Fabrication of large-area two-dimensional colloidal crystals.

Two-dimensional crystalline colloidal arrays (CCAs) have numerous applications, such as photonic crystal materials and sensors. Recently, numerous methods have been developed to prepare well-ordered 2D assemblies. These methods include spin coating, dip coating, and electrophoretic deposition of colloidal particles on planar substrates to form 2D nanoparticle arrays. Particle assembly at interfaces can also be used to form 2DCCAmonolayers. 2D particle arrays have previously been prepared on water surfaces. For example, Giersig and co-workers reported on the self-assembly of polystyrene (PS) latex particles on a water surface by applying a particle suspension to a clean silicon wafer, immersing the silicon wafer in water, and then consolidating the particle array by addition of surfactant. Through this method a few square centimeter 2D arrays were obtained. Retsch et al. used a similar approach to prepare close-packed 2D colloidal crystals by floating and re-depositing colloidal monolayers at the air/water interface. Qi and co-workers modified this approach by dropping an aqueous ethanol particle suspension onto a solid substrate that was surrounded by water. The dispersion spread on the water surface, resulting in monolayer 2D films up to about 16 cm in area. Pan et al. developed a method to fabricate closepacked monolayer arrays of latex particles on a stirred water surface. Then they transferred the arrays onto a substrate. The maximum sizes of the 2D array developed by the above methods appear limited to tens of square centimeters. There is a need to develop methods to prepare 2D arrays of larger sizes. For example, Jeong et al. reported amethod to form largearea 2D arrays based on a wire-wound rod coating method. Recently, Vogel et al. presented a method to produce closepacked monolayers by using a glass slide to add colloid suspensions to the air–water interface. Close-packed monolayer patches crystallize upon spreading of an ethanolic PS colloid dispersion. Upon further addition of colloid, the patches eventually assemble into a monolayer covering the complete water surface. Our group fabricated large 2D arrays by spreading alcohol/water solutions of colloidal particles on top of mercury surfaces. We obtained wellordered 2D arrays by self-assembly of colloidal particles on mercury as the solvent evaporated. These 2D arrays could easily be attached to hydrogel films. Although this method was very successful in preparing large-area 2D arrays, the use of mercury as a substrate is undesirable because of its toxicity. Here, we report on fast fabrication of large 2D arrays of colloidal particles at the air/water interface by a needle tip flow method. The preparation of the 2D monolayer CCAs by this approach is illustrated in Figure 1a. The tip of a sharpened