(692h) Towards 3D Nanopatterning in Polymeric Gels Using Photochromic Switches

Rapid, high throughput patterning in bulk polymeric systems with nanoscale resolution in three dimensions has long remained a coveted target for materials scientists. State of the art fabrication techniques capable of nanoscale resolution like electron beam lithography are associated with high setup and usage costs. Optical interference lithography has been an attractive technique to cheaply and rapidly pattern three dimensional features in polymer photoresists despite both the resolution and feature size being limited by diffraction. In the past few years, Stimulated Emission Depletion Microscopy (STED) inspired lithography schemes have shown the ability to direct-write features well below the diffraction limit using visible light. However, the high light thresholds required for effective photoinhibition renders them unsuitable to be used for interference lithography and limits their use to point by point writing.

Recently, we have shown that combining the reversibly saturable photoisomerization of spirothiopyran with thiol-Michael addition chemistry can be used to formulate a super-resolution writing system with the desired low light thresholds for parallel patterning. To study the kinetics and optimize the minimum feature sizes realizable with our spirothiopyran writing chemistry, a 1D super-resolution patterning prototype system was designed for self-assembled monolayers on glass substrates. The switchable photoresponsive surfaces fabricated are capable of direct writing various maleimide functionalized molecules in solution. By exploring the kinetics of photoisomerization of covalently bound spirothiopyran and subsequently tuning its microenvironment, we experimentally demonstrate large area nanopatterning with 80 nm resolution and molecular thickness using our 2-color interference lithography setup. The lateral feature size of the written patterns is shown to be tunable by controlling the relative intensity of the initiation and inhibition wavelengths. The reversible nature of photoinhibition in our system also makes it amenable for multiple phase shifted exposure, offering an avenue for sub-diffraction resolution between patterned lines. Finite element photokinetic modeling of our material system demonstrates the potential to extend our 2-color interference lithography setup to fabricate nanostructured gels with uniform spatial features in up to 20 μm thick photoresists. To this end, we have adapted the spirothiopyran writing chemistry for polyethylene glycol monomers to synthesize photoresponsive gels. Efforts are currently underway in our laboratory to apply spirothiopyran mediated 2-color interference lithography for 3D nanopatterning in these polymeric gels.