Evolution of granular structure and the enhancement of electron field emission properties of nanocrystalline and ultrananocrystalline diamond films due to plasma treatment process.

The present work reports the plasma post treatment (PPT) process that instigates the evolution of granular structure of nanocrystalline diamond (NCD), consequently conducing the enhancement of the electron field emission (EFE) properties. The NCD films contain uniform and nano-sized diamond grains (~20 nm) with negligible thickness for grain boundaries that is distinctly different from the microstructure of ultra nano crystalline (UNCD) films with uniformly sized ultra-nano-diamond grains (~5 nm) having relatively thick grain boundaries (~0.1 nm). The turn-on of the electron field emission (EFE) process occurs at (Eo)NCD = 24.1 V/µm and (Eo)UNCD = 18.6 V/µm for the pristine NCD and UNCD materials, respectively. The starting diamond films granular structure largely influenced the microstructure evolution behavior and EFE properties of the materials subject to plasma annealing. The CH4/(Ar-H2) ppt-process leads to formation of hybrid granular structured diamond (HiDNCD) via isotropic conjoining of nano-sized diamond grains, , whereas the CH4/N2 ppt-process leads to the formation of acicular granular structured diamond films (NNCD) via inducing aeolotropic growth of nano-diamond grains. While both of the HiDNCD films contain hybrid granular structure, which contain larger proportion of nanographite phase and result in improved EFE properties, viz. (Eo)HiD-UNCD = 7.7 V/µm. In contrast, when the films were CH4/N2 ppt-processed, the acicular diamond grains were formed for NUNCD films, but these pretends no carbon nanoclusters attached to the diamond grains and the nano-graphitic layers encasing diamond cores are not crystallized very well; as compared with NNCD films. Therefore, the NNCD films exhibit slightly inferior EFE properties than the NUNCD films, viz. (Eo)N-UNCD = 5.3 V/µm and (Eo)N-NCD = 11.8 V/µm. The difference in EFE properties for ppt-processed NCD and UNCD films corresponds to the dissimilar granular structure evolution behavior due to these films that is, in turn, resulted from the distinct different microstructure of the pristine NCD and UNCD films.