Effect of CH4 concentration on the growth behavior, structure, and transparent properties of ultrananocrystalline diamond films synthesized by focused microwave Ar/CH4/H2 plasma jets

► Effects of CH4 concentration on growth and properties of UNCD films were studied. ► Nanodiamond films transformed from UNCD to NCD by increasing CH4 concentration. ► Grain size and nondiamond contents in nanodiamond films concurrently increased. ► Modification of UNCD films was reconfirmed by transmittance studies.