Electric-Double-Layer-Modulation Microscopy

The electric double layer (EDL) formed around charged nanostructures at the liquid-solid interface determines their electrochemical activity and influences their electrical and optical polarizability. We experimentally demonstrate that restructuring of the EDL at the nanoscale can be detected by dark-field scattering microscopy. Temporal and spatial characterization of the scattering signal demonstrates that the potentiodynamic optical contrast is proportional to the accumulated charge of polarizable ions at the interface and that its time derivative represents the nanoscale ionic current. The material specificity of the EDL formation is used in our work as a label-free contrast mechanism to image nanostructures and perform spatially resolved cyclic voltammetry on an ion-current density of a few attoamperes, corresponding to the exchange of only a few hundred ions.

Figure 1

Measurement of the potentiodynamic contrast of the EDL on a rough surface. (a) The setup for measurement of the PDOC and the electrochemical cell configuration. (b) A typical scattering image from the ITO surface (scale bar $2{\mu }_m$). (c) The scattering intensity from the grain, annotated by a square in (b), plotted while changing the surface potential in a triangle-shape waveform. (d) The normalized intensity change plotted as a function of the cell potential for 100 cycles (light symbols) and the average of all cycles after correction for drift (bold symbols) for two different sweeping amplitudes. (e) An atomic-force-microscope scan of the ITO surface (scale bar $4{\mu }_m$).

Figure 2

The optical signal from the electric double layer under a variable surface potential: The measured electric current through the cell for an alternating voltage of (a) a triangle-shape and (b) a square waveform between $+1$ and $-1$ V. In both (a) and (b), the measured current is shown using green dots and the applied potential is shown using pink dots. The integrated current (green circles), which corresponds to the accumulated charge at the interface, shows the same temporal behavior as the measured scattering intensity (blue circles, averaged value over several cycles). The exponential change of the current toward equilibrium corresponds to the charging time of the electrochemical cell.

Figure 3

(a) The average PDOC contrast for the same ITO grain for three different salts and (b) the electric cyclic voltagram of the ITO substrate of the same measurements as in (a). The measured salts are $\mathrm{Na}\mathrm{Cl}$ (blue circles), $\mathrm{Na}\mathrm{Br}$ (red squares), and $\mathrm{Na}\mathrm{I}$ (orange triangles).

Figure 4

(a) A dark-field scattering image of the ITO substrate after the deposition of $\mathrm{Cr}$ through a $\mathrm{Si}\mathrm{N}$ stencil (scale bar $4{\mu }_m$). The deposition locations are on a triangular lattice. (b) The blue color scale shows the average covariance of each pixel intensity with the PDOC curve of a $\mathrm{Cr}$ particle, while the red color scale shows the covariance with the PDOC curve and an ITO grain. The corresponding reference PDOC curves are depicted in panels (c) and (d). All pixels with a covariance of less than 0.1 with either of the two references are colored in white for clarity.