Emergent Optoelectronic Properties through Controlling Nonuniform Charge Carrier Profiles within a Plasmonic Semiconductor Nanocrystal

Plasmonic nanocrystals (NCs) can act as a focusing lens that captures incident light at wavelengths near the localized surface plasmon resonance peak frequency (w_LSPR) and confines 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 w_LSPR in the visible range, whereas doped semiconductor NCs have w_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 w_LSPR arises from a lower charge carrier concentration in doped semiconductors as compared to metals, which also promotes another phenomenon: non-uniform intra-NC charge density.

In this work, I discuss how 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 and therefore we aim to demonstrate understanding and control over it. We show that, to a certain degree, the thickness of this depletion layer can be controlled with size and doping concentration. Going one step further, we employ radial control over dopant placement to not only tune the depletion layer thickness, but the entire intra-NC carrier concentration profile. In fact, we are able to engineer a doping profile that promotes a secondary plasmonic absorption mode within a single nanocrystal. This work illustrates the versatility of doped metal oxides for applications in infrared plasmonics.