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A tough material commonly used in coatings is diamond-like carbon (DLC), that is, amorphous carbon with content in four-fold coordinated C higher than $70%, and its composites with metal inclusions. This study aims to offer useful guidelines for the design and development of metalcontaining DLC coatings for solar collectors, where the efficiency of the collector depends critically on the performance of the absorber coating. We use first-principles calculations based on density functional theory to study the structural, electronic, optical, and elastic properties of DLC and its composites with Ag and Cu inclusions at 1.5% and 3.0% atomic concentration, to evaluate their suitability for solar thermal energy harvesting. We find that with increasing metal concentration optical absorption is significantly enhanced while at the same time, the composite retains good mechanical strength: DLC with 70-80% content in four-fold coordinated C and small metal concentrations (<3 at. %) will show high absorption in the visible (absorption coefficients higher than 10 5 cm À1) and good mechanical strength (bulk and Young's modulus higher than 300 and 500 GPa, respectively).

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We present in this work a theoretical framework based on the tight-binding method, which is able to probe at a local atomic level the optoelectronic response of nanomaterial systems and link it to the associated disorder. We apply this methodology to carbon nanocomposites containing diamond nanocrystals. We find that significant structural and topological disorder exists at the interface between the nanodiamonds and the embedding amorphous carbon matrix. This can be quantitatively probed by extracting the Urbach energies from the optical parameters. Disorder in the nanocrystals appears in their outer shell near the interface and is manifested as bond length and angle distortions. Energetics and stability analysis show that nanodiamonds embedded in matrices with high density and high fraction of fourfold coordinated atoms are more stable.

We study theoretically the equilibrium structure, as well as the response under external load, of characteristic carbon-based materials. The materials considered include diamond, amorphous carbon (a-C), "amorphous diamond" and nanocomposite amorphous carbon (na-C). A universal bulk-modulus versus density curve is obeyed by all structures we consider. We calculate the dependence of elastic constants on the density. The strength of a-C was found to increase in roughly a linear manner, with increasing concentration of four-fold atoms, with the maximum stress of the strongest a-C sample being about half that of diamond. The response of na-C to external load is essentially identical to the response of the embedding a-C matrix.