Although metals are commonly shiny and highly reflective, we here show that thin metal films appear black when deposited on a dielectric with antireflective moth-eye nanostructures. The nanostructures were tapered and close-packed, with heights in the range 300-600â €‰nm, and a lateral, spatial frequency in the range 5-7â €‰Î 1/4m â '1. A reflectance in the visible spectrum as low as 6%, and an absorbance of 90% was observed for an Al film of 100â €‰nm thickness. Corresponding experiments on a planar film yielded 80% reflectance and 20% absorbance. The observed absorbance enhancement is attributed to a gradient effect causing the metal film to be antireflective, analogous to the mechanism in dielectrics and semiconductors. We find that the investigated nanostructures have too large spatial frequency to facilitate efficient coupling to the otherwise non-radiating surface plasmons. Applications for decoration and displays are discussed.
Bibliographical noteFunding Information:
The work was supported by the NanoPlast project funded by the Danish National Advanced Technology Foundation (File No.: 007-2010-2) and by the European Commission via the FP7 MMP Integrated project PLAST4FUTURE (NMP2-SE-2012-314345). The work was also supported by the Danish Agency for Science, Technology and Innovation (International Network Programme, Israel-Danish international collaboration, Framework Grant 1370-00124A).