Rationalizing the Impact of Surface Depletion on Electrochemical Modulation of Plasmon Resonance Absorption in Metal Oxide Nanocrystals

Dynamic control over the localized surface plasmon resonance (LSPR) makes doped metal oxide nanocrystals (NCs) promising for several optoelectronic applications including electro-chromic smart windows and redox sensing. Metal oxide NCs such as tin-doped indium oxide display tunable infrared LSPRs via electrochemical charge injection and extraction as a function of the externally applied potential. In this work we have employed dispersion phase electrochemical charging/discharging to study the mechanism behind the optical modulation on an individual NC scale. The optical modulation of the LSPR is dominated by a sharp variation in intensity during reduction and oxidation along with an only modest shift in the LSPR frequency. With a core-shell modeling approach, in which an active NC core surrounded by a depleted shell is assumed, we were able to reproduce the trends in and main features of our experimental results. The shell thickness depends on the applied potential and we extracted the temporal evolution of the shell thickness together with the variation of the Drude parameters until equilibrium was reached. The variation of the core versus shell volume fraction as a function of electrochemical potential reinforces the importance of the depletion layer in highly doped NCs and uncovers important implications on their near and far field plasmonic properties.