Weakly perturbative imaging of interfacial water with submolecular resolution by atomic force microscopy

Scanning probe microscopy has been extensively applied to probe interfacial water in manyinterdisciplinary fields but the disturbance of the probes on the hydrogen-bonding structureof water has remained an intractable problem. Here, we report submolecular-resolutionimaging of the water clusters on a NaCl(001) surface within the nearly noninvasive region bya qPlus-based noncontact atomic force microscopy. Comparison with theoretical simulationsreveals that the key lies in probing the weak high-order electrostatic force between thequadrupole-like CO-terminated tip and the polar water molecules at large tip-water distances.This interaction allows the imaging and structural determination of the weakly bondedwater clusters and even of their metastable states with negligible disturbance. This work mayopen an avenue for studying the intrinsic structure and dynamics of ice or water on surfaces,ion hydration, and biological water with atomic precision.