Nanostructuring tetrahedral carbon films


Student thesis: Master's Thesis

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  • Wing Yan LUK


Awarding Institution
Award date4 Oct 2004


Tetrahedral amorphous carbon (ta-C) is extremely hard and smooth material on a nanoscale, and possesses a set of other extraordinary properties such as high electron field emission and good electrochemical properties. Because of these unique properties great interest has been stimulated in both scientific and industrial fields. In term of these aspects, the presented work aims at the controlled and systematic fabrication, modification and characterization of the novel nanostructure induced in ta-C films by energetic ions. The formation of novel nanostructures in the matrix of tetrahedral amorphous carbon (ta- C) films was induced by energetic ion beam exposure to different ion doses. The structural diversity was investigated not only on the ion dose but also energy and types of ions. At lower energy implantation carbon ion beams were use, however, at higher energy ion beam irradiation, argon ion beams were employed. Tetrahedral amorphous carbon (ta-C) films were deposited by a Filtered Cathodic Vacuum Arc (FCVA) and subsequently implanted by carbon ion beams being extracted from a Metal Vapor Vacuum Arc (MEVVA) ion source and accelerated to either 25 or 50 keV. In the second case of argon ion irradiation the energy was preset to 850 keV, which provided the range of argon ions larger than the thickness of ta-C films. The novel materials engineered with such ion beam processes then were examined with different characterization techniques. The structural analysis of Raman spectra indicates that originally abundant sp3 carbon atomic bonding of ta-C is gradually converted to a graphitic phase during the course of ion bombardment. The local order, growth and clustering the sp2 bonded carbon atoms in the ta-C films by ion implantation is also indicated by Raman spectroscopy. Highresolution transmission electron microscopy and transmission electron diffraction were used to evaluate the nanostructures induced in ta-C matrix. The analysis of implanted amorphous carbon films on an atomic scale shows the formation of structure with the higher degree of order. The graphitic basal planes are formed preferably along the ion tracks. The critical rearrangement is observed at 0.24 displacements per atom (dpa) when the onset of the transformation occurs. Unlike amorphisation of crystal structure by ion bombardment, the initially amorphous phase with short ordered sp3 bonding is nanostructured to the higher degree of an ordered structured using proper ion energies and doses. The structural rearrangement gives rise to the change in resistivity varying from original 108 to 10-4 Ωcm at the highest dose (1017 ions/cm2) of carbon ion implantation and to 10-3 Ωcm of the highest dose (1017 ions/cm2) of argon irradiated ta-films. Resistivity measurements indicate fundamental changes in the conduction mechanism with the increase in ion dose. The work discusses that the transformation process is associated with the change from interband to intraband absorption due to overlapping conduction and valence bands, which is the indicative of transforming a semi-conducting material to semi-metallic nanocomposite material. The optical bandgap Eg can be correlated to the structural properties. Carbon ion implantation to carbon sp3 bonded network leads to higher density of π-bonded clusters and expansion in their size, which is associated with inducing new nanostructures and consequent narrowing the optical bandgap. The hardness of carbon implanted ta-C films, investigated by nano-indentation, decreases with the increase in ion dose due to the graphitization formally sp3-rich carbon films. Xray photoelectron spectroscopy analysis of plasmon loss peaks in dependence on the ion dose and particle energy reveal variation of sp2 and sp3 content in nanostructured ta-C films. For example, carbon ion implantation at 25 keV to a dose of 1017 ions/cm2 reduces the content of sp3 hybridized bonding from original 83% in as deposited ta-C films to 52%. However, the argon irradiation at 850 keV to the same dose (1017 ions/cm2) decreases the content of sp3 bonding even to 30% only. On the other hand the root mean square (RMS) roughness of the nanostructured films increased with increasing ion dose and was 39.1 nm at the argon ion dose of 1 × 1017 ion/cm2 as found by Atomic Force Microscopy (AFM). The nanostructured material also becomes electrically conductive, optically opaque and retains considerable amount of sp3 bonding and high hardness above 15 GPa. Since the structuring of ta-C by energetic ions yield carbon material enriched in sp2 bonding configuration which is possible obtained by exposure of polymeric material, polymethyl metacrylate (PMMA) was also irradiated with 850 keV argon ion beam to different doses and compared with the results obtained at ta-C exposure. In summary this study shows the methodology of formation novel nanostructures in ta-C matrices, and provides understanding, mechanism and prediction of their formation. The work indicates controlled and practical tailoring nanostructures suitable for a variety of potential industrial applications.

    Research areas

  • Diamond thin films, Nanostructured materials