, develop a fluid guided growth approach and use MoSe~2~ as a demo system to understand and design the APCVD process. During the experiment, the carrier gas, Ar/H~2~ (85/15), enters the reaction tube from the inlet and carries the sublimated Se to react with MoO3 and generate MoSe2 crystals. The distribution of MoSe~2~ deposition on the silicon substrate is observed using optical microscopy. TEM and Raman spectroscopy have been used to identify the monolayer character of deposited crystals.

We numerically simulated the entire process that happens in the reaction tube. By applying computational fluid dynamics (CFD) analysis, the chemical reaction and the fluid flow have been integrated to investigate the mixing and reaction of precursors. The simulation, along with the experimental results, allows us to examine the role of the precursor mixing, the shear stress, and the fluid velocity in the outcome of the APCVD process.

The integration of experiment and CFD analysis in the full-reactor scale elucidates the mechanism involved in the APCVD process and guides the structure design and optimization. Our study shows that under the same experimental parameters, the change of the growth setup can significantly influence the evolution of the flow field and hence change the growth behavior. The factors that influence the APCVD process include but not limited to the precursor concentration distribution, fluid velocity (magnitude and direction), fluid shear stress over the substrate, as well as the mixing of Se and MoO~3~.  Higher and uniform concentration of precursors, low shear stress, and small flow deviation to the substrate can create a stable growth environment for large-scale monolayer 2D materials. With a minimum modification of the growth setup, we have successfully obtained inch-scale monolayer MoSe~2~.

This unprecedented success of achieving scalable 2D materials through fluidic design lays the foundation for designing new CVD systems to achieve the scalable synthesis of nanomaterials.


Fluid Guided Chemical Vapor Deposition Growth for Large-Scale Monolayer Two-Dimensional Materials

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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