Formation of Ultrathin Liesegang Patterns

For many years, it has been believed that self-organized periodic ring structures known by the name of Liesegang patterns (LPs) are formed only in quite thick media, typically thicker than at least several micrometers. Actually growing LPs in ultrathin films is extremely difficult because of the drying of film and susceptibility to rapid capillary wetting. The present work reports how we obtain successful LPs in ultrathin films of 65 nm thick. The key parameters are temperature control and the introduction of equilibrium water vapor in the sample environment. Atomic force microscope images clearly showed that the LPs are composed of 300-600 nm laterally coagulated particles. We have also evaluated the densities and thicknesses of the ultrathin films by X-ray reflectivity. During the present research, new patterns, which are different from ordinary LPs, have been discovered for the first time in the outermost part of the whole pattern. Studying LPs in ultrathin films may help to forge a better understanding of the mechanism underlying the intriguing phenomenon. Because of nanoscale scale thicknesses, self-organized periodic structures including so-called LPs will open up new opportunities in nanotechnologies.