**3.1 Chemical vapor deposition technique (CVD)**

This method is among the most used thin film coating methods, and it was utilized to make nanocrystalline diamond layer. More specifically, carbon atoms

**Figure 2.** *Synthesis of Nanodiamonds.*

are deposited during the breakdown of a gaseous mixture and carbon-containing molecules, most commonly methane CH4 (excess hydrogen). The gas phase decomposes into radicals like H• and CH3•, which are required for diamond formation, via a heated filament or microwave plasma. A continuous ND film is created on a membrane, usually a thin layer of silicon covered with a μm powder that functions as a nanodiamond deposition seed. Depending on the relative concentration of CH4/H2, the size of the coils wrapping the film ranges from 10 microns to a few nanometers.

Microcrystalline diamonds are formed by a low concentration of CH4, while a large quantity of CH4/H2 lowers particle size to the order of 10 of nm at 15% CH4/H2. After the hydrogen is absorbed by H• radicals, the carbon atoms on topmost of the diamond seeds are left with their bonds dangling. The CH3 molecules are then used to fill these bonds. New carbon can be caught together and finally bound in a diamond chains when this process occurs in two nearby regions [11].
