Enhancement of the VIS-NIR absorption in a sulfurated-high-entropy film

This paper introduces a simple method for a high-entropy film to achieve a high optical absorptance in the visible-near-infrared (VIS-NIR) range by means of a sulfuration process. The as-deposited (NiCrCuFeSi)O films were annealed together with S powders in a vacuum and then desulfurized under the conditions of argon flow protection. Rutherford backscattering spectroscopy results confirm that the S element appears in the sulfurated films (NiCrCuFeSi)OS (referred to as (NCCFS)OS below). Furthermore, the distribution of elements along the depth of the films is no longer uniform but gradient. X-ray diffraction indicates that the (NCCFS)OS films have the same face-centered cubic structure as the as-deposited films, while the CuS and SiO2 type phases may be generated after sulfuration. Scanning electron microscopy reveals the formation of island-like structures on the surface of the (NCCFS)OS films. The VIS-NIR spectrum shows that the (NCCFS)OS films achieve a high absorptance with low reflectance and low transmittance. After sulfuration, the average absorptance of the (NCCFS)OS films can reach up to 0.9 in the range of 300-1700 nm. The high absorptance of the (NCCFS)OS films may be due to joint contributions of the formation of CuS, gradient distribution of elements, and island-like structures.