Fullerene-like carbon nitride thin solid films

Sammanfattning: The subject of this thesis is to establish synthesis-structure-property- performance relations for nano structured carbon nitride thin solid films. This material consists of bent and frequently intersecting nitrogen-containing graphite basal planes, where nitrogen plays an important role for the initiation of inter-planar cross-links and the induction of curvature via pentagon formation, much like in Buckminster Fullerenes - hence, the terminology "fullerene-like carbon nitride". The work presented establishes the existence of a substantial number of preformed CxNy species as a key requirement for the fullerene-like structure evolution. These species are stabilised upon aligned incorporation into evolving sheets against subsequent desorption via C2N2, which has a selective effect for the formation of planar structures. It is shown that besides the existence of precursors, the morphology of the sheet-like structures in terms of plane alignment and extension depends strongly on the selectivity of the etching process and, thus, is defined by the formation probability of C2N2. This is determined by the reaction pathways and, thus, the number and complexity of the arriving CxNy species and the ad-atom (molecule) mobility as given by substrate temperature and ion assist energy. Reactive magnetron sputtering of carbon in a nitrogen-containing atmosphere is shown to comply with the requirements for the fullerene-like structure evolution. Thus, the dependence of the resulting structures in terms of plane extension, alignment and cross-linking on the variation in chemistry and flux of preformed species (N2 fraction in the discharge) as well as ad-atom mobility (substrate temperature, ion flux and energy) was established. Fullerene-like CNx is a predominantly sp2-hybridised material with 20-25 at% nitrogen structurally incorporated either substitutional in a graphite sheet or in a pyridine-like manner, which initiates curvature and cross-linking, respectively. This structure in turn retains the extraordinary in-plane strength of the hexagonal carbon network and extends it into three dimensions via buckling and inter-planar cross-linking. It is shown in this thesis that the fullerene-like microstructure gives rise to an outstanding mechanical behaviour, with a high compliance due to the high compressibility of the inter planar lattice spacing, but a low tendency to plastic deformation due to strong intra and inter-planar bonds. Thus, this class of materials deforms by reversible buckling and compression of sheets via bond angle deflection and rotation rather than bond breaking, whereas the mechanical response is defined by the extension, alignment and crosslinking of sheets. The high compliance and low plasticity alongside with the low coefficient of friction provides for a promising thin film material for tribological applications.