Silver iodide surface structure and ice nucleation ability investigated by noncontact AFM

Abstract

Silver iodide (AgI) is used as a cloud seeding material due to its ability to nucleate ice efficiently, which is explained by the good lattice match between AgI and hexagonal ice. The basal (0001) cleavage plane of AgI deviates from the lattice of hexagonal ice by as little as 2.5%. However, AgI consists of stacked planes of positively charged Ag+ alternating with negatively charged I–. Cleaving a AgI crystal along the (0001) plane thus exposes Ag+ and I– terminated surfaces. Both terminations are polar and inherently unstable. We present atomically resolved noncontact atomic force microscopy (NC-AFM) images that show how AgI(0001) surfaces compensate for this non-zero electric dipole perpendicular to the surface. Both Ag and I terminated surfaces form reconstructions, whose structure affects their ice nucleating abilities. NC-AFM images of UHV cleaved surfaces exposed to water vapor reveal that ice forms an epitaxial layer only on the Ag terminated surface, whereas on the I termination ice forms three-dimensional clusters. These atomic-level observations could enhance our understanding of ice formation processes in the atmosphere.