(272d) Influence of Crystalline Anisotropy on Localized Surface Plasmon Resonance of Semiconductor Nanocrystals

Doped semiconductor nanocrystals have been shown to host a tunable localized surface plasmon resonance (LSPR) over a wide optical range depending upon controlled dopant and carrier concentration. Metal nanoparticles such as gold and silver, have high free electron densities (~10²³/cm³) creating resonances with Ï?_lsp in the visible region whereas, in doped semiconductor nanocrystals, a highly variable carrier density, (10¹⁸~10²²/cm³) enables Ï?_lsp over the entire infrared region. Employing anisotropic particle shapes in metal nanoparticles and nanostructures have given an additional control over Ï?_lsp. For instance, in metal nanostructures, it has allowed LSPR band spitting and realization of resonance in near-infrared for high aspect ratio nanorods. In semiconductor nanocrystals, studies so far have focused on tuning LSPR frequency (Ï?_lsp) and the influence of anisotropic nanocrystal shape and intrinsic crystal structure remain poorly explored.

Unlike typical plasmonic metals like Ag or Au, doped semiconductors can have anisotropic crystal structures for example, Cu_2-xS (layered) or Cs:WO₃ (hexagonal), which is the focus of our current study. Here in this study, we have shown that colloidally synthesized hexagonal phase Cs:WO₃ nanocrystals exhibit strong aspect ratio-dependent LSPR absorption peaks that can only be explained via a cooperative influence of crystalline and shape anisotropies. We demonstrate the dominant influence of crystalline anisotropy, which uniquely causes strong LSPR band-splitting into two distinct peaks with comparable intensities. This finding highlights the limitations of conventional treatments of LSPR that assume isotropic dielectric constants and attribute multimodal peaks uniquely to shape anisotropy effects. This understanding extends our ability to rationally tune LSPR lineshape and near-field enhancement via synthetic control of shape and crystalline anisotropies of semiconductor nanocrystals. In particular, the demonstrated multimodal LSPR with near-equal intensities of h-Cs:WO₃ nanocrystals covers the near-infrared (NIR) region of great importance in photonic, solar, and clinical applications while maintaining high visible transparency due to its wide band gap.