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ABSTRACT Despite their high power density, microsupercapacitors (MSCs) are impractical for many energy storage applications due to their limited energy density. Their energy density can be increased by shaping the electrodes into 3D structures with high specific surface area (SSA). Direct printing of nanoporous 3D electrodes is a promising approach for achieving high SSA. However, conventional nanoscale 3D printing is too slow due to point‐by‐point processing. Here, we have employed the projection two‐photon lithography technique to fabricate nanoporous 3D electrodes via a rapid layer‐by‐layer mechanism. The 3D MSC electrodes are engineered as an array of nanoporous polymeric micropillars that are printed with customizable spacing and count over a 0.25 cm²area. After printing, these micropillars are conformally coated with titanium nitride to form conductive 3D electrodes, which exhibit a specific capacitance of 361 μF/cm². This is two orders of magnitude higher than the capacitance of the flat surface and exceeds the capacitance of both traditional bare electrodes, such as single‐wall carbon nanotubes (< 100 μF/cm²), and electrodes produced by photo‐polymerization 3D printing (˜200 μF/cm²). As our work demonstrates that high energy density 3D electrodes can be rapidly fabricated, it significantly expands the utility of MSCs as miniaturized energy storage devices. 

Two-photon lithography (TPL) is a laser-based additive manufacturing technique that enables the printing of arbitrarily complex cm-scale polymeric 3D structures with sub-micron features. Although various approaches have been investigated to enable the printing of fine features in TPL, it is still challenging to achieve rapid sub-100 nm 3D printing. A key limitation is that the physical phenomena that govern the theoretical and practical limits of the minimum feature size are not well known. Here, we investigate these limits in the projection TPL (P-PTL) process, which is a high-throughput variant of TPL, wherein entire 2D layers are printed at once. We quantify the effects of the projected feature size, optical power, exposure time, and photoinitiator concentration on the printed feature size through finite element modeling of photopolymerization. Simulations are performed rapidly over a vast parameter set exceeding 10,000 combinations through a dynamic programming scheme, which is implemented on high-performance computing resources. We demonstrate that there is no physics-based limit to the minimum feature sizes achievable with a precise and well-calibrated P-TPL system, despite the discrete nature of illumination. However, the practically achievable minimum feature size is limited by the increased sensitivity of the degree of polymer conversion to the processing parameters in the sub-100 nm regime. The insights generated here can serve as a roadmap towards fast, precise, and predictable sub-100 nm 3D printing. 

Abstract Two-photon lithography (TPL) is a photopolymerization-based additive manufacturing technique capable of fabricating complex 3D structures with submicron features. Projection TPL (P-TPL) is a specific implementation that leverages projection-based parallelization to increase the rate of printing by three orders of magnitude. However, a practical limitation of P-TPL is the high shrinkage of the printed microstructures that is caused by the relatively low degree of polymerization in the as-printed parts. Unlike traditional stereolithography (SLA) methods and conventional TPL, most of the polymerization in P-TPL occurs through dark reactions while the light source is off, thereby resulting in a lower degree of polymerization. In this study, we empirically investigated the parameters of the P-TPL process that affect shrinkage. We observed that the shrinkage reduces with an increase in the duration of laser exposure and with a reduction of layer spacing. To broaden the design space, we explored a photochemical post-processing technique that involves further curing the printed structures using UV light while submerging them in a solution of a photoinitiator. With this post-processing, we were able to reduce the areal shrinkage from more than 45% to 1% without limiting the geometric design space. This shows that P-TPL can achieve high dimensional accuracy while taking advantage of the high throughput when compared to conventional serial TPL. Furthermore, P-TPL has a higher resolution when compared to the conventional SLA prints at a similar shrinkage rate.