Nanoscale probing of local dielectric changes at the interface between solids and aqueous saline solutions

The mobility of dissolved ions and charged molecules at interfaces underpins countless processes in science and technology. Experimentally, this is typically measured from the averaged response of the charges to an electrical potential. High-resolution Atomic Force Microscopy (AFM) can image single adsorbed ions and molecules at solid-liquid interfaces, but probing the associated dynamics remains highly challenging. One possible strategy is to investigate the response of the species of interest to a highly localized AC electric field in an approach analogous to dielectric spectroscopy. The dielectric force experienced by the AFM tip apex is modulated by the dielectric properties of the sample probed, itself sensitive to the mobilities of solvated charges and dipoles. Previous work successfully used this approach to quantify the dielectric constant of thin samples, but with limited spatial resolution. Here we propose a strategy to simultaneously map the nanoscale topography and local dielectric variations across a range of interfaces by conducting high-resolution AFM imaging concomitantly with electrical AC measurements in a multifrequency approach. The strategy is tested over a 500 MHz bandwidth in pure liquids with different dielectric constants and in saline aqueous solutions. In liquids with higher dielectric constants, the system behaves as inductive-resistive-capacitive but the adjunction of ions removes the inductive resonances and precludes measurements at higher frequencies. High-resolution imaging is demonstrated over single graphene oxide (GrO) flakes with simultaneous but decoupled dielectric measurements. The dielectric constant is consistent and reproducible across liquids, except at higher salt concentrations where frequency-dependent effects occur. The results suggest the strategy is suitable for nanometre-level mapping of the dielectric properties of solid-liquid interfaces, but more work is needed to fully understand the different physical effects underpinning the measurements.