Confined Monolayer Ice Between CaF2 (111) and Graphene: Structure and Stability

Shi-Qi Li,Shi Qiu,Maodu Chen,Junfeng Gao,Hongsheng Liu

doi:10.3389/fphy.2021.740627

Abstract

Water monolayer can form in layered confined systems. Here, CaF2 (111) and graphene are chosen as modeling systems to explore the structure and stability of confined monolayer water. First, water molecules tend to intercalate into a confined space between graphene and CaF2, rather than on a bare surface of graphene. Water molecules can move fast in the confined space due to a low diffusion barrier. These water molecules are likely to aggregate together, forming monolayer ice. Four ice phases including ice II, ice III, ice IV, and ice Ih are compared in this confined system. Intriguingly, all the ice phases undergo very small deformation, indicating the 2D monolayer ice can be stable in the CaF2–graphene–confined system. Beyond, projected band structures are also plotted to understand the electronic behavior of these confined ice phases. Nearly all the bands originated from confined ices are flat and locate about 2–3 eV below the Fermi level. Binding energy calculations suggest that the stability sequence in this confined system as follows: Ih-up ≈ Ih-down ≈ II &lt; IV &lt; III. Our results bring new insights into the formation of water monolayer production in such a confined condition.

Highlights

The behavior of water at surfaces and a nanoconfined space [1] in two, three, or one dimensions is significantly different from that in bulk ice [2,3,4]
Ice III tends to be confined between CaF2 (111) and graphene under a wide T and P span
We have theoretically studied the structure and stability of monolayer ice phases confined between CaF2 (111) and graphene

Summary

Introduction

The behavior of water at surfaces and a nanoconfined space [1] in two, three, or one dimensions is significantly different from that in bulk ice [2,3,4]. The subtle interplay between the water–substrate and water–water interactions brings about many new distinctive ice configurations on different substrates [14,15,16,17,18]. In vacuum and on weakly interacting substrates, Xu et al [14] found a helical ice monolayer with every six water molecules helically arranged along the normal of the basal plane by performing an intensive structural search based on ab initio calculations. On Au(111), a two-dimensional (2D) interlocked ice consisting of two flat hexagonal water layers in which the hexagons in two sheets are in registry is imaged by non-contact atomic force microscopy and identified by density functional calculations [15]

Methods

Results

Conclusion