Ultra-Thin Metal Films for Enhanced Solar Absorption
  This paper presents modelled results for optical absorption in ultra-thin films of nickel, gold and silver across the solar spectrum. It is found in the case of nickel there is an optimum thickness for maximum solar absorption around 10-13nm. It is believed that this is a result of the real and imaginary parts of its refractive index being of similar magnitude across the solar spectrum which can give rise to very strong thin film absorption. 
N. Ahmad, J. Stokes, N. A. Fox, M. Teng and M. J. Cryan
physics.optics