There are two main approaches to form structures in the nanotechnology field of research, top-down and bottom-up. The top down approach uses machines to create nanostructures without control at the atomic level. Examples of top down mechanisms are photolithography, two-photon lithography, holography, electron beam lithography photoembossing and techniques originally developed for imaging purposes such as scanning tunneling microscopy and atomic force microscopy. The bottom-up approach uses the self-assembling properties of some materials, starting with atoms and molecules as building blocks to build up nanostructures. Common materials used in bottom-up approaches are block-copolymers, colloids, amphiphiles and liquid crystals. The top-down approaches have the disadvantage of not achieving control at the molecular level while the bottom-up approaches fail when trying to assemble them in bigger and more complex structures because of the randomness of the self assembly. This research focused on combining both types of approaches to overcome their limitations. We use liquid crystals (LCs) as self assembling materials and holography and lithography for the top-down techniques. The LC materials used contain reactive moieties which make them susceptible for polymerization. Usually the method of choice to polymerize LC is photopolymerization, because the temperature within certain limits can be freely chosen establishing that the molecular order associated to a particular LC phase is maintained during the process. A photoinitiator, a molecule that dissociates when exposed to light with the proper wavelength, is used to initiate the polymerization reaction. In common lithography a mask determines which areas will be exposed and thus polymerized. Our new technique, however, makes use of a new kind of material, a dichroic photoinitiator. The dissociation of this material is dependent of the polarization light that is used for excitation which must be parallel to the transition moment of the initiator molecule, i.e. for our rod-like shaped photoinitiator molecules parallel to their longitudinal axis. As the rod-like initiator molecules also align with their host LC molecules, polymerization can be controlled locally by LC orientation without the use of a mask or interference. Among other types of LC order the periodic orientation of cholesteric LC materials makes them perfect as a template for nanostructuring and microstructuring with a dichroic photoinitiator. The size of the created structures is determined by the cholesteric pitch and the cholesteric pitch can be easily modified by changing the concentration of its chiral dopant. This thesis presents an in depth characterization of the dichroic photoinitiators, as well as the creation of two and three dimensional microstructures by means of using the techniques previously mentioned.
|Qualification||Doctor of Philosophy|
|Award date||17 Apr 2008|
|Place of Publication||Eindhoven|
|Publication status||Published - 2008|