Fig. 1: Atomic structure of Cs⁺-water monolayer.

a Constant-current STM images of 2D Cs⁺-water layer on Au(111). Upper: The 2D Cs⁺-water layer is composed of single-stranded zigzag Cs⁺ chains. Lower left: The zoomed-in STM image of a 2D cation-water layer. Lower right: The line profile across the edge showing the heights of the layers, 1.7 (water molecules)−2.2 Å (Cs⁺ cations). b Constant-height AFM (Δf) images at the tip heights of 70 pm (left) and −140 pm (right). c Simulated AFM images at the tip heights of 12.45 Å (left) and 11.00 Å (right). d, e Top (d) and side (e) views of the structural model of 2D Cs⁺-water structure on the Au(111) surface. Au, Cs, H, and O atoms are denoted as yellow, cyan, white, and red spheres, respectively. The bridging water molecules are indicated by black (white) arrows in d (b and c). The flat-lying water molecules are indicated by red arrows in b–d. The water hexagons are indicated by black (red) dashed lines in d (b and c). The tip heights in b are referenced to the STM set point on the Au substrate (100 mV, 10 pA). The tip heights in c are defined as the vertical distance between the apex atom of the metal tip and the outmost atom of Au substrate. The oscillation amplitude of experimental and simulated images is 100 pm.