Strain Relief during Ice Growth on a Hexagonal Template

Heterogeneous
ice nucleation at solid surfaces impacts many areas
of science, from environmental processes, such as precipitation, to
microbiological systems and food processing, but the microscopic mechanisms
underpinning nucleation remain unclear. Discussion of ice growth has
often focused around the role of the surface in templating the structure
of water, forcing the first layer to adopt the registry of the underlying
substrate rather than that of ice. To grow a thick ice film, water
in the first few ice layers must accommodate this strain, but understanding
how this occurs requires detailed molecular-scale information that
is lacking. Here we combine scanning tunneling microscopy, low-energy
electron diffraction, and work-function measurements with electronic
structure calculations to investigate the initial stages of ice growth
on a Pt alloy surface, having a lattice spacing 6% larger than ice.
Although the first layer of water forms a strictly commensurate hexagonal
network, this behavior does not extend to the second layer. Instead,
water forms a 2D structure containing extended defect rows made from
face-sharing pentamer and octamer rings. The defect rows allow the
majority of second-layer water to remain commensurate with the solid
surface while compensating lateral strain by increasing the water
density close to that of an ice surface. The observation of octamer–pentamer
rows in ice films formed on several surfaces suggests that the octamer–pentamer
defect motif acts as a flexible strain relief mechanism in thin ice
films, providing a mechanism that is not available during the growth
of strained films in other materials, such as semiconductors.