Publisher Correction: The effect of hydration number on the interfacial transport of sodium ions.

In this Letter, the links to Supplementary Videos 5, 7, 9 and 10 were incorrect, and there were some formatting errors in the Supplementary Video legends. These errors have been corrected online.

The effect of hydration number on the interfacial transport of sodium ions.

Ion hydration and transport at interfaces are relevant to a wide range of applied fields and natural processes1-5. Interfacial effects are particularly profound in confined geometries such as nanometre-sized channels6-8, where the mechanisms of ion transport in bulk solutions may not apply9,10. To correlate atomic structure with the transport properties of hydrated ions, both the interfacial inhomogeneity and the complex competing interactions among ions, water and surfaces require detailed molecular-level characterization. Here we constructed individual sodium ion (Na+) hydrates on a NaCl(001) surface by progressively attaching single water molecules (one to five) to the Na+ ion using a combined scanning tunnelling microscopy and noncontact atomic force microscopy system. We found that the Na+ ion hydrated with three water molecules diffuses orders of magnitude more quickly than other ion hydrates. Ab initio calculations revealed that such high ion mobility arises from the existence of a metastable state, in which the three water molecules around the Na+ ion can rotate collectively with a rather small energy barrier. This scenario would apply even at room temperature according to our classical molecular dynamics simulations. Our work suggests that anomalously high diffusion rates for specific hydration numbers of ions are generally determined by the degree of symmetry match between the hydrates and the surface lattice.

Probing structural superlubricity of two-dimensional water transport with atomic resolution.

Low-dimensional water transport can be drastically enhanced under atomic-scale confinement. However, its microscopic origin is still under debate. In this work, we directly imaged the atomic structure and transport of two-dimensional water islands on graphene and hexagonal boron nitride surfaces using qPlus-based atomic force microscopy. The lattice of the water island was incommensurate with the graphene surface but commensurate with the boron nitride surface owing to different surface electrostatics. The area-normalized static friction on the graphene diminished as the island area was increased by a power of ~-0.58, suggesting superlubricity behavior. By contrast, the friction on the boron nitride appeared insensitive to the area. Molecular dynamic simulations further showed that the friction coefficient of the water islands on the graphene could reduce to <0.01.

A qPlus-based scanning probe microscope compatible with optical measurements.

We design and develop a scanning probe microscope (SPM) system based on the qPlus sensor for atomic-scale optical experiments. The microscope operates under ultrahigh vacuum and low temperature (6.2 K). In order to obtain high efficiency of light excitation and collection, two front lenses with high numerical apertures (N.A. = 0.38) driven by compact nano-positioners are directly integrated on the scanner head without degrading its mechanical and thermal stability. The electric noise floor of the background current is 5 fA/Hz1/2, and the maximum vibrational noise of the tip height is below 200 fm/Hz1/2. The drift of the tip-sample spacing is smaller than 0.1 pm/min. Such a rigid scanner head yields small background noise (oscillation amplitude of ∼2 pm without excitation) and high quality factor (Q factor up to 140 000) for the qPlus sensor. Atomic-resolution imaging and inelastic electron tunneling spectroscopy are obtained under the scanning tunneling microscope mode on the Au(111) surface. The hydrogen-bonding structure of two-dimensional (2D) ice on the Au(111) surface is clearly resolved under the atomic force microscope (AFM) mode with a CO-terminated tip. Finally, the electroluminescence spectrum from a plasmonic AFM tip is demonstrated, which paves the way for future photon-assisted SPM experiments.

Visualizing Eigen/Zundel cations and their interconversion in monolayer water on metal surfaces.

The nature of hydrated proton on solid surfaces is of vital importance in electrochemistry, proton channels, and hydrogen fuel cells but remains unclear because of the lack of atomic-scale characterization. We directly visualized Eigen- and Zundel-type hydrated protons within the hydrogen bonding water network on Au(111) and Pt(111) surfaces, using cryogenic qPlus-based atomic force microscopy under ultrahigh vacuum. We found that the Eigen cations self-assembled into monolayer structures with local order, and the Zundel cations formed long-range ordered structures stabilized by nuclear quantum effects. Two Eigen cations could combine into one Zundel cation accompanied with a simultaneous proton transfer to the surface. Moreover, we revealed that the Zundel configuration was preferred over the Eigen on Pt(111), and such a preference was absent on Au(111).

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 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.