Probing Dielectric Screening in the Electrochemical Double Layer with Surface Plasmon Resonant Grating Structures

We use surface plasmon resonance (SPR) metal nanostructures to probe dielectric screening within the electric double layer (EDL) of ionic liquids and aqueous electrolytes. By observing voltage-induced changes in the SPR, we can directly measure the dielectric screening (i.e., Debye screening) produced within the EDL as a function of the ion concentration. As a function of the applied potential, we observe shifts in the resonant angle (i.e., Δθ) of the grating that can be related to the change in the local index of refraction (i.e., Δnlocal) of the electrolyte with a minimum resolution of Δnlocal = 0.001. At high ion concentrations, we observe a dramatic decrease in the double layer's ability to screen charge, which contradicts standard Debye-Hückel theory. Detailed electromagnetic simulations (using the finite-difference time-domain (FDTD) modeling approach) relate the observed shifts in the plasmon resonance (Δθ) and change in reflection (ΔR) to the changes in Δnlocal of the electrolyte and the thickness (h) of the EDL, thus providing new insight into the structure of the double layer and the probe depth associated with these voltage-induced changes. We believe that this technique can be applied more broadly as an in situ diagnostic tool to monitor the electrochemical environment under working electrochemical conditions and to provide insight into elusive interfacial phenomena. This approach is reminiscent of early work from the 1980s on surface plasmons for studying electrochemical interfaces, where surface plasmons were excited using the Kretschmann geometry. However, in this study, we exploit the surface sensitivity of the SPR phenomenon with a voltage-differential spectroscopy approach to investigate the anomalous dielectric screening behavior recently observed in ionic liquid-based solutions and high-concentration aqueous electrolytes.