Dual-Band LSPR of Tungsten Bronze Nanocrystals Tunable over NIR and SWIR Ranges

The optical range of localized surface plasmon resonances (LSPR) has been extended over the infrared region by using doped semiconductor nanocrystals as host materials. The variable free career density and large variety of compositions of semiconductors have provided new methodologies to tune the LSPR wavelength. However, the tuning of LSPR by controlling the anisotropic morphology of nanoparticles remains poorly explored in semiconductor hosts. There is also a lack of material with finely tunable LSPR in the near-infrared (NIR) and short-wavelength infrared (SWIR) regions, which are particularly interesting for applications to energy, imaging, telecommunication, and biomedical technologies. Here, we present synthetic methods to tailor the morphology of cesium-doped hexagonal tungsten bronze (Cs-HTB) nanocrystals exhibiting strong multi-band LSPR depending sensitively on the aspect ratio (AR) of the particles. The LSPR band-splitting behavior is quantitatively analyzed as a function of the AR from 0.5 (platelets) to 6.2 (rods). Computational modeling, taking into account the anisotropic dielectric function of Cs-HTB, coincides unambiguously with experiments confirming the dominant effects of both the shape and crystalline anisotropies on LSPR. The wide range of controllable ARs enables the LSPR of a single material composition to span the entire NIR−SWIR region. We demonstrate that these highly tunable plasmonic nanocrystals can be used to customize solar irradiance through windows in order to optimize the efficiency of energy consumption in buildings.