A Convenient Method to Produce Close- and Non-close-Packed Monolayers using Direct Assembly at the Air-Water Interface and Subsequent Plasma-Induced Size Reduction

An extremely facile approach to produce close-packed colloidal monolayers over large areas using direct assembly at the air-water interface is presented. The influence of small amounts of sodium dodecyl sulfate (SDS) as well as the influence of the pH value of the subphase on the quality of the resulting monolayer is investigated. It is found that small amounts of SDS at the interface influence capillary forces and form a soft barrier that facilitates the crystallization process. Increased electrostatic repulsion arising from a higher pH of the subphase induced a higher order using carboxylic acid functionalized particles. The deposited close-packed monolayers were subjected to plasma treatment in order to shrink the colloids and produce non-close packed monolayers with lattice spacing and symmetry reflecting the order of the initial close-packed monolayer. A detailed examination of etching conditions and their influence on the shrinkage of the particles was performed, including effects of plasma power, composition, flow rates as well as polymeric-and substrate material. The monolayers exhibit vivid coloration, which is determined by their size and packing density. UV-Vis-NIR spectroscopy was used to investigate the change of monolayer color during the size reduction of the individual particles. A simple theoretical model was elaborated to explain the optical properties. Finally, the non-close-packed monolayers were used as masks to produce gold nanostructures to exemplify the versatility of the monolayer architectures in nanosphere lithography.