Evidence of nanodiamond forming in the gas phase: experiment and activation energy analysis

The mechanism of ballas like nano diamond (ND) formation still remains elusive, and this work attempts to analyze its formation in the framework of activation energy (E_a). ND thin films were grown from H₂/CH₄/N₂ plasma in a 2.45 GHz chemical vapor deposition system. The E_a is calculated corresponding to the thickness and mass growth rate, and renucleation rate, while using substrate temperature (~1000-1300 K) in all the calculations. While these values of ~10 kcal/mol match with the E_a for ND formation throughout literature, they are far off compared to ~50 kcal/mol for single crystal diamond (SCD) formation, concluding thus far, that the processes involved are different. To further elaborate this, we modified the substrate preparation and sample collection method while keeping the growth parameters constant. Electron microscopy and Raman spectroscopy of the collected sample found that ND self nucleates in the plasma and flows to the substrate which acts as a mere collection plate. The E_a values for all the ND films are now re-calculated using the approximated gas temperature (~2000-3000 K), giving values closer to SCD formation. It suggests that the formation process for ND and SCD could be the same, but it happens in the gas phase for ND and directly on the substrate for SCD.