[22p-D315-1~17] 3.1 Basic optics and frontier of optics

▲ [22p-D315-6] Crystallization and Polymorphic Transition of NaClO₃ via Plasmonic Trapping

〇AnChieh Cheng¹, Christophe Pin¹, Teruki Sugiyama², Keiji Sasaki¹ (1.Hokkaido Univ., 2.NYCU)

Keywords:plasmonic trapping, crystallization, polymorphic transition

The localized surface plasmon resonances of metal nanostructures enable the excitation of hotspots beyond the diffraction limit of the light that can be used to optically trap various nanomaterials with high precision. Previously we demonstrated enantioselective control in chiral crystallization of sodium chlorate (NaClO₃) by plasmonic trapping and successfully achieved significant crystal enantiomeric excess of more than 50 %. In that work, we used triangular trimer gold nanostructures with side length of 230 nm as a plasmonic structure, which enhanced electric field was not generated at the nanogap in the center of the trimer but at the outside of each triangle. Here, we design a triangular trimer gold nanostructures with side lengths of 170 nm that confine the enhanced electric field at the nanogap of the trimer. We investigate the possibility for both the metastable crystallization and the polymorphic transition of NaClO₃ to occur depending on the type of trimer. The 1064-nm CW laser with circular polarization at 1.0 MW/cm² was focused on a single triangular trimer structure. Crystallization was induced with almost 100% probability using both types of plasmonic structures, and the precipitated crystals were attributed to be the metastable achiral phase because of their birefringence. Noted, when 170 nm gold nanostructures were used, the crystallization time was much longer, and the crystal size was smaller than when 230 nm trimer were used. With further laser irradiation on 230 nm trimers, the birefringence of the crystals disappeared, indicating a polymorphic transition from the metastable achiral phase to the stable chiral phase. Intriguingly, this phenomenon has never been observed in the case of 170 nm trimer. These results will contribute to the understanding of the crystallization mechanism of NaClO₃ by plasmonic trapping and the elucidation of the mechanism of subsequent chirality control.