Amplification of Chirality in Photopatterned 3D Nanostructures of Chiral/Achiral Mixtures

Abstract

The dispersion of a chiral polymer in a polymerizable matrix can amplify the chirality of the material, and a helical conformation of the chiral material within the polymerized SU-8 excessively increased the circular dichroism. Here, we demonstrate the fabrication of three-dimensional nanostructures of chiral/achiral mixtures by two-photon lithography. The irradiation of light and annealing caused local changes in the chiral material and finally led to the enhancement of the optical properties. The demonstration of a photopatternable chiral material could expand the usage of optical materials for various applications.

1. Introduction

In general, the term chirality has a broad meaning and chirality in nature is an omnipresent phenomenon. Normally, in chemistry, chirality can be found in organic compounds consisting of one or two carbon atoms with four substituents [1]. In 1850, Pasteur observed double sodium–ammonium salts and found that their crystals had a tiny facet on one of the edges, oriented to either the right or left. He separated these two types of crystals and identified that they were optically active in solution, with equal rotations in absolute magnitude but opposite direction [2,3,4]. Likewise, when two molecules have the same formula but a different state of symmetry of one atom, they are referred to as enantiomers, which are chiral molecules—non-superimposable mirror images of each other [5], as shown in Figure 1.

Because the optical activity of natural materials is weak, researchers have tried to find enhancements for the characteristics of the materials, which has resulted in the discovery that optical activity can be affected by various factors such as light intensity, electric fields, magnetic fields and temperature [6,7,8,9]. As a reaction to these factors, the optical activity can be changed to the same order of magnitude when phase transitions are involved, and for some cases, the circular birefringence can become large enough to satisfy a negative refractive index [10,11,12]. In terms of the chirality parameter κ, a material’s chirality can be changed. When κ is large enough, a negative index of the material can be realized. If the κ of the chiral material is larger than the square root of the product of real parts of the ε and μ, which are the permittivity and permeability, respectively, a negative refraction will occur:

$$\sqrt{\epsilon \cdot \mu }<\mathsf{\kappa }$$

(1)

$$\mathrm{n}=\sqrt{\epsilon \cdot \mu }-\mathsf{\kappa }$$

(2)

where ε, μ, κ, and n are permittivity, permeability, the chirality parameter and the refractive index of the material, respectively [13].

π-conjugated conductive polymers, such as polyfluorenes and their copolymers, have received interest [14,15,16,17] as candidates for various applications; solar cells [18,19,20,21], light-emitting diodes [22,23,24,25], bio-imaging [26,27], optical switches [28,29,30,31], optoelectronics [32] and various synthetic methods of inducing enantioselective interactions of nanoparticles with chiral molecules have been studied [33,34,35]. Oh et al. introduced a synthesized chiral polymer, poly(fluorene-alt-benzothiadiazole) (PFBT), with the addition of gold nanoparticles (AuNPs) and showed that the annealing of the polymer caused an increase in the optical activity [36]. Compared with pure films, the annealed films showed more than three orders of magnitude higher values of chirality, which is an effect that is caused by the structural and plasmonic enhancement of the chirality. Later, the amplification of chirality in a solid phase caused by the dispersion of a chiral polymer in a polymerizable matrix was reported [37]. The helical conformation of PFBT within a polymerized SU-8 matrix excessively enhanced the CD. In addition, the photopatterning of the PFBT/SU-8 mixture was demonstrated successfully using UV exposure with a size of a few hundred μm [38].

Two-photon polymerization, using a two-photon absorption process, is a fabrication technique for nanopatterns and three-dimensional (3D) structures [39]. This method for nano- and 3D structures is of great interest in various fields, including nanotechnology, biotechnology, photonic crystals and functional applications such as micro-/nano-fluidic devices and micro-/nano-electromechanical systems [40]. Since PFBT/SU-8 showed good optical activity and the potential for usage in applications as a photopatternable material, understanding its characteristics and expanding on its usage are highly required. In this study, we successfully demonstrated the fabrication of PFBT/SU-8 using a two-photon lithography technique. The size of the patterned structures was 2–3 μm, and after patterning, it still showed greatly enhanced optical activity compared with pure PFBT films. By demonstrating a 3D photopatternable chiral material with a size of a few μm, we can expand its usage in various areas.

2. Materials and Methods

Preparation of materials: The PFBT was prepared based on published procedures, with palladium-catalyzed Suzuki polycondensation used as a final procedure [41]. To produce a high-molecular-weight PFBT (M_n = 13,000, M_w = 30,000), synthesized PFBT was purified by Soxhlet extraction and a fluorene monomer was attached by (S)-3,7-dimethyloctyl substituents for the introduction of the chirality. For the photopatterning, SU-8 2025 (Kayaku Advanced Materials, Inc., Westborough, MA, USA) was selected, which is a highly branched polymeric epoxy resin, and the photoinitiator PC-2506 (Polyset Co., Mechanicville, NY, USA) was also added. The mixture ratio of PFBT, SU-8 2025 and PC-2506 was 1:30:0.3. Before the addition of the SU-8 2025, the PFBT was first dissolved in tetrahydrofuran (THF) (Sigma-Aldrich, St. Louis, MO, USA) for 24 h using a magnetic stirrer. After dissolving the PFBT, SU-8 was added to PFBT/THF solution and mixed for 48 h with a magnetic stirrer. For comparison, several samples were prepared with different solid contents of the SU-8 photoresist, with mixtures of 10%, 20%, 30%, 40% and 50%.

Laser writing: The two-photon lithography system used an 800 nm writing-wavelength Ti:sapphire laser (Mira 900, Coherent, Santa Clara, CA, USA), as shown in Figure 2. This system had a piezo stage, so the substrate on the stage could move in the x, y and z directions to make designable 3D structures. The prepared PFBT/SU-8 solution was spin-coated on the glass and baked for 80 min at 60 °C to stabilize and remove the air in the coated solution, followed by baking at 95 °C for 8 min and then cooling down for 3 h. To fabricate the pattern, we designed an ‘L’ shape using a computer and the PFBT/SU-8-coated substrate, exposed by a femtosecond laser with a cumulative dose of 0.2–0.8 W·cm². After exposure, the substrate was post-baked at 65 °C for 10 min, followed by heating at 95 °C for 10 min to accelerate the crosslinking speed of the sample, then cooling down for 2 h. For the development of the laser-exposed sample, propylene glycol methyl ether acetate (PGMEA) (Sigma-Aldrich, St. Louis, MO, USA) was used; the sample was submerged in PGMEA for 3 h to dissolve the uncrosslinked areas. Isopropyl alcohol (IPA) (Sigma-Aldrich, St. Louis, MO, USA) and deionized water were used to rinse the sample, and finally, the sample was annealed at 150 °C for 15 min in the glove box, then cooled down to room temperature. The PFBT/SU-8-coated samples and patterned PFBT/SU-8 samples were measured using circular dichroism (CD) spectroscopy (J-815, Jasco, Great Dunmow, UK) for a comparison of the CD intensity, and the photopatterned structures were analyzed using scanning electron microscopy (SU-4000, Hitachi, Tokyo, Japan).

3. Results and Discussion

Understanding the physiochemical interactions between nanocrystals and supramolecular systems that can amplify chirality is complicated, and CD intensity has been used to understand chirality in molecular systems. When chiral molecules absorb circularly polarized light, the CD also increases; therefore, measuring the CD intensity is an effective method for understanding the chirality [42,43,44,45]. To investigate how the enhancement of the optical activity varied based on the solid content of SU-8, PFBT/SU-8 solutions with a solid content of SU-8 from 10% to 50% (mixture ratio of PFBT:SU-8 = 1:40) were prepared and spin-coated on the glass substrates. Figure 3 shows the measured CD intensity of the coated PFBT/SU-8 films, without annealing and with annealing. The black squares stand for the PFBT/SU-8 substrates without annealing and the red circles stand for the PFBT/SU-8-coated substrates with annealing. As mentioned above, the chiroptical characteristics of molecules can be induced by annealing, and as Figure 3 shows, the measured maximum CD intensity of the overall samples clearly shows the enhancement of chirality after annealing.

After the annealing of the PFBT/SU-8 films, the CD increased 2.16–2.83 times and the maximum increased CD for the solutions with 10%, 20%, 30%, 40% and 50% solid contents were 131 mdeg, 498 mdeg, 635 mdeg, 848 mdeg and 644 mdeg, respectively. By using the two-photon lithography setup, the PFBT/SU-8-coated films were exposed to a femtosecond laser to make an ‘L’ shape and developed by PGMEA to dissolve the unpolymerized area. After measuring the CD of the developed samples, they were annealed at 150 °C for 15 min and measured by CD spectroscopy again. In Figure 3, the blue triangles stand for the CD of the patterned samples without annealing and the green inverted triangles stand for the CD of the patterned samples with annealing. After annealing, the CD of all samples increased, with the 30% sample showing the best result. The main explanation for the CD enhancement is a change in the supramolecular ordering, which causes an increased helicity in the mixtures [37]. By annealing the sample, the helical fibrils of the patterned structure became liberated from the mixture and had a larger freedom, which caused the supramolecular conformation of the material. Among the samples, the sample with a solid content of 30% had the most ideal conditions for changing the supramolecular conformation, and consequently, it showed the best CD enhancement after annealing. On the other hand, the maximum CD of the annealed samples with 30%, 40% and 50% solid content were relatively lower than the maximum CD of the PFBT/SU-8 films with the same solid content of SU-8. Compared with pure PFBT film, the CD of the patterned PFBT/SU-8 structures were enhanced by 10–25 times and the solution with a solid content of 30% showed the greatest CD (632 mdeg) among them. Previous studies have described that developed PFBT/SU-8 structures after annealing show exceptionally large, enhanced CD through the formation of chiral aggregates [37]. In this case, the dispersed PFBT in the SU-8 formed nanocomposites, which played a key role in the enhancement of the CD. In addition, by using a UV light, a PFBT/SU-8 structure was fabricated and the patterned structure had a 30 times enhanced CD compared to a pure film without nanocomposites [38]. This study showed that without residual nanocomposites, polymerized PFBT/SU-8 mixtures that go through a development and annealing process still receive an advantage from the formation of chiral aggregation; therefore, the CD can be enhanced. Otherwise, although the structure patterned by two-photon lithography still showed an enhanced CD, the maximum CD intensity was lower than that of the structures fabricated by UV lithography. Figure 4 shows the schematic of the two-photon lithography and UV lithography. Two-photon absorption is a process of optical absorption whereby two photons are simultaneously absorbed by the material [46]. Unlike UV lithography, the nonlinear dependency of the polymerization caused by irradiation at a certain intensity can produce specific localized areas of the polymer, and consequently, it can produce 3D structures [47]. While UV lithography polymerizes the exposed area from the top down, two-photon lithography only polymerizes the local area where the intensity of the irradiated light reaches a threshold value, and in this experiment, the height of the two-photon lithography-exposed sample was lower than the height of the UV-exposed sample. In other words, two coated substrates will have the same thickness as the polymerizable materials, but the height of the patterned structures can be different when samples are exposed to different methods, such as two-photon lithography and UV lithography. Figure 5 shows the patterned PFBT/SU-8 structures fabricated by two-photon lithography. The patterned structures were defined very well and there were no residual nanocomposites. The height of the fabricated structure was 3 μm, which is relatively low considering that we previously reported a UV-patterned chiral structure that had a 15 μm height. As reported, the thickness of the PFBT/SU-8 structures affected the enhancement of the CD intensity [37], and this may explain why the enhancement of the CD for the patterned structure by two-photon lithography was relatively lower than that of the UV-patterned structure. In addition, based on our results, mixing a polymeric matrix with chiral oxides such as vanadium dioxide (VO₂), the α-phase of chromium oxide (α-Cr₂O₃) or zinc oxide (ZnO) [48,49] and applying our approach may possibly expand the usage of these materials to applications in various fields.

4. Conclusions

In conclusion, we reported on the photopatterned 3D structures of chiral/achiral mixtures fabricated with two-photon lithography. The size of the structures was around 3 μm and there were no residual nanocomposites on the unexposed area after the development process. After annealing, the patterned structures clearly showed an enhancement in the chirality, which resulted from the formation of chiral aggregation. Compared with pure PFBT film, the patterned structures showed a 25 times enhanced CD when formulated with a 30% SU-8 solid content in the mixture and a 1:40 PFBT:SU-8 ratio. The irradiation of light and annealing caused local changes in the chiral material, and finally led to the enhancement of the optical characteristics. The demonstration of a near-nm size of the photopatternable chiral material can expand the usage of optical materials to various applications. Further studies are required to obtain a better understanding of polarization dependency and the effect of temperature on the materials.