## 3 Results and Discussions  
Figure 2A shows patterned PFBT/SU-8 structures after the UV exposure and development process presented in our previous work [26]. As shown schematically in Figure 2B, the material in nominally unexposed (nominally unpolymerized) areas was not completely removed, and a residual film remained between the patterned features. This suggests that PFBT, a strong absorber, served to increase the photosensitivity of the film. Use of a high exposure dose (overexposure) and leakage of light under the photomask, due to imperfect contact of the photomask with the film, may cause unwanted crosslinking of the polymer. Heating, due to absorption by PFBT, could also initiate unwanted cross-linking. That unintentionally-cross-linked material was then not removed during development. To achieve a clean photo patterning, without any residual film between patterned features, we ensured the good contact of the photomask, optimized the photoresist composition, and optimized the UV exposure dose. Because of the different molecular sizes and coefficients of thermal expansion of PFBT and SU-8, making PFBT/SU-8 films with smooth surfaces was challenging. Smooth films were produced by first baking the PFBT/SU-8 film at 65 \({}^{\circ}\)C for 60 min then at 95 \({}^{\circ}\)C for 10 min. This pre-baking step  
Figure 1: Photodubrication process for poly(fluorene-alk-benzothiazole) (PFBT/SU-8) nanocomposite microstructures. reduces the solvent concentration in the spin-coated PFBT/SU-8 layer and 60 min of pre-baking time in a semifipid state may promote mixing between PFBT and SU-8. With smoother, flatter films from this improved pre-bake process, better contact between the photomask and substrate was achieved during UV exposure. The ratio of SU-8 to PFBT was also optimized to achieve the highest CD signal while maintaining clean patterning. First, we tested PFBT/SU-8 mass ratios from 1:20 to 1:80 at a fixed solid content of 30% SU-8. The 1:10 and 1:20 ratios did not form well-mixed films and produced poor quality patterns. Films using a 1:40 ratio could be patterned well and showed higher CD than those using 1:60 and 1:80 ratios. We then fixed the PFBT/SU-8 mass ratio at 1:40 and varied the amount of SU-8 (solid content) in the solution used for spin-coating. For solid content of 20% or less, well-defined patterns could not be obtained. Films prepared using 40% solid content SU-8 showed lower CD than those spun from 30% solid content. Thus, a 1:40 PFBT:SU8 mass ratio and 30% SU-8 were selected as the best combination for good quality patterning and high CD. The UV exposure dose of 225 mJ/cm\({}^{2}\) used here is about half the dose used in our previous work. Figure 2D shows an SEM image of a single feature from a photo patterned chiral polymer nanocomposite produced using these optimized conditions. Note that the unexposed area, inside the ring, was perfectly removed and the boundary of the structure was cleanly fabricated as well, in contrast to the structure shown in Figure 2C, produced using the same mask. Figure 2: PFBT/SU-8 pattern using the standard SU-8 exposure and development process as reported in our previous work [26]. Black bar scale = 100 \(\upmu\)m. (**A**) Top view scanning electron microscope (SEM) image, (**B**) schematic cross sectional view of patterned PFBT/SU-8. (**C**) Higher magnification, low angle SEM picture of the sample shown in (**A**), and (**D**) higher magnification SEM image of a sample produced by the optimized method presented here, patterned with the same mask. Figure 3 shows various patterns produced in this PFBT/SU-8 nanocomposite at the same optimized conditions. In all cases, the patterns were clean, high-fidelity replicas of the mask used to create them. The patterned structures are flat and uniform, showing no evidence of PFBT aggregation, and suggesting that the PFBT and SU-8 are well mixed at a molecular level in these patterned films.