Assembly of Mesoscale Particles over Large Areas and Its Application in Fabricating Tunable Optical Filters

This paper describes a method for the crystallization of mesoscale particles over areas as large as ∼1 cm2. We injected an aqueous dispersion of spherical particles into a cell formed by two glass substrates and a square frame of photoresist that had been patterned on the surface of one of the substrates. One side of the frame had channels on its surface that could retain the particles while letting the solvent flow through. External gas pressure and sonication drove the particles into a cubic-close-packed (ccp) assembly with the (111) face parallel to the surfaces of the glass substrates. The smallest particles that have been crystallized by this method are polystyrene beads of ∼60 nm in diameter. The procedure presented here offers a number of attractive features: (i) It is relatively fast. For example, polystyrene beads of 0.48 μm in diameter can be crystallized into a 25 layer assembly over an area of ∼1 cm2 in ∼48 h. (ii) It has a tight control over the surface morphology and the number of layers of the crystalline assemblies. (iii) It works for aqueous dispersions of a wide range of spherical particles regardless of their chemical compositions and/or surface properties (such as charge densities and chemical functional groups). We have also demonstrated fabrication of tunable optical filters from these crystalline assemblies of particles. Each crystalline assembly effectively rejects a narrow wavelength interval (as determined by the Bragg condition) in the spectral region ranging from ultraviolet to near-infrared. The tunability (in terms of wavelength) of the filter can be achieved by changing the angle between the incident light and the normal to the surface of the filter, or by using particles with different diameters.