  Coating metal nanotips with a negative electron affinity material like
hydrogen-terminated diamond bears promise for a high brightness photocathode.
We report a recipe on the fabrication of diamond coated tungsten tips. A
tungsten wire is etched electrochemically to a nanometer sharp tip, dip-seeded
in diamond suspension and subsequently overgrown with a diamond film by
plasma-enhanced chemical vapor deposition. With dip-seeding only, the seeding
density declines towards the tip apex due to seed migration during solvent
evaporation. The migration of seeds can be counteracted by nitrogen gas flow
towards the apex, which makes coating of the apex with nanometer-thin diamond
possible. At moderate gas flow, diamond grows homogeneously at shaft and apex
whereas at high flow diamond grows in the apex region only. With this
technique, we achieve a thickness of a few tens of nanometers of diamond
coating within less than 1 $\mu$m away from the apex. Conventional transmission
electron microscopy (TEM), electron diffraction and electron energy loss
spectroscopy confirm that the coating is composed of dense nanocrystalline
diamond with a typical grain size of 20 nm. High resolution TEM reveals
graphitic paths between the diamond grains.
