Tabletop microwave capillary reactor for nanodiamond synthesis

Low-temperature plasmas have been used to develop hallmark processes in the microelectronics industry. Revolutionized by the discovery of size-dependent electronic properties in semiconductor nanocrystals, continuous flow-through reactors have been designed for nanoparticle synthesis with high production rates and simple means of sample collection. Well-vetted synthesis routes to produce semiconductor nanocrystals like diamond cubic Si, Ge, SiGe, etc. have been developed. However, diamond nanocrystals themselves have eluded these attempts.

In the present work, an operating flow-through tabletop microwave chemical vapor deposition reactor is presented that can achieve nanodiamond synthesis, as evident from material characterization. The reactor consists of a quartz tube placed inside a compact cylindrical 2.45 GHz cavity. Precursor gases (H₂ and CH₄) flow through the tube to generate a stable plasma discharge. The input operating power is <100 W and samples are collected on a substrate placed in the tube at varying distances from the plasma core. A reactor-scale plasma model with H₂/CH₄ plasma chemistry is solved for the spatial distribution of C_xH_y (where x = 1,2 and y = 0-3) radicals. The electromagnetic absorption in the plasma and the corresponding gas heating and electron distribution functions are calculated. It is shown that the reactor is extremely efficient in producing atomic H and CH₃ radicals owing to the gas temperature of 2,000 K – all key parameters in producing sp³ phase carbon.