Dynamic morphology of mesoscopic pseudoisocyanine J aggregates on mica induced by humidity treatments

The effect of humidity treatments on the surface morphology of spin-coated pseudoisocyanine (PIC) dye films on mica was investigated by absorption spectroscopy, fluorescence microscopy, and atomic force microscopy (AFM). We demonstrated that the humidity treatment caused dynamic changes of mesoscopic J aggregate morphology due to the mobile nature of PIC molecules on mica surface. Under dry condition, lots of microcrystalline PIC J aggregates were isolatedly distributed on mica. In a low-humidity treatment (32% relative humidity (RH)), characteristic branch-like and patchy leaf-like J aggregate islands could be observed together with non-emissive disordered aggregates. The J aggregate islands grew to possess large patchy leaf-like morphology in a high-humidity treatment of 65% RH. The mobile properties of PIC on a mica surface under humid environments can be explained by capillary condensation of water around hydrophilic microcrystals on the surface and consequent disjoining pressure.