Fabrication Of Ordered Microstructures Based On Colloidal Crystals

Nano- and micro-featured structures have been attracted great attentions due to size-dependent physical effects and their potential applications, such as microvials, biosensors, microelectronic, micromechanical, and optical devices. In the previous studies, a variety of methods now exist to create nanoscale structures, especially colloidal crystal with ordered arrays of interstices affords opportunities for fabricating multifarious patterns and functional materials. Furthermore, combining with chemical deposition, selective etching, physical vapor deposition, polymerization, dewetting, or edge spreading lithography technologies, porous materials, ordered patterns and novel microstructures have been fabricated in the voids or on the surface of colloid spheres and exhibit predominant optical, electric and magnetic properties etc. As the self-assembly approach let nature do its work, the constituent components self-assemble following their favorite ways of either minimum local or global free energy or favored kinetics, to achieve close-packed structures of mechanical stability. This kind of close-packed structure showed its bottleneck on the fabrication of non-close-packed (ncp) structure. In this thesis, we fabricated two-dimensional (2D) ncp colloidal crystals using"lift-up"soft lithography technique. Based on these units, we fabricate a series of ordered microstructures. We believe this cheap, convenient, and widely applicable method will offer us a new way for micro-machining and functionality realization. In chapter 2, we prepared highly ordered 2D ncp microwell arrays on a PDMS surface by replica molding using colloidal crystals as templates. The feature sizes of the microwells in width could be flexibly controlled by the sphere interstices of the templates. The effects of the sphere interstices and chemical composition of the templates on the wetting behavior of PDMS prepolymer were studied. The wetting behavior of PDMS prepolymer on the rough surface is explained using Cassie-Baxter theory, because of the Wenzel roughness factor of the templates (r >1.7) and the high air ratio at the interface between the solid and prepolymer. The average sizes of the microwells in depth increase while the contact angles decrease with increasing sphere interstices. The prediction correlates well with experimental data.When the templates adsorb (CH3)2Cl2Si molecules, the same network structure as PDMS prepolymer, formed on the surface, can enhance the intermiscibility between PDMS prepolymer and the templates, that is, the PDMS prepolymer contact angle. Consequently, the depths of the microwells increase. When hydrophilic hydroxyl (-OH) groups are introduced onto the surfaces of the templates by plasma etching, surface roughness is induced on the PDMS molds. The distinct surface energy between PDMS prepolymer and templates leads to enhancement of the adhesion force and makes the PDMS molds break during the peeling process. The microwells on the PDMS surface here can find applications in cell patterning in the biology field or in microcontact printing as stamps. Filling the microwells with other materials may also lead to photonic or storage applications by reason of their small sizes.In chapter 3, we demonstrate a facile and efficient way to fabricate three-imensional (3D) colloidal crystals of bi-directional modulation via two steps. First, 2D ncp silica sphere arrays, which fabricated via coupling lift-up soft lithography and modified microcontact imprint (μCP), are incorporated into photoresist bulk by polymerization. The 2D ncp colloidal sphere arrays incorporated into photoresist could use as assembly units in subsequent experiments, furthermore the sphere interstice could modulate. Second, the 2D ncp colloidal crystals assemble via in situ layer-by-layer photopolymerization technology (LBLP). The space between two layer colloidal crystals could be adjusted by changing the viscosity of photoresist monomer and the adding machine pressure in LBLP process. This strategy not only bodes well the fabrication of 3D bulk material via 2D microstructure, but also is favored by the integration of various spheres (or particles) with different patterns and functions into a case due to the layer-by-layer assembly, which could facilitate the exploration in fabrication of 3D structures and optical applications.In chapter 4, we describe a novel and simple way to fabricate large scale ordered silica-bowl and silica-ring arrays or isolated ones by RIE using photoresist as a mask. With different etching time, the obtained nanostructure can be transformed from nanobowl to nanoring, and its feature size can also be precisely controlled. The changing process can be divided into three steps: (1) increasing of the opening diameter of the silica bowl; (2) increasing the depth of the silica bowl; (3) changing of geometry, from bowl to ring. Furthermore, we fabricated silver-silica composite nanobowls via deposition technology, which exhibit SERS enhancement. The obtained structures compositing with functional material have many potential applications in microvials, assembly, and biology simulation.