Mechanism behind the formation of self-assembled nanosized clusters in diamond-like carbon nanocomposites

Non-hydrogenated high sp³ content diamond-like carbon (DLC) has been subjected to intensive research efforts over the last three decades owing to its superior material properties. By introducing some metals/dopants into DLC film during deposition, the microstructure and properties of the films can be further modified, where studies have demonstrated it as an effective method to reduce the internal stress and consequently improve the adhesion and mechanical properties of the film. Recently, there are reports that some of the dopants actually formed independent clusters of nano-dimensional scale within the carbon matrix. This nanocluster formation was reported for both elements such as Ni, Cu, Er and composites such as ZnO. This study investigates the mechanism and effects of metal ions during carbon deposition. With a combined analysis using analytical methods (Saha´s equation and coefficient of absorption, ?_p), the presence of metals, e.g. Cu and Er are observed to increase the energy absorbed during deposition by the nanocomposite film. We complement the analytical solution with known theories such as the thermal spike model and TRIM Simulation, and we compare the findings with experimental results from Cu- and Er-containing carbon nanocomposite films obtained from pulsed laser deposition and filtered cathodic vacuum arc. Using well established characterization techniques such as Atomic Force Microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Microscratch test (MST), we show that as the ion energies increase with the increasing metal content, more energy was delivered to the surface of the films during growth process. This excess energy allows the energetic metal species to diffuse and bond with each other. The diffusion of the metal species leads to the formation of nano-clusters (islands) thus causing the roughness to increase. Both XPS and TRIM analysis further hinted that the presence of energetic metal sp- ecies may further force the carbon ions to react with the interface forming silicon carbide bonds.