Origin of a needle-like granular structure for ultrananocrystalline diamond films grown in a N2/CH4 plasma

Microstructural evolution as a function of substrate temperature (T-S) for conducting ultrananocrystalline diamond (UNCD) films is systematically studied. Variation of the sp(2) graphitic and sp(3) diamond content with T-S in the films is analysed from the Raman and near-edge x-ray absorption fine structure spectra. Morphological and microstructural studies confirm that at T-S = 700 degrees C well-defined acicular structures evolve. These nanowire structures comprise sp(3) phased diamond, encased in a sheath of sp(2) bonded graphitic phase. T-S causes a change in morphology and thereby the various properties of the films. For T-S = 800 degrees C the acicular grain growth ceases, while that for T-S = 700 degrees C ceases only upon termination of the deposition process. The grain-growth process for the unique needle-like granular structure is proposed such that the CN species invariably occupy the tip of the nanowire, promoting an anisotropic grain-growth process and the formation of acicular structure of the grains. The electron field emission studies substantiate that the films grown at T-S = 700 degrees C are the most conducting, with conduction mediated through the graphitic phase present in the films.