Atomic-Level Viscosity Distribution in the Hydration Layer

The viscosity of solvation structures is crucial for the development of energy-efficient biofunctional and electrochemical devices. Elucidating their subnanoscale distributions can cause the formation of a sustainable energy society. Here, we visualize the site-specific three-dimensional damping distribution on a ${\mathrm{CaCO}}_3$ surface composed of binary ion species using ultra-low-noise frequency modulation atomic force microscopy. With the support from molecular dynamics simulation, we found a strikingly large damping at the calcium sites, which demonstrates the capability of this methodology to visualize atomic-scale viscosity in the hydration layers. Our finding will expedite the evolutions of various functional devices.

Figure 1

Overview of the study. (a) Top and side views of the crystal structures of calcite (104). The broken blue rectangle represents the unit cell ($0.81\times 0.50{\mathrm{nm}}^2$). (b),(c) Representations of simultaneously acquired 3D conservative force (b) and damping (c) maps, in which the bottom faces show reconstructed constant $\mathrm{\Delta }f$ images at 500 Hz.

Figure 2

Constant-$\mathrm{\Delta }f$ atomic resolutions. (a)–(c) Experimental reconstructed topography (b) and damping image (b) at the contour surface at $\mathrm{\Delta }f=4\mathrm{kHz}$, which roughly corresponds to 0.6 nN, and crystal structure (c). Scale bars, 1 nm (a)–(c).

Figure 3

Constant-height atomic resolutions. (a)–(h) Horizontal distributions of experimental (a),(c),(e),(g) and simulated (b),(d),(f),(h) forces and (a),(b),(e),(f) damping (c),(g), and $D_w$ (d),(h) in the second (third) (a)–(d) and fourth (e)–(h) layers. The insets in (a),(c),(e),(g) and show Fourier-filtered images. The inverted color-scale bar is used in (d),(h). Scale bars, 1 nm (a)–(h).

Figure 4

Site dependencies of force and damping. (a)–(d) Experimental (a),(c) and simulated (b),(d) vertical force (a),(b), damping (c), and $D_w$ (d) maps on a calcite surface extracted from the 3D data along the broken lines in Figs. 3 and 3, respectively. The inverted color contrast is used in (d). (e),(f) Experimental force (e) and damping (f) profiles at the individual sites of calcite. Scale bars in (a)–(d) are the same.