Ordered Silica Microspheres Unsymmetrically Coated with Ag Nanoparticles, and Ag‐Nanoparticle‐Doped Polymer Voids Fabricated by Microcontact Printing and Chemical Reduction

Abstract

Colloid particles usually have charges or nanoparticles uniformly distributed over their surface. The design and preparation of unsymmetrically coated colloid particles have been a long-standing challenge in surface and colloid science. These particles would remedy some limitations of their spherical counterparts for potential applications in modeling the behaviors of highly irregular colloids that are more commonly found in industrial products, in the fields that require lattices with lower symmetries and high complexities, and as potential building blocks in generating three-dimensional (3D) photonic crystals with complete bandgaps. However, due to the thermodynamic limitations of the reaction, there have been only a few reports about the preparation of microspheres unsymmetrically coated with nanoparticles, which typically involve Langmuir–Blodgett techniques, the evaporation of metals on colloidal particles, controlled phase separation, and using the gas/liquid and liquid/solid interface action. Although some of these methods are effective to fabricate unsymmetrically coated particles, challenges in this field still exist. For example, the assembly of these particles into ordered arrays, enhancement of the unsymmetrical coating density, and precise control of the tropism of these particles have not been well developed. Two-dimensional (2D) structured arrays and patterns are also important due to their potential applications in engineering microelectronic and optoelectronic devices, the fabrication of biological and chemical sensors, and for controlled crystallization. Existing, elegant approaches that involve lithography, imprinting, and soft lithography techniques have been successfully applied to create patterned surfaces in microelectronic and plastic electronics. Among them, soft lithography encompasses a set of flexible methods for patterning materials. As a branch of soft lithography, microcontact printing (lcp) has also been used to modify solid surfaces with different properties, such as charge nature and wettability, to direct the deposition of colloidal microspheres on special regions of substrates. Recently, lift-up soft lithography and modified microcontact printing methods have been developed to pattern colloidal crystals. In this communication, the fabrication of ordered silica microspheres unsymmetrically coated with Ag nanoparticles using lift-up soft lithography and chemical reduction is reported. Taking advantage of the flexibility of lcp, these microspheres are easily transferred onto polymer-coated solid substrates and precisely realize a tropism conversion. By etching away the silica microspheres, ordered Ag-nanoparticle-doped polymer voids are obtained. These silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids can also be used as templates to fabricate ordered Ag-nanoparticle-doped polymer and gold composite voids with different morphologies. Compared with previous methods, it is believed that some progress in preparing ordered microspheres unsymmetrically coated with nanoparticles and nanoparticle-doped composite voids has been made, in that: a) the unsymmetrical coating density on the microspheres is increased, b) the ordered array of these unsymmetrically coated microspheres has been realized, c) the tropism of these ordered unsymmetrically coated microspheres can be well controlled, and d) ordered nanoparticle-doped polymer or polymer and metal composite voids with different morphologies can be easily obtained. Due to this progress, this method will provide a powerful platform for fabricating ordered, versatile, colloidal microspheres unsymmetrically coated with nanoparticles, and hybrid patterns. Figure 1 outlines the procedure for preparing ordered silica microspheres unsymmetrically coated with Ag nanoparticles and Ag-nanoparticle-doped polymer voids. First, monodisperse silica microspheres are assembled into colloidal crystals on a silicon wafer. Using the lift-up soft lithography technique, a single layer of close-packed silica microspheres are then transferred onto the surface of a poly(dimethylsiloxane) (PDMS) stamp. After depositing Ag nanoparticles on these microspheres by chemical reduction and spin-coating a thin C O M M U N IC A TI O N S