Crack patterns in superlattices made of maghemite nanocrystals.

Here, 11 nm γ-Fe2O3 nanocrystals characterized by a low size distribution of 5% and dispersed in chloroform are deposited onto a substrate as well-defined films. The film thickness is controlled by the initial concentration of the colloidal solution. After drying, the film shows a network of well-defined cracks. It is demonstrated using scanning electron microscopy and small-angle X-ray diffraction that the nanocrystals within the film are self-assembled in ordered lattices. It is shown that the nanocrystals self-assemble in superlattices before the cracks appear. In contrast to what was previously observed with a disordered film of maghemite nanocrystals with large size distribution, the top surface of the films is covered with patterns formed before crack formation. The cracks preferentially follow the direction of the hexagonal array observed on the top surface of the film by using a field emission gun scanning electron microscope. The average crack distance as a function of the film height follows the same linear scaling law as in amorphous nanocrystal assemblies. The size of superlattices of nanocrystals evolves with film height, but is usually significantly smaller than the average crack distance except on the border of the films.