Title: Submolecular-resolution non-invasive imaging of interfacial water with atomic force microscopy,
Abstract: Scanning probe microscopy (SPM) has been extensively applied to probe
interfacial water in many interdisciplinary fields but the disturbance of the
probes on the hydrogen-bonding structure of water has remained an intractable
problem. Here we report submolecular-resolution imaging of the water clusters
on a NaCl(001) surface within the nearly non-invasive region by a qPlus-based
noncontact atomic force microscopy. Comparison with theoretical simulations
reveals that the key lies in probing the weak high-order electrostatic force
between the quadrupole-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 bonded water clusters and even of their metastable
states without inducing any disturbance. This work may open up new possibility
of studying the intrinsic structure and electrostatics of ice or water on bulk
insulating surfaces, ion hydration and biological water with atomic precision.