Enhancing the plasma illumination behaviour of microplasma devices using microcrystalline/ultra-nanocrystalline hybrid diamond materials as cathodes

Abstract

The properties of capacity-type microplasma devices were significantly enhanced due to the utilisation of hybrid diamond films as cathodes. The performance of the microplasma devices was closely correlated with the electron field emission (EFE) properties of the diamond cathode materials. The nanoemitters, which were prepared by growing duplex-structured diamond films [microcrystalline diamond (MCD)/ultra-nanocrystalline diamond (UNCD)] on Si-pyramid templates via a two-step microwave plasma enhanced chemical vapour deposition (MPE-CVD) process, exhibited improved EFE properties (E₀ = 5.99 V μm⁻¹, J_e = 1.10 mA cm⁻² at 8.50 V μm⁻¹ applied field), resulting in superior microplasma device performance (with a lower threshold field of 200 V mm⁻¹ and a higher plasma current density of 7.80 mA cm⁻²) in comparison with UNCD film devices prepared using a single-step MPE-CVD process. The superior EFE properties of the duplex-structured MCD–UNCD films relative to those of the UNCD films can be attributed to the unique granular structure of the diamond films. High-resolution transmission electron microscopy reveals that the MCD–UNCD films consisted of abundant graphitic phases located at the periphery of large diamond aggregates and at the boundaries between the ultra-small diamond grains. The presence of the graphite phase is presumed to be the prime factor that renders these films more conductive and causes these films to exhibit higher EFE properties, thus resulting in the improved plasma illumination properties of the microplasma devices.