Atomic-resolution three-dimensional hydration structures on a heterogeneously charged surface

Abstract

Local hydration structures at the solid–liquid interface around boundary edges on heterostructures are key to an atomic-level understanding of various physical, chemical and biological processes. Recently, we succeeded in visualising atomic-scale three-dimensional hydration structures by using ultra-low noise frequency-modulation atomic force microscopy. However, the time-consuming three-dimensional-map measurements on uneven heterogeneous surfaces have not been achieved due to experimental difficulties, to the best of our knowledge. Here, we report the local hydration structures formed on a heterogeneously charged phyllosilicate surface using a recently established fast and nondestructive acquisition protocol. We discover intermediate regions formed at step edges of the charged surface. By combining with molecular dynamics simulations, we reveal that the distinct structural hydrations are hard to observe in these regions, unlike the charged surface regions, possibly due to the depletion of ions at the edges. Our methodology and findings could be crucial for the exploration of further functionalities.

Atomic-Level Viscosity Distribution in the Hydration Layer

“…Nevertheless, site-specific damping measurements on several atom-chemical species at different heights have yet to be achieved. In the present study, we demonstrate this challenging experiment using a recently developed nondestructive observation protocol [7,8]. Consequently, by combining molecular dynamics (MD) simulations with our AFM observations, we discover that site-specific damping with a shoulderlike peak found in AFM data reflects the locally reduced water self-diffusion coefficient (D w ).…”

“…We first obtained a time-averaged 3D water density map, and assigned numbers to the density peaks in the averaged profile. We then calculated 3D force data using a solvent tip approximation model [30], which has accurately reproduced the experimental force data [7,8]. In the simulated average force profile [ Fig.…”

“…The apparent surface rendered in the crosssectional maps (boundaries between the white and green regions) of Fig. 1(b) was somewhat rumpled because of several vertical positions of the atoms, which indicated that our established nondestructive observation protocol [7,8] is essential for the tip to obtain deep access to the individual sites. To compare the damping acquired on the individual sites at the same tip-sample distance, we first reconstructed a constant Δf plane as a topographic image that could have been obtained slightly above the threshold plane.…”

“…Recently, we developed an ultra-low-noise liquid 3D frequency-modulation atomic force microscopy (FM-AFM) technique, which has enabled the evaluation of hydration distributions [5][6][7][8]. Furthermore, FM-AFM has an advantage in separately detecting the conservative and dissipative forces as frequency shift (Δf) and dissipative damping, respectively [9][10][11][12].…”

“…Since the details of the 3D experiment and simulation have been elaborated in our previous reports [7,8], we now briefly outline them. We used a home-built FM-AFM system with a LabVIEW PXI controller (National Instruments).…”