Tunable multiscale infrared plasmonics with metal oxide nanocrystals




Agrawal, Ankit, Ph. D.

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Degenerately doped semiconductor nanocrystals (NC) exhibit a localized surface plasmon resonance (LSPR) that falls in the near- to mid-IR range of the electromagnetic spectrum. Unlike metal, the metal oxide LSPR characteristics can be further tuned by doping, and structural control, or by in situ electrochemical or photochemical charging. Here, we illustrate how intrinsic NC attributes like its crystal structure, shape and size, along with band structure and surface properties affects the LSPR properties and its possible applications. First, the interplay of NC shape and the intrinsic crystal structure on the LSPR was studied using model systems of In:CdO and Cs:WO₃, the latter of which has an intrinsic anisotropic crystal structure. For both systems, a change of shape from spherical to faceted NCs led to as anticipated higher near field enhancements around the particle. However, with Cs:WO₃, presence of an anisotropic hexagonal crystal structure, leads to additional strong LSPR band-splitting into two distinct peaks with comparable intensities. Second, plasmon-molecular vibration coupling, as a proof of concept for sensing applications, was shown using newly developed F and Sn codoped In₂O₃ NCs to couple to the C-H vibration of surface-bound oleate ligands. A combined theoretical and experimental approach was employed to describe the observed plasmon-plasmon coupling, the influence of coupling strength and relative detuning between the molecular vibration and LSPR on the enhancement factor, and the observed Fano lineshape by deconvoluting the combined response of the LSPR and molecular vibration in transmission, absorption, and reflection. Third, plasmon modulation through dynamic carrier density tuning was investigated using thin films of monodisperse ITO NCs with various doping level and sizes along with an in situ electrochemical setup. From the combination of the in-situ spectroelectrochemical analysis and optical modeling, it was found that often-neglected semiconductor properties, such as band structure modification upon doping and surface chemistry, strongly affect the LSPR modulation behavior. The influence of band structure and effects like Fermi level pinning by surface defect states were shown to cause a surface depletion layer that alters the LSPR properties, namely the extent of LSPR modulation, near field enhancement, and sensitivity of the LSPR to the surrounding.


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