2025 Volume 29 Issue 3-4 Pages 33-38
Clay mineral nanosheets obtained by exfoliation are approximately 1 nm thick, two-dimensional materials bearing negative charges originating from isomorphic substitution. These nanosheets are widely used in constructing supramolecular complexes through electrostatic interactions. Although the formation of such complexes critically depends on the nanosheet’s surface charge distribution, its local variations remain poorly understood. In this study, we aimed to visualize the surface charge distribution of clay mineral nanosheets using annular dark-field scanning transmission electron microscopy (ADF-STEM). To this end, heavy alkali metal ions, Rb+ and Cs+, were introduced as elemental markers via ion exchange, and their distributions were visualized as bright spots in both monolayer and multilayer regions. The experimental observations closely matched image simulations, confirming that the bright spots correspond to the marker ions. However, under continuous electron beam irradiation—especially at an accelerating voltage of 300 kV—the markers exhibited noticeable mobility. These findings highlight the need for structural strategies to immobilize marker ions within the interlayer space or nanosheet framework to enable stable and reliable visualization of surface charge distributions.
