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Ying Jiang and collaborators report the growth of the thinnest ice sheet in Nature

Water ices are ubiquitous in nature. The structure and growth of ice plays critical roles in an incredibly broad spectrum of materials science, tribology, biology, atmospheric science. As early as 1920s, Dennison, Bragg and Barnes studied the crystal structure of ice using X-ray diffractions, which represent the first attempts to learn about ice structures at molecular scale. For the following nearly 100 years, massive experiments and theoretical calculations have led to the discovery of eighteen crystalline phases (three-dimensional phases), among which the hexagonal ice (Ice Ih) is the most common ice in the biosphere (Fig. 1a and b). However, whether the ice can exist at two dimensions (2D) has been under longstanding debate. At the monolayer limit, the 2D ice usually contains high-density unsaturated hydrogen bonds, and is thus unstable compared with the 3D case. Although 2D ice can be stabilized through the interaction with the substrate, such a "2D ice" largely relies on the structure and symmetry of the substrate, and cannot be considered as the genuine 2D ice.

Figure 1. a, The thick sheet floated on the Rose ocean in Antarctica. b, The crystal structure of Ice Ih. c, The 2D ice obtained in this work (rendered from experimental image).

 

Now, the teams led by Prof. Ying Jiang, Prof. Limei Xu and Prof. Enge Wang of International Center for Quantum Materials (ICQM) of Peking University, in collaboration with Prof. Xiao Cheng Zeng of Department of Physics of University of Nebraska-Lincoln, successfully grow a 2D bilayer hexagonal ice (named "2D ice I") and image the 2D ice growth at the edges with atomic resolution, through a combined study using scanning probe microscope (SPM), density functional theory (DFT) calculations and molecular dynamics (MD) simulations. This work is published in Nature on January 2nd, 2020.

The key step is to choose the hydrophobic Au(111) surface as the substrate, which only weakly interacts with the water molecules. By carefully tuning the temperature and water pressure, the researchers were able to grow a single-crystal 2D ice fully wetting the surface (Fig. 1c). For the following, they used a qPlus-based atomic force microscopy (AFM) with CO-functionalized tip, which relies on the competition between the high-order electrostatic force and Pauli repulsion force, to image the 2D ice island (Fig. 2). The submolecular-resolution images reveal an interlocked bilayer-ice structure, consisting of two flat hexagonal water layers. In each water layer, half of the water molecules are lying flat while the other half are vertical with one O-H either upward or downward. The vertical water in one layer donates an H bond to the flat water in the other layer, leading to a fully saturated H-bonding structure. Such a stand-alone 2D ice was first predicted before by Koga et al. in 1997 using MD simulations, but direct imaging of its atomic structure has been lacking until now. Therefore, it is the first genuine 2D ice confirmed both by theory and experiment, i.e., named 2D ice I.