Contrôle de la forme et de l’assemblage de nanocristaux de CsxWO3-δ pour la plasmonique infrarouge

CsxWO3-δ nanocrystals exhibit localized Surface Plasmon Resonance (LSPR) spectra in the near-infrared (NIR: 700-1000 nm) region. The positions of their two LSPR peaks are dependent on their aspect ratio, which is defined as the ratio between the length and the width of the nanocrystals. By flexibly controlling the aspect ratio, the LSPR spectra can be tailored to fit specific spectral requirements for targeted applications.However, the previous hot-injection synthesis has not been straightforward, especially for creating CsxWO3-δ nanocrystals with strong shape anisotropy. In this study, we have enhanced the existing synthesis method into a facet-selective continuous growth process by utilizing seed-mediated 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 eludicated the 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 visible-adjacent 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 a plasmonic nanocrystal that has both peaks functioning outside the visible region. Random assemblies of CsxWO3-δ nanoplatelets produced an unexpected strong absorbance peak centered at lower wavelengths than the LSPR peaks. While its excitation mechanism needs to be further explored, we attribute this peak to be due to bulk plasmon resonance, whose photoexcitation has been accepted to be impossible from classical electrodynamics.