Anisotropic Origins of Localized Surface Plasmon Resonance in n-Type Anatase TiO₂ Nanocrystals

Colloidal nanocrystals of anatase TiO2 exhibit localized surface plasmon resonance (LSPR) in the mid-infrared upon carrier accumulation through synthetic doping or electrochemical reduction. However, the energy and intensity of LSPR in anatase TiO2 nanocrystals is anomalously low compared to those of other transparent conductive oxides with similar bulk conductivity. Here, the electronic origin of LSPR energy and intensity in TiO2 nanocrystals is quantified by measuring infrared transmittance of dilute dispersions of doped nanocrystals and ex situ charged thin films. Optical modeling of infrared spectra reveals that TiO2 nanocrystals can accommodate carrier concentrations exceeding 1.5 × 1021 cm–3 upon charging, but the large effective mass along the anatase c-axis is found to diminish the infrared absorption of TiO2 nanocrystals. The respective effects of crystalline anisotropy, synthetic doping, and electrochromic charging on LSPR in TiO2 nanocrystals are investigated, revealing promising new avenues to engineer this material for plasmonic applications.