Reversible Tuning of the Surface Plasmon Resonance of Indium Tin Oxide Nanocrystals by Gas-Phase Oxidation and Reduction

Heavily doped oxide nanocrystals exhibit a tunable localized surface plasmon resonance (LSPR) in the infrared, a property that is promising for applications in photonics, spectroscopy, and photochemistry. Nanocrystal carrier density and, thus, spectral response are adjustable via chemical reaction; however, the fundamental processes that govern this behavior are poorly understood. Here, we study the time dependence of the LSPR supported by indium tin oxide (ITO) nanocrystals during O₂ and N₂ annealing with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that the LSPR red-shifts upon oxidation in O₂ and blue-shifts to its original position upon reduction in N₂. A reaction-diffusion model allows us to rationalize the underlying physicochemical processes and quantitatively connect nanocrystal redox chemistry, solid-state diffusion, carrier density, and the LSPR.