Extended Data Fig. 1: Height-dependent experimental and simulated ∆f images with quadrupole/neutral tips.

a-e, Experimental (upper) and simulated ∆f images with quadrupole (d_z², q = −0.1 e, middle) and neutral (q = 0 e, lower) tips. Simulated images acquired by the quadrupole tip agree better with the experimental results. At a larger tip height, the H-up water molecules are visualized as individual depressions both by quadrupole and neutral tips (middle and lower panels in a), due to long-range vdW attraction from the higher O atoms (~13 pm). However, the electrostatic force between the quadrupole tip and H atoms can enlarge the attractive interaction, leading to much larger force contrast. When the tip height is further decreased, the H-up water molecules evolve into bright protrusions due to the Pauli repulsion. Meanwhile, because of the electrostatic repulsion, O-up water molecules also show bright feature with the quadrupole tip, indicated by red dashed circles in upper and middle panels in c. In contrast, the O-up water molecules are revealed as dark features by the neutral tip, indicated by the red dashed circle in the lower panel in c. Tip heights are given above each image. f-h, Experimental (f, oscillation amplitude, 40 pm) and simulated (g,h, oscillation amplitude, 200 pm) frequency-shift (∆f) versus distance curves above an H-up water molecule (red line) and an O-up water molecule (black line). The experimental (f) and simulated (g, the quadrupole tip) curves show qualitatively similar behavior, where the turning point of the H-up water molecule occurs at a larger tip height and its minimum ∆f is deeper than that of the O-up water molecule. The curve acquired by the neutral tip (h) shows the opposite behavior, where the minimum ∆f of the O-up water molecule is deeper. Such a difference arises from the attractive (repulsive) force between the positively (negatively) charged H (O) and the negatively charged CO tip.