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The interfaces between solids and water films in air play fundamental roles in physicochemical phenomena, biological functions, and nano-fabrication. Though the properties of the interfaces have been considered to be irrelevant to the water film thickness, we found distinctive mechanical features of the interface between a cleaved muscovite mica surface and a thin water film grown in humid air, dissimilar to those in bulk water, using frequency-modulation atomic force microscopy. The thin water film grew with quasi-stabilized hydration networks of water molecules, tightly bound each other at the interface, to a thickness of ~2 nm at near-saturating humidity. Consequently, defective structures of the hydration networks persisted vertically through the hydration layers at the interface, and K+ ions on the cleaved surface remained without dissolution into the water film. The results provide atomistic insights into thin water films in regard to epitaxial-like growth from vapour and the motion of water molecules and ions therein.

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The Nb-doped rutile titanium dioxide TiO2(110)-(1 × 1) surface, which was obtained after cleaning by cycles of Ar+ sputtering and annealing in ultrahigh vacuum, was examined by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscope (STM) techniques. The angle-resolved XPS measurement revealed that Nb in a pentavalent state was concentrated in the near-surface region. Preferential removal of O by the Ar+ sputtering induced enrichment of Nb and Ti on the surface, and the Nb cations with a low diffusivity in bulk TiO2 remained in the near-surface region in the subsequent annealing. Atom-sized spots assignable to either Nb atoms incorporated into the rutile lattice or Nb adatoms were observed in the STM images. The density of the spots was almost one-third of that of the surface Nb atoms estimated from the XPS results, which indicated that a large part of the near-surface Nb atoms was in the interstitial sites and was invisible in the STM images. The Nb interstitials were segregated to the surface to form oxide particles when the surface was annealed in O2. The height of the Nb adatom in STM images was reversibly changed, dependent on the adatom–adatom distances. The change in the image height of the adatoms was attributed to the change in the oxidation state.