The effects of CH4 concentration (0.5-5%) on the growth mechanisms, nanostructures, and optically transparent properties of ultrananocrystalline diamond (UNCD) films grown from focused microwave Ar/CH4/H-2 (argon-rich) plasma jets were systematically studied. The research results indicated that the grain size and surface roughness of the diamond films increased with increasing CH4 concentration in the plasma jet, however, the nondiamond contents in films would not be correspondingly decreased resulting from the dispersed diamond nanocrystallites in the films synthesized at higher CH4 concentration. The reason is due to that the relative emission intensity ratios of the C-2/H-alpha and the CH/C-2 in the plasma jets were increased and decreased with increasing CH4 concentration, respectively, to lower the etching of nondiamond phase and the renucleation of diamond during synthesis. The studies of transmission electron microscopy demonstrated that, while the CH4 introduction of 1% into the plasma jet produced the UNCD films with a spherical geometry (4-8 nm) and the CH4 introduction of 5% into the plasma jet led to the elongated (similar to 90 nm in length and similar to 35 nm in width) grains in the nanocrystalline diamond (NCD) films with a dendrite-like geometry. The transmittance of diamond films was decreased gradually by films transition from UNCD to NCD, resulting from the enhanced surface roughness and nondiamond contents in films to concurrently increase the light scattering and absorption during photon transmission. (c) 2013 Elsevier B.V. All rights reserved.