Black gold layers: preparation via thermal evaporation, material and optical properties, and application potential for gas sensors

Abstract

Nanostructured black metal (BM) layers represent a valuable material for many technological applications. Their unique properties resulting from their structure have led to their use as electromagnetic absorbers, gas sensing devices, and electronics. Although black gold films have been extensively studied, there is still a gap in the use of evaporated BM films for sensor applications. This paper compares the material and optical properties of reflective and black gold films, both of which were prepared by applying the thermal evaporation technique in a residual argon atmosphere. The impact of the nanostructured surface is discussed from the perspectives of morphology, chemical composition, emissivity, reflectivity, and absorbance. Microscopic analyses revealed that black gold comprises nanograins aggregated into a cauliflower-like structure. Positron annihilation spectroscopy reported a high concentration of pores in the black gold layer with a mean size of 1.1 nm. This highly porous structure allows high light absorbance in the visible wavelength range and strong electron plasma interactions in the infrared wavelength range. Preliminary results for gas sensing response to NO₂, CH₃CH₂OH, NH₃ and CO reveal the promising use of black gold coating for sensor applications.