We present STM observations of the first layer of water on Pt(111). Because scanning parameters typically used for high-resolution imaging can easily disturb the fragile water layer, acquiring images is particularly challenging. Nonetheless, employing an extremely small tunneling current, we were able to extract enough structural detail to decipher how individual water molecules arrange themselves.

At a growth temperature of 140K, we found large regions consisting of the √37 and √39 phases previously observed in diffraction experiments [1]. The main characteristic of both are triangularly-shaped regions, which appear as depressions in STM. The triangles are embedded in a hexagonal lattice of water molecules, which, remarkably, is rotated by approximately 30° relative to the √3x√3-R30° structure that is the usual starting point for discussions of ice films. We propose that the triangular regions consist of a central hexagon of water molecules surrounded by alternating pentagons and heptagons. Twelve water molecules in the center of this (“575757” di-interstitial) defect lie approximately flat, with O atoms directly atop Pt atoms. Elsewhere, the water molecules lie “H-down," i.e., with an H atom beneath the O. Image simulations based on DFT calculations are consistent with this non-conventional model.

The dramatic structural differences between the √39 phase and 3-D ice consisting of stacked unrotated classic bilayers suggests that 3-D islands do not grow on top of the wetting layer, rather the wetting-layer molecules substantially rearrange when 3-D ice nucleates.

 

This work was supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract DE-AC04-94AL85000.

 

[1] For example, A. Glebov, A. P. Graham, A. Menzel and J. P. Toennies, J. Chem. Phys. 106, 9382 (1997).