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The enhancement of the plasma illumination characteristics of capacitive-type plasma devices (CP-devices) utilizing diamond-coated Si-nanotips as cathodes was systematically investigated. The enhanced electron field emission (EFE) properties of the diamond films resulted in improved plasma illumination characteristics of the devices. Microcrystalline diamond films grown using ultrananocrystalline diamond as a nucleation layer (MCD/UNCD) possessed a lower turn-on field for inducing the EFE process with a higher EFE current density and resulted in a better plasma illumination performance for the CP-devices compared with those made from MCD films grown directly on Si-substrates without the nucleation layer. Transmission electron microscopy revealed that, in a two-step microwave plasma enhanced chemical vapor deposition process, the second step altered the granular structure of the UNCD nuclear layer instead of growing a layer of large-grain diamond film on top of the UNCD nucleation layer, resulting in a duplex microstructure. The MCD/UNCD films contained large diamond aggregates evenly distributed among the ultrasmall-grain matrix, with the induction of a few layers of graphite, surrounding the large aggregates. The presence of the graphene-like phase is presumed to be the prime factor resulting in the superior EFE properties of the MCD/UNCD films and the better plasma illumination characteristics of the CP-devices.