Tensile properties of polymer nanowires fabricated via two-photon lithography

Abstract

Two-photon lithography enables fabrication of complex 3D structures with nanoscale features. However, its utility is limited by the lack of knowledge about the process-property relationship. Here, we have designed micro-electro-mechanical systems (MEMS)-based miniaturized tensile testers to measure the stress-strain response of the individual polymer nanowires. Measurements demonstrate that geometrically indistinguishable nanowires can exhibit widely varying material behavior ranging from brittle to soft plastic based on processing conditions. In addition, a distinct size-scaling effect was observed for post-processed nanowires wherein thinner nanowires have up to 2 times higher properties. The process-property characterization presented here will be critical for predictive design of functional 3D structures with nanoscale features.

Fig. 1. Micro-electro-mechanical systems (MEMS)-based miniaturized tensile tester. (a) Optical image of the unpackaged tensile tester illustrating the actuator and the two sensors. (b and c) Scanning electron microscope images of the sensor pads and the polymer nanowire printed across the two stages. (d) Schematic representation of printing of a nanowire directly on top of a MEMS sensor via two-photon lithography.

Fig. 2. Stress–strain curves for quasi-static tensile tests on nanowires printed with IP-DIP photopolymer. Label LS denotes low-speed (100 μm s⁻¹) and HS denotes high-speed (10 mm s⁻¹) writing conditions. The label G denotes as-printed green-state condition whereas the label P denotes photochemical post-curing condition. Inset is a zoom-in of the curves around zero strain. Error bars quantify the combined 1-standard deviation uncertainty arising from the tensile tester force-displacement measurement uncertainty, print-to-print variation in nanowire cross-sectional area, and the measurement uncertainty in the length of nanowires.

Fig. 3. Width and height of green-state nanowires versus the time-averaged laser power. Nanowires were written in IP-DIP resist at a writing speed of 10 mm s⁻¹. Time-averaged writing power of 50 mW for the beam corresponds to a peak intensity of 2.43 TW cm⁻² at the center of the focused light spot. Error bars on width and height quantify the 1-standard deviation of the uncertainty across multiple nanowires printed under identical conditions.

Fig. 4. Material properties of nanowires printed with IP-DIP resist versus their size. Size of nanowires was controlled by varying the laser power in the range of 16 mW to 50 mW while writing with a constant speed of 10 mm s⁻¹. (a) Young's modulus and yield strength versus nanowire width. (b) Toughness up to 20% strain versus nanowire width. Error bars quantify the combined 1-standard deviation uncertainty arising from the tensile tester force-displacement measurement uncertainty, print-to-print variation in nanowire cross-sectional area, and the measurement uncertainty in the length of nanowires. Data points correspond to properties that were evaluated from a single tensile test under each condition.