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 nanosized diamond grains (∼20 nm) with negligible thickness for grain boundaries that is distinctly different from the microstructure of ultrananocrystalline (UNCD) films with uniformly sized ultrananodiamond grains (∼5 nm) having relatively thick grain boundaries (∼0.1 nm). The turn-on of the electron field emission (EFE) process occurs at ( E) = 24.1 V/μm and ( E) = 18.6 V/μm for the pristine NCD and UNCD materials, respectively. The granular structure of the starting diamond films largely influenced the microstructure evolution behavior and EFE properties of the materials subject to plasma annealing. The CH/(Ar-H) ppt-process leads to formation of a hybrid granular structured diamond (HiD and HiD) via isotropic conjoining of nanosized diamond grains, whereas the CH/N ppt-process leads to the formation of acicular granular structured diamond films (N and N) via inducing aeolotropic growth of nanodiamond grains. While both of the HiD and HiD films contain hybrid granular structure, the HiD films contain a larger proportion of nanographite phase and result in improved EFE properties, viz. ( E) = 7.7 V/μm and ( E) = 12.3 V/μm. In contrast, when the films were CH/N ppt-processed, the acicular diamond grains were formed for N and N films; however, carbon nanoclusters attached to the diamond grains of N films and the nanographitic layers encasing diamond cores are not crystallized very well, as compared with N films. Therefore, the N films exhibit slightly inferior EFE properties than the N films, viz. ( E) = 5.3 V/μm and ( E) = 11.8 V/μm. The difference in EFE properties for ppt-processed NCD and UNCD films corresponds to the dissimilar granular structure evolution behavior in these films that is, in turn, due to the distinct different microstructure of the pristine NCD and UNCD films.