Observation of liquid-solid transition of nanoconfined water by a quantum sensor.

Nanoconfined water exhibits many abnormal properties compared to bulk water. The origin of those anomalies has been attributed to the dramatically changed water structure under confinement, but still remains controversial due to the lack of experimental techniques for nanoconfined water molecules. Here, we addressed this issue by combining qPlus-type scanning probe microscopy (qPlus-SPM) with quantum sensing based on a single shallow nitrogen-vacancy (NV) center in diamond [1]. Using the local electric field from a metallic tip, we improved the NV's sensitivity up to the level of a single proton [2, 3]. Then we characterize both the dynamics and structure of confined water by NV-based nanoscale nuclear magnetic resonance (NMR). We observed a liquid-solid phase transition of nanoconfined water at ambient temperature with a critical confinement size of ~2 nm, below which the water diffusion was significantly suppressed and the hydrogen-bonding network of water became structurally ordered [4]. These findings are promising to present a unified picture to resolve the longstanding debates and uncertainties regarding the origins of various anomalous properties, such as superlubricity, ultrafast transport, and ultralow dielectric constant.