Two-dimensional bilayer ice structure on Au(111) probed by noncontact atomic force microscope.

The initial stages of heterogeneous ice nucleation at surfaces are relevant to many fields as diverse as atmospheric chemistry, astrophysics and biology. To date, scanning tunneling microscope (STM) has been ideal to provide molecular-scale description of water-solid interaction. However, a majority of the STM works have been done focused on hydrophilic surfaces, such as Pt, Ru, Pd, NaCl, MgO etc. The atomic–scale structures of ice overlayers on hydrophobic surfaces are still lacking. The main difficulty lies in the perturbation of the probe on the fragile water structures on the hydrophobic surfaces because of the weak water-surface interaction.
Here, we reported the observation and characterization of a two-dimensional (2D) ice structure on the highly hydrophobic Au(111) surface using a qPlus-based noncontact atomic force microscope (NC-AFM) with a CO-terminated tip. Our work revealed a peculiar buckled bilayer ice structure with disordered proton distribution, which is modulated by the underlying inhomogeneous surface potential. Those results are further substantiated by density functional theory (DFT) calculations. This work open up the possibility of probing ice growth on hydrophobic surfaces with atomic precision.