The hexagonal crystal structure of cesium-doped tungsten oxide leads to anisotropy in nanocrystal morphology (Figure ). The XRD pattern confirms that the NCs are the hexagonal phase of tungsten oxide with the cesium cations located in hexagonal channels (Figure ). The presence of these cations is crucial to stabilizing the otherwise metastable hexagonal crystal structure. The cesium cations also act as interstitial dopants, with the excess charge of the cations being compensated by electrons in the conduction band of the material. Various alkali metal doped tungsten oxide phases, commonly known as tungsten bronzes, have been shown to be metallic conductors due to the high concentration of free electrons generated by the interstitial cations. When nanocrystalline materials with high charge carrier concentrations are illuminated, these electrons oscillate in resonance with the electric field of light, producing localized surface plasmon resonance. LSPR in conducting metal oxide NCs results in strong absorption of near infrared (NIR) light (Figure ). The LSPR of the dispersion of NCs shows two distinct peaks, which correspond to two unique plasmon modes, a result of the anisotropy in particle shape and electronic structure (effective mass and damping) between the two unique crystallographic directions of the hexagonal crystal structure. The LSPR spectrum of the NCs broadens when they are deposited as a film due to plasmonic coupling between the NCs, but the bimodal structure is still apparent (Figure ). The NIR peak of the LSPR and wide bandgap, in the UV, allows for high transparency in the visible region, with only the tail of the LSPR spilling over into red edge of the visible, leading to the blue coloration of the NCs. After the removal of the surface-capping ligands by annealing in a furnace under air, the NC films can be reversibly electrochemically cycled to induce an electrochromic response, while recovering the transparent state upon oxidation. The change in optical density (OD) with electrochemical reduction varies depending on the electrolyte cation used to compensate the injected charge (Figure ). TBA + electrolytes have been previously used to enforce a purely capacitive charging mechanism. In our case, capacitive charging in a TBA + electrolyte results in a modulation spectrum (ΔOD) that matches very closely to the LSPR of the pristine material. Comparing this response to the electrochromism due to intercalation of the smaller cations, we see significant differences in the spectral line shape.