Surface defects in polyelectrolyte multilayers: Effects of drying and deposition cycle

Layer-by-layer deposition has been used extensively to fabricate ultrathin films, yet some fundamental aspects of this seemingly simple methodology remain unclear, such as defects and factors controlling their formation. In this work we use charged gold nanoparticles as a probe to assess charge distribution on polyelectrolyte multilayer surfaces. This straightforward approach revealed that while excess charges are present rather uniformly throughout most of the surface, there are isolated domains free of excess charges, i.e. surface defects. Analysis of defect size distribution indicated that the defects are formed via desorption of individual polyelectrolyte chains from the surface. It was further revealed that intermediate drying in between polyelectrolyte depositions can reduce surface defect by >90%, resulting in more coherent films. Area fraction of defects was found to decrease with the number of layers deposited. These surface defects result in significant contact angle hysteresis.