In many applications, thin films are coated onto the surface of materials to modify their mechanical, thermal, electrical, and optical properties. The coatings must be highly ordered two-dimensional crystals with a specific lattice spacing which is application dependent. For example, to make a highly-efficient anti-reflection coating for solar cells, the surface is textured with a periodic pattern with the wavelength comparable to that of solar radiation. This can be achieved by coating the surface by a thin film with a monolayer of particles embedded on its surface. The particle size and the spacing between particles can be selected for obtaining the desired pattern. These particle coated thin films can also be used for making masks for nano-lithography, textured hydrophobic surfaces, and membrane filters with regular pores. The membrane filters could be used to precisely control the mass transfer rate of a drug-delivery patch, and to separate proteins and other macromolecules based on their sizes.
Capillarity-driven clustering at fluid interfaces is a well-established method for forming monolayers, but those monolayers are poorly ordered. In the proposed technique an electric field in the direction normal to the interface is applied to control the self-assembly process so to obtain a virtually defect-free monolayer of electrically neutral nano-particles. When there are two or more types of particles present, there is a hierarchical order in the assembled monolayers which can be optimized by selecting a suitable fluid. The technique also allows us to vary the lattice spacing as appropriate to the application. The monolayer is then frozen onto the surface of a thin flexible film which can be transferred to the surface of a material to modify its surface properties. The technique is easy to implement and can be applied to a broad range of particle sizes and types with a high level of controllability which will be advantageous in many applications.
