STED-Inspired Nanolithography beyond (Meth-)Acrylates

In the year 2014, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry, among others, to Stefan Hell. He found that diffraction, inevitably caused by focusing light (Abbe 1873), does not limit resolution in fluorescence microscopy, contrary to common expectation. The trick suggested by Stefan Hell was based on using quantum chemical transitions within a dye molecule to optically deplete the excited state in the outer rim of the focal area via stimulated emission. Stimulated Emission Depletion (STED) microscopy was coined as the technical term for this technology. In the meantime, this idea has been transferred to three-dimensional, optical lithography. In this case, a photoresist, comprising liquid monomers and photo-initiators, is excited optically in the focal spot. This results in local polymerization. When the focus is moved through the resist, three- dimensional solid objects can be created. However also in this case, diffraction limits the minimal size of the focal spot to the wavelength. Consequently, diffraction compromises resolution and feature size in optical lithography, as well. If, however, the photo-initiators are depleted in the outer rim of the focus prior to initiating a polymerization reaction, resolution can be improved, despite the diffraction in optical lithography. This is referred to as STED-inspired optical lithography. Over the last 15 years, this method was applied very successfully by several groups, including the team of Prof. Klar. However, the materials, that could be optically nanostructured this way, were limited to acrylates and similar materials, the polymerization of which is radically initiated. One of the well-known members of this class of materials is plexiglass. Two other groups of polymers eluded the benefits of this technology, so far. Up to now, it was neither possible to three-dimensionally nanostructure cationic nor oxidative polymers with STED-inspired lithography. The former includes epoxides, which play an outstanding role in the semiconductor industry. The latter comprises pi- conjugated, conducting polymers which are essential for plastic-electronics. The current project aims for closing these two gaps. Finding suitable photo-starters and investigating their photo-dynamic behavior will be key as well as finding suitable optics and lasers. First in the world, the group of Prof. Klar has recently found promising routes to achieve these goals, forming the basis of this current FWF project.