Sub Diffractional STED-Inspired Cationic Lithography

Stimulated emission depletion (STED) broke the diffraction limit of resolution in fluorescence microscopy and it has been proposed that a STED-confined excitation volume should be equally applicable to spatially control chemical reactions on the nanometre scale. Meanwhile, this prediction has been experimentally realized using free radical polymerization of mostly (meth)acrylates. In this contribution, we will present concepts of how to achieve sub 100 nm structure sizes in STED-inspired epoxide cationic lithography by using a modified system of photosensitizers and initiators. In particular, we use thioxanthones as sensitizers and a sulfonium salt as initiator. The thioxanthones turned out to be optically depletable within the triplet system by transient state absorption depletion (TAD). Making use of this depletion mechanism in the outer rim of the excitation focus, we were able to write sub-diffractional, 125 nm wide Epoxide features with isopropyl thioxanthone, and, most recently, even sub-100 nm features using 2-chlorothioxanthone.