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The modification on microstructure of diamond films due to the incorporation of H2 species into the Ar/CH4 plasma was systematically investigated. While the hydrogen-free plasma produced the ultrananocrystalline diamond films with equiaxed grains (about 5 nm in size), the hydrogen-containing plasma resulted in platelike grains (about 100×300 nm2 in size). The size of the platelike grains increased with the H2 content in the plasma. Transmission electron microscopy and optical emission spectroscopy reveal that only 0.1%H2 incorporated in the Ar/CH4 plasma is sufficient for inducing the formation of platelike grains, suggesting that the platelike grains are formed via the competition between the attachment and the etching of hydrocarbons onto the existing diamond surfaces. In Ar plasma, the diamond grains were always passivated with hydrocarbons and the active carbon species in the plasma can only renucleate to form nanocrystalline diamond grains. Incorporation of H2 species in the plasma leads to partial etching of hydrocarbons adhered onto the diamond grains, such that active carbon species in the plasma can attach to diamond surface anisotropically, resulting in diamond flakes and dendrites geometry.

Enhanced electron field emission (EFE) properties have been observed for ultrananocrystalline diamond (UNCD) films grown on Au-coated Si (UNCD/Au-Si) substrates. The EFE properties of UNCD/Au-Si could be turned on at a low field of 8.9 V/μm, attaining EFE current density of 4.5 mA/cm(2) at an applied field of 10.5 V/μm, which is superior to that of UNCD films grown on Si (UNCD/Si) substrates with the same chemical vapor deposition process. Moreover, a significant difference in current-voltage curves from scanning tunneling spectroscopic measurements at the grain and the grain boundary has been observed. From the variation of normalized conductance (dI/dV)/(I/V) versus V, bandgap of UNCD/Au-Si is measured to be 2.8 eV at the grain and nearly metallic at the grain boundary. Current imaging tunneling spectroscopy measurements show that the grain boundaries have higher electron field emission capacity than the grains. The diffusion of Au into the interface layer that results in the induction of graphite and converts the metal-to-Si interface from Schottky to Ohmic contact is believed to be the authentic factors, resulting in marvelous EFE properties of UNCD/Au-Si.