Abstract
Scanning probe microscopy 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 noninvasive 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 with negligible disturbance. This work may open an avenue for studying the intrinsic structure and dynamics of ice or water on surfaces, ion hydration, and biological water with atomic precision.
Highlights
Scanning probe microscopy 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
The atomic force microscopy (AFM) images of the water tetramers taken with a COterminated tip at large tip–water distance show prominent internal features, which resemble the electrostatic potential distribution within the cyclic tetramer
From the simulations (Fig. 3e, j), we found that those chiral structures obtained with the Cl-tip arise from the pronounced tip relaxation at close tip–water distances, which is determined by the complex interplay between the Pauli and the electrostatic interaction
Summary
Scanning probe microscopy 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. 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 with negligible disturbance. An intrinsic problem of SPM is that all the probes inevitably induce perturbation to the fragile water structure, due to the excitation of the tunneling electrons and the tip–water interaction forces, especially under the close-imaging condition applied in order to achieve ultrahigh spatial resolution. The multipole electrostatic force between the CO-tip and water is rather weak, allowing precise structural determination of the weakly bonded water clusters and even their metastable states with negligible disturbance
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