Diamond films with different grain sizes in the range of ~ 9 nm to ~ 50 μm have been deposited on silicon substrates using a homemade microwave plasma chemical vapor deposition reactor by varying the deposition parameters. The surface morphologies have been examined by scanning electron microscope and atomic force microscope, which show the secondary nucleation intensity and surface defects of the diamond films increase with the decrease of the diamond grain size. Although X-ray diffraction spectra show the absence of graphitic carbon features, the Raman and X-ray photoelectron spectroscopy show the sp2/sp3-bonded carbon ratios increase with the decrease of the diamond grains. The CH4 percentage in plasma during deposition plays a crucial role in the formation of diamond films with different grain sizes and sp2 contents, which in turn determines the electron field emission behavior of the corresponding diamond films. The smaller the grain size of the diamond, the higher is the grain boundary density, which can provide more electron emission sites and form conductive networks for electron transport. The ultra-nanocrystalline diamond film shows needle-like cluster structures and optimum electrical performance. The corresponding electron field emission behavior can be turned on at a field of 6.71 V/μm and attain a current density of 16.28 μA/cm2 at an applied field of 11.31 V/μm.
