Selective Assembly of Colloidal Particles on a Nanostructured Template Coated with Polyelectrolyte Multilayers

Abstract

The development of twoand three-dimensional (2D/3D) structures made up of colloidal particles has received considerable attention with respect to their potential applications in biochip devices and sensors, optoelectronic devices, and photonic bandgap materials. These device applications require precisely controlled and uniformly self-assembled structures over large areas. Template-assisted organization with a physically or chemically patterned substrate has been a promising approach due to its advantages in creating longrange order with fewer defects, and the engineering of a new crystalline structure. Microor nanoscale physical patterns on a template can be prepared with photolithography, electron-beam lithography, and ion-beam etching. The resulting template allows accurate positioning and control of colloidal clusters on the nanoscale. For example, Yin et al. and Xia et al. fabricated well controlled sizes and shapes of colloidal clusters by combining topological templating and capillary forces. Cui et al. demonstrated that the physical template can be applied to the integration of nanoparticles, and developed colloidal nanocrystals on lithographically patterned substrates. However, these methods are limited to arrays of monofunctional particles due to their deficiency in differentiating between different functionalized particles. On the other hand, chemical templating has been achieved by developing self-assembled monolayers and polyelectrolyte multilayers using soft lithographic and photolithographic techniques, and these approaches can allow the introduction of two or more different functional groups on the substrate for creating multicomponent particle arrays. For example, Zheng et al. demonstrated the fabrication of two differently charged particle arrays on patterned polyelectrolyte multilayer templates; however, these processes still have some limitations with respect to the precise spatial control of particles on the chemically patterned template. As particle size is decreased, there are also challenges with flat 2D chemical patterns due to the limited region of contact between the particle and surface. Here, we examine a combination of physical and chemical patterning to achieve an optimized degree of spatial control, pattern stability, and selectivity. In this work, we present a technique for preparing chemically modified nanotemplates and their use in the development of sub-micrometer-scale particle arrays via self-organization on the surface. Recently, we developed nanostructured polymer molds using high-modulus (∼ 2 GPa at 25 °C) UV-curable photopolymers with favorable mechanical properties and excellent replication characteristics. In this previous work, we introduced this material for the preparation of physical templates of nanostructures and created functional chemical patterns on the template, using electrostatic layer-by-layer (LBL) assembly and polymer-on-polymer stamping processes. In the LBL assembly process, thin bilayers on the template are created via alternate adsorption of oppositely charged polyelectrolytes. Figure 1 illustrates the sample fabrication process employed to realize the nanotemplating and selective assembly of colloidal particles on the surface. A nanostructured template was prepared by molding the UV-curable polyurethanebased photopolymers on a silicon master substrate. Then, the template was coated with polyelectrolyte multilayers using the LBL deposition process to provide a charged functionality. The polyelectrolytes used here were poly(allylamine hydrochloride) (PAH) or poly(diallyldimethylammonium chloride) (PDAC) as polycations and sulfonated polystyrene (SPS) as a polyanion. The photopolymer surface was plasma treated prior to assembly and the polycation was absorbed first on the surface. The LBL process was repeated until 10.5 bilayers of (PAH–SPS) or (PDAC–SPS) were constructed, finishing with an outermost layer of PAH or PDAC to create a positively charged surface (layer thickness = 10.2 or 4.2 nm). Next, a flat sheet of poly(dimethylsiloxane) (PDMS) was immersed in an aqueous solution of SPS to allow adsorption of a layer of polyanion on the surface. Although C O M M U N IC A TI O N