Towards Quantitative Interpretation of 3D Atomic Force Microscopy at Solid-Liquid Interfaces

TL;DR

This paper reviews 3D atomic force microscopy at solid-liquid interfaces, focusing on interpreting force maps to quantitatively reveal atomic-scale liquid density distributions and their relation to interfacial structures.

Contribution

It clarifies how to interpret 3D-AFM force maps quantitatively, linking force oscillations to liquid density and entropy modulation at interfaces.

Findings

Force perturbations reflect intrinsic liquid density profiles.

Oscillatory forces relate to probe-modulated liquid entropy.

Quantitative atomic-scale liquid density can be derived from force maps.

Abstract

Three-dimensional atomic force microscopy (3D-AFM) has been a powerful tool to probe the atomic-scale structure of solid-liquid interfaces. As a nanoprobe moves along the 3D volume of interfacial liquid, the probe-sample interaction force is sensed and mapped, providing information on not only the solid morphology, but also the liquid density distribution. To date 3D-AFM force maps of a diverse set of solid-liquid interfaces have been recorded, revealing remarkable force oscillations that are typically attributed to solvation layers or electrical double layers. However, despite the high resolution down to sub-angstrom level, quantitative interpretation of the 3D force maps has been an outstanding challenge. Here we will review the technical details of 3D-AFM and the existing approaches for quantitative data interpretation. Based on evidences in recent literature, we conclude that the perturbation-induced AFM force paradoxically represents the intrinsic, unperturbed liquid density profile. We will further discuss how the oscillatory force profiles can be attributed to the probe-modulation of the liquid configurational entropy, and how the quantitative, atomic-scale liquid density distribution can be derived from the force maps.