Fabricating Functional Microstructures On The Basis Of Colloidal Crystals

New features that revealed by micro/nano-scale functional materials, as well as the special nature caused by the interaction between nanostructures have provided the developing motive for miniaturization of feature size and attracted great attentions. The abilities of preparing micro/nano-structure have become the key factors to fabricate nano-scale devices and new materials. Facing these demands, a large number of technologies for designing micro/nano-structures have developed rapidly. Conventional lithography, electron beam lithography, scanning probe, X-ray lithography techniques have been widely used to construct patterns. However, there are many limits, such as inefficient, costly and not suitable for mass production. Soft lithography, which included microcontact printing, replica molding, microtransfer molding, micromolding in capillaries and solvent-assisted micromolding, is procedurally simple, low-cost, high-resolution and high-repetition-the utilization. An elastomeric stamp or mold is the key element that transfers the pattern to the substrate, which limits their application to a certain extent. With the development of colloidal crystals, the ordered structure of colloidal crystals becomes an important research of patterning technologies. Especially, the combination of the colloidal crystals as masks and deposition, vapor deposition, plasma etching can be employed to build a series of two-dimensional or three-dimensional ordered patterns. It is an important step to promote the development of pattern technology. In this paper, we prepared non-close-packed (ncp) colloidal crystals with controlled lattice spacing and lattice structures. Combined these ncp colloidal crystals as templates with plasma etching techniques, functional arrays were fabricated and the properties of these arrays were characterized.In chapter 2,followed by a modified microcontact printing (μcp) technique we have reported, the as-prepared 2D ncp microsphere arrays were transferred onto a flat substrate coated with a thin film of poly(vinyl alcohol) (PVA). After removing the PVA film by calcination, the ncp arrays that fell on the substrate without being disturbed could be lifted up, deformed, and transferred again by another PDMS stamp, therefore the lattice feature could be changed step by step. Since the solvent-swelling behavior of PDMS is isotropic, the lattice spacing of colloidal crystal increase uniformly in all directions. By controlling the cycling times, the lattice spacing of the ncp arrays can be increased gradually to a desired value. Alternatively, with mechanical deformation of PDMS, the lattice spacing will only be enlarged along the stretching direction, which will change the lattice structure. Combination of these two strategies allows for continuous changing of both the lattice spacing and the crystal lattice structure, which will greatly enrich the complexity of 2D ncp colloidal crystals. It is well known that the lattice structure and lattice spacing act on manipulating the photonic bandgaps of photonic crystal, which play a significant role in the realization of integrated optical circuit devices. Our method provides the possibility for the preparation of all five Bravais lattice structures and the regulation of lattice parameters in the same lattice structure. Based on the experimental results, the correspondence relationships between experimental operation and lattice structures can be quantified via curve simulation. We can implement experimental steps to obtain any given ncp hexagonal array designed. Besides the 2D deformation behavior of PDMS, patterned PDMS stamp can be deformed vertically under capillary force and mechanical stress. By applying the deformation to lift-up-transfer process, one-dimensional arrays were obtained on the substrate. We simply fix all other conditions and tune the elastic module of PDMS elastomer and orientation differences between PDMS and 2D colloidal crystal, a variety of one-dimensional arrays were fabricated. These novel structures could be used as molds in colloidal crystal for patterning other materials such as porous inorganic films and nanowires as well as prototype models in theory simulation fields for optical materials.In chapter 3, based on the established technique to fabricate ncp colloidal crystals, we further expanded the template application of ncp colloidal crystals. Combined with the reactive ion etching, fluorescent polymer film can be developed into ncp fluorescent nanorod arrays. Compared with the fluorescent polymer film, the fluorescence intensity of nanorod arrays is increased significantly. We have two mechanisms for the improved fluorescence intensity associated with ncp arrays. The surface patterns allow Bragg scattering and prevent light trapping, permitting the internal spontaneous emission to escape. On the other hand, the ncp arrays can create band structure. The band structure essentially relies on the coupling of light that is trapped within the high index device layers to free-pace via the creation of leaky modes by periodic structuring. The ability to control the dispersion of these leaky modes by tailoring the band structure also provides a powerful mechanism to redirect the emitted light into certain preferred directions, where it can be detected with greater efficiency. Under the same swelling process, we employed microspheres with different diameters and obtained nanorod arrays with the same period/diameter. Subsequently, by employing microspheres with the same diameter, nanorod arrays with different lattice spacing were obtained. We observed the effect of nanorod diameter and lattice spacing on fluorescent intensity. Furthermore, we analyzed the fluorescent intensity of our nanorod arrays using FDTD simulation software and calculation of photonic band gap structure. The theoretical results were consistent with our experiment completely, which confirmed the correctness of our experiment. This technology of tuning florescence intensity can be applied to enhance the LED efficiency.In chapter 4, we report a novel technique for generating ordered arrays of fluorescent polymer barcode nanorods by reactive ion etching using ncp colloidal microsphere arrays as masks. Ncp microsphere arrays are transferred onto the surface of fluorescent polymer multilayers, which are spin-coated in turn on a substrate. The exposed polymers are then etched away selectively, leaving color-encoded nanorods with well-preserved fluorescent properties on the substrate. By changing the process of spin-coating, the amount of polymer in each layer could be tuned freely, which determines the relative fluorescence intensity of the barcode nanorods. Moreover, the shape of the nanorods is hierarchical according to different etching speeds of various materials, which also endow the nanorods with shape-encoded characters. We attempted a variety of water-soluble or oil-soluble polymer and obtained many nanorods. Similar to optical (distinguishable metals/numbers of stripes) coding demonstrated by Natan and co-workers, our barcode system can generate a number of codes as high as (mn+mceil(n/2))/2 (n: number of segments, m: number of different polymers). Increasing the kinds of polymers and the segments, more resolvable codes could be achieved, which demonstrated the controllability of our system. Furthermore, we can detach these nanorod arrays from the substrate and form dispersion of coding materials. After detaching, the desired shape and structure were well preserved with cleaving nearly from the bottom. The different polymer sections on the nanorods could be clearly distinguished from the differences in both shape and fluorescence of these nanorods. It is worth noticing here that not only the colors but also the approximate profiles of the pattern were successfully reproduced after releasing, as indicated from the microscopy images. Therefore, a series of nanorods with green, green/blue, red/blue/green color encoding could be fabricated. Such a library of color encoding nanorods could be very useful for the study of geometric and chemical anisotropic effects on product tracking and biodetection. Our method can also be extended to fabricate non-fluorescent multi-segmented and humidity-sensitive nanorod arrays that are similar to butterfly wings.