Excitations of free and localized electrons at nearby energies in reduced cesium tungsten bronze nanocrystals

The low-energy optical absorption mechanism of tungsten oxides and tungsten bronzes is unresolved, with the primary models reported involving free-electron and polaron excitations. Herein, a new mechanism is proposed, based on a systematic and detailed analysis of optical profiles in Cs-doped hexagonal tungsten bronze nanoparticles with varied amounts of oxygen vacancy (VO) and Cs dopant. The Drude–Lorentz analysis of absorption bands, incorporating a newly-developed Mie scattering integration method, has clarified the observed absorption profiles as consisting of three peaks of anisotropic plasmon and polaron exitations. The behavior of the deconvoluted components indicates that VO and Cs+ provide localized and delocalized electrons, respectively, both contributing to the collective plasmon resonance against external electromagnetic waves.