In the present study, metal nitride based sputtered thin film coatings such as Mo/Si, CrN/Si, and Mo:CrN/Si were investigated for their solar selective surface and mechanical applications. Despite a large number of literature is available in the area of solar selective applications of metal nitride based thin film coatings, these materials are still to be commercialized for their practical device applications. In view of this, we chose metal nitride based thin film coatings e.g., Mo, CrN and Mo:CrN to realize their structural, morphological, elemental compositions, optical, dielectric and mechanical properties in as-deposited, and annealed conditions. Detail analyses of these features were carried out using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-Ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy and FTIR, nanoindentation and finite element modeling (FEM). In addition to these, the first principle based density functional theory (DFT) integrated with the B3LYP hybrid functional plus LANL2DZ basis sets based infrared (IR), Raman and ultraviolet-visible (UV-Vis) analysis were also carried out to probe the electronic structural and optical properties of pristine and Mo-doped CrN clusters in the non-crystalline phase.
Optical analysis showed that in the visible range of the solar spectrum, the CrN coatings exhibit the highest solar absorptance of 66% while the lowest thermal emittance value of 5.67 was recorded for the CrN coating doped with Mo. As a result, the highest solar selectivity of 9.6, and the energy band-gap of 2.88 eV were achieved with the Mo-doped CrN coatings. On the other hand, optical studies of the annealed coatings showed that with the rise in annealing temperature up to 700 °C, the solar absorptance of CrN coatings increased from 61% to 89% and slightly decreased at 800 °C, while the optical band-gap energy dropped from 2.62 to 1.38 eV but slightly increased to 1.48 eV at 800 °C. Nanoindentation results indicated that as the annealing progresses, the hardness and elastic modulus values of CrN coatings are lowered. Further optical studies of Mo-doped CrN coatings showed that as the annealing temperature increased up to 700 oC, the solar absorptance is increased from 55% to its maximum value of 86%, and the optical band-gaps were dropped from 2.48 to 1.14 eV. Nanoindentation and finite element modeling studies of Mo-doped CrN coatings indicated that as the annealing progresses, both the hardness and elastic modulus values of these coatings reduced. A first principle based DFT simulation on Cr4N4, Cr4MoN3, Cr4Mo2N2, Cr3MoN4, Cr3Mo2N3, and Cr2Mo2N4 clusters indicated that the Cr4Mo2N2 structure was chemically and energetically the most stable species among the six clusters considered. The DFT based electronic analysis revealed that Cr4MoN3 and Cr3Mo2N3 clusters possess magnetic susceptibility while the infrared (IR), Raman and ultraviolet-visible (UV-Vis) studies indicated that the clusters formed by Cr4N4 and Cr4Mo2N2 are naturally stable and able to function as light harnessing materials to be used in solar selective surfaces
