Subdiffraction Multiphoton Polymerization of PEDOT

Direct laser writing via multiphoton polymerization (MPP) lithography has significantly advanced micro- and nanofabrication, yet its applicability remains largely confined to radical polymerization of acrylates and methacrylates. Here, we demonstrate that the ketocoumarin 7-diethylamino-3-thenoylcoumarin (DETC), widely used as a photoinitiator in subdiffractional radical polymerization lithography inspired by stimulated emission depletion (STED) microscopy, also effectively initiates oxidative step-growth polymerization of EDOT, enabling direct lithographic formation of sub-100 nm PEDOT nanostructures using visible light. Two approaches for subdiffraction MPP are pursued: (i) slow scanning with low excitation power, allowing for the influx of oxygen into the illumination point spread function (PSF), leading to chemical quenching of the DETC triplet state, and (ii) STED-inspired transient state absorption depletion lithography, where the DETC triplet states are optically quenched in the outer rim of the excitation PSF. Approach (i) yields 65 nm linewidths at the cost of an ultra-slow scan speed of 2.5 μm/s, while approach (ii) yields 105 nm wide lines at 100 μm/s. Raman spectroscopy suggests that conductivity in MPP-written PEDOT wires arises only when graphitic phases are formed.