Continuous Anisotropic Growth of Plasmonic Cs_<i>x</i>WO_3−δ Nanocrystals into Rods and Platelets

Shape control during nanocrystal synthesis enables tunable physicochemical properties that emerge at the nanoscale. While extensive efforts have been devoted to controlling shapes in various systems such as plasmonic metal nanoparticles or semiconductor quantum dots, the shape control of plasmonic doped semiconductor nanocrystals remains less explored and limited. Here, we report the synthesis of Cs_<i>x</i>WO_3-δ nanocrystals with exquisite shape control achieved through a continuous injection synthesis combined with precursor-mediated facet-selective growth. We demonstrate that the anisotropic growth of Cs_<i>x</i>WO_3-δ nanocrystals is strongly dependent on the precursor injection rate, which we attribute to the material's intrinsic structural anisotropy and the contrasting reaction kinetics on different crystallographic facets. Furthermore, we reveal that the presence of halide ions in the reaction medium is critical for passivating and suppressing the growth of Cs-exposed basal planes. By systematically modulating the shape aspect ratio, we achieved an extended range of nanocrystal morphologies, leading to a 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 the 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 the tunable LSPR properties of heavily doped plasmonic semiconductor systems.