Supracrystals are self-assembled materials that possess 3D superlattices. They are known to have a range of physical properties which are highly specific and often show unexpected behavior. Many properties of thick supracrystals (over 1000 layers) are known, and the optical properties are known to be governed by dipolar interactions within the lattice. But how the optical properties of thinner supracrystals are affected in certain circumstances is lesser known.

Marie-Paule Pileni, CEA/IRAMIS, France, and Claire Deeb, University of Paris-Sud, France, and colleagues at Sorbonne University, France, have investigated how cracks in silver supracrystal films, of 27 to 180 layers in thickness, affect the optical properties of the supracrystal. The researchers first fabricated oleylamine coated silver nanocrystals and synthesized them into supracrystals using wet chemical and deposition techniques.

The researchers found that the optical transmission of wavelengths in the 300–800 nm range was reduced as as the supracrystal film thickness increased. The researchers explain this unexpected behavior by the presence of wetting layers at the bottom of the cracks produced by a shrinking process during the formation of supracrystal films. This was confirmed by both theoretical and experimental data. The crack widths were also found to increase when the film thickness was increased, and this subsequently led to more complex wetting layers below the cracks.

• Influence of Cracks on the Optical Properties of Silver Nanocrystals Supracrystal Films,
  Jingjing Wei, Claire Deeb, Jean-Luc Pelouard, Marie-Paule Pileni,
  ACS Nano 2018.
  https://doi.org/10.1021/acsnano.8b07435