The role of nano-graphite phase on the enhancement of electron field emission properties of ultrananocrystalline diamond films

Here, we demonstrated with examples that the presence of nano-graphite phase markedly enhanced the electron field emission (EFE) properties of the ultrananocrystaline diamond films. The nano-graphite phase was induced by different mechanism. In the two-step microwave plasma enhanced CVD (MPECVD) process, the nano-graphite phase was induced at the interfaces of large-grain and ultra-small-grains regions, which was clearly illustrated by transmission electron microscopy (TEM). The EFE process was thus markedly enhanced, viz. low turn-on field of 9.2 V/μm with large emission current density of 800 μA/cm2 at 27.5 V/μm was achieved. In the Au-ion irradiation of UNCD films, the presence of nano-graphite phase was not clearly resolved by TEM but was strongly inferred by the Raman spectroscopy. The induction of graphitic phase was also believed to be the factor, resulting in marvelous EFE properties for these UNCD films. In the two-step MPECVD and Au-ion irradiation processes, the nano-graphite phase was presumably induced by the grain growth phenomenon. However, the most marked enhancement on the EFE properties for the UNCD films is resulted from the utilization of CH4/H2 plasma to replace the CH4/Ar plasma in MPECVD process. The nano-graphite phase was induced, forming an encapsulation layer surrounding the needle-like diamond grains. The EFE process of the nitrogen-doped UNCD films can be turned on at 8.3 V/μm, achieving 2160.0 μA/cm2 EFE capacities at an applied field of 18.0 V/μm.

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► Presence of nano-graphite phase markedly enhanced EFE properties for UNCD films.
► Nano-graphite phase can be induced by Au-ion irradiation or novel MPECVD processes.
► Novel processes include CH4/Ar–CH4/(Ar–H2) two-step or CH4/N2 one-step MPECVD.