Nonuniform charge carrier distribution within plasmonic semiconductor nanocrystals




Gibbs, Stephen Larkin

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Plasmonic nanocrystals (NCs) can act as focusing lenses that capture incident light at wavelengths near the localized surface plasmon resonance peak frequency (ω [subscript LSPR]) and confine it into nanoscopic volumes. This concentration of photon energy manifests in enhanced electric fields, increased temperature, and even electron transfer near the nanocrystal surface. Traditional plasmonic materials, metals, have ω [subscript LSPR] in the visible range, whereas doped semiconductor nanocrystals have ω [subscript LSPR] tunable through the infrared. This has helped to expand plasmon-enhanced processes to include those that lie within infrared wavelengths such as molecular sensing, photothermal therapy, and infrared driven photochemistry. The infrared ω [subscript LSPR] arises from a lower charge carrier concentration in doped semiconductors as compared to metals, which also promotes another phenomenon: nonuniform intra-NC charge carrier concentration. Depletion regions near the NC surface create an insulating shell, nearly devoid of charge carriers, that surrounds the higher carrier concentration, plasmonic core. This depletion layer can hinder all forms of plasmon-enhancement. Through careful colloidal synthetic techniques, quantitative spectroscopy, and Drude theory modelling, I demonstrated understanding and control over the thickness of this depletion layer with varied size and doping concentration. Going one step further, I employed radial control over dopant placement to not only tune the depletion layer thickness, but the entire intra-NC carrier concentration profile. In fact, I engineered a doping profile that promoted a secondary plasmonic absorption mode within a single nanocrystal. Engineering the intra-NC doping profile is uniquely achievable for doped semiconductor, which opens up the door to improved infrared plasmonic devices.


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