PhD defence by Jisoo Oh

Congratulations to Jisoo for her successful PhD defence on 2nd December 2024. Jisoo is starting her postdoctoral research at Sandrine Ithurria's group at ESPCI Paris in January 2025.

Thesis title: Shape control and assembly of CsxWO3-δ nanocrystals for infrared plasmonics

Abstract: CsxWO3-δ nanocrystals exhibit localized Surface Plasmon Resonance (LSPR) spectra in the near-infrared (NIR: 700-1000 nm) region. The two LSPR peak positions are dependent on their aspect ratio (AR=length/width). In this study, we have improved the synthesis method of the nanocristals through a facetselective continuous growth by utilizing seedmediated continuous injection synthesis and facet-selective ionic ligands. This approach

allowed us to either increase or decrease the aspect ratio of the nanocrystal seeds. By analyzing the injection-rate-dependent growth trends, we have described a growth scenario of this material. The LSPR spectra of CsxWO3-δ nanocrystals with the expanding AR values ranging from 0.2 to 21 covers from the visibleadjacent NIR to the mid-wave infrared (MWIR: 3000-5000 nm) region.

Assemblies of plasmonic nanocrystals often exhibit properties distinct from individual

nanocrystals. Directional assemblies of CsxWO3-δ platelets and rods have been created

by using the ligand-assisted and external electric field-assisted assembly methods, respectively. The former assembly enabled us to study the plasmonic coupling behavior of CsxWO3-δ platelets, while the later assembly allowed us to demonstrate polarized LSPR for the first time with plasmonic nanocrystals with both LSPR peaks outside the visible region. Random assemblies of CsxWO3-δ nanocrystals produced an unexpected strong absorbance peak centere at lower wavelengths than the LSPR peaks. While the excitation mechanism needs to be further explored, we attribute this peak to be due to bulk plasmon resonance.