Real-space imaging of interfacial water with submolecular resolution

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.