Abstract
This critical overview analyzes the relations between deposition conditions and structure for hydrogen-free carbon films, prepared by vacuum arc deposition. The manifold of film structures can be roughly divided into graphitic, nanostructured and amorphous films. Their detailed characterization uses advantageously sp3 fraction, density, Raman peak ratio and the mechanical properties (Young’s modulus and hardness). Vacuum arc deposition is based on energetic beams of carbon ions, where the film growth is mainly determined by ion energy and surface temperature. Both parameters can be clearly defined in the case of energy-selected carbon ion deposition, which thus represents a suitable reference method. In the case of vacuum arc deposition, the relation of the external controllable parameters (especially bias voltage and bulk temperature) with the internal growth conditions is more complex, e.g., due to the broad energy distribution, due to the varying “natural” ion energy and due to the surface heating by the ion bombardment. Nevertheless, some general trends of the structural development can be extracted. They are critically discussed and summarized in a hypothetical structural phase diagram in the energy-temperature plane.
Highlights
Carbon atoms have the peculiarity to realize bonding states with different dimensionality with their four valence electrons: sp3, sp2 and sp1 bonds
Physical vapor deposition (PVD) allows the preparation of pure carbon films without any other elements as hydrogen and of structures with a broad combination of the various bonding types, especially in sp2- and sp3-bonded arrangements (Carbynelike sp1 configurations are only realized by low-energy deposition, e.g., via gasphase condensation, which is not considered in this overview)
The released and recorded electrons have for carbon films only a very small escape depth, leading to an information depth of about 3 nm for Auger electron spectroscopy (AES) and 10 nm for X-ray photoelectron spectroscopy (XPS) [20]
Summary
Carbon atoms have the peculiarity to realize bonding states with different dimensionality with their four valence electrons: sp, sp2 and sp bonds. The structural characterization demands information on the dimensions, the preferred orientations and the distortions of the graphene stacks and on the usually existing nanoporosity Essential properties such as hardness and electrical resistivity are determined by the three-dimensional long-range connectivity of the sp network. It represents a composite of graphitic hexagons, of further sp bonded atoms, disordered in olefinic chains and dimers, and of some sp3-bonded atoms. Amorphous carbon films with high sp fractions s ≥ 50% (ta-C, tetrahedrally bonded amorphous carbon) are of special interest They are superhard with hardness values above 40 GPa. Structure extended stacks of graphene lamellae, possibly with preferential orientation, possibly with nano-/microporosity amorphous sp2 + graphitic hexagons (+sp3), possibly with nanoporosity amorphous sp3 + sp2 + (graphitic hexagons). Coatings 2022, 12, 109 are less complex. (3) a broad field of carbon structures can be realized by modified deposition conditions
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