The mobility of ions and charged molecules at interfaces underpins countless processes in science and technology. However, most experimental studies probing the diffusion of molecules or the response of ions to an applied electrical potential rely on averaging, often spatially and temporally. Probing the behaviour of single ions or molecules not only requires sub-nanometer spatial resolution, it also implies the ability to probe phenomena on the nano- to microsecond timescale. High-resolution Atomic Force Microscopy (AFM) is able to achieve sufficient resolution, but probing the associated dynamics remains typically limited to the millisecond to second time domain.
Here I present recent experimental developments aimed at tackling this problem. For molecules naturally diffusing along the interface, AFM can be augmented with ultrafast actuators (>50kHz) to achieve mechanical probing of the local diffusion process beyond the natural velocity of moving molecules. Probing the dynamics of ions in more challenging. One possible strategy is to investigate the response of the species of interest to a highly localized AC electric field over a range of relevant frequencies in an approach analogous to dielectric spectroscopy . We developed a strategy able to combine electrical AC measurements with high-resolution AFM at interfaces immersed in ionic solutions. Reliable contrasts can be obtained over a given sample, but the physical principles enabling and dominating the dielectric measurements remains unclear. This is problematic because it precludes a quantitative interpretation of the results. To better understand the Physics at play, we explore the impact of the setup geometry and tunable parameters on the measurements and on the range of the dielectric force.
 G. Gramse, et al. Nanotechnology 360 (2013) 415709.