Atmospheric-Pressure Flame Vapor Deposition of Nanocrystalline Diamonds: Implications for Scalable and Cost-Effective Coatings

Nanocrystalline diamonds (NCDs) are one of the many carbon allotropes that have attracted great attention for the advancement of many technologies owing to their superior mechanical, thermal, and optical properties. Yet, their synthesis must be improved for availability at low costs and their widespread application. Here, we report the atmospheric-pressure flame vapor deposition (FVD) synthesis of NCD particles and thin films over an area of more than 27 cm2 using methane-hydrogen-air flat flames. Synthesis at atmospheric pressure is beneficial as it can lower costs and be more time-efficient when compared to the batch-by-batch synthesis of low-pressure and high-pressure processes. Also, the abundance of methane gas available can further lower costs and improve scalability, while generating lower flame temperatures to mitigate the need of extensive cooling. Notably, the FVD method unlocks conditions for diamond growth beyond the previously considered diamond-growth region of the C-H-O phase diagram. By modeling the flame radical species as a guidance, we experimentally demonstrate that the FVD growth of NCDs can be facilely controlled by tuning the reactant gas composition, substrate material, and seeding density. Moreover, we show that the addition of an external electric bias was influential in controlling the porosity and thickness of the NCD films. Overall, with the low cost and simplicity for operation without the need of vacuum, this atmospheric-pressure FVD approach will offer opportunities to facilitate the scaling-up of NCD synthesis for applications in optical, tribological, thermal, and biomedical coatings.