  The structure and properties of water or ice are of great interest to
researchers due to their importance in the biological, cryopreservation and
environmental fields. Hexagonal ice (Ih) is a common ice phase in nature and
has been extensively studied; however, microstructural investigations at the
atomic or molecular scale are still lacking. In this paper, the fine structure
of quasi-2-dimensional ice Ih films was directly examined using cryogenic
transmission electron microscopy. Two types of thin Ih films were observed:
perfect single crystals growing along the [0001] direction and crystals with
stacking faults, including both basal (BSF) and prismatic (PSF) ones, along the
orientation of [11-20]; these results were further confirmed by theoretical
calculations. Importantly, for the first time, the stacking faults in ice Ih
were directly visualized and resolved. In light of the extension behavior of
the chair conformation composed of the water molecules, we elucidated the
formation mechanism of BSF in Ih, namely, the Ic phase. This study not only
determined the structural characteristics of ice structure at the molecular
scale but also provided important concepts for researchers to more fully
understand the growth kinetics of ice crystals at the atomic scale.
