Microsphere Lithography on Hydrophobic Surfaces for Generating Gold Films that Exhibit Infrared Localized Surface Plasmon Resonances

Abstract

Evaporation induced self-assembly is an established method for producing close-packed two-dimensional sphere masks on hydrophilic surfaces such as glass. In sphere lithography, gold or silver is deposited over sphere masks to generate a film-over-nanospheres or a nanoprism array that can be used as a sensing surface in localized surface plasmon extinction and surface enhanced Raman spectroscopy experiments. Sphere lithography is less commonly used to prepare sphere masks on hydrophobic surfaces associated with infrared window materials, in part because it is challenging to find solvents with wetting and evaporation characteristics that are appropriate for such surfaces. This wetting challenge can be overcome with appropriate surfactants. However, surfactant residues are then left behind on the sensing surface. We report methods for depositing monolayer crystalline sphere masks onto CaF2 windows that minimize surfactant residue by either using ethanol as a volatile cosolvent that enhances wetting, or by increasing the concentration of colloid to compensate for the reduced attractions between spheres and surface. The rate of evaporation of solvents from the colloid drop is controlled by fixing the headspace partial pressure of ethanol and/or water.