Water/solid interfaces are vital to our daily lives and are also a central theme across an incredibly wide range of scientific disciplines. Resolving the internal structure, that is, the O-H directionality, of water molecules adsorbed on solid surfaces has been one of the key issues of water science yet it remains challenging. Using a low-temperature scanning tunnelling microscope, we report submolecular-resolution imaging of individual water monomers and tetramers on NaCl(001) films supported by a Au(111) substrate at 5 K. The frontier molecular orbitals of adsorbed water were directly visualized, which allowed discrimination of the orientation of the monomers and the hydrogen-bond directionality of the tetramers in real space. Comparison with ab initio density functional theory calculations reveals that the ability to access the orbital structures of water stems from the electronic decoupling effect provided by the NaCl films and the precisely tunable tip-water coupling.

Proton transfer through hydrogen bonds plays a fundamental role in many physical, chemical and biological processes 1-5. Proton dynamics is susceptible to quantum tunnelling, which typically involves many hydrogen bonds simultaneously, leading to correlated many-body tunnelling 6-9. In contrast to the well-studied incoherent single-particle tunnelling, our understanding of many-body tunnelling is still in its infancy. Here we report the real-space observation of concerted proton tunnelling in a cyclic water tetramer using a cryogenic scanning tunnelling microscope. This is achieved by monitoring the reversible interconversion of the hydrogen-bonding chirality of the water tetramer with a chlorine-terminated scanning tunnelling microscope tip. We found that the presence of the Cl anion at the tip apex may either enhance or suppress the concerted tunnelling process, depending on the details of the coupling symmetry between the Cl ion and the protons. Our work opens up the possibility of controlling the quantum states of protons with atomic-scale precision. Many-body correlated tunnelling of protons is extremely sensitive to coupling with the atomic-scale environment owing to the demanding phase coherence between the protons. Spectroscopic techniques are not able to provide such local information because of the limitation of spatial resolution 5-7,10,11. Recently, the scanning tunnelling microscope (STM) has proved to be ideal for probing the intramolecular and intermolecular proton dynamics at the singlemolecule level 12-16. However, most of these studies are focused on classical over-barrier proton hopping, whereas through-barrier quantum tunnelling of protons has yet to be explored. Furthermore, tracking proton motion along hydrogen bonds requires the ability to resolve the hydrogen-bonding directionality 17. Although it was recently demonstrated that non-contact atomic force microscopy (NC-AFM; ref. 18) and state-of-the-art STM-based techniques 19 are able to visualize individual hydrogen bonds with unprecedented resolution, the asymmetry of the hydrogen bond and the extent of quantum delocalization of the shared proton has not been directly distinguishable. Here we show the possibility of directly visualizing the concerted tunnelling of four protons in an individual hydrogen-bonded water tetramer adsorbed on a Au-supported NaCl(001) film. This is achieved by monitoring the reversible interconversion of the hydrogen-bonding directionality and associated chirality of the tetramer using a submolecular orbital-imaging technique 20. To investigate the effect of the atomic-scale environment on the tunnelling process in a well-controlled manner, we functionalize the STM tip apex with a single chlorine anion and then tune the Cl-proton electric coupling in three dimensions with picometre

We report the quantitative assessment of nuclear quantum effects on the strength of a single hydrogen bond formed at a water-salt interface, using tip-enhanced inelastic electron tunneling spectroscopy based on a scanning tunneling microscope. The inelastic scattering cross section was resonantly enhanced by "gating" the frontier orbitals of water via a chlorine-terminated tip, so the hydrogen-bonding strength can be determined with high accuracy from the red shift in the oxygen-hydrogen stretching frequency of water. Isotopic substitution experiments combined with quantum simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the relatively strong ones. However, this trend can be completely reversed when a hydrogen bond is strongly coupled to the polar atomic sites of the surface.

Water-solid interactions are of broad importance both in nature and technology. The hexagonal bilayer model based on the Bernal-Fowler-Pauling ice rules has been widely adopted to describe water structuring at interfaces. Using a cryogenic scanning tunnelling microscope, here we report a new type of two-dimensional ice-like bilayer structure built from cyclic water tetramers on an insulating NaCl(001) film, which is completely beyond this conventional bilayer picture. A novel bridging mechanism allows the interconnection of water tetramers to form chains, flakes and eventually a two-dimensional extended ice bilayer containing a regular array of Bjerrum D-type defects. Ab initio density functional theory calculations substantiate this bridging growth mode and reveal a striking proton-disordered ice structure. The formation of the periodic Bjerrum defects with unusually high density may have a crucial role as H donor sites in directing multilayer ice growth and in catalysing heterogeneous chemical reactions on water-coated salt surfaces.

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