1, The University of California, Santa Barbara, Santa Barbara, California, United States
Degenerately doped metal oxide nanocrystals demonstrate a tunable localized surface plasmon resonance (LSPR) that can be integrated into a diverse range of optoelectronic applications. The energy and intensity of LSPR absorption can shift dramatically with defect composition, geometry and induced charge, allowing for dynamic modulation and application-specific spectral profiles. Anatase TiO2 has recently been explored as a plasmonic metal oxide, following observation of an infrared (IR) LSPR feature in niobium-doped anatase nanocrystals (de Trizio et al, Chem. Mater., 2013). The energy and intensity of this IR LSPR feature in TiO2 can be modulated by changing the free electron concentration with electrochemical charging in thin films (Dahlman et al, J. Am. Chem. Soc., 2015). However, the low energy and broad peak profile of LSPR in Nb-doped TiO2 nanocrystals contrasts with the material’s high electronic conductivity as a film, which is comparable to standard transparent conductive oxides such as tin-doped indium oxide. This talk will investigate the LSPR of charged and Nb-doped anatase TiO2 nanocrystals by ex situ infrared spectroscopy, and extract free carrier properties from simulated optical spectra.
Electronic properties of metal oxide plasmonics can be difficult to investigate for materials with LSPR energies in the mid-IR, because a different set of detectors, solvents and substrates must be used for IR transmittance or reflectance measurements. The concentration and scattering rates of free carriers in unbiased nanocrystals in solution are extracted from fits of FTIR transmittance spectra using the Drude model of carrier transport and Mie approximation of LSPR. The optical conductivity of Nb-doped TiO2 nanocrystals is found to be significantly lower than the electronic conductivity in similar Nb-doped thin films, suggesting that anisotropic carrier transport in TiO2 diminishes its optical response. Electrochemically-induced free carrier properties are explored through FTIR transmittance spectra of ex situ charged thin films of TiO2 nanocrystals. The optical transmittance of charged films is fitted to a layered optical model and compared for different charging potentials and capacities. Electrochemical reduction is found to induce greater variations in carrier concentration than synthetic doping alone, and demonstrates different scattering behavior. The role of interparticle coupling and effective medium effects in films are explored as explanations for the divergent behavior of charged and unbiased nanocrystals. These measurements reveal the different behavior of synthetic and charging-induced free carriers in plasmonic metal oxides, and provide a robust strategy for investigating electronic properties in semiconductor nanocrystals by IR optical measurements.