Sub-diffractional infrared absorption of two-dimensional water.

Several theoretical frameworks predict that water's vibrational dynamics and dielectric properties undergo dramatic changes when confined to a monolayer. However, experimental validation has been limited, often relying on extrapolations from bulk water or multilayered systems at solid-liquid interfaces. Here, we overcome these challenges by using Ångström-scale slit-like capillaries formed within van der Waals heterostructures, enabling infra-red (IR) signal enhancement necessary to probe the vibrational signature of two-dimensional (2D) water using synchrotron IR micro spectroscopy. This enhancement arises from the high reflectivity of atomically flat graphite combined with the resonating waveguide modes accommodated by hexagonal boron nitride (hBN). For the extreme monolayer state, we show that the measured changes in water's intramolecular vibrational modes reflects a significant frustration of the average hydrogen-bonding network. Furthermore, supported by DFT-MD, our data reveal the average structure of monolayer water and its evolution towards the bulk-state based on the population of the manyfold water clusters combined. Evidence of increased density, despite such disrupted hydrogen-bonding network, suggests the emergence of an unusually discontinuous phase. Our findings provide the first direct experimental evidence of water's vibrational signatures and hydrogen-bonding network in 2D, offering new insights into the fundamental properties of confined water.