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Introduction
Electron field emission (EFE) is considered as the only electron emission process compatible with the vacuum electronics due to the fast response time, low power consumption, cathode-ray-tube-like colors, and wide viewing angles.[1,2] However, incorporation of cold field emitters in practical electronic devices is still quite challenging due to the stringent requirements of long cathode lifetime with high stability. Since the discovery of the excellent EFE properties of Q-carbon composite structure,[3] there has been a significant increase of interest in studying the emission stability of this material for practical device applications. The carbon-based field emitters such as carbon nanotubes, diamond, nano-diamond, and diamond-like carbon (DLC), have been investigated, however, reliable commercial devices such as field emission lighting elements, frictionless motors, flat panel displays, etc. are still challenging to develop due to several barriers.[4] Over the decades, diamond has been considered as one of the most promising materials for cold-cathode applications owing to its negative electron affinity (NEA).[5,6] However, the ideal single crystal diamond cannot provide the required amount of electrons for the field emission, and the band structure of diamond is unsuitable for the transport of those electrons to the surface. Usually, the diamond surface is terminated by hydrogen to obtain NEA for enhanced EFE properties. But exposing the hydrogen-terminated diamond surface to high fields and to oxygen ambient for several days is enough to replace some of the surface hydrogen with different oxygen groups. This diminishes the NEA and thereby reduces the percentage of the emitting surface area, and worsens the EFE performance.[7] Therefore, researchers have tried nanometer size or highly defective crystallites having much degraded physical properties than the crystalline diamond, such as polycrystalline diamond, nitrogen-incorporated ultrananocrystalline diamond, and DLC.[8–11] The EFE characteristics from the DLC and amorphous carbon can be compared with vacuum breakdown at the high electric field and the electron affinity was found to be positive in these materials, which is undesirable for good field emission characteristics.[12,13] The field emission from DLC film has been found to be related to the surface roughness.