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Recent advances in probing two-dimensional materials confined water by scanning probe microscopy

Interfacial water is ubiquitous in nature and industry, which plays a crucial role in areas such as wetting, corrosion, lubrication and electrochemistry. The structure and dynamic behavior of interfacial water have been proven to be different from those of bulk water. Therefore, to deeply understand the unique properties of interfacial water, structural imaging with atomic resolution is of great importance. Up to now, owing to its high temporal and spatial resolution, scanning probe microscopy (SPM) has been extensively applied to investigate water molecules at solid interfaces. However, most of those SPM investigations are conducted at cryogenic temperature or under ultra-high vacuum, as under ambient conditions the capillary force between SPM tip and water molecules will perturb the structures of interfacial water on solid surface. Thanks to the two-dimensional (2D) materials which can be utilized as ultra-thin coatings owing to its atomic thickness, good flexibility, impermeability and chemical stability. The graphene coating could protect and fix the structures of interfacial water, which enables the structural imaging by SPM. The 2D material confined water expands the scope of SPM research on interfacial water, promoting the development of interface science. The structure and dynamics of 2D material confined water can be influenced by the structure and hydrophilicity of the interface materials, as well as environment factors such as humidity, temperature and pressure. In recent years, a great deal of works have been conducted with SPM to explore the properties of 2D material confined water and remarkable research results have been achieved. In this review, we will introduce recent progress in probing 2D materials confined water by SPM under ambient conditions, including detecting the microscopic structures of 2D confined water, probing the dynamic behaviors of 2D confined water as well as nanomanipulation of 2D confined water. Furthermore, the effects of confined water on the optical, electrical and frictional properties of the 2D material coatings are also discussed. Finally, a perspective on the directions of future SPM studies of 2D material confined water and the challenges faced by this field are presented. We hope that this review will inspire interest in 2D confined water and provide some new ideas for researchers in this field.