Methods for seeding silicon substrates with pretreatments and the in situ generation of diamond nuclei by the bias enhanced nucleation (BEN) have been studied to examine their effects on the nucleation density as well as the morphology of grown and seeded sides, the growth rate, and the quality of nanocrystalline diamond films. Pretreatments including mechanical abrasion by diamond paste, exposure of the silicon substrate to a hydrocarbon plasma, and ultrasonication of silicon in solvents with suspended nano- or micro-diamond powders were studied. With an optimized diamond seeding or nucleation process, ultra-thin, smooth and homogenous nanocrystalline diamond films can be fabricated.