(6gc) Ionic and Electronic Transport Properties in Bulk and Nano Metal Oxides

Agrawal, A., The University of Texas at Austin
Research Interests: Ionic transport, Solid State electrolyte, Optical properties, Plasmonics, Photonics

Teaching Interests: Transport Phenomenon, Material Science, Mathematics, Chemical Engineering Fundamentals

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 LSPR, the metal oxide LSPR can be tuned by doping, and structural control, or by in situ electrochemical or photochemical charging. While synthetic work for this class of materials has progressed significantly, quantitative correlation between their intrinsic properties to the final LSPR properties such as extinction and near field enhancement remains poorly explored, thus prohibiting their use in applications from plasmon derived smart windows to sensors. 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. This new insight demonstrated how the LSPR of semiconductor NCs can be controlled with shape, size and now crystal structure to tune the LSPR from near IR to mid-IR.

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. Electron energy loss spectroscopy was used to map the near field enhancement around these NCs responsible for coupling between the LSPR and molecular vibrations. 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.