Diamond thin films have been deposited on mirror-polished silicon substrates in a microwave plasma chemical vapor deposition reactor under fixed pressure but different flow rates of the mixed methane and hydrogen gas supply. The films synthesized were characterized by scanning electron microscopy, atomic force microscopy, Raman spectroscopy, and X-ray diffraction. Microcrystalline diamond (MCD) thin films were obtained at relatively low gas flow rates (30 sccm to 300 sccm), while nanocrystalline diamond (NCD) thin films with cauliflower-like morphology were obtained at higher gas flow rates (above 300 sccm). The formation of NCD at the high flow rates may be attributed to the enhancement of diamond secondary nucleation arising from the increase in the flux rate of carbon containing radicals reaching the diamond growth surface. Measurements of field electron emission from the diamond films have shown that the NCD films exhibit lower field emission thresholds and higher current density compared with the MCD films. Furthermore, the surface roughness of the NCD thin films grown at high gas flow rate is effectively reduced by applying positive biasing on the diamond growth stage. (c) 2006 Elsevier B.V. All rights reserved.