Ultrananocrystalline diamond, a new nanocarbon enabled by plasma enhanced chemical vapor deposition

Summary form only given. The idea of making use of the highly energetic carbon dimer molecular species, C₂, to nucleate and grow diamond was conceived in 1991. Importantly, this approach obviates the presence of atomic hydrogen to perform sequential hydrogen abstraction reactions, a requirement essential for conventional CVD diamond synthesis. The C₂ process uniquely results in the synthesis of ultrananocrystalline diamond (UNCD) films composed of 3-5 nm crystallites that along with fullerenes and nanotubes constitute the fascinating triad of new nanocarbons. Surface to volume considerations in UNCD dictate that about 10% of the carbon in the films is at grain boundaries where sp²+x carbon-carbon bond rehybridization occurs reminiscent of the bonding in fullerenes and nanotubes. The UNCD films are very smooth and have a Young´s modulus close to that of single crystal diamond. They can be conformally coated even on AFM tips and are potentially useful as NEMS and MEMS devices, coatings for rotating seals, hermetic sealing of retinal implants as well as other applications. Microwave plasma processes play a crucial role in many aspects of UNCD film properties such as the profound changes in electrical conductivity correlated with the formation of CN species when nitrogen, N₂, is added to the synthesis gas. The ratios of the plasma generated CN and C₂ molecules can be precisely controlled. By doing so, we are able to control the n-type conductivity of the films over many orders of magnitude. The fascinating recently determined relationship CN/C2=log(Ktimeselectrical conductivity) will be elucidated in terms of non-equilibrium plasma induced reactions, equilibrium reactions dictated by thermodynamics and changes in film microstructure. Various applications of highly conducting n-type UNCD will be enumerated