Continuous anisotropic growth of NIR-plasmonic CsxWO3-δ nanocrystals into rods and platelets

Shape control during nanocrystal synthesis enables tunable physico-chemical properties that emerge at the nanoscale. While extensively studied in plasmonic metal nanoparticles and semiconductor quantum dots, shape control remains less explored in plasmonic heavily doped semiconductor nanocrystals. Here, we report the synthesis of cesium-doped tungsten oxide (CsxWO3-δ) nanocrystals with exquisite shape control achieved through continuous injection synthesis combined with precursor-mediated facet-selective growth. We demonstrate that the anisotropic growth of CsxWO3-δ nanocrystals strongly depends on the precursor injection rate, which we attribute to the material’s intrinsic structural anisotropy and contrasting reaction kinetics on different crystallographic facets. Furthermore, we reveal that halide ions in the reaction medium are critical for passivating and suppressing growth of Cs-exposed basal planes. By systematically modulating the shape aspect ratio, we achieved an extended range of nanocrystal morphologies, leading to broad tunability of LSPR spectra, spanning the entire near-infrared region and extending into the mid-infrared. Computational simulations effectively reproduce the observed shape-dependent optical properties and highlight size-dependent damping behavior consistent with the free electron model. These findings provide a robust experimental methodology for shape control in structurally anisotropic nanocrystals and offer theoretical insights into tunable LSPR properties of heavily doped plasmonic semiconductor systems.