(386h) Chiral Self-Assembled Structures of Bowtie Shape: Synthesis, Optical Properties, and Deep Learning-Based Modeling.

Chiral nanostructures offer new solutions in many technological fields, including biosensing, chiral catalysis, enantioselective separation, and quantum computing. While many factors make chiral inorganic nanostructures significant to these areas, the unique characteristic that drives research in this area is their ability to exhibit high chiroptical activity. To date, many researchers are focused on materials combining nano-, meso-, and micron-scale structural elements since such hierarchical structures can rotate light polarization in a wide spectral range. However, top-down approaches, such as ion-beam lithography, extensively used for the growth of such helices, are limited for the fabrication of nanoscale elements and have a low mass production efficiency.

Here we developed a facile bottom-up approach for the formation of microscale helices of bowties shape. Depending on the enantiomer of chiral molecules used, bowties have a right- or left-handed twist with 100% enantiomeric excess. Transmission electron microscopy of bowties allowed to establish their assembly mechanism and growth. Bowties are formed by self-assembly of nanoparticles ~ 3.4 nm diameter that are arranged into stacked twisted nanosheets (Figure 1a). The latter have a thickness of about 100 nm and a 50 nm separation between each other. Careful adjustment of synthesis conditions allows growing bowties with desired length, width, thickness, and twist, which, in turn, lead to the strong chiroptical activity that could be carefully tuned in a wide spectral range. Computations of bowties chiroptical activity has shown that the circular dichroism and g-factor spectra experience a blue-shifting tendency as the pitch of bowties decreases (Figure 1b). To predict bowties optical properties, we also developed an artificial neural network that prognosticates g-factor values and spectra peak positions with reasonable accuracy (Figure 1c).

In summary, we report here the self-assembly of bowties consisting of twisted stacked nanosheets. The morphological properties of bowties are tuned by varying the chemical conditions tuning their optical properties. We believe that our results open new horizons for designing chiroptical materials and metamaterials with a strong and tunable polarization rotation essential for multiple emerging technologies.