Microstructural evolution as a function of substrate temperature (T S) for conducting ultrananocrystalline diamond (UNCD) films is systematically studied. Variation of the sp2 graphitic and sp3 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 °C well-defined acicular structures evolve. These nanowire structures comprise sp3 phased diamond, encased in a sheath of sp2 bonded graphitic phase. T S causes a change in morphology and thereby the various properties of the films. For T S = 800 °C the acicular grain growth ceases, while that for T S = 700 °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 °C are the most conducting, with conduction mediated through the graphitic phase present in the films.

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Grain boundaries and microstructures of ultrananocrystalline diamond (UNCD) films are engineered at nanoscale by controlling the substrate temperature (TS) and/or by introducing H2 in the commonly used Ar/CH4 deposition plasma in a microwave plasma enhanced chemical vapor deposition system. A model for the grain growth is proposed. The films deposited at low TS consist of random/spherical shaped UNCD grains with well-defined grain boundaries. On increasing TS, the adhering efficiency of CH radical onto diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nanosize diamond clusters break due to hydrogen abstraction activated, rendering the diamond phase less passivated. This leads to the C2 radical further attaching to the diamond lattice, resulting in the modification of grain boundaries and promoting larger sized clustered grains with a complicated defect structure. Introduction of H2 in the plasma at low TS gives rise to elongated clustered grains that is attributed to the presence of atomic hydrogen in the plasma, preferentially etching out the t-PA attached to nanosized diamond clusters. On the basis of this model a technologically important functional property, namely tribology of UNCD films, is studied. A low friction of 0.015 is measured for the film when ultranano grains are formed, which consist of large fractions of grain boundary components of sp(2)/a-C and t-PA phases. The grain boundary component consists of large amounts of hydroxylic and carboxylic functional groups which passivates the covalent carbon dangling bonds, hence low friction coefficient. The improved tribological properties of films can make it a promising candidate for various applications, mainly in micro/nanoelectro mechanical system (M/NEMS), where low friction is required for high efficiency operation of devices.

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The temperature dependent electron spin resonance (ESR) studies on Bi(1−x)CaxMnO3 (x=0.4, 0.45, and 0.5) having a triclinic structure were carried out to understand the spin dynamics, short-range magnetic interactions, and micromagnetic phase formation in these materials. The phase transition into charge-ordered (CO) state takes place at temperatures TCO=313K (for x=0.4 and 0.5) and at 317K (for x=0.45) samples. The data are explained in view of the existence of mixed valence clusters of Mn3+–Mn4+ ions coupled by strong short-range ferromagnetic (FM) double-exchange interactions in the paramagnetic state. The orbital ordering changes from FM to antiferromagnetic (AFM) type ∼190K (for x=0.4 and 0.5) and ∼180K (for x=0.45). The Néel Temperatures TN are 163K (for x=0.4 and 0.5) and 153K (for x=0.45). The temperature independence of ln (DI) double integrated intensity of the ESR resonance below 125K is ascribed to the existence of FM ordered microinhomogeneities embedded in the AFM ordered bulk sample. The TCO and TN values are found to be weakly dependent on the composition. The temperature dependence of ESR linewidth near TN has been analyzed in view of the spin-spin relaxation near magnetic critical points, as given by Seehra and Huber [AIP Conf. Proc. 24, 261 (1975)].